The Binding and Effects of Boron-Containing Compounds on G Protein-Coupled Receptors: A Scoping Review

Abstract

Boron-containing compounds (BCCs) have emerged as potential drugs. Their drug-like effects are mainly explained by their mechanisms of action in enzymes. Nowadays, some experimental data support the effects of specific BCCs on GPCRs, provided there are crystal structures that show them bound to G protein-coupled receptors (GPCRs). Some BCCs are recognized as potential ligands of GPCRs—the drug targets of many diseases. Objective: The aim of this study was to collecte up-to-date data on the interactions of BCCs with GPCRs. Methods: Data were collected from the National Center of Biotechnology Information, PubMed, Global Health, Embase, the Web of Science, and Google Scholar databases and reviewed. Results: Some experimental reports support the interactions of BCCs with several GPCRs, acting as their labels, agonists, or antagonists. These interactions can be inferred based on in silico and in vitro results if there are no available crystal structures for validating them. Conclusions: The actions of BCCs on GPCRs are no longer hypothetical, as the existing evidence supports BCCs’ interactions with and actions on GPCRs.

1. Introduction

Boron-containing compounds (BCCs) have emerged as potential drugs [1,2]. The reported drug effects of these compounds are mainly based on their mechanisms of action in enzymes; furthermore, they are reported to be ligands of several types of receptors. Currently, five BCCs are marketed as drugs for humans: bortezomib and ixazomib, which act on the proteasome (for cancer); crisaborole, which acts on phosphodiesterase 4 (PDE4, for skin inflammatory disorders); tavaborole, which acts on protein synthesis by inhibiting leucyl-tRNA synthetase in some fungi (as an antimycotic); and vaborbactam, which acts as a beta-lactamase inhibitor (used in some genitourinary infections). The interest in developing new BCCs as potential drugs is supported by multiple studies reporting the attractive features of BCCs. Among these features is the higher affinity of BCCs compared to their corresponding boron-free analogues, providing them with a higher ability to interact with the drug target; furthermore, BCCs exhibit increased stability in physiological conditions, and some organisms find it difficult to break them down due to their boron–carbon or boron–oxygen bonds, which are probably linked to the higher bioavailability and longer half-life of BCCs compared to their corresponding boron-free analogues [1].

Nowadays, the reasons for the increasing interest in BCCs in drug development include their potential actions in high-global-burden diseases, their effects on mammalian metabolism, and their neurological and cardiovascular effects [2,3,4,5]. Some of these effects are explained based on their interactions with G protein-coupled receptors (GPCRs) [6].

Based on their sequence homology, GPCRs are classified into six classes. These classes and their prototype members are as follows: Class A (rhodopsin-like), Class B (secretin receptor family), Class C (metabotropic glutamate), Class D (fungal mating pheromone), Class E (cyclic AMP), and Class F (frizzled/smoothened) [7]. In fact, to the best of our knowledge, a handful of studies have reported the binding of BCCs to GPCRs of Class A, either by the competitive displacement of some well-known ligands in GPCRs or by the presumed effects of the well-known GPCR–ligand complexes. A small amount of data is available that supports the actions of other classes or subfamilies.

However, there is no evidence of the interactions of BCCs with a specific residue of GPCRs. Moreover, no available crystal structure shows the binding of BCCs to GPCRs, even though they are recognized as drug targets in many diseases. Some observed effects of BCCs are strongly associated with their action on these targets. In this sense, this scoping review is focused on establishing the state of the art by analyzing the available information from in silico, in vitro, and in vivo studies regarding the potential actions of BCCs resulting from their binding with GPCRs.

2. Materials and Methods

All data were collected from the PubMed, Global Health, Embase, the Web of Science, Clinical trials, and Google Scholar databases and reviewed. The search was performed using the terms “(boron OR organoboron OR carboranes OR boron-containing compounds) AND (GPCR OR G-protein coupled receptor OR metabotropic receptor)”. All articles were carefully reviewed to identify if the reported information was original or referenced any other document regarding the possible action of a BCC on GPCRs. The PRISMA-ScR protocol was used, and the filled PRISMA checklist is provided in the Supplementary Materials, highlighting the search process and scope, while a flow diagram is presented in Figure 1.

3. Results

3.1. Experimental Evidence of Effects of BCCs on Family A GPCRs

3.1.1. Catecholamine (Dopamine and Adrenergic) Receptors

The first report in this field could be of the use of boron–dipyrromethene (BODIPY) derivatives as fluorescent ligands of adrenoceptors. In 1998, Daly et al. found a compound structurally related to prazosin (named BODIFY-FL-prazosin, quinazolinyl piperazine borate-dipyrromethene or QAPB) that could bind to α1-adrenoceptors with nanomolar affinity and modify inositol phosphate generation (by blocking phenylephrine induction); thus, it was described as an antagonist of that receptor [8]. Moreover, one year later, his research group reported that confocal visualization showed that the clustered component was located mainly intracellularly. In rat basilar artery smooth muscle cells, the intracellular binding sites were located close to the nuclear membrane, which is interesting because of the known distribution of receptors in some tissues [9]. In the following years, the researchers described the binding and distribution of adrenoceptors in the aortae of rats [10]. Baker et al. reported the binding to β2-adrenoceptors, and it stimulated and increased cyclic AMP production in CHO-K1 cells that expressed human β2-adrenoceptors. These effects were reduced by the well-known ligand ICI118551. The authors concluded that the compound named BODIPY-TMR CGP12177 or BODIPY-TMR-CGP is a long-acting ligand of β1,β2-adrenoceptors and a fluorescent β2-adrenoceptor agonist that can be used to label β2-adrenoceptors in the plasma membrane of living cells which express it [11].

