Radiopharmaceuticals for assessment of multidrug resistance P-glycoprotein-mediated drug transport activity

PET Radiopharmaceuticals for Imaging Chemotherapy-Induced Cardiotoxicity

Purpose of Review

Currently, cardiotoxicity is monitored through echocardiography or multigated acquisition scanning and is defined as 10% or higher LVEF reduction. The latter stage may represent irreversible myocardium injury and limits modification of therapeutic paradigms at earliest stages. To stratify patients for anthracycline-related heart failure, highly sensitive and molecularly specific probes capable of interrogating cardiac damage at the subcellular levels have been sought.

Recent Findings

PET tracers may provide noninvasive assessment of earliest changes within myocardium. These tracers are at nascent stages of development and belong primarily to (a) mitochondrial potential-targeted and (b) general ROS (reactive oxygen species)-targeted radiotracers. Given that electrochemical gradient changes at the mitochondrial membrane represent an upstream, and earliest event before triggering the production of the ROS and caspase activity in a biochemical cascade, the former category might offer interrogation of cardiotoxicity at earliest stages exemplified by PET imaging, using ¹⁸F-Mitophos and ⁶⁸Ga-Galmydar in rodent models.

Summary

Both categories of radiotracers may provide tools for monitoring chemotherapy-induced cardiotoxicity and interrogating therapeutic efficacy of cardio-protectants.

68Ga-Galmydar: A PET imaging tracer for noninvasive detection of Doxorubicin-induced cardiotoxicity

Background

Cancer patients undergoing Doxorubicin (DOX) treatment are susceptible to acute and chronic cardiac anomalies, including aberrant arrhythmias, ventricular dysfunction, and heart failure. To stratify patients at high risk for DOX -related heart failure (CHF), diagnostic techniques have been sought. While echocardiography is used for monitoring LVEF and LV volumes due to its wide-availability and cost-efficiency, it may not identify early stages of the initiation of DOX-induced systolic heart failure. To address these limitations, PET tracers could also provide noninvasive assessment of early and reversible metabolic changes of the myocardium.

Objective

Herein, we report a preliminary investigation of ⁶⁸Ga-Galmydar potential to monitor Dox-induced cardiomyopathy in vivo, ex vivo, and in cellulo employing both nuclear- and optical imaging.

Methods and results

To assess ⁶⁸Ga-Galmydar ability for monitoring DOX-induced cardiomyopathy, microPET imaging was performed 5 d post treatment of rats either with a single dose of DOX (15 mg/kg) or vehicle as a control (saline) and images were co-registered for anatomical reference using CT. Following tail-vein injection of the radiotracer in rats at 60 min, micro-PET/CT static scan (10 min acquisition), ⁶⁸Ga-Galmydar demonstrated 1.91-fold lower uptake in hearts of DOX-treated (standard uptake value; SUV: 0.92, n = 3) rats compared with their vehicle treated (SUV: 1.76, n = 3) control counterparts. For correlation of PET imaging data, post-imaging quantitative biodistribution studies were also performed, wherein excised organs were counted for γ activity, and normalized to injected dose. The post imaging pharmacokinetic data also demonstrated heart uptake values of 2.0 fold lower for DOX treated rats(%ID/g; DOX: 0.44 ± 0.1, n = 3) compared to their vehicle-treated controls (%ID/g; Control: 0.89 ± 0.03, n = 3, p = 0.04). Employing the fluorescent traits of Galmydar, live cell fluorescence imaging indicated a gradual decrease in uptake and retention of Galmydar within mitochondria of H9c2 cells following DOX-treatment, while indicating dose-dependent and time-dependent uptake profiles. Following depolarization of electronegative transmembrane gradients at the mitochondrial membrane, the uptake of the probe was decreased in H9c2 cells, and the uptake profiles were found to be identical, using both fluorescence and radiotracer bioassays. Finally, the decreased uptake of the metalloprobe in H9c2 cells also correlated with caspase-3 expression resulting from DOX-induced cardiotoxicity and cell death.

Conclusions

⁶⁸Ga-Galmydar could provide a noninvasive assessment of DOX-related and likely reversible metabolic changes at earliest stages. Further studies with other chemotherapeutics (potentially capable of inducing cardiomyopathy) are underway.

New 99mTc Radiotracers for Myocardial Perfusion Imaging by SPECT

OBJECTIVE

Myocardial Perfusion Imaging (MPI) with radiotracers is an integral component in evaluation of the patients with known or suspected coronary artery diseases (CAD). 99mTc-Sestamibi and 99mTc-Tetrofosmin are commercial radiopharmaceuticals for MPI by single photon-emission computed tomography (SPECT). Despite their widespread clinical applications, they do not meet the requirements of an ideal perfusion imaging agent due to their inability to linearly track the regional myocardial blood flow rate at >2.5 mL/min/g. With tremendous development of CZT-based SPECT cameras over the past several years, the nuclear cardiology community has been calling for better perfusion radiotracers with improved extraction and biodistribution properties.

METHODS

This review will summarize recent research efforts on new cationic and neutral 99mTc radiotracers for SPECT MPI. The goal of these efforts is to develop a 99mTc radiotracer that can be used to detect perfusion defects at rest or under stress, determine the regional myocardial blood flow, and measure the perfusion and left ventricular function.

RESULTS

The advantage of cationic radiotracers (e.g. 99mTc-Sestamibi) is their long myocardial retention because of the positive molecular charge and fast liver clearance kinetics. 99mTc-Teboroxime derivatives have a high initial heart uptake (high first-pass extraction fraction) due to their neutrality. 99mTc- 3SPboroxime is the most promising radiotracer for future clinical translation considering its initial heart uptake, myocardial retention time, liver clearance kinetics, heart/liver ratios and SPECT image quality.

CONCLUSION

99mTc-3SPboroximine is an excellent example of perfusion radiotracers, the heart uptake of which is largely relies on the regional blood flow. It is possible to use 99mTc-3SPboroximine for detection of perfusion defect(s), accurate quantification and determination of regional blood flow rate. Development of such a 99mTc radiotracer is of great clinical benefit for accurate diagnosis of CAD and assessing the risk of future hard events (e.g. heart attack and sudden death) in cardiac patients.

Live-cell fluorescence imaging: assessment of thioflavin T uptake in human epidermal carcinoma cells

Thioflavin T (ThT), a positively charged heterocyclic small molecule, is a widely used fluorescent marker of amyloid pathophysiology to confirm the cause of death in post mortem brain tissue of Alzheimer's disease (AD) patients. Literature precedents indicate that current positron emission tomography (PET) agents, such as 11C-PIB and 18F-flutemetamol, share significant structural similarity with ThT, a lipophilic dye which does not traverse the blood-brain barrier (BBB) to enable the detection of Aβ plaques in vivo. While vital for maintaining normal physiology and healthy brain function, the BBB comprises brain endothelial cells sealed via paracellular protein complexes, bound by an extracellular matrix forming tight junctions thus controlling the delivery of molecules into the brain. The human P-glycoprotein (Pgp/ABCB1, 170 kD plasma membrane protein), belonging to the ABC family of efflux transporter proteins, also lines the luminal surface of brain endothelial cells thus poised to secrete its recognized substrates into the blood. Herein, we postulate that thioflavin T (ThT), due to its physico-chemical attributes, such as moderate lipophilicity and protonated nitrogen, could very well be recognized as a transport substrate of Pgp (P-glycoprotein, ABCB1) thus restricting its permeation into the brain. To evaluate whether or not ThT is indeed recognized by Pgp as its transport substrate thus limiting its BBB permeability, herein, we evaluate cellular accumulation profiles of ThT and PiB (a similar structural uncharged mimetic) in human epidermal carcinoma KB-3-1 (Pgp-) and MDR KB-8-5 (Pgp+) cells, using live-cell fluorescence imaging. While ThT penetrates KB-3-1 cells, it gets excluded from KB-8-5 cells, and also indicates LY335979-induced uptake in Pgp-expressing cells. Furthermore, the cellular uptake profiles of PiB are not impacted by the expression of Pgp under identical conditions. These data show that uptake profiles of ThT have been modified by the expression of Pgp in these cells, and are inversely proportional to the expression of the transporter protein located on the plasma membrane of these cells. Combined data demonstrate that ThT is efficiently recognized by Pgp as its transport substrate.

