The evaluation of biological interactions using response surface methodology

Response surface methodology was employed in the statistical analysis of the combination exposures of genotoxic agents, bischloroethylnitrosourea with cis-diamminedichloroplatinum (II) and cis-diamminedichloroplatinum (II) with X rays. The measured endpoint in each case was sister chromatid exchanges in V79 Chinese hamster cells. The combination experiments employed a factorial design in which cells were treated, in various concentration combinations, with two agents simultaneously. Bis-chloroethylnitrosourea and cis-diamminedichloroplatinum (II) each exhibited curvilinear concentration-related increases in sister chromatid exchanges. X rays exhibited a dose-dependent increase in sister chromatid exchanges. For the cis-diamminedichloroplatinum (II)/X ray combinations, response surface methodology indicates a less-than-additive interaction, suggested by the non-parallel concentration-response curves of one agent at varying concentrations of the other, and a slight dose-dependent increase in sister chromatid exchanges due to X rays alone. Both cis-diamminedichloroplatinum and bis-chloroethylnitrosourea exhibited concentration-related increases in sister chromatid exchanges, cis-diamminedichloroplatinum (II) being 8-10 times (dependent on what level of effect was compared) more potent than bis-chloroethylnitrosourea. For the cis-diamminedichloroplatinum (II)/bis-chloroethylnitrosourea combinations, an increasingly less-than-additive interaction was detected. The analysis of these combinations demonstrates the strength of response surface methodology, a collection of mathematical and statistical techniques for detecting, analyzing and describing the biological effects resulting from exposures to multiple cytotoxic agents. The descriptive ability of these procedures is shown to be useful in that it leads to the suggestion of hypotheses regarding mechanisms of action.