Short-term stability studies of ampicillin and cephalexin in aqueous solution and human plasma: Application of least squares method in Arrhenius equation

A limited number of studies with application of the Arrhenius equation have been reported to drugs and biopharmaceuticals in biological fluids at frozen temperatures. This paper describes stability studies of ampicillin and cephalexin in aqueous solution and human plasma applying the Arrhenius law for determination of adequate temperature and time of storage of these drugs using appropriate statistical analysis. Stability studies of the beta-lactams in human plasma were conducted at temperatures of 20 °C, 2 °C, −20 °C and also during four cycles of freeze-thawing. Chromatographic separation was achieved using a Shimpak C18 column, acetonitrile as organic modifier and detection at 215 nm. LC-UV–MS/MS was used to demonstrate the conversion of ampicillin into two diastereomeric forms of ampicilloic acid. Stability studies demonstrated degradation greater than 10% for ampicillin in human plasma at 20 °C, 2 °C and −20 °C after 15 h, 2.7 days, 11 days and for cephalexin at the same temperatures after 14 h, 3.4 days and 19 days, respectively, and after the fourth cycle of freezing–thawing. The Arrhenius plot showed good prediction for the ideal temperature and time of storage for ampicillin (52 days) and cephalexin (151 days) at a temperature of −40 °C, but statistical analysis (least squares method) must be applied to avoid incorrect extrapolations and estimated values out uncertainty limits.