In 2008, Soriano-Ursúa et al. reported the first BCCs derived from salbutamol (albuterol), which induced the relaxation of smooth muscles in the guinea pig trachea. Theoretical assays accompanied by experimental assays showed that this compound had multiple interactions with the binding site in the in silico assays using the reported crystalline structures of the β2-adrenoceptor [12]. After that, these authors reported two additional BCC salbutamol derivatives, all with micromolar affinity in binding assays as well as higher efficacy and potency on cells expressing human receptors than their boron-free precursor and which were stable at biological conditions. Later, they studied the differences in the binding affinities determined on the guinea pig and human homologous receptors; moreover, they suggested some differences in the mode of binding at the orthosteric site predicted by in silico assays [13]. Moreover, the authors described slight differences in molecular dynamic assays, particularly for the role of the boron atom, in the interactions of the compound Politerol with the β2-adrenoceptors of each species, affecting the interactions and movements of the fifth to seventh transmembrane domains, known to be key in receptor activation (details discussed below).

The use of boron-containing clusters was reported by Louie et al. [14] as they screened carboranes for binding with serotonin, σ, and adrenergic receptors, as well as dopamine, serotonin, and norepinephrine transporters. They reported several metal–carborane derivatives, one of them acting as a probe on α-adrenoceptors with mean Ki values ranging from 17 to 435 nm, but with less affinity for other amine receptors and low specific binding [14].

Recently, a BCC exerting effects that occurred presumably through interactions with D2DR was reported (Figure 2 was obtained from comparing docking simulations). In fact, a boroxazolidone derivative structurally related to levodopa was reported that limited the neuronal damage and disrupted the performance of mice administered with MPTP (a known parkinsonism inducer), while in silico assays suggest its binding with submicromolar affinity. Its beneficial effects were diminished by the coadministration of the well-known D2DR antagonist risperidone, validating that boroxazolidone acted as a D2DR ligand [6,15].

3.1.2. Other Class A GPCRs

Among the subtypes of receptors, Class A represents the major transmembrane receptor involved in physiological processes such as vision, smell, cellular homeostasis, and specific responses for hormones and neurotransmitters. The scope of BCCs is beyond the labeling of receptors; their scope is enhanced by the observations that some BCCs can modulate their conformational dynamics. Their applications are expanding, since this subtype of GPCRs is a key target for the development of novel pharmacological therapies due to their high distribution and functionality. Thus, recent research has demonstrated that BBCs have potential as therapeutic and diagnostic agents.

In this sense, it was observed that, in GPR30, the administration of 0.4 mmol/L of boric acid enhances the splenic lymphocyte proliferation and the immune response via the expression of IL-2 and IFN-γ, whereas the administration of 40 mmol/L of boric acid causes opposite effects, such as cell apoptosis, in a dose-dependent manner. The selective inhibition of the receptor with the compound G-15 confirmed that GPR30 is the key mediator of these effects [16]. Also, carborane clusters were recently reported as GPCR ligands. Peptide agonists modified with carborane motifs and directed to the ghrelin receptor 1a (GhrR) demonstrate high chemical stability and efficient activation. The increased expression of the ghrelin receptor in various cancer cells makes it a viable target for BNCT. The combination of synthetic, boron-modified peptide ligands with the GhrR represents a suitable system for BNCT [17,18]. On the other hand, peptide conjugates based on neuropeptide Y and their optimization for maximizing the carborane charge showed a significant enhancement in the solubility of their chemical structures without any modifications to their affinity to the Y1 receptor, increasing the number of BCCs for targeted therapy. Another class of peptide conjugates that incorporated 80 atoms of boron per molecule and specifically acted on the gastrin-releasing peptide receptor (GRPR) was reported by Hoppenz et al. These compounds retain affinity and selectivity to the receptor and showed the selective internalization of tumoral cells and low cytotoxic effects. They can be used as a boron-loading source in a non-invasive approach like BNCT [19]. However, another feature of BCCs is that they act like energy reservoirs, which is the case for some BODIPYs that exhibit fluorescence and stability in acid media, as reported elsewhere [20]. The use of BODIPYs in GPCRs has efficiently demonstrated that these chemical structures could be used for treatments that involve the receptor GPR54, like type 2 diabetes (T2D), in which this receptor is overexpressed. Sharma et al. described peptide–boron difluoride formazanate conjugates with the ability to label the growth hormone secretagogue receptor (GHSR-1a); these compounds were not only different from the conventional BODIPY core but also shared their structure with known molecules that could activate the receptor [21]. The use and activities of BCCs in adenosine receptors have been explored. Representative studies were conducted by Fernandes et al. on the A2a adenosine receptor bound by a BODIPY ligand, which modifies the dynamics of the receptor, assessed using single-molecule Förster resonance energy transfer assays and fluorescence correlation spectroscopy [22]. Furthermore, the study by Bednarska-Szczepaniak et al. showed boron cluster-containing compounds with a high affinity for the A3AR receptor, favoring their selectivity over other adenosine receptor subtypes such as A1 and A2A. This specificity is attributed to the unique properties of boron clusters, which include the formation of non-covalent bonds with specific regions of the receptor. Compared to the phenyl counterparts, the boron derivatives had lower affinity but a significantly improved selectivity profile, which could be exploited in the development of more precise therapies [23].

