Prodrug for bioreductive activation-independent delivery of menahydroquinone-4: Human liver enzymatic activation and its action in warfarin-poisoned human liver

The N,N-dimethyglycine esters of menahydroquinone-4 (1-mono, 1; 4-mono, 2; 1,4-bis, 3) were established in previous reports as prodrugs that could ach ieve the systemic bioreductive activation-independent delivery of menahydro quinone-4 (MKH), the active form of menaquinone-4 (MK-4), in rat. The prese nt study was undertaken to investigate if the prodrugs could undergo cleava ge to parent drug (MKH) by a human tissues enzyme catalyzed hydrolytic path way, the mechanism of the prodrugs for vitamin K-dependent carboxylation in human liver and their action in the warfarin poisoned human liver. The hyd rolysis of the esters was shown to be catalyzed by esterases located in hum an liver but not in human plasma. The susceptibility of the esters to under go human liver esterase hydrolysis was affected by the esterified position: 1 > 2 > 3. By using a human liver microsomal test system, the stimulation of vitamin K-dependent carboxylation with the prodrugs was determined. The prodrug could stimulate the carboxylation activity in the absence of dithio threitol, an artificial activator of the reductive activation pathway of MK -4. The carboxylation activity of the prodrug was strongly inhibited in the presence of eserine, an esterase inhibitor. The prodrug could also stimula te the carboxylase under warfarin-poisoned conditions, where the vitamin K cycle was strongly inhibited. The results confirmed that the prodrug could generate MKH in human liver (active site), and that the resultant MKH could act as a cofactor for the carboxylase without reductive activation process es of MK-4 to MKH. Such bioreductive activation-independent vitamin K-depen dent carboxylation characteristic of the prodrug leads to enhanced pharmaco logical efficacy in the treatment of hypoprothrombinaemia induced in patien ts with coumarin and cephalosporin therapies.