10 grams (0.0115 moles) of tilmicosin and 0.0253 moles of various carboxylic acids (such as, for example, decanoic, lauric, linoleic, or myristic acids, in individual assays) were taken in a flask and made up to a final volume of 100 ml with N-methyl-pyrrolidone and stirred for 60 minutes to obtain a clear solution. One ml aliquots of these solutions were sealed in dialysis bags, and the bags were suspended in flasks containing 150 ml of phosphate-buffered saline, pH 7.4. The salt was observed to precipitate in the bag within about 1 hour. Aliquots of saline were removed at various intervals and tilmicosin was determined by HPLC. The results with decanoic acid (C-10) and lauric acid (C-12) salts in FIG. 3 show that tilmicosin continued to diffuse out of the bag for more than 120 hours. Longer chain length acids caused a slower release of tilmicosin. Micotil® (Eli Lilly, Indianapolis, Ind.), a phosphate salt of tilmicosin, is readily soluble and rapidly diffuses from the bag.
