Endogenous peptides and proteins include well characterized families of neuropeptide transmitters, neuropeptide modulators, hormones, and fragments of functional proteins, which are essential in many biological processes. The peptides exert potent biological actions in virtually all systems in the body (see figure for examples).
Pharmaceutical products which mimic the effects of endogenous peptide ligands are call peptidomimetics. Some examples of peptidomimetics and their corresponding endogenous ligand include desmopressin – vasopressin, octreotide – somatostatin, and insulin glargine – insulin. Drugs which block the receptors for endogenous peptide ligands can be peptide or non-peptide molecules. Examples include naloxone (opioid receptors), aprepitant (substance P receptors), and losartan (angiotensin II type 1).