Role of plasma protein binding in brain drug delivery

描述（由申请人提供）：药物给脑受到血脑屏障（BBB）（即神经血管单元）的限制，这明显影响许多中枢神经系统（CNS）疾病的治疗。在限制脑内药物传递的各种血脑屏障因子中，最不清楚的是血浆蛋白结合的作用，这涉及脑血流、脑毛细血管糖萼和质膜以及毛细血管循环中游离和结合药物浓度之间的复杂相互作用。大多数药物与血浆蛋白显著结合：大约一半与90%或更多的血浆蛋白结合。对一些人来说，血浆蛋白结合显著地减少了大脑的摄取和分布，而对另一些人来说，影响很小或没有影响。绝大多数血脑屏障药物转运研究报告称，脑吸收比血浆中药物的游离部分更大，并表明在毛细血管循环中发生特殊的相互作用，导致体内“增强解离”。净效应是，这阻碍了药物开发和临床分析中对中枢神经系统药物渗透的理解和预测。这项资助的主要假设是，许多药物的脑摄取可以使用改良的Crone-Renkin模型来预测，该模型除了结合药物的游离摄取和与大脑的交换外，还结合了药物解离和与脑毛细血管中的血浆蛋白的重新结合。这一假设将通过四个具体目标来实现：(1)利用精心控制的原位大鼠脑灌注技术，通过一个简单的模型，可以预测许多药物的初始单向脑内药物摄取，该模型基于四个容易确定的参数——动脉输入药物浓度、动脉输入药物的游离分数（fu）、血脑屏障通透性-游离（未结合）药物的表面面积积（PSU）和通过脑血管的流体流速(F)；(2)评估药物解离和血浆蛋白再结合的速率，以及它们对在规定流速下表现出高毛细管萃取的药物进入大脑的初始药物摄取的影响；(3)利用灌注技术证明血浆蛋白结合直接影响药物在脑内的稳态分布；(4)利用长酶白蛋白敲除大鼠验证体内也存在同样的关系，并且稳态下的脑药物浓度最终由血浆游离药物浓度和脑药物分布体积驱动。本研究将提供一种新的机制来区分在脑摄取方面表现出限制性和非限制性血浆蛋白结合作用的药物，将提供一种合理的方法来确定与血浆蛋白结合显著的CNS药物剂量，并将有助于在CNS药物开发中选择具有最佳脑递送的药物。这项工作对公共卫生至关重要的是，它将提高我们容易预测和识别跨越血脑屏障治疗中枢神经系统疾病的新药的能力。

DESCRIPTION (provided by applicant): Drug delivery to brain is limited by the blood-brain barrier (BBB) (i.e., neurovascular unit) which markedly impacts treatment for many central nervous system (CNS) diseases. Of the various BBB factors that limit brain drug delivery, one of the least well understood is the contribution of plasma protein binding, which involves a complex interplay between brain blood flow, the brain capillary glycocalyx and plasma membrane, and the free and bound drug concentrations in the capillary circulation. Most drugs bind significantly to plasma proteins: approximately half bind 90% or more. For some, plasma protein binding dramatically reduces brain uptake and distribution, whereas for others there is little or no effect. The great majority of BBB drug transport studies report greater brain uptake than can be accounted for based upon the free Fraction of drug in plasma and have suggested that special interactions occur in the capillary circulation leading to "enhanced dissociation" in vivo. The net effect is that this has impeded understanding and prediction of CNS drug penetration in drug development and clinical analysis. The primary hypothesis of this grant is that brain uptake for many drugs can be predicted using a modified Crone-Renkin model that incorporates drug dissociation and rebinding to plasma proteins in the brain capillary in addition to free drug uptake and exchange with brain. This hypothesis will be pursued through four specific aims: (1) to confirm using a carefully controlled in situ rat brain perfusion technique that initial, unidirectional drug uptake into brain can be predicted for many drugs with a simple model based upon four readily determined parameters - the arterial input drug concentration, the free fraction (fu) of drug in the arterial input, the apparent BBB permeability-surface area product (PSU) to free (unbound) drug, and the flow rate of fluid through the brain vasculature (F); (2) to evaluate the rates of drug dissociation and rebinding to plasma protein and their influence on initial drug uptake into brain for drugs that exhibit high capillary extraction at defined flow rate; (3) to show using the perfusion technique that plasma protein binding directly affects the steady state distribution of drug in brain; and (4) to validate using the Nagase albumin knockout rat that this same relationship holds in vivo and that brain drug concentration at steady state is driven ultimately by the plasma free drug concentration and the brain drug distribution volume. This research will provide a novel mechanism to distinguish drugs that exhibit restrictive vs nonrestrictive plasma protein binding effects on brain uptake, will provide a rational means upon which to base CNS drug-dosing for agents that bind significantly to plasma proteins, and will assist in selection of agents with optimal brain delivery in CNS drug development. The critical importance of this work to public health is that it will improve our ability to readily predict and identify new drug agents that cross the BBB for the treatment of CNS diseases.