Signaling Pathways and Novel Inhibitors in Chronic Lymphocytic Leukemia

Wnt-target genes include Myc, LEF, cyclinD1, COX-2, and MMP. Gene expression profiling from our laboratory and other groups have identified the overexpression of these wnt-target genes and support this pathway activation in CLL cells.²⁰ This is a promising signalling pathway and an active area of research for developing inhibitors that will have a growth inhibitory effect on CLL leukemic cells. GSK3b inhibitors and other drugs that re-express epigenetically silenced Wnt antagonist genes have been shown to inhibit this pathway activity in CLL cells in vitro.

Notch Pathway Activation

High-throughput exome sequencing has identified recurring mutations in a number of genes, including NOTCH1.²¹ Analysis of additional CLL patients confirmed activating NOTCH1 mutations in 10% to 15% of CLL patients and were also associated with poor outcome.²² This pathway is activated by ligands such as Jagged and Delta-like, which interact with the Notch receptor, which is then cleaved by γ-secretases. The cleaved intracellular domain of the NOTCH1 receptor in combination with other factors activates transcription of target genes, including Myc and HES1 (Figure). Besides the mutations that generate a truncated protein or may stabilize the pathway, the Notch pathway is also constitutively active in CLL specimens.²³ Notch stimulation increases activity of prosurvival pathways and genes such as NFκB that resist apoptotic signals. The pathway can be inhibited by γ-secretase inhibitors (GSIs), which reduce the levels of cleaved NOTCH1 protein and downregulated Notch target genes. This pathway is also able to modulate the microenvironment stimuli as the GSIs inhibit responses to chemokines such as CXCL12 and inhibit migration and invasion.²⁴

Newer Theraputic Agents

Work on signaling mechanisms paid dividends in CLL with the recent development of 2 inhibitors. Ibrutinib (BTK inhibitor) and idelalisib (PI3K inhibitor) are being studied in clinical trials, and both drugs block the BCR and microenvironment signaling pathways, thereby inhibiting the growth of CLL cells.

BTK Inhibitor: Ibrutinib

The activity of BTK is critical for a number of CLL signaling pathways, and it is a component of the initial signaling complex or signalosome that is formed with BCR signaling. Studies have shown that inhibiting this kinase blocks a number of pathways, including ERK, NFκB, and others. The drug ibrutinib blocks this kinase by forming a covalent bond and inhibiting its enzyme activity. This orally bioavailable drug showed activity in phase 1 trials in different B-cell malignancies.²⁵ In a phase 2 study, high-risk CLL patients were given 2 different doses of this inhibitor, and the overall response rate was 71% with an overall survival at 26 months of 83%.¹¹ Responses were seen in all patients irrespective of clinical and genetic risk factors. Based on these findings, the drug was approved for clinical use in patients with relapsed or refractory disease. Recently, there are data on the use of this drug as frontline therapy in elderly patients, and the drug was well tolerated.²⁶ There are additional ongoing trials to compare this drug with other agents, including chlorambucil (in chemotherapy-naïve patients) and ofatumumab (in relapsed or refractory patients).

PI3 Kinase p110 Delta Inhibitor: Idelalisib

The crucial finding for the development of this inhibitor was the over-expression of the delta isoform of PI3K p110 in B-cell malignancies.¹⁴ The drug CAL-101 selectively inhibits this constitutively active isoform and induces apoptosis in a number of B-cell malignancies.^15,27 In the phase 1 trial, this inhibitor was evaluated in relapsed/refractory patients at multiple dose levels.²⁸ There was inhibition of PI3K signaling with an overall response rate of 72%, and a partial response rate of 39% was observed in CLL patients. This was followed by a randomized, placebo-controlled phase 3 study in which patients with myelosuppression, decreased renal function, or other illnesses were treated with either rituximab alone or with rituximab and idelalisib.²⁹

At the time of reporting, the median progression-free survival (PFS) was 5.5 months in the placebo arm and was not reached in the idelalisib arm. Overall response rates were higher in the idelalisib group (81% vs 13%) with similar toxicity profiles in the 2 groups. This drug is now being extensively studied in combination with bendamustine and other anti-CD20 antibodies in clinical trials.

A unique toxicity observed with both these inhibitors is the initial lymphocytosis. In the case of ibrutinib, this was seen in a majority of patients (77%) and at the same time there was a response in the nodal disease, implying a redistribution of leukemic cells from the tissues to the peripheral blood.³⁰

A potential explanation is that these drugs inhibit signaling via chemokines and other components of the microenvironment and by inhibiting the homing signals, allows leukemic cells to move out of their niche areas. This was analyzed in a recent study that compared clinical and biochemical parameters of patients who had a complete or partial response with ibrutinib compared with a “partial response except for lymphocytosis.”³⁰ Patients with “partial response except for lymphocytosis” were found to have favorable prognostic factors, and the persisting leukemic cells were not clonally different from the original cells. The progression free survival of patients with “partial response except for lymphocytosis” was also similar to the subgroup with no prolonged lymphocytosis.