PORT-77 is a potent inhibitor of ABCG2 that reduces efflux of protoporphyrin IX from erythrocytes of patients with protoporphyria and reduces plasma protoporphyrin IX levels in healthy human subjects Free

Background:

PORT-77 is a novel, orally bioavailable inhibitor of the ATP-binding cassette subfamily G member 2 (ABCG2, also known as BCRP) transport protein that is under development for the treatment of erythropoietic protoporphyria (EPP) and X-linked protoporphyria (XLP).

EPP and XLP are rare, genetic photodermatoses that result in the accumulation of protoporphyrin IX (PPIX), primarily in the marrow reticulocytes and secondarily in circulating RBCs, plasma, and bile. When activated by sunlight through the skin, PPIX generates singlet oxygen free radicals that lead to cutaneous damage and excruciating pain. PPIX is cleared through the bile, and it can accumulate in the biliary system and damage cholangiocytes and hepatocytes, leading to cholestatic liver disease that can be rapidly progressive.

The ABCG2 transport protein mediates the transport of PPIX from RBCs into the plasma. We hypothesize that inhibition of ABCG2 by PORT-77 decreases PPIX efflux from circulating erythroid cells and reduces plasma PPIX concentrations, potentially ameliorating PPIX-mediated skin phototoxicity and liver damage. Previously shared preclinical data support a protective role of PORT-77 in the relevant EPP mouse model, BALB/c Fech^m1Pas (Shah et al., 2024).

Here, we first examine the in vitro effects of PORT-77 on PPIX efflux from EPP and XLP patients' RBCs, and second, compare plasma PPIX levels before and after dosing with PORT-77 in healthy human volunteers without EPP or XLP.

Methods:

Fresh whole blood samples were obtained from EPP and XLP patients (EPP N=2, XLP N=1) and baseline PPIX levels were measured by LC-MS/MS. RBCs were then isolated and incubated with increasing concentrations of PORT-77. PPIX was extracted from the conditioned media and measured using the same LC-MS/MS assay.

Plasma samples from healthy adult volunteers in an ongoing Phase 1 study (NCT06346509) were collected before and after dosing with PORT-77 at ascending concentrations. Samples from single-dose (N=64) and 14-day (N=40) dosing regimens were analyzed by LC-MS/MS.

Given the exquisite light lability of the PPIX molecule, study protocols included amber blood collection tubes, rapid freezing, processing under filtered light, and storage and shipping in opaque containers.

Results:

Whole blood PPIX levels were 889 and 598 μg/dL for the EPP donors, and 319 μg/dL for the XLP donor (normal levels for those without EPP/XLP: <80 ug/dL whole blood and <1 μg/dL plasma). PORT-77 demonstrated dose-dependent inhibition of PPIX efflux from these RBCs with >50% reduction in media PPIX compared to vehicle control, with an IC50 (half-maximal inhibitory concentration) in the low-nanomolar range. In the healthy volunteer study, PORT-77 was well-tolerated, with no serious adverse events and no safety or tolerability signals identified to date. Mean baseline plasma PPIX concentrations were 0.15 μg/dl (range 0.01 – 0.88 μg/dL). Oral dosing of PORT-77 resulted in a rapid and dose-dependent reduction of plasma PPIX concentrations, with >50% reduction from baseline as early as 6 hours after a single dose. Once daily oral dosing of PORT-77 for 14 days resulted in a sustained reduction of plasma PPIX concentrations lasting for 48 hours after the last dose, with no subsequent rebound above pretreatment levels through the final assessment.

Conclusions:

These in vitro studies using RBCs from EPP and XLP patients and in vivo observations in healthy human volunteers provide the first-ever human evidence that PORT-77-mediated inhibition of ABCG2 is sufficient to reduce efflux of PPIX from RBCs and lower plasma PPIX levels, respectively. These data in healthy human volunteers and patient-derived RBCs support further investigation of PORT-77 in the ongoing Phase 2 study (NCT06971900) of individuals with EPP.