PI3K isoforms in cell signalling and vesicle trafficking.

PI3Ks are a family of lipid kinases that phosphorylate intracellular inositol lipids to regulate signalling and intracellular vesicular traffic. Mammals have eight isoforms of PI3K, divided into three classes. The class I PI3Ks generate 3-phosphoinositide lipids, which directly activate signal transduction pathways. In addition to being frequently genetically activated in cancer, similar mutations in class I PI3Ks have now also been found in a human non-malignant overgrowth syndrome and a primary immune disorder that predisposes to lymphoma. The class II and class III PI3Ks are regulators of membrane traffic along the endocytic route, in endosomal recycling and autophagy, with an often indirect effect on cell signalling. Here, we summarize current knowledge of the different PI3K classes and isoforms, focusing on recently uncovered biological functions and the mechanisms by which these kinases are activated. Deeper insight into the PI3K isoforms will undoubtedly continue to contribute to a better understanding of fundamental cell biological processes and, ultimately, of human disease.

A small-molecule PI3Kα activator for cardioprotection and neuroregeneration.

Harnessing the potential beneficial effects of kinase signalling through the generation of direct kinase activators remains an underexplored area of drug development1-5. This also applies to the PI3K signalling pathway, which has been extensively targeted by inhibitors for conditions with PI3K overactivation, such as cancer and immune dysregulation. Here we report the discovery of UCL-TRO-1938 (referred to as 1938 hereon), a small-molecule activator of the PI3Kα isoform, a crucial effector of growth factor signalling. 1938 allosterically activates PI3Kα through a distinct mechanism by enhancing multiple steps of the PI3Kα catalytic cycle and causes both local and global conformational changes in the PI3Kα structure. This compound is selective for PI3Kα over other PI3K isoforms and multiple protein and lipid kinases. It transiently activates PI3K signalling in all rodent and human cells tested, resulting in cellular responses such as proliferation and neurite outgrowth. In rodent models, acute treatment with 1938 provides cardioprotection from ischaemia-reperfusion injury and, after local administration, enhances nerve regeneration following nerve crush. This study identifies a chemical tool to directly probe the PI3Kα signalling pathway and a new approach to modulate PI3K activity, widening the therapeutic potential of targeting these enzymes through short-term activation for tissue protection and regeneration. Our findings illustrate the potential of activating kinases for therapeutic benefit, a currently largely untapped area of drug development.

Intermittent PI3Kδ inhibition sustains anti-tumour immunity and curbs irAEs.

Phosphoinositide 3-kinase δ (PI3Kδ) has a key role in lymphocytes, and inhibitors that target this PI3K have been approved for treatment of B cell malignancies1-3. Although studies in mouse models of solid tumours have demonstrated that PI3Kδ inhibitors (PI3Kδi) can induce anti-tumour immunity4,5, its effect on solid tumours in humans remains unclear. Here we assessed the effects of the PI3Kδi AMG319 in human patients with head and neck cancer in a neoadjuvant, double-blind, placebo-controlled randomized phase II trial (EudraCT no. 2014-004388-20). PI3Kδ inhibition decreased the number of tumour-infiltrating regulatory T (Treg) cells and enhanced the cytotoxic potential of tumour-infiltrating T cells. At the tested doses of AMG319, immune-related adverse events (irAEs) required treatment to be discontinued in 12 out of 21 of patients treated with AMG319, suggestive of systemic effects on Treg cells. Accordingly, in mouse models, PI3Kδi decreased the number of Treg cells systemically and caused colitis. Single-cell RNA-sequencing analysis revealed a PI3Kδi-driven loss of tissue-resident colonic ST2 Treg cells, accompanied by expansion of pathogenic T helper 17 (TH17) and type 17 CD8+ T (TC17) cells, which probably contributed to toxicity; this points towards a specific mode of action for the emergence of irAEs. A modified treatment regimen with intermittent dosing of PI3Kδi in mouse models led to a significant decrease in tumour growth without inducing pathogenic T cells in colonic tissue, indicating that alternative dosing regimens might limit toxicity.

Signaling by Fyn-ADAP via the Carma1-Bcl-10-MAP3K7 signalosome exclusively regulates inflammatory cytokine production in NK cells.

