Keynote address: cellular reduction of nitroimidazole drugs: potential for selective chemotherapy and diagnosis of hypoxic cells

Optimized Carbohydrate-Based Nanogel Formulation to Sensitize Hypoxic Tumors

Solid tumors are often poorly vascularized, which impairs oxygen supply and drug delivery to the cells. This often leads to genetic and translational adaptations that promote tumor progression, invasion, metastasis, and resistance to conventional chemo-/radiotherapy and immunotherapy. A hypoxia-directed nanosensitizer formulation of a hypoxia-activated prodrug (HAP) was developed by encapsulating iodoazomycin arabinofuranoside (IAZA), a 2-nitroimidazole nucleoside-based HAP, in a functionally modified carbohydrate-based nanogel, facilitating delivery and accrual selectively in the hypoxic head and neck and prostate cancer cells. Although IAZA has been reported as a clinically validated hypoxia diagnostic agent, recent studies have pointed to its promising hypoxia-selective anti-tumor properties, which make IAZA an excellent candidate for further exploration as a multimodal theranostic of hypoxic tumors. The nanogels are composed of a galactose-based shell with an inner core of thermoresponsive (di(ethylene glycol) methyl ethyl methacrylate) (DEGMA). Optimization of the nanogels led to high IAZA-loading capacity (≅80-88%) and a slow time-controlled release over 50 h. Furthermore, nanoIAZA (encapsulated IAZA) displayed superior in vitro hypoxia-selective cytotoxicity and radiosensitization in comparison to free IAZA in the head and neck (FaDu) and prostate (PC3) cancer cell lines. The acute systemic toxicity profile of the nanogel (NG1) was studied in immunocompromised mice, indicating no signs of toxicity. Additionally, growth inhibition of subcutaneous FaDu xenograft tumors was observed with nanoIAZA, demonstrating that this nanoformulation offers a significant improvement in tumor regression and overall survival compared to the control.

Designing Hypoxia-Responsive Nanotheranostic Agents for Tumor Imaging and Therapy

Hypoxia, a common feature of most solid tumors, plays an important role in tumor proliferation, metastasis, and invasion, leading to drug, radiation, and photodynamic therapy resistance, and resulting in a sharp reduction in the disease-free survival rate of tumor patients. The lack of sufficient blood supply to the interior regions of tumors hinders the delivery of traditional drugs and contrast agents, interfering with their accumulation in the hypoxic region, and preventing efficient theranostics. Thus, there is a need for the fabrication of novel tumor theranostic agents that overcome these obstacles. Reports, in recent years, of hypoxia-responsive nanomaterials may provide with such means. In this review, a comprehensive description of the physicochemical and biological characteristics of hypoxic tumor tissues is provided, the principles of designing the hypoxia-responsive tumor theranostic agents are discussed, and the recent research into hypoxia-triggered nanomaterials is examined. Additionally, other hypoxia-associated responsive strategies, the current limitations, and future prospects for hypoxia-responsive nanotheranostic agents in tumor treatment are discussed.

Radiosynthesis of the tumor hypoxia marker [18F]TFMISO via O-[18F]trifluoroethylation reveals a striking difference between trifluoroethyl tosylate and iodide in regiochemical reactivity toward oxygen nucleophiles

