Liposomal formulations of anticancer copper(II) thiosemicarbazone complexes

α-N-Heterocyclic thiosemicarbazones such as triapine and COTI-2 are currently investigated as anticancer therapeutics in clinical trials. However, triapine was widely inactive against solid tumor types. A likely explanation is the short plasma half-life time and fast metabolism. One promising approach to overcome these drawbacks is the encapsulation of the drug into nanoparticles (passive drug-targeting). In a previous work we showed that it was not possible to stably encapsulate free triapine into liposomes. Hence, in this manuscript we present the successful preparation of liposomal formulations of the copper(II) complexes of triapine and COTI-2. To this end, various drug-loading strategies were examined and the resulting liposomes were physico-chemically characterized. Especially for liposomal Cu-triapine, a decent encapsulation efficacy and a slow drug release behavior could be observed. In contrast, for COTI-2 and its copper(II) complex no stable loading could be achieved. Subsequent in vitro studies in different cell lines with liposomal Cu-triapine showed the expected strongly reduced cytotoxicity and DNA damage induction. Also in vivo distinctly higher copper plasma levels and a continuous release could be observed for the liposomal formulation compared to free Cu-triapine. Taken together, the here presented nanoformulation of Cu-triapine is an important step further to increase the plasma half-life time and tumor targeting properties of anticancer thiosemicarbazones.

Nickel and zinc complexes of testosterone N4-substituted thiosemicarbazone: Selective cytotoxicity towards human colorectal carcinoma cell line HCT 116 and their cell death mechanisms

Two new Schiff base ligands (TE and TF) were prepared from conjugation of testosterone with 4-(4-ethylphenyl)-3-thiosemicarbazide and 4-(4-fluorophenyl)-3-thiosemicarbazide, respectively. Their nickel (NE and NF) and zinc (ZE and ZF) complexes were reported. X-ray crystallography revealed a distorted square planar geometry was adopted by NE. The compounds demonstrated excellent selectivity towards the colorectal carcinoma cell line HCT 116 despite their weak preferences towards the prostate cancer cell lines (PC-3 and LNCaP). Against HCT 116, all these compounds were able to arrest cell cycle at G₀/G₁ phase and induce apoptosis via mitochondria-dependent (TE, NE, and TF) and extrinsic apoptotic pathway (ZE, NF, and ZF). Moreover, only ZE was able to act as topoisomease I poison and halt its enzymatic reactions although all compounds presented excellent affinity towards DNA.

In vitro activity, ultrastructural studies and in silico pharmacokinetic properties of indol-3-yl-thiosemicarbazones derivatives and analogues against juvenile and adult worms of S. mansoni

The present work aimed to carry out in vitro biological assays of indol-3-yl derivatives thiosemicarbazones (2a-e) and 4-thiazolidinones (3a-d) against juvenile and adult worms of S. mansoni, as well as the in silico determination of pharmacokinetic parameters for the prediction of the oral bioavailability of these derivatives. All compounds were initially screened at a concentration of 200 μM against S. mansoni adult worms and the results evidenced the good activity of compounds 2b, 2d and 3b, which caused 100% mortality after 24, 48 and 72 h, respectively. Subsequent studies with these same compounds revealed that compound 2b was able to reduce the viability of the parasites by 85% and 83% at concentrations of 200 and 100 μM, respectively. In relation to the juvenile worms, all compounds (2b, 2d and 3b) were able to cause mortality, but compound 2b demonstrated better activity causing 100% mortality in 48 h. Additionally, it was possible to observe reduction in the viability of juvenile worms of 85%, 81% and 64% at concentrations of 200, 100 and 50 μM, respectively. Several ultrastructural damages were observed when adult and juvenile S. mansoni worms were exposed to compound 2b (200 μM) that was characterized by extensive destruction by the integument, which may justify the mortality rate of cultured parasites. In the DNA interaction assay, fragmentation of the genetic material of adult worms when treated with compound 2b (200 μM) was evidenced, indicating the apoptosis process as mechanism of parasite death. Regarding pharmacokinetic properties, all derivatives are according to the required parameters, predicting good oral bioavailability for the studied compounds. The results presented in this study reveal the good activity of compound 2b in both adult and juvenile worms of S. mansoni, pointing this compound as promising in the development of further studies on schistosomicidal activity.

Imaging of changes in copper trafficking and redistribution in a mouse model of Niemann-Pick C disease using positron emission tomography

Niemann-Pick C disease (NPC) is an autosomal recessive lysosomal storage disorder resulting from mutations in the NPC1 (95% of cases) or NPC2 genes. Disturbance of copper homeostasis has been reported in NPC1 disease. In this study we have used whole-body positron emission tomography (PET) and brain electronic autoradiography with copper-64 (64Cu), in the form of the copper(II) bis(thiosemicarbazonato) complex 64Cu-GTSM, to image short-term changes in copper trafficking after intravenous injection in a transgenic mouse model of NPC1 disease. 64Cu-GTSM is taken up in all tissues and dissociates rapidly inside cells, allowing monitoring of the subsequent efflux and redistribution of 64Cu from all tissues. Significantly enhanced retention of 64Cu radioactivity was observed in brain, lungs and blood at 15 h post-injection in symptomatic Npc1-/- transgenic mice compared to wildtype controls. The enhanced retention of 64Cu in brain was confirmed by electronic autoradiography, particularly in the midbrain, thalamus, medulla and pons regions. Positron emission tomography imaging with 64Cu in selected chemical forms could be a useful diagnostic and research tool for the management and understanding of NPC1 disease.

Studies of copper trafficking in a mouse model of Alzheimer's disease by positron emission tomography: comparison of 64Cu acetate and 64CuGTSM

Alzheimer's disease can involve brain copper dyshomeostasis. We aimed to determine the effect of AD-like pathology on 64Cu trafficking in mice, using positron emission tomography (PET imaging), during 24 hours after intravenous administration of ionic 64Cu (Cu(ii) acetate) and 64Cu-GTSM (GTSMH2 = glyoxalbis(thiosemicarbazone)). Copper trafficking was evaluated in 6-8-month-old and 13-15 month-old TASTPM transgenic and wild-type mice, by imaging 0-30 min and 24-25 h after intravenous administration of 64Cu tracer. Regional 64Cu distribution in brains was compared by ex vivo autoradiography to that of amyloid-β plaque. 64Cu-acetate showed uptake in, and excretion through, liver and kidneys. There was minimal uptake in other tissues by 30 minutes, and little further change after 24 h. Radioactivity within brain was focussed in and around the ventricles and was significantly greater in younger mice. 64CuGTSM was taken up in all tissues by 30 min, remaining high in brain but clearing substantially from other tissues by 24 h. Distribution in brain was not localised to specific regions. TASTPM mice showed no major changes in global or regional 64Cu brain uptake compared to wildtype after administration of 64Cu acetate (unlike 64Cu-GTSM) but efflux of 64Cu from brain by 24 h was slightly greater in 6-8 month-old TASTPM mice than in wildtype controls. Changes in copper trafficking associated with Alzheimer's-like pathology after administration of ionic 64Cu are minor compared to those observed after administration of 64Cu-GTSM. PET imaging with 64Cu could help understand changes in brain copper dynamics in AD and underpin new clinical diagnostic imaging methods.

Phase I trial of daily triapine in combination with cisplatin chemotherapy for advanced-stage malignancies

PURPOSE

Advanced-stage malignancies have increased deoxyribonucleotide demands in DNA replication and repair, making deoxyribonucleotide supply a potential exploitable target for therapy based on ribonucleotide reductase (RNR) inhibition.

METHODS

A dose-finding phase I trial was conducted of intravenous (i.v.) triapine, a small-molecule RNR inhibitor, and cisplatin chemotherapy in patients with advanced-stage solid tumor malignancies. Patients received dose-finding levels of i.v. triapine (48-96 mg/m²) and i.v. cisplatin (20-75 mg/m²) on 1 of 3 different schedules. The primary endpoint was to identify the maximum tolerated dose of a triapine-cisplatin combination. Secondary endpoints included the rate of triapine-cisplatin objective response and the pharmacokinetics and bioavailability of a single oral triapine dose. (Clinicaltrials.gov number, NCT00024323).

RESULTS

The MTD was 96 mg/m² triapine daily days 1-4 and 75 mg/m² cisplatin split over day 2 and day 3. Frequent grade 3 or 4 adverse events included fatigue, dyspnea, leukopenia, thrombocytopenia, and electrolyte abnormalities. No objective responses were observed; 5 (50%) of 10 patients treated at the MTD had stable disease. Pharmacokinetics indicated an oral triapine bioavailability of 88%.

CONCLUSIONS

The triapine-cisplatin combination may be given safely in patients with advanced-stage solid tumor malignancies. On the basis of these results, a phase I trial adequately powered to evaluate oral triapine bioavailability in women with advanced-stage uterine cervix or vulvar cancers is underway.

Novel Zinc(II) Complexes [Zn(atc-Et)₂] and [Zn(atc-Ph)₂]: In Vitro and in Vivo Antiproliferative Studies

Cisplatin and its derivatives are the main metallodrugs used in cancer therapy. However, low selectivity, toxicity and drug resistance are associated with their use. The zinc(II) (Zn(II)) thiosemicarbazone complexes [Zn(atc-Et)₂] (1) and [Zn(atc-Ph)₂] (2) (atc-R: monovalent anion of 2-acetylpyridine N4-R-thiosemicarbazone) were synthesized and fully characterized in the solid state and in solution via elemental analysis, Fourier transform infrared (FTIR), ultraviolet-visible (UV-Vis) and proton nuclear magnetic resonance (¹H NMR) spectroscopy, conductometry and single-crystal X-ray diffraction. The cytotoxicity of these complexes was evaluated in the HepG2, HeLa, MDA-MB-231, K-562, DU 145 and MRC-5 cancer cell lines. The strongest antiproliferative results were observed in MDA-MB-231 and HepG2 cells, in which these complexes displayed significant selective toxicity (3.1 and 3.6, respectively) compared with their effects on normal MRC-5 cells. In vivo studies were performed using an alternative model (Artemia salina L.) to assure the safety of these complexes, and the results were confirmed using a conventional model (BALB/c mice). Finally, tests of oral bioavailability showed maximum plasma concentrations of 3029.50 µg/L and 1191.95 µg/L for complexes 1 and 2, respectively. According to all obtained results, both compounds could be considered as prospective antiproliferative agents that warrant further research.

