Kawai R, Carson R E, Dunn B, Newman A H, Rice K C, Blasberg R G
Nuclear Medicine PET Program, NIDDK, National Institutes of Health, Bethesda, Maryland.
J Cereb Blood Flow Metab. 1991 Jul;11(4):529-44. doi: 10.1038/jcbfm.1991.102.
Brain distribution of the opiate receptor antagonist, cyclofoxy (CF), was evaluated at equilibrium in rats. A combination of i.v. injection and constant i.v. infusion was used to administer CF over a wide dose range (2.4-450 nmol/rat). Kinetic simulations and experimental results showed that this administration schedule accomplishes "true" tissue-blood equilibrium of CF within 60 min. To estimate the receptor-ligand binding parameters, we assumed that the CF concentration at the receptor site is identical to that in plasma water at equilibrium, and can be calculated from measured blood data after corrections for radiolabeled metabolites and plasma protein binding. This assumption was supported by CSF and plasma water measurements at equilibrium. Regional KD, Bmax, and a nonspecific tissue binding equilibrium constant (Keq) were estimated by fitting the tissue and plasma water concentrations to a single receptor model; the estimated values were 1.4-2.9 nM, 15-74 pmol/g of tissue, and 5.2-8.0, respectively. They are in good agreement with previous in vitro measurements (Rothman and McLean, 1988) as well as in vivo estimates from i.v. injection experiments (Sawada et al., 1990c). The conventional method to estimate the receptor-ligand binding parameters using data from cerebellum to approximate nonspecific tissue binding was found to be unacceptable. Although cerebellum is a brain region with no opiate receptors in rats, small differences in nonspecific tissue binding in different brain regions resulted in significant overestimations of KD and Bmax with this method. Receptor-active and -inactive enantiomers [18F-CF and 3H-CF)] were simultaneously administered to the same animal and the receptor-bound CF concentration could be accurately measured; this method was used to estimate Bmax from a single study in a single animal and has potential for direct application in human studies using positron emission tomography.
在大鼠体内平衡状态下评估了阿片受体拮抗剂环磷酰氧（CF）的脑部分布。采用静脉注射和持续静脉输注相结合的方式，在较宽剂量范围（2.4 - 450 nmol/大鼠）内给予CF。动力学模拟和实验结果表明，这种给药方案在60分钟内可实现CF的“真正”组织 - 血液平衡。为了估计受体 - 配体结合参数，我们假定受体部位的CF浓度与平衡时血浆水的浓度相同，并且可以在对放射性标记代谢物和血浆蛋白结合进行校正后，根据测得的血液数据计算得出。平衡时脑脊液和血浆水的测量结果支持了这一假设。通过将组织和血浆水浓度拟合到单一受体模型来估计区域KD、Bmax和非特异性组织结合平衡常数（Keq）；估计值分别为1.4 - 2.9 nM、15 - 74 pmol/g组织和5.2 - 8.0。它们与先前的体外测量结果（Rothman和McLean，1988年）以及静脉注射实验的体内估计值（Sawada等人，1990c）高度一致。发现使用来自小脑的数据来近似非特异性组织结合以估计受体 - 配体结合参数的传统方法不可接受。尽管小脑是大鼠脑中没有阿片受体的区域，但不同脑区非特异性组织结合的微小差异导致使用该方法时KD和Bmax被显著高估。将受体活性和非活性对映体[18F-CF和3H-CF)]同时给予同一只动物，并且可以准确测量受体结合的CF浓度；该方法用于从对单只动物的单一研究中估计Bmax，并且有可能直接应用于使用正电子发射断层扫描的人体研究。
