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, which have been carried out collaboratively with Dr. Tito Fojo, 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 therapeutic strategy. 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. The project can be viewed as high risk with potentially high gain for multiple tumor types and thus very appropriate for the NCI intramural program. 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 following closure of a multinational trial for toxicity. 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 was 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. 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 has just completed maximum accrual, and patients were treated without major toxicity. We have been encouraged by the disease responses in patients with nonsmall cell lung cancer. We completed accrual in cervical and ovarian cancer without evidence of major activity or toxicity. 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 Dr. 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 is open and accruing patients. 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. In addition to the PET-sestamibi trial, we have discussed collaborations with Dr. Robert Innis and Dr. Karen Kurdziel aimed at evaluating drug accumulation using PET agents 11C-N-desmethyl-loperamide and 18F-paclitaxel. These PET studies offer the opportunity to move the field forward in a significant way. In addition to the tariquidar studies, our group was approached by CBA Pharma, a small company developing a Pgp inhibitor CBT-1. This is an oral agent that has been in clinical trials. However, inhibition of Pgp by this agent was not previously confirmed in patients. We have completed a clinical trial examining surrogate markers of Pgp inhibition including altered sestamibi uptake and increased rhodamine uptake in circulating CD56+ mononuclear cells in patients treated with paclitaxel and CBT-1. This small trial was completed in approximately 1 year. Our laboratory also maintains an interest in studying 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 new compound class, the dimethane sulfonates, has been continuously in preclinical development at DTP and one, NSC-281612, was approved for Phase I testing. The Phase I trial is now open at the NIH clinical center. The study is a multi-institutional Phase 1 trial with one site at University of Pittsburgh and the other at Hershey Medical Center. One of the goals in the Phase I trial will be the development of biomarkers to evaluate the presence of DNA damage in tumor cells or surrogate tissues following treatment with the DMS compound. Given the difficulties in developing drug resistance modulators for resistance reversal, an alternate strategy is to select drugs for know targets that are not substrates for multidrug transporters. The recent FDA approval of ixabepilone is one example of such a strategy. This tubulin-targeted compound was developed in part because of its lack of interaction with P-glycoprotein in the hopes that it would be effective where taxanes were limited by drug efflux mechanisms. A similar strategy characterizes the development of diflomotecan, a second generation topoisomerase I inhibitor that, in contrast to the FDA approved agents topotecan and irinotecan, is not a substrate for efflux by the multidrug transporter ABCG2. Our goal is to develop this compound for use in drug resistant tumors including lung, pancreas, and the subset of breast cancer known as triple negative or basal.
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