Comparative serum levels following oral administration of calcium acetate tablet formulations

A bioequivalence study was performed between two brands of calcium acetate tablets, Phoslo? and Procal?, in healthy Chinese volunteers. The in vivo concentrations of calcium were determined by atomic absorbance spectrophotometry (AA) at 422.7 nm. The intra- and inter-day mean values were within ± 10 % of the actual values. Good linearity and reproducibility of the AA analysis of calcium were obtained. Since calcium is an endogenous substance, the amount of calcium excreted in urine was used as an index throughout the study. Subjects were randomly assigned to receive a single 667 mg dose of, Phoslo? or Procal? calcium acetate tablets after controlling their diet for three days with overnight fasting on the last control day. The mean control levels of calcium from days 1 to 3 were 104.62 mg and 105.41 mg for Phoslo? and Procal?, respectively. The mean increase in the 24 hour calcium level was 74.59 ± 10.69 mg for Phoslo? and 68.10 ± 30.12 mg for Procal?. These values corresponded to 44.1 % and 40.3 % recovery from the 169 mg administered dose of the two brands of calcium acetate tablets. There was a maximum excretion rate of calcium after one dose was given. a The mean time corresponding to the maximum excretion rate (T(max)) for Phoslo? and Procal? were 78.17 ± 5.23 and 77.53 ± 5.48 hours, respectively. No significant difference in T(max), cumulative excreted amount, and net excreted amount, obtained from the urine data after dosing, were observed between the two brands of calcium acetate tablets according to ANOVA analysis. The net excretion rate abruptly increased after calcium acetate administration as reflected in the T(max) value. However, the large variation of the confidence interval and low power value observed for excreted calcium indicated a large inter-subject variation. Careful control of diet, fluid intake, physical activity, and urine collection should be strictly obeyed by the volunteers throughout the study.