Hu M, Roland K, Ge L, Chen J, Li Y, Tyle P, Roy S
Department of Pharmaceutical Sciences, College of Pharmacy, Washington State University, Pullman, Washington 99164-6510, USA.
J Pharm Sci. 1998 Jul;87(7):886-90. doi: 10.1021/js970251e.
The purpose of this study was to determine the intestinal absorption characteristics of AG337, a mechanism-based inhibitor of thymidylate synthase, using a perfused rat intestinal model. Effects of site, pH, temperature, concentration, Na+, and inhibitors on the absorption of AG337 were determined, after the compound was shown to be stable in buffers of various pH, blank perfusate, and intestinal homogenate. The results indicated that absorption of AG337 was temperature-, pH-, Na+-, concentration-, and site-dependent. The best site of absorption is duodenum, where the absorption was 3-10 times (p < 0. 05) higher than absorption at jejunum, ileum, and colon. Among the four pH's studied, the best 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 decreased to 4 degreesC. Permeability of AG337 also decreased about 75% when the concentration was raised to 100 microM. These results suggest that a nutrient carrier may be involved in the transport of AG337. To determine the carrier responsible for the absorption of AG337, its absorption was determined in the presence of various inhibitors at different concentrations. The results indicated that transport of AG337 was inhibited significantly (p < 0.01) by 100 microM of adenine, hypoxanthine, and xanthine. The transport was also inhibited significantly (p < 0.01) by a mixture of 100 microM each of adenine, hypoxanthine, and xanthine, but not by a mixture of 100 microM each of thymine and uracil. A higher concentration of hypoxanthine resulted in increased inhibition. In contrast, 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 Pw of 0.8. In conclusion, absorption of AG337 in the rat intestine has been shown to be mainly via a purine base carrier with a significant nonsaturable component.
本研究旨在利用灌注大鼠肠道模型确定胸苷酸合成酶的基于机制的抑制剂AG337的肠道吸收特性。在证明该化合物在各种pH值的缓冲液、空白灌注液和肠匀浆中稳定后，测定了部位、pH值、温度、浓度、Na+和抑制剂对AG337吸收的影响。结果表明，AG337的吸收与温度、pH值、Na+、浓度和部位有关。最佳吸收部位是十二指肠，其吸收量比空肠、回肠和结肠高3至10倍（p < 0.05）。在所研究的四个pH值中，最佳pH值为6.5（p < 0.05）。在无Na+时吸收降低80%，当灌注液温度降至4℃时吸收降低75%。当浓度升至100μM时，AG337的渗透率也降低约75%。这些结果表明，一种营养载体可能参与了AG337的转运。为了确定负责AG337吸收的载体，在不同浓度的各种抑制剂存在下测定其吸收。结果表明，100μM的腺嘌呤、次黄嘌呤和黄嘌呤可显著抑制（p < 0.01）AG337的转运。由100μM的腺嘌呤、次黄嘌呤和黄嘌呤组成的混合物也可显著抑制（p < 0.01）转运，但由100μM的胸腺嘧啶和尿嘧啶组成的混合物则无此作用。较高浓度的次黄嘌呤导致抑制作用增强。相比之下，核苷转运体的典型抑制剂双嘧达莫和硝基苄基硫代肌苷（NBMPR）并未显著降低AG337的转运。结果还表明，AG337的吸收有一个显著的非饱和成分，非饱和Pw为0.8。总之，已证明AG337在大鼠肠道中的吸收主要通过嘌呤碱载体，且有一个显著的非饱和成分。
