Synthesis of high specific activity 125I- and 123I-labelled enantiomers of 2,5-dimethoxy-4-iodophenylisopropylamine (DOI)

Development of Fluorescent Probes that Target Serotonin 5-HT2B Receptors

Some 5-HT2B fluorescent probes were obtained by tagging 1-(2,5-dimethoxy-4-iodophenyl)-propan-2-amine (DOI) with a subset of fluorescent amines. Some of the resulting fluorescent ligands showed excellent affinity and selectivity profiles at the 5-HT2B receptors (e.g. 12b), while retain the agonistic functional behaviour of the model ligand (DOI). The study highlighted the most salient features of the structure-activity relationship in this series and these were substantiated by a molecular modelling study based on a receptor-driven docking model constructed on the basis of the crystal structure of the human 5-HT2B receptor. One of the fluorescent ligands developed in this work, compound 12i, specifically labelled CHO-K1 cells expressing 5-HT2B receptors and not parental CHO-K1 cells in a concentration-dependent manner. 12i enables imaging and quantification of specific 5-HT2B receptor labelling in live cells by automated fluorescence microscopy as well as quantification by measurements of fluorescence intensity using a fluorescence plate reader.

Experimental evaluation of the generalized vibrational theory of G protein-coupled receptor activation

Recently, an alternative theory concerning the method by which olfactory proteins are activated has garnered attention. This theory proposes that the activation of olfactory G protein-coupled receptors occurs by an inelastic electron tunneling mechanism that is mediated through the presence of an agonist with an appropriate vibrational state to accept the inelastic portion of the tunneling electron's energy. In a recent series of papers, some suggestive theoretical evidence has been offered that this theory may be applied to nonolfactory G protein-coupled receptors (GPCRs), including those associated with the central nervous system (CNS). [Chee HK, June OS (2013) Genomics Inform 11(4):282-288; Chee HK, et al. (2015) FEBS Lett 589(4):548-552; Oh SJ (2012) Genomics Inform 10(2):128-132]. Herein, we test the viability of this idea, both by receptor affinity and receptor activation measured by calcium flux. This test was performed using a pair of well-characterized agonists for members of the 5-HT₂ class of serotonin receptors, 2,5-dimethoxy-4-iodoamphetamine (DOI) and N,N-dimethyllysergamide (DAM-57), and their respective deuterated isotopologues. No evidence was found that selective deuteration affected either the binding affinity or the activation by the selected ligands for the examined members of the 5-HT₂ receptor class.

Pharmacokinetics and brain distribution in non human primate of R(-)[123I]DOI, A 5HT(2A/2C) serotonin agonist

Our goal was to synthesize with high specific activity R(-)-1-(2,5-Dimethoxy-4-[123I]iodophenyl)-2-aminopropane [R(-)[123I]DOI], an in vitro potent and selective 5-HT(2A/2C) serotonin agonist, and study in vivo its plasma pharmacokinetics and brain distribution in baboon by SPECT. The purpose was to evaluate this radiotracer as a potential tool in discerning the role of the agonist high affinity state of 5-HT(2) receptors in depression and other neurological disorders. The radiotracer was prepared by electrophilic radioiodination of the N-trifluoroacetyl precursor of R(-)-1-(2,5-Dimethoxyphenyl)-2-aminopropane [R(-)DMA-TFA] with high-purity sodium [123I]iodide in the presence of chloramine-T, followed by amino deprotection with KOH in isopropanol (labeling yield: 73%, radiochemical yield: 62%, radiochemical purity: 99%). In vivo studies in baboon showed high accumulation of radioactivity in thalamus, the frontoparietal cortex, temporal, occipital and the striatum regions, with slightly lower accumulation in the midbrain and cerebellum. Ketanserin did not displaced the radioactivity in any of these brain regions. Plasma metabolite analysis was performed using methanol protein precipitation, the methanol fractions contained from 68% to 92% of the mixture of a labeled metabolite and parent compound. The recovery coefficient of unmetabolized R(-)[123I]DOI was 68%. The percent parent compound present in the extracted fraction, measured by HPLC, decreased gradually with time from 99.8% to 0.3% still present after 4.7 hours post injection whereas the percentage of the only one detected metabolite increased conversely. Free fraction determination (f(1)), was 31 +/- 0.9% (n = 3). For comparison purposes, ex-vivo brain distribution, displacement and metabolite analysis was also carried out in rodents. Although R(-)[123I]DOI displayed good brain uptake and localized in serotonergic areas of the brain, its target to non target ratio and its insensitivity to ketanserin displacement suggest high nonspecific uptake, therefore non potentially useful as brain imaging radiotracer for visualization of the agonist high-affinity state of 5-HT(2A) receptors and for visualizing 5-HT(2C) receptors by SPECT.

