Predictors of inhibitor eradication by primary immune tolerance induction in severe haemophilia A with high responding inhibitors

Background Immune tolerance induction (ITI) is the only proven strategy to eradicate factor VIII inhibitors in patients with haemophilia A (HA). Aim To identify patients and treatment options with the highest chance of inhibitor eradication by primary ITI. Patients and methods In the frame of the Italian ITI Registry, carried out from 1995 to 2015 (last follow-up 2018), 137 primary ITI courses in severe HA patients (90/137 with poor prognosis) were analysed for predictors of outcome (complete/partial response or failure). Sixty-six of them (48%) were prospectively evaluated. Results ITI was successful in 91/137 patients (66.4%) and 70 (51.1%) achieved complete response within 11 months (median). Historical peak titres <= 200 BU/ml (P = .033), inhibitor titres <= 5 BU/ml at ITI start (P = .001), peak titres <= 100 BU/ml during ITI (P < .001) and missense mutations and small insertions/deletions of FVIII gene (P = .027) predicted complete inhibitor eradication. A score that considers the cumulative number of these variables predicted complete response with positive predictive values up to .81 at ITI start and .91 during ITI, respectively. Patients who had no bleeding (OR, 3.45, 95% CI: 1.4-8.6) nor other adverse events (OR 2.6, 95%CI: 1.3-5.3) during ITI had higher chances of complete response. During the 120-month follow-up (median), 2/70 patients who had achieved complete response relapsed (2.9%). Conclusions This Registry, with a centralized review of outcomes, homogeneous data collection (half of which prospective) and long-term follow-up, provides insights for optimizing ITI, with a rationale for further studies in the currently evolving scenario of inhibitor management in HA patients.