Two drugs of current clinical interest in this group are vinblastine and vincristine, alkaloids derived from the periwinkle plant (Vinca rosea). The precise action of these drugs is not well understood and many studies of their effects on cells are in progress. They appear to act as mitotic inhibitors, that is, not interfering with DNA synthesis but blocking division of the cell.

Vinblastine often produces injury to the normal bone marrow and depresses the white blood cells and platelets. In contrast, vincristine causes less severe damage to the normal bone marrow but produces neurotoxic and gastrointestinal effects.

The marked differences in clinical effects of the two drugs, despite their similarity in chemical structure, cannot as yet be explained. It has been postulated that the drugs may form several active metabolites in the body and that these may be responsible for their different activities. Another suggestion is that the differences in their effects may be ascribed to a variation in their ability to enter different types of cells, after which they may form a common metabolite responsible for anticancer and toxic effects. Thus, it is suggested that vincristine may be effective in acute childhood leukemia and toxic to the nervous system because of the relative ease with which it can enter leukemia and nervous system cells. On the other hand, vinblastine, which rarely produces neurotoxicity or remission in childhood leukemia, may have little ability to penetrate the walls of the nervous system or acute leukemia cells, but would be more useful in Hodgkin's disease.

Plant Alkaloid Action

Among the active cancer drugs are vincristine and vinblastine, alkaloids derived from the periwinkle plant (Vinca rosea). Scientists believe that these drugs, as illustrated above with the suggested action of vincristine, act as mitotic inhibitors; that is, they do not interfere with DNA synthesis as other drugs do, but block division of the cell.