Chester A. Mathis

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.

[125I]5-iodo-6-nitroquipazine: a potent and selective ligand for the 5-hydroxytrrptamine uptake complex. II. In vivo studies in rats

In search of a potent and selective radioiodinated ligand for the 5-hydroxytryptamine (serotonin or 5-HT) uptake complex, we synthesized and evaluated the in vitro properties of [125I]5-iodo-6-nitroquipazine. The binding properties and pharmacological profile of this radioligand were studied in rat brain homogenates, and it was found to display high affinity and selectivity for the serotonin uptake complex. Scatchard analysis of the binding data indicated a single population of sites with a Kd of 23 +/- 6 pM and a Bmax of 430 +/- 50 fmol/mg protein (mean +/- S.E.M., n = 7). Inhibitors of serotonin uptake were the most efficient competitors for [125I]5-iodo-6-nitroquipazine binding with Ki values similar in rank order and magnitude to those obtained in studies of other established serotonin uptake blockers. Inhibitors of dopamine and norepinephrine uptake as well as a wide variety of postsynaptic receptor agents were relatively ineffective in inhibiting [125I]5-iodo-6-nitroquipazine binding to rat brain membranes. Serotonin was the only monoaminergic neurotransmitter capable of effectively competing for [125I]5-iodo-6-nitroquipazine binding sites and gave a Ki value of 2.8 +/- 0.6 microM. Lesions of the serotonergic system with p-chloroamphetamine resulted in a dramatic loss (> 90%) of [125I]5-iodo-6-nitroquipazine binding to rat cortical membranes. Non-radiolabeled 5-iodo-6-nitroquipazine potently inhibited the binding of [3H]paroxetine to serotonin reuptake sites in rat cortical membranes with a Ki of 0.17 +/- 0.06 nM.(ABSTRACT TRUNCATED AT 250 WORDS)

Quantitative in vitro and ex vivo autoradiography of the α2-adrenoceptor antagonist [3H]atopamezole

Abstract Atipamezole is an α-adrenoceptor antagonist with high affinity and selectivity for the α 2 -receptor. In vitro autoradiography of [ 3 H]atipamezole on rat and human brain sections revealed a pattern virtually identical to the one observed using 3 H-labeled antagonists such as idazoxan or methoxyidazoxan or agonists such as p-aminoclonidine and 5-bromo-6-(2-imidazolin-2-yl-amino-quinoxaline (UK-14304). In vivo studies of [ 3 H]atipamezole demonstrated good penetration into the brain (0.3–1.8% injected dose/g at 5 min, depending upon brain region). In addition, [ 3 H]atipamezole displayed rapid in vivo clearance of nonspecific binding such that at 1 h following an i.v. injection of the drug (100 μCi/animal, rat tail vein administration) the pattern of radioactivity in the brain correlated very well with the receptor distribution as seen by in vitro autoradiography. Atipamezole binding in vivo was displaceable by idazoxan. These results demonstrate the potential of suitably labeled atipamezole for regional studies of brain α 2 -adrenoceptors in vitro and in vivo, in experimental animals as well as in human positron emission tomography (PET) studies.

Autoradiographic localization of binding sites for 125I-DOI, a new psychotomimetic radioligand, in the rat brain

The selective, high-affinity binding of various psychotomimetic phenylisopropylamine derivatives to 5HT 2 receptors has been previously reported. Optimal stereospecific selectivity was detected for 4-bromoor 4-iodo-R-(-)-2,5-dimethoxy phenylisopropylamines. Both of these derivatives are potent hallucinogenic agents in man (Glennon et al., 1986). Using membrane binding techniques, these workers have characterized the 5HT 2 agonist properties of [3H](+)-DOB, which selectively labels a guanylate-sensitive high-affinity state of the receptor (Titeler et al., 1985). The low specific activity of [3H]DOB renders it impractical for use in autoradiographic receptor localization. The iodine-containing analog DOI (4iodo-2,5-dimethoxyphenylisopropylamine), however, possesses a stereospecific 5HT 2 selectivity comparable to DOB (Glennon et al., 1986), suggesting that 125I-labelled DOI could be used for autoradiographic localization of 5HT 2 receptors. Although 1311or 123I-labelled analogs of DOI have been used in vivo for brain imaging and metabolic studies (Sargent et al., 1984), the utifization of 125I-DOI for in vitro autoradiography has not been previously reported. We have initiated investigations of 125I-DOI as a ligand for autoradiography and report here our preliminary results. The R ( ) enantiomer of t25I-DOI was synthesized to a radiochemical purity of 99 + %. The

Binding Potency of Paroxetine Analogues for the 5-Hydroxytryptamine Uptake Complex*

