Targeting PI3Kδ in cancer: a setback or the end

The phosphoinositide 3-kinase delta (PI3Kδ) is a member of the class I lipid kinase family based on its sequence homology in kinase domains and its substrate selectivity (1,2). First isolated from a cDNA library made from monocytes, the PI3Kδ gene is selectively expressed in leukocytes. Signaling through this kinase is via cytokine receptors (3,4). Subsequent functional studies revealed impaired B and T cell antigen receptor signaling in mouse models which have a nonfunctional kinase allele (4). Development of pan-kinase and delta selective inhibitors marked the beginning of a new era in the treatment of B cell malignancy. Meanwhile, the role of PI3Kδ in Treg function prompted exploration of it as a tool for breaking the immune tolerance to boost antitumor immunotherapy. The science behind the translational efforts was captured in this review. (5) Unfortunately, clinical use of this class of drug in management of B cell malignancies and research came to halt when all FDA approvals of this drug class were withdrawn.( 6) Here, we discuss the initial approval of four PI3K inhibitors. We present preclinical studies supporting use of the inhibitor in modulating the tumor Idelalisib is an orally bioavailable ATP-competitive kinase inhibitor given continuously that targets the PI3K p110 isoform δ (PI3Kδ) with high potency and selectivity. In an open labeled single arm phase 2 study, 125 patients with indolent lymphoma who had not responded to rituximab or alkylating agents or who had recurrence after 6 months were given 150 mg of drug twice daily until disease progression or withdrawal. Overall response rate (ORR), which was the primary end point, was 57% with 6% complete response. Median progression-free survival (PFS) was 11 months. Severe adverse events included neutropenia (27%), increased liver function tests (13%), diarrhea (13%) and pneumonia (7%). As the first-in-class drug, idelalisib's initial approval for B cell malignancies in July 2014 included indications in relapsed/recurrent CLL, SLL and follicular lymphoma (FL) (7). The approval in FL and in SLL was an accelerated one that mandated postmarketing trials which suffered from slow accrual. The regular approval was then noted to have a death rate due to drug toxicity more than double that of the control arm in a randomized trial in 2016 and FDA mandated a box warning on the label (8). The sponsor voluntarily withdrew idelalisib from the market in 2022 due to inability to complete the confirmation study.Copanlisib, which is considered a pan-PI3K inhibitor, received accelerated approval in 2017 for relapsed FL. The approval was based on CHRONOS-3, a multicenter, randomized, double blinded, placebo-controlled phase 3 trial of 652 patients with CD20 positive indolent lymphoma which recurred 6-12 months before the last dose of rituximab. Patients were randomized into receiving either copanlisib and rituximab or rituximab and placebo at a 3:1 ratio and were observed for PFS. The drug was given intravenously on days 1, 8, and 15 every 4 weeks. The trial showed a PFS advantage of 21.5 months with copanlisib plus rituximab over rituximab plus placebo (13.8 months). The most common serious adverse events (SAEs) were hyperglycemia (56% vs 8%) and hypertension (40% vs 9%). The drug was well tolerated overall with only one death from pneumonitis (9). Interim analysis, however, showed no overall survival (OS) advantage. In 2021, the agreement between the FDA and the drug maker was voluntarily withdrawn based on CHRONOS-3 results pending new study results from the ongoing CHRONOS-4. Unfortunately, CHRONOS-4 was not a positive study in relapsed indolent lymphoma showing no PFS or OS benefit in patients receiving rituximab and bendamustine plus copanlisib compared to rituximab and bendamustine plus placebo (32.9 m vs 33.3 m; HR: 1.13; 95% CI: 0.80-1.60; P=0.71) (10). Indeed, the curve of OS showed potential harm in patients receiving copanlisib compared with placebo. Copanlisib was voluntarily withdrawn from the market for having no efficacy.Duvelisib is a dual inhibitor of PI3Kδ, γ developed for treatment of hematologic malignancies. In 2018, the FDA approved durvelisib in relapsed/refractory (R/R) CLL/SLL after two or more lines of treatment based on the DUO trial (11). The DUO trial is a global phase 3 trial comparing the efficacy of monotherapy for relapsed/refractory CLL/SLL with duvelisib (25 mg twice a day orally) or ofatuzumab. Patients were randomized in a 1:1 ratio into either treatment. The primary end point, PFS, was 13.3 months for duvelisib versus 9.9 months for ofatumumab (HR: 0.52; 95% CI: 0.39-0.69; P<0.0001). The most common adverse events were diarrhea, neutropenia, pyrexia, anemia and cough. The ORR was significantly higher in the duvelisib arm vs ofatumumab (74% vs 45%). The final analysis, however, showed no OS benefit (HR: 1.09; 95% CI: 0.79-1.51). The duvelisib approval in R/R FL in 2018 was an accelerated one based on the response rates at 6 and 12 months and the approval mandated additional post marketing trial which never occurred. Finally, the approval in FL was voluntarily withdrawn in 2022. Further