Ibrutinib and venetoclax as primary therapy in symptomatic, treatment-naïve Waldenström macroglobulinemia.

ABSTRACT: Concurrent Bruton tyrosine kinase and BCL2 inhibition has not yet been investigated in Waldenström macroglobulinemia (WM). We performed an investigator-initiated trial of ibrutinib and venetoclax in symptomatic treatment-naïve patients with MYD88-mutated WM. Patients received ibrutinib 420 mg once daily (cycle 1), followed by a ramp-up of venetoclax to 400 mg daily (cycle 2). The combination was then administered for 22 additional 4-week cycles. The attainment of very good partial response (VGPR) was the primary end point. Forty-five patients were enrolled in this study. The median baseline characteristics were as follows: age 67 years, serum IgM 43 g/L, and hemoglobin 102 g/L. Seventeen patients (38%) carried CXCR4 mutations. Nineteen patients (42%) achieved VGPR. Grade 3 or higher adverse events included neutropenia (38%), mucositis (9%), and tumor lysis syndrome (7%). Atrial fibrillation occurred in 3 (9%), and ventricular arrhythmia in 4 (9%) patients that included 2 grade 5 events. With a median follow-up of 24.4 months, the 24-month progression-free survival (PFS) and overall survival (OS) rates were 76% and 96%, respectively, and were not impacted by CXCR4 mutations. The median time on therapy was 10.2 months, and the median time after the end of therapy (EOT) was 13.3 months. Eleven of the 12 progression events occurred after EOT, and the 12-month PFS rates after EOT were 79%; 93% if VGPR was attained, and 69% for other patients (P = .12). Ibrutinib and venetoclax induced high VGPR rates and durable responses after EOT, although they were associated with a higher-than-expected rate of ventricular arrhythmia in patients with WM, leading to early study treatment termination. This trial was registered at www.clinicaltrials.gov as #NCT04273139.

Dysfunctions of innate and adaptive immune tumor microenvironment in Waldenström macroglobulinemia.

Waldenström macroglobulinemia (WM) is a rare subtype of non-Hodgkin lymphoma characterized by malignant lymphoplasmacytic cells in the bone marrow (BM). To dissect the pathophysiology of WM, we evaluated clonal cells by mapping of B cell lymphomagenesis with adaptive and innate immune tumor microenvironment (TME) in the BM of WM patients using mass cytometry (CyTOF). In-depth immunophenotypic profiling of WM cells exhibited profound expansion of clonal cells in both unswitched and switched memory B cells and also plasma cells with aberrant expression variations. WM B lymphomagenesis was associated with reduction of most B cell precursors assessed with the same clonally restricted light chain and phenotypic changes. The immune TME was infiltrated by mature monocytes, neutrophils and adaptive T cells, preferentially subsets of effector T helper, effector CTL and effector memory CTL cells that were associated with superior overall survival (OS), in contrast to progenitors of T cells and myeloid/monocytic lineage subsets that were suppressed in WM cohort. Moreover, decrease in immature B and NKT cells was related to worse OS in WM patients. Innate and adaptive immune subsets of WM TME were modulated by immune checkpoints, including PD-1/PD-L1&PD-L2, TIGIT/PVR, CD137/CD137-L, CTLA-4, BTLA and KIR expression. The response of ibrutinib treatment to the reduction of clonal memory B cell was associated with high levels of immature B cells and effector memory CTL cells. Our study demonstrates that CyTOF technology is a powerful approach for characterizing the pathophysiology of WM at various stages, predicting patient risk and monitoring the effectiveness of treatment strategies.

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 years). Following discontinuation of ibrutinib, a rapid increase in serum immunoglobulin M level was observed in 60% (29/48) of evaluable patients, of whom ten 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% confidence interval [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 (odds ratio [OR] 0.20, 95% CI: 0.05-0.73) and salvage therapy ≤7 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 (hazard ratio 0.08, 95% CI: 0.02-0.38). TP53 mutations were associated with shorter OS, while acquired BTK C481S mutations had no impact. Our findings reveal that continuation of ibrutinib until subsequent treatment is associated with improved disease control and clinical outcomes.

Bone marrow involvement and subclonal diversity impairs detection of mutated CXCR4 by diagnostic next-generation sequencing in Waldenström macroglobulinaemia.

