A Review of Randomized Controlled Trials of Hereditary Angioedema Long-Term Prophylaxis with C1 Inhibitor Replacement Therapy: Alleviation of Disease Symptoms Is Achievable

**Background:** Hereditary angioedema (HAE) is a genetic disorder characterized by unpredictable, potentially life-threatening angioedema attacks due to C1 esterase inhibitor (C1-INH) deficiency or dysfunction. Long-term prophylaxis aims to reduce attack frequency, severity, and disease burden. C1-INH replacement therapy is a physiological approach that restores equilibrium to the contact and kallikrein/kinin system. This review examines RCT data on C1-INH replacement for long-term HAE prophylaxis, focusing on dose-response relationships and the potential for achieving attack freedom.

**Methods:** The review analyzed three randomized, double-blind, placebo-controlled crossover trials of C1-INH replacement therapies for long-term HAE prophylaxis. The intravenous pdC1-INH trial (phase 3) enrolled 22 patients (≥6 years) who received pdC1-INH 1000 U or placebo every 3–4 days for two consecutive 12-week periods. The subcutaneous pdC1-INH trial (phase 3) enrolled 90 patients (≥12 years) randomized to pdC1-INH 40 IU/kg or 60 IU/kg versus placebo twice weekly for two consecutive 16-week periods with a 2-week washout. The rhC1-INH trial (phase 2) enrolled 32 patients (≥13 years) who received rhC1-INH 50 IU/kg twice weekly, once weekly (plus placebo once weekly), or placebo twice weekly for three consecutive 4-week periods with 1-week washouts. Baseline attack frequency varied: the rhC1-INH trial included the most severe population (mean 17.9 attacks in prior 3 months; range 12–33), while the subcutaneous pdC1-INH trial had lower baseline rates (mean 8.8–10.8 attacks in prior 3 months).

**Key Results:** All three trials demonstrated significant reductions in HAE attack frequency with C1-INH prophylaxis versus placebo. In the intravenous pdC1-INH trial, time-normalized monthly attack rates were 2.09 with pdC1-INH vs 4.24 with placebo (50.8% reduction; p<0.001). In the subcutaneous pdC1-INH trial, monthly attack rates were 1.19 (40 IU/kg) and 0.52 (60 IU/kg) vs 3.61 and 4.03 for respective placebo groups (within-patient differences: -2.42 and -3.51; both p<0.001), corresponding to 55% and 84% reductions. In the rhC1-INH trial, monthly attack rates were 2.7 (twice weekly) and 4.4 (once weekly) vs 7.2 for placebo (mean differences: -4.4 and -2.8; p<0.0001 and p<0.0004), with per-protocol reductions of 72.1% and 44.4%. Dose-response was evident: ≥90% attack reduction was achieved by 23% (IV pdC1-INH 1000 U), 43% (SC pdC1-INH 40 IU/kg), 58% (SC pdC1-INH 60 IU/kg), 9% (rhC1-INH once weekly), and 26% (rhC1-INH twice weekly) of patients. ≥70% reduction was achieved by 67% (40 IU/kg) and 83% (60 IU/kg) with subcutaneous pdC1-INH. All therapies were well tolerated with no anaphylactic reactions or thrombotic complications reported.

**Clinical Implications:** The data demonstrate that C1-INH replacement therapy provides dose-dependent efficacy in reducing HAE attack frequency, with higher doses achieving greater proportions of patients reaching clinically meaningful thresholds including ≥90% reduction. The subcutaneous formulation (60 IU/kg twice weekly) showed the highest rates of near-complete attack control. These findings support the goal of attack freedom as an achievable target even for patients with moderate-to-severe baseline disease. However, direct comparisons between trials are limited by differences in study design, patient populations, baseline severity, and pharmacokinetic profiles (plasma half-lives: ~69 hours for SC pdC1-INH, ~56 hours for IV pdC1-INH, ~2.4 hours for rhC1-INH). The short half-life of rhC1-INH despite good efficacy raises questions about the correlation between plasma concentrations and clinical effectiveness. Further research is needed on the impact of baseline attack frequency, genetic variation in bradykinin pathways, and optimal dosing frequency to minimize breakthrough attacks.