GPCRs constitute the largest class of known targets in commercial drug discovery today—collectively the targets of at least 30% of currently marketed drugs. Yet, as heptahelical transmembrane proteins involved in divergent and manifold signaling pathways, they can confound conventional drug discovery techniques.
The recent “Congress on G Protein-Coupled Receptors in Drug Discovery” meeting highlighted key approaches from companies pursuing developments in biased agonism, structure elucidation for ligand prediction, allosteric modulation, and advancement of GPCRs from preclinical to clinical studies, particularly with small molecule drug candidates.
Guido Zaman, Ph.D., who heads assay development and pharmacology support for GPCR and enzyme projects at MSD/Merck, presented data on low molecular weight β–arrestin biased ligands of a class B GPCR, the parathyroid hormone receptor PTH-R1.
“Identification of small molecule drugs for GPCRs is not trivial,” Dr. Zaman said, citing a general paucity of structural information on most GPCRs. Further, class B GPCRs, whose native activating ligands are proteins and large peptides, can prove particularly recalcitrant to pharmacological intervention by small molecules.
Dr. Zaman’s lab and others are focused on bypassing these challenges by means of biased ligand binding, specifically through manipulation of β-arrestin. Because β-arrestin attenuates G protein coupling to GPCRs either by binding-site occlusion or by promoting cellular internalization of the GPCR, it is a key intervention point in GPCR-related drug discovery.
Binding of either β-arrestin or G protein will be associated with discrete downstream signaling cascades; depending on the particular disease pathway or side-effect profile, either could be desired. As Dr. Zaman noted, in the case of PTH, some models in the literature have shown that an anabolic (tissue-building) response is mediated by β-arrestin, and an osteoclast (bone-degrading) pathway is mediated through G proteins.
Precise and informative assays are thus critical to understanding these complex interactions such that they may be harnessed for effective pharmaceutical development. Dr. Zaman explained that MSD has refined a two-part assay system that confirms biased β-arrestin signaling recruitment: a live-cell, high-content green fluorescent protein imaging assay and a beta-galactosidase enzyme fragment complementation assay. The former permits study of the receptor in nonmodified form; the latter allows cost-effective, high-throughput measurements on standard chemiluminescence readers, he remarked.
MSD has developed an in silico modeling approach that Dr. Zaman said is predictive of small molecule interactions with GPCRs, for in silico selection of compound libraries. Such developments may contribute to a move away from ultrahigh-throughput screening while allowing for increased specificity, always a key concern with any drug candidate but of particular importance with GPCRs given their involvement with numerous signaling pathways.
MSD is using these approaches to develop low molecular weight compounds whose effects mimic those of natural hormones. Its most advanced project in this area, focused on gonadotropins, is currently in the clinic.