| Sumario: | Purpose: Calcium L-threonate is a novel drug that was developed for the treatment of osteoporosis and as a calcium supplement. However, calcium bioavailability of this drug is unknown due to lack of effective evaluation methods. In this study, we sought to measure the bioavailability of calcium L-threonate with a double-label stable isotope method. Methods: Fourteen healthy Chinese subjects were enrolled in the clinical study and were given 300 mg calcium L-threonate tablets containing 40 mg Ca after an intravenous injection of 4 mg Ca solution (as calcium chloride). Fractional urine samples were collected at the following time intervals: 0-3, 3-6, 6-9, 9-13, 13-24, 24-36 and 36-48 h. The abundance ratios of Ca/Ca and Ca/Ca in the urine were determined with thermal-ionization mass spectrometry (TI-MS). The calcium bioavailability was estimated by calculating the true fractional calcium absorption (TFCA) using the abundance ratios of Ca/Ca and Ca/Ca. Results: The bioavailability of calcium L-threonate in 14 healthy Chinese subjects was 26.49 ± 9.39 %. There was good agreement between TFCA from the 24 to 36 h and the 36 to 48 h urine pool, indicating that calcium balance was achieved at 24 h after dosing. The TFCA of the subjects did not statistically correlate with total urinary calcium excretion (0-48 h). There were no serious adverse events in this study. Conclusions: The bioavailability of calcium L-threonate in humans was successfully determined by estimating TFCA with the double-label stable isotope method, thus providing a useful approach for the evaluation of bioavailability of calcium formulations.
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