Evidence for microtubule target engagement in tumors of patients receiving ixabepilone.

PURPOSE Microtubule-stabilizing agents, such as taxanes, have been shown to be effective anticancer drugs. alpha-Tubulin, a basic unit of microtubules, can undergo several posttranslational modifications after assembly into stabilized microtubules, including acetylation and detyrosination. These modifications have been observed in cell cultures after exposure to microtubule stabilizers. Our objective was to develop a straightforward and dependable assay to show tubulin target engagement in tumor tissue after treatment of patients with ixabepilone(BMS-247550; Ixempra). EXPERIMENTAL DESIGN Levels of posttranslationally modified alpha-tubulin were assessed in lysates of cultured malignant cell lines, as well as in both tumor tissue and peripheral blood mononuclear cells derived from patients before and after treatment with ixabepilone. Modification-specific antibodies permitted quantitative Western blot analysis. RESULTS In cultured cell lines, the levels of detyrosinated (glu-terminated) and acetylated alpha-tubulin increased after microtubule stabilization induced by ixabepilone. ixabepilone treatment also induced a 2-fold to 25-fold increase in detyrosinated alpha-tubulin levels in 11 of 13 serial biopsies and a 2-fold to 100-fold increase in acetylated alpha-tubulin in 11 of 12 serial biopsies obtained from patients receiving ixabepilone. Overall, little or no difference in tubulin modifications were observed between the before and after ixabepilone treatment in lysates from their peripheral blood mononuclear cells at the time point examined. CONCLUSION Assessing the levels of detyrosinated and/or acetylated alpha-tubulin seems to provide a simple and reliable assay to show target engagement by the microtubule-stabilizing agent ixabepilone. Such analyses may provide further understanding of therapeutic success or failure of microtubule-stabilizing agents in cancer therapy.