Microwave gallium-68 radiochemistry for kinetically stable bis(thiosemicarbazone) complexes: structural investigations and cellular uptake under hypoxia

We report the microwave synthesis of several bis(thiosemicarbazones) and the rapid gallium-68 incorporation to give the corresponding metal complexes. These proved kinetically stable under 'cold' and 'hot' biological assays and were investigated using laser scanning confocal microscopy, flow cytometry and radioactive cell retention studies under normoxia and hypoxia. (68)Ga complex retention was found to be 34% higher in hypoxic cells than in normoxic cells over 30 min, further increasing to 53% at 120 min. Our data suggests that this class of gallium complexes show hypoxia selectivity suitable for imaging in living cells and in vivo tests by microPET in nude athymic mice showed that they are excreted within 1 h of their administration.

Predicting location of recurrence using FDG, FLT, and Cu-ATSM PET in canine sinonasal tumors treated with radiotherapy

Dose painting relies on the ability of functional imaging to identify resistant tumor subvolumes to be targeted for additional boosting. This work assessed the ability of FDG, FLT, and Cu-ATSM PET imaging to predict the locations of residual FDG PET in canine tumors following radiotherapy. Nineteen canines with spontaneous sinonasal tumors underwent PET/CT imaging with radiotracers FDG, FLT, and Cu-ATSM prior to hypofractionated radiotherapy. Therapy consisted of 10 fractions of 4.2 Gy to the sinonasal cavity with or without an integrated boost of 0.8 Gy to the GTV. Patients had an additional FLT PET/CT scan after fraction 2, a Cu-ATSM PET/CT scan after fraction 3, and follow-up FDG PET/CT scans after radiotherapy. Following image registration, simple and multiple linear and logistic voxel regressions were performed to assess how well pre- and mid-treatment PET imaging predicted post-treatment FDG uptake. R(2) and pseudo R(2) were used to assess the goodness of fits. For simple linear regression models, regression coefficients for all pre- and mid-treatment PET images were significantly positive across the population (P < 0.05). However, there was large variability among patients in goodness of fits: R(2) ranged from 0.00 to 0.85, with a median of 0.12. Results for logistic regression models were similar. Multiple linear regression models resulted in better fits (median R(2) = 0.31), but there was still large variability between patients in R(2). The R(2) from regression models for different predictor variables were highly correlated across patients (R ≈ 0.8), indicating tumors that were poorly predicted with one tracer were also poorly predicted by other tracers. In conclusion, the high inter-patient variability in goodness of fits indicates that PET was able to predict locations of residual tumor in some patients, but not others. This suggests not all patients would be good candidates for dose painting based on a single biological target.

A dual radiolabelling approach for tracking metal complexes: investigating the speciation of copper bis(thiosemicarbazonates) in vitro and in vivo

Copper(II)bis(thiosemicarbazonato) complexes such as [(64)Cu]Cu-ATSM continue to be investigated for positron emission tomography (PET) imaging of tumour hypoxia. However, the currently proposed mechanisms for the mode of action of these complexes are unable to account fully for their observed biological behaviour. In order to examine the roles of the copper metal and the ligand, we designed a pair of (123)I/(64)Cu-copper bis(thiosemicarbazonates), radiolabelled at either the metal or at the ligand. In vitro cellular retention studies of the orthogonal pair demonstrate for the first time that retention under hypoxia involves dissociation of the copper bis(thiosemicarbazone) complex, consistent with the previously suggested mechanism of reductive trapping of copper. In contrast, in vivo biodistribution and dynamic PET/SPECT imaging of the orthogonally labelled complexes underline our previous findings for [(64)Cu]Cu-ATSM and [(64)Cu]Cu-acetate, providing further support for the important contribution of copper metabolism in the in vivo hypoxia selectivity of Cu-ATSM. This dual radiolabelling approach may find applications for determining the speciation of other metal complexes in vitro and in vivo.

Mononuclear dioxomolybdenum(VI) thiosemicarbazonato complexes: Synthesis, characterization, structural illustration, in vitro DNA binding, cleavage, and antitumor properties

Four dioxomolybdenum(VI) complexes were synthesized by reacting [MoO2(acac)2] with N-ethyl-2-(5-bromo-2-hydroxybenzylidene) hydrazinecarbothioamide (1), N-ethyl-2-(5-allyl-3-methoxy-2-hydroxybenzylidene) hydrazinecarbothioamide (2), N-methyl-2-(3-tert-butyl-2-hydroxybenzylidene) hydrazinecarbothioamide (3), and N-ethyl-2-(3-methyl-2-hydroxybenzylidene) hydrazinecarbothioamide (4). The molecular structures of 1, 2, and all the synthesized complexes were determined using single crystal X-ray crystallography. The binding properties of the ligand and complexes with calf thymus DNA (CT-DNA) were investigated via UV, fluorescence titrations, and viscosity measurement. Gel electrophoresis revealed that all the complexes cleave pBR 322 plasmid DNA. The cytotoxicity of the complexes were studied against the HCT 116 human colorectal cell line. All the complexes exhibited more pronounced activity than the standard reference drug 5-fluorouracil (IC50 7.3μM). These studies show that dioxomolybdenum(VI) complexes could be potentially useful in chemotherapy.

Cardiac hypoxia imaging: second-generation analogues of 64Cu-ATSM

Myocardial hypoxia is an attractive target for diagnostic and prognostic imaging, but current approaches are insufficiently sensitive for clinical use. The PET tracer copper(II)-diacetyl-bis(N4-methylthiosemicarbazone) ((64)Cu-ATSM) has promise, but its selectivity and sensitivity could be improved by structural modification. We have therefore evaluated a range of (64)Cu-ATSM analogs for imaging hypoxic myocardium.

METHODS

Isolated rat hearts (n = 5/group) were perfused with normoxic buffer for 30 min and then hypoxic buffer for 45 min within a custom-built triple-γ-detector system to quantify radiotracer infusion, hypoxia-dependent cardiac uptake, and washout. A 1-MBq bolus of each candidate tracer (and (18)F-fluoromisonidazole for comparative purposes) was injected into the arterial line during normoxia, and during early and late hypoxia, and their hypoxia selectivity and pharmacokinetics were evaluated. The in vivo pharmacokinetics of promising candidates in healthy rats were then assessed by PET imaging and biodistribution.

RESULTS

All tested analogs exhibited hypoxia sensitivity within 5 min. Complexes less lipophilic than (64)Cu-ATSM provided significant gains in hypoxic-to-normoxic contrast (14:1 for (64)Cu-2,3-butanedione bis(thiosemicarbazone) (ATS), 17:1 for (64)Cu-2,3-pentanedione bis(thiosemicarbazone) (CTS), 8:1 for (64)Cu-ATSM, P < 0.05). Hypoxic first-pass uptake was 78.2% ± 7.2% for (64)Cu-ATS and 70.7% ± 14.5% for (64)Cu-CTS, compared with 63.9% ± 11.7% for (64)Cu-ATSM. Cardiac retention of (18)F-fluoromisonidazole increased from 0.44% ± 0.17% during normoxia to 2.24% ± 0.08% during hypoxia. In vivo, normoxic cardiac retention of (64)Cu-CTS was significantly lower than that of (64)Cu-ATSM and (64)Cu-ATS (0.13% ± 0.02% vs. 0.25% ± 0.04% and 0.24% ± 0.03% injected dose, P < 0.05), with retention of all 3 tracers falling to less than 0.7% injected dose within 6 min. (64)Cu-CTS also exhibited lower uptake in liver and lung.

CONCLUSION

(64)Cu-ATS and (64)Cu-CTS exhibit better cardiac hypoxia selectivity and imaging characteristics than the current lead hypoxia tracers, (64)Cu-ATSM and (18)F-fluoromisonidazole.

Application of 62Cu-diacetyl-bis (N4-methylthiosemicarbazone) PET imaging to predict highly malignant tumor grades and hypoxia-inducible factor-1α expression in patients with glioma

BACKGROUND AND PURPOSE

Hypoxic tissue evaluation in glioma is important for predicting treatment response and establishing antihypoxia therapy. In this preliminary study, (62)Cu-ATSM PET was used to determine its validity as a biomarker for distinguishing tumor grade and tissue hypoxia.

MATERIALS AND METHODS

(62)Cu-ATSM PET was performed in 22 patients with glioma, and the (62)Cu-ATSM SUV(max) and T/B ratio were semiquantitatively evaluated. (62)Cu-ATSM uptake distribution was qualitatively evaluated and compared with MR imaging findings. HIF-1α expression, a hypoxia marker, was compared with (62)Cu-ATSM uptake values.

RESULTS

The (62)Cu-ATSM SUV(max) and T/B ratio were significantly higher in grade IV than in grade III gliomas (P = .014 and .018, respectively), whereas no significant differences were found between grade III and grade II gliomas. At a T/B ratio cutoff threshold of 1.8, (62)Cu-ATSM uptake was predictive of HIF-1α expression, with 92.3% sensitivity and 88.9% specificity. The mean T/B ratio was also significantly higher in HIF-1α-positive glioma tissue than in HIF-1α-negative tissue (P = .001). Using this optimal threshold of T/B ratio, (62)Cu-ATSM PET showed regional uptake in 61.9% (13/21) of tumors within the contrast-enhanced region on MR imaging, which was significantly correlated with presence of a necrotic component (P = .002).

CONCLUSIONS

Our results demonstrated that (62)Cu-ATSM uptake is relatively high in grade IV gliomas and correlates with the MR imaging findings of necrosis. Moreover, the (62)Cu-ATSM T/B ratio showed significant correlation with HIF-1α expression. Thus, (62)Cu-ATSM appears to be a suitable biomarker for predicting highly malignant grades and tissue hypoxia in patients with glioma.

Radiolabeled Cu-ATSM as a novel indicator of overreduced intracellular state due to mitochondrial dysfunction: studies with mitochondrial DNA-less ρ0 cells and cybrids carrying MELAS mitochondrial DNA mutation

OBJECTIVES

Radiolabeled Cu-diacetyl-bis (N(4)-methylthiosemicarbazone) (*Cu-ATSM), including (60/62/64)Cu-ATSM, is a potential imaging agent of hypoxic tumors for positron emission tomography (PET). We have reported that *Cu-ATSM is trapped in tumor cells under intracellular overreduced states, e.g., hypoxia. Here we evaluated *Cu-ATSM as an indicator of intracellular overreduced states in mitochondrial disorders using cell lines with mitochondrial dysfunction.

