Opioid Peptides and Their Glycoconjugates: Structure-Activity Relationships

Since the discovery of the opioid receptors and their endogenous ligands, it has been widely recognized that endogenous opioid peptides produce a large spectrum of central and peripheral effects which include spinal and supraspinal analgesia, dependence, effects on gastrointestinal, renal and hepatic functions, cardiovascular and immunological responses, respiratory depression, and are also involved in neurological disorders. The intention of this review is to identify and highlight the acomplishments to date in the design of receptor-selective opioid peptide analogues as well as of their glycoconjugates which led to the potent ligands most useful as pharmacological tools or have potential for therapeutic applications. In the first part of the article, following a brief description of opioid receptors, and characteristics of m, d and k types, endogenous peptide ligands and their physiological roles are presented. The chapter concludes with presentation of the bioactivity profiles of peptide analogues with agonist properties and with high selectivity for distinct opioid receptor types. In the second part, synthetic efforts aimed at developing opioid peptide glycoconjugates and the progress made in this area are reviewed. Impact of the incorporated carbohydrate moiety on the receptor selectivity, conformation, stability and bioavailability of the parent peptides is discussed.