The MRI hypoxia marker trifluoromisonidazole (TFMISO) [1-(2-nitro-1H-imidazol-1-yl)-3-(2,2,2-trifluoroethoxy)propan-2-ol] was successfully labeled with (18)F to expand its role into a bimodal PET/MRI probe. (18)F-Labeling was achieved via a three-step procedure in which 2,2,2-[(18)F]trifluoroethyl p-toluenesulfonate prepared by (18)F-(19)F exchange served as the [(18)F]trifluoroethylating agent. The O-[(18)F]trifluoroethylation reaction proceeded efficiently to give the intermediate 1,2-epoxy-3-(2,2,2-[(18)F]trifluoroethoxy)propane, with approximately 60% of (18)F incorporated from the tosylate precursor, which was condensed with 2-nitroimidazole to yield [(18)F]TFMISO. Approximately 40% of the [(18)F]trifluoroethyl tosylate precursor was converted into the final product. In stark contrast, 2,2,2-[(18)F]trifluoroethyl iodide failed to produce [(18)F]TFMISO, giving instead 1,1-[(18)F]difluoro-2-iodoethoxy and 1-[(18)F]fluoro-2-iodovinyloxy analogs of [(18)F]TFMISO. Thus, this investigation has identified 2,2,2-[(18)F]trifluoroethyl tosylate as an excellent [(18)F]trifluoroethylating agent, which can convert efficiently an alcohol into the corresponding [(18)F]trifluoroethyl ether.

Parametric mapping of [18F]fluoromisonidazole positron emission tomography using basis functions

In this study, we show a basis function method (BAFPIC) for voxelwise calculation of kinetic parameters (K(1), k(2), k(3), K(i)) and blood volume using an irreversible two-tissue compartment model. BAFPIC was applied to rat ischaemic stroke micro-positron emission tomography data acquired with the hypoxia tracer [(18)F]fluoromisonidazole because irreversible two-tissue compartmental modelling provided good fits to data from both hypoxic and normoxic tissues. Simulated data show that BAFPIC produces kinetic parameters with significantly lower variability and bias than nonlinear least squares (NLLS) modelling in hypoxic tissue. The advantage of BAFPIC over NLLS is less pronounced in normoxic tissue. K(i) determined from BAFPIC has lower variability than that from the Patlak-Gjedde graphical analysis (PGA) by up to 40% and lower bias, except for normoxic tissue at mid-high noise levels. Consistent with the simulation results, BAFPIC parametric maps of real data suffer less noise-induced variability than do NLLS and PGA. Delineation of hypoxia on BAFPIC k(3) maps is aided by low variability in normoxic tissue, which matches that in K(i) maps. BAFPIC produces K(i) values that correlate well with those from PGA (r(2)=0.93 to 0.97; slope 0.99 to 1.05, absolute intercept <0.00002 mL/g per min). BAFPIC is a computationally efficient method of determining parametric maps with low bias and variance.

Pharmacokinetic analysis of hypoxia (18)F-fluoromisonidazole dynamic PET in head and neck cancer

This study used pharmacokinetic analysis of (18)F-labeled fluoromisonidazole ((18)F-FMISO) dynamic PET to assist the identification of regional tumor hypoxia and to investigate the relationship among a potential tumor hypoxia index (K(i)), tumor-to-blood ratio (T/B) in the late-time image, plasma-to-tissue transport rate (k(1)), and local vascular volume fraction (beta) for head and neck cancer patients.

METHODS

Newly diagnosed patients underwent a dynamic (18)F-FMISO PET scan before chemotherapy or radiotherapy. The data were acquired in 3 consecutive PET/CT dynamic scan segments, registered with each other and analyzed using pharmacokinetics software. The (K(i), k(1), beta) kinetic parameter images were derived for each patient.

RESULTS

Nine patients' data were analyzed. Representative images of (18)F-FDG PET (of the tumor), CT (of the anatomy), and late-time (18)F-FMISO PET (of the T/B) and parametric images of K(i) (potentially representing tumor hypoxia) are shown. The patient image data could be classified into 3 types: with good concordance between the parametric hypoxia map K(i) and high T/B, with concordant findings between the parametric hypoxia map and low T/B, and with ambiguity between parametric hypoxia map and T/B. Correlation coefficients are computed between each pair of T/B, K(i), k(1), and beta. Data are also presented for other potential hypoxia surrogate measures, for example, k(3) and k(1)/k(2).