Inflammation is a critical component of the immune response. However, acute or chronic inflammation can be highly destructive. Uncontrolled inflammation forms the basis for allergy, asthma and various autoimmune disorders. Here we identified a signaling pathway that was exclusively responsible for the production of inflammatory cytokines but not for cytotoxicity. Recognition of tumor cells expressing the NK cell-activatory ligands H60 or CD137L by mouse natural killer (NK) cells led to efficient cytotoxicity and the production of inflammatory cytokines. Both of those effector functions required the kinases Lck, Fyn and PI(3)K (subunits p85α and p110δ) and the signaling protein PLC-γ2. However, a complex of Fyn and the adaptor ADAP exclusively regulated the production of inflammatory cytokines but not cytotoxicity in NK cells. That unique function of ADAP required a Carma1-Bcl-10-MAP3K7 signaling axis. Our results have identified molecules that can be targeted to regulate inflammation without compromising NK cell cytotoxicity.

The p110δ isoform of the kinase PI(3)K controls the subcellular compartmentalization of TLR4 signaling and protects from endotoxic shock.

Lipopolysaccharide activates plasma-membrane signaling and endosomal signaling by Toll-like receptor 4 (TLR4) through the TIRAP-MyD88 and TRAM-TRIF adaptor complexes, respectively, but it is unclear how the signaling switch between these cell compartments is coordinated. In dendritic cells, we found that the p110δ isoform of phosphatidylinositol-3-OH kinase (PI(3)K) induced internalization of TLR4 and dissociation of TIRAP from the plasma membrane, followed by calpain-mediated degradation of TIRAP. Accordingly, inactivation of p110δ prolonged TIRAP-mediated signaling from the plasma membrane, which augmented proinflammatory cytokine production while decreasing TRAM-dependent endosomal signaling that generated anti-inflammatory cytokines (interleukin 10 and interferon-ß). In line with that altered signaling output, p110δ-deficient mice showed enhanced endotoxin-induced death. Thus, by controlling the 'topology' of TLR4 signaling complexes, p110δ balances overall homeostasis in the TLR4 pathway.

PI3K signalling: the path to discovery and understanding.

Over the past two decades, our understanding of phospoinositide 3-kinases (PI3Ks) has progressed from the identification of an enzymatic activity associated with growth factors, GPCRs and certain oncogene products to a disease target in cancer and inflammation, with PI3K inhibitors currently in clinical trials. Elucidation of PI3K-dependent networks led to the discovery of the phosphoinositide-binding PH, PX and FYVE domains as conduits of intracellular lipid signalling, the determination of the molecular function of the tumour suppressor PTEN and the identification of AKT and mTOR protein kinases as key regulators of cell growth. Here we look back at the main discoveries that shaped the PI3K field.

Positive correlation between transcriptomic stemness and PI3K/AKT/mTOR signaling scores in breast cancer, and a counterintuitive relationship with PIK3CA genotype.

A PI3Kα-selective inhibitor has recently been approved for use in breast tumors harboring mutations in PIK3CA, the gene encoding p110α. Preclinical studies have suggested that the PI3K/AKT/mTOR signaling pathway influences stemness, a dedifferentiation-related cellular phenotype associated with aggressive cancer. However, to date, no direct evidence for such a correlation has been demonstrated in human tumors. In two independent human breast cancer cohorts, encompassing nearly 3,000 tumor samples, transcriptional footprint-based analysis uncovered a positive linear association between transcriptionally-inferred PI3K/AKT/mTOR signaling scores and stemness scores. Unexpectedly, stratification of tumors according to PIK3CA genotype revealed a "biphasic" relationship of mutant PIK3CA allele dosage with these scores. Relative to tumor samples without PIK3CA mutations, the presence of a single copy of a hotspot PIK3CA variant was associated with lower PI3K/AKT/mTOR signaling and stemness scores, whereas the presence of multiple copies of PIK3CA hotspot mutations correlated with higher PI3K/AKT/mTOR signaling and stemness scores. This observation was recapitulated in a human cell model of heterozygous and homozygous PIK3CAH1047R expression. Collectively, our analysis (1) provides evidence for a signaling strength-dependent PI3K-stemness relationship in human breast cancer; (2) supports evaluation of the potential benefit of patient stratification based on a combination of conventional PI3K pathway genetic information with transcriptomic indices of PI3K signaling activation.

Endothelial phosphoinositide 3-kinase-ß inactivation confers protection from immune-mediated vascular injury.

