Phosphoinositide 3-kinase Enzymes Research Articles

PI3K Mutation Profiles on Exons 9 (E545K and E542K) and 20 (H1047R) in Mexican Patients With HER-2 Overexpressed Breast Cancer and Its Relevance on Clinical-Pathological and Survival Biological Effects.

Background: Trastuzumab resistance is associated with overexpressing the human epidermal growth factor receptor 2 (HER-2), which results from the altered phosphoinositide 3-kinase (PI3K) pathway in breast cancer patients. Objective: We quantified the frequency of PI3K enzyme single and double-point mutations in Mexican patients with HER-2 overexpressing breast cancer and its association with clinical-pathological variables. Methods: We embedded HER-2 breast samples in paraffin from 60 patients, extracted their DNA, and evaluated PI3K mutations in 49 HER-2-positive breast tumors. We focused on mutations for one exon 20 (H1047R) and two exon 9 PI3K (E545K, E542K) hotspots and characterized them as single and double-point mutations. The mean patient follow-up was 86 months. Results: Of 49 patients who tested positive for HER-2 breast cancer, 14.28% showed mutations in PI3K, 71.42% single-point, and 28.56% double-point mutations. We found single-point mutations in H1047R (42.85%) and E545K (28.57%). Only two patients exhibited double-point mutations: one in E542K/E545K and another in H1047R/E545K (14.28% each). Although we observed lower survival in patients with mutations in PI3K, we did not find a significant association between these factors (p = 0.191). However, single and double-point mutations in PI3K were significantly associated with the clinical stages of diagnosis and tumor size (p = 0.027 and p = 0.04, respectively). Conclusion: Single and double-point mutations in PI3K are related to tumor size and advanced clinical-pathological traits in Mexican patients with HER-2 overexpression, and future molecular studies are necessary to understand these findings.

Discovery of Novel Phosphoinositide-3-Kinase α Inhibitors with High Selectivity, Excellent Bioavailability, and Long-Acting Efficacy for Gastric Cancer.

Phosphoinositide-3-kinase (PI3K) overexpressed in many tumors is a promising target for cancer therapy. However, due to toxicity from the ubiquitous expression of PI3K in many tissues, the development of PI3K inhibitors with high selectivity and low toxicity has become an urgent need for tumor treatment. Herein, based on the HipHop, we designed and synthesized a series of 6-(4,6-dimorpholino-1,3,5-triazin-2-yl)benzo[d]oxazol-2-amine derivatives as potent, selective, and long-acting PI3Kα inhibitors. Compound 27 was determined with potent PI3Kα inhibitory activity (IC50 = 4.4 nM), which exhibited excellent selectivity for homologous PI3K enzymes and a 370 kinome panel. Meanwhile, 27 featured favorable stability (T1/2 > 10 h) and high bioavailability (130%). Importantly, compound 27 exerted great antigastric cancer activity in vivo when combined with taxol. Collectively, these characteristics suggested 27 to be a promising PI3K agent for cancer treatment.

Insights into the value of statistical models, solvent, and relativistic effects for investigating Re complexes of 2-(4'-aminophenyl)benzothiazole: a potential spectroscopic probe.

Cancer affects a major part of the worldwide population, and, to minimize deaths, the diagnosis in the early stages of the disease is fundamental. Thus, to improve diagnosis and treatment new potential spectroscopic probes are crucial. Benzothiazole derivates present antitumor properties and are highly selective and interact strongly with the enzyme phosphoinositide 3-kinase (PI3K), which was associated with cell proliferation and breast cancer cells. In this paper, the rhenium shielding tensors (187Re(σ)) and hydrogen and carbon chemical shifts (1H(δ) and 13C(δ)) of the Re(CO)3(NNO) complex conjugated with 2-(4'-aminophenyl)benzothiazole (ReABT) were evaluated. A statistical HCA model was used to analyze the best DFT protocol to compute σ and δ values and to evaluate the relativistic effects, both in the basis set and Hamiltonian as well as the functionals M06L or PBE0. The best protocol was applied to obtain 187Re(σ) of the ReABT complex in different environments (gas phase, solution, and in the active site of the PI3K enzyme). The results point out that 187Re(σ) values of the ReABT complex change significantly when the complex is docked in the PI3K enzyme.

