Development of a bio-relevant pH gradient dissolution method for a high-dose, weakly acidic drug, its optimization...

Magnesium orotate dihydrate (MOD) is a weakly acidic drug (pKa 2.83) which belongs to Biopharmaceutical Classification System (BCS) Class I at a dose of 500 mg and to BCS Class II at a dose of 1,000 mg. It is clinically prescribed at a dose of 3,000 mg (in two to three divided doses). The aim of the present study was to develop a bio-relevant pH gradient dissolution method for MOD in order to evaluate whether its clinically practiced therapeutic dose may be absorbed or not. The developed method revealed that MOD undergoes slow, but complete dissolution within 180 minutes, corresponding to the time to achieve maximum serum concentration (Tmax) in vivo. Optimization studies revealed that a rotational speed of 75 rpm provided reliable results (relative standard deviation of less than 20% up to a 10-minute time point, and less than 10% for the other time points), and MOD underwent complete dissolution within the testing timeframe at this rotational speed. Based on a pharmacokinetics study and the Wagner Nelson method, the relative extent of MOD absorption, when administered at a high dose equivalent to a human dose of 1,524 mg in Wistar rats in comparison to its oral suspension, was greater than 90%. In vitro – in vivo correlation, established through a deconvolution method, showed excellent correlation between percent of drug dissolved and percent of drug absorbed (R² = 0.9303). Therefore, even when MOD is administered at a single high dose, it can undergo slow but complete dissolution and absorption in vivo.