Synthesis and in vivo evaluation of [(11)C]CGP62349, a new GABA(B) receptor antagonist

Synthesis and Evaluation of a New 18F-Labeled Radiotracer for Studying the GABAB Receptor in the Mouse Brain

New GABA_B agonists, fluoropyridyl ether analogues of baclofen, have been synthesized as potential PET radiotracers. The compound with highest inhibition binding affinity as well as greatest agonist response, ( R)-4-amino-3-(4-chloro-3-((2-fluoropyridin-4-yl)methoxy)phenyl)butanoic acid (1b), was radiolabeled with ¹⁸F with good radiochemical yield, high radiochemical purity, and high molar radioactivity. The regional brain distribution of the radiolabeled ( R)-4-amino-3-(4-chloro-3-((2-[¹⁸F]fluoropyridin-4-yl)methoxy)phenyl)butanoic acid, [¹⁸F]1b, was studied in CD-1 male mice. The study demonstrated that [¹⁸F]1b enters the mouse brain (1% ID/g tissue). The accumulation of [¹⁸F]1b in the mouse brain was inhibited (35%) by preinjection of GABA_B agonist 1a, suggesting that the radiotracer brain uptake is partially mediated by GABA_B receptors. The presented data demonstrate a feasibility of imaging of GABA_B receptors in rodents and justify further development of GABA_B PET tracers with improved specific binding and greater blood-brain barrier permeability.

The potential efficacy of GABAB antagonists in depression

Evidence for the potential utility of GABA_B antagonists has been assembled from a variety of sources, including clinical experience with the GABA_B agonist baclofen, murine genetic GABA_B knock-outs, rodent studies of GABA_B receptor expression and function following treatment with antidepressant therapies, animal models of depression, and some functional and post mortem data from human subjects. Definitive testing of GABA_B antagonists in depression, however, still awaits the development of potent, selective and brain-penetrant compounds for human testing.

Development of new carbon-11 labelled radiotracers for imaging GABAA- and GABAB-benzodiazepine receptors

Two quinolines identified as positive allosteric modulators of γ-aminobutyric acid (GABA)(A) receptors containing the α(2) subunit, 9-amino-2-cyclobutyl-5-(6-methoxy-2-methylpyridin-3-yl)-2,3-dihydro-1H-pyrrolo[3,4-b]quinolin-1-one (4) and 9-amino-2-cyclobutyl-5-(2-methoxypyridin-3-yl)-2,3-dihydro-1H-pyrrolo[3,4-b]quinolin-1-one (5), were radiolabelled at the methoxy position with carbon-11 (half-life=20.4 min). These quinolines represent a new class of potential radiotracers for imaging the benzodiazepine site of GABA(A) receptors with positron emission tomography (PET). Both radiotracers were reliably isolated following reaction of their respective pyridinone/pyridinol tautomeric precursors with [(11)C]CH(3)I in clinically useful, formulated quantities (2.9% and 2.7% uncorrected radiochemical yield, respectively, relative to [(11)C]CO(2)) with high specific activities (>70 GBq μ mol(-1); >2 Ci μ mol(-1)) and high radiochemical purities (>95%). The radiosyntheses reported herein represent rare examples of selectively isolating radiolabelled compounds bearing [(11)C]2-methoxypyridine moieties. Although both radiotracers demonstrated promising imaging characteristics based on preliminary ex vivo biodistribution studies in conscious rodents, higher brain uptake was observed with [(11)C]5 and therefore this radiotracer was further evaluated. Carbon-11 labelled 5 readily penetrated the brain (>1 standard uptake value in cortical regions at 15 min post-injection of the radiotracer), had an appropriate regional brain distribution for GABA(A) receptors that appeared to be reversible, and did not show any appreciable radiometabolites in rat brain homogenates up to 15 min post-injection. Preadministration of flumazenil (1, 10 mg kg(-1)) or 5 (5 mg kg(-1)) effectively blocked >50% of [(11)C]5 binding to the GABA(A) receptor-rich regions, thereby suggesting that this radiotracer is worthy of further evaluation for imaging GABA(A) receptors. Additionally (R,S)-N-(1-(3-chloro-4-methoxyphenyl)ethyl)-3,3-diphenylpropan-1-amine, 6, an allosteric modulator of GABA(B) receptors, was efficiently labelled in one step using [(11)C]methyl iodide. Ex vivo biodistribution studies in conscious rats showed low brain uptake, therefore, efforts are underway to discover alternative radiotracers to image GABA(B). In conclusion, [(11)C]5 is worthy of further evaluation in higher species for imaging GABA(A) receptors in the central nervous system.

