Acetylator phenotype and serum levels of sulfapyridine in patients with inflammatory bowel disease

Summary Twenty-eight outpatients receiving sulfasalazine for inflammatory bowel disease were monitored. Assessment of acetylator phenotype according to the percentage of acetylated sulfapyridine in serum provided a clear distinction between rapid and slow acetylators. In comparison, the percentage of acetylated sulfapyridine in saliva or urine was a less precise index of phenotype. Determination of saliva concentrations of sulfapyridine and N⁴-acetylsulfapyridine did not provide a reliable estimate of serum levels. Slow acetylators had significantly higher serum concentrations of sulfapyridine (21.9±14.0 [SD] µg/ml) than rapid acetylators (8.8±4.3 µg/ml) and had a higher incidence of toxicity (not statistically significant,p>0.05). Serum concentrations of sulfapyridine were significantly higher in patients with symptoms of toxicity (23.2±15.9 µg/ml) than those without (13.9±9.5 µg/ml) (p<0.05). However, serum concentrations of total sulfapyridine (sulfapyridine plus N⁴-acetylsulfapyridine) were not significantly different in patients with (32.9±21.2 µg/ml) or without (22.8±12.0 µg/ml) toxicity (p>0.05). For all patients serum concentrations of sulfapyridine (total sulfapyridine) ranged from 3.5 to 73.1 (5.7 to 95.1) µg/ml in patients with controlled disease and 6.3 to 38.0 (14.0 to 54.7) µg/ml in patients with active disease. A significant correlation between clinical status of disease and serum drug concentrations was only apparent for rapid acetylators (p<0.05). The daily sulfasalazine dosage (mg/kg of body weight, log value) and serum drug concentrations (log values) were highly correlated (p<0.05). For clinical evaluation of inflammatory bowel disease patients determination of serum sulfapyridine concentrations appears to be more important for monitoring toxicity than therapeutic efficacy of sulfasalazine. Assessment of acetylator status appears to be useful for predicting serum sulfapyridine levels in patients receiving sulfasalazine therapy.