While the specific set of challenges associated

While the specific set of challenges associated with deorphanization and transforming each poorly characterized GPCR into a useful drug target will be unique, much can be learned by considering how these issues have been addressed for other receptors. One group of poorly characterized GPCRs that have recently received substantial interest as potential therapeutic targets for a wide range of pathologies are the GPCRs activated by free fatty acids (FFAs) (Hirasawa et al., 2008a, Hirasawa et al., 2008b, Milligan et al., 2006, Rayasam et al., 2007). These include the three receptors currently classified as the FFA family, FFA1, FFA2, and FFA3, as well as two additional receptors GPR120 and GPR84 (Milligan et al., 2006, Stoddart et al., 2008a, Stoddart et al., 2008b). Herein, we describe the experimental challenges that have arisen in attempting to develop FFA receptors into therapeutic targets, how these have been, at least partially, addressed and what challenges still remain to be resolved before the full therapeutic value of these receptors can be realized.
Deorphanization of the Receptors for FFAs
Uncovering the Pharmacology of FFA Receptors
Synthetic Ligands for FFA Receptors The identification of selective synthetic ligands is critical to elucidate the pharmacology and therapeutic potential of any GPCR in vitro and in vivo. This is particularly important for the FFA receptors given the poor potencies observed for the endogenous FFAs at these receptors. In addition, the significant overlap among the endogenous ligands of the two LCFA receptors FFA1 and GPR120 (Suzuki et al., 2008), as well as the overlap in ligands for the two SCFA receptors FFA2 and FFA3 (Milligan et al., 2009), makes uncovering the individual functions of these receptors in vivo particularly challenging. Considering the potential therapeutic relevance of these receptors, significant effort has been expended in order to identify novel synthetic ligands. There are several general approaches that can be taken to identify novel ligands for a receptor. Among them are both the direct screening approaches we described above in relation to the deorphanization of the FFA receptors, and the structure-based drug design and modeling approaches described in relation to uncovering the mode of ligand binding to these receptors. For experimental screening studies, however, programs are generally modified at this axl inhibitor stage to be much more extensive, high-throughput screens, and to concentrate on large libraries of synthetic chemicals rather than endogenously generated ligands. An extension to each of these is through ligand-based drug design, where once an initial ligand has been identified, a SAR based on the ligand is generated by chemically modifying the original ligand and assessing the activity of the various derivatives (Klabunde & Hessler, 2002).
Therapeutic Potential for FFA Receptors The final step in developing a GPCR into a viable drug target is the identification and subsequent validation of its therapeutic potential in the treatment of a specific pathology. Typically, the tissue expression pattern for the receptor is the first piece of information available and this is often used to infer likely biological function of the receptor. From there, a variety of in vitro and in vivo approaches must be taken in order to define the function of the receptor and ultimately uncover its therapeutic utility. While none of the FFA receptors are yet targets of clinically approved medicines, many show great promise at various stages in this process.
Conclusion Uncovering the pharmacology, biological functions, and resulting therapeutic potential of the FFA receptors has presented significant experimental challenges. In particular, the low potencies observed for all endogenous FFAs at these receptors and the lack of available tools, including suitable radioligands for binding assays and antagonists for many of the receptors, have made establishing the pharmacology of the FFA receptors difficult. Overlap in the endogenous ligands and the lack of suitably selective synthetic ligands for both the LCFA receptors FFA1 and GPR120, and the SCFA receptors FFA2 and FFA3 has complicated the effort to establish specific biological functions for these receptors in vivo. This has been further confounded by conflicting studies on the function of these receptors in vivo utilizing knockout mice. Indeed, this has resulted in a situation where at least for FFA1 both agonists and antagonists have been proposed by different groups for treatment of the same condition. Despite this, interest in these receptors has remained high, in particular, for FFA1, FFA2, and GPR120 as targets for the treatment of obesity and type 2 diabetes, as well as FFA2 for inflammatory conditions including IBD, asthma, and arthritis. The most critical next step is likely to be development of novel ligands for these receptors with suitable selectivity to fully elucidate their functions in vivo. It will then be possible to perform proof-of-principle studies to validate the receptors as drug targets. Only when this has been completed will the full therapeutic potential of the free fatty acid receptors be uncovered.