Endogenous opioid peptides have been studied extensively as potential therapeutics for the treatment of pain.
The major problems of using natural opioid peptides as drug candidates are their poor receptor specificity, metabolic instability
and inability to reach the brain after systemic administration. A lot of synthetic efforts have been made to opioid
analogs with improved pharmacological properties. One important structural modification leading to such analogs is cyclization
of linear sequences. Intramolecular cyclization has been shown to improve biological properties of various bioactive
peptides. Cyclization reduces conformational freedom responsible for the simultaneous activation of two or more receptors,
increases metabolic stability and lipophilicity which may result in a longer half-life and easier penetration across
biological membranes. This review deals with various strategies that have been employed to synthesize cyclic analogs of
opioid peptides. Discussed are such bridging bonds as amide and amine linkages, sulfur-containing bonds, including
monosulfide, disulfide and dithioether bridges, bismethylene bonds, monosulfide bridges of lanthionine and, finally, carbonyl
and guanidine linkages. Opioid affinities and activities of cyclic analogs are given and compared with linear opioid
peptides. Analgesic activities of analogs evaluated in the in vivo pain tests are also discussed.