Pabinafusp Alfa is a recombinant fusion glycoprotein developed as a blood-brain-barrier-penetrating enzyme replacement therapy for mucopolysaccharidosis type II (MPS II), also known as Hunter syndrome. It is also known by the development code JR-141 and is marketed in Japan as Izcargo for I.V. Infusion 10 mg. PMDA approved pabinafusp alfa in 2021 as a new active ingredient and classified both the drug product and drug substance as biological products.
PMDA describes pabinafusp alfa as a fusion of a humanized anti-human transferrin receptor antibody with human iduronate-2-sulfatase. The molecule consists of two A-chains containing 219 residues each and two B-chains containing 975 residues each. PMDA also provides a calculated protein-moiety molecular weight of 265,110.93 Da, while the product definition gives an approximate molecular weight of 300,000 Da, reflecting the glycoprotein's structural/glycosylation characteristics.
Pabinafusp alfa is designed to address one of the major limitations of conventional intravenous enzyme replacement therapy for MPS II: limited penetration of therapeutic enzyme into the central nervous system.
The antibody portion binds to human transferrin receptor 1 (TfR1) expressed on brain microvascular endothelial cells. This facilitates receptor-mediated transcytosis across the blood-brain barrier. Following transport into the central nervous system, the iduronate-2-sulfatase component can be taken up by cells and participate in the degradation of accumulated glycosaminoglycans.
PMDA reports that the molecule demonstrates binding to human TfR, cellular uptake and enzyme activity. Its hTfR binding affinity was measured by bio-layer interferometry with a KD of 1.22 × 10⁻¹⁰ mol/L, while M6P-receptor binding affinity was 9.22 × 10⁻⁹ mol/L by surface plasmon resonance.
Pabinafusp alfa is indicated for mucopolysaccharidosis type II (MPS II). MPS II is an X-linked lysosomal storage disorder caused by deficient or reduced activity of iduronate-2-sulfatase, resulting in accumulation of glycosaminoglycans, particularly heparan sulfate and dermatan sulfate.
The Japanese approved product is Izcargo for I.V. Infusion 10 mg. PMDA states that the usual dosage is 2.0 mg/kg once weekly by intravenous infusion. The approved product is a lyophilized powder for injection, with each vial containing 12.5 mg of pabinafusp alfa (genetical recombination).
Pabinafusp alfa requires extensive biological characterization because it is a large, glycosylated fusion protein. PMDA reports characterization of:
PMDA also reports an enzyme-activity characterization with Km = 32.6 mmol/L and Vmax = 16.9 μmol/min/mg under the specified assay conditions.
Pabinafusp alfa is manufactured using a CHO-cell recombinant expression system. The drug-substance process includes cell culture, harvesting, chromatographic purification, viral inactivation, virus removal and filtration/storage steps. PMDA reports commercial-scale process validation and an extensive viral-clearance evaluation.
The drug-substance control strategy includes content, description, identification, glycan profiles, pH, purity by SE-HPLC, host-cell protein, bacterial endotoxins, sialic acid content, M6P content, TfR binding activity, enzyme activity and UV-Vis assay.
Because Pabinafusp alfa is a recombinant fusion protein, conventional small-molecule tests such as melting point, residue on ignition and specific rotation are not appropriate universal API release parameters.
| Parameter | Specification |
|---|---|
| Appearance | white to off-white crystalline powder |
| Identification | IR & HPLC compliant |
| Assay (HPLC) | 98.0% – 102.0% |
| Loss on Drying | NMT 2.0% |
| Residue on Ignition | NMT 0.5% |
| Heavy Metals | NMT 10 ppm |
| pH (1% Solution) | 2.0 – 4.0 |
| Individual Impurity | NMT 0.5% |
| Water Content | NMT 1.0% |