The determination of the key molecular determinants of insulin function is important not only for examining the downstream pathways leading to its physiological effects but also the development of new, higher performing clinical compounds. Insulin’s bridging cystine residues serve a structural role in maintaining the receptor-binding domain in its correct conformation, yet less is known about other possible functional roles including disulfide exchange at the receptor binding site, proteolytic degradation and/or formulation stability. It is now very clear that these bridges play a significant role in regulating insulin’s activity.

This paper will discuss the design, synthesis and biological evaluation of dicarba insulin analogues. Using a homogeneous catalysis approach, three dicarba insulin analogues (two unsaturated (C=C, E and Z isomers) and one saturated (CH₂-CH₂)) can be generated from a single linear precursor. Not all linear peptides, however, undergo facile metathesis and performing metathesis on insulin sequences was found to be particularly troublesome. During the course of our study, the application of microwave irradiation, chaotropic salts, turn-inducing pseudoproline residues, interrupted SPPS-catalysis methods, and tandem catalysis were all investigated to overcome competing aggregation phenomena and low metathesis/hydrogenation yields.

This work culminated in viable syntheses of target dicarba-insulin analogues. The insulin receptor competition binding and receptor kinase activation assays which followed showed that many of the dicarba insulin analogues possessed identical metabolic activity to insulin. Several of the analogues were also equipotent in [3H]-2-deoxyglucose uptake assays and in vivo glucose tolerance tests. However, unlike insulin, and its short and long acting variants, several of the dicarba analogues possess significantly lower mitogenic potency as measured in DNA synthesis assays.

The generation of diaminosuberic acid analogues of cystine-containing peptides via homogeneous catalysis provides a powerful tool for examining structure-activity relationship. This presentation highlights how homogeneous catalysis can be used to solve complex biological problems.