Evaluation of synergism between the aminoglycoside antibiotic streptomycin and the cardiovascular agent amlodipine

Amlodipine, a cardiovascular drug, exhibited remarkable antibacterial action in vitro against 504 bacterial strains belonging to both Gram positive and Gram negative genera, as well as in vivo against a mouse-virulent bacterium. Based on such findings, the present study was undertaken to determine whether the efficacy of this non-antibiotic drug could be enhanced in the presence of any antibiotic. Twelve bacterial strains, sensitive to amlodipine as well as to 6 antibiotics, viz., benzyl penicillin, streptomycin, chloramphenicol, tetracycline, erythromycin and ciprofloxacin were chosen. Disc diffusion test with amlodipine and streptomycin revealed marked synergism between the combination, compared with their individual effects. The synergism was found to be statistically significant (p<0.01). To assess the degree of synergy, the checkerboard analysis was performed. The fractional inhibitory concentration (FIC) index of this combination turned out to be 0.24, which confirmed synergism. This antibiotic-non-antibiotic pair was then administered to mice, challenged with S. typhimurium to determine whether this was effective in vivo. Statistical analysis of the mouse protection tests suggested that the combination was highly synergistic (p<0.001), according to Student's t-test. This synergistic drug combination may help us in enhancing the scope of prolonged antibiotic therapy in various types of infections, and might open a new therapeutic approach to combat drug resistance in bacterial diseases.