CXCR4 mutations impact disease presentation and treatment outcomes in Waldenström macroglobulinaemia (WM). Non-uniform testing for CXCR4 mutations may account for discordant findings in WM clinical trials. We compared two approaches used in these trials for detection of the most common CXCR4 (S338X) variant: targeted next-generation sequencing (NGS) using unselected bone marrow (BM) samples, and combined allele-specific polymerase chain reaction (AS-PCR) and Sanger sequencing with unselected and CD19-selected BM samples. Our findings showed that targeted NGS frequently yielded false-negative results. Both CD19 selection and AS-PCR markedly improved detection of CXCR4S338X mutations. Sensitivity was adversely impacted by low BM involvement and CXCR4 mutation clonality.

Partial response or better at six months is prognostic of superior progression-free survival in Waldenström macroglobulinaemia patients treated with ibrutinib.

Ibrutinib is associated with durable responses in patients with Waldenström macroglobulinaemia (WM). We hypothesized that response depth is predictive of progression-free survival (PFS) in WM patients treated with ibrutinib. Using landmark analyses, we evaluated response depth in two cohorts of WM patients treated with ibrutinib monotherapy. The learning cohort was composed of 93 participants from two clinical trials, and the validation cohort of 190 consecutive patients treated off clinical trial. Rates of partial response (PR) or better at six months in learning and validation cohorts were 64% and 71% respectively (P = 0·29). In the learning cohort, three-year PFS rates for patients who attained PR or better at six months versus not were 81% and 57% respectively (P = 0·009). In the validation cohort, three-year PFS rates for patients who attained PR or better at six months versus not were 83% and 54% respectively (P = 0·008). In multivariate analyses, attaining PR or better at six months was associated with superior PFS in the learning [hazard ratio (HR) 0·38; P = 0·01] and validation cohorts (HR 0·18; P = 0·004). Attaining PR at six months on ibrutinib emerges as an intermediate outcome of interest and should be validated as surrogate for PFS in clinical trials evaluating Bruton tyrosine kinase inhibitors in WM.

Genomic evolution of ibrutinib-resistant clones in Waldenström macroglobulinaemia.

Ibrutinib is highly active in Waldenström macroglobulinaemia (WM) patients, but disease progression can occur due to acquired mutations in BTK, the target of ibrutinib, or PLCG2, the protein downstream of BTK. However, not all resistant patients harbour these alterations. We have performed a whole-exome sequencing study to identify alternative molecular mechanisms that can drive ibrutinib resistance. Our findings include deletions on chromosomes 6q, including homozygous deletions, and 8p, which encompass key regulators of BTK, MYD88/NF-κB, and apoptotic signalling. Moreover, we have identified recurring mutations in ubiquitin ligases, innate immune signalling, and TLR/MYD88 pathway regulators in ibrutinib-resistant WM patients.

Extracellular vesicle-mediated transfer of constitutively active MyD88L265P engages MyD88wt and activates signaling.

The link between inflammation and cancer is particularly strong in Waldenström macroglobulinemia (WM), a diffuse large B-cell lymphoma wherein the majority of patients harbor a constitutively active mutation in the innate immune-signaling adaptor myeloid differentiation primary response 88 (MyD88). MyD88Leu265Pro (MyD88L265P) constitutively triggers the myddosome assembly providing a survival signal for cancer cells. Here, we report detection and a functional role of MyD88 in the extracellular vesicles (EVs) shed from WM cells. MyD88L265P was transferred via EVs into the cytoplasm of the recipient mast cells and macrophages, recruiting the endogenous MyD88 that triggered the activation of proinflammatory signaling in the absence of receptor activation. Additionally, internalization of EVs containing MyD88L265P was observed in mice with an effect on the bone marrow microenvironment. MyD88-loaded EVs were detected in the bone marrow aspirates of WM patients thus establishing the physiological role of EVs for MyD88L265P transmission and shaping of the proinflammatory microenvironment. Results establish the mechanism of transmission of signaling complexes via EVs to propagate inflammation as a new mechanism of intercellular communication.

BTKCys481Ser drives ibrutinib resistance via ERK1/2 and protects BTKwild-type MYD88-mutated cells by a paracrine mechanism.