Blood-Brain Barrier Driven Pharmacoresistance in Amyotrophic Lateral Sclerosis and Challenges for Effective Drug Therapies

The blood-brain barrier (BBB) is essential for proper neuronal function, homeostasis, and protection of the central nervous system (CNS) microenvironment from blood-borne pathogens and neurotoxins. The BBB is also an impediment for CNS penetration of drugs. In some neurologic conditions, such as epilepsy and brain tumors, overexpression of P-glycoprotein, an efflux transporter whose physiological function is to expel catabolites and xenobiotics from the CNS into the blood stream, has been reported. Recent studies reported that overexpression of P-glycoprotein and increase in its activity at the BBB drives a progressive resistance to CNS penetration and persistence of riluzole, the only drug approved thus far for treatment of amyotrophic lateral sclerosis (ALS), rapidly progressive and mostly fatal neurologic disease. This review will discuss the impact of transporter-mediated pharmacoresistance for ALS drug therapy and the potential therapeutic strategies to improve the outcome of ALS clinical trials and efficacy of current and future drug treatments.

67Ga-metalloprobes: monitoring the impact of geometrical isomers on accumulation profiles in rat cardiomyoblasts and human breast carcinoma cells

Geometrically similar monocationic gallium(iii) complexes and their radiolabeled SPECT counterparts were obtained from Schiff base precursor ligands using ligand exchange reactions to evaluate the impact of cis and trans-isomers on their cellular accumulation profiles in rat cardiomyoblasts (H9c2(2-1)) and human breast carcinoma (MCF-7neo) cells. ⁶⁷Ga-metalloprobes comprising trans-phenolates showing an overall octahedral geometry and exhibiting uniform spatial distribution of positive charges on their molecular surface show steady-state accumulation in H9c2(2-1) and MCF-7neo cells, and localize in the mitochondria of the cells. Importantly, the surrogate geometrically similar and monocationic metalloprobe counterparts possessing the cis arrangement of phenolates do not show cellular uptake in H9c2(2-1) and MCF-7neo cells. Exploiting their modest fluorescent traits, live cell imaging indicates that trans-isomers of metalloprobes localize within the mitochondria of cells following their penetration, thereby indicating the excellent correlation of radiotracer data and live-cell microscopy results. Overall, these results indicate that the cell uptake profiles of metalloprobes within this class are mediated by the spatial distribution of charges over their molecular surface and hydrophobicity.

Metalloprobes: Fluorescence imaging of multidrug resistance (MDR1) P-Glycoprotein (Pgp)-mediated functional transport activity in cellulo

Radiolabeled metalloprobes offer sensitive tools for evaluating quantitative accumulation of chemical entities within pooled cell populations. Although beneficial in translational nuclear imaging, this method precludes interrogation of effects resulting from variations at a single cell level, within the same segment of cell population. Compared with radiotracer bioassays, fluorescence imaging offers a cost-efficient technique to assess accumulation of metalloprobes at a single cell level, and determine their intracellular localization under live cell conditions. To evaluate, whether or not radiotracer assay and fluorescence imaging provide complementary information on utility of metalloprobes to assess functional expression of P-glycoprotein (Pgp) on plasma membrane of tumor cells, imaging studies of fluorescent cationic Ga(III)-ENBDMPI (bis(3-ethoxy-2-hydroxy-benzylidene)-N,N'-bis(2,2-dimethyl-3-amino-propyl)ethylenediamine) and its neutral counterpart Zn(II)-ENBDMPI are performed. While the uptake profiles of the cationic metalloprobe are inversely proportional to expression of Pgp in tumor cells, the accumulation profiles of the neutral Zn(II)-ENBDMPI in non-MDR and MDR cells are not significantly impacted. The cationic Ga(III)-ENBDMPI maps with Mito-Tracker Red, thereby confirming localization within mitochondria of non-MDR (Pgp-) cells. Depolarization of both plasmalemmal and mitochondrial potentials decreased retention of the cationic Ga(III)-ENBDMPI within the mitochondria. Additionally, LY335979, an antagonist-induced accumulation of the cationic Ga(III) metalloprobe in MDR (Pgp+) cells indicated specificity of the agent. Compared with traits of Ga(III)-ENBDMPI as a Pgp recognized substrate, Zn(II)-ENBDMPI demonstrated uptake in both MDR and non-MDR cells thus indicating the significance of overall molecular charge in mediating Pgp recognition profiles. Combined data indicate that live cell imaging can offer a cost-effective methodology for monitoring functional Pgp expression.

(67/68)Galmydar: A metalloprobe for monitoring breast cancer resistance protein (BCRP)-mediated functional transport activity

INTRODUCTION

For stratification of chemotherapeutic choices, radiopharmaceuticals capable of imaging breast cancer resistance protein (BCRP/ABCG2)-mediated functional transport are desired. To accomplish this objective, Galmydar, a fluorescent and moderately hydrophobic Ga(III) cationic complex and its (67/68)Ga-radiolabeled counterparts were interrogated in HEK293 cells stably transfected with BCRP and their WT counterparts transfected with empty vector. Additionally, the sensitivity and specificity of (68)Ga-Galmydar to evaluate functional expression of BCRP at the blood-brain barrier (BBB) was investigated in gene-knockout mdr1a/1b(-/-) (double knockout, dKO) and mdr1a/1b(-/-)ABCG2(-/-) (triple knockout, tKO) mouse models.

METHODS

For radiotracer uptake assays and live cell fluorescence imaging, either (67)Ga-Galmydar or its unlabeled counterpart was incubated in HEK293 cells transfected with BCRP (HEK293/BCRP) and their WT counterparts at 37°C under a continuous flux of CO2 (5%) in the presence or absence of Ko143, a potent BCRP antagonist, and cellular uptake was measured to assess the sensitivity of Galmydar to probe BCRP-mediated functional transport activity in cellulo. For assessing the potential of Galmydar to enable diagnostic imaging of targeted tissues in vivo, the (67)Ga-radiolabeled counterpart was incubated in either human serum albumin or human serum at 37°C and the percentage of unbound (67)Ga-Galmydar was determined. To evaluate the sensitivity of (68)Ga-Galmydar for molecular imaging of BCRP-mediated efflux activity in vivo, microPET/CT brain imaging was performed in dKO and tKO mice and their age-matched WT counterparts, 60min post-intravenous injection.

RESULTS

(67)Ga-Galmydar shows uptake profiles in HEK293 cells inversely proportional to BCRP expression, and antagonist (Ko143) induced accumulation in HEK293/BCRP cells, thus indicating target sensitivity and specificity. Furthermore, employing the fluorescent characteristics of Galmydar, optical imaging in HEK293/BCRP cells shows an excellent correlation with the radiotracer cellular accumulation data. (67)Ga-Galmydar shows > 85% unbound fraction and presence of parental compound in human serum. Finally, microPET/CT imaging shows higher retention of (68)Ga-Galmydar in brains of dKO and tKO mice compared to their age-matched WT counterparts, 60min post-intravenous tail-vein injection.

CONCLUSIONS

Combined data indicate that Galmydar could provide a template scaffold for development of a PET tracer for imaging BCRP-mediated functional transport activity in vivo.

