Quantitative MYD88 L265P Analysis Represents a Powerful Tool for Assessing Disease Response and Evaluating Clinical Trial Performance in Waldenstrom's Macroglobulinemia

Serum IgM measurements represent the current standard for assessing changes in disease burden in Waldenström's Macroglobulinemia (WM). Many agents used to treat WM can significantly affect serum IgM levels without impacting the underlying burden of clonal lymphoplasmacytic cells thereby producing discordant findings. Such agents can raise (CD20-directed monoclonal antibodies, IMiDs), or lower (BTK-, proteasome-, MTOR- inhibitors) serum IgM levels, and impact categorical response assessment. Somatic mutations in MYD88 are found in 95-97% of WM patients, and support tumor growth through activation of multiple pro-survival pathways that include HCK/BTK, SYK and ERK. Nearly all MYD88 mutations in WM are of the L265P variant. Previous studies by us and others have shown that serial quantitative measurements of both bone marrow (BM) and peripheral blood (PB) MYD88 L265P (c.978 T>C) by allele-specific PCR can be used to assess changes in underlying BM disease burden (Blood 121:2051-2058; Leukemia 28:1698-1704). However, the utility of using quantitative allele-specific MYD88 L265P (qMYD88 L265P) analysis has not been studied in prospective clinical trials. As such, we performed a comprehensive study of qMYD88 L265P response assessment utilizing BM and PB CD19-selected tissue across 5 prospective clinical studies in WM: Ixazomib, Dexamethasone and Rituximab (IDR; NCT02400437) Ibrutinib monotherapy (IBR; NCT02604511); Venetoclax monotherapy (VEN; NCT02677324); Ibrutinib plus Ulocuplomab (IBR/ULO; NCT03225716); and Ibrutinib plus Venetoclax (IBR/VEN; NCT04273139). Changes in MYD88 L265P ΔCt were assessed as reported above with ΔCt = Mutant Ct - Wild-type Ct, and higher ΔCt values indicating a lower mutant allele burden, with each ΔCt unit reduction representing a 50% decrease in mutant MYD88 L265P burden. Changes in MYD88 L265P ΔCt were compared to changes in underlying BM disease burden and categorical response assessment using the current (IWWM-11) response criteria (Semin Hematol. 60:97-106). Across all five trials, BM (r=0.52; p