[Precipitation of piritramide and cefazolin. Study of the dependence on concentration and pH]

Drug incompatibility might lead to precipitation with subsequent serious complications, such as transient pulmonary embolism. Recently, incompatibility of the opioid piritramide with cephalosporin antibiotics was described. As both drugs are frequently administered in a perioperative setting, the present study addressed the question whether the precipitation effect depends on the piritramide concentration or on the pH of the solution. Moreover, it was tested whether the precipitate reversibly dissolves in a physiological saline solution.

Piritramide was diluted to the final test concentrations in 0.9 % sodium chloride solution. Precipitation tests were performed by combining 1 ml of the respective piritramide dilution with 1 ml of cefazolin (20 mg/ml) in a syringe. Precipitation was detected by visual inspection as an opaque whitish appearance of the mixture. Each concentration was tested 5 times. The pH values of the tested piritramide concentrations were determined using a 3-point calibrated pH meter. The precipitate formed in 1 ml of cefazolin (20 mg/ml) and 1 ml of piritramide (5 mg/ml) was diluted in 3 ml physiological saline.

The piritramide concentrations 5 mg/ml, 3.75 mg/ml and 3 mg/ml precipitated in the presence of cefazolin (20 mg/ml), while the concentrations 1.875 mg/ml, 1 mg/ml and 0.5 mg/ml did not produce a precipitate. To exclude the possibility that changes in pH of the tested dilutions might be responsible for these findings, the pH values of the piritramide dilutions were measured. The mean pH values of the concentrations 5 mg/ml, 3.75 mg/ml, 3 mg/ml, 1.875 mg/ml and 1 mg/ml did not differ significantly (pH 3.89 ± 0.004, n = 26, tested by ANOVA). However, the mean pH of 0.5 mg/ml was significantly different from the other tested dilutions (pH 3.98 ± 0.02, n = 6; p < 0.01 by ANOVA). After diluting the precipitate of piritramide and cefazolin in physiological saline the whitish precipitate completely dissolved and the resulting solution became clear (n = 5).

The results imply a concentration dependence of the precipitation with cefazolin, while a correlation with pH changes could not be detected. In cases of co-administration of cephalosporins and piritramide, a piritramide concentration of 1 mg/ml seems to be safe and does not form a precipitate. As the precipitate could be reversed by diluting in saline solution it is most likely that a proton switch between the carboxylic acid moiety of cefazolin and the amino group of piritramide causes the precipitation.