Human disposition, metabolism and excretion of etamicastat, a reversible, peripherally selective dopamine β-hydroxylase inhibitor

Case Study 10: A Case to Investigate Acetyl Transferase Kinetics

Major routes of metabolism for marketed drugs are predominately driven by enzyme families such as cytochromes P450 and UDP-glucuronosyltransferases. Less studied conjugative enzymes, like N-acetyltransferases (NATs), are commonly associated with detoxification pathways. However, in the clinic, the high occurrence of NAT polymorphism that leads to slow and fast acetylator phenotypes in patient populations has been linked to toxicity for a multitude of drugs. A key example of this is the observed clinical toxicity in patients who exhibit the slow acetylator phenotype and were treated with isoniazid. Toxicity in patients has led to detailed characterization of the two NAT isoforms and their polymorphic genotypes. Investigation in recombinant enzymes, genotyped hepatocytes, and in vivo transgenic models coupled with acetylator status-driven clinical studies have helped understand the role of NATs in drug development, clinical study design and outcomes, and potential roles in human disease models. The selected case studies herein document NAT enzyme kinetics to explore substrate overlap from two human isoforms, preclinical species considerations, and clinical genotype population concerns.

Glucuronidation: driving factors and their impact on glucuronide disposition

Glucuronidation is a well-recognized phase II metabolic pathway for a variety of chemicals including drugs and endogenous substances. Although it is usually the secondary metabolic pathway for a compound preceded by phase I hydroxylation, glucuronidation alone could serve as the dominant metabolic pathway for many compounds, including some with high aqueous solubility. Glucuronidation involves the metabolism of parent compound by UDP-glucuronosyltransferases (UGTs) into hydrophilic and negatively charged glucuronides that cannot exit the cell without the aid of efflux transporters. Therefore, elimination of parent compound via glucuronidation in a metabolic active cell is controlled by two driving forces: the formation of glucuronides by UGT enzymes and the (polarized) excretion of these glucuronides by efflux transporters located on the cell surfaces in various drug disposition organs. Contrary to the common assumption that the glucuronides reaching the systemic circulation were destined for urinary excretion, recent evidences suggest that hepatocytes are capable of highly efficient biliary clearance of the gut-generated glucuronides. Furthermore, the biliary- and enteric-eliminated glucuronides participate into recycling schemes involving intestinal microbes, which often prolong their local and systemic exposure, albeit at low systemic concentrations. Taken together, these recent research advances indicate that although UGT determines the rate and extent of glucuronide generation, the efflux and uptake transporters determine the distribution of these glucuronides into blood and then to various organs for elimination. Recycling schemes impact the apparent plasma half-life of parent compounds and their glucuronides that reach intestinal lumen, in addition to prolonging their gut and colon exposure.

Distribution and pharmacokinetics of etamicastat and its N-acetylated metabolite (BIA 5-961) in dog and monkey

1. The disposition etamicastat was evaluated in the Cynomolgus monkey after intravenous and oral administration of [(14)C]-etamicastat. The pharmacokinetics of etamicastat and its N-acetylated metabolite BIA 5-961 were also evaluated in monkeys and dogs. 2. In the monkey, 7 days after intravenous and oral administration of [(14)C]-etamicastat, 76.6-91.1% of the etamicastat-related radioactivity had been excreted mainly in urine. The radioactivity peaked in plasma between 4- and 8-h post-dosing followed by a quick decline and a slow terminal phase (half-life of 68.7 h). The calculated oral bioavailability for etamicastat was 46.1%. Etamicastat was quickly absorbed in monkeys and dogs with a half-life ranging from 5.2 to 9.9 h in monkeys and 6.9 to 11.4 h in dogs over. 3. The N-acetylated metabolite of etamicastat, represented 4-7% of the extent of exposure of etamicastat in the monkey, but was not found detectable in dogs. Gender did not influence etamicastat exposure and the concentration versus time curves fitted a dose-dependent pharmacokinetics in the dog, but not in the monkey. 4. In conclusion, etamicastat is rapidly absorbed and primarily excreted via urine in monkeys. Similarly, to humans, monkeys, unlike dogs, N-acetylate etamicastat and evidence that etamicastat pharmacokinetics is less than dose proportional.