Long-Term Efficacy and Tolerability of Pegzilarginase in Arginase 1 Deficiency: Results of Two International Multicentre Open-Label Extension Studies.

Arginase 1 deficiency (ARG1-D) is an autosomal recessive urea cycle disorder characterised by chronic hyperargininaemia, progressive spasticity, loss of mobility, and cognitive dysfunction. Standard of care (SOC), based on dietary protein restriction, rarely prevents progression. Pegzilarginase, a recombinant human enzyme, is the first approved disease-modifying therapy. We report outcomes from Study 102A (n = 14; up to 5 years) and the PEACE long-term extension (LTE) (n = 31; up to 3 years). Weekly pegzilarginase was administered with SOC. Outcomes included functional mobility (2-/6-minute walk tests [2MWT/6MWT], Gross Motor Function Measure [GMFM] D/E), spasticity (Modified Ashworth Scale [MAS]), plasma arginine, guanidino compounds, and safety. In PEACE, LTE arms were named according to initial 24-week double-blind treatment: placebo-pegzilarginase or pegzilarginase-pegzilarginase. Of 39 evaluable participants, 37 (95%) met composite response or achieved maximum score in ≥ 1 motor function domain. In 102A, mean 6MWT improved to 68.2 m (+19%; n = 12); GMFM-D/E increased by 2.7/3.7. In PEACE (pegzilarginase-pegzilarginase; placebo-pegzilarginase), 2MWT improved to 16.5 m (+25%; n = 6) and 13.5 m (+16%; n = 2); GMFM-D/E improved by 4.3/6.0 (n = 6) and 5.3/11.3 (n = 3). Spasticity improved in 21/25 (84%), with 12 reaching MAS 0 (no spasticity). Pegzilarginase sustained plasma arginine control: in 102A, means were 118 μmol/L at Week 192 (n = 9); in PEACE, < 115 μmol/L through Week 96 (n = 11). Guanidinoacetic acid normalised; N-acetylarginine approached normal; argininic acid and α-keto-δ-guanidinovaleric acid declined by > 50%. Most adverse events were mild/moderate; no treatment-related discontinuations or persistent antibodies occurred. Pegzilarginase produced sustained improvements in mobility, spasticity and biochemical control, supporting early intervention, long-term use and disease modification in ARG1-D.