Comparison of action of paclitaxel and poly(L-glutamic acid)paclitaxel conjugate in human breast cancer cells

The new anticancer agent poly(l-glutamic acid)-paclitaxel (PG-TXL) is a con jugate of paclitaxel and the water-soluble polyglutamate carrier. The obser vation that PG-TXL appears to possess antitumor activity superior to free p aclitaxel in preclinical studies suggests that PG-TXL might possess favorab le pharmacokinetic properties and/or have a mechanism of action different f rom that of paclitaxel. The purpose of this study was to compare the pharma cological action of PG-TXL and free paclitaxel in a panel of breast cancer cell lines with emphasis on their ability to induce apoptosis, their effect s on cell cycle progression, and their cellular uptake. Morphological analy sis and biochemical characterizations demonstrated that both compounds have similar abilities to induce apoptosis in cells expressing wild-type p53 (M CF-7) or mutant p53 (MDA-MB435 and MDA-MB453). Although MCF-7 cells were le ss sensitive to each compound than MDA-MB435 and MDA-MB453 cells, transfect ion experiments demonstrated that p53 did not appear to play a significant role in drug-induced cell death with either agent. Flow cytometry analysis further revealed that both free paclitaxel and PG-TXL induced a characteris tic G(2)/M arrest in the cell cycle, consistent with the disturbance of mic rotubule polymerization as their mechanism of action. Western blot analysis showed that paclitaxel and PG-TXL downregulated HER2/neu expression in a s imilar fashion. HPLC analysis revealed that paclitaxel was released from th e PG-TXL conjugate in vitro. The released paclitaxel, not the glutamic acid polymer, was subsequently transported into the cells. These results sugges t that PG-TXL exerts its anticancer activity by continuous release of free paclitaxel, and that the favorable pharmacokinetics and drug distribution o f the PG-TXL conjugate in vivo are likely the main factors contributing to its superior anticancer activity.