Inflammatory Bowel Disease Pharmacology, Part 1

Sulfasalazine consists of mesalamine (5-aminosalicylic acid) linked by an azo bond to sulfapyridine, a sulfonamide carrier. Colonic bacteria cleave this azo bond, releasing mesalamine locally at the site of mucosal inflammation. The therapeutic activity resides entirely in the mesalamine component; sulfapyridine is merely a carrier that prevents premature absorption in the small intestine, but it is also responsible for most of the adverse effects.

Adverse effects attributable to sulfapyridine include nausea, headache, reversible oligospermia, and hemolytic anemia in patients with glucose-6-phosphate dehydrogenase deficiency. Sulfapyridine competitively inhibits intestinal folate absorption, so folic acid 1 milligram daily is required for all patients on sulfasalazine — particularly important in women of childbearing potential. Screen for glucose-6-phosphate dehydrogenase deficiency before starting.

Mesalamine formulations were developed to deliver the active moiety without the sulfapyridine carrier. pH-dependent release coatings dissolve at specific luminal pH thresholds to target drug release to the terminal ileum and colon. Multi-matrix technology (once-daily tablets) releases mesalamine slowly throughout the entire colon and substantially improves adherence compared with older three or four times daily regimens. Rectal suppositories, enemas, and foam preparations deliver high local concentrations to the distal colon with minimal systemic absorption and are preferred for proctitis and left-sided disease.

For left-sided and extensive ulcerative colitis, combining oral plus rectal mesalamine produces higher mucosal drug concentrations and better remission rates than either route alone. This combination is first-line for mild to moderate active ulcerative colitis. The role of aminosalicylates in Crohn's disease is limited — they are not as effective as in ulcerative colitis and have no demonstrated benefit for preventing post-operative recurrence.

Mesalamine inhibits cyclo-oxygenase enzymes, reducing prostaglandin synthesis at the mucosal level. It also suppresses nuclear factor kappa-B transcription factor activation in epithelial and immune cells, reducing expression of interleukin-1, interleukin-6, tumor necrosis factor-alpha, and other pro-inflammatory cytokines. Reactive oxygen species production by mucosal neutrophils is reduced. The net effect is attenuation of mucosal inflammation without the systemic immunosuppression of corticosteroids or immunomodulators.

Azathioprine is non-enzymatically converted to 6-mercaptopurine, making the two drugs functionally equivalent at equivalent doses. 6-mercaptopurine then enters three competing metabolic pathways. The anabolic pathway via hypoxanthine-guanine phosphoribosyltransferase generates 6-thioguanine nucleotides — the active immunosuppressive metabolites incorporated into lymphocyte DNA, terminating proliferation and inducing apoptosis. The catabolic pathway via xanthine oxidase converts 6-mercaptopurine to inactive thiouric acid. A third pathway via thiopurine methyltransferase converts 6-mercaptopurine to 6-methylmercaptopurine, an inactive metabolite associated with hepatotoxicity at high concentrations.

Thiopurine methyltransferase activity is genetically polymorphic. Approximately 89 to 94 percent of patients have normal activity; 6 to 11 percent are intermediate metabolizers; and 0.3 percent are poor metabolizers carrying two non-functional alleles. In poor metabolizers, the methyltransferase catabolic pathway is absent, channeling all 6-mercaptopurine through the anabolic hypoxanthine-guanine phosphoribosyltransferase pathway and generating extremely high thioguanine nucleotide concentrations that cause severe, potentially fatal myelosuppression at standard doses. Thiopurine methyltransferase phenotyping or genotyping is mandatory before starting azathioprine or 6-mercaptopurine. Poor metabolizers require approximately 10 percent of standard dosing or should avoid thiopurines. Intermediate metabolizers require dose reduction of 30 to 50 percent.

NUDT15 is a second pharmacogenomic predictor of thiopurine myelotoxicity independent of thiopurine methyltransferase, particularly important in East Asian populations where the NUDT15 R139C variant has an allele frequency of approximately 10 percent versus less than 0.2 percent in Europeans. The Clinical Pharmacogenomics Implementation Consortium guidelines recommend testing for both thiopurine methyltransferase and NUDT15 before initiating thiopurines, with dose adjustment or alternative drug selection based on results.

Allopurinol inhibits xanthine oxidase, the catabolic enzyme that converts 6-mercaptopurine to inactive thiouric acid. Co-administration of allopurinol at full-dose azathioprine or 6-mercaptopurine blocks the catabolic pathway and causes massive accumulation of thioguanine nucleotides, producing potentially fatal myelosuppression. This is one of the most dangerous drug interactions in gastroenterology. If allopurinol must be used in a patient on a thiopurine — for example as a rescue strategy to redirect metabolism toward thioguanine nucleotides in patients with high 6-methylmercaptopurine levels — the azathioprine dose must be immediately reduced to 25 to 33 percent of the original dose. Prescribing allopurinol for gout without checking for thiopurine co-therapy is a potentially lethal error.

Thiopurines are steroid-sparing maintenance agents, not induction agents. Therapeutic thioguanine nucleotide accumulation takes 3 to 6 months, meaning thiopurines cannot rapidly control active disease. Corticosteroids or biologics are required for induction while thiopurines are co-initiated for long-term maintenance. Combining azathioprine with an anti-tumor necrosis factor biologic reduces formation of antibodies against the biologic and is more effective than either agent alone in Crohn's disease. White blood cell monitoring every 3 months is standard during maintenance therapy.