Orally bioavailable imidazoazepanes as calcitonin gene-related peptide (CGRP) receptor antagonists: discovery of MK-2918

Amide Bond Bioisosteres: Strategies, Synthesis, and Successes

The amide functional group plays a key role in the composition of biomolecules, including many clinically approved drugs. Bioisosterism is widely employed in the rational modification of lead compounds, being used to increase potency, enhance selectivity, improve pharmacokinetic properties, eliminate toxicity, and acquire novel chemical space to secure intellectual property. The introduction of a bioisostere leads to structural changes in molecular size, shape, electronic distribution, polarity, pK_a, dipole or polarizability, which can be either favorable or detrimental to biological activity. This approach has opened up new avenues in drug design and development resulting in more efficient drug candidates introduced onto the market as well as in the clinical pipeline. Herein, we review the strategic decisions in selecting an amide bioisostere (the why), synthetic routes to each (the how), and success stories of each bioisostere (the implementation) to provide a comprehensive overview of this important toolbox for medicinal chemists.

Blocking the CGRP Pathway for Acute and Preventive Treatment of Migraine: The Evolution of Success

The pivotal role of calcitonin gene-related peptide (CGRP) in migraine pathophysiology was identified over 30 years ago, but the successful clinical development of targeted therapies has only recently been realized. This Perspective traces the decades long evolution of medicinal chemistry required to advance small molecule CGRP receptor antagonists, also called gepants, including the current clinical agents rimegepant, vazegepant, ubrogepant, and atogepant. Providing clinically effective blockade of CGRP signaling required surmounting multiple challenging hurdles, including defeating a sizable ligand with subnanomolar affinity for its receptor, designing antagonists with an extended confirmation and multiple pharmacophores while retaining solubility and oral bioavailability, and achieving circulating free plasma levels that provided near maximal CGRP receptor coverage. The clinical efficacy of oral and intranasal gepants and the injectable CGRP monoclonal antibodies (mAbs) are described, as are recent synthetic developments that have benefited from new structural biology data. The first oral gepant was recently approved and heralds a new era in the treatment of migraine.

Applications of amide isosteres in medicinal chemistry

Isosteric replacement of amide groups is a classic practice in medicinal chemistry. This digest highlights the applications of most commonly employed amide isosteres in drug design aiming at improving potency and selectivity, optimizing physicochemical and pharmacokinetic properties, eliminating or modifying toxicophores, as well as providing novel intellectual property of lead compounds.

CGRP – a target for acute therapy in migraine: Clinical data

Background

A better understanding of the mechanisms underlying the migraine attack has reinforced the concept that migraine is a complex brain disease, and has paved the way for the development of new migraine specific acute treatments. In recent years, targeting the calcitonin gene-related peptide and its receptors has been one of the most promising pharmacological strategies for both acute and preventive treatment of migraine.

Findings

Randomized double-blind placebo-controlled trials have demonstrated the superiority of small molecule calcitonin gene-related peptide receptor antagonists (gepants) over placebo in treating acute migraine attacks measured as the two-hour pain free endpoint. Gepants also improved migraine associated symptoms, such as nausea, photophobia and phonophobia. Two of the class have had their development stopped because of hepatotoxicity, which is emerging as being due to metabolites. Gepants have a good tolerability and can be safely used in patients with stable cardiovascular disease.

Conclusion

Exciting results have been obtained targeting the calcitonin gene-related peptide pathway to abort acute migraine attacks, thus reinforcing the relevance of mechanism-based treatments specific for migraine.