Barron-González et al. have explored the action of a boroxazolidone tryptophan derivative named borolatonin. This compound induces changes in the behavior and the brains of male and female rats with hormonal deprivation in a way similar to that of melatonin. In fact, the cognitive deficit and neuronal loss, as well as the formation of amyloids and the changes in BDNF after hormonal deprivation, were diminished with both melatonin and borolatonin administrations [24]. The docking assays determined that the binding of borolatonin is similar to that of melatonin on the MT1 and MT2 melatonin receptors (Figure 3), as well as the interactions of key residues for the activation of these receptors.

3.2. Effects of BCCs on Family B and Family C GPCRs

3.2.1. Interaction and Effects of BCCs on Family B GPCRs

Family B GPCRs, also known as secretin receptors, are proteins that interact with polypeptide hormones, which are molecules with intercellular communication functions and are proteins that closely regulate stress response and longevity. They are present in mammals, Caenorhabditis elegans, and Drosophila melanogaster, but they have not been found in fungi, plants, or prokaryotes [25]. In 1991, the receptor for the intestinal hormone secretin was cloned for the first time by Dr. Nagata’s team, who found that the activation of the receptor regulates the concentration of intracellular cAMP [26]. Since that date, 18 genes from Drosophila, 6 from Caenorhabditis, and more than 33 human genes have been identified that encode a member of family B GPCRs. Currently, there are three subtypes of receptors belonging to this family: B1, B2, and B3 [27].

The B1 subfamily is the largest group of family B GPCRs. The receptors included in this subfamily are the following: the secretin receptor, responsible for the secretion of bicarbonate, zymogens, and potassium at the pancreatic level; the VPAC1 receptor, involved in T lymphocyte differentiation, neuromodulation, and circadian rhythm; the glucagon receptor, which modulates hepatic glycogenolysis, gluconeogenesis, and pancreatic insulin secretion; Receptor for Growth Hormone Releasing Hormone (GHRH), which is closely linked to the synthesis and release of growth hormone from the pituitary gland; and the glucagon-like peptide-1 receptor, which is responsible for the pancreatic secretion of glucagon and insulin, among other functions [26]. The prototype B2 subfamily receptors are the mucin-like hormone receptor (EMR1) and the leukocyte cell surface antigen CD97. On the other hand, the B3 subfamily is represented by proteins related to the Methuselah receptors present mainly in Drosophila [28].

The chemical–pharmaceutical industry has produced peptides for emulating the structure of natural peptides acting as ligands of GPCRs, increasing their affinity for the targeted receptors and increasing their bioavailability [29]. In this sense, the addition of boron to some peptides reduces their adverse effects, improves their metabolic stability by reducing their enzymatic degradation, and increases their activity on the receptor, because boron can form covalent bonds with nucleophilic amino acid residues of the binding pocket, facilitating the formation of hydrogen bonds with extramembrane and transmembrane residues and resulting in improvement in the strength and stability of the interactions with the receptor [1,30].

These results suggest the advantages of BCCs as ligands of GPCRs, albeit specific results of BCCs acting on family B GPCRs are lacking. Therefore, their use could lead to great advances in the treatment of endocrine diseases or to improvements in patient symptoms. An example of potential application of these BCCs is their action on the gastrointestinal tract for treating both metabolic disorders and diseases associated with the limited absorption of nutrients. Secretin is synthesized and released in the S cells of the intestinal mucosa of the duodenum and jejunum in response to various luminal nutrients, resulting in the activation of secretin receptors in the pancreatic acinar cells and, consequently, the production of pancreatic juice rich in bicarbonate [31]. Hence, BCCs that behave as agonists of secretin receptors could benefit patients with pancreatic insufficiency and chronic pancreatitis, resulting in greater absorption of nutrients and a reduction in clinical symptoms [29].

In a similar way, BCCs (peptides or small molecules) that activate GHRH receptors in the somatotroph cells of the pituitary gland could be beneficial for patients with a short stature who have issues with the synthesis and release of growth hormone. In contrast, BCCs designed to act as the antagonists of the GHRH receptor would be useful in cases of acromegaly and gigantism, which result from a high concentration of growth hormone [32]. As additional example of the application of BCCs on these receptors is the addition of boron to polypeptides that behave as GLP-1 receptor agonists to improve the quality of life of patients suffering from type 2 diabetes, as these BCCs could considerably reduce the plasma glucose concentration and could help restore the sensitivity of beta cells to secretagogues. Furthermore, it has been proven that some BCCs affect incretin levels [3,33].

Although reports on the effects of BCCs on family B GPCRs are lacking, Tian et al. reported the ability of BCCs to bind and label the glucagon receptor. They performed a confocal fluorescent microscopic study of the intracellular bio-orthogonal labeling of the third intracellular loop (ICL3) of the glucagon receptor. Their labeling strategy involved the site-specific introduction of a strained alkene amino acid into the ICL3 through genetic code expansion, followed by a highly specific inverse electron-demand Diels–Alder reaction with fluorescent tetrazine–BODIPY probes. These probes afforded the successful fluorescent labeling of GCGR-ICL3 and yielded high-background fluorescence due to its intracellular retention [34].

