Conjugation to a cell-penetrating peptide drives the tumour accumulation of the GLP1R antagonist exendin(9-39)

Abstract

Abstract Purpose Exendin, an analogue of the Glucagon-Like Peptide 1 (GLP1), is an excellent tracer for molecular imaging of pancreatic beta cells and beta cell-derived tumours. The commonly used form, exendin-4, activates the GLP1 receptor and causes internalisation of the peptide-receptor complex. As a consequence, injection of exendin-4 can lead to adverse effects such as nausea, vomiting and hypoglycaemia and thus requires close monitoring during application. By comparison, the antagonist exendin(9–39) does not activate the receptor, but its lack of internalisation has precluded its use as a tracer. Improving the cellular uptake of exendin(9–39) could turn it into a useful alternative tracer with less side-effects than exendin-4. Methods We conjugated exendin-4 and exendin(9–39) to the well-known cell-penetrating peptide (CPP) penetratin. We evaluated cell binding and internalisation of the radiolabelled peptides in vitro, and their biodistribution in vivo. Results Exendin-4 showed internalisation irrespective of the presence of the CPP, whereas for exendin(9–39) only the penetratin conjugate internalised. Conjugation to the CPP also enhanced the in vivo tumour uptake and retention of exendin(9–39). Conclusion We demonstrate that penetratin robustly improves internalisation and tumour retention of exendin(9–39), opening new avenues for antagonist-based in vivo imaging of GLP1R.