METHODS

Mitochondrial DNA-less ρ(0)206 cells; the parental 143B human osteosarcoma cells; the cybrids carrying mutated mitochondria from a patient of mitochondrial myopathy, encephalopathy, lactic acidosis and stroke-like episodes (MELAS) (2SD); and that carrying wild-type one (2SA) were used. Cells were treated under normoxia or hypoxia, and (64)Cu-ATSM uptake was examined to compare it with levels of biological reductant NADH and NADPH.

RESULTS

ρ(0)206 cells showed higher (64)Cu-ATSM uptake than control 143B cells under normoxia, whereas (64)Cu-ATSM uptake was not significantly increased under hypoxia in ρ(0)206 cells. Additionally, (64)Cu-ATSM uptake showed correlate change to the NADH and NADPH levels, but not oxygenic conditions. 2SD cells showed increased (64)Cu-ATSM uptake under normoxia as compared with the control 2SA, and (64)Cu-ATSM uptake followed NADH and NADPH levels, but not oxygenic conditions.

CONCLUSIONS

(64)Cu-ATSM accumulated in cells with overreduced states due to mitochondrial dysfunction, even under normoxia. We recently reported that (62)Cu-ATSM-PET can visualize stroke-like episodes maintaining oxygen supply in MELAS patients. Taken together, our data indicate that *Cu-ATSM uptake reflects overreduced intracellular states, despite oxygenic conditions; thus, *Cu-ATSM would be a promising marker of intracellular overreduced states for disorders with mitochondrial dysfunction, such as MELAS, Parkinson's disease and Alzheimer's disease.

Feasibility of dose painting using volumetric modulated arc optimization and delivery

PURPOSE

Dose painting strategies are limited by optimization algorithms in treatment planning systems and physical constraints of the beam delivery. We investigate dose conformity using the RapidArc optimizer and beam delivery technique. Furthermore, robustness of the plans with respect to positioning uncertainties are evaluated.

METHODS

A head & neck cancer patient underwent a [(61)Cu]Cu-ATSM PET/CT-scan. PET-SUVs were converted to prescribed dose with a base dose of 60 Gy, and target mean dose 90 Gy. The voxel-based prescription was converted into 3, 5, 7, 9, and 11 discrete prescription levels. Optimization was performed in Eclipse, varying the following parameters: MLC leaf width (5 mm and 2.5 mm), number of arcs (1 and 2) and collimator rotation (0, 15, 30 and 45 degrees). Dose conformity was evaluated using quality volume histograms (QVHs), and relative volumes receiving within ±5% of prescribed dose (Q(0.95-1.05)). Deliverability was tested using a Delta4(®) phantom. Robustness was tested by shifting the isocenter 1 mm and 2 mm in all directions, and recalculating the dose.

RESULTS

Good conformity was obtained using MLC leaf width 2.5 mm, two arcs, and collimators 45/315 degrees, with Q(0.95-1.05)=92.8%, 91.6%, 89.7% and 84.6%. Using only one arc or increasing the MLC leaf width had a small deteriorating effect of 2-5%. Small changes in collimator angle gave small changes, but large changes in collimator angle gave a larger decrease in plan conformity; for angles of 15 and 0 degrees (two arcs, 2.5 mm leaf width), Q(0.95-1.05) decreased by up to 15%. Consistency between planned and delivered dose was good, with ∼90% of gamma values <1. For 1 mm shift, Q(0.95-1.05) was decreased by 5-15%, while for 2 mm shift, Q(0.95-1.05) was decreased to 55-60%.

CONCLUSIONS

Results demonstrate feasibility of planning of prescription doses with multiple levels for dose painting using RapidArc, and plans were deliverable. Robustness to positional error was low.

Some thoughts on the mechanism of cellular trapping of Cu(II)-ATSM

Cu(II)-ATSM continues to be investigated, both in the laboratory and in the clinic, as a tumor hypoxia imaging agent. However, meaningful interpretation of these images requires a more complete understanding of the mechanism by which the tracer is trapped within the cell. Cu(II)-ATSM is a simple molecule and its biochemical interaction with cells is similarly simple, mainly based upon redox chemistry. Here we suggest that the trapping mechanism is biphasic. The first phase is a reduction/oxidation cycle involving thiols and molecular oxygen. This can be followed by interaction with proteins in the mitochondria leading to more permanent retention of the tracer. The uptake mechanism is complicated by this second step because of the changes in the cell resulting from hypoxia, such as an increase in nicotinamide adenine dinucleotide (NADH) redox state and differences in cellular biochemistry and cell proteomes. These changes may lead to differences in the extent of trapping and retention of the (64)Cu in different cell types. For example, copper uptake might be increased in cells with lower pH due to the lower stability of metal bis(thiosemicarbazones) under acidic conditions. Reaction rates with cellular reductants also vary with pH, which differs between cellular organelles. For Cu(II)-ATSM to reach its full potential, more complete characterization of the mechanism of cellular trapping in different cell types is required.

A phase I study of Triapine in combination with doxorubicin in patients with advanced solid tumors

PURPOSE

To assess the maximum-tolerated dose (MTD), dose-limiting toxicity (DLT), pharmacokinetics and antitumor activity of Triapine administered in combination with doxorubicin.

STUDY DESIGN

Patients were treated with doxorubicin intravenously (IV) on day 1 and Triapine IV on days 1-4 of a 21-day cycle. The starting dose (level 1) was doxorubicin 60 mg/m(2) and Triapine 25 mg/m(2). PK analysis was performed at various time-points before and after treatment.

RESULTS

Twenty patients received a total of 49 courses of treatment on study. At dose level 2 (doxorubicin 60 mg/m(2), Triapine 45 mg/m(2)), two patients experienced DLTs (febrile neutropenia, grade 4 thrombocytopenia). An additional three patients were enrolled at dose level 1 without initial toxicity. Enrollment then resumed at dose level 2a with a decreased dose of doxorubicin (45 mg/m(2)) with Triapine 45 mg/m(2). The two patients enrolled on this level had two DLTs (diarrhea, CVA). Enrollment was planned to resume at dose level 1; however, the sixth patient enrolled to this cohort developed grade 5 heart failure (ejection fraction 20%, pretreatment EF 62%) after the second course. Thus, doxorubicin and Triapine were reduced to 45 and 25 mg/m(2), respectively (level 1a), prior to resuming enrollment at dose level 1, the MTD. The main drug-related toxicity was myelosuppression. Non-hematologic toxicities included mild-to-moderate fatigue, grade 3 diarrhea and grade 4 CVA. There was one treatment-related death due to heart failure. While no objective responses were observed, subjective evidence of clinical activity was observed in patients with refractory melanoma and prostate cancer.

CONCLUSIONS

Pretreated patients with advanced malignancies can tolerate the combination of Triapine and doxorubicin at doses that achieve subjective clinical benefit with the main treatment-related toxicities being myelosuppression and fatigue. The MTD was determined to be doxorubicin 60 mg/m(2) on day 1 and Triapine 25 mg/m(2) on days 1-4 of a 21-day cycle.

Species dependence of [64Cu]Cu-Bis(thiosemicarbazone) radiopharmaceutical binding to serum albumins

INTRODUCTION

Interactions of three copper(II) bis(thiosemicarbazone) positron emission tomography radiopharmaceuticals with human serum albumin, and the serum albumins of four additional mammalian species, were evaluated.

METHODS

64Cu-labeled diacetyl bis(N4-methylthiosemicarbazonato)copper(II) (Cu-ATSM), pyruvaldehyde bis(N4-methylthiosemicarbazonato)copper(II) (Cu-PTSM) and ethylglyoxal bis(thiosemicarbazonato)copper(II) (Cu-ETS) were synthesized and their binding to human, canine, rat, baboon and porcine serum albumins quantified by ultrafiltration. Protein binding was also measured for each tracer in human, porcine, rat and mouse serum.

RESULTS

The interaction of these neutral, lipophilic copper chelates with serum albumin is highly compound- and species-dependent. Cu-PTSM and Cu-ATSM exhibit particularly high affinity for human serum albumin (HSA), while the albumin binding of Cu-ETS is relatively insensitive to species. At HSA concentrations of 40 mg/ml, "% free" (non-albumin-bound) levels of radiopharmaceutical were 4.0+/-0.1%, 5.3+/-0.2% and 38.6+/-0.8% for Cu-PTSM, Cu-ATSM and Cu-ETS, respectively.

CONCLUSIONS

Species-dependent variations in radiopharmaceutical binding to serum albumin may need to be considered when using animal models to predict the distribution and kinetics of these compounds in humans.

Assessment of Cu-ETS as a PET radiopharmaceutical for evaluation of regional renal perfusion

The copper(II) complex of ethylglyoxal bis(thiosemicarbazone) (Cu-ETS) was evaluated as a positron emission tomography (PET) radiopharmaceutical for assessment of regional renal perfusion.

METHODS

The concordance of renal flow estimates obtained with 11- and 15-microm microspheres was confirmed in four immature farm pigs using co-injected (46)Sc- and (57)Co-microspheres administered into the left ventricle. With the use of both immature farm pigs (n=3) and mature Göttingen minipigs (n=6), regional renal radiocopper uptake following intravenous [(64)Cu]Cu-ETS administration was compared to microsphere measurements of renal perfusion. The distribution and kinetics of [(64)Cu]Cu-ETS were further studied by PET imaging of the kidneys. The rate of [(64)Cu]Cu-ETS decomposition by blood was evaluated in vitro, employing octanol extraction to recover intact [(64)Cu]Cu-ETS.

RESULTS

The co-injected 11- and 15-microm microspheres provided similar estimates of renal flow. A linear relationship was observed between the renal uptake of intravenous [(64)Cu]Cu-ETS and regional renal perfusion measured using microspheres. [(64)Cu]Cu-ETS provided high-quality PET kidney images demonstrating the expected count gradient from high-flow outer cortex to low-flow medulla. When incubated with pig blood in vitro at 37 degrees C, the [(64)Cu]Cu-ETS radiopharmaceutical was observed to decompose with a half-time of 2.8 min.

CONCLUSION

Cu-ETS appears suitable for use as a PET radiopharmaceutical for evaluation of regional renal perfusion, affording renal uptake of radiocopper that varies linearly with microsphere perfusion measurements. Quantification of renal perfusion (in ml min(-1) g(-1)) with [(60,61,62,64)Cu]Cu-ETS will require correcting the arterial input function for the fraction of blood radiocopper remaining present as the intact Cu-ETS radiopharmaceutical, since the Cu-ETS chelate has limited chemical stability in blood. Rapid octanol extraction of blood samples appears suitable as an approach to capturing the actual blood concentration of [(60/61/62/64)Cu]Cu-ETS.

