Cyclic amide derivatives as potential prodrugs. Synthesis and evaluation of N-hydroxymethylphthalimide esters of some non-steroidal anti-inflammatory carboxylic acid drugs

Abstract

N-Hydroxymethylphthalimide (HMPhI) esters 5a-d of some nonsteroidal anti-inflammatory drugs were synthesized and evaluated as potential prodrugs with the aim of depressing the gastrotoxicity of the parent drugs by temporarily masking the carboxylic acid function. The ester prodrugs were synthesized through condensation of N -hydroxymethylphthalimide and the mixed carboxylic-carbonic anhydride intermediate or the corresponding acid chloride of the parent acid. Their structures were confirmed by 1H-NMR spectra and the purity has been assessed by TLC and elemental analyses. An HPLC method has been developed for investigation of the hydrolysis kinetics in aqueous buffer solutions and in 80% rabbit plasma. The lipophilicity parameters log P and log k′ were determined and showed that the prodrugs were found to be more lipophilic (log P > 2) than the parent drugs. A considerable chemical stability of all compounds ( t 1/2 = 4.7–21.9 h) has been observed in non-enzymatic simulated gastric fluid (hydrochloric acid buffer of pH 1.3), while at pH 7.4 only prodrugs 5b-d are sufficiently stable ( t 1/2 ~ 3–5 h). Meanwhile, rapid conversion to the parent drugs 3a-d was observed in 80% rabbit plasma ( t 1/2 ~ 1.0–11.5 min). Potential ulcerogenicity on rat stomach mucosa of the prodrugs and the parent drugs after oral administration for 4 days was studied using a scanning electron microscope. Gross observations and scanning electro-micrographs showed that the prodrugs are significantly less irritating to gastric mucosa than the parent NSAIDs. This result suggests that N-hydroxymethylphthalimide esters may be useful as nonulcerogenic prodrug forms for acidic NSAIDs.