A successful drug development program requires a complete understanding of the clinical pharmacology of the agents being evaluated. The Clinical Pharmacology Program (CPP) has as its primary interest the use of pharmacokinetic and pharmacodynamic concepts in the development of novel anticancer agents. The CPP is directly responsible for the pharmacokinetic/pharmacodynamic analysis of numerous Phase I and II clinical trials conducted within the NCI. In addition, the CPP provides direct pharmacokinetic support for many studies performed elsewhere in the extramural community. Within the section, we utilize compartmental and noncompartmental approaches to define the disposition of agents. Also, we are often required to characterize the plasma protein binding properties and metabolism of new agents through in vitro techniques. Several of our clinical trials have used adaptive control with a feedback mechanism to target particular plasma concentrations (e.g., suramin, CAI). The drugs with which the CPP has had its greatest experience include: suramin, phenylacetate, phenylbutyrate, TNP-470, PMEA, AZT, PSC 833, CAI, DAB486IL2, IgG-RFB4-SMPT-dgA CD22, IgG-HD37-SMPT-dgA CD19, ormaplatin, UCN-01, flavopiridol, thalidomide, 9AC, intraperitoneal cisplatin, intraperitoneal carboplatin, docetaxel, and paclitaxel. Currently, we are characterizing the interaction between ketoconazole and docetaxel and understand the pharmacokinetics of MS275, perifosine, depsipeptide, sorafenib, nelfinavir, bevacizumab, and clopidegral. Influence of a dual ABCB1 and ABCG2 inhibitor on imatinib disposition. Imatinib, a tyrosine kinase inhibitor currently approved for treatment of several malignancies, has been shown to be a substrate for multiple efflux-transporter proteins, including ABCB1 (P-glycoprotein) and ABCG2 (BCRP). The effect of inhibiting these transporters on tissue exposure to imatinib remains unclear. To assess the role of these transporters on drug disposition, 50 mg/kg imatinib was administered to Balb/C mice, 30 minutes after receiving tariquidar (10 mg/kg), an inhibitor of both ABCB1 and ABCG2, or vehicle, via oral gavage. Quantitative determination of imatinib in mouse plasma, liver and brain was performed using a newly-developed and validated liquid-chromatography-mass spectrometric method. Results: Exposure to imatinib was 2.2-fold higher in plasma, liver and brain in mice that received tariquidar, as compared to those that received the vehicle (P = 0.001). The peak plasma concentration did not increase substantially, suggesting that tariquidar is affecting the distribution, metabolism and/or excretion of imatinib, rather than absorption. Though tariquidar increased the absolute exposure of imatinib, the brain-to-plasma ratio of imatinib was unaffected. This study suggests that intentional inhibition of ABCB1 and ABCG2 function at the blood-brain barrier is unlikely to significantly improve clinical outcome of imatinib with currently used dosing regimens.
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