Heart transplant and recipient survival are limited by immune cell-mediated injury of the graft vasculature. We examined the role of the phosphoinositide 3-kinase-ß (PI3Kß) isoform in endothelial cells (EC) during coronary vascular immune injury and repair in mice. In minor histocompatibility-antigen mismatched allogeneic heart grafts, a robust immune response was mounted to each wild-type, PI3Kß inhibitor-treated, or endothelial-selective PI3Kß knockout (ECßKO) graft transplanted to wild-type recipients. However, microvascular EC loss and progressive occlusive vasculopathy only developed in control, but not PI3Kß-inactivated hearts. We observed a delay in inflammatory cell infiltration of the ECßKO grafts, particularly in the coronary arteries. Surprisingly, this was accompanied by an impaired display of proinflammatory chemokine and adhesion molecules by the ECßKO ECs. In vitro, tumor necrosis factor α-stimulated endothelial ICAM1 and VCAM1 expression was blocked by PI3Kß inhibition or RNA interference. Selective PI3Kß inhibition also blocked tumor necrosis factor α-stimulated degradation of inhibitor of nuclear factor kappa Bα and nuclear translocation of nuclear factor kappa B p65 in EC. These data identify PI3Kß as a therapeutic target to reduce vascular inflammation and injury.

PI3KC2ß inactivation stabilizes VE-cadherin junctions and preserves vascular integrity.

Endothelium protection is critical, because of the impact of vascular leakage and edema on pathological conditions such as brain ischemia. Whereas deficiency of class II phosphoinositide 3-kinase alpha (PI3KC2α) results in an increase in vascular permeability, we uncover a crucial role of the beta isoform (PI3KC2ß) in the loss of endothelial barrier integrity following injury. Here, we studied the role of PI3KC2ß in endothelial permeability and endosomal trafficking in vitro and in vivo in ischemic stroke. Mice with inactive PI3KC2ß showed protection against vascular permeability, edema, cerebral infarction, and deleterious inflammatory response. Loss of PI3KC2ß in human cerebral microvascular endothelial cells stabilized homotypic cell-cell junctions by increasing Rab11-dependent VE-cadherin recycling. These results identify PI3KC2ß as a potential new therapeutic target to prevent aggravating lesions following ischemic stroke.

Long-term treatment of cancer-prone germline PTEN mutant mice with low-dose rapamycin extends lifespan and delays tumour development.

PTEN is one of the most commonly inactivated tumour suppressor genes in sporadic cancer. Germline heterozygous PTEN gene alterations also underlie PTEN hamartoma tumour syndrome (PHTS), a rare human cancer-predisposition condition. A key feature of systemic PTEN deregulation is the inability to adequately dampen PI3-kinase (PI3K)/mTORC1 signalling. PI3K/mTORC1 pathway inhibitors such as rapamycin are therefore expected to neutralise the impact of PTEN loss, rendering this a more druggable context compared with those of other tumour suppressor pathways such as loss of TP53. However, this has not been explored in cancer prevention in a model of germline cancer predisposition, such as PHTS. Clinical trials of short-term treatment with rapamycin have recently been initiated for PHTS, focusing on cognition and colon polyposis. Here, we administered a low dose of rapamycin from the age of 6 weeks onwards to mice with heterozygous germline Pten loss, a mouse model that recapitulates most characteristics of human PHTS. Rapamycin was well tolerated and led to a highly significant improvement of survival in both male and female mice. This was accompanied by a delay in, but not full blockade of, the development of a range of proliferative lesions, including gastro-intestinal and thyroid tumours and endometrial hyperplasia, with no impact on mammary and prostate tumours, and no effect on brain overgrowth. Our data indicate that rapamycin may have cancer prevention potential in human PHTS. This might also be the case for sporadic cancers in which genetic PI3K pathway activation is an early event in tumour development, such as endometrial cancer and some breast cancers. To the best of our knowledge, this is the first report of a long-term treatment of a germline cancer predisposition model with a PI3K/mTOR pathway inhibitor. © 2022 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of The Pathological Society of Great Britain and Ireland.

Idelalisib inhibits experimental proliferative vitroretinopathy.

