Statistical analysis of the in-vitro action of combination antiviral agents

Similar to the chemotherapy of bacterial infections, with the advanced development of virostatics a combination of antiviral agents is supposed to be introduced into the causal treatment of viral illness. The present studies explain a method for in vitro testing of virostatic combinations in cell cultures. The principle of the method is based on the checkerboard-technique used to test antibiotic combinations. As example the combination of trisodium phosphonoformate with bromovinyl-2'-deoxyuridine, acyclovir or 2-hexadecylglycero-3-phosphocholine was tested against herpes simplex virus, type 1. For evaluation of the test results the so-called reduction dose 50 (RD50) was introduced. The RD50 corresponds to this substance concentration, which reduces alone or in combination with a second virostatic the virus concentration used in the test system to its TCID50. With analogy to the calculation of infectious doses the computation of the RD50 was performed by using the method of Spearman and Kaerber. The calculated values allow the comparability of the antiviral activity of substances and their combinations. Corresponding to testing of antibiotics the further analysis of combinations was carried out by calculation of fractional inhibitory concentration (FIC), synergy factors (SF), and the construction of isobolograms. In this way, indifferent, synergistic and antagonistic effects of substance combination should be determined.