Cyclic amide derivatives as potential prodrugs II: N- hydroxymethylsuccinimide- / isatin esters of some NSAIDs as prodrugs with an improved therapeutic index

Ester prodrugs of aspirin la, ibuprofen 1b, naproxen 1c and indomethacin 1d were synthesized using N-Hydroxymethylsuccinimide (HMSI) 3 and N- hydroxymethylisatin (HMIS) 4 as promoieties to reduce their gastrointestinal toxicity and improve bioavailability. Additionally, the kinetics of hydrolysis of the synthesized prodrugs 5a-d and 6a-d were studied at 37°C in non-enzymatic simulated gastric fluid (SGF; hydrochloric acid buffer pH = 1.2); 0.02 M phosphate buffer (pH = 7.4); 80% human plasma and 10% rat liver homogenate. The results indicate higher chemical stability of the ester prodrugs in non-enzymatic SGF (t(1/2) ≃ 6.5-18.6 h) and rapid conversion to the parent drugs in 80% human plasma (t(1/2) ≃ 11.4-235 min) as well as in 10% rat liver homogenates (t(1/2) ≃ 12.0-90.0 min). As a general pattern, the HMSI esters 5a-d revealed higher chemical stability than the corresponding HMIS analogues 6a-d. The pH-rate profile of 5c and 6a indicated maximum stability of the former at pH = 1.2-8.0 and of the latter at pH = 1.2-4.0. The distribution coefficient (D7.4) values of the prodrugs 5a-d, 6a-d and the parent drugs 1a-d in an n-octanol/phosphate buffer (pH =7.4) system indicated enhanced lipophilic properties of the prodrugs. Furthermore, the HMIS ester prodrugs 6a-d are more lipophilic than the corresponding HMSI derivatives 5a-d. In vivo ulcerogenicity studies using scanning electron microscopy on stomach specimens of rats treated with an oral dose for 4 d revealed that the synthesized ester prodrugs are significantly less irritating to gastric mucosa than the parent drugs. These results suggested HMSI and/or HMIS esters possess good potential as prodrugs with an improved therapeutic index for oral delivery of NSAIDs.