The Atlanta Protocol: Immune Tolerance Induction in Pediatric Patients with Hemophilia a and Inhibitors on Emicizumab

Introduction: The formation of neutralizing anti-factor VIII (fVIII) antibodies, called inhibitors, is a challenging complication in hemophilia A care. While novel non-factor therapies significantly reduce bleeding symptoms in patients with hemophilia A and inhibitors, the absence of fVIII tolerance remains unchanged. Additionally, there are concerns regarding the hemostatic efficacy and safety of bypassing agents necessary for the management of breakthrough bleeds in patients with inhibitors on these novel therapies. Immune tolerance induction (ITI) remains the primary method for eradicating inhibitors and restoring the hemostatic response to fVIII. Here we report on the response to ITI in pediatric patients following initiation of the bispecific humanized monoclonal antibody Emicizumab.

Methods: Patients between the ages of 1 - 11 years old with hemophilia A and an active inhibitor (≥0.6 Bethesda Units (BU)/mL) treated at the Emory Pediatric Hemophilia Treatment Center were included in this analysis. In general, ITI was initiated after four weekly loading doses of Emicizumab with standard or extended half-life recombinant (rfVIII) or plasma-derived (pdfVIII) fVIII at 100 units/kg three times per week. Factor choice was determined by the fVIII product used for the ITI attempt prior to initiation of Emicizumab or a standard half-life third generation rfVIII product if no prior ITI attempts. FVIII pharmacokinetics during ITI including the estimated fVIII half-life, fVIII incremental recovery of expected, and inhibitor titers were measured with chromogenic-based assays using bovine reagents. Bleeding symptoms and treatment regimens for bleeds/procedures were also monitored.

Results: Eight patients are currently receiving ITI according to this Atlanta Protocol. Details regarding this cohort are outlined in Table 1. Three of the 8 patients required central venous access for ITI fVIII infusions. These eight patients have historical peak inhibitor titers, defined as the highest inhibitor titer prior to this ITI regimen, ranging from 2.0 - 198 BU/mL. Four patients have just recently started ITI. The other 4 patients (patients 1, 2, 4, and 5) have been treated with Emicizumab and ITI for a median duration of 15 weeks (range 13-18 weeks). Two of the 4 patients are on ITI with a standard half-life third generation rfVIII product, 1 is on a standard half-life third generation B domain deleted (BDD) rfVIII product, and 1 is on an extended half-life (EHL) rfVIII-Fc fusion product. Their last measured inhibitor titers after initiation of ITI range from 0.3 - 3.7 chromogenic BU (CBU). All inhibitor titers declined with the start of ITI, and none of the 4 patients experienced an amnestic response (Figure 1). The fVIII incremental recovery improved in 3 of the 4 patients from a median of 30% of expected (range 17-69%) prior to Emicizumab with ITI to 85% of expected (range 67-85%) at the last clinical evaluation. The 1 patient remaining has an fVIII incremental recovery <10% of expected at the last clinical evaluation. Only one patient has required treatment with a single dose of recombinant activated factor VII (rfVIIa) for a right knee hemarthrosis during ITI without complication. There were 3 additional mild bleeding symptoms in 2 patients that did not require intervention including 1 trauma-induced forehead hematoma that self-resolved and 2 episodes self-resolving epistaxis lasting <5 minutes. Two of the 4 patients reported no bleeding symptoms. There were no episodes of thrombosis or thrombotic microangiopathy.

Conclusions: This is the first report detailing the Atlanta Protocol for ITI in hemophilia A patients with inhibitors receiving Emicizumab prophylaxis. Although early, these results suggest that ITI can be safely administered in these patients and is able to achieve continued improvement in clinical indicators of tolerance.

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