Recent therapeutic advances in chronic lymphocytic leukemia

Managing chronic lymphocytic leukemia in 2020: an update on recent clinical advances with a focus on BTK and BCL-2 inhibitors

The therapeutic landscape of chronic lymphocytic leukemia (CLL) underwent a paradigm shift in 2014 with the approval of ibrutinib, which binds covalently to the C481 residue of Bruton's tyrosine kinase (BTK) and irreversibly inhibits it. A number of large, phase 3 trials conducted in both the frontline and the relapsed/refractory settings resulted in the approval of ibrutinib for all CLL. Indeed, the role of chemoimmunotherapy in CLL is fast dwindling. The limitations of ibrutinib, e.g. the development of resistance-conferring C481 BTK mutations and the toxicity issues of atrial fibrillation and bleeding, in particular, have also become apparent with longer-term follow-up. This has spurred the development of second-generation, irreversible inhibitors with greater selectivity for BTK and third-generation, reversible BTK inhibitors to address C481 site mutations. The last 3 years have also witnessed enormous growth in the therapeutic role of the B-cell lymphoma 2 (BCL-2) antagonist venetoclax, initially approved (in 2016) only for patients with relapsed, 17p-deleted CLL. Venetoclax, in combination with CD20 antibodies, is currently approved for both treatment-naïve and relapsed/refractory patients, regardless of genomic subtype. Robust results have also been reported for ibrutinib plus venetoclax, and "triple" combinations of a BTK inhibitor, venetoclax, and obinutuzumab are now being pursued. The major questions facing the field at present are how best to select patients for BTK inhibitor monotherapy versus venetoclax/obinutuzumab upfront, what to do after failure of both BTK inhibitor(s) and venetoclax, and the ideal way to integrate measurable residual disease data into decisions regarding treatment choice, duration, and discontinuation.

Resistance-Associated Mutations in Chronic Lymphocytic Leukemia Patients Treated With Novel Agents

Inhibitors of B-cell receptor signaling, ibrutinib and idelalisib, and BCL-2 antagonist, venetoclax, have become the mainstay of treatment for chronic lymphocytic leukemia (CLL). Despite significant efficacy in most CLL patients, some patients develop resistance to these agents and progress on these drugs. We provide a state-of-the-art overview of the acquired resistance to novel agents. In 80% of patients with ibrutinib failure, acquired mutations in BTK and PLCG2 genes were detected. No distinct unifying resistance-associated mutations or deregulated signaling pathways have been reported in idelalisib failure. Acquired mutations in the BCL2 gene were detected in patients who had failed on venetoclax. In most cases, patients who have progressed on ibrutinib and venetoclax experience resistance-associated mutations, often present at low allelic frequencies. Resistance-associated mutations tend to occur between the second and fourth years of treatment and may already be detected several months before clinical relapse. We also discuss the development of next-generation agents for CLL patients who have acquired resistant mutations to current inhibitors.

Trends in the risk of second primary malignancies among survivors of chronic lymphocytic leukemia

With improving survivorship in chronic lymphocytic leukemia (CLL), the risk of second primary malignancies (SPMs) has not been systematically addressed. Differences in risk for SPMs among CLL survivors from the Surveillance, Epidemiology, and End Results (SEER) database (1973–2015) were compared to risk of individual malignancies expected in the general population. In ~270,000 person-year follow-up, 6487 new SPMs were diagnosed with a standardized incidence ratio (SIR) of 1.2 (95% CI:1.17–1.23). The higher risk was for both solid (SIR 1.15; 95% CI:1.12–1.18) and hematological malignancies (SIR 1.61; 95% CI:1.5–1.73). The highest risk for SPMs was noted between 2 and 5 months after CLL diagnosis (SIR 1.57; 95% CI:1.41–1.74) and for CLL patients between 50- and 79-years-old. There was a significant increase in SPMs in years 2003–2015 (SIR 1.36; 95% CI:1.3–1.42) as compared to 1973–1982 (SIR 1.19; 95% CI:1.12–1.26). The risk of SPMs was higher in CLL patients who had received prior chemotherapy (SIR 1.38 95% CI:1.31–1.44) as compared to those untreated/treatment status unknown (SIR 1.16, 95% CI:1.13–1.19, p < 0.001). In a multivariate analysis, the hazard of developing SPMs was higher among men, post-chemotherapy, recent years of diagnosis, advanced age, and non-Whites. Active survivorship plans and long-term surveillance for SPMs is crucial for improved outcomes of patients with a history of CLL.

