Key aspects of modern GPCR drug discovery

G-protein-coupled receptors (GPCRs) are the largest and most versatile cell surface receptor family with a broad repertoire of ligands and functions. We've learned an enormous amount about discovering drugs of this receptor class since the first GPCR was cloned and expressed in 1986, such that it's now well-recognized that GPCRs are the most successful target class for approved drugs. Here we take the reader through a GPCR drug discovery journey from target to the clinic, highlighting the key learnings, best practices, challenges, trends and insights on discovering drugs that ultimately modulate GPCR function therapeutically in patients. The future of GPCR drug discovery is inspiring, with more desirable drug mechanisms and new technologies enabling the delivery of better and more successful drugs.

Abstract 668: The translational biology of small molecule GPR65 inhibitors: shared effects between mouse models and human primary tumors highlight the unique transformative potential of targeting a genetically validated innate immune checkpoint

Whilst the advent of Immune Checkpoint Blockade has revolutionized the management of cancer, a significant proportion of patients have limited or absent response to these therapies. A key cause of this immune insensitivity is the hostile solid tumor microenvironment (TME) dominated by immunosuppressive myeloid cells. We previously identified the acid sensing G protein coupled receptor (GPCR), GPR65, as a primary determinant of these suppressive cells. In mice, genetic deletion of Gpr65 or oral administration of small molecule GPR65 inhibitors in vivo causes a profound repolarization of immunosuppressive tumor associated macrophages, an increase in infiltrating effector cells and potent anti tumor effects in syngeneic models. In TCGA data, across all tumors, patients homozygous for a hypomorphic coding variant in GPR65 (I231L) show increased overall survival, providing compelling genetic evidence of the clinical potential of GPR65 inhibition. To further explore the translational potential of GPR65 we employed a range of techniques to define the human biology of this receptor in different contexts. At the mechanistic level, single cell RNA sequencing (scRNAseq) of human PBMCs obtained from healthy donors demonstrated a pronounced effect of low pH on the myeloid compartment, with a clear polarization of these cells toward an immunosuppressive character and modulation of GPR65 expression. In parallel, pharmacological inhibition of GPR65 in human monocyte derived macrophages exposed to low pH demonstrated that equivalent gene expression changes are primarily due to GPR65 activation. To examine the relevance of these findings to the intact acidic human TME, we performed studies in fresh primary human tumor histocultures from clear cell renal cell carcinoma (ccRCC) patients with immunohistochemically confirmed high macrophage infiltration and carbonic anhydrase 9 (CA9) expression. In these cultures, GPR65 inhibition caused a dose dependent suppression of a geneset closely overlapping with that modulated by GPR65 in primary macrophages. Furthermore, we observed a marked decrease in immune suppressive IL10 secretion with coincident elevation of specific proinflammatory chemokines. Consistent with these findings, in vivo administration of a small molecule GPR65 inhibitor elicited similar changes in human CA9 expressing RCC PDX tumors implanted in myeloid boosted CD34+ stem cell engrafted NCG mice. In summary, inhibition of GPR65 provides a unique and genetically validated approach to favorably modify the immunosupressive TME with features highly conserved between mouse and human contexts. We propose that GPR65 inhibition holds significant clinical promise, with specific evidence around ccRCC as a potential standout indication.

Citation Format: Barbara Cipriani, Alastair Corbin, David Miller, Alan Naylor, Faraz Khan, Gavin Milne, Barbara Young, Rupert Satchell, Sourav Sarkar, Mussa Quareshy, Anastasia Nika, Preeti Singh, Gavin Knox, Darryl Turner, Satish Sankaran, Nandini Pal Basak, Toszka Bohn, Tobia Bopp, Surya Koturan, Bo Sun, Benjamin Fairfax, Tom McCarthy, Stuart Hughes. The translational biology of small molecule GPR65 inhibitors: shared effects between mouse models and human primary tumors highlight the unique transformative potential of targeting a genetically validated innate immune checkpoint [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 668.