Novel heterozygous PIK3CD mutation presenting with only laboratory markers of combined immunodeficiency

Introduction: Phosphatidylinositol-4,5-Bisphosphate 3-Kinase Catalytic Subunit Delta (PIK3CD) is one part of a heterodimer forming the enzyme phosphoinositide 3-kinase (PI3K), found primarily in leukocytes. PIK3CD generates phosphatidyl-inositol 3,4,5-trisphosphate (PIP3), and is involved in cell growth, survival, proliferation, motility, and morphology. An increasing number of patients have been described with heterozygous PIK3CD gain-of-function (GOF) mutations, leading to combined immunodeficiency with both B- and T-cell dysfunction. Patients suffer recurrent respiratory infections, often associated with bronchiectasis and ear and sinus damage, as well as severe recurrent or persistent infections by herpesviruses, including EBV-induced lymphoproliferation. Aim: To present the clinical phenotypic variability of a novel PI3KCD mutation within a family. Methods: Patient information was collected prospectively and retrospectively from medical records. Comprehensive immune work up, genetic, and signaling evaluation was performed. Results: We describe here 2 patients, daughter and mother, with heterozygous PIK3CD mutation identified by whole exome sequencing and Sanger confirmation. The child was screen-positive by newborn screening for severe combined immunodeficiency (SCID). Cellular assays revealed an increase in the baseline phosphorylation of T cells in the patient. Furthermore, both patients had hyper-activation of the catalytic domain, resulting in increased phosphorylation of AKT upon activation. Discussion: GOF mutations affecting the PIK3CD gene are associated with an increased risk for lymphoproliferation leading to Activated PIK3-delta syndrome (APDS). The clinical course of APDS is highly variable, ranging from combined immunodeficiency with recurrent infections, autoimmune complications, and requiring stem cell transplantation, through isolated antibody deficiency, to asymptomatic adults. Our patient is the first to be identified by newborn screening for SCID. Surprisingly, the clinical course has so far been unremarkable, as well, the mother appears to be completely asymptomatic. Nevertheless, the persistent lymphopenia indicates PIK3CD dysfunction. Because of the wide gap between laboratory findings and clinical manifestations, this kindred poses both a diagnostic as well treatment challenge. Statement of novelty: We report here a novel PIK3CD mutation diagnosed due to abnormal newborn screen for SCID.

Development of Multi-Target Chemometric Models for the Inhibition of Class I PI3K Enzyme Isoforms: A Case Study Using QSAR-Co Tool

The present work aims at establishing multi-target chemometric models using the recently launched quantitative structure–activity relationship (QSAR)-Co tool for predicting the activity of inhibitor compounds against different isoforms of phosphoinositide 3-kinase (PI3K) under various experimental conditions. The inhibitors of class I phosphoinositide 3-kinase (PI3K) isoforms have emerged as potential therapeutic agents for the treatment of various disorders, especially cancer. The cell-based enzyme inhibition assay results of PI3K inhibitors were curated from the CHEMBL database. Factors such as the nature and mutation of cell lines that may significantly alter the assay outcomes were considered as important experimental elements for mt-QSAR model development. The models, in turn, were developed using two machine learning techniques as implemented in QSAR-Co: linear discriminant analysis (LDA) and random forest (RF). Both techniques led to models with high accuracy (ca. 90%). Several molecular fragments were extracted from the current dataset, and their quantitative contributions to the inhibitory activity against all the proteins and experimental conditions under study were calculated. This case study also demonstrates the utility of QSAR-Co tool in solving multi-factorial and complex chemometric problems. Additionally, the combination of different in silico methods employed in this work can serve as a valuable guideline to speed up early discovery of PI3K inhibitors.

PI3K Signaling in Chronic Obstructive Pulmonary Disease: Mechanisms, Targets, and Therapy.

Chronic Obstructive Pulmonary Disease (COPD) is a progressive respiratory disorder characterized by irreversible chronic inflammation and airflow obstruction. It affects more than 64 million patients worldwide and it is predicted to become the third cause of death in the industrialized world by 2030. Currently available therapies are not able to block disease progression and to reduce mortality, underlying the need for a better understanding of COPD pathophysiological mechanisms to identify new molecular therapeutic targets. Recent studies demonstrated that phosphoinositide 3-kinase (PI3K) signaling is prominently activated in COPD and correlates with an increased susceptibility of patients to lung infections. PI3Ks have thus emerged as promising alternative drug targets for COPD and a wide array of pan-isoform and isoform-selective inhibitors have been tested in preclinical models and are currently being evaluated in clinical studies. Here, we summarize the recent knowledge on the involvement of PI3K enzymes in the pathophysiology of COPD, and we discuss the most recent results arising from the preclinical as well as the clinical testing of PI3K inhibitors as novel therapeutics for COPD.

The Specificity of EGF-Stimulated IQGAP1 Scaffold Towards the PI3K-Akt Pathway is Defined by the IQ3 motif

