Influence of the guanine nucleotide phosphorylation state and of Mg2+ ions on the interaction of vinzolidine/tubulin 6 S: a fluorescence quenching study

New semisynthetic vinca alkaloids: chemical, biochemical and cellular studies

A new semisynthetic anti-tumour bis-indol compound, KAR-2 [3'-(beta-chloroethyl)-2',4'-dioxo-3,5'-spiro-oxazolidino-4-dea cetoxy-vinblastine] with lower toxicity than vinca alkaloids used in chemotherapy binds to calmodulin but, in contrast to vinblastine, does not exhibit anti-calmodulin activity. To investigate whether the modest chemical modification of bis-indol structure is responsible for the lack of anti-calmodulin potency and for the different pharmacological effects, new derivatives have been synthesized for comparative studies. The synthesis of the KAR derivatives are presented. The comparative studies showed that the spiro-oxazolidino ring and the substitution of a formyl group to a methyl one were responsible for the lack of anti-calmodulin activities. The new derivatives, similar to the mother compounds, inhibited the tubulin assembly in polymerization tests in vitro, however their inhibitory effect was highly dependent on the organization state of microtubules; bundled microtubules appeared to be resistant against the drugs. The maximal cytotoxic activities of KAR derivatives in in vivo mice hosting leukaemia P388 or Ehrlich ascites tumour cells appeared similar to that of vinblastine or vincristine, however significant prolongation of life span could be reached with KAR derivatives only after the administration of a single dose. These studies plus data obtained using a cultured human neuroblastoma cell line showed that KAR compounds displayed their cytotoxic activities at significantly higher concentrations than the mother compounds, although their antimicrotubular activities were similar in vitro. These data suggest that vinblastine/vincristine damage additional crucial cell functions, one of which could be related to calmodulin-mediated processes.

Effects of antimitotic agents on tubulin-nucleotide interactions

The interaction of antimitotic drugs with guanine nucleotides in the tubulin-microtubule system is reviewed. Antimitotic agent-tubulin interactions can be covalent, entropic, allosteric or coupled to other equilibria (such as divalent cation binding, alternate polymer formation, or the stabilization of native tubulin structure). Antimitotics bind to tubulin at a few common sites and alter the ability of tubulin to form microtubules. Colchicine and podophyllotoxin compete for a common overlapping binding site but only colchicine induces GTPase activity and large conformational changes in the tubulin heterodimer. The vinca alkaloids, vinblastine and vincristine, the macrocyclic ansa macrolides, maytansine and ansamitocin P-3, and the fungal antimitotic, rhizoxin, share and compete for a different binding site near the exchangeable nucleotide binding site. The macrocyclic heptapeptide, phomopsin A, and the depsipeptide, dolastatin 10, bind to a site adjacent to the vinca alkaloid and nucleotide sites. Colchicine, vinca alkaloids, dolastatin 10 and phomopsin A induce alternate polymer formation (sheets for colchicine, spirals for vinblastine and vincristine and rings for dolastatin 10 and phomopsin A). Maytansine, ansamitocin P-3 and rhizoxin inhibit vinblastine-induced spiral formation. Taxol stoichiometrically induces microtubule formation and, in the presence of GTP, assembly-associated GTP hydrolysis. Analogs of guanine nucleotides also alter polymer morphology. Thus, sites on tubulin for drugs and nucleotides communicate allosterically with the interfaces that form longitudinal and lateral contacts within a microtubule. Microtubule associated proteins (MAPs), divalent cations, and buffer components can alter the surface interactions of tubulin and thus modulate the interactions between antimitotic drugs and guanine nucleotides.

Qualitative study of the interaction mechanism of estrogenic drugs with tubulin

Synthetic estrogenic drugs (E-diethylstilbestrol, erythro-hexestrol and E,E-dienestrol) inhibit tubulin assembly and erythro-hexestrol and E,E-dienestrol lead to the formation of twisted ribbon structures. For the inhibitory effect on tubulin assembly, estrogenic drugs seem to interact directly with tubulin 6S on site(s) analogous to the colchicine-site, but independent of the GTP- and vinblastine-sites. This binding does not involve tubulin tryptophanyl residues or sulfhydryl groups. The influence of temperature, calcium and magnesium on the formation of twisted ribbon structures induced by the binding of estrogenic drugs to microtubular protein and tubulin has also been studied. This formation is strongly magnesium-dependent whereas preformed twisted ribbon structures are calcium- and chilling-insensitive.