The Discovery of Epidepride and Its Analogs as High-Affinity Radioligands for Imaging Extrastriatal Dopamine D2 Receptors in Human Brain

[(123)I]Epidepride, [(18)F]fallypride, and [(76)Br]isoremoxipride (FLB-457) and their corresponding [(11)C]labeled derivatives belong to a class of high-affinity radioligands for SPECT or PET imaging of dopamine D(2) receptors in the human brain. In contrast to previously used imaging agents, these ligands are capable of identifying extrastriatal dopamine D(2) receptors. The design of these substituted benzamides derive its origin from the atypical antipsychotic agent, remoxipride. Starting in the late 1970's, halogenated analogs of (S)-sulpiride were evaluated in binding assays and behavioral studies, leading to the discovery of remoxipride. Remoxipride was 10 times weaker than sulpiride in vitro but 50 times more potent in vivo. Search for a putative active metabolite of remoxipride led to the discovery of raclopride and eticlopride, the former becoming a useful radioligand as tritium or carbon-11 labeled form for receptor binding and PET studies, respectively. In the US, the mono-iodine analog of raclopride, [(123)I]iodobenzamide (IBZM), was found to have moderate putamen-to-cerebellum ratio in rat and human brain. Continued search for metabolites of remoxipride led to the discovery of its 3,6-dihydroxy derivative, NCQ-344, with an extremely potent in vivo activity in the rat. SAR studies of the metabolites of remoxipride led to the discovery of the 3-methoxy isomer, isoremoxipride (FLB-457) and its corresponding 6-hydroxy analog, FLB-463, both having affinities for the dopamine D(2) receptor in the 20-30 pM range. Later, the 5-[(123)I]iodo analog of FLB-463, [(123)I]ioxipride ([(123)I]NCQ-298), became a potential SPECT imaging agent. In the mean time, the deshydroxy analog of IBZM, [(125)I]iodopride, showed binding potential in the rat similar to [(125)I]IBZM. Epidepride was designed by combining the structure of isoremoxipride with that of iodopride. In 1988, epidepride was independently prepared and radiolabeled in three separate laboratories in Stockholm, Berkeley, and Nashville. Evaluation of seven [(125)I]iodine substituted analogs of raclopride, including IBZM, revealed the unusual high striatum-to-cerebellum ratio of 234 of [(125)I]epidepride in the rat. Subsequent SPECT images with [(123)I]epidepride demonstrated its ability to identify extrastriatal dopamine D(2) receptors in the human brain. Exploration of the structure of epidepride confirmed its exceptional properties, to be exceeded only by its N-allyl homolog, [(125)I]nalepride. The design by others of a series of potent 5-(3-[(18)F]fluoropropyl) substituted analogs of epidepride for PET imaging, lead to the discovery of [(18)F]fallypride. By elucidating the role of lipophilicity in the substituted benzamides, the excellent imaging characteristics of [(11)C]/[(123)I]epidepride, [(11)C]/[(76)Br]isoremoxipride and [(18)F]fallypride, could not only be explained but predicted with remarkable accuracy. By using the inverse product of the receptor affinity (K(D)), and the apparent partition constant of the radioligand (P((7.4))), estimates of maximal binding potential of any radioligand for imaging of any neurotransmitter receptor or transporter site seem possible.