CONCLUSION

There is a positive correlation of 0.86 between the average T/B and average hypoxia index K(i) of the region of interest. However, because of the statistical photon counting noise in PET and the amplification of noise in kinetic analysis, the direct correlation between the T/B and hypoxia of the individual pixel is not obvious. For a tumor region of interest, there is a slight negative correlation between k(1) and K(i), moderate positive correlation between beta and K(i), but no correlation between beta and k(1).

Evaluation of a compartmental model for estimating tumor hypoxia via FMISO dynamic PET imaging

This paper systematically evaluates a pharmacokinetic compartmental model for identifying tumor hypoxia using dynamic positron emission tomography (PET) imaging with 18F-fluoromisonidazole (FMISO). A generic irreversible one-plasma two-tissue compartmental model was used. A dynamic PET image dataset was simulated with three tumor regions-normoxic, hypoxic and necrotic-embedded in a normal-tissue background, and with an image-based arterial input function. Each voxelized tissue's time activity curve (TAC) was simulated with typical values of kinetic parameters, as deduced from FMISO-PET data from nine head-and-neck cancer patients. The dynamic dataset was first produced without any statistical noise to ensure that correct kinetic parameters were reproducible. Next, to investigate the stability of kinetic parameter estimation in the presence of noise, 1000 noisy samples of the dynamic dataset were generated, from which 1000 noisy estimates of kinetic parameters were calculated and used to estimate the sample mean and covariance matrix. It is found that a more peaked input function gave less variation in various kinetic parameters, and the variation of kinetic parameters could also be reduced by two region-of-interest averaging techniques. To further investigate how bias in the arterial input function affected the kinetic parameter estimation, a shift error was introduced in the peak amplitude and peak location of the input TAC, and the bias of various kinetic parameters calculated. In summary, mathematical phantom studies have been used to determine the statistical accuracy and precision of model-based kinetic analysis, which helps to validate this analysis and provides guidance in planning clinical dynamic FMISO-PET studies.

In vivo19F Magnetic Resonance Spectroscopy and Chemical Shift Imaging of Tri-Fluoro-Nitroimidazole as a Potential Hypoxia Reporter in Solid Tumors

PURPOSE

2-Nitro-alpha-[(2,2,2-trifluoroethoxy)methyl]-imidazole-1-ethanol (TF-MISO) was investigated as a potential noninvasive marker of tissue oxygen levels in tumors using (19)F magnetic resonance spectroscopy (MRS) and (19)F chemical shift imaging.

EXPERIMENTAL DESIGNS

In vitro data were obtained using high-performance liquid chromatography on tumor cells incubated under varying oxygen conditions to determine the oxygen-binding characteristics. In vivo data were obtained using a well-characterized hypoxic murine breast tumor (MCa), in addition to studies on a rat prostate tumor model (R3327-AT) implanted in nude mice. Detection of intratumor (19)F signal from TF-MISO was done using MRS for up to 10 h following a 75 mg/kg i.v. injection. Localized distribution of the compound in the implanted MCa tumor has been imaged using slice-selective two-dimensional chemical shift imaging 6 h after injection.

RESULTS

The in vitro results showed that TF-MISO preferentially accumulates in cells incubated under anoxic conditions. The in vivo (19)F MR spectral features (line width and chemical shift) were recorded as a function of time after injection, and the results indicate that the fluorine atoms are indeed sensitive to changes in the local environment while still providing a detectable MR signal. Ex vivo spectra were collected and established the visibility of the (19)F signal under conditions of maximum hypoxia. Late time point (>6 h) tumor tissue concentrations, as obtained from (19)F MRS, suggest that TF-MISO is reduced and retained in hypoxic tumor. The feasibility of obtaining TF-MISO tumor distribution maps in a reasonable time frame was established.

CONCLUSIONS

Based on the results presented herein, it is suggested that TF-MISO has the potential to be a valid magnetic resonance hypoxia imaging reporter for both preclinical hypoxia studies and hypoxia-directed clinical therapy.

