The HCK/BTK inhibitor KIN-8194 is active in MYD88-driven lymphomas and overcomes mutated BTKCys481 ibrutinib resistance

Activating mutations in MYD88 promote malignant cell growth and survival through hematopoietic cell kinase (HCK)-mediated activation of Bruton tyrosine kinase (BTK). Ibrutinib binds to BTKCys481 and is active in B-cell malignancies driven by mutated MYD88. Mutations in BTKCys481, particularly BTKCys481Ser, are common in patients with acquired ibrutinib resistance. We therefore performed an extensive medicinal chemistry campaign and identified KIN-8194 as a novel dual inhibitor of HCK and BTK. KIN-8194 showed potent and selective in vitro killing of MYD88-mutated lymphoma cells, including ibrutinib-resistant BTKCys481Ser-expressing cells. KIN-8194 demonstrated excellent bioavailability and pharmacokinetic parameters, with good tolerance in rodent models at pharmacologically achievable and active doses. Pharmacodynamic studies showed sustained inhibition of HCK and BTK for 24 hours after single oral administration of KIN-8194 in an MYD88-mutated TMD-8 activated B-cell diffuse large B-cell lymphoma (ABC DLBCL) and BCWM.1 Waldenström macroglobulinemia (WM) xenografted mice with wild-type BTK (BTKWT)- or BTKCys481Ser-expressing tumors. KIN-8194 showed superior survival benefit over ibrutinib in both BTKWT- and BTKCys481Ser-expressing TMD-8 DLBCL xenografted mice, including sustained complete responses of >12 weeks off treatment in mice with BTKWT-expressing TMD-8 tumors. The BCL_2 inhibitor venetoclax enhanced the antitumor activity of KIN-8194 in BTKWT- and BTKCys481Ser-expressing MYD88-mutated lymphoma cells and markedly reduced tumor growth and prolonged survival in mice with BTKCys481Ser-expressing TMD-8 tumors treated with both drugs. The findings highlight the feasibility of targeting HCK, a key driver of mutated MYD88 pro-survival signaling, and provide a framework for the advancement of KIN-8194 for human studies in B-cell malignancies driven by HCK and BTK.

Phase 1 study of ibrutinib and the CXCR4 antagonist ulocuplumab in CXCR4-mutated Waldenström macroglobulinemia

MYD88 and CXCR4 mutations are common in Waldenström macroglobulinemia (WM). Mutated CXCR4 (CXCR4Mut) impacts BTK-inhibitor response. We conducted a phase 1 trial of the CXCR4-antagonist ulocuplumab with ibrutinib in this first-ever study to target CXCR4Mut in WM. Ibrutinib was initiated at 420 mg/d with cycle 1 and continued until intolerance or progression; ulocuplumab was given cycles 1 to 6, with a 3 + 3 dose-escalation design. Each cycle was 4 weeks. Thirteen symptomatic patients, of whom 9 were treatment-naive patients were enrolled. Twelve were evaluable for response. At best response, their median serum immunoglobulin M declined from 5574 to 1114 mg/dL; bone marrow disease decreased from 65% to 10%, and hemoglobin increased from 10.1 to 14.2 g/dL (P < .001). The major and VGPR response rates were 100% and 33%, respectively, with VGPRs observed at lower ulocuplumab dose cohorts. Median times to minor and major responses were 0.9 and 1.2 months, respectively. With a median follow-up of 22.4 months, the estimated 2-year progression-free survival was 90%. The most frequent recurring grade ≥2 adverse events included reversible thrombocytopenia, rash, and skin infections. Ulocuplumab dose-escalation did not impact adverse events. The study demonstrates the feasibility of combining a CXCR4-antagonist with ibrutinib and provides support for the development of CXCR4-antagonists for CXCR4Mut WM. This trial was registered at www.clinicaltrials.gov as #NCT03225716.

Natural history of Waldenström macroglobulinemia following acquired resistance to ibrutinib monotherapy

Ibrutinib is highly active and produces long-term responses in patients with Waldenström macroglobulinemia (WM), but acquired resistance can occur with prolonged treatment. We therefore evaluated the natural history and treatment outcomes in 51 WM patients with acquired resistance to ibrutinib monotherapy. The median time between ibrutinib initiation and discontinuation was 2 years (range, 0.4-6.5). Following discontinuation of ibrutinib, a rapid increase in serum IgM level was observed in 60% (29/48) of evaluable patients, of whom 10 acutely developed symptomatic hyperviscosity. Forty-eight patients (94%) received salvage therapy after ibrutinib. The median time to salvage therapy after ibrutinib cessation was 18 days (95% CI 13-27). The overall and major response rates to salvage therapy were 56% and 44%, respectively, and the median duration of response was 48 months (95% CI 34-not reached). Quadruple-class (rituximab, alkylator, proteasome inhibitor, ibrutinib) exposed disease (OR 0.20, 95% CI 0.05-0.73) and salvage therapy 07 days after discontinuing ibrutinib (OR 4.12, 95% CI 1.07-18.9) were identified as independent predictors of a response to salvage therapy. The 5-year overall survival (OS) following discontinuation of ibrutinib was 44% (95% CI 26-75%). Response to salvage therapy was associated with better OS after ibrutinib (HR 0.08, 95% CI 0.02-0.38). TP53 mutations were associated with shorter OS, while acquired BTKC481S mutations had no impact. Our findings reveal that continuation of ibrutinib until subsequent treatment is associated with improved disease control and clinical outcomes.

