Stability of gentamicin sulfate injection following unit dose repackaging

The potency of gentamicin sulfate following repackaging in disposable syringes were determined by a modification of the agar diffusion bioassay, using a highly gentamicin-resistant strain of Psuedomonas aeruginosa to assay milligram quantities of active drug. Three factors were studied at 30, 60 and 90 days: (1) syringe character (glass vs plastic); (2) temperature (25 C vs 4 C); and (3) volume of aspirated sample. Glass syringes were superior at all time intervals. Storage in plastic for even 30 days resulted in unacceptable loss of potency (greater than 15%) and formation of a brown precipitate. Mass spectrophotometry indicated that this precipitate consisted mainly of esters of phthalic acid (plasticizer) and methylparaben (gentamicin preservative). Temperature had no effect on retained potency. A positive correlation between the percent potency lost and volume of gentamicin aspirated was demonstrated in both glass and plastic.