Fig. 1.

The concept of Combination Index (CI) based on the Loewe additivity model. (A) Loewe additivity with CI = 1 can be visualized as a straight line at a two-dimensional isobologram with the doses of drug 1 and drug 2 as coordinates. A synergistic (CI < 1) and antagonistic (CI > 1) drug combination will be positioned below and above the additivity line, respectively (adopted from [9]). (B) Loewe additivity model cannot directly assess such drug interaction in which a combination effect is higher than the achievable effect of the individual drugs, even though by intuition one would expect a clinically relevant synergy in such cases.