Tumor microenvironment in CLL and MCL: pathogenesis, targets, and therapy

National Heart, Lung, And Blood Institute

Investigators

Linked publications & trials

Abstract

Chronic lymphocytic leukemia (CLL) and mantle cell lymphoma (MCL) are tumors of mature B cells and are closely related biologically and in clinical behavior. Both are currently incurable with chemotherapy. BCR signaling has emerged as the pivotal pathway in the pathogenesis of both CLL and MCL. A major contribution from my group has been the first demonstration of active BCR signaling in CLL patients in vivo. Furthermore, we showed that BCR signaling and the consequent activation of the NF-B pathway occurs primarily in the lymph node microenvironment rather than in the peripheral blood or bone marrow. Similarly, we found direct in vivo evidence for activation of the BCR and canonical NF-B pathways in MCL that, in the absence of activating mutations, depends upon the lymph node microenvironment. These findings provide a mechanistic explanation for the surprising efficacy of Bruton tyrosine kinase inhibitors in treating this type of lymphoma. Mutations and single nucleotide polymorphisms in components of the BCR and NF-B pathways were associated with cell-autonomous signaling and influence the sensitivity of MCL cells to BCR signaling inhibitors. Small molecule inhibitors of BCR signaling have transformed clinical approaches. The most promising clinical results have been achieved with ibrutinib. Between 2012 and 2014, our phase 2 clinical trial with ibrutinib enrolled 84 patients. At the end of 2018, 41 (49%) patients remained on study. Over twenty publications from my group report data from this clinical study or from laboratory investigations into specimens obtained from participating patients. A unique aspect of this study is the inclusion of a cohort of 34 patients with TP53 alterations that were treated with single-agent ibrutinib. Historically, this groups of high-risk patients had a median survival of less than 3 years with intensive, multiagent chemoimmunotherapy. At best response, 30% achieved complete responses. At a median follow-up of 6.5 years, 17 (50%) patients remained on study including six with complete responses. Safety was consistent with previous reports. The most common reason for treatment discontinuation was disease progression (35%), followed by withdrawal of consents and deaths unrelated to CLL (6% each). Estimated 6-year progression-free and overall survival was 61% and 79%, respectively. Of twelve patients who progressed on ibrutinib, four had histologic transformation and eight had progressive CLL. BTK and/or PLCG2 (phospholipase C-gamma2 gene) mutations were detected in 83% of progressors. Most (75%) patients who progressed on ibrutinib switched to alternative targeted agents. The median post-progression survival was 25 months. These data underscore the durability of response with ibrutinib compared to chemoimmunotherapy. To improve tolerability and minimize off-target effects of ibrutinib, ACP-196 (acalabrutinib), a more BTK selective inhibitor was developed. Taking advantage of our PDX model, we contributed pre-clinical data supporting the translation of acalabrutinib into clinical trials. We investigated the safety, efficacy, and pharmacodynamics of acalabrutinib at 100 mg twice daily (BID) or 200 mg once daily (QD) in 48 patients with relapsed/refractory or high-risk treatment nave CLL. Acalabrutinib was well tolerated and yielded an overall response rate (ORR) of partial response or better of 95.8% and an estimated progression-free survival (PFS) rate at 24 months of 91.5% with BID dosing and an ORR of 79.2% and an estimated PFS rate at 24 months of 87.2% with QD dosing. Target occupancy is a measure of covalent binding of the drug to BTK and has been applied as a pharmacodynamic parameter in clinical studies of BTK inhibitors. However, the kinetics of de novo BTK synthesis, which determines occupancy, and the relationship between occupancy, pathway inhibition and clinical outcomes remained undefined. We found that BTK resynthesis was faster in CLL than in healthy volunteers. BID dosing maintained higher BTK occupancy and achieved more potent pathway inhibition compared to QD dosing. Small increments in occupancy attained by BID dosing relative to QD dosing compounded over time to augment downstream biological effects. In conclusion, acalabrutinib was an effective and safe treatment for relapsed or high-risk treatment nave CLL patients. Twice daily dosing increased BTK occupancy and the consequent downregulation of oncogenic pathways. Sustained and complete target occupancy is required for maximal inhibition of tumor cells although an absolute threshold for complete occupancy remains ill-defined. Extended follow-up is required to determine its potential impact on long-term clinical outcomes. These data were submitted to the as part of the new drug application package. Measurements of target occupancy support the twice daily dosing regimen as approved by the FDA. Randomized trials established the superiority of ibrutinib-based therapy over chemoimmunotherapy in chronic lymphocytic leukemia (CLL). Durability of progression-free survival (PFS) with ibrutinib can vary by patient subgroups. Clinical tools for prognostication and risk-stratification are needed. To develop a risk stratification tool that can assist in the selection of therapeutic strategies, we analyzed 804 CLL patients treated with ibrutinib across six clinical trials and correlated the clinical model with genetic biomarkers of ibrutinib resistance. Patients treated with ibrutinib on phase 2 and 3 trials sponsored by the manufacturer provided the discovery dataset and were subdivided into discovery and internal validation cohorts. An external validation cohort included 84 patients enrolled on our investigator-initiated phase 2 trial. Univariate analysis of 18 pre-treatment parameters was performed using PFS and overall survival (OS) endpoints. Multivariate analysis and machine learning algorithms identified four factors for a prognostic model that was validated in internal and external cohorts. Factors independently associated with inferior PFS and OS were: TP53 aberration, prior treatment, -2 microglobulin 5mg/L, and lactate dehydrogenase >250U/L. Each of these four factors contributed one point to a prognostic model that stratified patients into three risk groups: 3-4 points, high-risk; 2 points, intermediate-risk; 0-1 point, low-risk. The 3-year PFS for all 804 patients combined was 47%, 74% and 87% for the high-, the intermediate, and the low-risk group, respectively (P<0.0001). The 3-year OS was 63%, 83% and 93%, respectively (P<0.0001). The model remained significant when applied to treatment-nave and relapsed/refractory cohorts individually. For 84 patients in the external cohort, BTK and PLCG2 mutations were tested cross-sectionally and at progression. The cumulative incidences of mutations were strongly correlated with the model. In the external cohort, Richters transformation occurred in 17% of the high-risk group, and no patient in the low-risk group. In conclusion, the 4-factor model redefines high-risk disease among CLL patients treated with ibrutinib. High-risk patients should be considered for clinical trials testing novel regimens. Low-risk patients have favorable benefit-risk ratio with ibrutinib monotherapy. We are now using this information to guide intensified treatment approaches to patients in the high-risk category.