[(18)F]FMISO and [(18)F]FDG PET imaging in soft tissue sarcomas: correlation of hypoxia, metabolism and VEGF expression

Hypoxia imparts resistance to radiotherapy and chemotherapy and also promotes a variety of changes in tumor biology through inducible promoters. The purpose of this study was to evaluate the use of positron emission tomography (PET) imaging with fluorine-18 fluoromisonidazole (FMISO) in soft tissue sarcomas (STS) as a measure of hypoxia and to compare the results with those obtained using [(18)F]fluorodeoxyglucose (FDG) and other known biologic correlates. FDG evaluates energy metabolism in tumors while FMISO uptake is proportional to tissue hypoxia. FMISO uptake was compared with FDG uptake. Vascular endothelial growth factor (VEGF) expression was also compared with FMISO uptake. Nineteen patients with STS underwent PET scanning with quantitative determination of FMISO and FDG uptake prior to therapy (neo-adjuvant chemotherapy or surgery alone). Ten patients receiving neo-adjuvant chemotherapy were also imaged after chemotherapy but prior to surgical resection. Standardized uptake value (SUV) was used to describe FDG uptake; regional tissue to blood ratio (>or=1.2 was considered significant) was used for FMISO uptake. Significant hypoxia was found in 76% of tumors imaged prior to therapy. No correlation was identified between pretherapy hypoxic volume (HV) and tumor grade ( r=0.15) or tumor volume ( r=0.03). The correlation of HV with VEGF expression was 0.39. Individual tumors showed marked heterogeneity in regional VEGF expression. The mean pixel-by-pixel correlation between FMISO and FDG uptake was 0.49 (range 0.09-0.79) pretreatment and 0.32 (range -0.46-0.72) after treatment. Most tumors showed evidence of reduced uptake of both FMISO and FDG following chemotherapy. FMISO PET demonstrates areas of significant and heterogeneous hypoxia in soft tissue sarcomas. The significant discrepancy between FDG and FMISO uptake seen in this study indicates that regional hypoxia and glucose metabolism do not always correlate. Similarly, we did not find any relationship between the hypoxic volume and the tumor volume or VEGF expression. Identification of hypoxia and development of a more complete biologic profile of STS will serve to guide more rational, individualized cancer treatment approaches.

Supramolecular structures of metronidazole and its copper(II), cobalt(II) and zinc(II) coordination compounds

In this work we present the synthesis and structural and spectroscopic characterization of Cu(II), Co(II) and Zn(II) coordination compounds with the antibiotic metronidazole ([double bond]emni). Coordination to metal ions is through its imidazolic nitrogen, while the hydroxyethyl and nitro groups act as supramolecular synthons. [Co(emni)(2)Br(2)], and [Zn(emni)(2)X(2)] (X(-)=Cl, Br) stabilize zig-zag chains, and a 2D supramolecular structure is formed by inter-chain contacts through inter-molecular hydrogen-bonding. Pleated sheet or layers are formed by [Co(emni)(2)Cl(2)] and [Cu(emni)(2)Cl(H(2)O)](2)Cl(2), respectively. The dinuclear Cu(II) compound [Cu(emni)mu(O(2)CMe)(2)](2) gives a one-dimensional zig-zag arrangement. The contribution of metal ions in metronidazole coordination compounds is shown in the stabilization of the different aggregate structures.

Measuring hypoxia and predicting tumor radioresistance with nuclear medicine assays

