Morpholinoalkyl ester prodrugs of diclofenac: synthesis, in vitro and in vivo evaluation.

Morpholinoalkyl esters (HCl salts) of diclofenac (1) were synthesized and evaluated in vitro and in vivo for their potential use as prodrugs for oral delivery. Prodrugs were freely soluble in simulated gastric fluid (SGF) and pH 7.4 phosphate buffer and showed a minimum of a 2000-fold increase in solubility over the parent drug. All prodrugs were more lipophilic than 1 as indicated by n-octanol/pH 7.4 buffer partition coefficients, but less lipophilic than 1 in terms of n-octanol/SGF partition coefficients. Potentiometrically determined ionization constants (pKas) were in the range of 7.52 to 8.40 at 25 degrees C. The chemical and enzymatic hydrolyses of prodrugs were evaluated in SGF/pH 7.4 phosphate buffer and rat plasma, respectively, at 37 degrees C. All prodrugs were quantitatively hydrolyzed to 1 by either chemical and/or enzymatic means. An increase in carbon chain length rendered the prodrugs more stable at pH 7.4, but less stable in SGF. In general, the esters were hydrolyzed rapidly in rat plasma at 37 degrees C, the half-lives of hydrolysis being in the range of 4.85 to 23.49 min. Based on in vitro results, prodrug 2 was chosen to evaluate solid-state stability, bioavailability, and in vivo ulcerogenicity. At elevated temperatures, the solid-state decomposition of 2 followed biphasic kinetics, with rapid decomposition occurring initially. The extent, but not the rate, of absorption was significantly greater in rats for prodrug 2 than 1 following single dose oral administration. Prodrug 2 was significantly less irritating to gastric mucosa than 1 following single and chronic oral administration in rats.