A generator-produced gallium-68 radiopharmaceutical for PET imaging of myocardial perfusion

Lipophilic cationic technetium-99m-complexes are widely used for myocardial perfusion imaging (MPI). However, inherent uncertainties in the supply chain of molybdenum-99, the parent isotope required for manufacturing ⁹⁹Mo/^99mTc generators, intensifies the need for discovery of novel MPI agents incorporating alternative radionuclides. Recently, germanium/gallium (Ge/Ga) generators capable of producing high quality ⁶⁸Ga, an isotope with excellent emission characteristics for clinical PET imaging, have emerged. Herein, we report a novel ⁶⁸Ga-complex identified through mechanism-based cell screening that holds promise as a generator-produced radiopharmaceutical for PET MPI.

Interrogation of multidrug resistance (MDR1) P-glycoprotein (ABCB1) expression in human pancreatic carcinoma cells: correlation of 99mTc-Sestamibi uptake with western blot analysis

Histopathological studies indicate that ∼63% of pancreatic tumors express multidrug resistance (MDR1) P-glycoprotein (Pgp) and its polymorphic variants. However, Pgp expression detected at the mRNA or protein level does not always correlate with functional transport activity. Because Pgp transport activity is affected by specific mutations and the phosphorylation state of the protein, altered or less active forms of Pgp may also be detected by PCR or immunohistochemistry, which do not accurately reflect the status of tumor cell resistance. To interrogate the status of the functional expression of MDR1 Pgp in MiaPaCa-2 and PANC-1 cells, cellular transport studies using Tc-Sestamibi were performed and correlated with western blot analysis. Biochemical transport assays in human pancreatic carcinoma MiaPaCa-2 and PANC-1 cells, human epidermal carcinoma drug-sensitive KB-3-1 cells, and human breast carcinoma MCF-7 cells (negative controls), and human epidermal carcinoma drug-resistant KB-8-5 cells, human breast carcinoma stably transfected with Pgp MCF-7/MDR1Pgp cells, and liver carcinoma HepG2 cells (positive controls) were performed. Protein levels were determined using a monoclonal antibody C219. Tc-Sestamibi demonstrates accumulation in human pancreatic carcinoma MiaPaCa-2 and PANC-1 cells. Uptake profiles are not affected by treatment with LY335979, a Pgp inhibitor, and correlate with western blot analysis. These cellular transport studies indicate an absence of Pgp at a functional level in MiaPaCa-2 and PANC-1 cells. Because major pancreatic tumors originate from the pancreatic duct and Tc-Sestamibi undergoes a dominant hepatobiliary mode of excretion, it would not be a sensitive probe for imaging pancreatic adenocarcinomas. Following interrogation of the functional status of Pgp in other pancreatic carcinoma cells, chemotherapeutic drugs that are also MDR1 substrates could offer alternative therapeutics for treating pancreatic adenocarcinomas.

Radiosynthesis and biological evaluation of 68Ga-labeled colchicine conjugates

OBJECTIVE

Colchicine, a plant-derived alkaloid, is a known substrate for P-glycoprotein (Pgp), which confers multidrug resistance (MDR) to cancer cells and tumors, through enhanced efflux of chemotherapeutic drugs. Hence, radiolabeled colchicine can be a suitable probe for imaging of activity of Pgp transport in vivo and early diagnosis of MDR.

METHODS

In the present study, colchicine was hydrolyzed to desacetylcolchiceine for conjugation with p-SCN-Bn-DOTA and p-SCN-Bn-NOTA. The resulting conjugates, DOTA-desacetylcolchiceine and NOTA-desacetylcolchiceine, were radiolabeled with 68Ga. The radiotracers 68Ga-DOTA-desacetylcolchiceine (68Ga-1) and 68Ga-NOTA-desacetylcolchiceine (68Ga-2) were evaluated in vitro (MCF-7 and T47D breast cancer cell lines) and in vivo (biodistribution studies, Swiss mice bearing fibrosarcoma tumors).

RESULTS

The radiotracers prepared in >97% radiochemical yield showed good in vitro binding and significant inhibition with 100-fold cold colchicine (p<0.05). In vivo the tumor uptake reached maximum at 120 minutes postinjection (68Ga-1: 2.35%±0.39% injected dose per gram [ID/g]; 68Ga-2: 1.5%±0.31% ID/g). Of the two radiotracers 68Ga-2 cleared faster from blood (p<0.05) with lower uptake in nontargeted organs as compared with 68Ga-1.

CONCLUSIONS

The radiotracer 68Ga-2 has shown improved pharmacokinetic features over 68Ga-1 and the previously reported 99mTc(CO)3-colchicine radiotracer. The preliminary studies with 68Ga-2 indicate its potential for in vivo targeting of tumor. However, the efficacy of the radiotracer for imaging of multidrug-resistant states will be ascertained in future.

(99m)Tc sestamibi as a prognostic factor of response to first-line therapy and outcome in patients with malignant lymphoma

INTRODUCTION

Resistance to chemotherapy poses a major problem in cancer patients. Although of multifactorial origin, some of the implicated mechanisms also interfere with (99m)Tc-MIBI uptake and retention in cancer cells. The aim of the current study was to investigate the prognostic value of baseline (99m)Tc-MIBI imaging in lymphoma.

METHODS

(99m)Tc-MIBI SPECT was performed in 16 patients with Hodgkin lymphoma and 31 with non-Hodgkin lymphoma (NHL) before chemotherapy initiation. Early (20 minutes), late (120 minutes) tumor-to-background (T/B) ratios, and 2-hour (99m)Tc-MIBI washout were calculated. Follow-up data were obtained for a period of 45.5 ± 23.5 months. Study end points were response to first-line chemotherapy, lymphoma-related death (LRD), and time to disease progression.

RESULTS

Of the scintigraphic indices examined, the late T/B ratio correlated best with study end points. A cutoff value of 1.8 determined by receiver operating characteristic analysis discriminated poor from good response and LRD from survival with an accuracy of 87% and 81%, respectively. Kaplan-Meier survivor functions separated by this cutoff differed significantly for both time to disease progression and LRD (P = 0.0001 and P = 0.0015). In the Cox proportional hazards model, the late T/B ratio proved to have an independent and incremental value over clinical prognostic factors (age, lymphoma type, Ann Arbor stage, lactate dehydrogenase levels) and, in NHL patients, over the international prognostic index. Patients with high international prognostic index score could be further stratified into different prognostic categories.

CONCLUSION

The current study indicates that baseline (99m)Tc-MIBI SPECT can provide useful prognostic information in patients with lymphoma, particularly NHL, regarding therapy response and final outcome.

(99m)Tc(N)-DBODC(5), a potential radiolabeled probe for SPECT of multidrug resistance: in vitro study

[(99m)Tc(N)(DBODC)(PNP5)](+) [DBODC is bis(N-ethoxyethyl)dithiocarbamato; PNP5 is bis(dimethoxypropylphosphinoethyl)ethoxyethylamine], abbreviated as (99m)Tc(N)-DBODC(5), is a lipophilic cationic mixed compound investigated as a myocardial imaging agent. The findings that this tracer accumulates in mitochondrial structures through a mechanism mediated by the negative mitochondrial membrane potential and that the rapid efflux of (99m)Tc(N)-DBODC(5) from nontarget tissues seems to be associated with the multidrug resistance (MDR) P-glycoprotein (P-gp) transport function open up the possibility to extend its clinical applications to tumor imaging and noninvasive MDR studies. The rate of uptake at 4 and 37 °C of (99m)Tc(N)-DBODC(5) was evaluated in vitro in selected human cancer cell lines and in the corresponding sublines before and after P-gp and/or MDR-associated protein (MRP) modulator/inhibitor treatment using (99m)Tc-sestamibi as a reference. The results indicated that (1) the uptake of both (99m)Tc(N)-DBODC(5) and (99m)Tc-sestamibi is correlated to metabolic activity of the cells and (2) the cellular accumulation is connected to the level of P-gp/MRP expression; in fact, an enhancement of uptake in resistant cells was observed after treatment with opportune MDR inhibitor/modulator, indicating that the selective blockade of P-gp/MRP prevented efflux of the tracers. This study provides a preliminary indication of the applicability of (99m)Tc(N)-DBODC(5) in tumor imaging and in detecting P-gp/MRP-mediated drug resistance in human cancer. In addition, the possibility to control the hydrophobicity and pharmacological activity of this heterocomplex through the variation of the substituents on the ligands backbone without affecting the P2S2 coordinating sphere makes (99m)Tc(N)-DBODC(5) a suitable scaffold for the preparation of a molecular probe for single photon emission computed tomography of MDR.