Tumor cells at low oxygen tension are relatively radioresistant. The hypoxic fraction of individual tumors before, during and after radiotherapy is likely to have prognostic value but its diagnosis still awaits an accurate and acceptable assay. The recent indications that hypoxia can also induce the expression of specific genes and promote a more aggressive tumor phenotype makes its diagnosis even more important. Over 15 years ago, misonidazole, an azomycin-based hypoxic cell radiosensitizer, was found to link covalently to cellular molecules at rates inversely proportional to intracellular oxygen concentration. The use of bioreducible markers to positively label zones of viable hypoxic cells within solid tumors and to predict for tumor radioresistance was proposed. Several hypoxic markers have now been identified and their selective binding within tumors has been measured by both invasive and non-invasive assays. Research from our laboratory has emphasized both mechanistic and preclinical studies associated with nuclear medicine procedures for measuring tumor hypoxia and predicting tumor radioresistance. This report updates radiation oncologists about the status of nuclear medicine hypoxic marker research and development as of mid-1997. While several potential imaging agents have been identified, their testing and validation in appropriate human tumors will require focused research efforts by individual academic departments and, possibly, by clinical trials performed through cooperative groups. Since the prediction of hypoxia in individual tumors could strongly impact radiotherapy treatment planning, the radiation oncology research community is best positioned to execute the validation studies associated with these markers.

The role of specific reductases in the intracellular activation and binding of 2-nitroimidazoles

PURPOSE

To determine the relative effectiveness of specific cellular reductases for the activation and binding of 2-nitroimidazoles in vivo.

METHODS AND MATERIALS

Monkey kidney cells were transfected with recombinant plasmids to effect intracellular overexpression of P450 reductase and DT-diaphorase. The covalent binding of 2-nitroimidazoles to cellular macromolecules was measured as a function of time of cell incubation at various oxygen concentrations. The effect of allopurinol on cellular binding of radiolabeled 2-nitroimidazoles was also measured.

RESULTS

A 1,000-fold overexpression of DT-diaphorase resulted in a small but significant increase in 2-nitroimidazole binding rate. An 80-fold overexpression of cytochrome P450 reductase resulted in a 5-7-fold increase in the binding rate of 2-nitroimidazole. The inhibition of xanthine oxidase by allopurinol had no effect on 2-nitroimidazole binding rates. The amplification of P450 reductase activity within cells was always much larger than the resultant increase in 2-nitroimidazole binding rate, suggesting an enzyme kinetic process less than first order and possibly of 1/2-order.

CONCLUSION

These data suggest that cytochrome P450 reductase is the most important enzyme in these cells for reducing 2-nitroimidazoles to intermediates which can covalently bind to cellular macromolecules. Furthermore, since this cellular process demonstrates approximately 1/2-order kinetics, a tissue's capacity for binding 2-nitroimidazole drug in hypoxia should be proportional to the square root of its intracellular P450 reductase level.

Magnetic resonance imaging of human melanoma xenografts in vivo: proton spin-lattice and spin-spin relaxation times versus fractional tumour water content and fraction of necrotic tumour tissue

Proton nuclear magnetic resonance (1H-nmr) imaging is used routinely in clinical oncology to provide macroscopic anatomical information, whereas its potential to provide physiological information about tumours is not well explored. To evaluate the potential usefulness of 1H-nmr imaging in the prediction of tumour treatment resistance caused by unfavourable microenvironmental conditions, possible correlations between proton spin-lattice and spin-spin relaxation times (T1 and T2) and physiological parameters of the tumour microenvironment were investigated. Tumours from six human melanoma xenograft lines were included in the study. 1H-nmr imaging was performed at 1.5 T using spin-echo pulse sequences. T1- and T2-distributions were generated from the images. Fractional tumour water content and the fraction of necrotic tumour tissue were measured immediately after 1H-nmr imaging. Significant correlations across tumour lines were found for T1 and T2 versus fractional tumour water content (p < 0.001) as well as for T1 and T2 versus fraction of necrotic tumour tissue (p < 0.05). Tumours with high fractional water contents had high values of T1 and T2, probably caused by free water in the tumour interstitium. Fractional water content is correlated to interstitial fluid pressure in tumours, high interstitial fluid pressure being indicative of high vascular resistance. Tumours with high fractional water contents are thus expected to show regions with radiobiologically hypoxic cells as well as poor intravascular and interstitial transport of many therapeutic agents. T1 and T2 decreased with increasing fraction of necrotic tumour tissue, perhaps because complexed paramagnetic ions were released during development of necrosis. Viable tumour cells adjacent to necrotic regions are usually chronically hypoxic. Tumours with high fractions of necrotic tissue are thus expected to contain significant proportions of radiobiologically hypoxic cells. Consequently, quantitative 1H-nmr imaging has the potential to be developed as an efficient clinical tool in prediction of tumour treatment resistance caused by hypoxia and/or transport barriers for therapeutic agents. However, much work remains to be done before this potential can be adequately evaluated. One problem is that high fractional tumour water contents result in longer T1 and T2 whereas high fractions of necrotic tumour tissue result in shorter T1 and T2; i.e. the two parameters which are indicative of treatment resistance contribute in opposite directions. Another problem is that the correlations for T1 and T2 versus fraction of necrotic tumour tissue are not particularly strong.

