Toxicity of antibiotics and antifungals on cultured human corneal cells: effect of mixing, exposure and concentration.

Toxic effects of topical drugs may be masked by manifestations of the disease they cure. The toxicity of drug mixtures has not been thoroughly studied. We therefore investigated cytopathic effects on primary cultures of human corneal cells of six topical antimicrobials singly and in combinations of any two, to determine the combined toxicity ranking and the interaction between duration of exposure and concentration. Preconfluent cultures were exposed to fixed dilutions of single drugs, or to equal-dilution mixtures of two drugs, for 7 and 14 days. Diminishing concentrations of single drugs were applied sequentially to cultures for 14 days. The number of metabolically competent cells was assessed by measuring hexosaminidase and total protein. Toxic effects depended on substance, concentration and exposure. The scale of toxicity determined for single drugs after 7 days of exposure was: gentamicin > econazole > or = methicillin > or = clotrimazole > or = miconazole > or = chloramphenicol. After 14 days this order changed: in particular chloramphenicol showed a highly increased toxicity. The order of diminishing effects was: gentamicin > chloramphenicol > or = methicillin > miconazole > econazole > clotrimazole. A clear reduction in cytopathic effects was observed when drug concentration was decreased progressively only in cultures treated with gentamicin or methicillin. All drug combinations were more toxic than their components at equal dilution. Combinations containing chloramphenicol ranked most toxic overall, those containing econazole least. A tapering off combination regime did not improve cell survival. These in vitro toxicity data complement clinical studies and suggest ways in which topical drugs can be chosen to minimise toxic effects to corneal surface.