Polymyxin formulations

Although tyrothricia is too toxic for parenteral therapy, it was formerly sold in the United States as oral lo2enges. Modem tyrothricin formulations are composed of 70—80% tyrocidines and 30—20% linear gramicidins. Tyrocidines are not as active as linear gramicidins and are too toxic for any therapeutic use by themselves. The bactericidal linear gramicidins are used in the United States solely as an ophthalmic solution in combination with polymyxin B sulfate and neomycin sulfate. The linear gramicidin is used in this aqueous product as a substitute for bacitracin, which lacks stabiUty under such conditions. 

Table 2 shows the results obtained for four-hour and 24-hour exposure to AmB. LC-AmB had an IC50 above lOOmg/L after 24-hour exposure. This indicates that it has very low toxicity, comparable to the commercial formulation AmBisome (9). The toxicity increased with the time of exposure for all formulations (after a 48-hour exposure, the IC50 of LC-AmB was 86mg/L, data not shown). Very similar results were obtained in the presence of polymyxin B, eliminating the possibility that the toxicity was due to the contamination of the formulations with lipopolysaccharide (LPS) (22). 

Neomycin is available in numerous topical formulations, both 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. 

Colistin is used primarily for oral treatment of E. coli infections in calves and pigs. Commercial formulations of colistin consist of two main components colistin A and colistin B. Tire ratio between these two components is not constant, while small amounts of other related components are also present as minor constituents. Both polymyxin B and colistin have been frequently used orally in combination with bacitracin or neomycin. 

The ototopical antimicrobial preparations stated earlier suffice for most cases of otitis externa and selected cases of chronic suppurative otitis. However, these compounds have a limited effect in certain patients with resistant strains of bacteria, drug-induced allergies, or a tympanic membrane perforation that requires administration into the middle ear space. In the last case, ototopical preparations may cause pain because of the acidic pH or the presence of alcohol. Ototoxicity of neomycin, polymyxin B, and colistin is also of concern, and many otolaryngologists prefer topical ophthalmic preparations.f Ophthalmic preparations are discussed in the article Ocular Drug Formulation and Delivery in this volume. 

Neomycin is not usually administered parenter-ally to animals because of nephrotoxicity and ototoxicity. Only 3% of a dose of neomycin is absorbed following p.o. administration it is, therefore, used in the therapy of coliform enteritis in small and large animals. It is available as tablets, boluses and water additives, in many different combinations with antibiotics, corticosteroids and anticholinergic agents. It can also be used to decrease nitrogenous waste production by the normal gastrointestinal flora in animals with hepatic encephalopathy. Neomycin is not absorbed through the skin, so it is frequently utilized as the antibacterial constituent in ophthalmic formulations (especially in combination with bacitracin and polymyxin B) and in preparations for the treatment of otitis externa in small animals. 

Neosporin [Pfizer], Proprietary formulation of polymyxin B sulfate, neomycin sulfate, and gramicidin. 

The bacitracin zinc and polymyxin B sulfate ophthalmic ointment USP is a sterile antimicrobial ointment formulated for ophthalmic use. Bacitracin zinc is the zinc salt of bacitracin, a mixture of related cyclic polypeptides (mainly bacitracin A) produced by the growth of an organism of the lichenlformis group of Bacillus subtilis var Tracy. It has a potency of not less than 40 bacitracin units per milligram. Polymyxin B sulfate is the sulfate salt of... 

The molecular weight of polymyxin B was shown by the method of partial substitution to be 1,150 d 10 per cent. Quantitative amino acid analysis yielded the amino acids a,y-diaminobutyric acid, L-threonine, D-phenylalanine and L-leucine in the molar proportions 6 2 1 1. Because no free a-carboxyl- and no free a-amino groups could be detected, polymyxin Bi had to be of a cyclic nature . Neither pepsin nor trypsin was found to attack the molecule and therefore partial hydrolysis and separation of the fragments was used for the elucidation of the structure. Of the 14 fragments isolated and identified, seven key peptides were necessary for proposing four tentative structures for polymyxin Two were formulated with a ring... 

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