Clinical development of P glycoprotein modulators in oncology.

The last two decades have witnessed dramatic advances into the mechanisms of drug resistance in cancer. The identification of P glycoprotein (Pgp) as a specific mechanism led to the initial hope and expectation that it would be possible to modulate this and increase sensitivity to drug therapy. Clinical trials using first- and second-generation Pgp modulators did establish proof of principle that in some settings, clinical drug resistance could be overcome with the addition of a Pgp modulator-for example, clinical resistance to paclitaxel, a Pgp substrate, in women with ovarian cancer was shown to be overcome in approximately 20% with the addition of PSC 833, a highly effective Pgp modulator. However, evolutionary and adaptive redundancy in resistance mechanisms have tempered clinical results, even with very effective second- and third-generation modulators. The lessons from oncology establish sound methodology for the evaluation of Pgp modulators for safety, tolerability and efficacy in Phase I, II and III clinical trials. This review will focus on some of the early-phase clinical trials with earlier and newer Pgp modulators, either as single agents or in combination with chemotherapy.