Phase I and pharmacokinetic study of Triapine, a potent ribonucleotide reductase inhibitor, in adults with advanced hematologic malignancies

Triapine, a potent inhibitor of ribonucleotide reductase, has demonstrated anti-leukemia activity in pre-clinical models. We conducted a Phase I study of Triapine administered as a 2 h infusion for 5 days in 25 adults with advanced leukemias. We established that Triapine at 96 mg/m2 once a day can be given safely on days 1-5 and 15-19 or 1-5 and 8-12 of a 4-week cycle. When administered twice a day on days 1-5 and 8-12, the maximum tolerated dose of Triapine appears to be 64 mg/m2, although the true criteria for DLT were not met by protocol definition. No CR or PR were observed, but 76% of patients had a >50% reduction in white blood cell counts. At all dose levels, the peak plasma concentration of Triapine (2.2-5.5 microM) was above levels required to achieve in vitro/in vivo leukemia growth inhibition. Based on these data, we conclude that Triapine warrants further investigation in hematologic malignancies.

Production and tumour uptake of [64Cu]Pyruvaldehyde-bis (N4-methylthiosemicarbazone) for PET and/or therapeutic purposes

BACKGROUND

Copper-64 (T(1/2)=12.7 degrees h) is an important radionuclide used both in PET imaging and therapy. [(64)Cu]-pyruvaldehyde- bis(N(4)-methylthiosemicarbazone) ([64Cu]-PTSM) has already been used in the detection of cerebral and myocardial blood flow. In this study, a simple production method and tumor accumulation of [(64)Cu]-PTSM in fibrosarcoma-bearing mice were reported.

MATERIAL AND METHODS

Cu-64 was produced via the 68Zn(p, alpha n)(64)Cu nuclear reaction. [(64)Cu]-PTSM was prepared using in-house made PTSM ligand and [(64)Cu]cuprous acetate and injected to fibrosarcoma-bearing mice.

RESULTS

Copper-64 was prepared in chloride form ( approximately 200 mCi, > 95% chemical yield at 180 degrees microA for 1.1 h irradiation, radionuclidic purity > 96%, copper-67 as impurity). The solution of (64)Cu- PTSM was prepared in > 80% radiochemical yield and more than 98% radiochemical purity. A significant tumor uptake was observed 2 hours post injection in tumor-bearing mice (tumor/muscle: 9, tumor/blood: 6).

CONCLUSION

[(64)Cu]-PTSM was prepared on a radiopharmaceutical scale using readily available zinc-68, with high quality and was shown to possess application in the therapy and/or imaging of fibrosarcoma.

64Cu-azabicyclo[3.2.2]nonane thiosemicarbazone complexes: radiopharmaceuticals for PET of topoisomerase II expression in tumors

Topoisomerase II (Topo-II) is an essential enzyme in the DNA replication process and is the primary cellular target for many of the most widely used and effective anticancer agents. It has been reported that thiosemicarbazones (TSCs) are potent antitumor agents that inhibit Topo-II. The aim of this study was to investigate the relationship between the in vitro and in vivo behavior of novel (64)Cu-TSC complexes and the expression of Topo-II activity.

METHODS

Four (4)N-azabicyclo[3.2.2]nonane TSC derivatives (EPH142, EPH143, EPH144, and EPH270) were successfully radiolabeled with (64)Cu, to form lipophilic cations of the general formula [(64)Cu(L)]Cl, and the partition coefficient (logP) values were determined. One agent [(64)Cu-EPH270](+) was observed in vitro in cultured cell studies. The kinetics of 2 compounds, [(64)Cu-EPH144](+) and [(64)Cu-EPH270](+), were examined in mice bearing L1210 tumors and small-animal PET was conducted in mice bearing L1210 and PC-3 tumors, which expressed high and low levels of Topo-II, respectively. All data were compared with the activity and levels of Topo-II, as determined by a commercially available assay kit and western blot analysis.

RESULTS

The 4 complexes were radiolabeled by incubation of (64)CuCl(2) with the ligand in ethanolic solution. The complexes were isolated in high radiochemical purity, as determined by radio-thin-layer chromatography and radio-high-performance liquid chromatography. The compounds were shown to be lipophilic with logP values ranging from 1.34 to 1.92. In biodistribution studies, good L1210 tumor uptake was noted ([(64)Cu-EPH144](+) at 1 h, 4.70 %ID/g [percentage injected dose per gram]; 4 h, 8.80 %ID/g; 24 h, 6.64 %ID/g; and [(64)Cu-EPH270](+) at 1 h, 2.58 %ID/g; 4 h, 6.00 %ID/g; 24 h, 4.80 %ID/g). Small-animal PET of animals with L1210 tumors (high Topo-II expressing) showed excellent tumor accumulation compared with that of animals with PC-3 tumors (low Topo-II expressing), and the L1210 tumor uptake was significantly reduced by coadministration of a Topo-II poison.

CONCLUSION

Here we describe the characterization of a new class of copper-radiolabeled TSC analogs. We demonstrate that the accumulation of the (64)Cu-compounds is related to the expression levels of Topo-II in tumor tissue.

PET imaging of tumour hypoxia

Tumour hypoxia represents a significant challenge to the curability of human tumours leading to treatment resistance and enhanced tumour progression. Tumour hypoxia can be detected by non-invasive and invasive techniques but the inter-relationships between these remains largely undefined. [¹⁸F]Fluoromisonidazole-3-fluoro-1-(2 ^′-nitro-1 ^′-imidazolyl)-2-propanol ([¹⁸F]MISO) and Cu-diacetyl-bis(N⁴-methylthiosemicarbazone (Cu-ATSM)-positron emission tomography (PET), and blood oxygen level-dependent (BOLD)-magnetic resonance imaging (MRI) are the lead contenders for human application based on their non-invasive nature, ease of use and robustness, measurement of hypoxia status, validity, ability to demonstrate heterogeneity and general availability; PET techniques are the primary focus of this review.

Double-tracer autoradiography with Cu-ATSM/FDG and immunohistochemical interpretation in four different mouse implanted tumor models

BACKGROUND

We studied the regional characteristics within tumor masses using PET tracers and immunohistochemical methods.

METHODS

The intratumoral distribution of (64)Cu-diacetyl-bis(N4-methylthiosemicarbazone) ([(64)Cu]Cu-ATSM) and [(18)F] 2-fluoro-2-deoxyglucose ((18)F]FDG) in mice with tumors of four different origins (LLC1, Meth-A, B16 and colon26) was compared with the immunohistochemical staining of proliferating cells (Ki67), blood vessels (CD34 or von Willebrand factor), and apoptotic cells (terminal deoxynucleotidyltransferase-mediated dUTP nick end labeling method).

RESULTS

With all cell lines, [(64)Cu]Cu-ATSM and [(18)F]FDG were distributed with different gradation in the tumor mass. The immunohistochemical study demonstrated that the high [(64)Cu]Cu-ATSM uptake regions were hypovascular and consisted of tumor cells arrested in the cell cycle, whereas the high [(18)F]FDG uptake regions were hypervascular and consisted of proliferating cells.

CONCLUSION

In our study, it was revealed that one tumor mass contained two regions with different characteristics, which could be distinguished by [(64)Cu]Cu-ATSM and [(18)F]FDG. Because hypoxia and cell cycle arrest are critical factors to reduce tumor sensitivity to radiation and conventional chemotherapy, regions with such characteristics should be treated intensively as one of the primary targets. [(64)Cu]Cu-ATSM, which can delineate hypoxic and cell cycle-arrested regions in tumors, may provide valuable information for cancer treatment as well as possibly for treating such regions directly as an internal radiotherapy reagent.

Intertumoral differences in hypoxia selectivity of the PET imaging agent 64Cu(II)-diacetyl-bis(N4-methylthiosemicarbazone)

Cu-Diacetyl-bis(N(4)-methylthiosemicarbazone) (Cu-ATSM) is a recently developed PET imaging agent for tumor hypoxia. However, its accuracy and reliability for measuring hypoxia have not been fully characterized in vivo. The aim of this study was to evaluate (64)Cu-ATSM as a hypoxia PET marker by comparing autoradiographic distributions of (64)Cu-ATSM with a well-established hypoxia marker drug, EF5.

METHODS

R3230 mammary adenocarcinomas (R3230Ac), fibrosarcomas (FSA), and 9L gliomas (9L) were used in the study. EF5 and Hoechst 33342, a vascular perfusion marker, were administered to the animal for immunohistochemical analysis. (64)Cu-ATSM microPET and autoradiography were performed on the same animal. The tumor-to-muscle ratio (T/M ratio) and standardized uptake values (SUVs) were characterized for these 3 different types of tumors. Five types of images-microPET, autoradiography, EF5 immunostaining, Hoechst fluorescence vascular imaging, and hematoxylin-and-eosin histology-were superimposed, evaluated, and compared.

RESULTS

A significantly higher T/M ratio and SUV were seen for FSA compared with R3230Ac and 9L. Spatial correlation analysis between (64)Cu-ATSM autoradiography and EF5 immunostained images varied between the 3 tumor types. There was close correlation of (64)Cu-ATSM uptake and hypoxia in R3230Ac and 9L tumors but not in FSA tumors. Interestingly, elevated (64)Cu-ATSM uptake was observed in well-perfused areas in FSA, indicating a correlation between (64)Cu-ATSM uptake and vascular perfusion as opposed to hypoxia. The same relationship was observed with 2 other hypoxia markers, pimonidazole and carbonic anhydrase IX, in FSA tumors. Breathing carbogen gas significantly decreased the hypoxia level measured by EF5 staining in FSA-bearing rats but not the uptake of (64)Cu-ATSM. These results indicate that some other (64)Cu-ATSM retention mechanisms, as opposed to hypoxia, are involved in this type of tumor.

CONCLUSION

To our knowledge, this study is the first comparison between (64)Cu-ATSM uptake and immunohistochemistry in these 3 tumors. Although we have shown that (64)Cu-ATSM is a valid PET hypoxia marker in some tumor types, but not for all, this tumor type-dependent hypoxia selectivity of (64)Cu-ATSM challenges the use of (64)Cu-ATSM as a universal PET hypoxia marker. Further studies are needed to define retention mechanisms for this PET marker.

