Paclitaxel Resistance: Molecular Mechanisms and Pharmacologic Manipulation

It has been approximately ten years since the Food and Drug Administration (FDA) approved paclitaxel for the treatment of platinum resistant epithelial ovarian carcinoma. Since the approval, the drug has found therapeutic applications in a variety of schedules and in a wide variety of epithelial malignancies. Its novel mechanism of action provided the hope that it would demonstrate anti-neoplastic activity in multidrug resistant tumor cells. Unfortunately, as with other chemotherapeutic drugs, resistance is commonly seen. Laboratory investigation has defined a wide variety of resistance mechanisms including overexpression of multidrug resistance (MDR-1) gene, molecular changes in the target molecule (betatubulin), changes in apoptotic regulatory and mitosis checkpoint proteins, and more recently changes in lipid composition and potentially the overexpression of interleukin 6 (IL-6). This review describes the in vitro molecular data that define and support the various mechanisms of resistance and critically evaluates the evidence for the participation of these mechanisms in clinically relevant paclitaxel resistance. This review also explores pharmacologic attempts to modulate paclitaxel resistance, principally through inhibition of the MDR-1 drug efflux pump. Future avenues for drug resistance research and its pharmacologic manipulation in the clinic are discussed.