Novel oxidative pathways of sulphapyridine and sulphadiazine by the turtle Pseudemys scripta elegans

Identification of cytochrome P450 and arylamine N-acetyltransferase isoforms involved in sulfadiazine metabolism

Sulfadiazine hydroxylamine has been postulated to be the mediator of the greatly increased rates of adverse reactions to sulfadiazine experienced by people with human immunodeficiency virus infection. Therefore, we investigated the in vitro human cytochrome P450 (P450) and N-arylamine acetyltransferase (detoxification) metabolism of sulfadiazine. Formation of both the hydroxylamine and 4-hydroxy sulfadiazine was NADPH-dependent in human liver microsomes (HLM). The average K(m) (+/-S.D.) and V(max) in HLM (n = 3) for hydroxylamine formation was 5.7 +/- 2.2 mM and 185 +/- 142 pmol/min/mg, respectively. Significant (p < 0.05) inhibition by selective P450 isoform inhibitor sulfaphenazole (2.1 microM; CYP2C9) indicated a role for CYP2C9 in the formation of the hydroxylamine. Hydroxylamine formation correlated strongly with tolbutamide 4-hydroxylation (CYP2C8/9) in HLM (r = 0.76, p < or = 0.004, n = 12). Fluconazole (CYP2C9/19 and CYP3A4 inhibitor at clinical concentrations) inhibited hydroxylamine formation, with one-enzyme model K(i) estimates ranging from 9 to 40 microM. Acetylation of sulfadiazine in human liver cytosol (HLC) correlated strongly with NAT2 activity as measured by sulfamethazine N-acetylation (r = 0.92, p < 0.001, n = 12). The average K(m) (+/-S.D.) and V(max) in HLC (n = 3) was 3.1 +/- 1.7 mM and 221.8 +/- 132.3 pmol/min/mg, respectively. The polymorphic acetylation of sulfadiazine may predispose slow acetylator patients to adverse reactions to sulfadiazine. On the basis of our K(i) estimates, clinical fluconazole concentrations of 25 microM would produce decreases of 40 to 70% in hepatic-mediated hydroxylamine production. Therefore, we predict that fluconazole may prove useful in the clinic as an in vivo inhibitor of sulfadiazine hydroxylamine formation to suppress adverse reactions to this drug.

Isolation, identification and determination of sulfadiazine and its hydroxy metabolites and conjugates from man and rhesus monkey by high-performance liquid chromatography

The following metabolites of sulfadiazine (S) were isolated from monkey urine by preparative HPLC: 5-hydroxysulfadiazine (5OH), 4-hydroxysulfadiazine (4OH) and the glucuronide (5OHgluc) and sulfate conjugate of 5OH (5OHsulf). The compounds were identified by NMR, mass and infrared spectrometry and hydrolysis by beta-glucuronidase. The analysis of S, the hydroxymetabolites (4OH, 5OH) and conjugates N4-acetylsulfadiazine (N4), 5OHgluc and 5OHsulf in human and monkey plasma and urine samples was performed using reversed-phase gradient HPLC with UV detection. In plasma, S and N4 could be detected in high concentrations, whereas the other metabolites were present in only minute concentrations. In urine, S, the metabolites and conjugates were present. The limit of quantification of the compounds in plasma varies between 0.2 and 0.6 microgram/ml (S 0.31, N4 0.40, 4OH 0.20, 5OH 0.37, 5OHgluc 0.33 and 5OHsulf 0.57 microgram/ml). In urine it varies between 0.6 and 1.1 micrograms/ml (S 0.75, N4 0.80, 4OH 0.60, 5OH 0.80, 5OHgluc 0.80 and 5OHsulf 1.1 micrograms/ml). The method was applied to studies with healthy human subjects and Rhesus monkeys. The metabolites 5OH, 5OHgluc and 5OHsulf were present in Rhesus monkey and not in man. Preliminary results of studies of metabolism and pharmacokinetics in Rhesus monkey and man are presented.

In vitro effects of sulfadiazine and its metabolites alone and in combination with pyrimethamine on Toxoplasma gondii

Sulfadiazine and the metabolites N4-acetyl-sulfadiazine, 4-OH-sulfadiazine, 5-OH-sulfadiazine, 5-OH-glucuronide-sulfadiazine, and 5-OH-sulfate-sulfadiazine were tested separately and in combination with pyrimethamine for the inhibitory activity on Toxoplasma gondii growth in vitro. Except for N4-acetyl-sulfadiazine, all sulfa compounds possessed anti-Toxoplasma activity. The addition of 0.05 micrograms of pyrimethamine per ml, a concentration which in itself is not inhibitory, potentiated the microbial activity of sulfadiazine and its metabolites 100 fold.

Comparison of the metabolism of four sulphonamides between humans and pigs

Pigs are unable to form N1-glucuronides of sulphadimethoxine and sulphamethomidine, while humans are able to do so. Pigs and humans are able to oxidise sulphapyridine and form the O-glucuronide. The double conjugate N4-acetylsulphapyridine-O-glucuronide is formed in humans but not in pigs. Sulphadiazine is mainly acetylated in both humans and pigs. A hypothesis about N1-glucuronidation is presented.