PI3Kδ inhibitor, GS-1101 (CAL-101), attenuates pathway signaling, induces apoptosis, and overcomes signals from the microenvironment in cellular models of Hodgkin lymphoma.

GS-1101 (CAL-101) is an oral PI3Kδ-specific inhibitor that has shown preclinical and clinical activity in non-Hodgkin lymphoma and chronic lymphocytic leukemia. To investigate the potential role of PI3Kδ in Hodgkin lymphoma (HL), we screened 5 HL cell lines and primary samples from patients with HL for PI3Kδ isoform expression and constitutive PI3K pathway activation. Inhibition of PI3Kδ by GS-1101 resulted in the inhibition of Akt phosphorylation. Cocultures with stroma cells induced Akt activation in HL cells, and this effect was blocked by GS-1101. Conversely, production of the stroma-stimulating chemokine, CCL5, by HL cells was reduced by GS-1101. GS-1101 also induced dose-dependent apoptosis of HL cells at 48 hours. Reductions in cell viability and apoptosis were enhanced when combining GS-1101 with the mTOR inhibitor everolimus. Our findings suggest that excessive PI3Kδ activity is characteristic in HL and support clinical evaluation of GS-1101, alone and in combination, as targeted therapy for HL.

The phosphoinositide 3'-kinase delta inhibitor, CAL-101, inhibits B-cell receptor signaling and chemokine networks in chronic lymphocytic leukemia.

In lymphocytes, the phosphoinositide 3'-kinase (PI3K) isoform p110δ (PI3Kδ) transmits signals from surface receptors, including the B-cell receptor (BCR). CAL-101, a selective inhibitor of PI3Kδ, displays clinical activity in CLL, causing rapid lymph node shrinkage and a transient lymphocytosis. Inhibition of pro-survival pathways, the presumed mechanism of CAL-101, does not explain this characteristic pattern of activity. Therefore, we tested CAL-101 in assays that model CLL-microenvironment interactions in vitro. We found that CAL-101 inhibits CLL cell chemotaxis toward CXCL12 and CXCL13 and migration beneath stromal cells (pseudoemperipolesis). CAL-101 also down-regulates secretion of chemokines in stromal cocultures and after BCR triggering. CAL-101 reduces survival signals derived from the BCR or from nurse-like cells, and inhibits BCR- and chemokine-receptor-induced AKT and MAP kinase (ERK) activation. In stromal cocultures, CAL-101 sensitizes CLL cells toward bendamustine, fludarabine, and dexamethasone. These results are corroborated by clinical data showing marked reductions in circulating CCL3, CCL4, and CXCL13 levels, and a surge in lymphocytosis during CAL-101 treatment. Thus, CAL-101 displays a dual mechanism of action, directly decreasing cell survival while reducing interactions that retain CLL cells in protective tissue microenvironments. These data provide an explanation for the clinical activity of CAL-101, and a roadmap for future therapeutic development.

CAL-101, a p110delta selective phosphatidylinositol-3-kinase inhibitor for the treatment of B-cell malignancies, inhibits PI3K signaling and cellular viability.

Phosphatidylinositol-3-kinase p110δ serves as a central integration point for signaling from cell surface receptors known to promote malignant B-cell proliferation and survival. This provides a rationale for the development of small molecule inhibitors that selectively target p110δ as a treatment approach for patients with B-cell malignancies. We thus identified 5-fluoro-3-phenyl-2-[(S)-1-(9H-purin-6-ylamino)-propyl]-3H-quinazolin-4-one (CAL-101), a highly selective and potent p110δ small molecule inhibitor (half-maximal effective concentration [EC(50)] = 8nM). Using tumor cell lines and primary patient samples representing multiple B-cell malignancies, we have demonstrated that constitutive phosphatidylinositol-3-kinase pathway activation is p110δ-dependent. CAL-101 blocked constitutive phosphatidylinositol-3-kinase signaling, resulting in decreased phosphorylation of Akt and other downstream effectors, an increase in poly(ADP-ribose) polymerase and caspase cleavage and an induction of apoptosis. These effects have been observed across a broad range of immature and mature B-cell malignancies, thereby providing a rationale for the ongoing clinical evaluation of CAL-101.