Silencing of HDAC6 as a therapeutic target in chronic lymphocytic leukemia

Although the treatment paradigm for chronic lymphocytic leukemia (CLL) is rapidly changing, the disease remains incurable, except with allogeneic bone marrow transplantation, and resistance, relapsed disease, and partial responses persist as significant challenges. Recent studies have uncovered roles for epigenetic modification in the regulation of mechanisms contributing to malignant progression of CLL B cells. However, the extent to which epigenetic modifiers can be targeted for therapeutic benefit in CLL patients remains poorly explored. We report for the first time that expression of epigenetic modifier histone deacetylase 6 (HDAC6) is upregulated in CLL patient samples, cell lines, and euTCL1 transgenic mouse models compared with HDAC6 in normal controls. Genetic silencing of HDAC6 conferred survival benefit in euTCL1 mice. Administration of isoform-specific HDAC6 inhibitor ACY738 in the euTCL1 aging and adoptive transfer models deterred proliferation of CLL B cells, delayed disease onset via disruption of B-cell receptor signaling, and sensitized CLL B cells to apoptosis. Furthermore, coadministration of ACY738 and ibrutinib displayed synergistic cell kill against CLL cell lines and improved overall survival compared with either single agent in vivo. These results demonstrate for the first time the therapeutic efficacy of selective HDAC6 inhibition in preclinical CLL models and suggest a rationale for the clinical development of HDAC6 inhibitors for CLL treatment, either alone or in combination with Bruton tyrosine kinase inhibition.

From Biology to Therapy: The CLL Success Story

Chemoimmunotherapy has been the standard of care for patients with chronic lymphocytic leukemia (CLL) over the last decade. Advances in monoclonal antibody technology have resulted in the development of newer generations of anti-CD20 antibodies with improved therapeutic effectiveness. In parallel, our knowledge about the distinctive biological characteristics of CLL has progressively deepened and has revealed the importance of B-cell receptor (BCR) signaling and upregulated antiapoptotic proteins for survival and expansion of malignant cell clones. This knowledge provided the basis for development of novel targeted agents that revolutionized treatment of CLL. Ibrutinib and idelalisib inhibit the Bruton tyrosine kinase (BTK) and phosphoinositide 3-kinase (PI3K) delta, respectively, thus interfering with supportive signals coming from the microenvironment via the BCR. These drugs induce egress of CLL cells from secondary lymphoid organs and remarkably improve clinical outcomes, especially for patients with unmutated immunoglobulin heavy-chain genes or with p53 abnormalities that do not benefit from classical treatment schemes. Latest clinical trial results have established ibrutinib with or without anti-CD20 antibodies as the preferred first-line treatment for most CLL patients, which will reduce the use of chemoimmunotherapy in the imminent future. Further advances are achieved with venetoclax, a BH3-mimetic that specifically inhibits the antiapoptotic B-cell lymphoma 2 protein and thus causes rapid apoptosis of CLL cells, which translates into deep and prolonged clinical responses including high rates of minimal residual disease negativity. This review summarizes recent advances in the development of targeted CLL therapies, including new combination schemes, novel BTK and PI3K inhibitors, spleen tyrosine kinase inhibitors, immunomodulatory drugs, and cellular immunotherapy.

Ex Vivo Pharmacological Profiling in Chronic Lymphocytic Leukemia Cells

In vitro drug combination studies are commonly used for CLL primary lymphocytes. An advancement in this method is to perform ex vivo drug testing where the first agent is administered to patients and second drug is tested in these patients' cells in vitro. These assays have been effective in identifying novel agents that work additively or synergistically. In this chapter, we provide a step-by-step protocol for ex vivo drug testing that can be used for combination strategies.