Resistance to the nitroheterocyclic drugs

The nitroheterocyclic drugs have been available since the early 1960's for the treatment of anaerobic protozoa. The application of these drugs has widened since then and they are presently used to treat anaerobic pathogenic bacteria and protozoa. The activity of the nitroheterocyclic drugs depends on the all-important nitro group attached to the imidazole or furan ring. Although the nitro radicals, generated by reduction of the parent drugs, are similar for both families of nitroheterocyclics, the nitroimidazoles and the nitrofurans, the electron potential of each is different and thus the mechanism of action depends on different pathways. The nitroimidazoles depend on reduction by ferredoxin or flavodoxin. The nitrofurans require nitroreductase activity, but the natural substrate of these enzymes has not been identified. Increased use of nitroheterocyclic drugs, in response to drug resistance to other commonly used antibiotics, has in turn resulted in drug resistance to a number of nitroheterocyclic drugs. Bacteroides strains and other bacteria, including Helicobacter, have developed resistance. Among the protozoa, Trichomonas has developed resistance to metronidazole via a number of mechanisms, especially a decrease in drug reduction, as a result of alterations in the electron transport pathways. Resistance to both types of nitroheterocyclic drugs has been reported in Giardia. Although resistance to these drugs is not widespread, their increased use world-wide as a prophylaxis and in chemotherapy will inevitably result in increased resistance in organisms commonly found in asymptomatic infections, including Trichomonas, Giardia and Entamoeba. However, the variety of substitutions which can be attached to the ring structures has led to a great variety of drugs being synthesised, some of which are many-fold more active than the commonly prescribed nitroheterocyclics. With careful administration of currently available drugs and continued interest in synthesising more active compounds, we can optimistically expect to have useful nitroheterocyclic drugs available for some time.

Bioreductive metabolism of AF-2[2-(2-furyl)-3-(5-nitro-2-furyl)acrylamide] combined with 2-nitroimidazoles. Implications for use as hypoxic cell markers

Metabolism of misonidazole under hypoxic conditions depletes the parent drug and causes about 4% of the reduced-drug-products to form adducts with cellular macromolecules (binding), and this process has been used to detect hypoxia in cells and tissues. The nitrofuran, AF-2 [2-(2-furyl)-3-(5-nitro-2-furyl)acrylamide] has been shown to increase both the metabolic depletion of misonidazole and its binding. In the present study, factors which might affect this process have been examined, in an in vitro system, to test the hypothesis that metabolic depletion of misonidazole could limit its ability to diffuse freely to the hypoxic cell population. Drastic reductions in glucose concentrations from their normal value of 5-10 mM to less than 0.5 mM had no significant effect on the metabolism of either misonidazole or AF-2. Similarly, glucose concentration did not influence the binding of misonidazole, even when concentrations of both oxygen (extreme hypoxia) and glucose were near zero--a very toxic biochemical environment. Similarly, the metabolism of the nitroheterocyclics had no effect on glucose consumption. The bioreductive depletion of misonidazole in extreme hypoxia appeared to be independent of drug concentration between 25 and 100 microM: this nearly zero-order rate of drug metabolism prevented the possibility of working at constant drug concentration. AF-2 exacerbated this effect by greatly enhancing the metabolic depletion of misonidazole. AF-2 was found to increase both the metabolic depletion and binding of misonidazole by the same factor. An unexpected finding was that metabolism of etanidazole, a 2-nitroimidazole closely related to misonidazole, was not enhanced by AF-2. Micromolar amounts of oxygen inhibited the reductive activation of AF-2, and also the interaction between AF-2 and misonidazole. Our results suggest that metabolic depletion of nitroheterocyclics could influence their ability to diffuse adequately to hypoxic tissues, particularly at the low drug concentrations that have been used to measure tissue hypoxia in vivo.