PET and SPECT radiotracers to assess function and expression of ABC transporters in vivo

Adenosine triphosphate-binding cassette (ABC) transporters, such as P-glycoprotein (Pgp, ABCB1), breast cancer resistance protein (BCRP, ABCG2) and multidrug resistance-associated proteins (MRPs) are expressed in high concentrations at various physiological barriers (e.g. blood-brain barrier, blood-testis barrier, blood-tumor barrier), where they impede the tissue accumulation of various drugs by active efflux transport. Changes in ABC transporter expression and function are thought to be implicated in various diseases, such as cancer, epilepsy, Alzheimer's and Parkinson's disease. The availability of a non-invasive imaging method which allows for measuring ABC transporter function or expression in vivo would be of great clinical use in that it could facilitate the identification of those patients that would benefit from treatment with ABC transporter modulating drugs. To date three different kinds of imaging probes have been described to measure ABC transporters in vivo: i) radiolabelled transporter substrates ii) radiolabelled transporter inhibitors and iii) radiolabelled prodrugs which are enzymatically converted into transporter substrates in the organ of interest (e.g. brain). The design of new imaging probes to visualize efflux transporters is inter alia complicated by the overlapping substrate recognition pattern of different ABC transporter types. The present article will describe currently available ABC transporter radiotracers for positron emission tomography (PET) and single-photon emission computed tomography (SPECT) and critically discuss strengths and limitations of individual probes and their potential clinical applications.

64Cu-labeled phosphonium cations as PET radiotracers for tumor imaging

Alteration in mitochondrial transmembrane potential (ΔΨ(m)) is an important characteristic of cancer. The observation that the enhanced negative mitochondrial potential is prevalent in tumor cell phenotype provides a conceptual basis for development of mitochondrion-targeting therapeutic drugs and molecular imaging probes. Since plasma and mitochondrial potentials are negative, many delocalized organic cations, such as rhodamine-123 and (3)H-tetraphenylphosphonium, are electrophoretically driven through these membranes, and able to localize in the energized mitochondria of tumor cells. Cationic radiotracers, such as (99m)Tc-Sestamibi and (99m)Tc-Tetrofosmin, have been clinically used for diagnosis of cancer by single photon emission computed tomography (SPECT) and noninvasive monitoring of the multidrug resistance (MDR) transport function in tumors of different origin. However, their diagnostic and prognostic values are often limited due to their insufficient tumor localization (low radiotracer tumor uptake) and high radioactivity accumulation in the chest and abdominal regions (low tumor selectivity). In contrast, the (64)Cu-labeled phosphonium cations represent a new class of PET (positron emission tomography) radiotracers with good tumor uptake and high tumor selectivity. This review article will focus on our recent experiences in evaluation of (64)Cu-labeled phosphonium cations as potential PET radiotracers. The main objective is to illustrate the impact of radiometal chelate on physical, chemical, and biological properties of (64)Cu radiotracers. It will also discuss some important issues related to their tumor selectivity and possible tumor localization mechanism.

Synthesis, molecular structure, and validation of metalloprobes for assessment of MDR1 P-glycoprotein-mediated functional transport

The human genome is known to consist of 49 ATP-binding cassette (ABC) transporter genes. Among these ABC proteins, overexpression of multidrug resistance (MDR1) P-glycoprotein (Pgp/ABCB1) is the best characterized barrier to successful chemotherapeutic treatments, impacts pharmacokinetics of numerous recognized drugs, and is also quickly emerging as an important target in the development of neurodegenerative diseases. Therefore, there exists an urgent need to seek radiopharmaceuticals, incorporated with generator-produced radionuclides to assist their widespread deployment, for noninvasive assessment of Pgp-mediated functional transport activity in vivo.

METHODS

gallium(III) complexes (5a and 5b) possessing octahedral geometry were synthesized, analytically characterized, and evaluated for their potential to serve as probes of Pgp-mediated functional transport activity in cellulo and in vivo. While unlabeled compounds (5a and 5b) were examined via cell cytotoxicity assays, the (67)Ga-labeled counterparts (6a and 6b) were evaluated via cell transport studies and quantitative biodistribution studies in mdr1a/1b((-/-)) gene-deleted mice and their wild-type (WT) counterparts.

RESULTS

cytotoxicity data of 5a and 5b displayed profiles modified by the expression of Pgp in drug-resistant cells. (67)Ga-metalloprobes (6a and 6b) showed high accumulation in human epidermal carcinoma drug-sensitive KB-3-1 cells (Pgp-), human breast carcinoma MCF-7 (Pgp-) cells; an inhibitor (LY335979, 1 microM) induced accumulation in multidrug resistant (MDR, Pgp+) KB-8-5, KB-8-5-11 cells, and stably transfected MCF-7/MDR1 cells, thus demonstrating their ability to interrogate Pgp-mediated functional transport activity in cellulo. In mdr1a/1b((-/-)) gene-deleted mice, the (67)Ga-metalloprobe (6b) showed 8-fold greater brain uptake and retention compared with WT counterparts and no significant difference in blood pharmacokinetics.

CONCLUSION

molecular imaging of the functional transport activity of MDR1 Pgp (ABCB1) with (67/68)Ga-metalloprobes could enable non-invasive monitoring of the blood-brain barrier, tumors, and tissues in vivo.

Modulating P-glycoprotein regulation: future perspectives for pharmacoresistant epilepsies?

Enhanced brain efflux of antiepileptic drugs by the blood-brain barrier transporter P-glycoprotein is discussed as one mechanism contributing to pharmacoresistance of epilepsies. P-glycoprotein overexpression has been proven to occur as a consequence of seizure activity. Therefore, blocking respective signaling events should help to improve brain penetration and efficacy of P-glycoprotein substrates. A series of recent studies revealed key signaling factors involved in seizure-associated transcriptional activation of P-glycoprotein. These data suggested several interesting targets, including the N-methyl-d-aspartate (NMDA) receptor, the inflammatory enzyme cyclooxygenase-2, and the prostaglandin E2 EP1 receptor. These targets have been further evaluated in rodent models, demonstrating that targeting these factors can control P-glycoprotein expression, improve antiepileptic drug brain penetration, and help to overcome pharmacoresistance. In general, the approach offers particular advantages over transporter inhibition as it preserves basal transporter function. In this review the different strategies for blocking P-glycoprotein upregulation, including their therapeutic promise and drawbacks are discussed. Moreover, pros and cons of the approach are compared to those of alternative strategies to overcome transporter-associated resistance. Regarding future perspectives of the novel approach, there is an obvious need to more clearly define the clinical relevance of transporter overexpression. In this context current efforts are discussed, including the development of imaging tools that allow an evaluation of P-glycoprotein function in individual patients.

