The effect of temperature and pH on the deacetylation of diamorphine in aqueous solution and in human plasma.

The effect of temperature on the kinetics of the deacetylation of diamorphine and 6-monoacetylmorphine was studied in human plasma. Diamorphine was rapidly and quantitatively degraded to 6-monoacetylmorphine with initial half-lives of 354, 18 and 3 min at temperatures of 4, 25 and 37 degrees C, respectively. Further deacetylation to morphine was not detected. In aqueous solution, diamorphine was quantitatively degraded to give 6-monoacetylmorphine as the major product and morphine as a minor product, the rate of deacetylation being dependent on temperature and pH. At pH 4.0 and 5.6 diamorphine had a half-life of greater than 14 days at all temperatures but at alkaline pH diamorphine was rapidly deacetylated. The rate of deacetylation of 6-monoacetylmorphine was consistently slower than that of diamorphine under identical conditions of pH and temperature. A method is described for the rapid stabilization and subsequent assay of diamorphine in plasma which will prevent errors in estimation of the drug due to unwanted hydrolysis.