Abstract— The in‐vitro inhibition constants (Ki) of 14 structural analogues of the potent 5‐hydroxytryptamine (5‐HT)‐uptake inhibitor paroxetine were determined to assess the structure‐affinity relationship of these derivatives. A goal of these studies was to determine those positions on paroxetine which could be derivatized without significantly decreasing the affinity of the drug for the binding site, so that radiolabels such as [18F]fluoroalkyl groups might be appended for future in‐vivo imaging studies of the 5‐HT uptake system. Using the methyl moiety as a steric probe for these studies, it was found that the rank order of potency of various methyl‐substituted paroxetine analogues for inhibiting the binding of [3H]paroxetine to the 5‐HT re‐uptake site was: 4′≅ 3′‐≅ 2″‐>2′‐≅ 1‐ >5″‐> 6″‐methyl. The in‐vitro equipotent molar ratios (EPMR, Ki(analogue)/Ki(paroxetine)) of the analogues were determined, and the EPMRs of the 4′‐,3′‐, and 2″‐methyl derivatives were 1·9, 2·2 and 2·2, respectively. The 4′‐ and 2″‐fluoromethyl and ‐fluoroethyl analogues were synthesized, and the EPMRs of the 4′‐ and 2″‐fluoromethyl derivatives were determined to be 2·0 and 3·5, and those of the 4′‐ and 2″‐ fluoroethyl analogues were 5·2 and 6·2, respectively. The 2″‐fluoromethyl analogue was unstable in aqueous solutions, and it is not a promising ligand for in‐vivo studies. The 4′‐fluoromethyl derivative was stable in aqueous solutions and, based upon its relatively high affinity, is a candidate for radiolabelling with 18F for in‐vivo positron emission tomography studies of the 5‐HT re‐uptake site.

A 122Xe122I generator for remote radio-iodinations

A 122Xe-122I generator system is described that produces 122I extraction efficiencies of approximately 60%. Radiocontaminants were less than 0.1% at the time of 122I removal following a 10 min ingrowth period. The chemical form of 122I was identified as [122I]iodide, and the [122I]iodide was remotely incorporated into radiopharmaceuticals for PET studies with an overall efficiency of as much as 40%.

Rapid brain scanning radiopharmaceutical

A method for detecting the blood flow in animals, particularly in the brain, is provided wherein a detectable amount of a novel radioactive compound of the formula I is administered to one animal: ##STR1## wherein R1 and R2 are independently alkyl of 1 to 6 carbon atoms or benzyl; R3 is alkyl of 1 to 6 carbon atoms, benzyl, cyclopropylalkyl of 4 to 6 carbon atoms, or cyanoalkyl of 2 to 6 carbon atoms; R4 is hydrogen, benzyl or alkyl of 1 to 6 carbon atoms; with the provisos that R4 is not isopropyl and when R4 is methyl, R3 is not benzyl; and X is a radioactive halogen.

Comparative evaluation of electrophilic aromatic iododemetallation techniques for labeling radiopharmaceuticals with iodine-122

A series of metallated arenes bearing sigma-bonded tin, mercury, germanium, boron, or silicon were evaluated as labeling substrates for the generator-produced positron emitter 122I(t1/2 = 3.6 min). Using dichloramine-T with 122I, radiochemical yields exceeding 90% were achieved after 1 min at 25 degrees C using stannylated or mercurated arenes in ethanol, or germylated arenes in acidic solvent. Stannylated or mercurated arenes resulted in high labeling yields even with deactivated aromatics in ethanol, but regiospecifically-iodinated products were obtained only from stannylated precursors due to differences in the syntheses of these organometallics. The implications of these results to the labeling of radiopharmaceuticals with 122I are discussed.

Synthesis of 1-[2′,5′-dimethoxy-4′-(β-fluoroethyl)phenyl]-2-aminopropane:studies related to 18F-labeled serotonin receptor ligands

Abstract Synthesis of the titled 2,5-dimethoxy-4-fluoroalkylamphetamine is reported. The highly functionalized aromatic nucleus of the key fluorination precursor was utimately derived from a low temperature aromatic halogen-lithium exchange reaction followed by alkylation of the resultant anion with ethylene oxide.

Fluoroalkylbenzenes: Synthesis of (S)-2.3-Dimethoxy-5-[(3-fluoro)propyll-6-hydroxy-N-](1-ethyl-2-pyr-rrolidinyl)methyllbenzamide

Abstract The synthesis of the titled fluoropropylsalicylamide ( 2 ) was achieved by a route which avoided the intramolecular cyclization reactions of ortho -[(3-fluoro)propyl]phenol substrates.