Minimizing liver uptake of cationic Tc radiotracers with ether and crown ether functional groups

Ischemia-related diseases, particularly coronary artery disease (CAD), account for the majority of deaths worldwide. Myocardial ischemia is a serious condition and the delay in reperfusion of ischemic tissues can be life-threatening. This is particular true in the aged population. Rapid and accurate early detection of myocardial ischemia is highly desirable so that various therapeutic regiments can be given before irreversible myocardial damage occurs. Myocardial perfusion imaging with radiotracers is an integral component in evaluations of patients with known or suspected CAD. (99m)Tc-Sestamibi and (99m)Tc-Tetrofosmin are commercial radiopharmaceuticals currently available for myocardial perfusion imaging. Despite their widespread clinical applications, both (99m)Tc-Sestamibi and (99m)Tc-Tetrofosmin do not meet the requirements of an ideal perfusion imaging agent, largely due to their high liver uptake. The intense liver uptake makes it difficult to interpret the heart activity in the inferior and left ventricular wall. Photon scattering from the high liver radioactivity accumulation remains a significant challenge for diagnosis of heart diseases. This review will summarize the most recent research efforts to minimize the liver uptake of cationic (99m)Tc radiotracers by using ether and crown ether-containing chelators. Fast liver clearance will shorten the duration of imaging protocols (< 30 min post-injection), and allow for early acquisition of heart images with high quality. Improvement of heart/liver ratio may permit better detection of the presence and extent of coronary artery disease. Identification of such a new radiotracer that allows for the improved noninvasive assessment of myocardial perfusion would be of considerable benefit in treatment of patients with suspected CAD.

Imaging of P-glycoprotein function and expression to elucidate mechanisms of pharmacoresistance in epilepsy

The issue of pharmacoresistance in epilepsy has received considerable attention in recent years, and a number of plausible hypotheses have been proposed. Of these, the so-called transporter hypothesis is the most extensively researched and documented. This hypothesis assumes that refractory epilepsy is associated with a localised over-expression of drug transporter proteins such as P-glycoprotein (Pgp) in the region of the epileptic focus, which actively extrudes antiepileptic drugs (AEDs) from their intended site of action. However, although this hypothesis has biological plausibility, there is no clinical evidence to support the assertion that AEDs are sufficiently strong substrates for transporter-mediated extrusion from the brain. The use of modern brain imaging techniques to determine Pgp function in patients with refractory epilepsy has started only recently, and may ultimately determine whether increased expression and function of Pgp or other efflux transporters are involved in AED resistance.

P-glycoprotein versus MRP1 on transport kinetics of cationic lipophilic substrates: a comparative study using [99mTc]sestamibi and [99mTc]tetrofosmin

The multidrug resistance (MDR) phenotype in cancer is closely related with the overexpression of P-glycoprotein (Pgp) and multidrug resistance protein-1 (MRP1). Although conferring resistance to a similar spectrum of drugs, these proteins present distinct transport mechanisms and have their own substrates. In this work, we compared the functional properties of Pgp and MRP1 in the transport kinetics of two cationic lipophilic tracers, [(99m)Tc]sestamibi and [(99m)Tc]tetrofosmin, in cellular models of resistance. Cellular transport kinetics of both tracers was evaluated in Small-cell lung cancer cell line H69 and in its drug-resistant sublines, H69LX4 and H69AR, overexpressing Pgp and MRP1, respectively. Studies were performed in the absence and in the presence of MDR modulators. Kinetic parameters extracted from time-activity curves were analyzed through receiver-operating characteristics curve analysis. The uptake and the efflux rate of both radiotracers were significantly higher (p < 0.05) in sensitive cells. However, MRP1 was more effective than Pgp in removing tracers from the intracellular medium. The addition of verapamil and PSC833 significantly reduced the efflux rate and restored the accumulation of both tracers in H69LX4 cells. Only verapamil was effective in the inhibition of MRP1; however, the effects were more pronounced with [(99m)Tc]sestamibi, when compared to [(99m)Tc]tetrofosmin. Outward transport of radiotracers by MRP1 was dependent on the intracellular glutathione levels. We concluded that both tracers can detect Pgp- and MRP1-mediated drug resistance, based on transport kinetics; however, MRP1 is more effective than Pgp on outward transport of radiotracers. We postulate that this finding can be useful to distinguish between the two resistance mechanisms.

Mechanism for myocardial localization and rapid liver clearance of Tc-99m-N-MPO: a new perfusion radiotracer for heart imaging

BACKGROUND

[Tc-99m-N(mpo)(PNP5)](+) (Tc-99m-N-MPO: Hmpo = 2-mercaptopyridine N-oxide and PNP5 = N-ethoxyethyl-N,N-bis[2-(bis(3-methoxypropyl)phosphino)ethyl]amine) is a new Tc-99m radiotracer useful for myocardial perfusion imaging. The main objective of this study is to elucidate the mechanism for myocardial localization and fast liver clearance of Tc-99m-N-MPO in comparison with Tc-99m-sestamibi ([Tc-99m-(MIBI)(6)](+): MIBI = 2-methoxy-2-methylpropylisonitrile).

METHODS AND RESULTS

Subcellular distribution of Tc-99m-N-MPO and Tc-99m-sestamibi was examined in the excised Sprague-Dawley (SD) rat myocardium. Biodistribution and planar imaging studies were performed using SD rats in the absence/presence of Cyclosporin-A. Due to negative plasma and mitochondrial potentials, 84.5% +/- 3.2% of Tc-99m-N-MPO was found in the mitochondrial fraction as compared to 88.0% +/- 1.5% of Tc-99m-sestamibi. There was no significant difference in their mitochondrial accumulation. Tc-99m-N-MPO was also able to retain its chemical integrity in rat myocardium. Pre-treatment of SD rats with Cys-A result in significant increase in the kidney and liver uptake of Tc-99m-N-MPO.

CONCLUSION

Tc-99m-N-MPO and Tc-99m-sestamibi share almost identical subcellular distribution and localization mechanism. The MDR transport function of hepatocytes and renal cells is responsible for the fast clearance kinetics of Tc-99m-N-MPO from liver and kidneys, respectively. Tc-99m-N-MPO is a very promising myocardial perfusion radiotracer with favorable biodistribution properties and rapid liver clearance.

Evaluation of (64)Cu(DO3A-xy-TPEP) as a potential PET radiotracer for monitoring tumor multidrug resistance

In this study, we evaluated the potential of (64)Cu(DO3A-xy-TPEP) (DO3A-xy-TPEP = (2-(diphenylphosphoryl)ethyl)diphenyl(4-((4,7,10-tris(carboxymethyl)-1,4,7,10-tetraazacyclododecan-1-yl)methyl)benzyl)phosphonium) as a PET (positron emission tomography) radiotracer for noninvasive monitoring of multidrug resistance (MDR) transport function in several xenografted tumor models (MDR-negative: U87MG; MDR-positive: MDA-MB-435, MDA-MB-231, KB-3-1, and KB-v-1). It was found that (64)Cu(DO3A-xy-TPEP) has a high initial tumor uptake (5.27 +/- 1.2%ID/g at 5 min p.i.) and shows a steady uptake increase between 30 and 120 min p.i. (2.09 +/- 0.53 and 3.35 +/- 1.27%ID/g at 30 and 120 min p.i., respectively) in the MDR-negative U87MG glioma tumors. (64)Cu(DO3A-xy-TPEP) has a greater uptake difference between U87MG glioma and MDR-positive tumors (MDA-MB-231: 1.57 +/- 0.04, 1.00 +/- 0.17, and 0.93 +/- 0.15; MDA-MB-435: 1.15 +/- 0.19, 1.12 +/- 0.20, and 0.81 +/- 0.11; KB-3-1: 1.45 +/- 0.31, 1.43 +/- 0.16, and 1.08 +/- 0.19; and KB-v-1: 1.63 +/- 0.47, 1.81 +/- 0.31, and 1.14 +/- 0.22%ID/g at 30, 60, and 120 min p.i., respectively) than (99m)Tc-Sestamibi. Regardless of the source of MDR, the overall net effect is the rapid efflux of (64)Cu(DO3A-xy-TPEP) from tumor cells, which leads to a significant reduction of its tumor uptake. It was concluded that (64)Cu(DO3A-xy-TPEP) is more efficient than (99m)Tc-Sestamibi as the substrate for MDR P-glycoproteins (MDR Pgps) and multidrug resistance-associated proteins (MRPs), and might be a more efficient radiotracer for noninvasive monitoring of the tumor MDR transport function. (64)Cu(DO3A-xy-TPEP) and (99m)Tc-Sestamibi share almost identical subcellular distribution patterns in U87MG glioma tumors. Thus, it is reasonable to believe that (64)Cu(DO3A-xy-TPEP), like (99m)Tc-Sestamibi, is able to localize in mitochondria due to the increased plasma and mitochondrial transmembrane potentials in tumor cells.

