Serum protein binding displacement: theoretical analysis using a hypothetical radiopharmaceutical and experimental analysis with 123I-N-isopropyl-p-iodoamphetamine.

INTRODUCTION The binding of radiopharmaceutical to serum proteins is thought to be an important factor that restricts its excretion and accumulation in tissue. We calculated the effect of inhibitors of serum protein binding using a hypothetical radiopharmaceutical. In vitro experiments and protein binding inhibitor-loaded monkey scintigraphy were then conducted using (123)I-N-isopropyl-p-iodoamphetamine (IMP) as the radiopharmaceutical. METHODS Free fraction ratios of radiopharmaceutical were calculated with one radiopharmaceutical, two serum proteins and two specific inhibitors in the steady state at various serum protein concentrations. In vitro protein binding inhibition studies using human, rat and monkey sera were performed with site-selective displacers of specific binding sites: 400 microM 6-methoxy-2-naphthylacetic acid (6MNA; a major nabumeton metabolite) as a serum albumin Site II inhibitor and 400 microM erythromycin (ETC) as an alpha(1)-acid glycoprotein (AGP) site inhibitor. Scintigraphy with or without 6MNA loading of monkeys was performed. RESULTS The theoretical findings roughly corresponded to the experimental results. Approximately 75% of IMP bound to serum albumin Site II and AGP in the species examined. The free fraction of IMP (25.0+/-0.6% for human, 22.8+/-0.4% for monkey, 23.7+/-0.3% for rat) increased with loading of specific protein binding inhibitors (6MNA: 28.0+/-0.3% for human, 24.5+/-0.7% for monkey, 24.3+/-0.2% for rat; ETC: 26.3+/-0.4% for human, 29.5+/-1.1% for monkey, 26.0+/-0.7% for rat) and was serum protein concentration dependant based on the results of calculations. Simultaneous administration of 6MNA and ETC produced a higher free fraction ratio of IMP (31.9+/-1.0% for human, 34.6+/-0.4% for monkey, 27.0+/-0.3% for rat) than summation of the single administrations of 6MNA and ETC (domino effect) in human, rat and monkey sera. Rapid cerebral accumulation was observed with 6MNA loading in monkey scintigraphy. CONCLUSIONS 6MNA appears to change the pharmacokinetics and brain accumulation of IMP in monkeys. Further studies in human are required.