A peptide whose ends are joined together in a ring — making it more resistant to degradation and often more selective for its target receptor.The N- and C-termini are linked by a peptide bond, or a side chain forms an intramolecular bond creating a ring. Cyclization constrains the peptide’s three-dimensional shape, improving receptor selectivity by limiting the range of conformations it can adopt. Also significantly increases resistance to proteolytic degradation — enzymes that break down linear peptides often cannot process the ring structure. Melanotan II, bremelanotide, and setmelanotide are all cyclic peptides.