Sulfasalazine Other names: Sulfasalazine 5-ASA

Therapeutic benefit of sulfasalazine in ulcerative colitis and Crohn’s Disease appears to be due to a local action of the sulfasalazine and its split product 5-aminosalicylic acid on the mucous membrane and deeper colonic structures. Pharmacological actions noted for these compounds include inhibition of neutrophil activation, free radical scavenging, inhibition of superoxide production, inhibition of bacterial growth. Sulfasalazine inhibits 15-Prostaglandin dehydrogenase and slows prostaglandin metabolism. Lipoxygenase release in inflammatory cells is also depressed. NK cells and T cell proliferation are inhibited.

Sulfasalazine has beneficial effects in the treatment of ulcerative colitis and maintenance of remission, and in the treatment of acute Crohn’s disease. Around 90% of a dose reaches the colon where bacteria split the drug into sulpyapyridine and mesalazine. These are active, and the unsplit sulfasalazine is also active on a variety of systems. Most sulfapyridine is absorbed, hydroxylated or glucuronidated and a mix of unchanged and metabolised sulfapyridine appears in the urine.

Some mesalazine is taken up and acetylated in the colon wall, such that renal excretion is mainly acetyl-mesalazine. Sulfasalazine is excreted unchanged in the bile and urine. Overall the drug and its metabolites exert immunomodulatory effects, antibacterial effects, effects on the arachidonic acid cascade and alteration of activity of certain enzymes. The net result clinically is a reduction in activity of the inflammatory bowel disease.

The enteric coated sulfasalazine is registered for the treatment of rheumatoid arthritis, where the effect resembles penicillamine or gold.

Oral administration

With regard to the use of sulfasalazine in bowel disease there is no evidence that systemic levels are of any relevance other than with regard to ADR incidence. Here levels of sulfapyridine over about 50µg/ml are associated with a substantial risk of ADRs, especially in slow acetylators.

For sulfasalazine given as a single 3g oral dose, peak serum levels of sulfasalazine occurred in 3-5 hours, elimination half life was 5.7 ±0.7 hours, lag time 1.5 hours. During maintenance therapy renal clearance of sulfasalazine was 7.3 ±1.7ml/min, for sulfapyridine 9.9 ±1.9 and acetyl-mesalazine 100 ±20. Free sulfasalazine first appears in plasma in 4.3 hours after a single dose with an absorption half life of 2.7 hours. The elimination half life was calculated as 18 hours. For mesalazine, only acetyl-mesalazine (not free mesalazine) was demonstrable, the acetylation probably largely achieved in the colon mucosa. After 3g sulfasalazine dose lag time was 6.1 ±2.3 hours and plasma levels kept below 2µg/ml total mesalazine. 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 125 ml/min corresponding to the GFR. Studies in volunteers suggest that sulfasalazine is handled in a similar manner whether given as suspension or tablets.

Rectal administration

There are considerable individual differences in the retention time of suppositories in volunteer studies. Consequently uptake values vary widely also. Given that the effect of the drug is almost certainly due to a local effect pharmacokinetics becomes less relevant to therapeutic action than to possible adverse effects related to systemic levels.

A study of five volunteers over three days following insertion of 2 × 0.5 g suppositories gave the following results:

Retention time: mean 8.9 hours (s.d. 5.2), serum concentration at 10 hours: sulfasalazine 1.7 mcg/ml (s.d. 0.46), sulfapyridine less than 1 mcg/ml. Percentage renal excretion: 10.2 (s.d. 4.3). Uptake as reflected by excretion is much below that of the oral rate and may explain the good tolerance of the dose form.

In two-year carcinogenicity studies in rats and mice, sulfasalazine showed some evidence of carcinogenicity. In rats, there was a small increase in the incidence of transitional cell papillomas in the urinary bladder and kidney. The tumours were judged to be induced mechanically by calculi formed in the urine rather than through a direct genotoxic mechanism. In the mouse study, there was a significant increase in the incidence of hepatocellular adenoma or carcinoma. The mechanism of induction of hepatocellular neoplasia has been investigated and attributed to species-specific effects of sulfasalazine that are not relevant to humans.

Sulfasalazine did not show mutagenicity in the bacterial reverse mutation assay (Ames test) or in the L51784 mouse lymphoma cell assay at the HGPRT gene. It did not induce sister chromatid exchanges or chromosomal aberrations in cultured Chinese hamster ovary cells, and in vivo mouse bone marrow chromosomal aberration tests were negative. However, sulfasalazine showed positive or equivocal mutagenic responses in rat and mouse micronucleus assays, and in human lymphocyte sister chromatid exchange, chromosomal aberration and micronucleus assays. The ability of sulfasalazine to induce chromosome damage has been attributed to perturbation of folic acid levels rather than to a direct genotoxic mechanism.

Based on information from non-clinical studies, sulfasalazine is judged to pose no carcinogenic risk to humans. Sulfasalazine use has not been associated with the development of neoplasia in human epidemiology studies.