The solubility of antibiotic and corticosteroid combinations

The use of collagen shields soaked in various combinations of medications has been advocated to enhance drug delivery to the cornea. Recently, severe corneal toxicity associated with aggregate formation in mixtures of gentamicin and methylprednisolone prompted our study of the effect of drug concentration, pH, and temperature on the solubility of several antibiotic and corticosteroid formulations commonly used to treat ocular disease. Selected combinations of cefazolin, vancomycin, tobramycin, gentamicin, methylprednisolone, and dexamethasone were evaluated. Mixtures of tobramycin and vancomycin produced no precipitates, but many spheroid aggregates were seen when methylprednisolone and gentamicin were combined. Although the effects of precipitate formation on drug bioavailability and toxicity have not been fully determined, until such information is available, the use of combinations of drugs that remain in solution during administration is recommended.