Abstract IA39: Investigating malignant transformation in Waldenstrom's macroglobulinemia

Waldenström's macroglobulinemia (WM) is an uncommon B-cell lymphoma that corresponds to the histopathologic classification of IgM-secreting lymphomplasmacytic lymphoma. It is also a disease characterized by a series of highly recurrent mutations. Whole-genome sequencing of CD19+ bone marrow cells from patients with WM led to the discovery of a somatic heterozygous c.978T&gt;C mutation in Toll-like receptor adaptor protein MYD88, resulting in a leucine-to-proline substitution p.Leu265Pro (L265P) in over 90% of WM patients. Somatic activating mutations in MYD88 induce constitutive homodimerization and downstream signaling through the nuclear factor kappa-light-chain-enhancer of activated B cells (NFKB) pathway independent of receptor activation. Other somatic events characteristic of WM include deletions in chromosome 6q21-23 and activating nonsense and/or frameshift mutations in the carboxyl-terminal tail of CXCR4, found in 40-60% and over 30% of WM patients, respectively. Both of these events are found predominantly in MYD88 mutant WM and are frequently restricted to a subclonal population.

Similar to related lymphomas, WM is thought to evolve from IgM-secreting monoclonal gammopathy of undermined significance (MGUS). The MYD88 mutations are easily detectible IgM MGUS stage, suggesting that it is a very early event in clonal development but may predate the malignant transformation into WM. This theory is support by several transgenic mouse models of mutant MYD88 that have demonstrated that p.Leu265Pro MYD88 mutation in B cells alone is not sufficient for lymphomagenesis (Knittel et al., Blood 2016; Sewastianik et al., Blood Adv 2019). Mutations in CXCR4 have been detected in IgM MGUS, though at lower rates than are typical of WM. Given that this and the transcriptional profile of CXCR4 activating mutations in WM are consistent with a pattern of suppression of tumor suppressors upregulated by mutant MYD88, it has been proposed as a possible transformation event from IgM MGUS to asymptomatic WM. Likewise, a number of careful genetic studies of IgM MGUS have suggested the emergence of somatic copy number alterations, particularly the deletions in 6q, to be related to the malignant transformation of WM (Schop et al., Cancer Genet Cytogenet 2006; Paiva et al., Blood 2015).

We therefore reanalyzed our whole-genome sequencing data of WM patients and performed allele-specific PCR and 6q copy number assays to study the rates and relationship of CXCR4 mutations and chromosome 6q deletions in untreated MYD88 mutated WM. Both our 30-patient whole-genome and our 25-patient PCR cohort found no relationship between symptomatic and asymptomatic WM and the somatic events, indicating that they both were occurring at an earlier stage of WM development. Both studies also found that the large clonal deletions in chromosome 6q were not observed in the presence of CXCR4 mutations (p&lt;0.01 for both). Using our previous RNASeq data of 55 WM patients, we compared the transcriptional signatures for CXCR4-mutated and 6q-deleted WM relative to WM patients with the MYD88 mutation alone, revealing an overlapping signature of 19 genes. Notably, the magnitude and relative direction of the changes were similar for CXCR4-mutated and 6q-deleted WM. Impacted genes included biologically relevant targets such as FAM110C, WNK2, CDK14, FOXO3, IGF2R, and HRK. Preliminary validation studies using qPCR and immunohistochemistry have confirmed these findings. We therefore propose that CXCR4 mutations and 6q deletions are not only appearing during the transformation from MGUS to WM but target a related set of genes that may play a critical role in the pathogenesis of MYD88 mutant WM. Functional studies to further characterize these genes and evaluate their potential as therapeutic targets for WM are ongoing.

Citation Format: Zachary R. Hunter, Maria Luisa Guerrera, Guang Yang, Nicholas Tsakmaklis, Xia Lu, Steven P. Treon. Investigating malignant transformation in Waldenstrom's macroglobulinemia [abstract]. In: Proceedings of the AACR Virtual Meeting: Advances in Malignant Lymphoma; 2020 Aug 17-19. Philadelphia (PA): AACR; Blood Cancer Discov 2020;1(3_Suppl):Abstract nr IA39.

Genomic Landscape of Waldenström Macroglobulinemia and Its Impact on Treatment Strategies

Next-generation sequencing has revealed recurring somatic mutations in Waldenström macroglobulinemia (WM), including MYD88 (95%-97%), CXCR4 (30%-40%), ARID1A (17%), and CD79B (8%-15%). Deletions involving chromosome 6q are common in patients with mutated MYD88 and include genes that modulate NFKB, BCL2, Bruton tyrosine kinase (BTK), and apoptosis. Patients with wild-type MYD88 WM show an increased risk of transformation and death and exhibit many mutations found in diffuse large B-cell lymphoma. The discovery of MYD88 and CXCR4 mutations in WM has facilitated rational drug development, including the development of BTK and CXCR4 inhibitors. Responses to many agents commonly used to treat WM, including the BTK inhibitor ibrutinib, are affected by MYD88 and/or CXCR4 mutation status. The mutation status of both MYD88 and CXCR4 can be used for a precision-guided treatment approach to WM.

