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 (t0.5 ∼
4 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 D7.4 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-PEG6 analogue
is presented that is stable in blood (t0.5 ∼ 18 h), has significant blood-pressure lowering effect,
and shows fast recovery of heart function in Langendorff assay.