Experimental reversal of P-glycoprotein-mediated multidrug resistance by pharmacological chemosensitisers

Abstract

INTRODUCTION CLINICAL DRUG resistance to chemotherapeutic agents is a major obstacle for their curative potential in the treatment of human cancers. Multidrug resistance (MDR) is a common mechanism of cellular resistance to the cytotoxic activity of many chemotherapeutic agents. Tumour cells selected for the MDR phenotype overexpress the MDRl gene product, Pglycoprotein (Pgp), a membrane-bound drug efflux pump which confers resistance to a broad range of commonly used chemotherapeutic drugs and other agents. Numerous compounds have been identified which inhibit the efflux activity of Pgp, and reverse cellular resistance to cytotoxic agents in experimental systems (1, 31. This suggests that clinical drug resistance in human turnours, which often overexpress Pgp, may be potentially circumvented through the concomitant administration to patients of a Pgp inhibitor and chemotherapeutic drugs. &p-mediated MDR is only one of many cellular mechanisms by which tumour cells may evade the cytotoxic effects of anticancer agents, but is one of the best understood and most intensively studied forms of mammalian drug resistance. Remarkable progress has been made in defining the cellular and biochemical pharmacology of MDR since the initial description of the MDR phenotype and its drug efflux characteristics [3, 41. The membrane protein responsible for drug efflux has been identified and its structure and function characterised, genes encoding for Pgp and other related drug transport proteins have been cloned, and Pgp has been shown to be functionally expressed in many human turnours, as well as in normal tissues. Of clinical relevance, a large number of drugs have been identified which inhibit the function of Pgp, termed chemosensitisers or MDR modulators, and trials have been designed and carried out in humans to test the potential for pharmacological inhibition of clinical MDR. Many of these chemosensirisers have proven toxic or ineffective in ~1210, and thus the identification of novel agents for potential clinical use has taken on great importance. This article will briefly summarise our current understanding of the pha~acolo~cal mechanism of Pgp inhibition, review in detail those agents most likely to have clinical impact on the circumvention of Pgp-mediated MDR in humans, and discuss novel approaches to the identification of more potent and less toxic chemosensitisers. CELLULAR PHARMACOLOGY AND IMOLECULAR BIOLOGY OF MDR The MDR phenotype was first described by Biedler and Riehm who noted that, following selection of mammalian tumour cells for resistance to a single cytotoxic drug, a broad crossresistance developed simultaneously to other structurally and functionally unrelated drugs [3]. Soon after, Dano noted that such drug-resistant cells displayed a decrease in accumulation of the anthracycline chemotherapeutic agent, daunomytin, due to its active outward transport [4]. Chemotherapeutic drugs now known to be affected by ‘~DRinclude doxorubicin, mitoxantrone, vincristine, vinblastine, W-16, paclitaxel and topotecan, but not drugs such as bleomycin, methotrexate, cisplatinum or alkylating agents. In addition, naturally occurring carcinogens, such as benzo(a)pyrene [5], and physiological substances such as hormonal steroids [6], also serve as substrates for the Pgp pump. The most consistent alteration found in MDR cell lines is an increased expression of a high molecular weight cell surface glycoprotein (Pgp) not detectable in drug-sensitive cells, associated with an energy-dependent mechanism for the decreased accumulation and retention of cytotoxic drugs [4, 7, 81. The presence of Pgp was found to correlate with both the degree of resistance and the relative decrease in drug accumulation [Q]. Monoclonal antibodies to Pgp and nucleic acid probes for the MDRl gene demonstrated that most MDR cell lines from rodent or human origin overexpress this gene E9, 10