Drug monitoring in inflammatory bowel disease: helpful or dispensable?

Thiopurines, methotrexate and the calcineurin inhibitors cyclosporin A and tacrolimus are classical immunosuppressive treatment modalities for inflammatory bowel disease (IBD). Since a high inter-patient variability exists in drug efficacy and toxicity, their application requires the knowledge of appropriate indications as well as strategies for individualization of dosage and monitoring for adverse events. Results of pharmacogenetic studies that examine the relationship between single-gene polymorphisms and associated effects on the pharmacokinetics and pharmacodynamics may be helpful for the optimization of individualized therapy. Although 85-95% of patients worldwide present with the homozygote thiopurine S-methyltransferase (TPMT) wild-type genotype and a normal enzyme activity, cost-benefit analyses suggest assessment of TPMT enzyme activity prior to thiopurine therapy for IBD to prevent life-threatening toxicity. Monitoring of 6-mercaptopurine metabolites is a helpful, but not an indispensable tool in thiopurine non-responders to discriminate poor adherence and under-dosing from pharmacogenetic thiopurine resistance and thiopurine refractory disease. Response to and adverse events of methotrexate therapy are hard to predict. Pharmacogenetic indices of methotrexate metabolization have been evaluated in rheumatoid arthritis (RA) but not in IBD yet. In contrast to RA, concentration of methotrexate polyglutamates correlates positively with non-response and adverse effects in IBD. Calcineurin inhibitor metabolism is mainly controlled by cytochrome P-450 isoenzymes 3A4/3A5 and P-glycoprotein that underlie a variety of gene polymorphisms and are susceptible to drug interactions. Independent from pharmacokinetic alterations a MDR1 polymorphism may predict cyclosporin failure in severe ulcerative colitis. Frequent monitoring of whole blood levels is required since efficacy and toxicity are dose-dependent.