ENPP1 inhibition as a therapeutic approach for later-onset hypophosphatasia.

Hypophosphatasia (HPP) is caused by loss-of-function mutations in the human ALPL gene that encodes tissue-nonspecific alkaline phosphatase (TNAP), whose deficiency results in the accumulation of the calcification inhibitor inorganic pyrophosphate (PPi), resulting in skeletal and dental hypomineralization. Enzyme replacement with mineral-targeted TNAP (asfotase alfa) improves skeletal mineralization but the almost daily injections of this biologic can lead to injection site reactions and discontinuation of treatment. Since PPi is produced by the enzymatic action of ectonucleotide pyrophosphatase/phosphodiesterase 1 (ENPP1) from ATP, we tested if ENPP1 could be a druggable target for the development of an alternative treatment for HPP, particularly for the non-lethal later-onset forms of HPP where enzyme replacement is not currently approved. We orally administered 30 and 100 mg/kg/d of an ENPP1 inhibitor, REV102, to the AlplPrx1/- mouse model of late-onset HPP, for 105 d and confirmed target engagement, as plasma PPi concentrations were markedly reduced. X-ray, micro computed tomography and bone morphometry indicated improvement in appendicular skeletal mineralization. This study suggests that the adult HPP phenotype could benefit from oral administration of ENPP1 inhibitors. LAY ABSTRACT Hypophosphatasia (HPP) is a soft bones disease caused by inactivating mutations in the gene (Alpl) encoding tissue-nonspecific alkaline phosphatase (TNAP), crucially important for skeletal and dental mineralization. Deficiency in TNAP function leads to the accumulation of its substrate, inorganic pyrophosphate (PPi), that acts as a potent calcification inhibitor, and this increase in PPi causes insufficient skeletal mineralization. We tested if pharmacologically inhibiting ENPP1, the enzyme that generates PPi, could lower PPi concentrations and ameliorate soft bone disease in a mouse model of later-onset HPP. The results were efficacious and point to the potential usefulness of this strategy to treat HPP.