64Cu-labeled 2-(diphenylphosphoryl)ethyldiphenylphosphonium cations as highly selective tumor imaging agents: effects of linkers and chelates on radiotracer biodistribution characteristics

Radiolabeled organic cations, such as triphenylphosphonium (TPP), represents a new class of radiotracers for imaging cancers and the transport function of multidrug resistance P-glycoproteins (particularly MDR1 Pgp) by single photon emission computed tomography (SPECT) or positron emission tomography (PET). This report presents the synthesis and biological evaluation of (64)Cu-labeled 2-(diphenylphosphoryl)ethyldiphenylphosphonium (TPEP) cations as novel PET radiotracers for tumor imaging. Biodistribution studies were performed using the athymic nude mice bearing subcutaneous U87MG human glioma xenografts to explore the impact of linkers, bifunctional chelators (BFCs), and chelates on biodistribution characteristics of the (64)Cu-labeled TPEP cations. Metabolism studies were carried out using normal athymic nude mice to determine the metabolic stability of four (64)Cu radiotracers. It was found that most (64)Cu radiotracers described in this study have significant advantages over (99m)Tc-Sestamibi for their high tumor/heart and tumor/muscle ratios. Both BFCs and linkers have significant impact on biological properties of (64)Cu-labeled TPEP cations. For example, (64)Cu(DO3A-xy-TPEP) has much lower liver uptake and better tumor/liver ratios than (64)Cu(DO3A-xy-TPP), suggesting that TPEP is a better mitochondrion-targeting molecule than TPP. Replacing DO3A with DO2A results in (64)Cu(DO2A-xy-TPEP) (+), which has a lower tumor uptake than (64)Cu(DO3A-xy-TPEP). Substitution of DO3A with NOTA-Bn leads to a significant decrease in tumor uptake for (64)Cu(NOTA-Bn-xy-TPEP). The use of DOTA-Bn to replace DO3A has little impact on the tumor uptake, but the tumor/liver ratio of (64)Cu(DOTA-Bn-xy-TPEP) (-) is not as good as that of (64)Cu(DO3A-xy-TPEP), probably due to the aromatic benzene ring in DOTA-Bn. Addition of an extra acetamido group in (64)Cu(DOTA-xy-TPEP) results in a lower liver uptake, but tumor/liver ratios of (64)Cu(DOTA-xy-TPEP) and (64)Cu(DO3A-xy-TPEP) are comparable due to a faster tumor washout of (64)Cu(DOTA-xy-TPEP). Substitution of xylene with the PEG 2 linker also leads to a significant reduction in both tumor and liver uptake. MicroPET imaging studies on (64)Cu(DO3A-xy-TPEP) in athymic nude mice bearing U87MG glioma xenografts showed that the tumor was clearly visualized as early as 1 h postinjection with very high T/B contrast. There was very little metabolite (<2%) detectable in the urine and feces samples for (64)Cu(DO3A-xy-TPEP), (64)Cu(DOTA-Bn-xy-TPEP)(-), and (64)Cu(NOTA-Bn-xy-TPEP). Considering both tumor uptake and T/B ratios (particularly tumor/heart, tumor/liver, and tumor/muscle), it was concluded that (64)Cu(DO3A-xy-TPEP) is a promising PET radiotracer for imaging the MDR-negative tumors.

Subcellular distribution and metabolism studies of the potential myocardial imaging agent [99mTc(N)(DBODC)(PNP5)]+

99mTc(N)-DBODC5 is the lead compound of a new series of monocationic 99mTc(N)-based potential myocardial imaging agents that exhibit original biodistribution properties. This study was addressed to elucidate the mechanisms of distribution, retention, and elimination of this promising 99mTc(N)-agent.

METHODS

The sex-related in vitro and in vivo stability and the subcellular distribution of 99mTc(N)-DBODC5 were investigated. Studies were performed by considering binding to the serum proteins; stability in rat serum, human serum, and rat liver homogenates; and the chemical integrity of the complex after extraction from rat tissues such as heart, liver, and kidney, as well as from intestinal fluids and urine. The effect of cyclosporin A on the in vivo pharmacokinetic properties of 99mTc(N)-DBODC5 was also evaluated. Subcellular distribution of 99mTc(N)-DBODC5 in ex vivo rat heart was determined by standard differential centrifugation techniques.

RESULTS

No significant in vitro serum protein binding and no notable biotransformation of the native compound into different species by the in vitro action of the serum and liver enzymes was evidenced. In vivo experiments showed that sex affects the pharmacokinetic profile of the 99mTc(N)-complexes including metabolism and excretion. Chromatographic profiles of 99mTc(N)-radioactivity extracted from tissues and fluids of female rats were always coincident with the control. Conversely, a small percentage of metabolized species was detected by high-performance liquid chromatography in liver extracts of male rats. Furthermore, administration of cyclosporin A caused a significant reduction of lung, liver, and kidney washout along with a considerable variation in activity distribution in the intestinal tract in both male and female rats, thus indicating a possible implication of Pgp transporters in determining the biologic behavior of 99mTc(N)-DBODC5. However, this phenomenon was more pronounced in females. Subcellular distribution studies showed that 86.3% +/- 7.4% of 99mTc(N)-DBODC5 was localized into mitochondrial fraction as a result of the interaction with the negative membrane potential.

CONCLUSION

Evidence showing that the new 99mTc(N)-myocardial tracers behave as multidrug resistance-associated protein P-glycoprotein substrates, combined with their selective mitochondrial accumulation, strongly supports the possibility that diagnostic application of 99mTc(N)-DBODC5 can be extended to tumor imaging and noninvasive multidrug resistance studies.

Tc-99m-N-MPO: novel cationic Tc-99m radiotracer for myocardial perfusion imaging

BACKGROUND

Technetium 99m-N-MPO ([Tc-99m-N(mpo)(PNP5)](+)) is a cationic Tc-99m nitrido complex. The objective of this study is to evaluate its potential as a new radiotracer for myocardial perfusion imaging.

METHODS AND RESULTS

Biodistribution studies were performed in Sprague-Dawley rats and guinea pigs to compare the myocardial uptake and excretion kinetics of Tc-99m-N-MPO from noncardiac organs, such as the liver and lungs, with those of the known cationic Tc-99m radiotracers: Tc-99m-N-DBODC5 and Tc-99m-sestamibi. Planar imaging was performed in Sprague-Dawley rats to evaluate the utility of Tc-99m-N-MPO as a myocardial perfusion imaging agent. Metabolism studies were carried out by use of both Sprague-Dawley rats and guinea pigs. In general, the heart uptake of Tc-99m-N-MPO was between that of Tc-99m-sestamibi and Tc-99m-N-DBODC5 over the 2-hour study period. However, the heart-liver ratio of Tc-99m-N-MPO (12.75 +/- 3.34) at 30 minutes after injection was more than twice that of Tc-99m-N-DBODC5 (6.01 +/- 1.45) and approximately 4 times higher than that of Tc-99m-sestamibi (2.90 +/- 0.22). The heart uptake and heart-liver ratio of Tc-99m-N-MPO and Tc-99m-sestamibi in guinea pigs were significantly lower than those obtained in Sprague-Dawley rats. The metabolism studies demonstrated no detectable Tc-99m-N-MPO metabolites in the urine and feces samples of the Sprague-Dawley rats at 120 minutes after injection. In guinea pigs no Tc-99m-N-MPO metabolites were detected in the urine at 120 minutes, but only approximately 60% of Tc-99m-N-MPO remained intact in the feces samples. In contrast, there was no intact Tc-99m-sestamibi detected in urine samples, and less than 15% of Tc-99m-sestamibi remained intact in the feces samples. Planar imaging studies indicated that clinically useful images of the heart may be obtained as early as 15 minutes after injection of Tc-99m-N-MPO.

