The Use of Avapritinib in Advanced Systemic Mastocytosis: Report of an Open-Label Compassionate Use Program in the United Kingdom

Background: Advanced systemic mastocytosis (AdvSM) is a rare haematological neoplasm occurring due to clonal proliferation of mast cells in extracutaneous organs. In up to 95% of cases, the driver mutation is KIT D816V. AdvSM encompasses aggressive SM (ASM), mast cell leukaemia and SM with an associated haematological neoplasm (SM-AHN). In this latter, the AHN component often dictates the clinical course, being most frequently associated with chronic myelomonocytic leukaemia (CMML). Treatment for AdvSM has evolved recently with the introduction of targetted tyrosine kinase inhibitors (TKI). Avapritinib, a highly selective inhibitor for KIT and PDGFRA, produced robust responses in measures of mast cell/disease burden in the PATHFINDER trial, leading to approval in USA and Europe. We report the clinical experience of AdvSM patients who have received avapritinib at 11 centers in the United Kingdom via an open-label compassionate use program. Methods: Avapritinib was commenced in AdvSM patients with progressive disease-related symptoms and end-organ damage. The initial dose was adjusted according to tolerability and kept continuously until disease progression or unmanageable toxicity. Results: In a cohort of 13 patients, with an age average of 68.8 years (range 57-76), 11 patients (84.6%) were diagnosed with SM-AHN; 81.8% with a concomitant diagnosis of CMML. 2 patients (15.4%) were diagnosed with ASM. All patients harbored the D816V mutation and 8 patients (61.5%) presented with additional mutations; 5 with SRSF2 and 1 with ASXL1. On application of prognostic scores; International Prognostic Scoring system for Mastocytosis (IPSM) 10 patients (76.9%) were classified as AdvSM-3/4 and Mutation-Adjusted Risk Score for Advanced Systemic Mastocytosis (MARS) 7 patients (53.8%) were stratified as high risk. Treatment: 10 patients (76.9%) received avapritinib as a first-line regime, and 9 patients (69.2%) started at a dose of 200 mg od. Three patients were previously treated with other regimes - one patient with midostaurin, one with cladribine and one with azacytidine. Response: Median duration of treatment was 503.7 days (range 75 - 1168 days). Baseline median blood parameters for this cohort were tryptase level of 168 ng/mL (range 91-811), alkaline phosphatase (ALP) of 437 IU/L (range 127-1235), albumin of 34.5 g/L (range 24-48) and a spleen size of 16.4 cm (range 12-26), measured by radiologically. At response evaluation, 9 patients (69.2%) had a tryptase level <20 ng/mL and 11 patients (84.6%) had normalised their ALP and albumin levels. None of the patients had a clinically enlarged spleen, with 9 patients (69.2%) having a normal spleen size on abdominal ultrasound. 8 patients had a complete pathological remission. The overall response rate (using the modified IWG-ECNM-MRT criteria) was 76.9% with a complete response (CR) or CR with partial haematologic recovery in 53.8% of patients. At the last follow-up, 3 patients (23.1%) died; 1 from SM progression, 1 from haematemesis and 1 due to progression of the AHN component but had a partial response of the SM with avapritinib. Toxicities:10 patients (76.9%) had haematological toxicities. Bleeding was reported in 4 patients (30.8%); 3 patients with grade 1/2 (1 mucocutaneous and 2 mild gastrointestinal bleeds), and 1 fatal gastrointestinal bleed. No intracranial bleeds were reported. Fluid retention was noted in 3 patients (23.1%). Less frequent side effects noted were nausea/vomiting (2 patients) and skin/hair discoloration (1 patient). 11 patients (84.6%) needed dose reduction of avapritinib due to haematological toxicity with 8 patients (72.7%) remaining at 100 mg od. 4 patients (36.4%) had to intermittently stop treatment until haematological recovery. Avapritinib was stopped in 2 patients (15.4%) due to disease progression or unmanageable toxicity. Conclusion: This small cohort of patients reflects the clinical heterogeneity of AdvSM and mirrors the PATHFINDER trial outcomes in 'real world 'experience in the UK. Avapritinib as 1st line was tolerated, producing sustained clinical and pathological responses in the majority of this group of non-selected patients with AdvSM with high risk disease. Dose adjustments due to anticipated myelosuppression were made, with 72.2% of patients maintained on 100mg Avapritinib od. 2 patients with SM-AHN are referred for HSCT as a curative treatment as a result of achieving a CR with Avapritinib.