Pharmacologic differences in taxanes leading to difference in clinical activity and toxicity

e13567 Background: Paclitaxel and docetaxel share major parts of their structures and mechanism of action, but differ in several aspects. Both paclitaxel and docetaxel act at microtubule causing tubulin polymer generation but exhibit pharmacodynamic differences. These pharmacological differences may account for the differences observed between taxanes in their clinical activity and toxicity. Methods: Review of articles providing details of taxane pharmacology was conducted to evaluate pharmacological basis. Results: Greater uptake and slower efflux of docetaxel from tumor cells leads to longer retention time; whereas equal uptake and efflux of paclitaxel shorter retention time for paclitaxel. Hence, apoptotic and mitotic responses of paclitaxel is complete within 4 days implying that more frequent (weekly) administration of paclitaxel will be superior. High affinity to β-tubulin with docetaxel (1.9) than with paclitaxel (1.0) allows higher inhibition of tumor growth. Secondly, drug concentration causing maximum polymerization is 0.2 µM and 0.4 µM for docetaxel and paclitaxel, respectively; indicating that docetaxel is twice as potent as paclitaxel in causing microtubule polymerization. Docetaxel arrests cells in the radiosensitive G2/M phase of the cell cycle making it a potent radiosensetizer than paclitaxel. Mechanisms consistent with allosteric modulation of the reductases increase doxorubicinol formation at clinically relevant concentrations of paclitaxel when it is administered with doxorubicin. Enhanced formation of doxorubicinol aglycone, a metabolite potentially involved in the reversible phase of cardiotoxicity causes higher cardiotoxicty when doxorubicin is administered with paclitaxel than with docetaxel. Non linear pharmacokinetics and hypersensitivity with paclitaxel and fluid retention with docetaxel is attributed to excepient of the drug rather than drug itself. Conclusions: Influx and efflux mechanism support superior weekly administration of Paclitaxel. Pharmacological differences between taxanes justify the incomplete resistance seen between taxanes. The incomplete cross resistance is specifically related to docetaxelactivity in paclitaxel resistant cases.