SYK is activated by mutated MYD88 and drives pro-survival signaling in MYD88 driven B-cell lymphomas

Activating MYD88 mutations promote pro-survival signaling through BTK and HCK, both targets of ibrutinib. Despite high response rates, complete responses to ibrutinib are lacking, and other MYD88 triggered pro-survival pathways may contribute to primary drug resistance. B-cell receptor (BCR) signaling has been observed in lymphomas driven by mutated MYD88, even without activating the BCR pathway mutations. We identified activated SYK (p-SYK), a component of BCR in complex with MYD88 in MYD88-mutated WM and ABC DLBCL lymphoma cells. Confocal microscopy confirmed co-localization of MYD88 with SYK in MYD88-mutated cells. Knockdown of MYD88 or use of a MYD88 signaling inhibitor abrogated SYK activation, while expression of mutated but not wild-type MYD88 amplified p-SYK in MYD88-mutated and wild-type lymphoma cells. Knockdown of SYK or use of inhibitors targeting SYK blocked p-STAT3 and p-AKT signaling in MYD88-mutated cells. Cell viability analysis showed that combining ibrutinib and SYK inhibitors triggered synthetic killing of MYD88-mutated lymphoma cells. Our findings extend the spectrum of mutated MYD88 pro-survival signaling to include SYK directed BCR cross talk in MYD88-mutated lymphomas. Targeting SYK in combination with ibrutinib produces synthetic lethality, providing a framework for the clinical investigation of ibrutinib with SYK inhibitors in MYD88-mutated lymphomas.

Autologous stem cell transplantation with in vivo purged progenitor cells shows long-term efficacy in relapsed/refractory follicular lymphoma

High-dose chemotherapy with autologous stem cell transplantation (ASCT) has been shown effective in the control of relapsed/refractory follicular lymphoma. We evaluate the long-term outcome of patients with relapsed or refractory follicular lymphoma treated with ASCT with in vivo purged progenitors cells. We report the long-term results of a prospective multicenter phase 2 trial on 124 relapsed/refractory follicular lymphoma patients treated with a program of anthracycline-based debulking chemotherapy, immunochemotherapy, mobilization of in vivo purged PBSC followed by ASCT. Median age was 52 years; 14% of patients had grade 3A histology. Debulking chemotherapy produced CR in 16% and PR in 71%, while 13% of patients progressed. After rituximab, cyclophosphamide, vincristine, prednisone (R-COP), CR was obtained in 60% and PR in 35%; 118 patients successfully mobilized PBSC and 117 proceeded to ASCT. The harvest in all the 32 molecularly informative patients was bcl-2 negative. TRM was 0%. The 5-year PFS was 54% and the 5-year OS was 83%. After a median f-up of 6.7 years (range 1.5-13.6), 54% are still in CR. These data show that prolonged PFS is achievable in relapsed/refractory patients with high dose autologous transplantation of in vivo purged progenitor cells.

The NOTCH pathway is recurrently mutated in diffuse large B-cell lymphoma associated with hepatitis C virus infection

Hepatitis C virus has been found to be associated with B-cell non-Hodgkin lymphomas, mostly marginal zone lymphomas and diffuse large B-cell lymphoma. Deregulation of signaling pathways involved in normal marginal zone development (NOTCH pathway, NF-κB, and BCR signaling) has been demonstrated in splenic marginal zone lymphoma. We studied mutations of NOTCH pathway signaling in 46 patients with hepatitis C virus-positive diffuse large B-cell lymphoma and in 64 patients with diffuse large B-cell lymphoma unrelated to HCV. NOTCH2 mutations were detected in 9 of 46 (20%) hepatitis C virus-positive patients, and NOTCH1 mutations in 2 of 46 (4%). By contrast, only one of 64 HCV-negative patients had a NOTCH1 or NOTCH2 mutation. The frequency of the NOTCH pathway lesions was significantly higher in hepatitis C virus-positive patients (P=0.002). The 5-year overall survival was 27% (95%CI: 5%-56%) for hepatitis C virus-positive diffuse large B-cell lymphoma patients carrying a NOTCH pathway mutation versus 62% (95%CI: 42%-77%) for those without these genetic lesions. By univariate analysis, age over 60 years, NOTCH2 mutation, and any mutation of the NOTCH pathway (NOTCH2, NOTCH1, SPEN) were associated with shorter overall survival. Mutation of the NOTCH pathway retained an independent significance (P=0.029). In conclusion, a subset of patients with hepatitis C virus-positive diffuse large B-cell lymphoma displays a molecular signature of splenic marginal zone and has a worse clinical outcome.