The Ectonucleotide Pyrophosphatase/Phosphodiesterase 1 (ENPP1) ectoenzyme regulates vascular intimal proliferation and mineralization of bone and soft tissues. ENPP1 variants cause Generalized Arterial Calcification of Infancy (GACI), a rare genetic disorder characterized by ectopic calcification, intimal proliferation, and stenosis of large- and medium-sized arteries. ENPP1 hydrolyzes extracellular ATP to pyrophosphate (PP_i) and AMP. AMP is the precursor of adenosine, which has been implicated in the control of neointimal formation. Herein, we demonstrate that an ENPP1-Fc recombinant therapeutic inhibits proliferation of vascular smooth muscle cells (VSMCs) in vitro and in vivo. Addition of ENPP1 and ATP to cultured VSMCs generated AMP, which was metabolized to adenosine. It also significantly decreased cell proliferation. AMP or adenosine alone inhibited VSMC growth. Inhibition of ecto-5′-nucleotidase CD73 decreased adenosine accumulation and suppressed the anti-proliferative effects of ENPP1/ATP. Addition of AMP increased cAMP synthesis and phosphorylation of VASP at Ser157. This AMP-mediated cAMP increase was abrogated by CD73 inhibitors or by A_2aR and A_2bR antagonists. Ligation of the carotid artery promoted neointimal hyperplasia in wild-type mice, which was exacerbated in ENPP1-deficient ttw/ttw mice. Prophylactic or therapeutic treatments with ENPP1 significantly reduced intimal hyperplasia not only in ttw/ttw but also in wild-type mice. These findings provide the first insight into the mechanism of the anti-proliferative effect of ENPP1 and broaden its potential therapeutic applications beyond enzyme replacement therapy.

Calcified aortic valve disease in its final stage leads to aortic valve stenosis, limiting cardiac function. To date, surgical intervention is the only option for treating calcific aortic valve stenosis. This study combined controlled drug delivery by nanoparticles (NPs) and active targeting by antibody conjugation. The chelating agent diethylenetriaminepentaacetic acid (DTPA) was covalently bound to human serum albumin (HSA)-based NP, and the NP surface was modified using conjugating antibodies (anti-elastin or isotype IgG control). Calcification was induced ex vivo in porcine aortic valves by preincubation in an osteogenic medium containing 2.5 mM sodium phosphate for five days. Valve calcifications mainly consisted of basic calcium phosphate crystals. Calcifications were effectively resolved by adding 1–5 mg DTPA/mL medium. Incubation with pure DTPA, however, was associated with a loss of cellular viability. Reversal of calcifications was also achieved with DTPA-coupled anti-elastin-targeted NPs containing 1 mg DTPA equivalent. The addition of these NPs to the conditioned media resulted in significant regression of the valve calcifications compared to that in the IgG-NP control without affecting cellular viability. These results represent a step further toward the development of targeted nanoparticular formulations to dissolve aortic valve calcifications.

Generalized arterial calcification of infancy (GACI) and autosomal recessive hypophosphatemic rickets type 2 (ARHR2) are age-related phenotypes of the rare genetic mineralization disorder, ENPP1 Deficiency, which evolve on a phenotypic continuum. To date, our understanding of the clinical spectrum of ENPP1 Deficiency is based on small studies or case reports, across which there is significant variability in clinical presentation, and limited duration of follow-up. From a previously published large retrospective natural history study, we performed a subgroup analysis to elucidate the most prevalent signs and symptoms of ENPP1 Deficiency diagnosed as GACI or ARHR2, to illustrate the onset and incidence of these complications over the lifetime, and to characterize the associated medical burden of disease. Of the 84 individuals with ENPP1 Deficiency analyzed, 51 had a recorded diagnosis of GACI, 11 were diagnosed with ARHR2, and 22 were diagnosed with both. We confirmed that those diagnosed with GACI presented predominantly with early-onset arterial calcification, respiratory distress, heart failure, and hypertension, necessitating acute inpatient care and leading to high (44%) infant mortality. Notably, we found that the majority (60.3%) of those with a history of GACI had prenatal ultrasound anomalies, including effusions, polyhydramnios, and hydrops fetalis. We estimated that 70% of individuals with ENPP1 Deficiency who survive to age 10 will have developed musculoskeletal complications, primarily rickets and/or osteomalacia. The clinical picture of ARHR2 in this study extended beyond skeletal deformities to include hearing impairment, joint involvement, and ongoing risk of cardiovascular problems. This study sheds light on the signs and symptoms of ENPP1 Deficiency in the real world, with implications for life-long patient monitoring.