3.2.2. Effects of BCCs on Family C GPCRs

Family C GPCRs have two unique characteristics compared to other GPCRs: (a) they have a large extracellular domain, far from the seventh transmembrane domain, which has the orthosteric sites; and (b) they form constitutive dimers that are characterized by unique activation modes [35]. Family C GPCRs include metabotropic glutamatergic receptors (mGluRs), GABA-B receptors, calcium-sensing receptors (CaSRs), sweet taste receptors, and pheromone receptors. The CaSR, GABA-B, and mGluR receptors are widely studied in the field of neuroscience and for controlling plasma calcium. On the other hand, the receptors related to sweet taste are relevant to the food industry because additives can be used that impact these receptors and increase the intensity of food flavor [36].

The mGluRs are divided into three groups. Members of group I (mGluR 1 and 5) couple to the Gq protein; those in group II (mGluR 2 and 3) and group III (mGluR 4, 6, 7, and 8) couple to the Gi protein. All of the above receptors are expressed profusely throughout the central nervous system and are involved in pathologies such as Parkinson’s disease, Fragile X Syndrome, Alzheimer’s disease, anxiety, and schizophrenia [37].

To the best of our knowledge, only one study has suggested the activity of BCCS on mGluRs. In fact, oxazaborolidine (BZQuin), which has been reported to actively enhance seizures, is suggested to be a ligand of these receptors based on docking assays; in line with this observation, its in vivo effects have been assessed, and a comparison with other drugs has been performed [38]. In in silico assays (Figure 4), BZQuin binds with high affinity to mGluR 1, 2, and 7, precisely at the L-glutamate binding site, with higher negative Gibbs binding energy than L-glutamate and D-aspartate. BZQuin positively modulates mGluRs and can be directly related to increased glutamatergic neurotransmission, since it increases motor activity and the number of seizure episodes. As the above compound is a positive modulator of some mGluRs, new BCCs can be synthesized with the aim of negatively modulating other mGluRs and providing improved models of Parkinson’s disease, anxiety, or epilepsy. The addition of boron to allosteric molecules could have several advantages, such as increased drug potency and selectivity, improved drug pharmacokinetic properties, and reduced drug toxicity.

mGluR5 is garnering increasing interest as a target for neurodegenerative diseases [39], and it has negative and positive allosteric domains (NAM and PAM, respectively) in its transmembrane domain, which can be targets for molecules that modulate the activity of the receptor. Allosteric agonists cannot activate or inhibit a receptor on their own [40]; however, in the presence of the orthosteric ligand, they can promote the total activation or inhibition of the receptor [41]. As the allosteric sites of mGluR5 are more conserved than orthosteric sites, fluorescent BCCs targeting allosteric sites allow the more precise localization of mGluR5 in the nervous system.

Therefore, the use of BCCs that directly interact with mGluR stand out above other molecules in fluorescence tests [42]. BODIPY’s high photochemical stability, molar absorptivity, and fluorescence yield have been applied for this purpose [43,44]. The addition of BODIPY to alkyne groups can generate fluorescent molecules that interact with the NAM and PAM of mGluR5 [45]. Based on the above facts, allosteric mGluR5 ligands are developed with the aim of showing improvement in various situations such as anxiety, depression, and drug abuse. Due to the identification of allosteric sites of mGluR5 through BCCs, it was possible to develop negative allosteric drugs such as Basimglurant, designed for the treatment of drug-resistant depression and Fragile X Syndrome [46].

Since the development of BCCs that behave as allosteric modulators is still in its initial phases, there are no studies related to their interaction with GABA-B and CaSR receptors. However, they will be key in addressing pathologies such as depression, epilepsy, hypo- and hyperparathyroidism, and the plasma control of extracellular calcium in the future.

3.3. Theoretical Assays Supporting Interactions of BCCs with GPCRs

The above sections have mentioned that docking assays showed the ability of BCCs to reach the orthosteric sites of some GPCRs complementarily to experimental assays. Moreover, other docking procedures and molecular dynamic simulations have yielded advances in the improved mode and favorable energy of BCCs in the linkage to specific GPCRs [47,48]. The following paragraphs provide examples found in these studies.

Various studies involving boron cluster conjugates with biologically important low-molecular-weight compounds have been performed to evaluate their potential bioactivity. In this case, the question that arose was whether the addition of a phenyl or boron cluster to adenosine derivatives would affect or modify their ligand affinity for A2A and A3 adenosine receptors, which are GPCRs. A study was carried out using PatchDock v1.3, and due to the limited availability of boron parameters, the carbon atom type was changed, and the A3 receptor was modeled using the Local Meta-Threading-Server, LOMETS v3, while the crystallized A2A structure was downloaded (PDB ID: 2YDO). Blind docking was performed, and it was seen that, in the case of the A2A receptor, the adenosine derivatives with a phenyl cluster had a better affinity, with interactions such as those seen with amino acid residues T88 and I275, while for the derivatives with a boron cluster, interactions were observed with the extracellular residues of phenylalanine and leucine in the second extracellular loop, resulting in the cluster being localized in a small pocket, thus producing unfavorable interactions. In the case of the A3 receptor, the addition of the boron cluster produces higher affinity and interactions, with more interactions with transmembrane amino acids L85, S181, Ile186, and W243, which could be due to a more geometrically rigid structure and the reduced mobility of the binding pocket, which helps the ligand to bind to the inner surface of the channel [49].

