Melanocortin receptors (primarily MC2R on the adrenal cortex)

Class

Repository corticotropin injection (purified porcine ACTH/adrenocorticotropic hormone analog gel)

Pathway

Stimulates endogenous corticosteroid (cortisol) production via the hypothalamic-pituitary-adrenal axis

Melanocortin receptors are a family of five G-protein-coupled receptors (MC1R–MC5R) that respond to melanocortin peptides derived from a common precursor, proopiomelanocortin. Adrenocorticotropic hormone (ACTH) is the natural ligand most selective for MC2R, which is expressed almost exclusively on cells of the adrenal cortex. Binding of ACTH to MC2R triggers the adrenal gland to synthesize and release corticosteroids, principally cortisol, as part of the hypothalamic-pituitary-adrenal (HPA) axis that governs the body's stress and inflammatory response. Because corticosteroids broadly suppress inflammation and immune activity, stimulating their endogenous production is a therapeutic strategy in conditions marked by excessive inflammation or autoimmune activity, including certain rheumatologic, dermatologic, renal, and neurologic disorders. Unlike synthetic corticosteroids that are administered directly, an ACTH-based therapy works upstream, prompting the patient's own adrenal glands to produce cortisol along with other adrenal steroid hormones in physiologic ratios. ACTH also retains modest affinity for other melanocortin receptor subtypes found on immune and other tissues, raising the possibility of direct, steroid-independent anti-inflammatory signaling. This dual route of action, indirect via adrenal steroidogenesis and potentially direct via non-adrenal melanocortin receptors, distinguishes ACTH-based approaches from simple corticosteroid replacement.