guanosine-triphosphate and laulimalide

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 17beta-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.

Topics: Bridged Bicyclo Compounds, Heterocyclic; Drug Synergism; Glutamic Acid; Guanosine Triphosphate; Humans; Lactones; Macrolides; Microtubule-Associated Proteins; Models, Molecular; Paclitaxel; Taxoids; Temperature; Tubulin; Tumor Cells, Cultured

Previous work has shown that laulimalide, a sponge-derived natural product, resembles paclitaxel in enhancing tubulin assembly and in its effects on cellular microtubules. The two compounds, however, seem to have distinct binding sites on tubulin polymer. Nearly equimolar amounts of tubulin, laulimalide, and paclitaxel are recovered from microtubules formed with both drugs. In the present study, we searched for differences between laulimalide and paclitaxel in their interactions with tubulin polymer. Laulimalide was compared with paclitaxel and epothilone A, a natural product that competes with paclitaxel in binding to microtubules, for assembly properties at different temperatures and for effects of GTP and microtubule-associated proteins on assembly. Although minor differences were observed among the three drugs, their overall effects were highly similar, except that aberrant assembly products were observed more frequently with paclitaxel and that the polymers formed with laulimalide and epothilone A were more stable at 0 degrees C. The most dramatic difference observed between laulimalide and epothilone A was that only laulimalide was able to enhance assembly synergistically with paclitaxel, as would be predicted if the two drugs bound at different sites in polymer. Because stoichiometric amounts of laulimalide and paclitaxel can cause extensive tubulin assembly, maximum synergy was observed at lower temperatures under reaction conditions in which each drug alone is relatively inactive. Laulimalide-induced assembly, like paclitaxel-induced assembly, was inhibited by drugs that inhibit tubulin assembly by binding at either the colchicine- or vinblastine-binding site. When radiolabeled GTP is present in a reaction mixture with either laulimalide or paclitaxel, nucleotide hydrolysis occurs with incorporation of radiolabeled GDP into polymer.

Topics: Animals; Antineoplastic Agents, Phytogenic; Biopolymers; Cattle; Drug Synergism; Guanosine Triphosphate; Hydrolysis; Macrolides; Microtubule-Associated Proteins; Microtubules; Paclitaxel; Taxoids; Tubulin; Tubulin Modulators