VPB0227 - Laboratory monitoring in haemophilia A patients with factor VIII inhibitor treated with emicizumab

Background: Emicizumab, a humanized bispecific antibody for haemophilia A (HA) treatment, is known to affect the activated partial thromboplastin time (aPTT) making not reliable the aPTT-based tests, such as the “classic” Nijmegen-Bethesda assay (clot-based) for measuring FVIII inhibitor titre. The use of chromogenic assay with bovine regents (insensitive to Emicizumab) can avoid misinterpretation of results. A mixture of two recombinant anti-idiotype monoclonal antibodies (anti-Emi; Chugai and Nara Medical University) for preventive neutralization of the drugs could be an option when the chromogenic assay is not available.

Aims: To evaluate the efficacy of neutralizing anti-Emi by comparing the results obtained with aPTT-based and chromogenic assays on samples from HA patients treated with emicizumab.

Methods: Fifty plasma samples of HA patients with inhibitor were analysed for FVIII inhibitor titre using Nijmegen-Bethesda assay measuring residual FVIII both with the one-stage (Actin FS-Siemens, Germany) and the chromogenic assay (with bovine regents, Siemens). Emicizumab concentration was measured by using a modified one-stage assay calibrated with a specific Calibrator (r2 Diagnostics Inc., US), before and after the addition of anti-Emi (1:20). All the tests were performed on Sysmex CS-2400 (Sysmex, Kobe, Japan).

Results: The results are reported in Table as mean [range]. Emicizumab concentration, ranging from 100.4 to 0.9 µg/mL before neutralization, was < 0.5 µg/mL in all the samples measured after the neutralization.
Inhibitor titre was < 0.5 UB in almost all the samples at medium and low inhibitor titre when measured with the one-stage before neutralization. Good agreement was found between inhibitor titre measured with chromogenic assay and with one-stage assay after neutralization, as shown by Bland-Altman plot (Figure)

Conclusion(s): These results showed the efficacy of the anti-Emi that could be used in the laboratory monitoring of inhibitor titre in HA patients treated with emicizumab when the one-stage is the only available assay.