2.6. Combination index calculations

The median-effect analysis of Chou-Talalay Method was used to determine additive, synergistic or antagonistic effect for combination therapy with JWH-133 and IR700DX-6T-PDT. In vitro therapeutic effect data were used to calculate combination index (CI) via CompuSyn (Combosyn, Inc., Paramus, NJ) software.

C_A,x and C_B,x are the concentrations of drug A and drug B used in combination to achieve x% drug effect. IC_x,A and IC_x,B are the concentrations for single agents to achieve the same effect(44). CI values <1, =1, and >1 indicate synergistic, additive, and antagonistic effects, respectively.

The isobologram analysis, similar to CI analysis, evaluates the interaction of two drugs. First, when drug A or B is used alone, the concentrations required to produce a defined single-agent effect (e.g., IC50) are placed on the x and y axes in a two-coordinate plot, corresponding to (C_A, 0) and (0, C_B), respectively. These two points are then connected and the line is defined as the line of additivity. Second, when the two drugs are used together, concentrations of them needed to provide the same effect, denoted as (c_A, c_B), are placed in the same plot. Synergy, additivity, or antagonism are indicated when (c_A, c_B) is located on the lower left, on, or on the upper right of the line of additivity, respectively.