CONCLUSION

The combination of favorable organ biodistribution and myocardial uptake with rapid liver clearance makes Tc-99m-N-MPO a very promising myocardial perfusion radiotracer worthy of further evaluation in various preclinical animal models.

Metalloprobes: synthesis, characterization, and potency of a novel gallium(III) complex in human epidermal carcinoma cells

Multidrug resistance (MDR) mediated by overexpression of the MDR1 gene product, P-glycoprotein (Pgp), represents one of the best characterized barriers to chemotherapeutic treatment in cancer and may be a pivotal factor in progression of Alzheimer's disease (AD). Thus, agents capable of probing Pgp-mediated transport could be beneficial in biomedical imaging. Herein, we synthesized and structurally characterized a gallium(III) complex (5) of the naphthol-Schiff base ligand. The crystal structure revealed octahedral geometry for the metallodrug. Cytotoxicity profiles of 5 were evaluated in KB-3-1 (Pgp-) and KB-8-5 (Pgp+) human epidermal carcinoma cell lines. Compared with an LC(50) (the half-maximal cytotoxic concentration) value of 1.93 microM in drug-sensitive (Pgp-) cells, the gallium(III) complex 5 demonstrated an LC(50) value>100 microM in drug-resistant (Pgp+) cells, thus indicating that 5 was recognized by the Pgp as its substrate, thereby extruded from the cells and sequestered away from their cytotoxic targets. Radiolabeled analogues of 5 could be beneficial in noninvasive imaging of Pgp-mediated transport in vivo.

Evaluation of novel cationic 99mTc(I)-tricarbonyl complexes as potential radiotracers for myocardial perfusion imaging

This report describes the evaluation of three cationic (99m)Tc(I)-tricarbonyl complexes--[(99m)Tc(CO)(3)(L)](+) (L=N-methoxyethyl-N,N-bis[2-(bis(3-ethoxypropyl)phosphino)ethyl]amine (ME-PNP), N-[15-crown-5)-2-yl]-N,N-bis[2-(bis(3-ethoxypropyl)phosphino)ethyl]amine (15C5-PNP) and N-[18-crown-6)-2-yl]-N,N-bis[2-(bis(3-ethoxypropyl)phosphino)ethyl]amine (18C6-PNP))--as potential radiotracers for myocardial perfusion imaging. Biodistribution, imaging and metabolism studies were performed using Sprague-Dawley rats. It was found that bisphosphine ligands have a significant impact on the biodistribution characteristics and clearance kinetics of their cationic (99m)Tc(I)-tricarbonyl complexes. Among the three radiotracers evaluated in this study, [(99m)Tc(CO)(3)(15C5-PNP)](+) has a very high initial heart uptake and is retained in the rat myocardium for >2 h. It also shows rapid clearance from the liver and lungs. The heart/liver ratio of [(99m)Tc(CO)(3)(15C5-PNP)](+) is approximately 2.5 times better than that of (99m)Tc-sestamibi at 30 min postinjection. [(99m)Tc(CO)(3)(15C5-PNP)](+) is almost identical to (99m)TcN-DBODC5 with respect to heart uptake, heart/lung ratio and heart/liver ratio. Results from metabolism studies show that there is no significant metabolism for [(99m)Tc(CO)(3)(15C5-PNP)](+) in the urine, but it does show a small metabolite peak (<10%) in the radio high-performance liquid chromatography chromatogram of the feces sample at 120 min postinjection. Results planar imaging studies demonstrate that [(99m)Tc(CO)(3)(15C5-PNP)](+) has a much better liver clearance profile than (99m)Tc-sestamibi and might give clinically useful images of the heart as early as 30 min postinjection. [(99m)Tc(CO)(3)(15C5-PNP)](+) is a very promising candidate for more preclinical evaluations in various animal models.

Single photon emission computed tomography and positron emission tomography imaging of multi-drug resistant P-glycoprotein--monitoring a transport activity important in cancer, blood-brain barrier function and Alzheimer's disease

Overexpression of multi-drug resistant P-glycoprotein (Pgp) remains an important barrier to successful chemotherapy in cancer patients and impacts the pharmacokinetics of many important drugs. Pgp is also expressed on the luminal surface of brain capillary endothelial cells wherein Pgp functionally comprises a major component of the blood-brain barrier by limiting central nervous system penetration of various therapeutic agents. In addition, Pgp in brain capillary endothelial cells removes amyloid-beta from the brain. Several single photon emission computed tomography and positron emission tomography radiopharmaceutical have been shown to be transported by Pgp, thereby enabling the noninvasive interrogation of Pgp-mediated transport activity in vivo. Therefore, molecular imaging of Pgp activity may enable noninvasive dynamic monitoring of multi-drug resistance in cancer, guide therapeutic choices in cancer chemotherapy, and identify transporter deficiencies of the blood-brain barrier in Alzheimer's disease.

Ether and crown ether-containing cationic 99mTc complexes useful as radiopharmaceuticals for heart imaging

While radiopharmaceutical research has been focused on the development of target-specific radiotracers for early detection and radiotherapy of cancers in the last decade, there is a limited effort on new cationic 99mTc radiotracers for heart imaging. This review will summarize some of the most recent developments in ether- and crown ether-containing cationic 99mTc radiotracers that have a fast liver clearance with a heart/liver ratio substantially better than that of 99mTc-Sestamibi and 99mTc-Tetrofosmin, the two commercial 99mTc radiopharmaceuticals currently available for myocardial perfusion imaging. Fast liver clearance might shorten the duration of imaging protocols (<30 min post-injection), and allow for early acquisition of heart images of high quality. Improvement of heart/liver ratio may permit better detection of the presence and extent of coronary artery disease. Identification of such a new radiotracer that allows for the improved non-invasive delineation of myocardial perfusion would be of considerable benefit in treatment of patients with suspected coronary artery disease.

