Immune Tolerance Induction (ITI) in Paediatric and Adult Patients with Haemophilia a (HA) and High-Titre (HT) Factor VIII (FVIII) Inhibitor - Real Life Data

Introduction: In Poland, the principles for the management of rare bleeding disorders as well as financing of medication are regulated by the National Program for the Management of Haemophilia and Related Bleeding Disorders. One objective of the Program for 2012-2018 was to include the financing of ITI for all haemophilia patients with inhibitor. The qualification for ITI was performed by a physician from a Haemophilia Treatment Centre (HTC).

Goal: the aim of the study is/was prospective assessment of the efficacy of ITI in real life setting.

Patients and Methods: The study comprised 30 patients with severe and one with moderate HA and HT FVIII inhibitors. Factor VIII activity (FVIII:C) was measured by one-stage clotting assay and Factor VIII inhibitors by a modified Nijmegen-Bethesda assay and expressed in Bethesda Units/ml (BU/ml). Cut-off level for positive inhibitor was ≥0.5BU/ml. The following data was collected: historical peak inhibitor titre (HPT), pre-ITI titre (measured 1 month prior to ITI), peak titre recorded during the course of ITI, time between inhibitor detection and ITI start, and ITI duration. During ITI course blood samples were collected every 4 weeks. Treatment regimen with dosing and type of FVIII concentrate was at the discretion of the physician in each HTC. ITI success was defined as inhibitor titre below 0.5 BU/ml with FVIII half-life ≥6h and FVIII recovery ≥66% of normal. Partial response to ITI was defined as clinical response to FVIII therapy with no anamnestic response despite the presence of FVIII inhibitor or FVIII half-life <6h and FVIII recovery <66% of normal.

Results: 31 patients with HA and inhibitor were divided into two groups: Group 1 comprised 12 adults with severe HA, median age 36 years (mean 40.1±15.1 and Group 2 included 19 boys, (<18 years) (18 with severe, one with moderate HA), median age 8 years (mean 8.1±4.3) The median (mean) duration time between inhibitor detection and ITI start was 108 months (102±91) in Group 1 and 10 months (9±10) in Group 2. The median (mean) HPT before ITI was 68 BU/ml (156±193) and 50 BU/ml (126±242) in Group 1 and 2, respectively. ITI is still ongoing in one patient (Group 1) and in 3 boys (Group 2); these 4 patients were excluded from outcome analysis. Two adult patients (16,6%) of Group 1 and 2 boys (10,5%) of group 2 discontinued ITI therapy for a variety of reasons such as death (one patient), poor compliance or loss to follow-up; those patients were however included in the outcome analysis.

The median (mean) duration of ITI was 24 months (30.8±31.7) (1-120) in Group 1 and 28 months (33.4±21.4) (6-84) in Group 2. The median (mean) peak titre during ITI was 40 BU/ml (225.7±185.7) in Group 1 and 85 (160±171.7) in Group 2. Immune tolerance to FVIII (success) was achieved in 3/11 adult (27.3%) and in 6/16 paediatric patients (37.5%). Median HPT, median time between inhibitor detection and ITI start, median peak inhibitor titre during ITI and median duration of ITI in patients who achieved immune tolerance were 380 BU/ml, 120 months, 8 BU/ml, 33 months in Group 1 and 22.5 BU/ml, 9.5 months, 205 BU/ml, 25 months in Group 2, respectively.

No partial response to ITI was observed in our patients. The most common initial ITI regimen in both groups was 100 IU/kg/d of FVIII (50 - 200 IU/kg/d). In 28/31 (90.3%) patients only plasma derived FVIII concentrates were used which contained various amounts of von Willebrand factor. In two patients recombinant FVIII was administered. Recombinant, plasma derived and EHL FVIII concentrates were used in the case of one boy. In two patients immunosuppressive therapy (prednisone or rituximab) was concomitantly applied.

Conclusions: it is our belief that the presented real-life data reflects i) the challenge of ITI therapy in clinical practice and ii) wide variety of approaches to ITI strategy in individual haemophilia treatment centres.