Rapid method for the detection of synergism in combinations of antiviral substances

The plaque inhibition method was modified in order to evaluate the effectiveness of various combinations of antiviral substances. One substance (A) diffuses from the centre of cell culture, the other (B) is incorporated into the agar overlay at subinhibitory concentration. The inhibitory effect of the combination (A + B) is demonstrated by the increase in size of the inhibitory zone in comparison with the control inhibitory zone produced by the substance A alone. The ratio of the diameter of the inhibitory zone with substance combination (A + B) to the diameter of single drug control zone (substance A) serves as index DI (degree of interaction). Quantitative evaluation of the degree of potentiation using isobolograms showed that DI greater than 1.5 indicate a synergistic effect of the respective combinations. This inexpensive method can serve for rapid selection of suitable combinations out of number of substances. Model experiments were performed with combinations of selected inhibitors of virus replication.