Ligands for expression cloning and isolation of GABA(B) receptors

The scope of the plenary lecture at the occasion of the Xth Meeting on Heterocyclic Structures in Medicinal Chemistry, Palermo 2002, is considerably larger than that of the main lecture at the XVIth International Symposium on Medicinal Chemistry, Bologna 2000, described by Froestl et al. in Farmaco 56 (2001) 101. Additional information is presented, in particular, on the reaction conditions for the 31 step synthesis of the combined affinity chromatography and photoaffinity radioligand [125I]CGP84963 and on the recent developments of the molecular biology of GABA(B) receptors.

Effects of gamma-aminobutyrate B receptor modulation on normal micturition and oxyhemoglobin induced detrusor overactivity in female rats

PURPOSE

Using baclofen (Sigma-Aldrich, Steinheim, Germany), a gamma-aminobutyrate B (GABA(B)) receptor agonist, and CGP62349 (AstraZeneca R & D Mölndal, Sweden), a GABA(B) receptor antagonist, in a rat model of conscious micturition we addressed certain questions, including whether there is a tonic GABA(B) receptor influence on normal bladder function, how baclofen affects normal and C-fiber activated micturition, and where the sites of GABA(B) receptor action are.

MATERIALS AND METHODS

Nonanesthetized female Sprague-Dawley rats were used. The bladder was catheterized and other catheters were placed intravenously, intrathecally or intracerebroventricularly. At 3 days the rats underwent cystometric investigation in a metabolic cage. Micturition was stimulated by infusing saline intravesically. Overactivity caused by C-fiber activation was induced by intravesical oxyhemoglobin. Drugs were given intravenously, intrathecally or intracerebroventricularly. Micturition parameters were recorded and compared before and after drug administration.

RESULTS

Baclofen at doses of 0.5 microg. intrathecally and 0.3 microg. intracerebroventricularly increased bladder capacity and threshold pressure. Overflow incontinence developed in 4 of 7 rats after 0.5 microg. baclofen intrathecally and in 5 of 7 after 1 microg. baclofen intracerebroventricularly. CGP62349 at a dose of 30 microg. intrathecally and intracerebroventricularly had a stimulatory effect on micturition, which was attenuated by baclofen. While intravenous baclofen at 1 mg. (-1)kg. was devoid of effects, intravenous baclofen at 4 mg. kg. (-1)tended to decrease micturition pressure, bladder capacity and micturition volume. Infusion volume decreased significantly, demonstrating a diuretic effect, which was abolished by pretreatment with subcutaneous desmopressin at 25 ng. kg. (-1). CGP62349 at 2 mg. kg. (-1) intravenously had a stimulatory effect on micturition, which was inhibited by baclofen. Intravesical oxyhemoglobin at 250 microM. induced bladder overactivity, which was attenuated by baclofen at 4 mg. kg. (-1) intravenously and abolished by baclofen 0.5 microg intrathecally.

CONCLUSIONS

In the normal rat stimulation of GABA(B) receptors, mainly in the central nervous system, inhibits micturition. Antagonism of GABA(B) receptors stimulates micturition, suggesting that the receptors are under tonic GABAergic influence. Baclofen intrathecally attenuated oxyhemoglobin induced detrusor overactivity, suggesting that the inhibitory actions of GABA(B) receptor agonists in the spinal cord may be useful for controlling micturition disorders caused by C-fiber activation in the urothelium and/or suburothelium.

PET in psychopharmacology

Emission tomography techniques and, in particular, positron emission tomography (PET) enable the in vivo study of several physiological and neurochemical variables in human subjects using methods originally developed for quantitative autoradiography. In particular, PET allows one to evaluate in human subjects: (a) the effect of specific neurochemical challenges on regional brain function at rest or under activation; (b) the activity of neurotransmitters and the regional expression of specific molecular targets during pathology including their modulation by drug treatment; (c) the kinetics of drug disposition and activity directly in the target organ. This is of primary interest in the field of biological psychiatry and in psychoactive drugs development, where it is particularly difficult to reproduce human diseases using animal models in view of the peculiarity of this field and the large heterogeneity of each psychiatric illness also inside the same clinical definition. The aim of this paper is to review the principal strategies and the main results of the use of PET in psychopharmacology.