Pharmacokinetics of the three radioiodinated dopamine D 2 receptor ligands [ 123I]IBF, [ 123I]epidepride and [ 123I]2′-ISP in nonhuman primates

Abstract

The pharmacokinetics of three radioiodinated high affinity dopamine D 2 receptor binding ligands—two benzamide neuroleptics (IBF and epidepride) and the butyrophenone neuroleptic analog 2′-iodospiperone (2′-ISP)—were measured in nonhuman primates. [ 123I]IBF and [ 123I]epidepride were prepared by iododestannylation of the corresponding tributylstannyl derivative, whereas [ 123I]2′-ISP was labeled by (NH[in4]) 2SO 4 mediated iodide for bromide exchange starting from 2′-bromospiperone. Labeled products were purified by HPLC and were obtained in > 93% radiochemical purity and > 7000 Ci/mmol sp. act. After i.v. injection in baboons, serial arterial plasma samples were extracted with ethyl acetate (IBF and epidepride) or denatured with methanol (2′-ISP) and analyzed by HPLC. For IBF, plasma levels of parent compound dropped to 50% of the plasma activity within 20 min post injection and the major radiometabolite was lipophilic. For epidepride, it took 30–40 min for parent content to reach 50% and the major radiometabolite was polar. For 2′-ISP, parent composition dropped to 60% after about 15 min. Arterial input curves for IBF and epidepride fit three-exponential models with terminal half-life of 54–76 and 50–59 min, respectively. Whole body images were acquired using the conjugate counting method. The distribution of all three agents was qualitatively similar, with major excretion through the hepatobiliary route. Peak whole brain uptake, observed within 20 min for all three tracers, was estimated as 7% injected dose for [ 123I]IBF, 8% for [ 123I]epidepride and 5% for [ 123I]2′-ISP.