Non-invasive assessment of human tumour hypoxia with 123I-iodoazomycin arabinoside: preliminary report of a clinical study

Non-invasive predictive assays which can confirm the presence or absence of hypoxic cells in human tumours show promise for understanding the natural history of tumour oxygenation, and improving the selection of patient subsets for novel radiotherapeutic strategies. Sensitiser adducts have been proposed as markers for hypoxic cells. Misonidazole analogues radiolabelled with iodine-123 have been developed for the detection of tumour hypoxia using conventional nuclear medicine techniques. In this pilot study, we have investigated one such potential marker, 123I-iodoazomycin arabinoside (123I-IAZA). Patients with advanced malignancies have undergone planar and single-photon emission computed tomographic (SPECT) imaging after intravenous administration of 123I-IAZA. We have observed radiotracer avidity in three out of ten tumours studied to date. Normal tissue activity of variable extent was also seen in the thyroid and salivary glands, upper aerodigestive tract, liver, intestine, and urinary bladder. Quantitative analysis of those images showing radiotracer avidity revealed tumour/normal tissue (T/N) ratios of 2.3 (primary small cell lung carcinoma), 1.9 (primary malignant fibrous histiocytoma) and 3.2 (brain metastasis from small cell lung carcinoma) at 18-24 h post injection. These preliminary data suggest that the use of gamma-emitter labelled 2-nitroimidazoles as diagnostic radiopharmaceuticals is feasible and safe, and that metabolic binding of 123I-IAZA is observed in some, but not all tumours. The inference that tumour 123I-IAZA avidity could be a non-invasive measure of tumour hypoxia deserves independent confirmation with needle oximetry.

Electrochemical characteristics of nitroheterocyclic compounds of biological interest. VIII. Stability of nitro radical anions from cyclic voltammetric studies

The stability of the one electron addition product of four biologically important nitroheterocyclic compounds has been examined electrochemically. Using cyclic voltammetry the tendency of the nitro radical anion to undergo disproportionation was studied by two methods of analysis. The first was based on determining the voltammetric time-constant required for half of the reduction product, RNO2-., to react further. The second concerned the minimum volume of dimethylformamide which had to be added to the aqueous electrolytic medium to give a specific cyclic voltammetric response. Both methods were found to compare well with the results obtained for RNO2-. stabilities using a theoretically derived procedure for a second order reaction following a charge-transfer step. The use of these alternative approaches for quantifying the reactivity of reduction products is discussed. The time-constant method in particular may be useful in studying complex reaction pathways.

Tumor hypoxia, reoxygenation and oxygenation strategies: possible role in photodynamic therapy

The concept of hypoxia and its role in tumor therapy are currently under re-evaluation. Poor oxygenation is no longer visualized as an independent feature promoting necrosis and resistance to treatments, but rather as one of the several interdependent microenvironmental parameters associated with impaired blood perfusion. Tumor cells display several survival strategies and remain clonogenic for long periods in nutrient-deprived situations. Reoxygenation may cause lethal damage, improve the response to therapy, or else allow the cell variants adapted to hypoxia to resume proliferation with enhanced aggressiveness and resistance to treatment. The blood supply parameters, oxygenation status and metabolism of malignant cells are discussed here from the standpoint of tumor photodynamic therapy. The role of the tumor interstitial fluid as oxygen- and sensitizer-carrier is discussed. Techniques for assessing tumor oxygenation and for mapping hypoxic territories are described. Strategies for locally improving the oxygenation levels or for selectively destroying the hypoxic populations are outlined.

