Synergistic Effects of Peloruside A and Laulimalide with Taxoid Site Drugs, but Not with Each Other, on Tubulin Assembly

Abstract

Previous studies on the drug content of pelleted tubulin polymers suggest that peloruside A binds in the laulimalide site, which is distinct from the taxoid site. In a tubulin assembly system containing microtubule-associated proteins and GTP, however, peloruside A was significantly less active than laulimalide, inducing assembly in a manner that was most similar to sarcodictyins A and B. Because peloruside A thus far seems to be the only compound that mimics the action of laulimalide, we examined combinations of microtubule-stabilizing agents for synergistic effects on tubulin assembly. We found that peloruside A and laulimalide showed no synergism but that both compounds could act synergistically with a number of taxoid site agents [paclitaxel, epothilones A/B, discodermolide, dictyostatin, eleutherobin, the steroid derivative 17␤-acetoxy-2-ethoxy-6-oxo-B-homo-estra-1,3,5(10)-trien-3-ol, and cyclostreptin]. None of the taxoid site compounds showed any synergism with each other. From an initial study with peloruside A and cyclostreptin, we conclude that the synergism phenomenon derives, at least in part, from an apparent lowering of the tubulin critical concentration with drug combinations compared with single drugs. The apparent binding of peloruside A in the laulimalide site led us to attempt construction of a pharmacophore model based on superposition of an energy-minimized structure of peloruside A on the crystal structure of laulimalide. Although the different sizes of the macrocycles limited our ability to superimpose the two molecules, atom correspondences that were observed were consistent with the difficulty so far experienced in creation of fully active analogs of laulimalide.

Low-dose laulimalide represents a novel molecular probe for investigating microtubule organization

“…4,5 It is also active in multidrug-resistant cancer cell lines overexpressing P-glycoprotein (P-gp) and is both effective in taxoid-resistant cell lines 5 and synergistic with taxoids. 6 These findings suggest that laulimalide may form the basis for a next-generation antimitotic agent, although we note lingering concerns over toxicity. 7 MT stabilizers interfere with MT dynamics and alter mitotic processes.…”

Mutations in the β-tubulin binding site for peloruside A confer resistance by targeting a cleft significant in side chain binding

“…This is known from studies that indicate synergy between peloruside A or laulimalide and other microtubulestabilizing drugs, all of which bind at the paclitaxel binding site, [9][10][11]23 as well as from direct binding studies that have shown that microtubules formed in vitro and taxol-saturated can still bind peloruside A and vice-versa. 8,21 These results demonstrated that the binding site for peloruside A and laulimalide was a new drug site on tubulin, but did not indicate where on tubulin it might be.…”

Microtubule Active Agents: Beyond the Taxane Frontier

“…It has the advantage of being less lipophilic than paclitaxel and binds to a-tubulin on the laulimalide-binding site (52). This binding site distinct from the taxanes raises the possibility of combining drugs that act on laulimalide and taxane-binding sites-a synergistic antiproliferative effect has already been seen in vitro (53).…”