The focus of our sections 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 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, and we have been particularly intrigued by the disease responses in patients with nonsmall cell lung cancer. 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 has not been confirmed in patients. We have confirmed activity of the agent in an ex vivo assay. 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 has been submitted for final review by the IRB. Our laboratory also maintains an interest in studying drug resistance in other model systems. Several years ago, in collaboration with the NCIs 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 one, NSC-281612, has been approved for Phase I testing. A protocol is in preparation, and an IND will be submitted. 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. A second aspect of this latter project is the evaluation of other mechanisms of drug resistance using Bioinformatics strategies. This project is carried out in collaboration with Dr. Wilfred Stein, Hebrew University, Jerusalem, and Dr. Thomas Litman, University of Copenhagen, Denmark. Two separate bioinformatics strategies identified cell adhesion as a mechanism of resistance, using gene expression data in clinical samples reported in publicly available databases. These experiments resonated with a literature already available suggesting that cell adhesion was an important mechanism of resistance. Interestingly, evaluation of in vitro models of cell adhesion -- such as spheroid formation-- have suggested that cells cultured in this manner have similar gene expression profiles to confluent cells cultured on plastic. This would suggest that spheroid cultures per se cannot serve as models of intrinsic drug resistance and that other in vitro models should be developed
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