Synthesis and pharmacological evaluation of ring-methylated derivatives of 3,4-(methylenedioxy)amphetamine (MDA)

The three isomeric ring-methylated derivatives of the well-known hallucinogen and entactogen MDA (1a) were synthesized and evaluated for pharmacological activity as monoamine-releasing agents and as serotonin agonists. The 2-methyl derivative 2a and the 5-methyl derivative 2b were found to be more potent and more selective than the parent compound in inhibiting [3H]-serotonin accumulation in rat brain synaptosomal preparations. Their activity in vivo was confirmed in rats trained to discriminate serotonin-releasing agents and hallucinogens from saline. The results indicate that compounds 2a,b are among the most potent 5-HT-releasing compounds known and show promise as lead compounds in the search for antidepressant drugs that release serotonin rather than inhibit its uptake.

Radioiodinated D-(+)-N1-ethyl-2-iodolysergic acid diethylamide: a ligand for in vitro and in vivo studies of serotonin receptors

Radioiodinated D-(+)-N1-ethyl-2-iodolysergic acid diethylamide ([125I]-EIL) has been evaluated as a ligand for in vitro and in vivo studies of cerebral serotonin 5-HT2 receptors. [125I]-EIL exhibited high affinity (KD = 209 pM) for 5-HT2 receptors with a high degree of specific binding (80-95%) in membranes from rat prefrontal cortex. The regional distribution of [125I]-EIL binding in vivo to seven areas of mouse brain correlated significantly (Rs = 0.93) with known densities of 5-HT2 receptors. In vivo specificity, defined by tissue to cerebellum radioactivity ratios, reached a maximum for frontal cortex at 6 hr (21.2) and persisted through 16 hr (8.8). Ketanserin, a 5-HT2 receptor antagonist, fully inhibited binding in a dose dependent fashion in all brain regions except cerebellum. By contrast, blockers for dopamine D2, alpha- or beta-adrenergic receptors did not significantly inhibit radioligand binding in any region. [125I]-EIL selectively labels 5-HT2 receptors in vivo with the highest specificity of any serotonergic ligand reported to date, indicating that [123I]-EIL should prove applicable to single photon emission computed tomography studies in living brain.

Differential interactions of indolealkylamines with 5-hydroxytryptamine receptor subtypes

Affinities of drugs for 21 indolealkylamine derivatives, some with putative hallucinogenic activity, were determined at 5-HT1A, 5-HT2A and 5-HT2B recognition sites, using radioligand competition studies. Nearly all of the derivatives displayed greatest potency for the 5-HT2A receptor, labelled by [125I]R-(-)DOI in the cortex of the rat. Most derivatives displayed 2-10 times lower affinity at the HT2B receptor labelled by [3H]ketanserin in bovine cortex. Derivatives lacking ring substituents displayed lower affinities for all of the recognition sites, compared to derivatives substituted in the 4- or 5-position of the indole ring. The 4-hydroxylated derivatives displayed 25-380-fold selectivity for the 5-HT2A site, vs the 5-HT1A site, while the 5-substituted derivatives displayed approximately equal potency at the 5-HT1A and 5-HT2A sites. Affinity of all the compounds at the 5-HT2B site was greater than 300 nM. The 6-substituted derivatives displayed greater than micromolar affinities for all of the 5-HT recognition sites examined. The size of the N,N-dialkyl substituent was a secondary determinant of affinity, with groups larger than N,N-diisopropyl resulting in a marked reduction in affinity at both the 5-HT2A and 5-HT1A recognition sites. This study demonstrated that hallucinogenic 4-hydroxy-indolealkylamines, like psychotomimetic phenylisopropylamines, bind potently and selectively to the 5-HT2A recognition site, labelled by [125I]R-(-)DOI. This provides further evidence indicating that this recently described subtype of the 5-HT2 receptor may partially mediate the action of hallucinogenic agents.