Other studies were performed due to the increased interest in the use of boron neutron capture therapy, a mode of cancer therapy; in this field, hydrophobic carborane clusters with their high boron content, stability, and chemically modifiable properties have high utility. Moreover, carboranes have shown their potential for other pharmaceutical applications, such as the use of derivatives of retinoic acid on GPCRs. Thus, some boron clusters have been proposed to be ligands of adenosine receptors A1, A2A, A2B, and A3. The A3 receptor was modeled using the LOMETS, and a directed docking procedure was performed to cover the intracellular and extracellular parts of the receptor, covering the entire extracellular part, binding pocket, and intracellular region. The boron parameters were added to the Autodock 4.2.6 parameters. With a further docking study and the analogous binding of the polyhedral with a phenyl ring, it was shown that carborane clusters with adenosine and 2′-deoxyadenosine pairs did bind to the entrance of the adenosine A3 receptor at the orthosteric site as potential agonists. The study compared the best compound with a carborane cluster against another compound with a phenyl cluster, leading to the potential modulation of neuroprotection, neurodegeneration, cellular proliferation, and cell death [23,50].

In another study, an array of histamine H1 receptor ligands were synthesized and explored, where a boron dipyrromethene-based fluorophore was incorporated into various ligands with a peptide linker and an orthosteric targeting moiety. In this case, it was seen that this fluorophore conferred high binding affinity to the ligand and that the peptide linker, which was optimized, helped the ligand to interact with key residues in the receptor. A GLIDE docking experiment was carried out using the H1R crystal structure (PDB ID: 3RZE) and prepared with the Schrödinger modeling suite. The missing atoms were modeled using PROPKA v3.5 and minimized using the OPLS3 force field. The proposed receptor residues interacting with the side groups were near or in the extracellular loop region of the receptor, i.e., D178, K179, K191, N443, and E447, assuming that the interactions formed were due to the disruption of the existing water hydrogen bonds. The addition of a phosphatidylcholine-based phospholipid bilayer and posterior docking show that it had a hydrophobic effect, with the fluorophore increasing the affinity to the binding site. In addition, it was found that F440 could act as a lid for the pharmacophore to help it in its binding position [51].

The number of studies exploring ligands for monoamine receptors to treat Alzheimer’s disease are increasing. Among these molecules, melatonin has an inverse correlation with the severity of neuropathology, and it has been reported to reduce the formation of β-plaques and neurofibrillary tangles. Its receptors, MT1 and MT2, are GPCRs that are involved in the cellular reduction of cAMP. An in silico study was conducted where both receptors were modeled from their human analogs, MT1 from 6ME5 of MT1–agomelatine and 6ME9 from MT2–ramelteon. The ligands were agomelatine, ramelteon, melatonin, and borolatonin, an adduct of tryptophan and 2-aminoethoxydiphenyl borate (2-APB). All compounds did bind to the orthosteric site, having interactions with residues T178, F179, A191, V192, F251, N255, A284, and Y285, with most interactions being of the Van der Waals and hydrophobic types (Figure 2). The different residues from the homolog receptors, with tryptophan being in position 254 in MT1 and 264 in MT2, appear to increase the number of interactions in the binding pockets, allowing ligands to form pi–pi interactions with sidechain residues in rat models. For borolatonin, there were interactions with the aromatic rings from the residues and hydrogen bonds were formed with the amine in the indole moiety, while the exposed oxygen interacted with the boron-containing ring. Interestingly, although the boron atom did not interact directly with the residues, all studied BCCs had a higher affinity than other known ligands of melatonin receptors [52].

The β-2 adrenoreceptor is another GPCR that has good study prospects due to the various pharmaceutical uses of its modulation, and its ligands are potential drugs that can be used to treat neurological, metabolic, and cardiovascular disorders and cancer. Considering previously known agonists that have been tested and developed using a guinea pig animal model, interactions with human or guinea pig receptors were studied using in silico assays. In this case, boron-containing albuterol derivatives were used (BR-AEA, Boronterol, and Politerol) based on previous reports that stated that BCCs demonstrated higher potency and efficacy than their carbon counterparts. The human β2AR was obtained (PDB ID: 3PDS), while the guinea pig was built using MODELLER v.10 using 3PDS and 3P0G as templates. Thus, for the docking study, the software Autodock was used with a blind procedure, and the calculation box was centered on the midpoint between the α-carbons of two conserved amino acids, D113 and S204. The three compounds were linked to the hydrophobic orthosteric site between TM3 and TM7, with their amine moiety interacting with D113 of both receptors, while their tert-butyl moiety was oriented to the upper segment of TM7 on the guinea pig receptor. In the human receptor, for BR-AEA and Boronterol, the boron-containing moiety was in the middle of the crevice between TM3 and TM7, while for Politerol, it was in the upper segment of TM3, with hydrogen bonds with S203, S204, and S207 in TM5 in both receptors. For the human receptor, BR-AEA and Politerol showed fewer interactions with N293. Their binding affinities were similar to those of albuterol [13]. These results did correlate with the results of previous studies in which a tetracoordinate boron atom was placed on albuterol derivatives that showed low toxicity, and the resulting compounds were bound to the guinea pig receptor. Politerol, which, due to the tetravalent boron atom and its negative charge, binds to hydroxyl moieties and forms key interactions with the orthosteric sites, including D113, S203, S207, W286, F289, and N312, was of special interest [33]. A molecular dynamics simulation was also performed where the human and guinea pig proteins were both used as apo-protein and ligand–protein complexes. They were embedded in a lipid bilayer membrane with CHARMM force fields at 1 ATM and at a temperature of 300 K. The simulation was carried out using AMBER v12 executable for 250 ns. In these simulations, Politerol was used and compared with the isoproterenol and albuterol complexes. In these simulations, stabilization was observed after 100 ns, with only small differences in Root Mean Square Deviation values, but the Root Mean Square Fluctuation (RMSF) values of the last 150 ns of the free system were more flexible than those of the Politerol-bound complexes, with the extracellular residues 170–195 and cytoplasmic residues 231 to 254 having greater flexibility; this indicates the stabilizing role of the BCC in the complex. Nevertheless, in the guinea pig results, no difference was found in the cumulative RMSF compared to that seen in the human results, with the Politerol complexes showing a bigger difference [13].

