Around 90% of a dose reaches the colon where bacteria split the drug into sulfapyridine (SP) and mesalazine (ME). These are also active, and the unsplit sulfasalazine (SASP) is also active on a variety of symptoms. Most SP is absorbed, hydroxylated or glucuronidated and a mix of unchanged and metabolised SP appears in the urine. Some ME is taken up and acetylated in the colon wall, such that renal excretion is mainly AC-ME. SASP is excreted unchanged in the bile and urine. Turning to mesalazine, in urine only AC-ME (not free ME) was demonstrable, the acetylation probably largely achieved in the colon mucosa. After a 3g SASP dose lag time was 6.1±2.3 hours and plasma levels kept below 2µg/ml total ME. Urinary excretion half life was 6.0±3.1 hours and absorption half life based on these figures 3.0±1.5 hours. Renal clearance constant was 125ml/min corresponding to the GFR.
Salazopyrin En-Tabs® (Sulfasalazine) UK Summary of Product Characteristics, Pfizer Limited, updated August 2010

Sulfasalazine is metabolised by intestinal bacteria to sulfapyridine (SP) and 5-ASA. Approximately 15% of a dose of sulfasalazine is absorbed as parent and is metabolised to some extent in the liver to the same two species. The observed plasma half life for IV sulfasalazine is 7.6 ±3.4 hrs. The primary route of metabolism of SP is via acetylation to form AcSP. The rate of metabolism of SP to AcSP is dependent upon acetylator phenotype. SP can also be metabolised to 5-hydroxy-sulfapyridine and N-acetyl-5-hydroxy-sulfapyridine. 5-ASA is primarily metabolised in both the liver and intestine to N-acetyl-5-aminosalicylic acid via a non-acetylation phenotype dependent route. Due to low plasma levels produced by 5-ASA after oral administration, reliable estimates of plasma half lives are not possible. Absorbed SP and 5-ASA and their metabolites are primarily eliminated in the urine either as free metabolites or glucuronide conjugates. The majority of 5-ASA stays within the colonic lumen and is excreted as 5-ASA and acetyl 5-ASA within the feces. The calculated clearance of sulfasalazine following IV administration was 1L/hr. Renal clearance was estimated to account for 37% of total clearance.
Sulfasalazine tablets USP US Prescribing Information, Watson Laboratories Inc, updated May 2010