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

Abstract

Ester prodrugs of aspirin 1a, 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 (D 7.4) 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.