IPT: A novel iodinated ligand for the CNS dopamine transporter

Abstract

An iodinated cocaine derivative, N‐(3′‐iodopropen‐2′‐yl)‐2β‐carbomethoxy‐3β‐(4‐chlorophenyl)tropane (IPT), was evaluated as a probe for in vitro and in vivo labeling of dopamine (DA) and serotonin (5‐HT) transporters in Sprague‐Dawley rat brain. Saturation analysis of [¹²⁵I]IPT in rat striatal homogenates, in two different buffer solutions, Tris‐HCl and phosphate, demonstrated a one‐site binding with affinities (K_d) of 0.25 ± 0.02 and 0.16 ± 0.02 nM and densities (B_max) of 939 ± 161 and 1,982 ± 137 fmol/mg protein, respectively. Competition by known DA transporter ligands showed a rank order of RTI‐55 > IPT > GBR12909 > mazindol > (−)cocaine. Binding to 5‐HT transporter sites was evaluated in rat cortical homogenates. Saturation experiment results showed a single site with a K_d value of 1.2 ± 0.2 nM and a B_max value of 100 ± 20 fmol/mg protein. The rank order of potency of several monoamine uptake inhibitors (paroxetine > fluoxetine > mazindol > R‐nisoxetine > GBR12909) suggests that [^l25I] IPT labels 5‐HT transporters in rat cortical homogenates. Both ex vivo and in vitro autoradiographic studies revealed high densities of [¹²⁵I]IPT binding sites in the caudate nucleus, putamen, olfactory tubercle and nucleus accumbens, areas known to be rich in dopaminergic innervation. Moderate accumulation of activity was also observed in the substantia nigra. The dorsal raphe, a region with a high density of 5‐HT innervation, was labeled using in vitro autoradiography with [¹²⁵I]IPT, but the labeling using ex vivo autoradiography was less prominent at 30 min postinjection and not noticeable at 60 min postinjection. Furthermore, systemic administration of IPT to rats increased the locomotor activity, a behavioral effect consistent with the in vitro and in vivo characteristics of compounds acting at dopaminergic sites. These results demonstrate that [¹²⁵I]IPT is a useful ligand for in vitro labeling of DA and 5‐HT transporters and a ligand selective for labeling of DA transport sites in vivo. When labeled with I‐123,[¹²³I]IPT holds promise as a SPECT imaging agent for studies of neuropsychiatric disorders that involve DA transporters.