A Tale of Two Isoforms: PI3K Delta and Gamma

isoforms

The phosphoinositide 3-kinases (PI3Ks) are a family of lipid kinases that are expressed in mammalian cells and are known to play an important role in intracellular signaling of different receptor tyrosine kinases and G-protein—coupled receptors.^1-4Comprehensive reviews of the PI3K signaling pathway and its role in normal and disease states have been previously published.^3,4Various isoforms of PI3Ks exist, including the delta (δ) and gamma (γ) isoforms, which show a preferential expression in cells of the immune system.¹Selective inhibitors of these kinases have been developed, and these agents have been used to investigate the biologic role of these isoforms in a variety of inflammatory processes and hematologic malignancies.^1,2

Role of PI3K δ in Normal and Disease States

Studies of genetically engineered mice that are either deficient in PI3K δ, or that express a catalytically inactive form, have linked this kinase to a broad range of immune cell functions, such as B-cell and T-cell activation and proliferation, and have also shown it to be a requirement for signaling in other immune cell types, such as monocytes and dendritic cells.^2,3,5-7In mice that express a catalytically inactive form of PI3K δ, for example, immune responses are attenuated, and antigen signaling in both B and T cells is impaired.⁶B cells in these mice also show reduced immunoglobulin production, and poor proliferative responses to B-cell receptor (BCR) stimulation or CD40 in vitro.⁷B cells that lack PI3K δ also show a diminished chemotaxis response to chemokines like CXCL13.⁸Importantly, these experimental observations from genetically engineered mice can be replicated using small-molecule inhibitors that are specific for the PI3K-δ isoform.⁹

The use of PI3K-δ inhibitors such as CAL-101 has also been examined in tumor cell lines and in patient samples representative of primary B-cell malignancies. These studies have demonstrated that constitutive activation of PI3K and activation of downstream effector molecules can be blocked with PI3K-δ inhibition, which ultimately results in the induction of apoptosis.¹⁰In addition, chronic lymphocytic leukemia (CLL) cells show high PI3K enzymatic activity and PI3K-δ expression, and these CLL cells can be induced to undergo apoptosis ex vivo using CAL-101, whereas this PI3K-δ inhibitor does not cause apoptosis in normal T cells or natural killer cells, nor does it inhibit other essential immune functions such as antibody-dependent cellular cytotoxicity.¹¹

Clinical data also show that PI3K δ can be effectively inhibited in vivo with CAL-101 treatment, and this has been correlated with transient lymphocytosis and reduced levels of circulating chemokines, including CCL-3, CCL-4, and CXCL-13 in patients with CLL.¹²

Role of PI3K γ in Normal and Disease States

Results from mouse models of asthma have suggested that inhibitors of the p110-γ isoform of PI3K inhibit innate immune cell migration and other lymphocyte functions, for example, eosinophil recruitment.^1,13Similar to mice with PI3K-δ gene inactivation, key immune cell functions, including migration, proliferation, and activation, are also observed in mice with a targeted disruption of the PI3K-γ gene.^3,8,14,15Macrophages from mice lacking PI3K γ, for example, show reduced migratory capability and accumulation in response to inflammatory and chemotactic stimuli; neutrophils from these mice also displayed impaired mobility and respiratory burst capability in response to chemotactic signals.^14,15T cells that are deficient in p110γ also display reduced chemotaxis to important lymphocyte chemokines such as CCL19, CCL21, and CXCL12.⁸

There is evidence that the PI3K-γ isoform also plays an important role in inflammatory cell recruitment to tumors, or tumor inflammation, which is known to create a microenvironment that is conducive to angiogenesis, tumor growth, and localized immunosuppression.¹⁶Tumor-derived chemoattractant signals have been found to directly activate the p110g isoform, leading to activation of the a4b1 integrin, and subsequent myeloid cell adhesion and invasion into tumors.¹⁶Importantly, small-molecule inhibitors of PI3K have been shown to suppress this inflammatory process and the associated tumor growth.^16,17

IPI-145 (Duvelisib): A Unique Mechanism of Action

Collectively, the above findings support differential and complementary roles for the PI3K-δ and -γ isoforms in important immune functions such as lymphocyte chemotaxis, activation, proliferation, and innate immune cell function.^1,8Dual inhibitors of both the PI3K-δ and -γ isoforms have also been developed, and there is evidence in some disease model systems that dual inhibition of both isoforms may, in some cases, be more effective than inhibiting either one alone. This has been demonstrated in a model of antibody-induced arthritis where dual inhibition appears to be most effective at blocking adaptive and innate immune responses.^1,18

Winkler et al in 2013 described duvelisib (IPI- 145), a dual inhibitor of PI3K δ,γ.^1,2The enzymatic half maximal inhibitory concentration (IC50) values with duvelisib for the γ isoform was reported to be 2.5 nM, while it was approximately 10-fold higher for the γ isoform, at 27 nM.^1,2Inhibition of both kinases with this agent thus has potential utility in both inflammatory conditions and in oncology.

Potential Clinical Application of Duvelisib in Lymphoma

At the 2014 American Society of Hematology (ASH) Conference, Peluso et al used selective inhibitors of the individual PI3K-δ and -γ isoforms, as well as duvelisib, to assess the role of these PI3K isoforms in B-cell malignancies.¹⁹Using in vitro model systems, they found that the proliferation of primary CLL cells stimulated with lymphokines from the tumor microenvironment (sCD40L, interleukin 2 [IL-2], and IL-10) could be potently inhibited using a PI3K-δγ—selective isoform inhibitor or duvelisib, whereas a PI3K-γ–selective inhibitor had little effect.¹⁹In contrast, T-cell migration induced by CXCL12 was effectively inhibited by a PI3K-γ—selective inhibitor, whereas a PI3K-δ–selective inhibitor had little effect, and duvelisib showed an intermediate activity in this experimental setting.¹⁹In other experiments, investigators found that PI3K-γ inhibition could effectively block the polarization of macrophages toward the more tumor- promoting M2 phenotype, whereas a PI3K-δ inhibitor had no effect.¹⁹Collectively, these findings highlighted the complementary roles of these two PI3K isoforms in regulating the interactions of the malignant B cell with its microenvironment, and the therapeutic potential for dual PI3K inhibitors, such as duvelisib, for diseases such as B-cell lymphomas.

These nonclinical data, coupled with recently reported clinical findings with duvelisib,^20-22have prompted initiation of a phase II study (DYNAMO) in patients with relapsed refractory (R/R) indolent non-Hodgkins lymphoma (NCT01882803); a phase III study in combination with rituximab (DYNAMO+R) in patients with previously treated follicular lymphoma (FL) (NCT02204982); and a phase I/II trial (CONTEMPO) in patients with previously untreated FL (NCT02391545). In addition, 2 clinical trials are under way in patients with CLL: DUO, a phase III trial in patients with R/R CLL (NCT02004522), and SYNCHRONY, a phase I/II trial in patients with CLL who were previously treated with a Bruton’s tyrosine kinase (BTK) inhibitor (NCT02292225).

References

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