Metabolism by N‐Acetyltransferase 1 In Vitro and in Healthy Volunteers: A Prototype for Targeted Inhibition

Abstract

Inhibition of drug metabolism is generally avoided but can be useful in limited circumstances, such as reducing the formation of toxic metabolites. Acetylation is a major pathway for drug elimination that can also convert substrates into toxic species, including carcinogens. Sulfamethoxazole, a widely used antibiotic, is metabolized via arylamine N-acetyltransferase 1. p-Aminosalicylate, used for antitubercular treatment, is also metabolized by N-acetyltransferase 1 and could potentially inhibit sulfamethoxazole metabolism. Human hepatocytes from 4 donors were incubated in vitro with sulfamethoxazole and paminosalicylate at clinically achievable concentrations. p-Aminosalicylate competitively reduced the acetylation of sulfamethoxazole in vitro by 61% to 83% at 200 microM. Four healthy volunteers were studied following doses of 500 mg sulfamethoxazole either alone or during administration of paminosalicylate (4 g ter in die). Plasma concentrations of paminosalicylate exceeded 100 microM. With each subject as his or her own control, p-aminosalicylate reduced by 5-fold the ratio of plasma concentrations of acetylsulfamethoxazole relative to parent drug (P < .001). Metabolic drug-drug interaction studies in vitro successfully predicted inhibition of acetylation via N-acetyltransferase 1 in vivo. Although no specific toxic species was investigated in this work, the potential was demonstrated for improving the therapeutic index of drugs that have toxic metabolites.

Effect of MMX&reg; mesalamine coadministration on the pharmacokinetics of amoxicillin, ciprofloxacin XR, metronidazole, and sulfamethoxazole: results from four randomized clinical trials

“…After oral administration, sulfamethoxazole is conjugated in the liver by NAT-1 and NAT-230,31 to the inactive N4-acetyl metabolite, which represents approximately 15% of circulating drug-related material in the blood 32. Approximately 80%–100% of administered sulfamethoxazole is rapidly excreted in urine by glomerular filtration and active tubular secretion,31,60 with 60% of the drug secreted as the N4-acetyl metabolite, and the remainder as unchanged drug or glucuronide 32.…”

“…Metronidazole is mainly oxidatively metabolized by cytochrome P450 to hydroxymetronidazole,28 with CYP2A6 as the principal enzyme involved 29. Sulfamethoxazole is conjugated in the liver by N-acetyltransferase (NAT)-1 and NAT-230,31 to its N4-acetyl metabolite 32. MMX mesalamine is not subject to oxidative metabolism, and in human microsomal incubations (IC 50 [half maximal inhibitory concentration] >100 μM), it does not appear to inhibit cytochrome P450 activity 33.…”

Reactive Intermediates and the Pathogenesis of Adverse Drug Reactions: The Toxicology Perspective

“…Because of their short half-life and high reactivity, detection of reactive metabolites via in vitro methods is particularly useful and includes GSH trapping and covalent protein binding. Generalized descriptions of some methods currently available for the in vitro Hydroxylamine metabolites further oxidized to nitroso-sulfamethoxazole [155][156][157][158][159][160][161] Terbinafine…”