G protein-coupled receptors (GPCRs) are transmembrane receptors that are generally highly conserved in sequence in the animal kingdom and are found in a wide range of cell types. They are involved in many physiological and pathological mechanisms including immune cell migration, adhesion and cell death. Their extracellular domain can have several post-translational modifications to allow interaction with its ligands.

The seven transmembrane segments are organized to create a cavity that contains a three-dimensional site for acceptance of a potential ligand, natural or drug. Once bound, the ligand induces a conformational change in the receptor that allows it to make contact with a G protein, which in turn modulates the activity of an enzyme or ion channel to transduce a signal and transmit it into the cell.

GPCRs, which are the target of at least 30% of existing small molecules drugs (NCE), are now gaining notoriety for a different class of therapeutic targets.

Since the discovery of transmembrane receptors, small molecule chemicals have been the class par excellence for targeting membrane proteins, mainly due to their small size allowing an easier interaction with receptors with extremely complex three-dimensional configurations and steric hindrances. Nevertheless, the clinical trials that followed the drug discovery process of small molecules have been detrimental to them in many respects.

Small chemical molecules present a risk of non-specific targeting of the target expressed on the cell surface and therefore a consequence of toxicity of healthy tissues, commonly called off-target toxicity phenomenon. Their small size also has the disadvantage of a short life span once in the body, and therefore implies the need for repeated injection. The penetration of these drugs into complex microenvironments such as solid tumors is not guaranteed either. Finally, small molecules do not necessarily involve the recruitment of the full range of immune cells and cytotoxic effects required.

As a result, the pharmaceutical industry has turned to the development of other types of drug modalities, most notably monoclonal antibodies. In May 2018, the FDA approved for the first time a therapeutic antibody related to the targeting of a GPCR, Amgen's erenumab, a CGRP-R antagonist for the treatment of migraine, followed very quickly by the approval of mogamulizumab (Potilegeo), a promising chemokine receptor CCR4 binder for CCR4-positive T-cell lymphomas, developed by Kyowa Kirin.

Current studies depict a total of 57 monoclonal antibodies in preclinical and clinical development for seven-transmembrane proteins with two different therapeutic mechanisms of action. The first aims at blocking the binding of the ligand to its transmembrane receptor in order to prevent signal transduction, the antibody entering directly into competition with the natural ligand of the GPCR. The second focuses on the ability to initiate an antibody-mediated killing response in order to eliminate unwanted cells, through CDC (complement-dependent-cytotoxicity) or ADCC (antibody-dependent cell-mediated cytotoxicity) mechanisms.

The term CDC refers to complement-dependent cell lysis (complement-dependent-cytotoxicity). It intervenes on the surface of cells targeted by therapeutic monoclonal antibodies by triggering the activation cascade of the classical pathway. Antibodies bind to their target via their Fab fragment, which leads to a modification of their constant fragment (Fc) which becomes capable of triggering the activation of C1q, C1r and C1s. This complex activates and then successively cleaves factors C4 and C2, then C3, thus joining the effect of complement activation by the alternate pathway. The third pathway, or lectin pathway, is activated by the binding of collectin, whose structure is close to that of C1q, on a cell surface, without the intermediary of antibodies. The activation sequence, or terminal steps, is the same for all three pathways and results in the polymerization of C9, creating pores in the target cell membrane.

ADCC is another form of cell death, primarily involving recognition by antibodies. This time, the Fc fragment recruits cells bearing a receptor for the Fc of immunoglobulins, macrophages or natural killer (NK) cells. Phagocytosis by the macrophage leads to the destruction of the target cell. NK cells induce target cell apoptosis via perforins and granzymes.

Although, thanks to the ADCC and CDC functions that we have just described, a monoclonal antibody can effectively suppress cancer cells, this efficiency can be further optimized by the creation of a new format derived from antibodies: antibody-drug conjugate (ADC).

Antibody Drug Conjugate is the unique assembly of three components combining the specificity of an antibody with the cytotoxicity of a chemical molecule through a linker. The conjugation technologies and payloads are the subject of many patents of companies developing ADCs.

The mechanism of action of an antibody-drug conjugate (ADC) breaks down as follows and present several challenges. First the monoclonal antibody binds to its membrane receptor on the cell surface, which then causes internalization of the ADC-receptor complex to form an endosome and then fuse with a cell lysosome. The antibody must have sufficient affinity for the target GPCR, while the density of the latter must be sufficiently high on the cell surface to allow the cytotoxic agent to reach its threshold concentration.

The chemical molecule is then released by lysosomal cleavage to allow the former to act on microtubule or DNA, and thus kill the target cell. It means that the linker must be stable in circulation, but easily releases the active form of the cytotoxic agent within the cell. Payloads must demonstrate a sufficient potency to be effective at low concentrations.

So far, the FDA has approved eight ADCs, systematically in the context of immunotherapy oncology treatment:

• Anti-CD33 gemtuzumab ozogamicin, 2001, by Pfizer
• Anti-CD30 brentuximab vedotin, 2011, by Seattle Genetics
• Anti-HER2 ado-trastizumab emtansine, 2013, by Roche
• Anti-CD22 inotuzumab ozogamicin, 2017, by Pfizer
• Anti-CD79b polutuzumab vedotin, 2019, by Roche
• Anti-HER2 fam-trastuzmab deruxtecan, 2019, by AstraZeneca
• Anti-Nectin-4 enfortumab vedotin, 2019, by Astellas and Seattle Genetics
• Anti-trop-2 sacituzumab govitecan, 2020, by Immunomedics
• Anti-BCMA belantamab, 2020, by GlaxoSmithKline.

In the context of GPCR, newly synthesized receptors are delivered to the cell surface from the Golgi complex. In unstimulated cells there is a relatively slow endocytosis of receptors from the surface into endosomes. But in the presence of agonist ligands and antibody, GPCRs initiate a conformational change and the rate of endocytosis is increased dramatically. Once the receptors reach endosomes, they can either be recycled back to the plasma membrane or routed to lysosomes for degradation.

This rapid internalization of GPCR - which we traditionally measure by FACS at SYnAbs - is a key feature in making it an effective drug target for ADCs. Another feature of interest could be the mutations of GPCRs, as exploited by the team of Jie Cui with peptide-drug conjugate format (1) since certain tumor cells highly express mutated GPCRs.

So far, we’ve numered six ADC approaches towards GPCR, currently at preclinical or clinical stages:

• OTSA101, anti-FZD10 developed by Oncotherapy Science,
• JBH 492, anti-CCR7 developed by Novartis,
• DS-6157, anti-GPR20 developed by Daiichi Sankyo,
• anti-CXCR4 developed by John Hopkins,
• anti-LGR5 developed by Texas Therapeutics Institute,
• anti-calcitonin receptor developed by Monash.

But since GPCRs are expressed in many diseases involving inflammation, we may soon witness the development of ADC strategy against GPCRs in the treatment of autoimmune diseases. To be continued.

REFERENCES

(1) Jie Cui, Yukimatsu Toh, Soohyun Park, Wangsheng Yu, Jianghua Tu, Ling Wu, Li Li, Joan Jacob, Sheng Pan, Kendra S. Carmon, and Qingyun J. Liu

Journal of Medicinal Chemistry 2021 64 (17), 12572-12581

DOI: 10.1021/acs.jmedchem.1c00395

Drug Conjugates of Antagonistic R-Spondin 4 Mutant for Simultaneous Targeting of Leucine-Rich Repeat-Containing G Protein-Coupled Receptors 4/5/6 for Cancer Treatment