analysis of the DUO trial's OS showed that the treatment arm crossed over to show potentially more death after ~45 months compared with the ofatumumab (HR: 1.06; 95% CI: 0.71-1.58).Umbralisib is a dual inhibitor of PI3Kδ/casein kinase-1ε, thought to exhibit improved selectivity for PI3Kδ compared with other PI3K inhibitors. In a phase IIb open labeled multi cohort trial enrolling 208 patients designed to evaluate the efficacy and safety of umbralisib in patients with R/R indolent non-Hodgkin's lymphoma who are CD20+ and not responding to at least one anti-CD20 treatment. Patients were given umbralisib 800 mg orally once a day until disease progression, unacceptable toxicity or withdrawal. The ORR was 47.1% and tumor reduction occurred in 86.4% of patients. The median time to response was 2.7-4.6 months. The median duration of response was not reached for marginal zone lymphoma (MZL), 11.1 months for FL, and 18.3 months for SLL. Median PFS was not reached for MZL, 10.6 months for FL, and 20.9 months for SLL. At least one serious AE occurred in 53.4% of patients and included neutropenia (11.5%), diarrhea (10.1%) and increased LFTs (~7%) with 31 patients discontinuing study due to SAEs (12). The FDA gave accelerated approval to umbralisib for patients with R/E (extranodal) FL who received three or more lines of treatment, or R/R MZL who had received one or more lines of anti-CD20 treatment. In a phase 3 trial comparing umbralisib plus ublituximab versus obinutuzumab and chlorambucil in untreated R/R CLL, the PFS was improved with HR 0.55 (13). However, interim analysis showed that OS was worse with HR 1.23 (14). The manufacturer eventually withdrew all applications from CLL, SLL, FL and MZL due to lack of OS benefit.PI3Ks phosphorylate the inositol ring of phosphatidylinositol (PtdIns) lipids. Three classes are documented, and Class I is noted for its role in regulating cell signaling at and downstream of the plasma membrane. Class I PI3Ks consist of a regulatory subunit (p85) and a catalytic subunit (p110), which exists in four isoforms: α, β, γ, and δ. Whereas p110α and p110β are ubiquitously expressed in different tissues, p110γ and p110δ are mainly concentrated in leukocytes. Widespread high throughput sequencing discovered that p110 mutations were present in 14% of all solid tumors, strengthening the belief that targeting PI3K in solid tumors is sound. (15) Initial successes in the clinics targeting PI3K in B cell malignancies using delta selective idelalisib ushered in an era of subtype targeting. The impact of different inhibitors on PI3K and PI3K function is studied in BaF3 cells. (16) It is noteworthy that Treg downregulation by PI3K inhibitor emerged as an approach of TIME manipulation (Figure 1).PI3Kδ is critical for transmission of signals from B/T cell receptors. The critical role of PI3Kδ in maintaining Treg proliferation and immunosuppressive function toward T conventional cells was demonstrated in 2006. (17) Tregs with non-functional PI3Kδ (D910A/D910A) had attenuated suppression function and failed to protect mice against experimental colitis in an adoptive T cell transfer. Further work showed that inactivation of PI3Kδ in mice protected them against formation of a broad range of solid tumors as well as hematological malignancy engraftment. Inactivation of PI3Kδ in Tregs genetically or pharmacologically achieved the same purpose (18). PI3K-AKT is essential for in vivo maintenance of murine Tregs. Genetic inactivation of PI3K D910A/D910A in a murine chronic lymphocytic leukemia (CLL) model resulted in defective Treg expansion and B cell receptor signaling. The TIME protected the mice from acquiring CLL and acute myeloid leukemia (AML). Reconstitution of the D910A/D910A mice with wild type reversed the leukemia resistance (19). PI3Kδ inhibition likely unleashed a potent T cell-mediated anti-tumor immune response. Inhibition of PI3Kδ in human T cells selectively inhibits Treg proliferation compared to T conventional cells. Inhibition of PI3Kδ also slows tumor growth (20). Human Tregs are highly sensitive to PI3Kδ specific inhibition. Idelalisib specifically inhibited human Tregs and was 4-13fold more potent in inhibiting Treg than conventional CD4 and CD8 T cells (21). PI3Kδ has a crucial role in shaping the TIME, especially the suppressive cells in head and neck cancer. PTEN loss and PI3Kδ activation was linked to weakened expression of MHCI and MHCII when induced by IFNγ, while PI3Kδ inhibitor was able to enhance it (22). Signal transduction via PI3K and its impact on immune cell function is illustrated in Figure 1.Consistent with a role of upregulation of autoimmunity by PI3K inhibitors, prolonged heavy suppression of PI3Kδ in Tregs is considered responsible for a subset of the adverse effects associated with PI3Kδ inhibitors approved and then withdrawn from the market which include, among others, colitis, hepatitis and pneumonitis.