SynergyFinder 2.0. implements four reference synergy models (HSA, Bliss, Loewe and ZIP), and their extensions to calculate synergy scores for higher-order combination data. These models quantify the degree of synergy either as the excess over the maximum single drug response (HSA), multiplicative effect of single drugs as if they acted independently (Bliss), expected response corresponding to an additive effect as if the single drugs were the same compound (Loewe), and expected response corresponding to the effect as if the single drugs did not affect the potency of each other (ZIP). More specifically, the following higher-order formulations were used to quantify the drug combination synergy (S) for the measured multi-drug combination effect between N drugs :
Here, are the measured responses of the single drugs, while a, b and n are the doses of the single drugs required to produce the combination effect . For the ZIP model, is the dose of Nth drug fitted with four-parameter log-logistic (4PL) function, whereas is the dose that produces the half-maximum effect (also known as relative or , depending on the readout), and is the shape parameter indicating the slope of the dose–response curve.