Acquired ibrutinib resistance due to BTKCys481 mutations occurs in B-cell malignancies, including those with MYD88 mutations. BTKCys481 mutations are usually subclonal, and their relevance to clinical progression remains unclear. Moreover, the signaling pathways that promote ibrutinib resistance remain to be clarified. We therefore engineered BTKCys481Ser and BTKWT expressing MYD88-mutated Waldenström macroglobulinemia (WM) and activated B-cell (ABC) diffuse large B-cell lymphoma (DLBCL) cells and observed reactivation of BTK-PLCγ2-ERK1/2 signaling in the presence of ibrutinib in only the former. Use of ERK1/2 inhibitors triggered apoptosis in BTKCys481Ser-expressing cells and showed synergistic cytotoxicity with ibrutinib. ERK1/2 reactivation in ibrutinib-treated BTKCys481Ser cells was accompanied by release of many prosurvival and inflammatory cytokines, including interleukin-6 (IL-6) and IL-10 that were also blocked by ERK1/2 inhibition. To clarify if cytokine release by ibrutinib-treated BTKCys481Ser cells could protect BTKWT MYD88-mutated malignant cells, we used a Transwell coculture system and showed that nontransduced BTKWT MYD88-mutated WM or ABC DLBCL cells were rescued from ibrutinib-induced killing when cocultured with BTKCys481Ser but not their BTKWT-expressing counterparts. Use of IL-6 and/or IL-10 blocking antibodies abolished the protective effect conferred on nontransduced BTKWT by coculture with BTKCys481Ser expressing WM or ABC DLBCL cell counterparts. Rebound of IL-6 and IL-10 serum levels also accompanied disease progression in WM patients with acquired BTKCys481 mutations. Our findings show that the BTKCys481Ser mutation drives ibrutinib resistance in MYD88-mutated WM and ABC DLBCL cells through reactivation of ERK1/2 and can confer a protective effect on BTKWT cells through a paracrine mechanism.

Deepening of response after completing rituximab-containing therapy in patients with Waldenstrom macroglobulinemia.

Rituximab-containing regimens are commonly used for frontline therapy in patients with symptomatic Waldenström macroglobulinemia (WM). We had observed that a portion of WM patients experienced deepening of response months to years after therapy completion. We carried a retrospective study aimed at describing this phenomenon. We gathered baseline data, and responses at end of induction, end of maintenance and best response. Deepening of response was defined as ≥25% decrease in serum IgM achieved at a later time from therapy completion. Of 178 patients included, 116 (65%) received maintenance therapy and 62 (35%) were observed. In patients who received maintenance, 44 (38%) had ≥25% decrease in serum IgM level after the end of maintenance with a median time from end of maintenance to lowest IgM level of 1.6 years (range 0.1-7.9 years). In patients who were observed, 19 (31%) had ≥25% decrease in serum IgM level after the end of induction with a median time from end of induction to lowest IgM level of 1.6 years (range 0.2-5.1 years). Baseline hemoglobin <11.5 g/dL, bone marrow involvement ≥50%, CXCR4 mutations and serum IgM ≥4000 mg/dL were associated with lower odds of deepening of response after therapy completion. Deepening of response was associated with better progression-free survival (PFS; HR 0.46, 95% CI 0.26-0.80; P = .006) and better survival after frontline treatment initiation (SAFTI; HR 0.21, 95% CI 0.06-0.73; P = .01). In conclusion, deepening of response occurs in one third of WM patients after completing rituximab-containing regimens and was associated with better PFS and SAFTI.

TP53 mutations are associated with mutated MYD88 and CXCR4, and confer an adverse outcome in Waldenström macroglobulinaemia.

Little is known about TP53 mutations in Waldenström Macroglobulinaemia (WM). We evaluated 265 WM patients for TP53 mutations by next-generation sequencing, and validated the findings by Sanger sequencing. TP53 mutations were identified and validated in 6 (2·6%) patients that impacted the DNA-binding domain. All six were MYD88- and CXCR4-mutated. Ibrutinib showed activity in patients carrying all three mutations. With a median follow-up of 18 months, 2 (33%) with biallelic TP53 inactivation died of progressive disease. TP53 mutations are rare in WM, and associate with MYD88 and CXCR4 mutations. WM patients with TP53 mutations show response to ibrutinib.

CXCR4 mutation subtypes impact response and survival outcomes in patients with Waldenström macroglobulinaemia treated with ibrutinib.

