Stable Factor IX Expression and Sustained Reductions in Factor IX Use 8 Years after Gene Therapy with CSL220 (Formerly AMT-060) in Adults with Hemophilia B

Introduction: CSL220 (formerly AMT-060) is an adeno-associated virus serotype 5 (AAV5) vector encoding a codon-optimized wild‑type human factor IX (FIX) gene, driven by a liver-specific promoter. CSL220 has an identical vector sequence to etranacogene dezaparvovec (CSL222), without the activity‑enhancing two nucleotide change in the human FIX coding sequence of the Padua FIX variant. The Phase I/II study included 10 patients with severe or moderately severe hemophilia B (FIX activity ≤2 IU/dL) who received a single intravenous infusion of CSL220 (5×10¹² gc/kg [Cohort 1; n=5] or 2×10¹³ gc/kg [Cohort 2; n=5]). Nine out of ten patients were FIX concentrate prophylaxis-free after CSL220 administration. Using the one-stage activated partial thromboplastin time (aPTT) assay (HemosIL® SynthASil reagent), mean (standard deviation, SD) and median (range) FIX activities were 6.5 (0.3) IU/dL and 6.5 (6.3–6.7) IU/dL, respectively, in Cohort 1 (n=2) and 6.6 (2.0) IU/dL and 7.2 (3.8–8.3) IU/dL in Cohort 2 (n=4) at completion of the phase I/II trial (NCT02396342), 5 years post-treatment with CSL220. Continued assessment of the efficacy and safety of CSL220 is on‑going in an extension study up to 10 years post‑administration (NCT05360706).

Methods: Patients who successfully completed all assessments during 5 years of follow-up were enrolled in the open-label, Phase I/IIb extension study. Here, we report the third year of follow‑up in the extension study; representing 8 years after CSL220 administration.

Four of five patients from Cohort 1 (including one patient who remained on FIX prophylaxis) and all five patients from Cohort 2 enrolled in the extension study.

Results: Endogenous FIX activity (one-stage aPTT assay) remained stable at Year 8; mean (SD) and median (range) FIX activity were 4.9 (1.2) IU/dL and 5.2 (3.6–6.0) IU/dL in Cohort 1 (n=3, excluding the patient who remained on prophylaxis), and 5.6 (1.2) IU/dL and 5.8 (3.7–7.0) IU/dL in Cohort 2 (n=5), respectively.

Mean (SD) annualized bleeding rate (ABR) for individual (excluding the patient who remained on prophylaxis) for Year 8 was 2.2 (2.2, n=3) and 1.0 (1.8, n=5) for Cohort 1 and 2, respectively. In Year 8, one patient in Cohort 1, and three patients in Cohort 2 experienced no bleeds. Mean (SD) annualized spontaneous bleeding rate (AsBR) was 1.5 (1.7, n=3) in Cohort 1 and 0.4 (0.9, n=5) in Cohort 2. Spontaneous bleeds were experienced by two patients in Cohort 1 and one patient in Cohort 2. One patient experienced one traumatic bleed in Cohort 1, and one patient experienced one traumatic bleed in Cohort 2.

Mean (SD) annualized FIX consumption during Year 8 (excluding surgeries and the patient who remained on FIX prophylaxis) was 17,817.1 (30,860.1) IU/year (or 189.5 [328.3] IU/kg/year) in Cohort 1 (n=3) and 12,302.4 (24,223.6) IU/year (or 149.3 [300.0] IU/kg/year) in Cohort 2 (n=5).

No new safety events were identified during Year 8, and no patient returned to continuous FIX prophylaxis.

Conclusions: Durability of factor expression is a key consideration in the decision-making process about gene therapy for patients and physicians. With just one amino acid difference in the expressed FIX protein, CSL220 is the precursor of etranacogene dezaparvovec (CSL222), which was the first gene therapy approved for the treatment of hemophilia B. This 8-year follow-up after CSL220 administration provides continued evidence for the durability, stability, and safety of FIX expression after AAV5-based gene therapy for the treatment of hemophilia B.