Comparison of methods for evaluating drug-drug interaction

Abstract

The goal of the present report is to compare several published methods of analyzing drug-drug interaction data. The compared methods are the curve-shift analysis, isobologram, combination index, and universal surface response analysis, and the comparison was based on analysis of published cytotoxicity data of combinations of two anti-folate agents. Major findings are as follows. The curve shift analysis enabled the inspection of the experimental data and visual evaluation of the approximate parallelism between the dose response curves. Isobologram analysis provided the range of concentration ratios where maximal synergy was obtained. The combination index analysis readily provided quantitative estimation of the extent of synergy or antagonism. The universal surface response method summarized drug-drug interaction in a single parameter, facilitating comparison of larger arrays of combinations. Only the curve shift analysis and the universal surface response method yielded a statistical estimate of differentiation between synergy, additivity, and antagonism. In summary, curve shift analysis, isobolograms, combination index analysis, and the universal response surface method are useful methods for analyzing drug-drug interaction, and provide complementary information.

Figure 1. Curve shift analyses

The experimental combination concentrations were normalized to IC₅₀-equivalents of single agents. Data were analyzed using nonlinear regression without weighting. The data points are mean values of five replicates. The lines are best-fitted regressed lines. A leftward shift of concentration-effect curves for combinations when compared to single agent curves indicates synergism, and a rightward shift indicates antagonism. T:A indicates trimetrexate-to-AG2034 ratios in their molar concentration. Experiment with ratio of T:A=1:50 has been performed twice; the second experiment is labeled T:A=1:50 repeat. Note that the legend gives the molar concentration ratios of the trimetrexate:AG2034 mixtures. However, the X axis (logarithmic scale) is the total concentration of trimetrexate plus AG2034 expressed in IC_{50, equivalents} as calculated by Eq. 4.

Figure 2. Isobologram analysis

The diagonal line is the line of additivity. Experimental data points, represented by dots, located below, on, or above the line indicate synergy, additivity, or antagonism, respectively.

Figure 3. Combination index curve analysis

CI values less than, equal to, or greater than 1 indicates synergy, additivity, or antagonism, respectively. The horizontal line at Combination Index=1 is the line of additivity. T:A ratio indicates trimetrexate to AG2034 ratio in their molar concentrations.