Ibrutinib is associated with response rate of 90% and median progression-free survival (PFS) in excess of 5 years in Waldenström macroglobulinaemia (WM) patients. CXCR4 mutations are detected in 30-40% of patients with WM and associate with lower rates of response and shorter PFS to ibrutinib therapy. Both frameshift (CXCR4FS ) and nonsense (CXCR4NS ) CXCR4 mutations have been described. The impact of these mutations on outcomes to ibrutinib have not been evaluated in WM patients. We studied consecutive patients with a diagnosis of WM, on ibrutinib therapy, for the presence of CXCR4FS and CXCR4NS mutations and evaluated the differences in response and PFS between groups. Of 180 patients, 68 patients (38%) had CXCR4 mutations; 49 (27%) had CXCR4NS and 19 (11%) had CXCR4FS mutations. In multivariate models, patients with CXCR4NS had lower odds of major response (Odds ratio 0·25, 95% confidence interval [CI] 0·12-0·53; P < 0·001) and worse PFS (Hazard ratio 4·02, 95% CI 1·95-8·26; P < 0·001) than patients without CXCR4 mutations. CXCR4FS was not associated with worse major response or PFS rates than patients without CXCR4 mutations. Our results suggest different response and PFS rates to ibrutinib for WM patients with CXCR4NS and CXCR4FS , and advocate in favour of CXCR4 mutational testing as well as CXCR4-directed therapy.

Acquired mutations associated with ibrutinib resistance in Waldenström macroglobulinemia.

Ibrutinib produces high response rates and durable remissions in Waldenström macroglobulinemia (WM) that are impacted by MYD88 and CXCR4WHIM mutations. Disease progression can develop on ibrutinib, although the molecular basis remains to be clarified. We sequenced sorted CD19+ lymphoplasmacytic cells from 6 WM patients who progressed after achieving major responses on ibrutinib using Sanger, TA cloning and sequencing, and highly sensitive and allele-specific polymerase chain reaction (AS-PCR) assays that we developed for Bruton tyrosine kinase (BTK) mutations. AS-PCR assays were used to screen patients with and without progressive disease on ibrutinib, and ibrutinib-naïve disease. Targeted next-generation sequencing was used to validate AS-PCR findings, assess for other BTK mutations, and other targets in B-cell receptor and MYD88 signaling. Among the 6 progressing patients, 3 had BTKCys481 variants that included BTKCys481Ser(c.1635G>C and c.1634T>A) and BTKCys481Arg(c.1634T>C) Two of these patients had multiple BTK mutations. Screening of 38 additional patients on ibrutinib without clinical progression identified BTKCys481 mutations in 2 (5.1%) individuals, both of whom subsequently progressed. BTKCys481 mutations were not detected in baseline samples or in 100 ibrutinib-naive WM patients. Using mutated MYD88 as a tumor marker, BTKCys481 mutations were subclonal, with a highly variable clonal distribution. Targeted deep-sequencing confirmed AS-PCR findings, and identified an additional BTKCys481Tyr(c.1634G>A) mutation in the 2 patients with multiple other BTKCys481 mutations, as well as CARD11Leu878Phe(c.2632C>T) and PLCγ2Tyr495His(c.1483T>C) mutations. Four of the 5 patients with BTKC481 variants were CXCR4 mutated. BTKCys481 mutations are common in WM patients with clinical progression on ibrutinib, and are associated with mutated CXCR4.

Response and survival for primary therapy combination regimens and maintenance rituximab in Waldenström macroglobulinaemia.

Waldenström macroglobulinaemia (WM) is a rare and incurable lymphoma. Comparative studies evaluating the efficacy of primary therapy in symptomatic WM patients have not been performed. In this study, we compared response and survival outcomes in WM patients who received primary therapy with cyclophosphamide-dexamethasone-rituximab (CDR), bortezomib-dexamethasone-rituximab (BDR) and bendamustine-rituximab (Benda-R), as well as maintenance rituximab following primary therapy. Analyses were adjusted for relevant clinical factors associated with response and survival. Maintenance rituximab was analysed as a time-varying covariate. Our study included 182 patients, of which 57 (31%) received Benda-R, 87 (48%) BDR and 38 (21%) CDR; 116 (64%) received maintenance rituximab. The median time to best response was shorter for Benda-R and BDR than CDR (18, 20 and 30 months, respectively). Benda-R and BDR were associated with better median progression-free survival (PFS) than CDR (5·5, 5·8 and 4·8 years, respectively), and better 10-year overall survival rates (OS; 95%, 96% and 81%, respectively). Maintenance rituximab was associated with higher rates of major response (97% vs. 68%), and better median PFS (6·8 years vs. 2·8 years) and 10-year OS rate (84% vs. 66%) when compared to not receiving maintenance. Benda-R, BDR and maintenance rituximab associate with higher response rates and longer survival in WM patients than CDR and no maintenance, respectively.

MYD88 wild-type Waldenstrom Macroglobulinaemia: differential diagnosis, risk of histological transformation, and overall survival.

