Quantification of an C-labelled β-adrenoceptor ligand, S-(−)CGP 12177, in plasma of humans and rats

β-Adrenoceptors in human lungs and heart can be imaged with the radioligand 4-[3-[(1,1-dimethyl)amino]-2-hydroxypropoxy]-1,3-dihydro-2H-benzimidazol-2-¹¹C-one (CGP 12177, [¹¹C]I). For quantification of receptor density with compartment models by adjustment of rate constants, an ‘input function’ is required which consists of the integral of the concentration of unmodified ligand in arterial plasma over time. A discrepancy in the literature regarding metabolic stability of [¹¹C]I prompted us to study metabolism in rats by reversed-phase HPLC (RP-HPLC) of trichloroacetic acid extracts of arterial plasma after i.v. injection of [¹¹C]I (> 11.1 TBq/mmol, 11 MBq/kg). Some plasma samples were also directly applied to an internal-surface reversed-phase (ISRP) column. In parallel experiments, tritiated [¹¹C]I was employed and methanol extracts of arterial plasma were analyzed by straight-phase TLC. The three methods were in excellent agreement. Unmodified [¹¹C]I decreased from > 98.5% (³H) or > 99.9% (¹¹C) initially to 57 ± 7% at 80 min post injection to formation of two polar metabolites. Using the RP-HPLC method, no metabolism was detectable in humans up to 30 min after injection of [¹¹C]I (1851 MBq). Deproteinization of plasma with acetonitrile resulted in the formation of a radioactive species (artifact) which eluted immediately after the void volume in RP-HPLC and which could be mistakenly interpreted as a metabolite. Plasma protein binding was low (ca. 30%) in both humans and rats. Association of the radioligand to blood cells suggested that equilibrium between receptor-bound and free radioligand was reached within 15 min after high-specific-activity injections, but only after more than 30 min after low-specific-activity injections.