Triapine and cytarabine is an active combination in patients with acute leukemia or myelodysplastic syndrome

Triapine, an iron chelator and a potent inhibitor of ribonucleotide reductase, has significant anti-leukemia activity. A phase I study of Triapine in combination with ara-C was conducted in 32 patients with refractory acute leukemia and high-risk MDS. Triapine (105 mg/m2/day 6-h infusion) was followed immediately by ara-C [100 (n=4), 200 (n=6), 400 (n=7), or 800 (n=8)mg/m2/day] as an 18-h infusion for 5 consecutive days. Dose-limiting toxicities (DLTs) were observed at the 800 mg/m2 ara-C dose level (one patient each with grade 4 mucositis; grade 4 neutropenic colitis, sepsis; grade 4 neuropathy; and grade 4 hyperbilirubinemia). Therefore, the study was amended to include an ara-C dose level of 600 mg/m2/day, no DLTs occurred in seven patients treated at this dose level. Mean Triapine C(max) and AUC were 1.13 microg/mL and 251.5 minmicrog/mL. Of 31 evaluable patients, 4 (13%) (3 AML, 1 Ph+ALL) achieved a CR (1 at a dose of 800 mg/m2; 2 at 600 mg/m2; 1 at 200mg/m2). The recommended phase II regimen is Triapine 105 mg/m2/day followed by ara-C 600 mg/m2/day for 5 consecutive days every 3-6 weeks.

Cell line-dependent differences in uptake and retention of the hypoxia-selective nuclear imaging agent Cu-ATSM

BACKGROUND

Cu-diacetyl-bis(N(4)-methylthiosemicarbazone) [Cu-ATSM] is a potential marker for tumor hypoxia that has been under evaluation for clinical use. In this study, we examined the mechanisms underlying the uptake of (64)Cu in cells incubated with (64)Cu-ATSM.

METHODS

The in vitro uptake of (64)Cu was determined as a function of oxygenation conditions and incubation time with (64)Cu-ATSM using four and two tumor cell lines of human origin and rodent origin, respectively. Additionally, the rate of (64)Cu efflux and Cu-ATSM metabolism was determined.

RESULTS

(64)Cu accumulation is rapid during the first 0.5-1 h of incubation. It is highest in anoxic cells but is also significant in normoxic cells. After this initial period, the level of intracellular (64)Cu varies depending on the cell line and the oxygenation conditions and, in some circumstances, may decrease. During the first 0.5-1 h, the ratio of (64)Cu levels between anoxic and normoxic cells is approximately 2:10 and that between hypoxic (0.5% O(2)) and normoxic cells is approximately 1:2.5, depending on the cell line. These ratios generally decrease at longer times. The (64)Cu-ATSM compound was found to be metabolized during incubation in a manner dependent on oxygenation conditions. Within 2 h under anoxic conditions, (64)Cu-ATSM could no longer be detected, although 60-90% of the amount of (64)Cu added as (64)Cu-ATSM was present in the medium. Non-ATSM (64)Cu was taken up by the cells, albeit at a much slower rate. Efflux rates of (64)Cu were found to be cell line dependent and appeared to be inversely correlated with the final (64)Cu uptake levels under anoxic conditions.

CONCLUSION

The uptake and retention of (64)Cu and their relation to oxygenation conditions were found to be cell line dependent. Given the complexities in the oxygen dependence and cell line-dependent kinetics of uptake and retention of Cu following exposure to Cu-ATSM, the clinical utility of this compound may be disease site specific.

QSAR studies of copper azamacrocycles and thiosemicarbazones: MM3 parameter development and prediction of biological properties

Genetic algorithms (GA) were used to develop specific copper metal-ligand force field parameters for the MM3 force field, from a combination of crystallographic structures and ab initio calculations. These new parameters produced results in good agreement with experiment and previously reported copper metal-ligand parameters for the AMBER force field. The MM3 parameters were then used to develop several quantitative structure-activity relationship (QSAR) models. A successful QSAR for predicting the lipophilicity (log P(ow)) of several classes of Cu(II)-chelating ligands was built using a training set of 32 Cu(II) radiometal complexes and 6 simple molecular descriptors. The QSAR exhibited a correlation between the predicted and experimental log P(ow) with an r(2) = 0.95, q(2) = 0.92. When applied to an external test set of 11 Cu(II) complexes, the QSAR preformed with great accuracy; r(2) = 0.93 and a q(2) = 0.91 utilizing a leave-one-out cross-validation analysis. Additional QSAR models were developed to predict the biodistribution of a smaller set of Cu(II) bis(thiosemicarbazone) complexes.

Basic characterization of 64Cu-ATSM as a radiotherapy agent

64Cu-diacetyl-bis(N4-methylthiosemicarbazone) (64Cu-ATSM) is a promising radiotherapy agent for the treatment of hypoxic tumors. In an attempt to elucidate the radiobiological basis of 64Cu-ATSM radiotherapy, we have investigated the cellular response patterns in vitro cell line models. Cells were incubated with 64Cu-ATSM, and the dose-response curves were obtained by performing a clonogenic survival assay. Radiation-induced damage in DNA was evaluated using the alkali comet assay and apoptotic cells were detected using Annexin V-FITC and propidium iodide staining methods. Washout rate and subcellular distribution of 64Cu in cells were investigated to further assess the effectiveness of 64Cu-ATSM therapy on a molecular basis. A direct comparison of subcellular localization of Cu-ATSM was made with the flow tracer analog Cu-pyruvladehyde-bis(N4-methylthiosemicarbazone). In this study, 64Cu-ATSM was shown to reduce the clonogenic survival rate of tumor cells in a dose-dependent manner. Under hypoxic conditions, cells took up 64Cu-ATSM and radioactive 64Cu was highly accumulated in the cells. In the 64Cu-ATSM-treated cells, DNA damage by the radiation emitted from 64Cu was detected, and inhibition of cell proliferation and induction of apoptosis was observed at 24 and 36 h after the treatment. The typical features of postmitotic apoptosis induced by radiation were observed following 64Cu-ATSM treatment. The majority of the 64Cu taken up into the cells remained in the postmitochondrial supernatant (the cellular residue after removal of the nuclei and mitochondria), which indicates that the beta- particle emitted from 64Cu may be as effective as the Auger electrons in 64Cu-ATSM therapy. These data allow us to postulate that 64Cu-ATSM will be able to attack the hypoxic tumor cells directly, as well as potentially affecting the peripheral nonhypoxic regions indirectly by the beta- particle decay of 64Cu.

Fluorescence studies of the intra-cellular distribution of zinc bis(thiosemicarbazone) complexes in human cancer cells

The uptake of zinc bis(thiosemicarbazone) complexes in human cancer cells has been studied by fluorescence microscopy and the cellular distribution established, including the degree of uptake in the nucleus.

Phase I and pharmacokinetic study of the ribonucleotide reductase inhibitor, 3-aminopyridine-2-carboxaldehyde thiosemicarbazone, administered by 96-hour intravenous continuous infusion

PURPOSE

3-aminopyridine-2-carboxaldehyde thiosemicarbazone (3-AP; Triapine; Vion Pharmaceuticals Inc, New Haven, CT) is a potent inhibitor of ribonucleotide reductase, with activity in preclinical tumor model systems. A phase I trial was initiated to determine the dose-limiting toxicities, maximum-tolerated dose, and pharmacokinetics of a 96-hour intravenous (IV) continuous infusion in patients with advanced cancer.

PATIENTS AND METHODS

Initially, courses were administered every 3 weeks, using an accelerated titration design. Subsequently, courses were administered every 2 weeks, and the dose was escalated in cohorts of three to six patients.

RESULTS

Twenty-one patients were enrolled, seven on the every-3-week schedule and 14 on the every-other-week schedule. Three of six patients at 160 mg/m(2)/d developed dose-limiting toxicities including neutropenia, hyperbilirubinemia, and nausea or vomiting. Based on these initial results, the dose for 3-AP was re-escalated beginning at 80 mg/m(2)/d but administered every 2 weeks. At 120 mg/m(2)/d, three of seven patients had dose-limiting but reversible asthenia, hyperbilirubinemia, and azotemia or acidosis; however, in the case of renal and hepatic adverse events, the events were related to pre-existing borderline abnormal organ function. Therefore, the recommended phase II dose for 3-AP administered by 96-hour IV infusion is 120 mg/m(2)/d every 2 weeks. Detailed pharmacokinetic studies demonstrated linear kinetics up to 160 mg/m(2), with substantial inter-patient variability. There was no correlation between dose and clearance (R(2) = 0.0137). There were no objective responses, but there was prolonged stabilization of disease or decreases in serum tumor markers associated with stable disease in four patients.

CONCLUSION

The 96-hour infusion of 3-AP is safe and well tolerated at the recommended phase II doses. Phase II trials of Triapine are ongoing.

Adsorptive stripping voltammetric behaviour of azomethine group in pyrimidine-containing drugs

The stripping voltammetric behaviour of buspirone hydrochloride (BUS) and piribedil (PIR), as models of pyrimidine-containing compounds, was studied using a hanging mercury drop electrode (HMDE). A sensitive adsorptive stripping voltammetric method for determination of such drugs is described. The voltammetric peaks were obtained at -1.23 and -1.22 V for BUS and PIR. respectively, which correspond to the reduction of the azomethine group of pyrimidine ring in Britton-Robinson buffer (pH 7). Factors such as pH of supporting electrolyte, accumulation potential and time and instrumental parameters were optimized. Calibration plots and regression data validation, accuracy, precision, limits of detection, limits of quantification, and other aspects of analytical merit are presented. The applicability of the method was evaluated through determination of BUS and PIR in tablet dosage forms. A preliminary study of the analysis of plasma samples, spiked with the investigated drug, after a simple extraction procedure is described.