MYD88 mutations are present in 95% of Waldenstrom Macroglobulinaemia (WM) patients, and support diagnostic discrimination from other IgM-secreting B-cell malignancies. Diagnostic discrimination can be difficult among suspected wild-type MYD88 (MYD88WT ) WM cases. We systematically reviewed the clinical, pathological and laboratory studies for 64 suspected MYD88WT WM patients. World Health Organization and WM consensus guidelines were used to establish clinicopathological diagnosis. Up to 30% of suspected MYD88WT WM cases had an alternative clinicopathological diagnosis, including IgM multiple myeloma. The estimated 10-year survival was 73% (95% confidence interval [CI] 52-86%) for MYD88WT versus 90% (95% CI 82-95%) for mutated (MYD88MUT ) WM patients (Log-rank P < 0·001). Multivariate analysis only showed MYD88 mutation status (P < 0·001) as a significant determinant for overall survival. Diffuse large B-cell lymphoma (DLBCL) was diagnosed in 7 (15·2%) and 2 (0·76%) of MYD88WT and MYD88MUT patients, respectively (Odds ratio 23·3; 95% CI 4·2-233·8; P < 0·001). Overall survival was shorter among MYD88WT patients with an associated DLBCL event (Log-rank P = 0·08). The findings show that among suspected MYD88WT WM cases, an alternative clinicopathological diagnosis is common and can impact clinical care. WM patients with MYD88WT disease have a high incidence of associated DLBCL events and significantly shorter survival versus those with MYD88MUT disease.

Ibrutinib in previously treated Waldenström's macroglobulinemia.

BACKGROUND: MYD88(L265P) and CXCR4(WHIM) mutations are highly prevalent in Waldenström's macroglobulinemia. MYD88(L265P) triggers tumor-cell growth through Bruton's tyrosine kinase, a target of ibrutinib. CXCR4(WHIM) mutations confer in vitro resistance to ibrutinib. METHODS: We performed a prospective study of ibrutinib in 63 symptomatic patients with Waldenström's macroglobulinemia who had received at least one previous treatment, and we investigated the effect of MYD88 and CXCR4 mutations on outcomes. Ibrutinib at a daily dose of 420 mg was administered orally until disease progression or the development of unacceptable toxic effects. RESULTS: After the patients received ibrutinib, median serum IgM levels decreased from 3520 mg per deciliter to 880 mg per deciliter, median hemoglobin levels increased from 10.5 g per deciliter to 13.8 g per deciliter, and bone marrow involvement decreased from 60% to 25% (P<0.01 for all comparisons). The median time to at least a minor response was 4 weeks. The overall response rate was 90.5%, and the major response rate was 73.0%; these rates were highest among patients with MYD88(L265P)CXCR4(WT) (with WT indicating wild-type) (100% overall response rate and 91.2% major response rate), followed by patients with MYD88(L265P)CXCR4(WHIM) (85.7% and 61.9%, respectively) and patients with MYD88(WT)CXCR4(WT) (71.4% and 28.6%). The estimated 2-year progression-free and overall survival rates among all patients were 69.1% and 95.2%, respectively. Treatment-related toxic effects of grade 2 or higher included neutropenia (in 22% of the patients) and thrombocytopenia (in 14%), which were more common in heavily pretreated patients; postprocedural bleeding (in 3%); epistaxis associated with the use of fish-oil supplements (in 3%); and atrial fibrillation associated with a history of arrhythmia (5%). CONCLUSIONS: Ibrutinib was highly active, associated with durable responses, and safe in pretreated patients with Waldenström's macroglobulinemia. MYD88 and CXCR4 mutation status affected responses to this drug. (Funded by Pharmacyclics and others; ClinicalTrials.gov number, NCT01614821.).

HCK is a survival determinant transactivated by mutated MYD88, and a direct target of ibrutinib.