Measurement of tumor hypoxia by invasive and non-invasive procedures: a review of recent clinical studies

The potential importance of hypoxic cells in cancer treatment response has been debated since their presence in human tumors was inferred by the classical studies of Thomlinson and Gray. Tumor cells which contain low concentrations of molecular oxygen display resistance to high energy photon irradiation and some chemotherapy regimens, in both in vitro and animal tumor studies. No diagnostic procedure is currently available for measuring the oxygenation status of human tumors at the time of diagnosis or throughout treatment. Recent studies with oxygen electrodes and sensitizer-adducts indicate a wide heterogeneity of oxygen levels within solid human tumors, even for tumors of similar histology and size. These studies suggest that to determine the relative importance of tumor hypoxia in treatment resistance, a "predictive assay" for monitoring tumor oxygenation status in individual patients will be required. Recently, several sophisticated techniques for measuring tumor oxygen levels and tumor metabolism have indicated both intertumor and intratumor heterogeneity of tumor oxygen levels and other metabolites. While providing useful information about human tumor biology, most of the invasive procedures are not appropriate as a standard diagnostic tool. Non-invasive measurements of 1) sensitizer-adducts by nuclear medicine procedures and 2) tumor energetics by 31P NMR spectroscopy might be developed as routine predictors of tumor oxygenation and possible treatment outcome.

Dynamic measurements of hexafluoromisonidazole (CCI-103F) retention in mouse tumours by 1H/19F magnetic resonance spectroscopy

Selective retention of hexafluoromisonidazole, CCI-103F, in RIF-1 and SCCVII tumours of C3H/Km mice has been measured by 1H/19F magnetic resonance spectroscopy (MRS) on a Bruker AM-400 multinuclear spectrometer. CCI-103F concentrations in tumours and in normal tissues were measured using an MRS technique in which the water component in the tissues serves as an internal concentration reference. The biodistribution and elimination half-life of the drug in the tissues after i.p. injections were determined. The plasma half-life of the drug (41 min) was measured by high-pressure liquid chromatography. The two tumour lines and liver have longer retention times with half-lives of 47, 129 and 81 min, respectively, while normal tissues, muscle and brain have little retention of CCI-103F and clear the drug very quickly. Dynamic measurements of CCI-103F retention in tumours by MRS may provide a non-invasive probe for assessing tumour hypoxia.

The potentiation of radiation damage by nicotinamide in the SCCVII tumour in vivo

We have continued our assessment of the ability of nicotinamide to sensitize tumours to radiation. Using the SCCVII carcinoma and estimating tumour response by either a regrowth delay or an in vivo/in vitro survival assay, it was found that a large single dose of nicotinamide (1000 mg/kg) increased radiation-induced tumour damage. This effect was observed regardless of whether the tumour was grown intramuscularly, subcutaneously or intradermally, or whether the nicotinamide was administered intraperitoneally, intravenously or orally. The enhancement was maximal when the drug was given between 30 min and 2 h prior to irradiation and resulted in enhancement ratios ranging from 1.1 to 1.7. Although the radiation response of tumours was dependent on tumour size, the radiation enhancement produced by nicotinamide was not. Utilizing the technique of labelling tumour cells with the fluorescent stain Hoechst 33342, we were able to identify the presence of both chronic and acutely hypoxic cells in this tumour model and obtained results suggesting that apart from reducing chronic hypoxia, nicotinamide may also have the ability to decrease the level of radioresistant acute hypoxia.