PORT-77, a Novel and Potent Inhibitor of ABCG2, Protects Against Skin Photo Toxicity and Liver Damage in a Mouse Model of Erythropoietic Protoporphyria (EPP)

Introduction

PORT-77 is an orally bioavailable inhibitor of the ABCG2 (also known as BCRP) transport protein and is under development for the treatment of EPP.

EPP is a rare, genetic form of photodermatosis resulting from autosomal recessive mutations in ferrochelatase (FECH), the enzyme in the heme biosynthesis pathway that inserts Fe2+ into protoporphyrin IX (PPIX) to form heme. The defect in FECH activity results in accumulation of PPIX in marrow reticulocytes and circulating RBCs, which is eventually released into the plasma, leading to its deposition in liver and skin. The accumulated PPIX in skin, when activated by sunlight exposure, generates singlet oxygen mediated free radicals which, in combination with other inflammatory processes, leads to cutaneous damage and excruciating pain. Notably, the transport of PPIX from RBCs to plasma is mediated by the transporter ABCG2. The role of ABCG2 in mediating PPIX efflux from RBCs has been established using FECH mutant mice (Fech ^m1Pas), a mouse model of EPP. Genetic ablation of ABCG2 in FECH mutant mice results in a substantial decrease in serum PPIX concentrations by compartmentalizing the PPIX inside RBCs, and a subsequent decrease in PPIX in skin and liver. Importantly, ABCG2 deficiency protects against EPP-associated phototoxicity and hepatotoxicity, suggesting that ABCG2 plays a critical role in the pathophysiology of EPP by mediating PPIX efflux from RBCs. Taken together, we hypothesized that pharmacological inhibition of ABCG2 would reduce plasma PPIX concentrations by decreasing efflux from RBCs and marrow reticulocytes. This is predicted to decrease PPIX levels in disease compartments including skin and liver and result in clinical improvement by reducing EPP-associated skin phototoxicity and liver damage.

Methods

In vitro testing of PORT-77 was performed using both biochemical and cell-based assays. In vivo efficacy of PORT-77 was investigated in the FECH mutant mice by evaluating EPP associated skin phototoxicity and liver damage. PPIX concentrations were measured by LC-MS/MS method.

Results

1. In vitro testing of PORT-77 showed dose dependent inhibition of human and mouse ABCG2 activity with IC₅₀ values in the low nanomolar range.
2. Oral dosing of PORT-77 in FECH mutant mice resulted in protection against light induced skin damage which was associated with ~50% reduction in serum PPIX concentration.
3. Chronic dosing (8-weeks) of PORT-77 in FECH mutant mice resulted in a marked reduction in liver PPIX levels and a reduction in various markers of progressive liver damage (liver enzymes, hydroxyproline levels, liver fibrosis, bile duct damage).
4. Chronic dosing of PORT-77 in FECH mutant mice did not adversely affect hematological parameters. Importantly, PORT-77 was safe and well tolerated.

Conclusions

In summary, in vitro pharmacology data and in vivo efficacy data in a mouse model of EPP provide a strong biological rationale for developing PORT-77 for the treatment of the patients with EPP. The safety and PK of PORT-77 are currently being evaluated in a phase 1 study.