Taxol, a plant-derived antimicrotubule agent, was originally isolated from the bark of the Pacific yew, Taxus brevifolia. This naturally occurring antineoplastic drug has been demonstrated to possess broad activity against a variety of human solid tumors, particularly in drug-refractory ovarian cancer and metastatic breast cancer (1). However, the exact mechanism of taxol's cytotoxicity against tumor cells is not entirely clear. Recent studies have demonstrated that taxol, besides causing microtubule bundling and mitotic arrest, is able to induce internucleosomal DNA fragmentation and typical morphological features of apoptosis in a number of solid tumor cells. These results clearly indicate that taxol, in addition to its classical activity against microtubules and cell cycle arrest, also possesses significant cell-killing activity by induction of apoptosis.

Although it is well recognized that taxol can cause both mitotic arrest and apoptotic cell death, it remains unclear whether taxol-induced cell death is a secondary event resulting from mitotic arrest or represents a novel mechanism of taxol against tumor cells.