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P-glycoprotein activity and therapeutic response in ulcerative colitis Catalina M. Cortada1, An¨ªbal Gil3, Silvina Goncalves3, Alicia Sambuelli3, Modesto C. Rubio2, Marta A. Carballo1 1 CIGETOX, Departamento de Bioqu¨ªmica Cl¨ªnica y 2 C¨¢tedra de Farmacologia, Facultad de Farmacia y Bioqu¨ªmica, UBA, 3 Hospital de Gastroenterolog¨ªa ¡°Dr. Carlos Bonorino Udaondo¡±, eldecata@yahoo.com.ar P-glycoprotein, encoded by ABCB1, is a transmembrane efflux pump that is involved in relevant clinical drug transport. First described as overexpressed in cancer refractoriness, it is now proposed to be involved in ADMET (absorption, distribution, metabolism, elimination and toxicity) of many clinically used drugs 1. Together with the activity of xenobiotic metabolizing enzymes, efflux mechanisms have become the subject of considerable interest in recent studies of gut mucosal defence and pharmacokinetics of drugs 2. Inflammatory bowel diseases are chronically relapsing disorders likely due to a combination of immunological, environmental and genetic factors. Ulcerative colitis (UC), a member of this group, is characterized by a contiguous inflammation of the colonic lamina propria 3. ABCB1 was proposed as a candidate gene for UC therapeutic response and pathogenesis. P-glycoprotein is expressed in lymphocytes and luminal epithelium of colon, the target tissue for treatment and pathogenesis of UC. Increased expression has been reported to be associated with steroid UC refractoriness. Conversely, deficient P-glycoprotein function has been postulated as a UC susceptibility/severity factor. The aim of the present work was to investigate the role of P-glycoprotein in therapeutic response of ulcerative colitis by studying its functionality in lymphocytes isolated from peripheral blood. Samples were taken from 27 patients with active colitis (Mayo score: severe n = 9; moderate n = 9; mild n = 9) classified clinically as refractory (n = 16) and responders (n = 11) to treatment. These patients were treated with 5-aminosalicylic acid derivatives, glucocorticoids and/or 6-mercaptopurine, depending on their clinical state. Efflux of rhodamine 123 (a fluorescent P-glycoprotein substrate) was studied by flow cytometry in the absence and presence of a P-glycoprotein inhibitor (verapamil, 100 ¦ÌM) 4. Data were expressed evaluating the behavior of two markers based on % of cells with maximum (M1)/minimum (M2) intracellular fluorescence, reflecting inactivity/activity of the pump. Results were compared with a group of healthy individuals (n = 68). Significant differences were observed in the absence and presence of verapamil inhibition, when comparing refractory vs. responders (p < 0.05) as well as refractory vs. healthy controls (p < 0.01). No differences were observed when comparing responders vs. controls (p > 0.05) (Kruskal-Wallis test and Dunn post-test). Rhodamine efflux assay was also performed in 9 patients who required therapeutic change; a significant reduction of rhodamine transport (p< 0.01) was observed without inhibitor when patients achieved clinical response. In summary, our results suggest a relevant role of P-glycoprotein in ulcerative colitis treatment response and the usefulness of P-glycoprotein functional assay in the early detection of individual therapeutic response. References 1.Schinkel AH, Jonker JW. Mammalian drug efflux transporters of the ATP binding cassette (ABC) family: an overview. Advanced Drug Delivery Reviews 2003, 55: 3-29. 2. Ho G-T, Moodie FM, Satsangi J. Multidrug resistance 1 gene (P-glycoprotein 170): an important determinant in gastrointestinal disease? Gut 2003, 52: 759-766. 3.Mestecly J, Lamm ME, McGhee JR, Bienenstock J, Mayer L, Strober W. Mucosal Immunology (Third Edition). Elsevier Academic Press, 2005. 4.Chaudhary PM, Mechetner EB, Roninson IB. Expression and Activity of the Multidrug Resistance P-glycoprotein in Human Peripheral Blood Lymphocytes. Blood 1992; 80: 2735-2739.