Proliferative vitreoretinopathy (PVR) is a fibrotic eye disease that develops after rhegmatogenous retinal detachment surgery and open-globe traumatic injury. Idelalisib is a specific inhibitor of phosphoinositide 3-kinase (PI3K) δ. While PI3Kδ is primarily expressed in leukocytes, its expression is also considerably high in retinal pigment epithelial (RPE) cells, which play a crucial part in the PVR pathogenesis. Herein we show that GeoMx Digital Spatial Profiling uncovered strong expression of fibronectin in RPE cells within epiretinal membranes from patients with PVR, and that idelalisib (10 µM) inhibited Akt activation, fibronectin expression and collagen gel contraction induced by transforming growth factor (TGF)-ß2 in human RPE cells. Furthermore, we discovered that idelalisib at a vitreal concentration of 10 µM, a non-toxic dose to the retina, prevented experimental PVR induced by intravitreally injected RPE cells in rabbits assessed by experienced ophthalmologists using an indirect ophthalmoscope plus a + 30 D fundus lens, electroretinography, optical coherence tomography and histological analysis. These data suggested idelalisib could be harnessed for preventing patients from PVR.

Treg-driven tumour control by PI3Kδ inhibition limits myeloid-derived suppressor cell expansion.

BACKGROUND: Recent studies have demonstrated that blocking the PI3Kδ signalling enzyme (by administering a small molecule inhibitor, PI-3065) can potently improve the anti-tumour T-cell response through direct inhibition of Tregs. This treatment also has a negative impact on MDSC numbers but the primary mechanism driving this effect has remained unclear. METHODS: The 4T1 breast cancer mouse model was used in combination with PI-3065 to gain insights into the effect of PI3Kδ inhibition on MDSCs. RESULTS: PI-3065 treatment resulted in a concomitant reduction in MDSC expansion and tumour size. However, targeting Tregs independent of PI-3065 was also associated with reduced tumour volume and MDSC numbers. Surgical removal of tumours resulted in a rapid and significant decline in MDSC numbers, whilst ex vivo studies using cells from PI-3065-treated mice demonstrated no direct effect of the inhibitor on MDSC activity. CONCLUSIONS: Our data suggest that MDSCs are not inhibited directly by PI-3065 treatment but that their reduced recruitment and immunosuppression within the tumour microenvironment is an indirect consequence of PI3Kδ-inhibition-driven tumour control. This indicates that PI3Kδ inhibition drives tumour immunity by breaking down multiple immunosuppressive pathways through both direct mechanisms (on Treg) and indirect mechanisms, secondary to tumour control (on MDSCs).

The emerging mechanisms of isoform-specific PI3K signalling.

Phosphoinositide 3-kinases (PI3Ks) function early in intracellular signal transduction pathways and affect many biological functions. A further level of complexity derives from the existence of eight PI3K isoforms, which are divided into class I, class II and class III PI3Ks. PI3K signalling has been implicated in metabolic control, immunity, angiogenesis and cardiovascular homeostasis, and is one of the most frequently deregulated pathways in cancer. PI3K inhibitors have recently entered clinical trials in oncology. A better understanding of how the different PI3K isoforms are regulated and control signalling could uncover their roles in pathology and reveal in which disease contexts their blockade could be most beneficial.

Loss of Phosphatidylinositol 3-Kinase Activity in Regulatory T Cells Leads to Neuronal Inflammation.

Class I PI3K enzymes are critical for the maintenance of effective immunity. In T cells, PI3Kα and PI3Kδ are activated by the TCR and costimulatory receptors, whereas PI3Kγ is activated by G protein-coupled chemokine receptors. PI3Kδ is a key regulator of regulatory T (Treg) cell function. PI3K isoform-selective inhibitors are in development for the treatment of diseases associated with immune dysregulation, including chronic inflammatory conditions, cancer, and autoimmune diseases. Idelalisib (PI3Kδ), alpelisib (PI3Kα), duvelisib (PI3Kδ/γ), and copanlisib (pan-PI3K) have recently been approved for use in cancer treatment. Although effective, these therapies often have severe side effects associated with immune dysregulation and, in particular, loss of Treg cells. Therefore, it is important to gain a better understanding of the relative contribution of different PI3K isoforms under homeostatic and inflammatory conditions. Experimental autoimmune encephalitis is a mouse model of T cell-driven CNS inflammation, in which Treg cells play a key protective role. In this study, we show that PI3Kδ is required to maintain normal Treg cell development and phenotype under homeostatic conditions but that loss of PI3Kδ alone in Treg cells does not lead to autoimmunity. However, combined loss of PI3Kα and PI3Kδ signaling resulted in increased experimental autoimmune encephalitis disease severity. Moreover, mice lacking PI3Kα and PI3Kδ in Treg cells developed spontaneous peripheral nerve inflammation. These results show a key role for PI3K signaling in Treg cell-mediated protection against CNS inflammation.

