Stability of Vincristine Complexes in Cytosols Derived from Xenografts of Human Rhabdomyosarcoma and Normal Tissues of the Mouse

The selective action of vincristine (VCR) has been correlated with longer retention of the drug in neoplastic tissue compared with normal tissues of the mouse (J. A. Houghton, L. G. Williams, P. M. Torrance, and P. J. Houghton, Cancer Res., 44:582-590, 1984). In order to examine the basis for this differential, the stability of drug-protein complexes was examined further. The stability of drug-protein complexes formed in cytosols derived from HxRhl 8 tumors, ileum, liver, kidney, skeletal muscle, blood, brain, spleen, lung, and bone marrow was examined. Protein-bound [³H]VCR was isolated by gel filtration of [³H]VCR-cytosol mixtures from each tissue except for ileum and Wood. Complexes formed in brain and HxRh18 cytosols were stable at 37° for at least 2 h; all other complexes were unstable. For liver, kidney, and muscle, half-times of complexes were in a similar order to the initial rates of elimination of [³H]VCR from these tissues in vivo but were of shorter duration. The HxRh18-[³H]-VCR complex was unstable at 37° in the presence of cytosols prepared from ileum, kidney, liver, and lung. Drug metabolism by these tissues was not detected in vitro. In the presence of heat-treated extracts from ileum or kidney, [³H]VCR complex was stable, suggesting that the destabilizing factor may be enzymic. Degradation of ¹²⁵iodinated tubulin, analyzed by polyacrylamide-sodium dodecyl sulfate gel electrophoresis, occurred in the presence of ileum but not skeletal muscle or brain cytosols. This correlated with the stability of HxRh18-[³H]VCR complexes. In the presence of kidney cytosol, however, the molecular weight of ^12Sl-tubulin remained unchanged, suggesting a different mechanism. Based upon data obtained, cytosols from normal tissues may be categorized into three classes: (a) those that formed stable complexes (brain); (b) those that formed unstable complexes but also destabilized preformed complex (ileum, kidney, liver, lung); and (c) tissues that formed unstable complexes but did not destabilize preformed complex (skeletal muscle, spleen, bone marrow, blood). The degree of instability of complexes formed in cytosols prepared from normal tissues appears to correlate with rapid loss of VCR from these tissues in vivo and hence may represent mechanism(s) for the selective action of this antineoplastic agent.