Inhibition of Glycine Transporter 1 Reduces Phototoxicity in a Mouse Model of Erythropoietic Protoporphyria (EPP)

Erythropoietic protoporphyria (EPP) is a genetic disorder typically caused by reduced activity of ferrochelatase (FECH), the enzyme that catalyzes the final step in heme biosynthesis. EPP phototoxicity occurs when the photosensitive metal-free protoporphyrin IX (PPIX) in erythrocytes and skin is exposed to irradiation from sunlight, absorbs photons, and produces reactive oxygen species (ROS) that damage the skin and subdermal layers. Patients with EPP suffer from acute and severe phototoxicity after sun exposure, significantly impacting their quality of life. Fech m1Pas /Fech m1Pas homozygous mice (EPP mice) retain approximately 5% residual FECH activity due to a severe homozygous loss-of-function mutation. EPP mice develop protoporphyria characterized by elevated PPIX levels in red blood cells (RBCs) and liver, which leads to liver fibrosis and cutaneous phototoxicity that manifests as skin lesions when these mice are exposed to light within the excitation wavelength of PPIX (395 to 410 nM) (Wang, 2019). Glycine transporter 1 (GlyT1) mediates the import of glycine, a precursor for heme synthesis, to erythroid cells. The GlyT1 inhibitor bitopertin has been shown to decrease whole-blood PPIX levels in patients with EPP in Phase 2 clinical studies (Dickey, 2024; Ross, 2024) and in animal models (Wu, 2022). This study evaluated whether GlyT1 inhibition can ameliorate phototoxicity via downregulating PPIX levels in Fechm1Pas/Fechm1Pas EPP mice. EPP mice were administered vehicle or a selective small molecule inhibitor of GlyT1, DISC-C, at 15 mg/kg by oral gavage twice daily for 18 days. GlyT1 inhibition decreased PPIX levels in RBCs by 43% and 37% on Day 14 and Day 18, respectively. On Day 14 of treatment, hair was shaved from the backs of EPP mice, and all mice were exposed to light at a wavelength of 395 nM (588±10% lumens/m2) for 30 minutes. Mice administered vehicle developed progressively worsening skin lesions with histopathological characterization of sterile abscesses, hyperkeratosis, and dermal lymphocyte infiltration. The skin lesions worsened over the observation period from Day 14 to Day 18, with 51.2% exposed skin area developing skin lesions at 4 days after light exposure (Day 18). In contrast, mice administered DISC-C developed skin lesions in 9.2% of exposed skin area, suggesting GlyT1 inhibition reduced phototoxicity in EPP mice. Additionally, a lower percentage of the area with skin lesions correlated with lower PPIX levels in RBCs. Collectively, this study demonstrated that oral GlyT1 inhibitor administration decreased PPIX levels in RBCs, resulting in diminished phototoxicity upon light exposure in the EPP mouse model. Overall, these data support the rationale for treating patients with EPP with GlyT1 inhibitors.