Calcitonin Gene-Related Peptide Antagonists and Therapeutic Antibodies

The calcitonin gene-related peptide (CGRP) receptor is composed of the calcitonin receptor-like receptor (CLR, a class B GPCR) and a single-pass membrane protein known as receptor activity modifying protein type 1 (RAMP1). The levels of the CGRP peptide increase during a migraine attack and infusion of CGRP can provoke a migraine attack. Consequently, there is much interest in inhibiting the actions of CGRP as a way to control migraine. Here we describe the development of small molecule antagonists designed to bind to the CGRP receptor to block its action by preventing binding of the CGRP peptide. We also describe the development of antibody drugs, designed to bind either to the CGRP receptor to block its action, or to bind directly to the CGRP peptide. The field has been very active, with one antibody drug approved and three antibody drugs in phase III clinical trial. Initial programs on the development CGRP antagonists were frustrated by liver toxicity but the current outlook is very promising with five small molecule antagonists in various stages of clinical trial.

Calcitonin gene-related peptide: physiology and pathophysiology

Calcitonin gene-related peptide (CGRP) is a 37-amino acid neuropeptide. Discovered 30 years ago, it is produced as a consequence of alternative RNA processing of the calcitonin gene. CGRP has two major forms (α and β). It belongs to a group of peptides that all act on an unusual receptor family. These receptors consist of calcitonin receptor-like receptor (CLR) linked to an essential receptor activity modifying protein (RAMP) that is necessary for full functionality. CGRP is a highly potent vasodilator and, partly as a consequence, possesses protective mechanisms that are important for physiological and pathological conditions involving the cardiovascular system and wound healing. CGRP is primarily released from sensory nerves and thus is implicated in pain pathways. The proven ability of CGRP antagonists to alleviate migraine has been of most interest in terms of drug development, and knowledge to date concerning this potential therapeutic area is discussed. Other areas covered, where there is less information known on CGRP, include arthritis, skin conditions, diabetes, and obesity. It is concluded that CGRP is an important peptide in mammalian biology, but it is too early at present to know if new medicines for disease treatment will emerge from our knowledge concerning this molecule.

Localization of CGRP receptor components, CGRP, and receptor binding sites in human and rhesus cerebellar cortex

The cerebellum is classically considered to be mainly involved in motor processing, but studies have suggested several other functions, including pain processing. Calcitonin-gene-related peptide (CGRP) is a neuropeptide involved in migraine pathology, where there is elevated release of CGRP during migraine attacks and CGRP receptor antagonists have antimigraine efficacy. In the present study, we examined CGRP and CGRP receptor binding sites and protein expression in primate cerebellar cortex. Additionally, mRNA expression of the CGRP receptor components, calcitonin receptor-like receptor (CLR) and receptor activity modifying protein 1 (RAMP1), was examined. In addition, expression of procalcitonin was studied. We observed high [(3)H]MK-3207 (CGRP receptor antagonist) binding densities in the molecular layer of rhesus cerebellar cortex; however, due to the limit of resolution of the autoradiographic image the exact cellular localization could not be determined. Similarly, [(125)I]CGRP binding was observed in the molecular layer and Purkinje cell layer of human cerebellum. CLR and RAMP1 mRNA was expressed within the Purkinje cell layer and some expression was found in the molecular layer. Immunofluorescence revealed expression of CGRP, CLR, and RAMP1 in the Purkinje cells and in cells in the molecular layer. Procalcitonin was found in the same localization. Recent research in the biology of cerebellum indicates that it may have a role in nociception. For the first time we have identified CGRP and CGRP receptor binding sites together with CGRP receptor expression through protein and mRNA localization in primate cerebellar cortex. These results point toward a functional role of CGRP in cerebellum. Further efforts are needed to evaluate this.

Metabolically Stable tert-Butyl Replacement

Susceptibility to metabolism is a common issue with the tert-butyl group on compounds of medicinal interest. We demonstrate an approach of removing all the fully sp(3) C-Hs from a tert-butyl group: replacing some C-Hs with C-Fs and increasing the s-character of the remaining C-Hs. This approach gave a trifluoromethylcyclopropyl group, which increased metabolic stability. Trifluoromethylcyclopropyl-containing analogues had consistently higher metabolic stability in vitro and in vivo compared to their tert-butyl-containing counterparts.