DETERMINATION OF ABSORPTION CHARACTERISTICS OF AG337, A NOVEL THYMIDYLATE SYNTHASE INHIBITOR, USING A PERFUSED RAT INTESTINAL MODEL

The purpose of this study was to determine the intestinal absorption c haracteristics of AG337, a mechanism-based inhibitor of thymidylate sy nthase, using a perfused rat intestinal model. Effects of site, pH, te mperature, concentration, Na+, and inhibitors on the absorption of AG3 37 were determined, after the compound was shown to be stable in buffe rs of various pH, blank perfusate, and intestinal homogenate. The resu lts indicated that absorption of AG337 was temperature; pH-, Na+-, con centration-, and site-dependent. The best site of absorption is duoden um, where the absorption was 3-10 times (p < 0.05) higher than absorpt ion at jejunum, ileum, and colon. Among the four pH's studied, the bes t was at pH 6.5 (p < 0.05). Absorption was 80% lower in the absence of Na+, and 75% lower when the temperature of the perfusate was decrease d to 4 degrees C. Permeability of AG337 also decreased about 75% when the concentration was raised to 100 mu M. These results Suggest that a nutrient carrier may be involved in the transport of AG337. To determ ine the carrier responsible for the absorption of AG337, its absorptio n was determined in the presence of various inhibitors at different co ncentrations. The results indicated that transport of AG337 was inhibi ted significantly (p < 0.01) by 100 mu M of adenine, hypoxanthine, and xanthine. The transport was also inhibited significantly (p < 0.01) b y a mixture of 100 mu M each of adenine, hypoxanthine, and xanthine, b ut not by a mixture of 100 mu M each of thymine and uracil. A higher c oncentration of hypoxanthine resulted in increased inhibition. In cont rast, prototypical inhibitors of nucleoside transporter, dipyridamole and nitrobenzylthioinosine (NBMPR), did not significantly decrease the transport of AG337. The results also showed that absorption of AG337 had a significant nonsaturable component, with a nonsaturable P-w of 0 .8. In conclusion, absorption of AG337 in the rat intestine has been s hown to be mainly via a purine base carrier with a significant nonsatu rable component.