Enhancing targeted radiotherapy by copper(II)diacetyl- bis(N4-methylthiosemicarbazone) using 2-deoxy-D-glucose

Most cancer deaths are a consequence of resistance to conventional chemotherapy and radiation therapy. This may be attributable to unique phenotypic characteristics of solid tumors. We have exploited two well-described characteristics of solid tumors commonly associated with treatment failure, high glucose use and hypoxia, to design a unique therapy based on the selective accumulation of two cytotoxic compounds, 2-deoxyglucose (2-DG) and copper(II)diacetyl-bis(N(4)-methylthiosemicarbazone) ((64)Cu-ATSM). (64)Cu-ATSM localizes to hypoxic regions of tumors and has been used for administering a high local dose of radiation therapy after uptake by cells. 2-DG, a glucose analog, selectively accumulates in cancer cells and interferes with energy metabolism, resulting in cancer cell death. 2-DG has been shown to potentiate the cytotoxic effect of ionizing radiation and certain chemotherapeutic agents. We have tested the effect of 2-DG on tumor response when combined with (64)Cu-ATSM in a mouse breast tumor model using the highly aggressive mouse mammary carcinoma cell line EMT-6. 2-DG administered up to 2 mg/g of body weight daily resulted in no weight loss or systemic symptoms. EMT-6 mammary tumors had similar uptake of [(18)F]fluoro-2-deoxyglucose before and after 2 weeks of 2-DG treatment as determined by microPET imaging, indicating that resistance to 2-DG uptake does not develop. Pretreatment of tumor-bearing mice with 2-DG resulted in increased uptake of (64)Cu-ATSM by tumors compared with nontreated mice. This effect was not observed with the nonhypoxia-specific agent copper(II)pyruvaldehyde-bis(N(4)-methylthiosemicarbazone. When 2-DG was combined with a single dose of (64)Cu-ATSM (2 mCi), tumor growth was inhibited approximately 60% compared with untreated mice, and animals survived approximately 50% longer than untreated mice or animals treated with each agent alone (32 versus 20 days). The maximum effect on tumor growth and survival was observed when 2-DG was administered daily for the lifetime of the mouse. Our results indicate that 2-DG potentiates the effect of (64)Cu-ATSM on tumoricidal activity and animal survival. We hypothesize that 2-DG alters the metabolic state of the cell, leading to increased uptake of (64)Cu-ATSM by the tumor. This would result in a higher local dose of radiotherapy. The continued presence of 2-DG would then prevent the repair of damaged cells, leading to inhibition of tumor growth. Our data indicate that the strategy of combining tumor-specific cytotoxic agents that function by differing mechanisms can result in an effective, selective, tumor-specific cell death with minimal effect on the host.

Intra-tumoral distribution of (64)Cu-ATSM: a comparison study with FDG

(64)Cu-labeled diacetyl-bis(N(4)-methylthiosemicarbazone) ((64)Cu-ATSM) is a promising agent for internal radiation therapy and imaging of hypoxic tissues. In the present study, the intra-tumoral distribution of (64)Cu-ATSM was investigated by comparing it to that of [(18)F]FDG and histological findings. VX2 tumors were implanted into Japanese white rabbits subcutaneously. (64)Cu-ATSM and [(18)F]FDG were co-injected intravenously and the tumor was dissected and cut into 1 mm thick slices 1 h after the injection. The uptake of (64)Cu-ATSM and [(18)F]FDG was measured using a dual-tracer autoradiographic technique. Histological cell biology was estimated from the optical microscopy of tumor sections. The major accumulation of (64)Cu-ATSM was observed around the outer rim of the tumor masses which consisted mainly of active cells and expected to be hypoxic. [(18)F]FDG was distributed more widely with highest levels in the inner regions where pre-necrotic cells were mainly observed. (64)Cu-ATSM appears to be useful for the detection of hypoxic but active tumor cell regions in vivo.

In vivo assessment of tumor hypoxia in lung cancer with 60Cu-ATSM

Tumor hypoxia is recognized as an important determinant of response to therapy. In this study we investigated the feasibility of clinical imaging with copper-60 diacetyl-bis( N(4)-methylthiosemicarbazone) ((60)Cu-ATSM) in patients with non-small-cell lung cancer (NSCLC) and also assessed whether pretreatment tumor uptake of (60)Cu-ATSM predicts tumor responsiveness to therapy. Nineteen patients with biopsy-proved NSCLC were studied by positron emission tomography (PET) with (60)Cu-ATSM before initiation of therapy. (60)Cu-ATSM uptake was evaluated semiquantitatively by determining the tumor-to-muscle activity ratio (T/M). All patients also underwent PET with fluorine-18 fluorodeoxyglucose (FDG) prior to institution of therapy. The PET results were correlated with follow-up evaluation (2-46 months). It was demonstrated that PET imaging with (60)Cu-ATSM in patients with NCSLC is feasible. The tumor of one patient had no discernible (60)Cu-ATSM uptake, whereas the tumor uptake in the remaining patients was variable, as expected. Response was evaluated in 14 patients; the mean T/M for (60)Cu-ATSM was significantly lower in responders (1.5+/-0.4) than in nonresponders (3.4+/-0.8) (P=0.002). However, the mean SUV for (60)Cu-ATSM was not significantly different in responders (2.8+/-1.1) and nonresponders (3.5+/-1.0) ( P=0.2). An arbitrarily selected T/M threshold of 3.0 discriminated those likely to respond to therapy: all eight responders had a T/M <3.0 and all six nonresponders had a T/M > or =3.0. Tumor SUV for FDG was not significantly different in responders and nonresponders (P=0.7) and did not correlate with (60)Cu-ATSM uptake (r=0.04; P=0.9). (60)Cu-ATSM-PET can be readily performed in patients with NSCLC and the tumor uptake of (60)Cu-ATSM reveals clinically unique information about tumor oxygenation that is predictive of tumor response to therapy.

Phase I and pharmacokinetic study of 3-aminopyridine-2-carboxaldehyde thiosemicarbazone (3-AP) using a single intravenous dose schedule

PURPOSE

To perform a phase I and pharmacokinetics study of 3-aminopyridine-2-carboxaldehyde thiosemicarbazone (3-AP) a new ribonucleotide reductase inhibitor using a single intravenous (2-h) schedule every 4 weeks. 3-AP was given at a starting dose of 5 mg/m(2) with escalation based on a modified Fibonacci scheme.

PATIENTS AND METHODS

A total of 27 patients with advanced cancer were entered into the study. Doses of 3-AP ranged from 5 mg/m(2) to 105 mg/m(2). Blood and urine samples were collected and 3-AP was measured by HPLC.

RESULTS

A total of 46 courses were evaluable. One patient developed grade 4 thrombocytopenia at the lowest dose level, and one patient had grade 3 anemia. Two patients developed grade 3 coagulation abnormalities. The only other toxicities of more than grade l occurring in more than 10% of patients were fever and asthenia. No toxicities were observed at the highest dose level. Peak serum concentration of 3-AP increased linearly with dose. No tumor responses were observed in this heavily pretreated population, although eight patients had stabilization of their disease.

CONCLUSIONS

Relevant tumor inhibitory concentrations were achieved without significant toxicity using doses up to 105 mg/m(2) on this single intravenous dose schedule. Prolonged administration schedules and combinations with other cytotoxic agents, strategies predicted to have greater antitumor efficacy according to preclinical studies, are under investigation.

Copper bis(thiosemicarbazone) complexes as hypoxia imaging agents: structure-activity relationships

Copper(II) bis(thiosemicarbazone) complexes labelled with Cu-60/62/64 are useful radiopharmaceuticals for imaging blood flow and hypoxic tissues in vivo. The aim of this study was to identify structure-activity relationships within a series of analogues with different alkyl substitution patterns in the ligand, in order to design improved hypoxia imaging agents and elucidate hypoxia selectivity mechanisms. Thirteen such complexes were synthesised and characterised spectroscopically and electrochemically. The uptake of each (labelled with Cu-64) in EMT6 tumour cells in vitro under normoxic and hypoxic conditions was studied. All complexes were taken up efficiently into cells, and some showed strong hypoxia selectivity, which was highly correlated with the Cu(II/I) redox potential. Redox potentials at the low end of the range were found to be essential for hypoxia selectivity. In turn, the redox potential was strongly correlated with alkyl substitution pattern, and the most important determinant of the redox potential was the number of alkyl groups on the diimine backbone of the ligand. Several complexes in the series warrant further evaluation as hypoxia imaging agents. The radioactivity uptake/release behaviour in the cells provides insight into possible mechanisms, and a model for hypoxia-selective intracellular trapping is discussed.

Copper-64-pyruvaldehyde-bis(N(4)-methylthiosemicarbazone) for the prevention of tumor growth at wound sites following laparoscopic surgery: monitoring therapy response with microPET and magnetic resonance imaging

Laparoscopic colectomy for curable colon cancer may result in the development of abdominal wall implants because of disseminated disease and the favorable environment of the wound site for cell implantation. Injection of disaggregated human GW39 colon cancer cells into the hamster peritoneum represents a model of tumor spillage that may occur during dissection, manipulation, resection, and extraction of tumor during surgery in the clinical setting. Using this well-established animal model, we tested the efficacy of (64)Cu-pyruvaldehyde-bis(N(4)-methylthiosemicarbazone) ((64)Cu-PTSM) in inhibiting tumor cell implantation in trocar wound sites. Anesthetized hamsters had four 5-mm trocars inserted through the anterior abdominal wall. GW39 cells ( approximately 3.2 x 10(4) cells in 0.5 ml) were injected into the peritoneum through a midline incision. Ten min later, hamsters were randomized to receive 5, 3, or 1 mCi of (64)Cu-PTSM through the same midline incision. High-resolution magnetic resonance imaging and microPET were used to monitor tumor volume and morphology after surgery. After 7 weeks, animals were sacrificed, and trocar and midline wounds were harvested for macroscopic and histological analysis. No macroscopic tumor was found in any of the group treated with 5 mCi of (64)Cu-PTSM, whereas 96% of the wound sites in the group treated with saline had macroscopic tumor growth (P < 0.001). This study demonstrates the therapeutic potential of (64)Cu-PTSM in inhibiting cancer cell implantation and growth at doses well below the maximum tolerated dose, with no signs of toxicity to the hamsters.

Retention mechanism of hypoxia selective nuclear imaging/radiotherapeutic agent cu-diacetyl-bis(N4-methylthiosemicarbazone) (Cu-ATSM) in tumor cells

The retention mechanism of the novel imaging/radiotherapeutic agent, Cu-diacetyl-bis(N4-methylthiosemicarbazone) (Cu-ATSM) in tumor cells was clarified in comparison with that in normal tissue in vitro. With Cu-ATSM and reversed phase HPLC analysis, the reductive metabolism of Cu-ATSM in subcellular fractions obtained from Ehrlich ascites tumor cells was examined. As a reference, mouse brain was used. To determine the contribution of enzymes in the retention mechanisms, and specific inhibitor studies were performed. In subcellular fractions of tumor cells, Cu-ATSM was reduced mainly in the microsome/cytosol fraction rather than in the mitochondria. This finding was completely different from that found in normal brain cells. The reduction process in the microsome/cytosol was heat-sensitive and enhanced by adding exogenous NAD(P)H, an indication of enzymatic reduction of Cu-ATSM in tumor cells. Among the known bioreductive enzymes, NADH-cytochrome b5 reductase and NADPH-cytochrome P450 reductase in microsome played a major role in the reductive retention of Cu-ATSM in tumors. This enzymatic reduction was enhanced by the induction of hypoxia. Radiocopper labeled Cu-ATSM provides useful information for the detection of hypoxia as well as the microsomal bioreductive enzyme expression in tumor.

