Promising combinations of drugs targeting apoptosis

The monoclonal leukemic B-cells of patients with chronic lymphocytic leukemia (CLL) circulate in blood and lymphatics between microenvironments, including lymph nodes, liver and spleen, bone marrow, and secondary lymphoid tissue. These long-lived cells are in a dynamic interplay of proliferation and apoptosis, with the microenvironments providing critical support and signaling for CLL cell growth and survival. CLL cell turnover and longevity ultimately lead to accumulation of the cells and the typical pattern of progressive disease is rising lymphocyte count, enlarging lymph nodes liver and spleen and progressive cytopenias, secondary to crowding out of normal hematopoietic elements in the bone marrow. Many critical surface and intracellular molecules participate in, and are responsible for, microenvironment interactions, proliferation, and resistance to physiological senescence-related apoptosis observed in this disease. The B-cell receptor (BCR) signaling pathway is one critical pathway for CLL pathophysiology, and protein members of this pathway have been targeted with small molecule inhibitors successfully for therapeutic intervention, particularly Bruton's tyrosine kinase (BTK) and phosphoinositide 3-kinase (PI3K). One mechanism of action for the inhibitors of BCR signaling is to disrupt and block the ability of CLL cells to interact with their microenvironment, resulting in release of CLL cells from these protective and stimulating niches. The era of targeted therapy for CLL was ushered in with development of the orally administered irreversible small molecule BTK inhibitor (BTKi), ibrutinib. Multiple phase 3 trials reported improved progression-free survival (PFS) with irreversible BTKi-based treatment comparing across the spectrum of patient age, comorbidities, and chemoimmunotherapy (CIT) intensity (Burger et al. 2020; Moreno et al. 2019; Shanafelt et al. 2019; Sharman et al. 2020; Woyach et al. 2018). BTKi-based treatment is highly effective at reducing the bulk of disease, especially in lymph nodes, and achieving disease control; deep remissions are uncommon, but can rarely occur with prolonged treatment, and treatment is typically continuous, until progression, which can be many years (Byrd et al. 2015; Coutre et al. 2017; O'Brien et al. 2014, 2018). Finally, BTKi-based treatments work by a p53-independent mechanism and are effective for biologically high-risk CLL, particularly del(17p) and mutated TP53 CLL, where CIT is not recommended (Ahn et al. 2020; O'Brien et al. 2014; Stilgenbauer et al. 2018). The malignant CLL cells commonly harbor characteristic chromosome aberrations; chromosome deletions are hallmark of the disease. The majority of patients have leukemia cells harboring deletion of a portion of the long arm of chromosome 13 [del(13q)]. Two important microRNA (miR) genes, miR15 and miR16, were identified in the critical minimal deleted region of del(13q) and were noted to be deleted or downregulated in more than two-thirds of all CLL cases (Calin et al. 2002). MicroRNAs are non-translated small RNAs that function to regulate gene expression. Both miR15 and miR16 bind and downregulate BCL2 transcript levels. The absence of miR15 and miR16 in cases with del(13q) leads to increased BCL2 expression (Cimmino et al. 2005) and resultant resistance to apoptosis. BCL2 is universally overexpressed by leukemia cells of patients with CLL. Small molecule inhibitor blockade of BCL2 protein favors the stoichiometry of BCL2 family members toward pro-apoptosis, thus exposure of CLL cells to a small molecule inhibitor potently induces apoptosis, but not in normal hematopoietic cells. Lowering of the apoptotic threshold in CLL cells through inhibition of BCL2 also establishes an opportunity to synergize other therapeutic agents. Additionally, this mechanism of action is independent of p53, making it particularly appealing for patients with high-risk disease which lacks normal TP53, such as those with del(17p) and patients who have mutated TP53. CLL is a chronic disease with heterogeneous clinical course and timeline between patients. No clinical trial has shown survival benefit with early treatment for CLL, including with targeted therapy, therefore, treatment is only initiated when patients develop active disease requiring treatment according to the standard International Working Group on CLL (iwCLL) criteria. Active CLL requiring treatment is manifest by progressive disease-related symptoms, such as fatigue, drenching night sweats, or unintentional weight loss; massively enlarged lymph nodes, liver or spleen; or progressive anemia or thrombocytopenia (Hallek et al. 2018). Patients who do not display active CLL can be monitored and observed, some for many years. Historically, standard treatment for CLL consisted of CIT, with the intent of achieving disease remission, followed by a period of time off treatment for the duration of the remission. Deeper response, such as with complete remission (CR), was correlated with longer remission duration and PFS. Disease relapse ultimately was expected and retreatment initiated with evidence of active, progressive disease. Depth of response, further reflected by level of measurable residual disease (MRD) status in blood and bone marrow, is an important endpoint for CIT-based treatment and fixed-duration targeted therapy. Undetectable MRD (uMRD) measured at a level of 10^-4 sensitivity (uMRD4) has been used as a treatment endpoint correlated with longer PFS, time-to-next treatment, and overall survival (OS) and certainly curative treatment could not be demonstrated without achieving uMRD status.