3.4. Role of Moieties Including Boron Atoms in Interactions with GPCRs

The role of specific moieties including the boron atom in the binding of some proteins has been explored recently [1,2]. Moreover, Gao et al. described a wide array of possibilities for BCCs enabling bioconjugation [53].

Hence, specifically on GPCRs, there are some interactions and mechanisms of binding that could be particularly utilized by BCCs. In this regard, it is well known that the key interactions of GPCRs and ligands confer selectivity, potency, and efficacy and are related to some residues that were identified forty years ago with punctual mutations and in the last twenty years with X-ray crystallography of relevant proteins [54]. In this sense, at least in the Class A receptors, the relevant residues for the first contact of endogenous ligands (often an amine moiety) are those in the 3.32 position (in agreement with the Ballesteros–Weinstein nomenclature) and the 7.43 position, often aspartic acid and asparagine, respectively. The second step involves the interactions with some serine residues in the fifth transmembrane domain. Consecutively, some interactions involve toggle switches and the conformational changes in the ionic lock that trigger conformational changes that modify distances among the third, fifth, sixth, and seventh domains related to the activation of classic and other pathways in the intracellular face of the receptor [55].

Thus, the interactions of BCCs could be similar to those of some compounds tested in classical pharmacology evaluation. Keeping in mind the possibilities of tricoordinate (Lewis acid) or tetracoordinate boron atoms (as are found in most biological applications), as well as the dynamic nature of B–O and B–N bond formation and the fact that a significant proportion of boron-enabled bioconjugations exhibit rapid reversibility, the interactions with GPCRs could be used to develop reversible and covalent ligands with at least one boron atom [2,54,56]. For example, in the first interaction with Asp, Glu, or Gln residues, the boronic compounds able to form iminoboronates are attractive, while the boronate ester that mediated the binding of biomolecules could be applied to the interactions with serine residues in the fifth transmembrane domain [12,13]. Also, the hydrophobic interactions with toggle switches (comprising phenylalanine residues in the sixth domain) could be useful for arylboronic acids or carboranes [57]. Additionally, the reaction with N-terminal cysteines could be useful to enhance the binding to some cysteines, in a similar way to the previous design of some covalent full agonists [58]. Figure 5 depicts examples of putative modes of interactions at key points at or near the orthosteric site of a GPCR. Additionally, some other interactions at allosteric sites have been proposed which expand the possibilities of BCCs for acting on GPCRs [59].

3.5. Clinical Trials and Effects of GPCRs in Humans

Clinical trials exploring the specific actions of a BCC on a GPCR are lacking. However, some effects observed in clinical studies could result from the action of BCCs on GPCRs. For example, it is difficult to exclude the involvement of GPCR modulation in the effects of some BCCs used for glioblastoma or neck and head tumors (albeit results are not reported in the Clinical trials database (https://clinicaltrials.gov, accessed on 2 January 2025); examples: NCT00115440, NCT00974987, and NCT00062348). The effects of some compounds such as phenylalanine have been suggested as a way of avoiding radiation in preclinical and clinical studies [60,61], while some structurally related compounds have been reported as potentially active against tumors by acting on the recently described GPCRs, such as GPR17 or GPR68.

Nguyen et al. [62] explored the development of novel indole derivatives as G protein-coupled receptor 17 (GPR17) agonists for glioblastoma treatment. Using molecular docking, they screened over 6000 indoline derivatives and identified the compound CHBC, a phenolic Mannich base synthesized via a multicomponent Petasis borono-Mannich reaction, as a promising candidate. CHBC effectively reduced the cAMP and calcium levels in glioblastoma cells, indicating successful GPR17 activation. In vitro assays revealed dose-dependent cytotoxicity with an IC₅₀ of approximately 85 μM, surpassing the effects of the known agonist MDL29,951. These findings highlight CHBC as a potential therapeutic agent targeting GPR17 in glioblastoma [62].

In addition, Adam et al. explored GPR68 (Ovarian Cancer G Protein-Coupled Receptor 1 or OGR1), a proton-sensing GPCR activated by extracellular acidity, as it plays a pivotal role in tumor microenvironments. Although the direct evidence of BCC modulation of GPR68 activity is limited, compounds like 2-APB and bortezomib are known to influence calcium signaling pathways, including those mediated by inositol trisphosphate (IP3). Given GPR68’s role in pH sensing and calcium regulation, the possibility of BCCs modulating its activity presents an intriguing area for future research. Understanding the interaction between BCCs and GPR68 could uncover novel therapeutic strategies targeting this receptor in oncology [63].

