John A. Karas

Insulin replacement therapy is essential for the management of diabetes. However, despite the relative success of this therapeutic strategy, there is still a need to improve glycaemic control and the overall quality of life of patients. This need has driven research into orally available, glucose‐responsive and rapid‐acting insulins. A key consideration during analogue development is formulation stability, which can be improved via the replacement of insulin's A6–A11 disulfide bond with stable mimetics. Unfortunately, analogues such as these require extensive chemical synthesis to incorporate the nonnative cross‐links, which is not a scalable synthetic approach. To address this issue, we demonstrate proof of principle for the semisynthesis of insulin analogues bearing nonnative A6–A11 cystine isosteres. The key feature of our synthetic strategy involves the use of several biosynthetically derived peptide precursors which can be produced at scale cost‐effectively and a small, chemically synthesised A6–A11 macrocyclic lactam fragment. Although the assembled A6–A11 lactam insulin possesses poor biological activity in vitro, our synthetic strategy can be applied to other disulfide mimetics that have been shown to improve thermal stability without significantly affecting activity and structure. Moreover, we envisage that this new semisynthetic approach will underpin a new generation of hyperstable proteomimetics.

The tachykinin neuropeptide substance P (SP) is the canonical agonist peptide for the neurokinin 1 receptor (NK 1 R). More recently, it has also been shown to activate the Mas‐related G protein‐coupled receptor X2 (MRGPRX2) receptor on mast cells (MCs), triggering degranulation and release of inflammatory mediators. SP undergoes rapid C‐terminal truncation in vivo by a number of proteases to generate the metabolites SP(1–9)‐COOH and in particular SP(1–7)‐COOH. While the C terminus of SP is critical for NK 1 R activation, studies have shown that the peptide polycationic N terminus is key for MRGPRX2 and mast cell activation. The study thus aimed to determine if the C‐terminally truncated metabolites of SP, SP(1–9)‐COOH, and SP(1–7)‐COOH retained stimulatory activity at MRGPRX2. SP, SP(1–9)‐COOH, and SP(1–7)‐COOH were synthesized and tested on HEK293 cells expressing NK 1 R or MRGPRX2, and LAD2 human mast cells, to determine the activity of SP and its metabolites in Ca 2+ mobilization, degranulation, and cytokine assays. As expected from prior studies, both C‐terminally truncated SP metabolites had essentially no activity at NK 1 R, even at very high concentrations. In contrast, the in vivo metabolite of SP, SP(1–9)‐COOH retained ability to activate MRGPRX2 across all parameters tested, albeit with reduced potency compared to intact SP. SP(1–7)‐COOH did not produce any significant MRGRPX2 activation. Our results suggest that the SP metabolite, SP(1–9)‐COOH, may play a regulatory role through the activation of MRGPRX2. However, given the relatively low potency of both SP and SP(1–9)‐COOH at MRGPRX2, additional work is needed to better understand the biological importance of this expanded SP/MRGPRX2 pathway.