Synthesis, chemical and enzymatic hydrolysis, and bioavailability evaluation in rabbits of metronidazole amino acid ester prodrugs with enhanced water solubility.
A series of amino acid esters (3a-e) have been synthesized and evaluated as potential prodrugs of metronidazole with the aim of improving aqueous solubility and therapeutic efficacy. The aqueous solubility and the lipophilicity (expressed as the log P value) of metronidazole and its esters were investigated. In general the prodrugs revealed enhanced water solubility compared with metronidazole. N,N-diethylglycinate hydrochloride (3a) and 4-ethylpiperazinoacetate (3e) derivatives displayed higher aqueous solubility, which exceeded that of the parent drug by factors of approximately 140 and 100, respectively. All the esters revealed lower log P values than metronidazole except for the 4-phenylpiperazinoacetate derivative (3f), which was 6.5-times more lipophilic than metronidazole. The hydrolysis kinetics of the esters were studied in aqueous phosphate buffer (pH 7.4) and 80% human plasma at 37 degrees C. In all cases the hydrolysis followed pseudo-first-order kinetics and resulted in a quantitative reversion to metronidazole as evidenced by HPLC analysis. The prodrugs exhibited adequate chemical stability (half-life, t1/2, 4-16 h) in aqueous phosphate solution of pH 7.4. In 80% human plasma they were hydrolysed within a few minutesto metronidazole. The esters 3d (methylpiperazinoacetate derivative) and 3f were exempted since their t1/2 values were approximately 2.5 and 8.5 h, respectively. A comparative pH-rate profile study of N,N-diethylglycinate hydrochloride (3a) and 4-ethyl-piperazinoacetate (3e) derivatives in aqueous buffer solution over the pH range 2.2-10 was investigated. The results indicated that 3a showed marked stability at pH 2-6 followed by accelerated hydrolysis at pH 7.4. The basic ester 3e was found to be less stable at lower pH values but exhibited comparative stability at physiological pH. Moreover, in-vivo experiments in rabbits revealed a higher metronidazole plasma level with sustained release characteristics within the prodrug-treated animals (10- and 2.5-fold) as compared with the parent drug-treated group. In conclusion, the designed amino acid esters 3a and 3c-e might be considered as good candidates for water-soluble prodrug forms of metronidazole.
Mahfouz
N M
NM
Department of Pharmaceutical Medicinal Chemistry, Faculty of Pharmacy, Assiut University, Egypt. nmahfouz@acc.aun.eun.eg
Hassan
M A
MA
D000596:Amino Acids / Q000737:chemistry*; D000818:Animals; D000890:Anti-Infective Agents / Q000138:chemical synthesis* / Q000493:pharmacokinetics; D001682:Biological Availability; D003692:Delayed-Action Preparations; D015195:Drug Design; D004952:Esters / Q000737:chemistry; D006863:Hydrogen-Ion Concentration; D006868:Hydrolysis; D007700:Kinetics; D008795:Metronidazole / Q000031:analogs & derivatives* / Q000138:chemical synthesis / Q000493:pharmacokinetics; D011355:Prodrugs / Q000138:chemical synthesis* / Q000493:pharmacokinetics; D011817:Rabbits; D012995:Solubility
D000596:Amino Acids; D000890:Anti-Infective Agents; D003692:Delayed-Action Preparations; D004952:Esters; D011355:Prodrugs; D008795:Metronidazole