(6)In early adoptive T cell transfer experiments, transfer of CD4+CD25-Th cells with tumor/selfreactive CD8+ T cells into CD4+ T cell-deficient hosts induced autoimmunity and regression of established melanoma. Transfer of Treg cells (phenotypically CD4+CD25+Foxp3+) was able to block autoimmunity and the efficacy of adoptive T cell immunotherapy. These findings reveal that the presence of naturally occurring Treg cells prevents Th cells from breaking down tolerance to persisting self-antigens and treatment of established tumors (23). Treg cells have become known for their crucial role in the regulation of cancer immunity (24).In ovarian cancer, recruitment of Tregs into the tumor was suspected to foster immune privilege. Higher Treg infiltration was associated with reduced overall survival (25). In hepatocellular carcinoma, increased Tregs impairs CD8+ T cell function and correlates with poor survival (26), while higher Treg infiltration seems to correlate with higher tumor burden and advanced stage in lung cancer (27). In gliomas, Treg tumor infiltration prefers the astrocyte lineage and higher grade tumors such as glioblastoma multiforme, although its prognostic value in survival was not demonstrated (28). In immunogenic tumors such as lung cancer, direct immunosuppression from Tregs, myeloid derived suppressor cells (MDSCs) and tumor associated macrophages is one of the important mechanisms of tolerance causing the tumor to either not respond to treatment from the beginning or to lose response to immune checkpoint inhibitors (ICI) overtime. Increased Tregs are linked to immune escape by dampening the CD8+ T cell tumor infiltration and cytotoxicity (29). In a systemic review and meta-analysis of 76 papers covering 15,512 cases encompassing 17 types of cancer, high Treg infiltration was found to be associated with poor overall survival (OS), with an odds ratio of 1.46 (P<0.001). This effect is particularly evident in immunogenic tumors such as cervical, renal, lung and melanoma which are among the cancers for which ICI received early approval (30). Clinically, increased circulating myeloid-derived suppressor cells (MDSCs) has been associated with poor response to anti-CTLA4 therapies in melanoma patients (31).PI3K inhibition with low-dose duvelisib improved T cell activation resulting in improved T cellmediated cytotoxicity when added to ICI in an animal model by inhibiting MDSCs (32). Adding the PI3K eganelisib broke the tolerance of the tumor model of 4T1 (breast) and B16 (melanoma) to combination anti-PD-1 and anti-CTLA4 resulting in complete remission of tumors in 30% and 80% of the animals, respectively (33). In a 4T1 breast cancer model, PI3K facilitated tumor growth and inhibited tumor immune surveillance. Genetic deletion of PI3Kγ or pharmacological inhibition could partially attenuate the effects. Treatment with a pan Class I PI3K inhibitor and ICI resulted in consistent inhibition of tumor growth compared with either agent alone (34).Intermittent dosing with copanlisib (pan inhibitor of PI3K) in vivo resulted in strong anti-tumor efficacy. Copanlisib increased tumor infiltration of activated T cells and macrophages, and increased CD8+ T cell/regulatory T cell and M1/M2 macrophage ratios. The combination of intermittently dosed copanlisib with the ICI antibody anti-PD-1 demonstrated enhanced anti-tumor efficacy in both ICI-sensitive and resistant mouse models. In an ICI-sensitive model, combination therapy resulted in complete remission and prevented tumor recurrence (35). An intermittent schedule of the PI3Kα/β/δ inhibitor, BAY1082439, overcame ICI resistance in PTEN-null prostate cancer models. It is not clear however, inhibition of one or all subunits contributed to this effect. BAY1082439 promoted clonal expansion of tumor-associated CD8+ T cells via Treg suppression. PI3K inhibitor-primed tumors become responsive to anti-PD-1 therapy illustrating how cold tumors can be turned into T cellinflamed ones, responding to ICI (36). Copanlisib use also induced diffused large B cell lymphoma ABC subtype to regress by blocking BCR dependent and independent NF-κB and AKT in an ibrutinib resistant model and combination use results in complete response (37).Taken together, the use of PI3K inhibitors represents an opportunity to break immune tolerance in the TIME in multiple cancer subtypes.From clinical trial experiences, a common theme has emerged. 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Indeed, the clinical trial to or no benefit cell lung cancer from single agent inhibitor after therapy their cancer or no It could also be in the cold such as prostate cancer, to immunogenic response to benefit from treatment with solid tumors are to the most resistance in TIME, this to be the most in which to study the toxicity of PI3K inhibitors in who have not received treatment for their CLL or of the head and neck or be at high to unacceptable and are not PI3K inhibitor with a different of has also emerged. a PI3Kδ inhibitor, was to regulatory T cell proliferation while having effects on conventional CD4+ T cells and no effect on CD8+ T cells. In and lung the tumors to and the infiltration of CD8 and while suppressive immune cells is in clinical phase in solid and hematologic tumors and and other solid tumor such as melanoma and cell lung cancer after the of the PI3Kδ targeting PI3Kδ signaling to be a and for B cell malignancies. 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