Role of plasma protein binding in brain drug delivery

描述(申请人提供)：药物输送到大脑受到血脑屏障(BBB)(即神经血管单位)的限制，这显著影响了许多中枢神经系统(CNS)疾病的治疗。在限制脑内药物传递的各种血脑屏障因素中，最不为人所知的是血浆蛋白结合的作用，它涉及脑血流量、脑毛细血管甘氨酸和质膜以及毛细血管循环中自由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：大约一半结合90%或更多。对于一些人来说，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，这几乎没有影响。绝大多数的血脑屏障药物转运研究报告了比基于血浆中药物的游离分数所能解释的更大的脑摄取，并表明在体内的毛细血管循环中发生了特殊的相互作用，导致了“增强的解离”。最终的结果是，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性的理解和预测。这项资助的主要假设是，可以使用改进的Crone-Renkin模型预测大脑对许多药物的摄取，该模型除了与大脑自由摄取和交换药物外，还包括药物在大脑毛细血管中的解离和重新结合到血浆蛋白。这一假说将通过四个具体的目标来实现：(1)利用精心控制的大鼠脑原位灌流技术证实，许多药物最初的单向脑吸收可以用一个简单的模型来预测，该模型基于四个容易确定的参数-动脉输入药物浓度、药物在动脉输入的游离分数(Fu)、对游离药物的表观血脑屏障通透性表面积乘积(PSU)和通过脑血管系统的液体流量(F)；(2)评估在规定流速下具有高毛细管萃取能力的药物的药物解离和与血浆蛋白重结合的速率及其对药物初始脑内摄取的影响；(3)利用灌流技术显示血浆蛋白结合直接影响药物在脑内的稳态分布；(4)使用NAGase白蛋白基因敲除大鼠验证体内同样的关系，稳态脑内药物浓度最终由血浆游离药物浓度和脑药物分布量驱动。这项研究将提供一种新的机制来区分对脑摄取具有限制性和非限制性血浆蛋白结合作用的药物，将为与血浆蛋白显著结合的药物提供合理的中枢神经系统给药依据，并将有助于在中枢神经系统药物开发中选择具有最佳脑递送作用的药物。这项工作对公共卫生的关键重要性在于，它将提高我们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.