Preclinical Characterization of PWT143, a Novel Selective and Potent Phosphatidylinositol 3-kinase Delta (PI3K delta) Inhibitor with Ex-Vivo Activity in Hematologic Malignancies.

Abstract 2907

Early clinical trials with small molecule inhibitors that target kinases in the B Cell Receptor (BCR) signaling pathway have demonstrated promising activity in B cell malignancies. These kinases include PI3K delta, Bruton's tyrosine Kinase (BTK) and spleen tyrosine kinase (SYK), with clinical validation demonstrated by the inhibitors GS-1101 (CAL-101), ibrutinib, and fostamatinib respectively. The clinical observations with GS-1101 and ibrutinib include rapid lymph node shrinkage and high lymph node response rate in refractory CLL as well clinical activity in indolent NHL and MCL, and in DLBCL for ibrutinib. In addition to B cells, PI3K delta is expressed in other hematopoietic cells such as T cells, mast cells and neutrophils, and plays a role in cellular signals transmitted by immunoreceptors such as FcεR, FcγR, and chemokine receptors. Therefore, inhibitors of PI3K delta may have utility in diverse hematological malignancies, in addition to those in B cells.

PWT143 is a highly selective PI3K delta inhibitor that has been selected as a development candidate. PWT143 exhibits low nanomolar potency in cellular phosphorylation assays against PI3K delta. PWT143 exhibits cellular selectivity of 2200-, 30- and 700-fold against the alpha, beta and gamma isoforms, with no activity against approximately 500 other kinases tested, including mTOR. Low nM potency has also been demonstrated in a whole blood functional assay of basophil activity which is encouraging for translation to the clinic.

The activity of PWT143 has been profiled in survival and proliferation assays in a panel of 12 human hematological malignancy cell lines, and compared to GS-1101, ibrutinib, and fostamatinib. The cell lines included DLBCL, Burkitts lymphoma, and lymphoblastic leukemias. PWT143 IC₅₀s ranged from 30 nM to 4 μM with the majority approximating 1μM, while the comparator molecules exhibited higher IC₅₀values: 250 nM to > 10 μM for GS-1101, 250 nM to 9.5 μM for ibrutinib, and 400 nM to > 10 μM for fostamatinib.

Viability/proliferation assays were also performed in peripheral blood cells freshly isolated from patients with various hematological malignancies. In CLL samples, PWT143 displayed IC₅₀ values < 100 nM in 3 of 4 cases, in marked contrast to GS-1101 or ibrutinib which exhibited IC₅₀ values >10 μM for the majority of these samples tested. Potent activity was also observed for PWT143 in primary AML samples with IC₅₀s in the 100 nM range or lower for 3 of 5 cases tested, but generally > 1 μM for GS-1101 or Ibrutinib. In previously frozen CLL and AML patient samples procured from commercial sources, PWT143 similarly exhibited several-fold lower IC₅₀values than GS-1101 or ibrutinib. These data suggest increased sensitivity of CLL and AML patient samples to PWT143.

The lack of activity of GS-1101 and ibrutinib at low micromolar concentrations in the primary cell assays is consistent with the published mechanism of action. Rather than a direct inhibition of tumor cell viability, the major axis of clinical activity is inhibition of stromal-tumor interactions mediated by BCR “inside-out” signaling which normally maintains tumor cells in the lymph node. Accordingly, the clinical activity of GS-1101, ibrutinib and fostamatinib is associated with marked lymphocytosis due to release of tumor cells from the lymph nodes into peripheral blood, observed in the initial weeks of treatment and often persists for many months.

The direct inhibition of viability by PWT143, as well as the established stromal-mediated mechanism of action of PI3K delta inhibitors, may translate to increased clinical activity for PWT143. PWT143 is a potent and selective PI3K delta inhibitor, and preclinical data indicate that it is an attractive candidate for clinical development.