GCG and obesity due to melanocortin 4 receptor deficiency: Latest approaches for obesity treatment focus on medicine targets in the central nervous system (e.g., the leptin-melanocortin axis, the opioid system, GLP-1/GLP-1 system, and FGF21/FGFR1c/b-Klotho axis), but only a few investigational agents will be able to meet the FDA regulatory criteria and advance onto the marketplace [18].