Radiosynthesis of N-(4-chloro-3-[11C]methoxyphenyl)-2-picolinamide ([11C]ML128) as a PET radiotracer for metabotropic glutamate receptor subtype 4 (mGlu4)

Abstract

N-(Chloro-3-methoxyphenyl)-2-picolinamide (3, ML128, VU0361737) is an mGlu4 positive allosteric modulator (PAM), which is potent and centrally penetrating. 3 is also the first mGlu4 PAM to show efficacy in a preclinical Parkinson disease model upon systemic dosing. As a noninvasive medical imaging technique and a powerful tool in neurological research, positron emission tomography (PET) offers a possibility to investigate mGlu4 expression in vivo under physiologic and pathological conditions. We synthesized a carbon-11 labeled ML128 ([11C]3) as a PET radiotracer for mGlu4, and characterized its biological properties in Sprague Dawley rats. [11C]3 was synthesized from N-(4-chloro-3-hydroxyphenyl)-2-picolinamide (2) using [11C]CH3I. Total synthesis time was 38±2.2 min (n = 7) from the end of bombardment to the formulation. The radioligand [11C]3 was obtained in 27.7±5.3% (n = 5) decay corrected radiochemical yield based on the radioactivity of [11C]CO2. The radiochemical purity of [11C]3 was >99%. Specific activity was 188.7±88.8 GBq/μmol (n = 4) at the end of synthesis (EOS). PET images were conducted in 20 normal male Sprague Dawley rats including 11 control studies, 6 studies blocking with an mGlu4 modulator (4) to investigate specificity and 3 studies blocking with an mGlu5 modulator (MTEP) to investigate selectivity. These studies showed fast accumulation of [11C]3 (peak activity between 1-3 min) in several brain areas including striatum, thalamus, hippocampus, cerebellum, and olfactory bulb following with fast washout. Blocking studies with the mGlu4 modulator 4 showed 22-28 % decrease of [11C]3 accumulation while studies of selectivity showed only minor decrease supporting good selectivity over mGlu5. Biodistribution studies and blood analyses support fast metabolism. Altogether this is the first PET imaging ligand for mGlu4, in which the labeled ML128 was used for imaging its in vivo distribution and pharmacokinetics in brain.

Co-operative binding assay for the characterization of mGlu4 allosteric modulators

“…Despite the important role of binding characteristics for drug development there is no fully characterized cell based binding assay method available for mGlu 4 allosteric modulators (Kil et al, 2014a, 2013; Le Poul et al, 2012; Motlagh et al, 2014; Patent US20120245153). Moreover, the binding affinity of specific compound can be considered as an intrinsic property of the molecule, the co-operative effect is unique and dependent on the orthosteric/allosteric ligand pair in a specific microenvironment (Langmead, 2011).…”

“…1) was selected as a candidate for tritium labeling and reference compound ([ 3 H] 3 ) based on published in vitro and in vivo activity data, good aqueous solubility, biodistribution, and drug properties (Kil et al, 2013). ML128 represents the most potent and selective mGlu 4 probe, which is also developed as an mGlu4 positron emission tomography (PET) tracer (Hopkins et al, 2010; Kil et al, 2013). Furthermore, ML128 is centrally penetrating upon systemic dosing, displays excellent pharmacokinetics and has strong anti-Parkinsonian activity in preclinical models (Hopkins et al, 2010).…”

“…1) suffer from receptor specificity and the search for molecules that behave as allosteric modulators has led to finding of small molecular scaffolds targeting to other distinct binding cavities of the mGlu 4 (Christopoulos, 2002). The targeting of distinct allosteric binding site of mGlus with small molecular scaffolds made it possible to non-invasively image these receptors using nuclear medicine technologies, which have increased the knowledge of mGlus and mGlu targeting drugs in multiple diseases (Black et al, 2010; Drouin-Ouellet et al, 2011; Kil et al, 2013, 2014a, 2014b; Wang et al, 2012). During the last decades there has been a substantial progress in identifying positive allosteric modulators (PAM) for mGlu 4 (Conn et al, 2009).…”

Synthesis and evaluation of N-(methylthiophenyl)picolinamide derivatives as PET radioligands for metabotropic glutamate receptor subtype 4

“…A specific mGlu 4 PET radioligand could be an important tool for understanding the role of mGlu 4 in healthy and disease conditions, and also for the development of new drugs targeting this receptor. Recently, we reported a carbon-11 labeled PET radioligand [ 11 C] 2 30 and a fluorine-18 labeled PET radioligand [ 18 F] 8 31 (Fig. 2).…”

Re-exploring the N-phenylpicolinamide derivatives to develop mGlu4 ligands with improved affinity and in vitro microsomal stability

“…Recently, we have reported a carbon-11 labeled PET ligand [ 11 C] 2 ( N -(4-Chloro-3-[ 11 C]methoxyphenyl)picolinamide) 25 , which was based on a reported mGlu 4 PAM 2 16 . In 2009, two research groups at Addex Pharma 26 and Vanderbilt University 16 have independently disclosed a series of small arylamide compounds as a new class of mGlu 4 PAMs.…”