Oncogenic PIK3CA promotes cellular stemness in an allele dose-dependent manner.

The PIK3CA gene, which encodes the p110α catalytic subunit of PI3 kinase (PI3K), is mutationally activated in cancer and in overgrowth disorders known as PIK3CA-related overgrowth spectrum (PROS). To determine the consequences of genetic PIK3CA activation in a developmental context of relevance to both PROS and cancer, we engineered isogenic human induced pluripotent stem cells (iPSCs) with heterozygous or homozygous knockin of PIK3CAH1047R While heterozygous iPSCs remained largely similar to wild-type cells, homozygosity for PIK3CAH1047R caused widespread, cancer-like transcriptional remodeling, partial loss of epithelial morphology, up-regulation of stemness markers, and impaired differentiation to all three germ layers in vitro and in vivo. Genetic analysis of PIK3CA-associated cancers revealed that 64% had multiple oncogenic PIK3CA copies (39%) or additional PI3K signaling pathway-activating "hits" (25%). This contrasts with the prevailing view that PIK3CA mutations occur heterozygously in cancer. Our findings suggest that a PI3K activity threshold determines pathological consequences of oncogenic PIK3CA activation and provide insight into the specific role of this pathway in human pluripotent stem cells.

Pancreatic cell plasticity and cancer initiation induced by oncogenic Kras is completely dependent on wild-type PI 3-kinase p110α.

Increased PI 3-kinase (PI3K) signaling in pancreatic ductal adenocarcinoma (PDAC) correlates with poor prognosis, but the role of class I PI3K isoforms during its induction remains unclear. Using genetically engineered mice and pharmacological isoform-selective inhibitors, we found that the p110α PI3K isoform is a major signaling enzyme for PDAC development induced by a combination of genetic and nongenetic factors. Inactivation of this single isoform blocked the irreversible transition of exocrine acinar cells into pancreatic preneoplastic ductal lesions by oncogenic Kras and/or pancreatic injury. Hitting the other ubiquitous isoform, p110ß, did not prevent preneoplastic lesion initiation. p110α signaling through small GTPase Rho and actin cytoskeleton controls the reprogramming of acinar cells and regulates cell morphology in vivo and in vitro. Finally, p110α was necessary for pancreatic ductal cancers to arise from Kras-induced preneoplastic lesions by increasing epithelial cell proliferation in the context of mutated p53. Here we identify an in vivo context in which p110α cellular output differs depending on the epithelial transformation stage and demonstrate that the PI3K p110α is required for PDAC induced by oncogenic Kras, the key driver mutation of PDAC. These data are critical for a better understanding of the development of this lethal disease that is currently without efficient treatment.

Pharmacological and cell-specific genetic PI3Kα inhibition worsens cardiac remodeling after myocardial infarction.

BACKGROUND: PI3Kα (Phosphoinositide 3-kinase α) regulates multiple downstream signaling pathways controlling cell survival, growth, and proliferation and is an attractive therapeutic target in cancer and obesity. The clinically-approved PI3Kα inhibitor, BYL719, is in further clinical trials for cancer and overgrowth syndrome. However, the potential impact of PI3Kα inhibition on the heart and following myocardial infarction (MI) is unclear. We aim to determine whether PI3Kα inhibition affects cardiac physiology and post-MI remodeling and to elucidate the underlying molecular mechanisms. METHODS AND RESULTS: Wildtype (WT) 12-wk old male mice receiving BYL719 (daily, p.o.) for 10 days showed reduction in left ventricular longitudinal strain with normal ejection fraction, weight loss, mild cardiac atrophy, body composition alteration, and prolonged QTC interval. RNASeq analysis showed gene expression changes in multiple pathways including extracellular matrix remodeling and signaling complexes. After MI, both p110α and phospho-Akt protein levels were increased in human and mouse hearts. Pharmacological PI3Kα inhibition aggravated cardiac dysfunction and resulted in adverse post-MI remodeling, with increased apoptosis, elevated inflammation, suppressed hypertrophy, decreased coronary blood vessel density, and inhibited Akt/GSK3ß/eNOS signaling. Selective genetic ablation of PI3Kα in endothelial cells was associated with worsened post-MI cardiac function and reduced coronary blood vessel density. In vitro, BYL719 suppressed Akt/eNOS activation, cell viability, proliferation, and angiogenic sprouting in coronary and human umbilical vein endothelial cells. Cardiomyocyte-specific genetic PI3Kα ablation resulted in mild cardiac systolic dysfunction at baseline. After MI, cardiac function markedly deteriorated with increased mortality concordant with greater apoptosis and reduced hypertrophy. In isolated adult mouse cardiomyocytes, BYL719 decreased hypoxia-associated activation of Akt/GSK3ß signaling and cell survival. CONCLUSIONS: PI3Kα is required for cell survival (endothelial cells and cardiomyocytes) hypertrophic response, and angiogenesis to maintain cardiac function after MI. Therefore, PI3Kα inhibition that is used as anti-cancer treatment, can be cardiotoxic, especially after MI.

