Singhal D, Ho N F, Anderson B D
Department of Pharmaceutics and Pharmaceutical Chemistry, University of Utah, Salt Lake City, Utah 84112, USA.
J Pharm Sci. 1998 May;87(5):569-77. doi: 10.1021/js9703582.
This study investigates the mechanisms of absorption and the role of intestinally localized purine salvage pathway enzymes on the ileal availabilities of 2',3'-dideoxyinosine (ddI), a substrate for purine nucleoside phosphorylase (PNP); 2'-fluoro-2',3'-dideoxyinosine (F-ddI), a non-PNP substrate; and 6-chloro-2',3'-dideoxypurine (6-Cl-ddP), an adenosine deaminase (ADA) activated prodrug of ddI. The potential for increasing the intestinal availability of 6-Cl-ddP through the use of ADA inhibitors, namely, 2'-deoxycoformycin (DCF) and erythro-9-(2-hydroxy-3-nonyl)adenine (EHNA), is also explored. Drug permeability coefficients across the intestinal epithelium were determined in in situ perfusions in the mesenteric vein cannulated rat ileum based on both drug appearance in blood (Pblood) and disappearance from the lumen (Plumen) and their paracellular and transcellular components were estimated by comparison to the permeabilities of two paracellular markers, mannitol and urea. Values of Pblood for ddI were determined to be (1.1 +/- 0.3) x 10(-6) cm/s, in close agreement with the value of (1.0 +/- 0.3) x 10(-6) cm/s obtained for F-ddI, a PNP resistant analogue of ddI having virtually the same molecular size and lipophilicity as ddI. This indicates that PNP may not play an important role in the low intestinal absorption of ddI. The Pblood for 6-Cl-ddP, (19 +/- 2) x 10(-6) cm/s, was 4.5-fold lower than Plumen, (84 +/- 12) x 10(-6) cm/s, which means that 77 +/- 6% of 6-Cl-ddP was metabolized during its intestinal transport, thus qualitatively accounting for the low oral bioavailability (7%) of 6-Cl-ddP observed in vivo in rats. Extensive intracellular metabolism of 6-Cl-ddP by ADA was confirmed by the high concentrations of ddI found both in the intestinal lumen and blood during 6-Cl-ddP perfusions and by a rate of ddI appearance in blood which was approximately 10-fold higher than ddI controls. Co-perfusion of the potent, hydrophilic ADA inhibitor DCF (Ki = 0. 001-0.05 nM) with 6-Cl-ddP led to only partial inhibition of intestinal ADA, while complete inhibition was obtained using the less potent but more lipophilic inhibitor EHNA (Ki = 1-20 nM). Hence, EHNA may be used to improve intestinal absorption of 6-Cl-ddP in vivo.
本研究调查了吸收机制以及肠道定位的嘌呤补救途径酶对2',3'-二脱氧肌苷（ddI，嘌呤核苷磷酸化酶（PNP）的底物）、2'-氟-2',3'-二脱氧肌苷（F-ddI，一种非PNP底物）和6-氯-2',3'-二脱氧嘌呤（6-Cl-ddP，ddI的腺苷脱氨酶（ADA）激活前体药物）回肠可用性的作用。还探讨了通过使用ADA抑制剂，即2'-脱氧助间型霉素（DCF）和赤型-9-(2-羟基-3-壬基)腺嘌呤（EHNA）来提高6-Cl-ddP肠道可用性的可能性。基于药物在血液中的出现情况（Pblood）和从肠腔中的消失情况（Plumen），在肠系膜静脉插管大鼠回肠的原位灌注中测定了药物跨肠上皮的渗透系数，并通过与两种细胞旁标记物甘露醇和尿素的渗透性比较来估计它们的细胞旁和跨细胞成分。ddI的Pblood值测定为(1.1±0.3)×10(-6) cm/s，与F-ddI（ddI的一种PNP抗性类似物，其分子大小和脂溶性与ddI几乎相同）获得的(1.0±0.3)×10(-6) cm/s值密切一致。这表明PNP可能在ddI的低肠道吸收中不发挥重要作用。6-Cl-ddP的Pblood为(19±2)×10(-6) cm/s，比Plumen（(84±12)×10(-6) cm/s）低4.5倍，这意味着77±6%的6-Cl-ddP在其肠道转运过程中被代谢，从而从定性上解释了在大鼠体内观察到的6-Cl-ddP低口服生物利用度（7%）。在6-Cl-ddP灌注期间，在肠腔和血液中均发现高浓度的ddI，以及血液中ddI的出现速率比ddI对照高约10倍，证实了ADA对6-Cl-ddP的广泛细胞内代谢。强效亲水性ADA抑制剂DCF（Ki = 0.001 - 0.05 nM）与6-Cl-ddP共同灌注仅导致肠道ADA的部分抑制，而使用效力较弱但更具脂溶性的抑制剂EHNA（Ki = 1 - 20 nM）则获得了完全抑制。因此，EHNA可用于改善6-Cl-ddP在体内的肠道吸收。