There are five boron-containing drugs approved by the FDA. In the clinical trials evaluating their use in humans, some of their effects could be linked to their actions on GPCRs. In this regard, bortezomib was developed as a dipeptide with bioactivity on the proteasome, but it also had affinity for other enzymes such as chymotrypsin and thrombin, while its affinity for GPCRs, which are targets for peptides, was not measured [64]. However, multiple clinical trials have shown the activity of the compounds on multiple neoplastic cells and their adverse effects, which could involve their actions on GPCRs. For example, a study on the autonomic nervous system dysfunction after bortezomib administration (NCT01314625) yielded no clear results. However, the effects of this drug are clearly linked to the vascular and nervous systems, with the probable involvement of GPCRs. In fact, the incidence of autonomic neuropathy of the digestive system induced by bortezomib is around 60%, and its severity is closely related to efficacy, advanced age, constipation, diabetes, fracture/spinal cord compression in bed, and history of alcoholism. The early detection and adjustment of treatment are key for reversibility. The autonomic neuropathy of the digestive system was significantly alleviated in most patients after the timely adjustment of the treatment regimen, and the administration of bortezomib could be continued [65]. Also, changes in the nervous system linked to neurodegeneration could be linked to effects produced by the binding to and modulation of glutamate receptors and S1P receptors, in addition to increased oxidative stress and increased inflammatory processes [66]. Additionally, Ghelardini et al. reported that the use of a selective antagonist of GluR5 (2-methyl-6-(phenylethynyl)-pyridine) inhibits the glutamatergic system, thus decreasing the development of painful peripheral chemoneuropathy induced by BTZ [67].

Another clinical trial, NCT04566328, in phase III, compared two therapeutic regimens in patients newly diagnosed with multiple myeloma. This study evaluated a four-drug combination (daratumumab–hyaluronidase, bortezomib, lenalidomide, and dexamethasone) against a three-drug regimen (daratumumab–hyaluronidase, lenalidomide, and dexamethasone). Results demonstrated that bortezomib could inhibit cancer cell growth by blocking key enzymes required for cellular proliferation. The addition of bortezomib to the three-drug regimen enhanced its effectiveness in reducing cancer progression or preventing recurrence compared to the three-drug regimen alone; these results were compared to a previous phase II study of the same research team, assessing the obtained results [68]. Similarly, the phase III clinical trial NCT05561387 focuses on induction regimens followed by either double- or single-drug maintenance therapy for newly diagnosed multiple myeloma in patients who are not undergoing stem cell transplantation. This study specifically targets frail or intermediate-fitness patients, defined by age, comorbidities, and functional status. The trial evaluates four drugs—bortezomib, lenalidomide, dexamethasone, and daratumumab–hyaluronidase—to identify the most effective combination for controlling and attenuating multiple myeloma while preventing relapse. Each drug in the regimen plays a distinct role: bortezomib inhibits enzymes essential for cancer cell growth; lenalidomide promotes the production of healthy blood cells while inducing cancer cell death; dexamethasone reduces inflammation and modulates the immune response; and daratumumab–hyaluronidase, a monoclonal antibody, targets and interferes with cancer cell proliferation. By testing various combinations, the trial aims to optimize therapeutic outcomes for this challenging patient population.

The topical boron-based phosphodiesterase 4 (PDE4) inhibitor crisaborole has been approved by the US Food and Drug Administration (FDA) for the treatment of mild-to-moderate atopic dermatitis (AD) in patients aged two years and older. In pivotal phase III clinical trials, crisaborole demonstrated superior efficacy compared to the vehicle control, achieving the primary endpoint of clear or almost clear skin with a more frequent ≥2-grade improvement in disease severity in both children and adults. Crisaborole offers a non-steroidal treatment option for eczema, providing an alternative to corticosteroids with a favorable safety profile [69,70]. Protease-activated receptor-2 (PAR2), a Class A G protein-coupled receptor (GPCR) expressed in various tissues, including the skin, is critical in the inflammatory processes associated with AD. PAR2 promotes Th2-mediated inflammation, delays skin barrier repair, and influences keratinocyte differentiation. Additionally, it is implicated in itch and pain transmission in the skin, making it a potential target in dermatological therapies. While the therapeutic effects of crisaborole have been well documented, its possible interaction with PAR2 remains an area of interest. A study by Peña et al. [71] indicated the efficacy of crisaborole in reducing adverse events compared to topical steroids, suggesting a possible link to PAR2 modulation [71,72].

A preclinical study exploring the combined effects of crisaborole and vitamin D in a mouse model of allergic contact dermatitis provides further insights. This research demonstrated that the combination significantly reduced inflammation and epidermal hyperkeratosis, indicating enhanced therapeutic potential. These findings suggest the broader anti-inflammatory role of crisaborole, particularly when combined with other agents. Despite these promising findings, the precise interaction of crisaborole with GPCRs, including PAR2, remains unclear. Crisaborole’s primary mechanism of action involves PDE4 inhibition, which modulates inflammatory pathways. Although GPCRs are integral to various physiological processes related to inflammation, the current evidence does not establish any direct interactions between crisaborole and these receptors. In conclusion, crisaborole represents a safe and effective treatment for mild-to-moderate AD through its PDE4-inhibitory effects. Emerging evidence, including its combination with vitamin D, underscores its potential for enhanced anti-inflammatory efficacy. However, further studies are warranted to elucidate the relationship between crisaborole and GPCRs, such as PAR2, and to explore its broader implications in dermatological therapies [73,74].

