Johnson M D, Anderson B D
Department of Pharmaceutics and Pharmaceutical Chemistry, University of Utah, Salt Lake City, Utah 84112, USA.
J Pharm Sci. 2000 Mar;89(3):322-35. doi: 10.1002/(SICI)1520-6017(200003)89:3<322::AID-JPS4>3.0.CO;2-1.
The biochemical and physiological mechanisms responsible for the limited central nervous system (CNS) uptake of dideoxynucleoside reverse transcriptase inhibitors currently used to treat HIV-1 infection in humans are poorly understood. In vitro models of the blood-brain barrier (BBB) offer an attractive alternative to in vivo or in situ animal studies for understanding the role of the blood-brain barrier in regulating brain tissue concentrations of these agents. In the present study, the kinetics of 2', 3'-dideoxyinosine (ddI) uptake and purine nucleoside phosphorylase (PNP) mediated catabolism in primary cultures of bovine brain microvessel endothelial cells (BBMECs) were determined in order to ascertain the importance of both transport and metabolism governing the CNS availability of this purine dideoxynucleoside. Initial rates of ddI uptake as a function of ddI donor concentration suggest the involvement of both passive diffusion and carrier-mediated processes. These studies confirm earlier in vivo findings that transporters may play a role in regulating the CNS concentration of ddI. Analysis of ddI uptake and metabolite accumulation in BBMECs over longer time intervals (beyond the intial rate region) provide substantial in vitro evidence for an enzymatic BBB for ddI. Simulations of the CNS availability of ddI derived from in vitro estimates of parameters for passive diffusion, carrier-mediation, and metabolism indicate that the fraction of ddI entering the BBB cells which actually reaches the brain parenchyma may be quite low (< 2%) due to metabolism by PNP localized within the BBB, consistent with the low CNS delivery of ddI observed in vivo. Transporters and metabolic enzymes within the BBB may function in coordinated fashion to reduce the CNS concentrations of both rapidly metabolized and poorly metabolized dideoxynucleosides.
目前用于治疗人类HIV-1感染的双脱氧核苷逆转录酶抑制剂在中枢神经系统（CNS）中的摄取有限，其生化和生理机制仍知之甚少。血脑屏障（BBB）的体外模型为体内或原位动物研究提供了一个有吸引力的替代方案，有助于理解血脑屏障在调节这些药物在脑组织中的浓度方面的作用。在本研究中，测定了牛脑微血管内皮细胞（BBMECs）原代培养物中2'，3'-双脱氧肌苷（ddI）摄取动力学和嘌呤核苷磷酸化酶（PNP）介导的分解代谢，以确定转运和代谢对这种嘌呤双脱氧核苷在CNS中可用性的重要性。ddI摄取的初始速率作为ddI供体浓度的函数表明，被动扩散和载体介导过程均有参与。这些研究证实了早期体内研究结果，即转运体可能在调节ddI的CNS浓度中发挥作用。对BBMECs中ddI摄取和代谢物积累在更长时间间隔（超出初始速率区域）的分析，为ddI的酶促血脑屏障提供了大量体外证据。根据被动扩散、载体介导和代谢参数的体外估计值模拟ddI在CNS中的可用性，结果表明，由于血脑屏障内局部存在的PNP代谢作用，进入血脑屏障细胞并实际到达脑实质的ddI比例可能相当低（<2 %），这与体内观察到的ddI在CNS中的低递送情况一致。血脑屏障内的转运体和代谢酶可能以协同方式发挥作用，以降低快速代谢和代谢不良的双脱氧核苷在CNS中的浓度。
