Optimizing PEG-Extended Apelin Analogues as Cardioprotective Drug Leads: Importance of the KFRR Motif and Aromatic Head Group for Improved Physiological Activity

Apelin is an important contributor to the renin-angiotensin axis, regulating cardiovascular, metabolic, and neurological functions. Apelin-17 has especially potent cardio-physiological effects but is rapidly degraded in human blood (t(0.5) similar to 18 h min). Angiotensin-converting enzyme 2 (ACE-2), neprilysin (NEP), and plasma kallikrein (KLKB1) cleave and inactivate it, with the latter cutting within the arginine-arginine site. Here, we show that analogues with an N-terminal polyethylene glycol (PEG) extension as well as peptide bond isosteres resist KLKB1 cleavage but that only the PEG-extended analogues significantly improve physiologically activity. The PEGylated analogues feature comparatively high log D-7(.4) values and high plasma protein binding, adding to their stability. An alanine scan of apelin-17 reveals that the integrity and conformational flexibility of the KFRR motif are necessary for cardio-physiological activity. An optimized Cbz-PEG(6) analogue is presented that is stable in blood (t(0.5) similar to 18 h), has significant blood-pressure lowering effect, and shows fast recovery of heart function in Langendorff assay.