Continuous angiotensin II infusion increases tumour: normal blood flow ratio in colo-rectal liver metastases

Insufficient blood flow within colo-rectal hepatic metastases is a factor which may limit drug delivery to, and thus the response of, these tumours to regional chemotherapy. Loco-regional flow may be manipulated pharmacologically to enhance the tumour blood flow relative to that of the normal liver. However, as yet, only transient effects have been studied. Patients receiving regional chemotherapy for unresectable hepatic disease were given a 45 min regional infusion of the vasoconstrictor Angiotensin II. Intrahepatic blood flow distribution was assessed serially by Positron Emission Tomography (PET) imaging together with the trapping tracer copper(II) pyruvaldehyde bis(N-4-methylthiosemicarbazone) (Cu-PTSM) labelled using copper-62. Eleven lesions in nine patients were studied, with no adverse effects. Prior to Angiotensin II administration tumour blood flow was generally found to be greater than that of liver (10/11 lesions; 8/9 patients; median TNR 1.3, iqr 0.9-2.5). A significant increase in relative flow to tumour was seen in response to 10 min Angiotensin II infusion in most cases (7/11 lesions; 7/9 patients; median TNR 2.1, iqr 1.4-4.1; P = 0.008), which appeared to be sustained throughout the 45 min infusion period (median TNR 1.85, iqr 1.3-3.8; P = 0.03). These effects were accompanied by transient elevation of mean arterial pressure, but no change in pulse rate. These observations suggest that prolonged regional vasoconstrictor administration could prove useful in the management of unresectable colo-rectal hepatic metastases, and that further development of vascular manipulation to enhance tumour targeting and drug delivery is warranted.

Copper-62 ATSM as a hypoxic tissue tracer in myocardial ischemia

Copper-62 labeled diacetyl-bis(N4-methylthiosemicarbazone) (62Cu-ATSM) has been proposed as a generator produced positron-emitting tracer for hypoxic tissue imaging. To clarify the usefulness of 62Cu-ATSM for myocardial ischemia, 62Cu-ATSM PET was performed in 7 patients with coronary artery disease. Increased myocardial uptake of 62Cu-ATSM was observed (myocardium/blood ratio: 3.09) in one patient with unstable angina, who had increased 18F-fluorodeoxyglucose (18F-FDG) uptake under the fasting condition. The other 6 patients, who were clinically stable, did not have increased 62Cu-ATSM uptake, although abnormal 18F-FDG uptake was seen in 4 patients. This preliminary study suggests that 62Cu-ATSM is a promising PET tracer for hypoxic imaging in acute ischemia.

Tumor uptake of copper-diacetyl-bis(N(4)-methylthiosemicarbazone): effect of changes in tissue oxygenation

We showed previously that, in vitro, copper-diacetyl-bis(N(4)-methylthiosemicarbazone) (Cu-ATSM) uptake is dependent on the oxygen concentration (pO2). We also showed that, in vivo, Cu-ATSM uptake is heterogeneous in animal tumors known to contain hypoxic fractions. This study was undertaken to confirm the pO2 dependence of this selective uptake in vivo by correlating Cu-ATSM uptake with measured tumor pO2.

METHODS

Experiments were performed with the 9L gliosarcoma rat model using a needle oxygen electrode to measure tissue pO2. Using PET and electronic autoradiography, Cu-ATSM uptake was measured in tumor tissue under various pO2 levels. The oxygen concentration within implanted tumors was manipulated by chemical means or by altering the inhaled oxygen content.

RESULTS

A good correlation between low pO2 and high Cu-ATSM accumulation was observed. Hydralazine administration in animals caused a decrease in the average tumor pO2 from 28.61 +/- 8.74 mm Hg to 20.81 +/- 7.54 mm Hg in untreated control animals breathing atmospheric oxygen. It also caused the tumor uptake of Cu-ATSM to increase by 35%. Conversely, in animals breathing 100% oxygen, the average tumor pO2 increased to 45.88 +/-15.9 mm Hg, and the tumor uptake of Cu-ATSM decreased to 48% of that of the control animals. PET of animals treated in a similar fashion yielded time-activity curves showing significantly higher retention of the tracer in hypoxic tissues than in oxygenated tissues.

CONCLUSION

These data confirm that Cu-ATSM uptake in tissues in vivo is dependent on the tissue pO2, and that significantly greater uptake and retention occur in hypoxic tumor tissue. Therefore, the possible use of Cu-ATSM PET as a prognostic indicator in the management of cancer is further validated.

A novel approach to overcome hypoxic tumor resistance: Cu-ATSM-guided intensity-modulated radiation therapy

PURPOSE

Locoregional tumor control for locally advanced cancers with radiation therapy has been unsatisfactory. This is in part associated with the phenomenon of tumor hypoxia. Assessing hypoxia in human tumors has been difficult due to the lack of clinically noninvasive and reproducible methods. A recently developed positron emission tomography (PET) imaging-based hypoxia measurement technique which employs a Cu(II)-diacetyl-bis(N(4)-methylthiosemicarbazone) (Cu-ATSM) tracer is of great interest. Oxygen electrode measurements in animal experiments have demonstrated a strong correlation between low tumor pO(2) and excess (60)Cu-ATSM accumulation. Intensity-modulated radiation therapy (IMRT) allows selective targeting of tumor and sparing of normal tissues. In this study, we examined the feasibility of combining these novel technologies to develop hypoxia imaging (Cu-ATSM)-guided IMRT, which may potentially deliver higher dose of radiation to the hypoxic tumor subvolume to overcome inherent hypoxia-induced radioresistance without compromising normal tissue sparing.

METHODS AND MATERIALS

A custom-designed anthropomorphic head phantom containing computed tomography (CT) and positron emitting tomography (PET) visible targets consisting of plastic balls and rods distributed throughout the "cranium" was fabricated to assess the spatial accuracy of target volume mapping after multimodality image coregistration. For head-and-neck cancer patients, a CT and PET imaging fiducial marker coregistration system was integrated into the thermoplastic immobilization head mask with four CT and PET compatible markers to assist image fusion on a Voxel-Q treatment-planning computer. This system was implemented on head-and-neck cancer patients, and the gross tumor volume (GTV) was delineated based on physical and radiologic findings. Within GTV, regions with a (60)Cu-ATSM uptake twice that of contralateral normal neck muscle were operationally designated as ATSM-avid or hypoxic tumor volume (hGTV) for this feasibility study. These target volumes along with other normal organs contours were defined and transferred to an inverse planning computer (Corvus, NOMOS) to create a hypoxia imaging-guided IMRT treatment plan.

RESULTS

A study of the accuracy of target volume mapping showed that the spatial fidelity and imaging distortion after CT and PET image coregistration and fusion were within 2 mm in phantom study. Using fiducial markers to assist CT/PET imaging fusion in patients with carcinoma of the head-and-neck area, a heterogeneous distribution of (60)Cu-ATSM within the GTV illustrated the success of (60)Cu-ATSM PET to select an ATSM-avid or hypoxic tumor subvolume (hGTV). We further demonstrated the feasibility of Cu-ATSM-guided IMRT by showing an example in which radiation dose to the hGTV could be escalated without compromising normal tissue (parotid glands and spinal cord) sparing. The plan delivers 80 Gy in 35 fractions to the ATSM-avid tumor subvolume and the GTV simultaneously receives 70 Gy in 35 fractions while more than one-half of the parotid glands are spared to less than 30 Gy.

CONCLUSION

We demonstrated the feasibility of a novel Cu-ATSM-guided IMRT approach through coregistering hypoxia (60)Cu-ATSM PET to the corresponding CT images for IMRT planning. Future investigation is needed to establish a clinical-pathologic correlation between (60)Cu-ATSM retention and radiation curability, to understand tumor re-oxygenation kinetics, and tumor target uncertainty during a course of radiation therapy before implementing this therapeutic approach to patients with locally advanced tumor.

Copper-64-diacetyl-bis(N4-methylthiosemicarbazone): An agent for radiotherapy

Systemic administration of hypoxia-selective (64)Cu-diacetyl-bis(N(4)-methylthiosemicarbazone) ((64)Cu-ATSM) has increased significantly the survival time of hamsters bearing human GW39 colon cancer tumors. Radiotherapy experiments were performed in animals bearing either 7-day-old (0.5-1.0 g) or 15-day-old (1.5-2.0 g) tumors. Studies compared animals treated with a single dose of 0, 4, 6, 7, 8, or 10 mCi of (64)Cu-ATSM (1 Ci = 37 GBq) with or without the vasodilator hydralazine. A multiple dose regimen of 3 x 4 mCi at 72-h intervals was studied also. Single doses of >6 mCi of (64)Cu-ATSM and the dose-fractionation protocol significantly increased the survival time of the hamsters compared with controls. The highest dose, 10 mCi of (64)Cu-ATSM, increased survival to 135 days in 50% of animals bearing 7-day-old tumors, 6-fold longer than control animals' survival (20 days), with only transient leucopenia and thrombocytopenia but no overt toxicity. Human absorbed doses were calculated from hamster biodistribution; the dose-critical organs were the lower large intestine (1.43 +/- 0.19 rad/mCi) and upper large intestine (1.20 +/- 0.38 rad/mCi). High-resolution MRI and positron-emission tomography using a therapeutic administration of 10 mCi were used to monitor tumor volume and morphology and to assess tumor dosimetry accurately, giving a tumor dose of 81 +/- 7.5 rad/mCi. (64)Cu-ATSM has increased the survival time of tumor-bearing animals significantly with no acute toxicity and thus is a promising agent for radiotherapy.