PI3Kδ inhibitor idelalisib in combination with BTK inhibitor ONO/GS-4059 in diffuse large B cell lymphoma with acquired resistance to PI3Kδ and BTK inhibitors.

Activated B-cell-like diffuse large B-cell lymphoma relies on B-cell receptor signaling to drive proliferation and survival. Downstream of the B-cell receptor, the key signaling kinases Bruton's tyrosine kinase and phosphoinositide 3-kinase δ offer opportunities for therapeutic intervention by agents such as ibrutinib, ONO/GS-4059, and idelalisib. Combination therapy with such targeted agents could provide enhanced efficacy due to complimentary mechanisms of action. In this study, we describe both the additive interaction of and resistance mechanisms to idelalisib and ONO/GS-4059 in a model of activated B-cell-like diffuse large B-cell lymphoma. Significant tumor regression was observed with a combination of PI3Kδ and Bruton's tyrosine kinase inhibitors in the mouse TMD8 xenograft. Acquired resistance to idelalisib in the TMD8 cell line occurred by loss of phosphatase and tensin homolog and phosphoinositide 3-kinase pathway upregulation, but not by mutation of PIK3CD. Sensitivity to idelalisib could be restored by combining idelalisib and ONO/GS-4059. Further evaluation of targeted inhibitors revealed that the combination of idelalisib and the phosphoinositide-dependent kinase-1 inhibitor GSK2334470 or the AKT inhibitor MK-2206 could partially overcome resistance. Characterization of acquired Bruton's tyrosine kinase inhibitor resistance revealed a novel tumor necrosis factor alpha induced protein 3 mutation (TNFAIP3 Q143*), which led to a loss of A20 protein, and increased p-IκBα. The combination of idelalisib and ONO/GS-4059 partially restored sensitivity in this resistant line. Additionally, a mutation in Bruton's tyrosine kinase at C481F was identified as a mechanism of resistance. The combination activity observed with idelalisib and ONO/GS-4059, taken together with the ability to overcome resistance, could lead to a new therapeutic option in activated B-cell-like diffuse large B-cell lymphoma. A clinical trial is currently underway to evaluate the combination of idelalisib and ONO/GS-4059 (NCT02457598).

Ala657 and conserved active site residues promote fibroblast activation protein endopeptidase activity via distinct mechanisms of transition state stabilization.

Fibroblast activation protein (FAP) and dipeptidyl peptidase-4 (DPP-4) are highly homologous serine proteases of the prolyl peptidase family and therapeutic targets for cancer and diabetes, respectively. Both proteases display dipeptidyl peptidase activity, but FAP alone has endopeptidase activity. FAP Ala657, which corresponds to DPP-4 Asp663, is important for endopeptidase activity; however, its specific role remains unclear, and it is unknown whether conserved DPP-4 substrate binding residues support FAP endopeptidase activity. Using site-directed mutagenesis and kinetic analyses, we show here that Ala657 and five conserved active site residues (Arg123, Glu203, Glu204, Tyr656, and Asn704) promote FAP endopeptidase activity via distinct mechanisms of transition state stabilization (TSS). The conserved residues provide marked TSS energy for both endopeptidase and dipeptidyl peptidase substrates, and structural modeling supports their function in binding both substrates. Ala657 also stabilizes endopeptidase substrate binding and additionally dictates FAP reactivity with transition state inhibitors, allowing tight interaction with tetrahedral intermediate analogues but not acyl-enzyme analogues. Conversely, DPP-4 Asp663 stabilizes dipeptidyl peptidase substrate binding and permits tight interaction with both transition state analogues. Structural modeling suggests that FAP Ala657 and DPP-4 Asp663 confer their contrasting effects on TSS by modulating the conformation of conserved residues FAP Glu204 and DPP-4 Glu206. FAP therefore requires the combined function of Ala657 and the conserved residues for endopeptidase activity.