Polymyxin B is a peptide antibiotic effective against Gram-negative organisms, including Pseudomonas aeruginosa, Escherichia coli, Enterobacter, and Klebsiella. Most strains of Proteus and Serratia are resistant, as are all Gram-positive organisms. Topical preparations may be compounded in either a solution or ointment base. Numerous prepackaged antibiotic combinations contain polymyxin B. Detectable serum concentrations are difficult to achieve from topical application, but the total daily dose applied to denuded skin or open wounds should not exceed 200 mg in order to reduce the likelihood of neurotoxicity and nephrotoxicity. Allergic contact dermatitis to topically applied polymyxin B sulfate is uncommon.
Neomycin and gentamicin are aminoglycoside antibiotics active against Gram-negative organisms, including E coli, Proteus, Klebsiella, and Enterobacter. Gentamicin generally shows greater activity against P aeruginosa than neomycin. Gentamicin is also more active against staphylococci and group A β-hemolytic streptococci. Widespread topical use of gentamicin, especially in a hospital environment, should be avoided to slow the appearance of gentamicin-resistant organisms.
Neomycin is available in numerous topical formulations, alone and in combination with polymyxin, bacitracin, and other antibiotics. It is also available as a sterile powder for topical use. Gentamicin is available as an ointment or cream.
Topical application of neomycin rarely results in detectable serum concentrations. However, in the case of gentamicin, serum concentrations of 1–18 mcg/mL are possible if the drug is applied in a water-miscible preparation to large areas of denuded skin, as in burned patients. Both drugs are water-soluble and are excreted primarily in the urine. Renal failure may permit the accumulation of these antibiotics, with possible nephrotoxicity, neurotoxicity, and ototoxicity.
Neomycin frequently causes allergic contact dermatitis, particularly if applied to eczematous dermatoses or if compounded in an ointment vehicle. When sensitization occurs, cross-sensitivity to streptomycin, kanamycin, paromomycin, and gentamicin is possible.
Systemic antibiotics traditionally used in the treatment of acne vulgaris have been shown effective when applied topically. Currently, two antibiotics are used topically for this indication: clindamycin phosphate and erythromycin base. Effectiveness of topical therapy is less than that achieved by its oral administration. Therefore, topical therapy is generally suitable only in mild to moderate cases of inflammatory acne.
Clindamycin has in vitro activity against Propionibacterium acnes; this has been postulated as the mechanism of its beneficial effect in acne therapy. Approximately 10% of an applied dose is absorbed, and rare cases of bloody diarrhea and pseudomembranous colitis have been reported following topical application.
The hydroalcoholic vehicle and foam formulation (Evoclin) may cause drying and irritation of the skin, with complaints of burning and stinging. The water-based gel and lotion formulations are well tolerated and less likely to cause irritation. Allergic contact dermatitis is uncommon. Clindamycin is also available in fixed-combination topical gels with benzoyl peroxide (Acanya, BenzaClin, Duac, Onexton) and with tretinoin (Velitin, Ziana).