Evaluation of 62Cu labeled diacetyl-bis(N4-methylthiosemicarbazone) as a hypoxic tissue tracer in patients with lung cancer

62Cu labeled diacetyl-bis(N4-methylthiosemicarbazone) (62Cu-ATSM) has been proposed as a generator-produced, positron-emitting tracer for hypoxic tissue imaging. From basic studies, the retention mechanism of 62Cu-ATSM is considered to be closely related to cytosolic/microsomal bioreduction, a possible system for hypoxic bioreductive drug activation. In order to evaluate the characteristics of 62Cu-ATSM, PET studies were performed in 4 normal subjects and 6 patients with lung cancer. 62Cu-ATSM cleared rapidly from the blood with little lung uptake (0.43+/-0.09, uptake ratio; divided by the arterial input function) in normal subjects. Intense tumor uptake of 62Cu-ATSM was observed in all patients with lung cancer (3.00+/-1.50). A negative correlation was observed between blood flow and flow-normalized 62Cu-ATSM uptake in three of four patients. In contrast, 62Cu-ATSM uptake was not related to that of 18F-fluorodeoxyglucose. The negative correlation between blood flow and flow normalized 62Cu-ATSM uptake suggests an enhancement of retention of 62Cu-ATSM by low flow. 62Cu-ATSM is a promising PET tracer for tumor imaging, which might bring new information for chemotherapeutic treatment as well as radiotherapy of hypoxic tumors.

Triapine (3-aminopyridine-2-carboxaldehyde- thiosemicarbazone): A potent inhibitor of ribonucleotide reductase activity with broad spectrum antitumor activity

Previous studies from our laboratories have shown that (a) Triapine() is a potent inhibitor of ribonucleotide reductase activity and (b) hydroxyurea-resistant L1210 leukemia cells are fully sensitive to Triapine. In an analogous manner, Triapine was similarly active against the wild-type and a hydroxyurea-resistant subline of the human KB nasopharyngeal carcinoma. Triapine was active in vivo against the L1210 leukemia over a broad range of dosages and was curative for some mice. This agent also caused pronounced inhibition of the growth of the murine M109 lung carcinoma and human A2780 ovarian carcinoma xenografts in mice. Optimum anticancer activity required twice daily dosing due to the duration of inhibition of DNA synthesis which lasted about 10 hr in L1210 cells treated with Triapine in vivo. DNA synthesis in normal mouse tissues (i.e. duodenum and bone marrow) uniformly recovered faster than that in L1210 leukemia cells, demonstrating a pharmacological basis for the therapeutic index of this agent. Triapine was more potent than hydroxyurea in inhibiting DNA synthesis in L1210 cells in vivo, and the effects of Triapine were more pronounced. In addition, the duration of the inhibition of DNA synthesis in leukemia cells from mice treated with Triapine was considerably longer than in those from animals treated with hydroxyurea. Combination of Triapine with various classes of agents that damage DNA (e.g. etoposide, cisplatin, doxorubicin, and 1-acetyl-1,2-bis(methylsulfonyl)-2-(2-chloroethyl)hydrazine) resulted in synergistic inhibition of the L1210 leukemia, producing long-term survivors of tumor-bearing mice treated with several dosage levels of the combinations, whereas no enhancement of survival was found when Triapine was combined with gemcitabine or cytosine arabinoside. The findings demonstrate the superiority of Triapine over hydroxyurea as an anticancer agent and further suggest that prevention by Triapine of repair of DNA lesions created by agents that damage DNA may result in efficacious drug combinations for the treatment of cancer.

Cytosolic/microsomal redox pathway: a reductive retention mechanism of a PET-oncology tracer, cu-pyruvaldehyde-bis(N4-methylthiosemicarbazone) (cu-PTSM)

OBJECTIVE

To clarify the retention mechanism of a PET imaging agent Cu-pyruvaldehyde-bis(N4-methylthiosemicarbazone) (Cu-62-PTSM) in tumor cells, reductive metabolism of non-radioactive Cu-PTSM in five cultured tumor cell lines, a tumor specimen and non-tumor tissues in vitro was evaluated by electron spin resonance spectrometry (ESR).

RESULTS

In the brain, mitochondrial electron transport enzyme reduced Cu-PTSM specifically. On the other hand, Cu-PTSM was not reduced in tumor mitochondria. The mitochondrial electron transport enzyme in tumor cells was not damaged, but NADH was considered to be depleted. In compensation for that, the tumor cells acquired complementary reduction activity in the microsome/cytosol. The reduction was enzymatic and NADH-dependent, possibly similar to the activation mechanism of bioreductive anticancer drugs.

CONCLUSION

Cu-PTSM and its derivatives are considered to be used as a marker for microsome/cytosol redox ability in PET oncology, although the physiological role of the redox enzyme system in tumor cells has not been clarified. The change in electron (NADH) flow in tumor cells might be a mechanism supporting aerobic glycolysis in tumor cells.

Synthesis and evaluation of copper radiopharmaceuticals with mixed bis(thiosemicarbazone) ligands

Four "mixed" bis(thiosemicarbazone) derivatives of pyruvaldehyde were synthesized that incorporate two dissimilar thiosemicarbazone functions. The corresponding [67Cu]copper(II) complexes were prepared and evaluated as possible copper radiopharmaceuticals. The pyruvaldehyde-based mixed bis(thiosemicarbazone) ligands, CH3C[=NNHC(S)NHMe]CH[=NNHC(S)NHEt] (1), CH3C[=NNHC(S)NHMe] CH[=NNHC(S)NEt2] (2), CH3C[=NNHC(S)NHMe]CH[=NNHC(S)-cyclo-N(CH2)5] (3), and CH3C [=NNHC(S)NHMe]CH[=NNHC(S)-cyclo-N(CH2)6] (4), were obtained by reaction of the appropriate thiosemicarbazide derivative with pyruvaldehyde-2-N4-methylthiosemicarbazone (CH3C[=NNHC(S) NHMe]CHO). The 67Cu-labeled copper(II) complexes of ligands 1-4 were prepared and screened in a rat model to assess the potential of each chelate as a 62Cu-radiopharmaceutical for imaging with positron emission tomography. The 67Cu-complexes of ligands 1-4 exhibit significant uptake into the brain and heart 1 min following intravenous administration to rats. For the 67Cu-complexes of ligands 2, 3, and 4, the cerebral and myocardial uptake of 67Cu is two-to-threefold lower at 2 h than at 1 min postinjection, due to significant biological clearance of these 67Cu-chelates. However, the 67Cu-complex of 1 affords cerebral and myocardial uptake and retention comparable to that of [67Cu]Cu-PTSM in this model. Although the kinetics of this new agent appear attractive, ultrafiltration studies using solutions of dog and human serum albumin reveal that the 67Cu-complex of ligand 1, like Cu-PTSM, interacts more strongly with human albumin than dog albumin. Thus, this new agent would appear to offer no advantage over Cu-PTSM as a 62Cu-labeled tracer for evaluation of regional tissue perfusion.

Evaluation of gallium-68 tris(2-mercaptobenzyl)amine: a complex with brain and myocardial uptake

Previous research into development of a gallium-radiolabeled agent that crosses the blood-brain barrier has met with limited success. In this study, we focused our attention on a Ga(III) complex of a 4-coordinate amine trithiolate tripod ligand, tris(2-mercaptobenzyl) amine (S3N). The Ga(III) S3N complex is small, neutral, and lipophilic, meeting the requirements for a potential brain imaging agent. The Ga-68 complex was easily formed with a radiochemical purity of >95%. In vitro stability of the Ga-S3N complex, determined in rat serum incubated at 37 degrees C, was greater than 95% intact at 2 h by silica gel and reversed-phase radio-thin layer chromatography. Biodistribution studies conducted in female Sprague-Dawley rats showed the complex cleared rapidly from the blood with initial high liver uptake followed by rapid washout. Significant uptake was observed in the brain, with brain:blood ratios increasing from 0.11 at 2 min postinjection to 3.8 at 60 min postinjection. Uptake was also observed in the heart going from a heart:blood ratio of 2.3 at 2 min postinjection to 11 at 60 min postinjection. Molecular mechanics were used to determine the coordination number, and demonstrated that the Ga(III) complex prefers to be 4-coordinate. Imaging studies with 68Ga-S3N in a Nemestrina macaque showed significant brain uptake, similar to other lipophilic agents. The extraction of 68Ga-S3N into the brains of both rodents and primates, higher than any 68Ga agent reported in the literature, suggests that this compound may have potential as a brain imaging agent for positron emission tomography.

Evaluation of 64Cu-ATSM in vitro and in vivo in a hypoxic tumor model

We have evaluated Cu-diacetyl-bis(N4-methylthiosemicarbazone) (Cu-ATSM), an effective marker for the delineation of hypoxic but viable tissue, in vitro in the EMT6 carcinoma cell line under varying degrees of hypoxia and compared it with the flow tracer 64Cu-pyruvaldehyde-bis(N4-methylthiosemicarbazone) (Cu-PTSM) and the hypoxic tracer 18F-fluoromisonidazole (MISO). We have also compared the uptake of Cu-ATSM and Cu-PTSM in vivo and ex vivo in a murine animal model bearing the EMT6 tumor.

METHODS

Uptake of 64Cu-ATSM, 64Cu-PTSM and 18F-MISO in vitro into EMT6 cells was investigated at the dissolved oxygen concentrations of 0, 1 x 10(3), 5 x 10(3), 5 x 10(4) and 2 x 10(5) ppm. Biodistribution performed at 1, 5, 10, 20 and 40 min compared 64Cu-ATSM with 64Cu-PTSM in BALB/c mice bearing EMT6 tumors. To determine long-term retention of 64Cu-ATSM, biodistribution was also performed at 1, 2 and 4 h. Ex vivo autoradiography of tumor slices after co-injection of 60Cu-PTSM (60Cu, T1/2 = 23.7 min) and 64Cu-ATSM (64Cu, t1/2 = 12.7 h) into the same animal was performed.

RESULTS

After 1 h, 64Cu-ATSM was taken up by EMT6 cells: 90% at 0 ppm, 77% at 1 x 10(3) ppm, 38% at 5 x 10(3) ppm, 35% at 5 x 10(4) ppm and 31% at 2 x 10(5) ppm. 18F-MISO also showed oxygen concentration dependent uptake, but with lower percentages than 64Cu-ATSM. 64Cu-PTSM showed 83%-85% uptake into the cells after 1 h, independent of oxygen concentration. Biodistribution data of 64Cu-ATSM and 64Cu-PTSM showed optimal tumor uptake after 5 and 10 min, respectively (0.76% injected dose (ID)/organ for 64Cu-ATSM and 1.11%ID/organ for 64Cu-PTSM). Ex vivo imaging experiments showed 60Cu-PTSM uniform throughout the EMT6 tumor, but heterogeneous uptake of 64Cu-ATSM, indicative of selective trapping of 64Cu-ATSM into the hypoxic tumor cells.

CONCLUSION

Cu-ATSM exhibits selectivity for hypoxic tumor tissue both in vivo and in vitro and may provide a successful diagnostic modality for the detection of tumor ischemia.