Objective: N-acetyltransferase 2 (NAT2) genotype-phenotype relation with sulfasalazine as probe drug by means of detailed genotype analysis and kinetic data evaluation.
Background: Though phenotype analysis of sulfasalazine metabolism has been described before, genotype investigations in this regard are scarce. The influence of different single point mutations on the metabolism of the sulfasalazine metabolite sulfapyridine (SP) should give more insight into the functionality of different alleles especially with those still under discussion.
Methods: In two bioavailability studies performed under comparable conditions with 24 healthy subjects of both genders equally distributed, plasma levels of SP and acetylsulfapyridine (Ac-SP) were determined after oral intake of enteric coated formulations of sulfasalazine (500 mg and 1,000 mg, respectively). The resulting metabolic ratios were calculated. NAT2 genotype was analyzed in parallel for all subjects deducing haplotype set as well as putative functional phenotype as (homozygous or heterozygous) rapid acetylator (RA) or slow acetylator (SA) and correlated with the PK results.
Results and discussion: RA genotype in the overall study population was seen with 45.5% (including 6.8% homozygous wildtype *4/*4) and SA genotype with 54.5%. Compared to RA genotype, apparent terminal elimination half-life of SP as well as of Ac-SP was prolonged in the SA genotype population, C(max) and AUC values of SP were higher whereas average C(max) value of Ac-SP was lower (with AUC only some tendency to lower values). In general, phenotype-genotype correlation was good with only few exceptions. Strongest functional effect on enzyme activity was noticed in slow acetylators carrying the 341T > C mutation, followed by 590G > A mutation whereas the influence of 857G > A was considerably less pronounced. Homozygous 803A > G mutation (lysine > arginine shift) did not reveal enzyme activity reduction.