Variation in selective toxicity: causes and consequences.

Variation in selective toxicity is a fundamental consequence of the neoplastia process and seriously limits the value of predictive tests. This variation is due to the biochemical heterogeneity of animal and human host-tumor systems, to the heterogeneity of drugs with respect to those factors which are required for maximum selective effect, and to the low therapeutic indices of many anticancer drugs. It is suggested that this problem can be overcome either by use of drugs with high therapeutic indices or by treatment based on the biochemical peculiarities of each individual host-tumor-drug system.