Highly potent indanamine serotonin uptake blockers as radiotracers for imaging serotonin uptake sites

Two highly potent indanamine serotonin (5-HT) uptake blockers, trans-3′-(4′-bromophenyl)-1-indanamine (trans-[¹¹C]DBPI or [¹¹C]Lu 19-056) and its iodo analog, trans-3′(4′-[¹²⁵I]iodophenyl)-1-indanamine (trans-[¹²⁵I]DIPI) were evaluated as radiotracers for imaging 5-HT uptake sites in vivo. Trans-[¹¹C]DBPI was synthesized by N-methylation of the normethyl precursor with [¹¹C]iodomethane. Trans-[¹²⁵I]DIPI was synthesized by iododestannylation of the tributyltin precursor with [¹²⁵I]NaI. Radiochemical yields for the [¹¹C] and [¹²⁵I] radiotracers were 34 and 40%, with specific activities of 4000 and 1800 mCi/μmol, respectively. In vitro, the iodo analog, trans-DIPI, showed an IC₅₀ value of 0.26 nM in inhibition of [³H]paroxetine binding to 5-HT uptake sites in rat cortex. The potency was found to be equivalent to that of paroxetine or McN5652. In vivo, after i.v. injection into mice, both radiotracers showed high uptake in brain (3–4% dose/whole brain at 15 min) and high accumulation into target tissues such as hypothalamus and olfactory tubercles (7–8% dose/g at 60 min). The binding was blocked by pre-injection of 5 mg/kg of paroxetine. While the in vivo distribution agreed with previously reported 5-HT uptake site distribution, the radiotracers showed high uptake in non-target tissues such as cerebellum, resulting in low target-to-non-target ratios (1.5–1.6 at 60 min). Since washout from non-target regions was slower than from target regions, longer-time observation with ¹²⁵I up to 6 h did not improve the ratios. HPLC analyses of mouse brain homogenates and blocking studies indicated that the high uptake in non-target regions is not the result of metabolism or any interaction of the radiotracers with those tissues via specific binding sites. In spite of low target-to-non-target ratios, target regions with high density of 5-HT uptake sites, such as the raphe nuclei, superior colliculi and substantia nigra, were visualized with trans-[¹²⁵I]DIPI by ex vivo autoradiography, since the radiotracer showed high specific binding (total minus nonspecific binding).