Background

Hemophilia B is a congenital rare bleeding disorder caused by a deficiency or mutation in the gene encoding clotting factor IX (Factor IX, FIX). Patients experience spontaneous bleeding in joints or soft tissues even from minor impacts, and repeated bleeding leads to irreversible arthropathy and chronic pain. The current standard of care is prophylaxis involving regular intravenous infusions of factor IX (FIX) to replace the deficiency; however, the physical burden of lifelong injections, high costs, and limited venous access have been major factors reducing treatment adherence.

To overcome these limitations, AAV (Adeno-associated virus) vector-based gene replacement therapies have been developed for adults. However, clinical application in adolescents has been approached with caution due to the risk of dilution of the introduced episomal therapeutic genes caused by continuous liver growth and cell division. In particular, the use of existing wild-type FIX genes necessitated high-dose vector administration to induce sufficient clotting activity, which has been identified as a cause of triggering capsid-related T-cell immune responses leading to liver toxicity. Therefore, there was an urgent need for clinical evidence demonstrating safety and efficacy in adolescents, alongside the introduction of high-activity variants capable of inducing potent coagulation activity even with low-dose vectors.

Key Findings

In this single-arm phase 1 trial, an AAV vector therapy carrying the 'Padua' gain-of-function variant FIX gene, which has approximately 5 to 8 times higher clotting activity than the wild-type protein, was administered via a single intravenous dose to 11 adolescents with severe to moderate hemophilia B. The researchers closely monitored the liver immune response in adolescent patients to optimize vector dosage, and long-term tracked changes in FIX activity and Annualized Bleeding Rate (ABR) before and after treatment.

In the clinical trial, no serious adverse events (SAEs) or severe complications such as vector-related hemolysis or thrombosis were reported among all 11 adolescent patients who completed treatment. In cases where transient elevations in liver enzyme levels were observed, stabilization was achieved without liver damage through short-term oral corticosteroid administration. Most importantly, as the patients' endogenous FIX activity was maintained at a level effective for bleeding prevention, they were able to discontinue existing regular clotting factor infusion therapy or significantly extend the dosing intervals. Accordingly, the patients' ABR values showed a significant decrease compared to pre-treatment levels, establishing clinical efficacy.

Significance and Outlook

These results are highly significant as they successfully expanded the application of high-activity Padua variant-based AAV therapy, which was previously limited to adults, to the adolescent period, where physical activity is high and the risk of joint damage increases rapidly. This is because it demonstrated the validity of a treatment strategy that can prevent joint damage early through intervention during adolescence, a period serving as a bridge between childhood and adulthood. It is also notable that the use of low doses secured coagulation activity within the therapeutic range, thereby mitigating concerns regarding systemic immune responses and hepatotoxicity associated with high-dose vector administration.

However, due to the nature of episomal gene delivery rather than permanent genomic integration, it is difficult to exclude the possibility that FIX expression levels may gradually decrease during long-term follow-up over several years in adolescent patients whose physical growth has not yet completely ceased. Additionally, issues such as the exclusion of patients possessing existing neutralizing antibodies, high drug prices, and the need for long-term safety data are challenges to be addressed in the process of future commercialization and health insurance entry. The researchers plan to confirm the durability of the treatment efficacy through additional long-term follow-up and subsequent phase 3 clinical trials.