Interactions of Halichondrin B and Eribulin with Tubulin

Compounds that modulate microtubule dynamics include highly effective anticancer drugs, leading to continuing efforts to identify new agents and improve the activity of established ones. Here, we demonstrate that [(3)H]-labeled halichondrin B (HB), a complex, sponge-derived natural product, is bound to and dissociated from tubulin rapidly at one binding site per alpha beta-heterodimer, with an apparent K(d) of 0.31 mu M. We found no HB-induced aggregation of tubulin by high-performance liquid chromatography, even following column equilibration with HB. Binding of [(3)H]HB was competitively inhibited by a newly approved clinical agent, the truncated HB analogue eribulin (apparent K(nu) 0.80 mu M) and noncompetitively by dolastatin 10 and vincristine (apparent K(i)'s, 0.35 and 5.4 mu M, respectively). Our earlier studies demonstrated that HB inhibits nucleotide exchange on beta-tubulin, and this, together with the results presented here, indicated the I-1B site is located on beta-tubulin. Using molecular dynamics simulations, we determined complementary conformations of HB and beta-tubulin that delineated in atomic detail binding interactions of HB with only beta-tubulin, with no involvement of the alpha-subunit in the binding interaction. Moreover, the HB model served as a template for an eribulin binding model that furthered our understanding of the properties of eribulin as a drug. Overall, these results established a mechanistic basis for the antimitotic activity of the halichondrin class of compounds.