Abstract 2162: Inhibition of GPR65 counteracts low pH induced immunosuppressive polarization of macrophages: In vitro and in vivo characterization of potent, selective and orally bioavailable small molecule GPR65 antagonists

The acidic tumour microenvironment (TME) and the abundance of tumour associated macrophages (TAMs) are key features of solid tumours that drive immune suppression, support tumour growth and limit the efficacy of approved therapies. We identified the pH sensing GPCR, GPR65, as a key determinant of low pH induced immune suppression in human cancers, particularly in TAMs, based on the following observations: 1) cancer patients who are homozygous for a hypomorphic coding variant in GPR65 (I231L) show a statistically significant increase in survival and altered expression of key immune system genes compared to other genotypes; 2) single cell RNA sequencing (scRNAseq) data from multiple solid tumors show that GPR65 and downstream pathway genes are ubiquitously expressed in myeloid and other innate immune cells in human cancers; and 3) low pH acting via GPR65 profoundly alters gene expression in human macrophages in vitro, bringing about a pronounced suppression of proinflammatory cytokines and a marked upregulation of tissue repair genes. These findings identify GPR65 as a novel innate immune check point, and suggest that GPR65 inhibition could be highly beneficial in cancer. Indeed, previous work has shown that genetic ablation of the GPR65 signaling pathway in B16.F10 tumour bearing mice upregulates immunostimulatory genes in TAMs and significantly reduces tumor growth1. Pathios has identified potent and selective antagonists of human GPR65 with excellent oral bioavailability and pharmacokinetics. In line with their potencies in recombinant cell systems, lead molecules are able to inhibit low pH induced cAMP elevations in primary human macrophages with IC50 values in the single digit nM range. In macrophages subjected to acidic conditions, the inhibitory effects on cAMP are accompanied by a reduction in the expression of anti inflammatory and tissue repair genes, and the enhancement of immunostimulatory genes. In particular, GPR65 inhibition counteracts the pronounced low pH induced suppression of key Type I/II interferon (IFN) response genes and chemokines such as CXCL9, CXCL10 and CXCL11. Following oral administration in tumour bearing mice, lead compounds up-regulate the expression of anti-tumor immune response genes at systemic exposures that significantly suppress GPR65 signalling in vitro. Results from tumour growth inhibition studies in mice with our GPR65 inhibitors will be also be presented. In summary, ‘Macrophage Conditioning’ via GPR65 inhibition holds substantial promise as a novel immunoncology strategy to counteract the immunosuppressive effects of the acidic TME on TAMs, and could be deployed either as monotherapy or in combination with T cell checkpoint inhibitors or other standard of care treatments. 1Nat. Immunol. 19:1319

Citation Format: Barbara Cipriani, David Miller, Alan Naylor, Gavin Milne, Barbara Young, Rupert Satchell, Suorav Sarkar, Zoe Smith, Rhoanne McPherson, Anastasia Nika, Preeti Singh, Toszka Bohn, Tobias Bopp, Tom McCarthy, Stuart Hughes. Inhibition of GPR65 counteracts low pH induced immunosuppressive polarization of macrophages: In vitro and in vivo characterization of potent, selective and orally bioavailable small molecule GPR65 antagonists [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 2162.