Activating mutations in MYD88 are present in ∼95% of patients with Waldenström macroglobulinemia (WM), as well as other B-cell malignancies including activated B-cell (ABC) diffuse large B-cell lymphoma (DLBCL). In WM, mutated MYD88 triggers activation of Bruton tyrosine kinase (BTK). Ibrutinib, a pleiotropic kinase inhibitor that targets BTK, is highly active in patients with mutated MYD88. We observed that mutated MYD88 WM and ABC DLBCL cell lines, as well as primary WM cells show enhanced hematopoietic cell kinase (HCK) transcription and activation, and that HCK is activated by interleukin 6 (IL-6). Over-expression of mutated MYD88 triggers HCK and IL-6 transcription, whereas knockdown of HCK reduced survival and attenuated BTK, phosphoinositide 3-kinase/AKT, and mitogen-activated protein kinase/extracellular signal-regulated kinase signaling in mutated MYD88 WM and/or ABC DLBCL cells. Ibrutinib and the more potent HCK inhibitor A419259, blocked HCK activation and induced apoptosis in mutated MYD88 WM and ABC DLBCL cells. Docking and pull-down studies confirmed that HCK was a target of ibrutinib. Ibrutinib and A419259 also blocked adenosine triphosphate binding to HCK, whereas transduction of mutated MYD88 expressing WM cells with a mutated HCK gatekeeper greatly increased the half maximal effective concentration for ibrutinib and A419259. The findings support that HCK expression and activation is triggered by mutated MYD88, supports the growth and survival of mutated MYD88 WM and ABC DLBCL cells, and is a direct target of ibrutinib. HCK represents a novel target for therapeutic development in MYD88-mutated WM and ABC DLBCL, and possibly other diseases driven by mutated MYD88.

Transcriptome sequencing reveals a profile that corresponds to genomic variants in Waldenström macroglobulinemia.

Whole-genome sequencing has identified highly prevalent somatic mutations including MYD88, CXCR4, and ARID1A in Waldenström macroglobulinemia (WM). The impact of these and other somatic mutations on transcriptional regulation in WM remains to be clarified. We performed next-generation transcriptional profiling in 57 WM patients and compared findings to healthy donor B cells. Compared with healthy donors, WM patient samples showed greatly enhanced expression of the VDJ recombination genes DNTT, RAG1, and RAG2, but not AICDA Genes related to CXCR4 signaling were also upregulated and included CXCR4, CXCL12, and VCAM1 regardless of CXCR4 mutation status, indicating a potential role for CXCR4 signaling in all WM patients. The WM transcriptional profile was equally dissimilar to healthy memory B cells and circulating B cells likely due increased differentiation rather than cellular origin. The profile for CXCR4 mutations corresponded to diminished B-cell differentiation and suppression of tumor suppressors upregulated by MYD88 mutations in a manner associated with the suppression of TLR4 signaling relative to those mutated for MYD88 alone. Promoter methylation studies of top findings failed to explain this suppressive effect but identified aberrant methylation patterns in MYD88 wild-type patients. CXCR4 and MYD88 transcription were negatively correlated, demonstrated allele-specific transcription bias, and, along with CXCL13, were associated with bone marrow disease involvement. Distinct gene expression profiles for patients with wild-type MYD88, mutated ARID1A, familial predisposition to WM, chr6q deletions, chr3q amplifications, and trisomy 4 are also described. The findings provide novel insights into the molecular pathogenesis and opportunities for targeted therapeutic strategies for WM.

Ibrutinib discontinuation in Waldenström macroglobulinemia: Etiologies, outcomes, and IgM rebound.

Ibrutinib is the first approved therapy for symptomatic patients with Waldenström macroglobulinemia (WM). The reasons for discontinuing ibrutinib and subsequent outcomes have not been previously evaluated in WM patients. We therefore conducted a retrospective review of 189 WM patients seen at our institution who received treatment with ibrutinib, of whom 51 (27%) have discontinued therapy. Reasons for discontinuation include: disease progression (n = 27; 14%), toxicity (n = 15; 8%), nonresponse (n = 5; 3%), and other unrelated reasons (n = 4; 2%). The cumulative incidence of ibrutinib discontinuation at 12, 24, 36, and 48 months from treatment initiation was 22%, 26%, 35%, and 43%, respectively. A baseline platelet count ≤100 K/µL and presence of tumor CXCR4 mutations were independently associated with 4-fold increased odds of ibrutinib discontinuation. An IgM rebound (≥25% increase in serum IgM) was observed in 37 patients (73%) following ibrutinib discontinuation and occurred within 4 weeks for nearly half of patients. The response rate to salvage therapy was 71%; responses were higher in patients without an IgM rebound and when salvage therapy was initiated within 2 weeks of stopping ibrutinib. Patients who discontinued ibrutinib due to disease progression versus nonprogression events had significantly shorter overall survival (21 versus 32 months; P = .046). Response to salvage therapy was associated with an 82% reduction in the risk of death following ibrutinib discontinuation. WM patients who discontinue ibrutinib require close monitoring, and continuation of ibrutinib until the next therapy should be considered to maintain disease control.