Peripheral blood smear

showing CLL

Image by Mary Ann Thompson

NEW YORK—Despite enormous advances in therapies for chronic lymphocytic leukemia (CLL) that target the B-cell receptor (BCR) signaling pathway, there is still room for improvement, according to investigators at the Mayo Clinic.

Bruton’s tyrosine kinase (BTK) and phosphoinositide-3 kinase delta (PI3Kδ) inhibitors are major players in mediating BCR signaling, yet both have off-target effects.

“These agents are not curative, and resistance develops,” explained Neil Kay, MD, of the Mayo Clinic in Rochester, Minnesota.

He described the Axl receptor tyrosine kinase and its inhibitor, TP-0903, at Lymphoma & Myeloma 2015, and discussed how it may be another promising target for CLL therapy.

Pros and cons of current therapies

Ibrutinib, a potent, irreversible, covalent inhibitor of BTK, inhibits interleukin 2 kinase, an essential enzyme in Th2 T cells.

“The potential benefit of this,” Dr Kay said, “is it shifts the balance between Th1 and Th2 T cells and potentially enhances antitumor immune responses.”

However, ibrutinib may cause off-target effects, including defects of platelet function, atrial fibrillation probably related to the inhibition of cardiac PI3K-AKT signaling, and decreased efficacy of anti-CD20 antibodies as mediated by natural killer cells.

In addition, long-term exposure to ibrutinib can induce signal mutations associated with resistance.

Idelalisib, a reversible inhibitor of the delta isoform of PI3K, modulates malignant B-cell signaling and inhibits the chemokines CCL3 and 4. It does not inhibit antibody-dependent cell-mediated cytotoxicity mediated by anti-CD20 antibodies, and it may stimulate antitumor responses.

However, idelalisib may also increase the incidence of diarrhea and colitis and can cause transaminitis and pneumonitis.

Axl receptor tyrosine kinase

So Dr Kay and fellow researchers explored whether CLL B cells express other active receptor tyrosine kinases (RTKs), and if so, what their functional implication is in CLL B-cell survival.

And what they detected is active Axl RTK and basic fibroblast growth factor receptor 3 (FGFR3) RTK in CLL B cells. The human Axl gene, a member of the TAM family of RTKs, is located on chromosome 19q13.2 and encodes a protein weighing 100–140 kD.

When Axl is activated, it initiates various signaling pathways, including cell survival, proliferation, apoptosis inhibition, migration, cell adhesion, and cytokine production. It does this through interactions with a number of signaling molecules, including PI3K and phospholipase C γ2 (PLCγ2), among others.

Most important, Axl is overexpressed in a number of human malignancies, including acute myeloid leukemia, and is associated with poor survivorship.

The research team immunoprecipitated Axl RTK from CLL B-cell lysates and examined them for co-localization by Western blot for the proteins PI3K, c-Src, Syk, PLCγ2, ZAP-70, and Lyn.

“What we think currently is that Axl provides a docking site for these key signaling molecules,” Dr Kay said.

The team found that, by inhibiting Axl with TP-0903, they were able to induce robust apoptosis in CLL B cells, including those from high-risk 17p– and 11q– CLL. They also observed that Axl inhibition resulted in significant reduction of the anti-apoptotic proteins Bcl-2, XIAP, and Mcl-1 and upregulation of the pro-apoptotic protein BIM in CLL B cells.

They then tested the BTK inhibitors ibrutinib and TP-4216 with and without the Axl inhibitor TP-0903 and found that concurrent administration of TP-0903 with the reversible BTK inhibitor TP-4216 had additive effects on apoptosis. This was not the case when it was added to ibrutinib.

“There was much more dramatic upregulation with the reversible BTK inhibitor,” Dr Kay emphasized.

He said Axl inhibition is a therapeutic opportunity in that these inhibitors are orally bioavailable and would be candidates for clinical testing. These inhibitors may minimize drug resistance and prevent the emergence of Richter’s transformation, he added.

The researchers received TP-0903 from Tolero Pharmaceuticals.

The search continues for additional targets in CLL