Blood distribution of mycophenolic acid.

RS-61443 (RS) a morpholinoethyl ester of mycophenolic acid (MPA), can be considered a prodrug, as immunosuppressive activity is expressed only after hydrolysis to MPA upon absorption. Little is known about the blood distribution of MPA; such information would have an impact on the medium used for analysis of the drug in clinical trials. This was investigated by spiking whole blood having an initial temperature of either 4 degrees or 22 degrees C with increasing amounts of MPA ranging from 100 to 10,000 micrograms/L. These drug concentrations span the range seen when immunosuppressive doses of the RS are administered. This was followed by incubation of the blood at 37 degrees C for 0-120 min prior to separation of the cells. The drug concentration was measured in the plasma and whole blood fractions by high-performance liquid chromatography. MPA was almost exclusively found in the plasma fraction and did not exhibit any temperature or concentration dependence. The free or unbound fraction of MPA over the same concentration range was determined by ultracentrifugation and demonstrated a concentration dependence ranging from 7.2 to 16.5% of total drug for a concentration range spanning 500-10,000 micrograms/L. The drug was found to be primarily associated with the non-albumin proteins in the plasma. Less than 10% of the drug was found to be bound to lipoproteins. The data suggest that from an analytical standpoint, plasma, rather than whole blood, would be the most suitable medium for analysis because of the higher concentrations of the drug found in this fraction.