SUMMARY: The FDA on April 17, 2018, approved TAVALISSE® (Fostamatinib tablets) for the treatment of thrombocytopenia in adult patients with chronic Immune Thrombocytopenic Purpura (ITP), who have had an insufficient response to a previous treatment. ITP can manifest as an acute self limited disease often seen in children (Acute ITP) or in a chronic form (Chronic ITP) seen in adults with the thrombocytopenia lasting for 6 months or longer.
TAVALISSE is an oral Tyrosine Kinase Inhibitor that targets SYK kinase (Spleen Tyrosine Kinase). SYK associates with Fcγ receptors (FcγR) on the cell surface of various inflammatory cells, including macrophages, which are in turn responsible for platelet clearance in ITP. Inhibition of SYK by the active metabolite of TAVALISSE® reduces the destruction of platelets by macrophages, that are activated in the immune system. TAVALISSE® is the first and only SYK inhibitor indicated for adults with Chronic ITP and the present study validated the therapeutic effect of SYK inhibition in an autoimmune disease.
The approval of TAVALISSE® was supported by data from two randomized placebo-controlled phase III trials and an open label extension study, as well as an initial proof-of-concept study. FIT-1 and FIT-2 are two identical, multicenter, randomized, double-blind, placebo-controlled, phase III trials that included 150 patients with persistent or Chronic ITP, who had an insufficient response to previous treatment, which included Corticosteroids, Immunoglobulins, Splenectomy, and/or a Thrombopoietin Receptor Agonist (TPO-RA). Patients were randomized 2:1 to receive either TAVALISSE® 100 mg orally twice daily or placebo for 24 weeks. Dose could be escalated to 150 mg orally twice daily after one month. The median age was 54 years and enrolled patients had three documented platelet counts of 30,000/µL or less. Patients had ITP for a median of 8.5 years prior to enrollment and the median baseline platelet count was 16,000/μL. The most common prior treatments for ITP were Steroids (94%), TPO-RAs (47%), Splenectomy (35%), and Rituximab (32%). The Primary endpoint was Stable platelet response of 50,000/ µL or more on at least 4 of the 6 biweekly visits between Weeks 14 and 24 of the study, without rescue treatment.
The Overall Response Rate with TAVALISSE® in the two trials, FIT-1 and FIT-2 was 29% versus 2% in the placebo group (P<0.001). Among the responders on TAVALISSE®, Stable Response (Primary end point) was noted in 18% versus 1% in the placebo group (P=0.007) and Intermediate Response (defined as at least 2 consecutive biweekly platelet counts of 50,000/μL or more, without rescue treatment) was noted in 11% versus 0% in the placebo group (P<0.001).
Among patients treated with TAVALISSE®, the median platelet counts at 24 weeks of follow up was 95,000/μL for those who had a Stable Response, 49,000/μL for those who had Intermediate Response, and 20,500/μL in non-responders. For those in the placebo group, however, median platelet counts only reached 17,500/μL. More than half of patients treated with TAVALISSE® (54%) had increment in platelet count (20,000/μL or more), compared with 29% of patients receiving placebo (P=0.005). The median time to response (platelet count of 50,000/μL or more) in those treated with TAVALISSE®, was two weeks. Rescue medications (including Platelet transfusions and intravenous Immunoglobulin) were required in 26% of responding patients in the TAVALISSE® group and for 45% of patients in the placebo group. Response rates were not influenced by Age, Sex, baseline platelet count, Splenectomy or prior treatment with a TPO-RA. Among patients who had received and later failed to respond to a TPO-RA before enrollment, 17% had a Stable Response to TAVALISSE®. Serious bleeding was not noted in the 29% of patients who achieved a response, whereas it occurred in 5.6% of non-responders and 10.2% of those receiving placebo.
Patients from FIT-1 and FIT-2 trials were also included in an open label expansion cohort (FIT-3). In this study, 23% of those who received placebo in FIT-1 or FIT-2 had a Stable platelet response to TAVALISSE®. The most common toxicities were rash, fatigue, nausea, diarrhea, abdominal pain, hypertension, liver function abnormalities and neutropenia.
It was concluded that TAVALISSE® is the first and only Spleen Tyrosine Kinase (SYK) inhibitor, and by its unique mechanism of action, is an important alternative for patients with difficult to treat chronic ITP. Bussel J, Mayer J, Cervinek L, et al. Treatment of primary adult chronic immune thrombocytopenia (CITP) with fostamatinib, an oral SYK inhibitor: results of two randomized, placebo-controlled phase 3 studies. Abstract #S435. Presented at the 22nd Congress of the European Hematology Association, June 24, 2017; Madrid, Spain.