In vivo nonspecific binding parameters of (R,R)-[125I]4IQNB estimated from the pharmacokinetics of the (S,S)-[125I]4IQNB stereoisomer.

To the Editor: A proposed method of determining nonspecific binding parameters for a receptor binding radioli­ gand is to measure the pharmacokinetics of a ste­ reoisomer whose affinity for receptor is lower (Wagner, 1986). We have investigated a specific ex­ ample to gain a better understanding of when the parameters derived from the stereoisomer may rea­ sonably be used to estimate the nonspecific binding of the original radioligand. Our analysis of data from a recent report (Sawada et aI., 1990) suggests that even a stereoisomer with a low relative affinity can exhibit a high degree of specific binding. Sawada et al. (1990) use a three-compartment ki­ netic model of (R,R)-3-quinuclidinyl 4-e 25 1]iodo­ benzilate [(R,R)-IQNB] binding to the muscarinic acetylcholine receptor (mAChR), taking the phar­ macokinetics of the lower-affinity stereoisomer, (S,S)-IQNB, to represent nonspecific binding. However, the toO-fold in vitro affinity difference that Sawada et al. (1990) cite may not be sufficient to ensure that the (S,S)-IQNB binding is nonspe­ cific. We have performed computer simulations of the in vivo pharmacokinetics of (S,S)-IQNB using metabolite-corrected plasma radioactivity (Sawada et aI., 1990) and a number of possible sets of kinetic rate constants that are consistent with a 100-fold difference in affinity. With every set of rate con­ stants, we have found a predominance of specific over nonspecific binding. The in vivo ratio that Sawada et al. (1990) call k51 k6 for (S,S)-IQNB is 40 times lower than the in vivo ratio k31 k4 for (R,R)-IQNB. Since the 40-fold ratio is close to the cited 100-fold ratio between the in vitro mAChR affinities of the two compounds, the values of k5 and k6 for (S,S)-IQNB may well describe specific binding to receptor. Therefore, we have relabeled k5 and k6 as k3 and k4 and have in­ terpreted them as possibly applying to the specific binding site. Our model contains tissue compart­ ments for receptor-bound ligand and free ligand. The simulated concentration of free ligand in tissue implicitly includes nonspecific tissue localization, as is true in any model that does not contain an explicit nonspecific compartment (Wong et aI., 1986; Zeeberg and Wagner, 1987).