Neomycin is an aminoglycoside compound that is relatively toxic and therefore not used systemically. Neomycin was first described by Waksman and Lechevalier in 1949 as a product of Streptomyces fradiae. Three chemically similar constituents have been described, neomycin A, B, and C. Purification of a similar compound was performed at Roussell Laboratories, after description by Decaris in France, and was called framycetin. Later, it appeared that framycetin was identical to neomycin B sulfate, and references in this chapter to neomycin thus include framycetin. Marketed formulations may contain various mixtures of the compounds or just one of the three forms, as is the case with framycetin-containing formulations. Like other aminoglycosides, neomycin exerts its action by inhibiting protein synthesis. The drug is used in various topical forms only and should never be given parenterally because of demonstrated toxicity.

PHARMACOKINETICS

Neomycin is poorly absorbed, and therefore systemic availability is poor. However, there is some resorption. A single oral dose of 4 g neomycin may produce a peak serum level of about 4.0 mg/ml . Enema-administered neomycin is absorbed to about the same extent. As neomycin is now given only topically, local concentrations are relatively high, and the concentration–time profile varies by location and formulation. Not much is known on the pharmacodynamics locally, and dosing is primarily by clinical experience.

TOXICITY

Neomycin is toxic when given systemically. Although the drug was available as a parenteral formulation in the past, it should not be given in its parenteral form as it causes irreversible ototoxicity. Animal studies have also demonstrated that neomycin is the most nephrotoxic of the commonly used aminoglycosides, and renal toxicity in animals can even be demonstrated after very large oral doses of the drug.