Updated results of the ASPEN trial from a cohort of patients with <i>MYD88</i> wild-type (<i>MYD88</i>^WT) Waldenström macroglobulinemia (WM).

e20056 Background: Inhibitors of Bruton tyrosine kinase (BTK) have shown significant activity in patients with WM harboring a mutation in the MYD88 gene. However, lower response rates and shorter progression-free survival have been reported in patients with WM who lack such mutations ( N Engl J Med. 2015;372:1430). The ASPEN trial evaluated zanubrutinib (ZANU), a potent and selective BTK inhibitor, in WM patients. Methods: In the ASPEN trial, bone marrow MYD88 mutations were assessed at study entry by a central laboratory (NeoGenomics). Based on the results of the MYD88 mutation assay, patients were assigned to cohort 1 ( MYD88 mutation) or cohort 2 ( MYD88 WT or mutation unknown). All cohort 2 patients received ZANU 160 mg twice daily until disease progression. The objective was to assess the safety and efficacy of ZANU in patients with MYD88 WT WM. Results: In total, 28 patients with 26 MYD88 WT WM were enrolled into cohort 2. The median age was 72 years; 5 patients were treatment-naïve (TN) and 23 patients were relapsed/refractory (R/R; ≥1 prior therapy). Most patients had intermediate- (39.3%) or high-risk (42.9%) disease by International Prognostic Scoring System for WM. With the median follow-up of 17.9 months, 2 patients discontinued ZANU due to adverse events, and 6 patients experienced disease progression; there were no cases of disease transformation. The overall response rate was 80.8%, with a major response rate of 50.0%, including a very good partial response (VGPR) rate of 26.9% (Table). Progression-free survival event-free rate at 12 months was 72.4%. The most frequently reported adverse events (AEs) were diarrhea, anemia, contusion, pyrexia, and upper respiratory tract infection. Major hemorrhage was reported in 2 patients, and atrial fibrillation was reported in 1 patient. There were no fatal AEs. Conclusions: ZANU showed clinically meaningful antitumor activity, including achieving major responses and durability of responses, and was considered well-tolerated with a low discontinuation rate due to AEs, in patients with MYD88 WT WM. ZANU demonstrated efficacy regardless of MYD88 mutational status in WM. Clinical trial information: NCT03053440 . [Table: see text]