Nicotinamide-N-methyltransferase inhibition improves cardiac function and structure in a heart failure with preserved ejection fraction mouse model.

HFpEF is a major and global disease with limited treatment options and novel therapeutics are eagerly awaited. A potential treatment option may be Nicotinamide N-methyltransferase (NNMT) inhibition. This study aimed to investigate the cardiac protective effects of the NNMT enzyme inhibitor AMO-NAM in a HFpEF mouse model. Aged (18-22 months old) female mice developed a cardiometabolic HFpEF phenotype using a multiple hit strategy with high-fat diet (HFD) and angiotensin II (AngII) infusion. NNMT inhibitor 4-amino-6-methoxynicotinamide (AMO-NAM) was added to HFD and mice were treated for four weeks. Cardiac function was assessed by echocardiography, molecular (RT-qPCR; O-link, LC-MS assay) and histological analyses (Masson staining; macrophage staining) were performed to evaluate AMO-NAMs drug-specific effects. We observed significant increases in 1-MNA (+121.6 %), the product of the reaction catalysed by NNMT, and its downstream metabolites levels, 2PY (+274.7 %), and 4PY (+296.4 %) in the left ventricle (LV) of the HFpEF model. Treatment with the AMO-NAM did not affect NAD levels in the HFpEF model but markedly decreased 1-MNA (-77.1 %), 2PY (-66.2 %), and 4PY (-71.1 %) levels. NNMT inhibition led to notable improvements in cardiac function, evidenced by enhanced global longitudinal strain and reversed peak longitudinal strain rate alongside significant reductions in LV hypertrophy and fibrosis. This was accompanied by decreased pro-inflammatory and pro-fibrotic gene expression in plasma and LV tissue and reduced macrophage infiltration in LV and visceral adipose tissue, highlighting the anti-inflammatory and anti-fibrotic effects of NNMT inhibition. Targeting the NNMT is cardioprotective and holds promise for treating HFpEF patients with an unfavorable cardiometabolic phenotype.