Development of radioligands for emission tomography imaging of dopamine D1 and benzodiazepine receptors

Abstract

Brain imaging of dopamine and benzodiazepine (BZ) receptors in vivo is of central interest in neuropsychiatric disorders such as schizoprenia, Huntington's disease and epilepsy.

The objective was to develop new radioligands for emission tomography of dopamine D1 and BZ receptors. A series of new compounds with origin from Novo Nordisk, belonging to the chemical classes of benzazepines and benzodiazepines, were labelled with the positron emitting radionuclides 11C and 76Br for positron emission tomography (PET), they--emitting radionuclide 123l for single photon emission computed tomography (SPECT) and the longer-lived radionuclides 1251 and 14C for receptor characterisation in vitro and metabolite studies. The time course for the brain distribution of radioactivity was measured by PET or SPECTin the monkey and human brain. Radioligand metabolism measured in plasma was determined by gradient HPLC.

11C-Labelling was performed by N-methylation of secondary amines or amides using [11C]methyl iodide or[11C]methyl triflate. 76Br-labelling was performed in a Cu+ assisted nucleophilic substitution reaction or by electrophilic iododestannylation. Radioiodinations were accomplished using different approaches such asiododediazonization, nucleophilic exchange of radioiodine for bromine, iododestannylation and direct electrophilic substitution on activated aromatic systems. 14C-labelled benzazepines were synthesized in several steps starting from [14C]methyliodide.

The slow wash-out of the D1 antagonist [76Br]NNC22-0010 from the monkey brain allowed PET measurements over several hours. Of the seven D1 radioligands examined for PET[11C]NNC 22-0215 was the most slowly metabolised as measured in monkey plasma. [11C]NNC 756 and[11C]NNC 112 had the highest striatum to cerebellum ratio. The ratio was 4-5 in human studies. The highly selective radioligand[11C]NNC 112 is the most promising radioligand since radioactivity not only in the striatum but also in neocortical regions reached equilibrium conditions during time of a PET study which is of advantage for quantification of D1 receptors in neuropsychiatric disorders.

The two structurally closely related BZ agonists NNC 13-8241 and NNC 13 8199 were labelled for in vitro studies and PET/SPECT examinations. In vitro and in vivo binding studies in rats showed that [1251]NNC 13-8241 binds to BZ receptors with high affinity and low degree of non-specific binding. SPECT studies with [1231]NNC 13-8241 and PET studies with[76Br]- and [11C]NNC 13 8199 confirmed the low degree of non-specific binding of these radioligands and demonstrated saturable and reversible binding for central BZ receptors in both monkey and man. The metabolism of the radioligands observed in monkey and human plasma was very slow. The results indicate that[1231]NNC 13-8241 and [76Br]NNC 13-8199 have potential as radioligands for human emission tomography studies using quantitative models for which a slow rate of metabolism is of advantage.