[125I]-2-(2,5-dimethoxy-4-iodophenyl)aminoethane ([125I]-2C-I) as a label for the 5-HT2 receptor in rat frontal cortex

Recent studies of 5-HT2 receptor binding have involved the use of radiolabeled agonists. This report describes the use of [125I]-2-(2,5-dimethoxy-4-iodophenyl)aminoethane ([125I]-2C-I) as a label for low-density 5-HT2 agonist binding sites. A nonhydrolyzable analog of GTP, GppNHp, was found to inhibit the high affinity binding of [125I]-2C-I. 5-HT and several 5-HT2 agonists and antagonists displayed high affinity for this site. In addition, a significant decrease in the Bmax value, but not the KD for [125I]-2C-I was observed at 37 degrees C as compared to that observed at 24 degrees C. Several structure-activity relationships were investigated for displacement of [125I]-2C-I, and the results are consistent with the importance of this receptor in the mechanism of action of hallucinogens. This study demonstrates the utility of [125I]-2C-I as a novel radioligand and provides further data that the 5-HT2 receptor is significantly linked to hallucinogenic activity for several compounds.

Common receptors for hallucinogens in rat brain: a comparative autoradiographic study using [125I]LSD and [125I]DOI, a new psychotomimetic radioligand

The S and R enantiomers of the psychotomimetic 5HT2 agonist DOI (2,5-dimethoxy-4-iodophenylisopropylamine) were labeled with 125I at high-specific activity. The regional distribution of binding sites for each of the enantiomers was investigated using in vitro quantitative autoradiography and compared to the regional distribution of [125I]LSD in the rat brain. Saturable, specific binding of the radioligands was determined in cortical membrane homogenates. All radioligands exhibited specific binding in localized regions throughout the rat brain. Binding of [125I]DOI enantiomers was completely displaced (greater than 90%) by 1 microM of the corresponding unlabeled enantiomer; [125I]LSD was completely displaced by 1 microM LSD. The choroid plexus showed the highest-density binding. Other regions showing high-density binding included the frontoparietal cortex (motor and somatosensory areas), anterior cingulate gyrus, lateral olfactory tubercle, nucleus accumbens, caudate nuclei, claustrum, nucleus of the lateral olfactory tract, dentate gyrus, mamillary nuclei, and motor trigeminal nuclei. In most regions, [125I]S-DOI, the less active enantiomer, exhibited 25-40% of the amount of total binding as [125I]R-DOI. In some regions, [125I]R-DOI and [125I]LSD had similar binding densities; in others, marked differences were apparent. The regional distribution of specific [125I]R-DOI binding sites correlated with the distribution of 5HT2 receptors reported in previous studies. DOI and its analogs may have potential clinical applications for in vivo localization of 5HT2-receptors using positron emission tomography (PET) and similar techniques.

Autoradiographic localization of 5HT2 receptors in rat brain using [125I]-DOI, a selective psychotomimetic radioligand

1. Binding sites for the R and S enantiomers of the 5HT2 agonist DOI (2,5-dimethoxy-4-iodophenylisopropylamine) were identified in rat brain using quantitative in-vitro autoradiography and compared with [125I]-LSD binding. 2. In most regions of the brain, binding density of the less active isomer [125I]S-DOI was 15 to 85% of that exhibited by the active [125I]R-DOI isomer. 3. Cortical membrane preparations exhibited two binding sites, of the enantiomers with high (KdH) and low (KdL) affinity constants of 1.2 +/- 0.02 nM and 29 +/- 7 nM for the [125I]R-DOI and 2.1 +/- 0.2 nM and 18 +/- 4 nM for [125I]S-DOI respectively. The respective high (BmaxH) and low (BmaxL) binding densities were 92 +/- 10 and 536 +/- 164 fmol/mg protein for the [125I]R-DOI and 67 +/- 19 and 245 +/- 60 fmol/mg protein for [125I]S-DOI. 4. Our results correlate with regional distribution of 5HT2 receptors reported in previous studies and indicate that DOI and its congeners have potential clinical applications for the in-vivo localization of 5HT2 receptors.