PARALLEL DOSE-RESPONSE CURVES IN COMBINATION EXPERIMENTS

A possible experimental design for combination experiments is to compa re the dose-response curve of a single agent with the corresponding cu rve of the same agent using either a fixed amount of a second one or a fixed dose ratio. No interaction is then often defined by a parallel shift of these curves. We have performed a systematic study for variou s types of dose-response relations both for the dose-additivity (Loewe additivity) and for the independence (Bliss independence) criteria fo r defining zero interaction. Parallelism between dose-response curves of a single agent and those of the same agent in the presence of a fix ed amount of another one is found for the Loewe-additivity criterion f or linear dose-response relations. For nonlinear relations, one has to differentiate between effect parallelism (parallel shift on the effec t scale) and dose parallelism (parallel shift on the dose scale). In t he case of Loewe additivity, zero-interaction dose parallelism is foun d for power, Weibull, median-effect and logistic dose-response relatio ns, given that special parameter relationships are fulfilled. The mech anistic model of competitive interaction exhibits dose parallelism but not effect parallelism for Loewe additivity. Bliss independence and L oewe additivity lead to identical results for exponential dose-respons e curves. This is the only case for which dose parallelism was found f or Bliss independence. Parallelism between single-agent dose-response relations and Loewe additivity mixture relations is found for examples with a fixed dose-ratio design. However, this is again not a general property of the design adopted but holds only if special conditions ar e fulfilled. The comparison of combination dose-response curves with s ingle-agent relations has to be performed taking into account both pot ency and shape parameters. The results of this analysis lead to the co nclusion that parallelism between zero interaction combination and sin gle-agent dose-response relations is found only for special cases and cannot be used as a general criterion for defining zero-interaction in combined-action assessment even if the correct potency shift is taken into account. (C) 1998 Society for Mathematical Biology.