Sequential targeting of PI3Kδ and LAG3 as an effective anti-cancer approach.

Emerging studies have demonstrated the potential of PI3Kδ blockade as an immunotherapy for solid tumours. In pre-clinical models, we recently demonstrated that anti-LAG3 immune checkpoint blockade vastly potentiated PI3Kδ-based immunotherapy, enabling successful tumour control in all treated mice.

Uninephrectomy and class II PI3K-C2ß inactivation synergistically protect against obesity, insulin resistance and liver steatosis in mice.

Uninephrectomy (UNx) in living kidney donors for transplantation is now routine clinical practice. While chronic kidney disease, due to bilateral kidney dysfunction, is associated with insulin resistance, liver steatosis, and type 2 diabetes, the metabolic impact of UNx remains unclear. To better understand the crosstalk between the kidney and insulin target tissues, we studied the metabolic consequences of UNx and the potential involvement of class II PI3K-C2ß, the inactivation of which has been reported to result in insulin sensitization. Mice underwent UNx or sham operation followed by either normal chow or high-fat diet (HFD). Seventeen weeks post-UNx, mice showed improved glucose tolerance, insulin sensitivity, and decreased HFD-induced liver steatosis. This was associated with an enhanced serum FGF21 and insulin-stimulated Akt signaling in the liver and muscle of both lean and obese mice. Remarkably, the combination of UNx and PI3K-C2ß inactivation protected against HFD-induced obesity and further potentiated the metabolic improvement observed in WT UNx mice correlating with a synergistic increase in metabolic tissues of (1) insulin-stimulated Akt signaling (2) FGFR1 and ßKlotho expression. We demonstrated a potential beneficial effect of kidney donation and more effectively with PI3K-C2ß inactivation to protect against metabolic disorders through a mutual insulin/FGF21 sensitization.

Transient Inhibition of PI3Kδ Enhances the Therapeutic Effect of Intravenous Delivery of Oncolytic Vaccinia Virus.

Tumor-targeting oncolytic viruses such as vaccinia virus (VV) are attractive cancer therapeutic agents that act through multiple mechanisms to provoke both tumor lysis and anti-tumor immune responses. However, delivery of these agents remains restricted to intra-tumoral administration, which prevents effective targeting of inaccessible and disseminated tumor cells. In the present study we have identified transient pharmacological inhibition of the leukocyte-enriched phosphoinositide 3-kinase δ (PI3Kδ) as a novel mechanism to potentiate intravenous delivery of oncolytic VV to tumors. Pre-treatment of immunocompetent mice with the PI3Kδ-selective inhibitor IC87114 or the clinically approved idelalisib (CAL-101), prior to intravenous delivery of a tumor-tropic VV, dramatically improved viral delivery to tumors. This occurred via an inhibition of viral attachment to, but not internalization by, systemic macrophages through perturbation of signaling pathways involving RhoA/ROCK, AKT, and Rac. Pre-treatment using PI3Kδ-selective inhibitors prior to intravenous delivery of VV resulted in enhanced anti-tumor efficacy and significantly prolonged survival compared to delivery without PI3Kδ inhibition. These results indicate that effective intravenous delivery of oncolytic VV may be clinically achievable and could be useful in improving anti-tumor efficacy of oncolytic virotherapy.