Abstract Body (Do not enter title and authors here): Background
Cyclic GMP (cGMP) preserves cardiac structure and function, and impaired cGMP signaling contributes to the pathogenesis of heart failure with reduced ejection fraction (HFrEF). cGMP levels are regulated by synthetic pathways triggered by NO or natriuretic peptides (NP) and degradatory mechanisms involving phosphodiesterases (PDEs; hydrolyse cGMP) and neprilysin (inactivate NP). Indeed, the expression/activity of PDEs and neprilysin are increased in pre-clinical models and patients with HFrEF.

Aim
To evaluate the therapeutic potential of dual inhibition of PDE5 and neprilysin in preventing the pathogenesis of HFrEF.

Methods
Wild type (WT), NO (GC-1α^-/-) and NP (GC-A^-/-) -sensitive guanylyl cyclase null mice underwent abdominal aortic constriction (AAC; increased afterload) or sham and received the PDE5 inhibitor sildenafil (Sild; 30mg/kg/day; p.o.) and/or the neprilysin inhibitor racecadotril (Race; 30mg/kg/day; p.o.). Changes in heart morphology/function were assessed via echocardiography. Hearts were sectioned and stained to determine cardiomyocyte size and fibrosis. Underlying molecular changes were investigated by qPCR.

Results
AAC induced cardiac dysfunction characterised by decreased ejection fraction (EF%: sham 65.8±3.9, AAC 53.6±5.2), increased left ventricle/body weight ratio (LV/BW: sham 3.26±0.07, AAC 4.37±0.08), and greater cardiac fibrosis (%: sham 0.9±0.09, AAC 5.2±1.04; all n=14-15; *P<0.05). In combination, sildenafil and racecadotril promoted cGMP signalling and ameliorated AAC-induced cardiac dysfunction, reversing compromised contractility (EF%: Sild 46.3±0.9, Race 54.5±0.8, Sild+Race 60.6±1.2), left ventricular hypertrophy (LV/BW: Sild 4.5±0.1, Race 3.9±0.2, Sild+Race 3.9±0.1), and cardiac fibrosis (%: Sild 1.6±0.2, Race 5.9±2.6, Sild+Race 1.5±0.5); dual therapy was broadly more effective than either drug alone (all n=15; *P<0.05 versus one or both monotherapies). AAC elevated mRNA expression of markers of cardiac dysfunction, hypertrophy and fibrosis (e.g. BNP, Col3/4A, MMP2) which were significantly reduced by combination treatment. The positive pharmacodynamic profile of dual therapy was maintained in GC-1α^-/- mice but absent in GC-A^-/- animals implying efficacy was underpinned by augmentation of NP bioactivity (rather than NO).

Conclusions
Concomitant blockade of PDE5 and NEP offers a novel approach for the treatment of HFrEF driven by synergistic augmentation of NP signaling.

Funding
British Heart Foundation (RG/F/23/110123)