The boron-based compound Talabostat (Val-boroPro) is an oral inhibitor of dipeptidyl peptidases, including fibroblast activation protein (FAP), which is highly expressed in tumor-associated fibroblasts. FAP plays a significant role in promoting the progression of colorectal cancer and other malignancies. Talabostat, the first clinical inhibitor of FAP enzymatic activity, has been evaluated for its therapeutic potential in various cancer settings [75]. In a phase II clinical trial (NCT04171219) involving patients with metastatic colorectal cancer, Talabostat demonstrated its potential as a therapeutic agent targeting FAP-expressing tumor stromata. Preclinical studies further highlighted its immunomodulatory effects. It was found to stimulate the production of cytokines and chemokines, which enhance T-cell-mediated immunity and exhibit T-cell-independent activity. For instance, in melanoma xenograft models (A375 and A2058), Talabostat reduced tumor size by over 60% in immunodeficient mice, emphasizing its role as an immune-modulating agent [76]. Talabostat has also been investigated in advanced non-small-cell lung cancer (NSCLC). A phase II trial (NCT00080080) combined Talabostat with docetaxel to assess its efficacy in improving treatment outcomes for this challenging patient population. The study reported a modest objective response rate (ORR) with partial tumor responses in some patients. However, there was no significant improvement in progression-free survival (PFS) or overall survival (OS) compared to docetaxel monotherapy. While the combination therapy was generally well tolerated, its lack of substantial clinical benefit limited its further development for treating NSCLC [77]. Despite its promising preclinical activity and immune-mediated mechanisms, Talabostat’s efficacy in FAP-expressing tumors has not been directly explored in clinical trials. While it primarily acts by inhibiting dipeptidyl peptidases, its broader impact on the tumor microenvironment suggests possible indirect interactions with GPCRs [78]. Talabostat’s stimulation of cytokine and chemokine production could influence immune responses and tumor–stroma interactions mediated through GPCR signaling pathways. These indirect effects might contribute to its anti-tumor activity. Although no direct interaction between Talabostat and GPCRs has been reported, future research should investigate its role in modulating GPCR-associated pathways. Such studies could uncover novel therapeutic mechanisms and expand the scope of Talabostat’s applications, particularly in cancers characterized by GPCR dysregulation [79].

Dutogliptin, a BCC, is a selective inhibitor of dipeptidyl peptidase-4 (DPP-4), an enzyme that plays a crucial role in glucose metabolism by degrading incretin hormones, such as glucagon-like peptide-1 (GLP-1) and gastric inhibitory polypeptide (GIP). These hormones are the essential regulators of insulin secretion [80]. By inhibiting DPP-4, dutogliptin enhances the activity of GLP-1 and GIP, allowing them to bind to their respective GPCRs, thereby enhancing insulin secretion through GPCR-mediated pathways and improving glycemic control in patients with T2D [81]. Concerning the interactions with GPCRs, the current scientific evidence does not indicate that dutogliptin directly interacts with or modulates GPCR function. Instead, its therapeutic effects are primarily mediated through the preservation of incretin hormone activity, which subsequently acts on the corresponding GPCR to facilitate insulin release and maintain glucose homeostasis [82]. In conclusion, dutogliptin enhances the activity of incretin hormones, which exert their effects via GPCRs to regulate insulin secretion and glucose balance. This mechanism highlights the therapeutic potential of targeting the DPP-4–incretin axis and GPCR signaling pathways in managing T2D.

The limitations of the current scoping review include the used approach and a lack of continuous assessment of the quality of the evidence obtained from all articles selected for the review from the included databases. Moreover, as no complex containing a BCC with a GPCR has been reported, the last section is speculative; however, some described details do support GPCR involvement. Furthermore, the limited reviewing of some included articles could have biased the perspective of the authors.

4. Conclusions and Future Directions

This scoping review shows the increasing evidence on the binding, labeling, and actions of BCCs on GPCRs. The relevance of this evidence is based on the fact that most currently used drugs exert their therapeutic actions through interactions with GPCRs (acting as agonists or antagonists, by means of orthosteric effects or allosteric effects and by biased signaling). Moreover, GPCR ligands are considered promising drugs for treating several diseases with high global incidence, such as neurodegenerative, cardiac, pulmonary, and metabolic diseases and cancer [83]. A few small molecules recently approved for human use (to treat those diseases) are GPCR ligands [84].

An increasing amount of data has demonstrated the biological actions of BCCs. Moreover, the knowledge on the interactions of BCCs with proteins is accumulating. Currently, the evidence on the actions of BCCs on GPCRs is limited to competitive analysis in the classic pharmacological binding approach using the displacement of well-known GPCR ligands. The theoretical approaches show the direct interactions of BCCs with the reported orthosteric and allosteric sites of some GPCRs. However, crucial evidence, such as the crystallography data of these compounds in protein complexes, is lacking, and evidence on the therapeutic actions of well-known ligands of GPCRs (including some marketed drugs) is also needed.

Future studies will allow us to assess the specific role of the boron atom in ligand molecules or the relevance of specific moieties containing the boron atom. These studies will also focus on the particular differences in the BCC interactions on GPCRs, as in the crystallized ligand–protein complexes. Moreover, the use of more than one boron moiety for developing new drugs targeting GPCRs could be considered.

The multiple observed and potential effects of BCCs due to their enhanced interactions with GPCRs call for enhanced efforts in designing new BCC ligands for the prevention, diagnosis, and treatment of human diseases with high global burdens, such as cancer and cardiovascular, pulmonary, neurological, and metabolic diseases.