Abstract 1631: GPR65 is a critical mediator of low pH induced immunosuppressive signalling in tumor associated macrophages: Human target validation of GPR65 as a novel innate immune checkpoint and discovery of potent, selective GPR65 antagonists

Tumor-associated macrophages (TAMs) are the major innate immune component in the microenvironment of solid tumors. These cells are highly heterogeneous and plastic but often display a pronounced immunosuppressive phenotype that supports primary tumor growth and metastasis. A recently identified determinant of the immunosuppressive properties of TAMs is the activation of the pH-sensing G protein-coupled receptor, GPR65, on these cells by the acidic microenvironment that is inherent to many advanced solid tumours1. Previous work in mouse macrophages has shown that GPR65 activation leads to an elevation of inducible cAMP early repressor (ICER), an isoform of the CREM gene, which in turn suppresses the expression of a host of proinflammatory mediators1. Consistent with a high expression of GPR65 and CREM in human myeloid cells, and particularly in human TAMs, we now show that equivalent immunosuppressive signaling is also present in human macrophages in response to low pH. Further substantiating GPR65 as an innate immune checkpoint in human cancers, we also show that subjects that are homozygous for a hypomorphic coding variant in GPR65 (I231L) have a significantly improved survival across a range of cancers compared to other genotypes. This survival advantage is maintained in patients with highly glycolytic tumors that would otherwise be predicted to respond poorly to immunotherapy, and is consistent with studies showing that genetic deletion of ICER attenuates the growth of anti-PD-1-resistant B16.F10 melanoma tumors in mice1. With a view to developing a therapeutic agent able to reverse low pH-dependent immunosuppression in TAMs, we identified potent and selective small molecule inhibitors of GPR65 from a screening campaign. These molecules were able to fully ablate the acidic pH-induced expression of ICER in human macrophages with downstream implications for pro-inflammatory cytokines known to support T cell anti-tumor responses. Additional medicinal chemistry optimization led to molecules exhibiting excellent oral bioavailability in preclinical species and systemic exposure that completely suppressed GPR65 signaling following oral dosing in mice. Profiling of these inhibitors in relevant mouse tumor models is currently ongoing. In conclusion, we have (i) demonstrated that GPR65 is a key determinant of low pH-induced immunosuppression in human macrophages and thus an important innate immune checkpoint in cancer, and (ii) identified selective small molecule inhibitors of GPR65 with potent in vivo activity. This work provides the basis for developing novel “macrophage conditioning” therapeutic agents that may have utility across a range of cancers, either as single agents or in combination with other approved immunoncology drugs. 1Nat Immunol 19:1319.

Citation Format: Barbara Cipriani, Alan Naylor, Gavin Milne, Barbara Young, Rupert Satchell, Sourav Sarkar, Zoe Smith, Louise Healy, John Unitt, Jessica Holien, Rhoanne McPherson, Anastasia Nika, Jessy Cartier, Tozska Bohn, Tobias Bopp, Tom McCarthy, Stuart Hughes. GPR65 is a critical mediator of low pH induced immunosuppressive signalling in tumor associated macrophages: Human target validation of GPR65 as a novel innate immune checkpoint and discovery of potent, selective GPR65 antagonists [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 1631.

A combinatorial biophysical approach; FTSA and SPR for identifying small molecule ligands and PAINs

Biophysical methods have emerged as attractive screening techniques in drug discovery both as primary hit finding methodologies, as in the case of weakly active compounds such as fragments, and as orthogonal methods for hit validation for compounds discovered through conventional biochemical or cellular assays. Here we describe a dual method employing fluorescent thermal shift assay (FTSA), also known as differential scanning fluorimetry (DSF) and surface plasmon resonance (SPR), to interrogate ligands of the kinase p38α as well as several known pan-assay interference compounds (PAINs) such as aggregators, redox cyclers, and fluorescence quenchers. This combinatorial approach allows for independent verification of several biophysical parameters such as KD, kon, koff, ΔG, ΔS, and ΔH, which may further guide chemical development of a ligand series. Affinity values obtained from FTSA curves allow for insight into compound binding compared with reporting shifts in melting temperature. Ligand-p38 interaction data were in good agreement with previous literature. Aggregators and fluorescence quenchers appeared to reduce fluorescence signal in the FTSAs, causing artificially high shifts in Tm values, whereas redox compounds caused either shifts in affinity that did not agree between FTSA and SPR or a depression of FTSA signal.