Effect of hypoxia on the cytotoxicity of taxoid- and nontaxoid-site tubulin-binding agents

Introduction

Tubulin-binding agents (TBAs) like the taxanes are critical adjuvant and palliative first-line therapies for the treatment of early, advanced and metastatic solid tumors of different lineages. A prominent feature of solid tumors is the development of hypoxic regions due to abnormal vasculature and limited supply of nutrition and oxygen. Via the hypoxia-inducible factors, hypoxic stress results in the activation of a series of pathways that contribute to the development of resistance to not only TBAs but also other anticancer drugs. Given the major clinical problem that represents resistance to taxanes, a number of new innovative therapeutic strategies are currently under development to overcome hypoxia-induced chemoresistance. Peloruside A (PEL) is a TBA isolated from the marine sponge Mycale hentscheli that has a similar mode of action to paclitaxel (PTX). It is of particular interest because it binds to a distinct site on b-tubulin that differs from that of PTX, which binds to the taxoid site. In this study, we compared the effects of PEL to the known effects of PTX on colorectal HCT116 cancer cells exposed to hypoxic stress.

Materials/methods

Cells were maintained at 37^oC in a controlled atmosphere of 95% air and 5% CO2. Hypoxia was induced by culturing cells at 1% O2 and 5% CO2. Pharmacological effects of PEL and PTX on HCT116 cells under normoxic and hypoxic conditions were determined by MTT cell proliferation assay, flow cytometry analysis, and intracellular tubulin polymerization assay.

Results and conclusions

Our preliminary results show an increase in the cytotoxicity of PEL in hypoxic HCT116 cells compared to cells cultured in the normoxic condition. Consistent with previous findings, hypoxia induced significant resistance to PTX in the hypoxic cells. The increased potency of PEL is likely due to greater ability of PEL to alter microtubule dynamics and induce apoptosis than PTX in hypoxic conditions. Hypoxia-induced changes in the microtubule dynamics are known to alter beta-tubulin susceptibility to PTX. Drugs that bind to an alternate site may have better outcome in the treatment of hypoxic tumor cells than PTX.