Evaluation of novel cationic (99m)Tc-nitrido complexes as radiopharmaceuticals for heart imaging: improving liver clearance with crown ether groups

This report describes the evaluation of a series of novel cationic (99m)Tc-nitrido complexes, [(99m)TcN(DTC)(PNP)]+ (DTC = crown ether-containing dithiocarbamates; PNP = bisphosphine), as potential radiotracers for myocardial perfusion imaging. Synthesis of cationic (99m)Tc-nitrido complexes was accomplished in two steps according to literature methods. Biodistribution studies were performed in rats. Planar images of Sprague-Dawley rats administered with 15+/-2 MBq of cationic (99m)Tc radiotracer were obtained using a PhoGama large field-of-view Anger camera. Samples from both urine and feces were analyzed by a reversed-phase radio-HPLC method. Results from biodistribution studies showed that most of the cationic (99m)Tc-nitrido complexes have a high initial heart uptake with a long myocardial retention. They also show a rapid clearance from the liver and lungs. Cationic complexes [(99m)TcN(L2)(L6)]+ and [(99m)TcN(L4)(L6)]+ show heart/liver ratios four to five times better than that of (99m)Tc-sestamibi due to their much faster liver clearance. Their heart uptake and heart/liver ratio are comparable to that of (99m)TcN-DBODC5 within the experimental error. These findings have been confirmed by the results from imaging studies. Radio-HPLC analysis of urine and feces samples indicated that there was very little metabolism of cationic (99m)Tc-nitrido complexes in rats under anesthesia. The key finding of this study is that lipophilicity remains the most important factor affecting both heart uptake and target-to-background (T/B) ratios. Crown ethers are very useful functional groups to improve the liver clearance of cationic (99m)Tc-nitrido complexes. It is the combination of the appropriate DTCs and bisphosphines that results in cationic (99m)Tc-nitrido complexes with high heart uptake and fast clearance from the liver at the same time. The fast liver clearance of [(99m)TcN(L2)(L6)]+ and [(99m)TcN(L4)(L6)]+ suggests that they might be used to obtain clinically useful images as early as 30 min postinjection. [(99m)TcN(L2)(L6)]+ and [(99m)TcN(L4)(L6)]+ are very promising candidates for further evaluation in more extensive preclinical animal models.

Automated synthesis of 18F analogue of paclitaxel (PAC): [18F]Paclitaxel (FPAC)

A positron-emitting paclitaxel (PAC) derivative could allow in vivo measurement of multidrug resistance in tumors and, therefore, predict a potential chemotherapeutic benefit to patients. [18F]Paclitaxel was produced using a 2-reaction vessel automated synthesizer followed by HPLC purification. Optimized reaction conditions resulted in radiochemical yields of 21.2+/-9.6% at end of bombardment, radiochemical purity >99%, and specific activity of 159+/-43 G Bq/micromol. [18F]Paclitaxel activities of 1.33+/-0.729 G Bq (n=7) were obtained in sterile, pyrogen-free solution for IV administration.

The application of Fe 3 O 4 nanoparticles in cancer research: A new strategy to inhibit drug resistance

Although much effort has been extended to the efficient cancer therapies, the drug resistance is still a major obstacle in cancer chemotherapeutic treatments. Almost 90% of the cancer therapy failure is caused by the relative problems. Recently, the application of drug coated polymer nanospheres and nanoparticles to inhibit the related drug resistance has attracted much attention. In this report, we have explored a novel strategy to inhibit the multidrug resistance of the targeted tumor cells by combining the unique properties of tetraheptylammonium capped Fe(3)O(4) magnetic nanoparticles with the drug accumulation of anticancer drug daunorubicin. Our results of confocal fluorescence and atomic force microscopy (AFM) as well as electrochemical studies demonstrate the remarkable synergistic effect of Fe(3)O(4) nanoparticles on drug uptake of daunorubicin in leukemia K562 cells. These observations indicate that the interaction between the magnetic nanoparticles Fe(3)O(4) and biologically active molecules on the membrane of leukemia cell lines may contribute to their beneficial effect on cellular uptake so that the synergistic enhanced effect of magnetic nanoparticles Fe(3)O(4) on drug uptake of drug resistance leukemia K562 cells could be observed upon application of the Fe(3)O(4) nanoparticles.

Assessing responses to cancer therapy using molecular imaging

Tumor responses to therapy in the clinic are still evaluated primarily from non-invasive imaging measurements of reductions in tumor size. This approach, however, lacks sensitivity and can only give a delayed indication of a positive response to treatment. Major advances in our understanding of the molecular mechanisms responsible for cancer, combined with new targeted clinical imaging technologies designed to detect the molecular correlates of disease progression and response to treatment, are set to revolutionize our approach to the detection and treatment of the disease. We describe here the imaging technologies available to image tumor cell proliferation and migration, metabolism, receptor and gene expression, apoptosis and tumor angiogenesis and vascular function, and show how measurements of these parameters can be used to give early indications of positive responses to treatment or to detect drug resistance and/or disease recurrence. Special emphasis has been placed on those applications that are already used in the clinic and those that are likely to translate into clinical application in the near future or whose use in preclinical studies is likely to facilitate translation of new treatments into the clinic.

Scintigraphic imaging of P-glycoprotein expression with a radiolabelled antibody

PURPOSE

P-glycoprotein (P-gp) is a membrane efflux pump protein that is involved in multidrug resistance (MDR). Tumour cells with high P-gp expression show poor response to cancer treatment with several chemotherapeutics. In vivo targeting and visualisation of P-gp expression would allow MDR to be evaluated non-invasively prior to treatment. The aim of this study was to investigate the feasibility of visualising P-gp expression in tumours using a monoclonal anti-P-gp antibody, 15D3.

METHODS

Nude BALB/c mice with subcutaneously growing human uterine sarcoma cell tumours with either high (MES-SA/Dx5 1977) or low (MES-SA 1976) P-gp expression were used. When tumours were 0.2-0.4 g, mice received (131)I-15D3 or (111)In-DTPA-15D3 monoclonal anti-P-gp antibody intravenously. Images were acquired up to 3 days p.i. and radioactivity concentration in various tissues was determined after euthanisation of the animals.

RESULTS

The images demonstrated that radioactivity accumulated to a higher concentration in high P-gp expressing tumours than in the low P-gp expressing MES-SA 1976 tumour. Furthermore, visualisation of the P-gp expressing tumours was superior with (111)In-DTPA-15D3 than with (131)I-15D3. After injection of (111)In-DTPA-15D3, the high P-gp expressing MES-SA/Dx5 1977 tumours were clearly visualised at 3 days p.i. The biodistribution data indicated that radioactivity concentration in the high P-gp expressing tumours was higher than in the tumours with low P-gp expression (20.78+/-1.42 %ID/g for MES-SA/Dx5 1977 tumours and 8.39+/-3.78 %ID/g for MES-SA 1976 tumours for (111)In-DTPA-15D3).

CONCLUSION

The (111)In-labelled monoclonal anti-P-gp antibody clearly visualised P-gp expression in a human uterine sarcoma tumour in nude mice.

Accelerator mass spectrometry allows for cellular quantification of doxorubicin at femtomolar concentrations

Accelerator mass spectrometry (AMS) is a highly sensitive analytical methodology used to quantify the content of radioisotopes, such as (14)C, in a sample. The primary goals of this work were to demonstrate the utility of AMS in determining total cellular [(14)C]anthracycline concentrations following administration of doxorubicin (DOX) and to develop a sensitive assay that is superior to high performance liquid chromatography (HPLC) for the quantification of [(14)C]anthracycline at the tumor level. In order to validate the sensitivity of AMS versus HPLC with fluorescence detection, we performed three studies comparing the cellular accumulation of DOX: one in vitro cell line study, and two in vivo xenograft mouse studies. Using AMS, we quantified cellular [(14)C]anthracycline content up to 4 h following in vitro exposure at concentrations ranging from 0.2 pg/ml (345 fM) to 2 microg/ml (3.45 microM) [(14)C]DOX. The results of this study show that, compared to standard fluorescence-based HPLC, the AMS method was over five orders of magnitude more sensitive. Two in vivo studies compared the sensitivity of AMS to HPLC using a nude mouse xenograft model in which breast cancer cells were implanted subcutaneously. After sufficiently large tumors formed, [(14)C]DOX was administered intravenously at two dose levels. Additionally, we tested the AMS method in a nude mouse xenograft model of multidrug resistance (MDR) in which each mouse was implanted with both wild type and MDR+ cells on opposite flanks. The results of the second and third studies showed that [(14)C]anthracycline concentrations were significantly higher in the wild type tumors compared to the MDR+ tumors, consistent with the MDR model. Although this method does not discriminate between parent drug and metabolites, the extreme sensitivity of AMS should facilitate similar studies in humans to establish target site drug delivery and to potentially determine the optimal treatment dose and regimen.