The focus of our section's research program is to develop therapeutic strategies aimed at overcoming drug resistance in cancer. Our research has been dedicated to the translation of drug resistance reversal strategies to the clinic. The design of our clinical trials has been enhanced by laboratory support that has allowed us to analyze clinical samples and interpret the clinical trial findings. A significant clinical trial effort has related to the inhibition of P-glycoprotein, an ABC transporter mediating resistance through outward transport of anticancer agents. These studies have been carried out collaboratively with Dr. Tito Fojo and evaluate the hypothesis that Pgp modulation may increase anticancer drug efficacy. In trials carried out across the globe, beginning with the failed first-generation trials that employed agents without sufficient potency, and continuing with the failed second-generation trials centered on valspodar with its accompanying need for anticancer agent dose reduction, there has been much disappointment in this field. Even a multinational randomized trial combining the new agent tariquidar with paclitaxel or vinorelbine closed early for toxicity. It must be stated that there is no convincing proof to date that this strategy will eventually be shown to provide clinical benefit and the resistance reversal paradigm remains a hypothesis. However, the failed earlier strategies do not negate strong evidence supporting continued development of Pgp antagonists.Current studies are evaluating the third generation inhibitor tariquidar (XR9576). In our completed Phase I interaction study with vinorelbine and tariquidar, total inhibition of Pgp-mediated drug efflux was observed in CD56+ cells, with persistence of inhibition for 48 hours after a single intravenous dose of tariquidar. 99mTc-sestamibi imaging was employed as a surrogate for altered drug accumulation in normal and tumor tissues. More than half of the patients had detectable increases in tumor uptake of 99mTc-sestamibi. Our goal in launching a new tariquidar trial was to gather more data regarding the safety of tariquidar in combination with a chemotherapeutic agent and to identify a combination that could be used as a single agent. Docetaxel was chosen as an excellent Pgp substrate with known efficacy that could be benefited by increasing drug accumulation in lung, cervical, or ovarian cancer. In planning an interaction trial of docetaxel with tariquidar, we selected an effective but conservative dose of docetaxel - 75 mg/m2 on a q-3-week schedule. The trial is designed with both pharmacokinetic and pharmacodynamic assays. To examine whether tariquidar interferes with docetaxel clearance, careful pharmacokinetics will be performed on a dose of docetaxel administered with and without tariquidar. To limit the length of time that a patient is treated without the modulator, the pharmacokinetic portion of the study is carried out on two 40 mg/m2 docetaxel doses, one week apart. The order of administration of tariquidar is randomized between the day 1 and day 8 doses. Patients begin therapy with 75 mg/m2 q-3-weeks in combination with tariquidar in the second cycle. In addition to pharmacokinetic analysis, 99mTc-sestamibi studies are performed in each enrolled patient with and without tariquidar, and our laboratory carries out CD56+ rhodamine assays in peripheral mononuclear cells. The trial is open and accruing patients without major toxicity, with 16 patients enrolled to date.Although the 99mTc-sestamibi studies provide good proof-of-concept showing increased radionuclide accumulation following tariquidar, the studies are poorly quantitative because they are planar images and background often overwhelms differences. Led by Peter Herscovitch, the Clinical Center PET department developed a method to label sestamibi with 94mTc for positron emission imaging, promising a more quantitative imaging agent. A clinical trial testing this agent has received IRB approval, with FDA approval pending. We anticipate enrolling patients from Tito Fojo's adrenocortical and renal cell cancer trials, as well as the tariquidar trial described above. It is our hope that the quantitative PET imaging will allow us to better answer the question of how much impact tariquidar can have on patient tumors. It can be argued that tariquidar is the most promising Pgp inhibitor to enter clinical development. Volunteer studies showed its lack of intrinsic toxicity, CD56+ assays suggest durability of Pgp inhibition, and minimal or no inhibition of cytochrome P450 has been detected. Pharmacokinetic interaction studies suggested minimal impact on the clearance of the three anticancer agents tested in combination with tariquidar to date: vinorelbine, doxorubicin, or paclitaxel. However, one concern is the possibility that inhibition of Pgp per se may increase toxicity to Pgp-expressing bone marrow stem cells. This could result in toxicity due to a pharmacodynamic effect without a pharmacokinetic interaction. Thus, the future development of Pgp inhibitors depends upon demonstration of safety as well efficacy. In addition to the tariquidar studies, our laboratory has maintained an interest in drug resistance in other model systems. Several years ago, in collaboration with the NCI's Developmental Therapeutics Program, we identified a number of compounds with selectivity against renal cell caner, based on COMPARE analysis using cytotoxicity data in the 60 cell line panel. These compounds were evaluated in our laboratory and the renal selectivity confirmed. One class of these compounds, the dimethane sulfonates, has been continuously in preclinical development at DTP and are nearing the point at which clinical trials can be considered.
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