Epidermal growth factor receptor (EGFR) and its downstream phosphoinositide 3-kinase (PI3K) pathway are commonly deregulated in cancer. Recently, we have shown that the IQ motif-containing GTPase-activating protein 1 (IQGAP1) provides a molecular platform to scaffold all the components of the PI3K-Akt pathway and results in the sequential generation of phosphatidylinositol-3,4,5-trisphosphate (PI3,4,5P3). In addition to the PI3K-Akt pathway, IQGAP1 also scaffolds the Ras-ERK pathway. To define the specificity of IQGAP1 for the control of PI3K signaling, we have focused on the IQ3 motif in IQGAP1 as PIPKIα and PI3K enzymes bind this region. An IQ3 deletion mutant loses interactions with the PI3K-Akt components but retains binding to ERK and EGFR. Consistently, blocking the IQ3 motif of IQGAP1 using an IQ3 motif-derived peptide mirrors the effect of IQ3 deletion mutant by reducing Akt activation but has no impact on ERK activation. Also, the peptide disrupts the binding of IQGAP1 with PI3K-Akt pathway components, while IQGAP1 interactions with ERK and EGFR are not affected. Functionally, deleting or blocking the IQ3 motif inhibits cell proliferation, invasion, and migration in a non-additive manner to a PIPKIα inhibitor, establishing the functional specificity of IQ3 motif towards the PI3K-Akt pathway. Taken together, the IQ3 motif is a specific target for suppressing activation of the PI3K-Akt but not the Ras-ERK pathway. Although EGFR stimulates the IQGAP1-PI3K and -ERK pathways, here we show that IQGAP1-PI3K controls migration, invasion, and proliferation independent of ERK. These data illustrate that the IQ3 region of IQGAP1 is a promising therapeutic target for PI3K-driven cancer.

Class I Phosphoinositide 3-Kinase PIK3CA/p110α and PIK3CB/p110β Isoforms in Endometrial Cancer.

The phosphoinositide 3-kinase (PI3K) signalling pathway is highly dysregulated in cancer, leading to elevated PI3K signalling and altered cellular processes that contribute to tumour development. The pathway is normally orchestrated by class I PI3K enzymes and negatively regulated by the phosphatase and tensin homologue, PTEN. Endometrial carcinomas harbour frequent alterations in components of the pathway, including changes in gene copy number and mutations, in particular in the oncogene PIK3CA, the gene encoding the PI3K catalytic subunit p110α, and the tumour suppressor PTEN. PIK3CB, encoding the other ubiquitously expressed class I isoform p110β, is less frequently altered but the few mutations identified to date are oncogenic. This isoform has received more research interest in recent years, particularly since PTEN-deficient tumours were found to be reliant on p110β activity to sustain transformation. In this review, we describe the current understanding of the common and distinct biochemical properties of the p110α and p110β isoforms, summarise their mutations and highlight how they are targeted in clinical trials in endometrial cancer.

Idelalisib.

Idelalisib (GS-1101, CAL-101, Zydelig®) is an orally bioavailable, small-molecule inhibitor of the delta isoform (p110δ) of the enzyme phosphoinositide 3-kinase (PI3K). In contrast to the other PI3K isoforms, PI3Kδ is expressed selectively in hematopoietic cells. PI3Kδ signaling is active in many B-cell leukemias and lymphomas. By inhibiting the PI3Kδ protein, idelalisib blocks several cellular signaling pathways that maintain B-cell viability. Idelalisib is the first PI3K inhibitor approved by the US Food and Drug Administration (FDA). Treatment with idelalisib is indicated in relapsed/refractory chronic lymphocytic leukemia (CLL), follicular lymphoma (FL), and small lymphocytic lymphoma (SLL). This review presents the preclinical and clinical activity of idelalisib with a focus on clinical studies in CLL.

The down-regulation of TAPP2 inhibits the migration of esophageal squamous cell carcinoma and predicts favorable outcome

Tandem pH domain-containing proteins TAPP1 and TAPP2 are adaptor proteins that specifically bind to phosphatidylinositol-3,4-bisphosphate, or PI(3,4)P2, a product of phosphoinositide 3-kinases (PI3K). Although PI3K enzymes have multiple functions in cell biology, including cell migration, the functions of PI (3, 4) P2 and its binding proteins are not well understood. Previously studies found that TAPP2 is highly expressed in primary leukemic B cells that have strong migratory capacity. However, the function and underlying mechanisms of TAPP2 in ESCC remain largely unknown. In the present study, we investigated the level of TAPP2 in human esophageal squamous cell carcinoma (ESCC) tissues and in corresponding adjacent non-tumor tissues by immunohistochemistry (IHC) and western blot analyses. TAPP2 protein level was increased in ESCC tissues compared with corresponding adjacent non-tumor tissues. In vitro experiments showed that under-expression of TAPP2 reduced ESCC cell TE1 migration by wound-healing assays and transwell migration assays, and it was concurrent with the decreased expression of the phosphorylation of AKT. Taken together, these findings suggested that TAPP2 serves as oncogenic gene in ESCC and may serve as a new target for ESCC therapy.

PI3K Signaling in Tissue Hyper-Proliferation: From Overgrowth Syndromes to Kidney Cysts.

The members of the PhosphoInositide-3 Kinase (PI3K) protein family are well-known regulators of proliferative signals. By the generation of lipid second messengers, they mediate the activation of AKT/PKB (AKT) and mammalian Target Of Rapamycin (mTOR) pathways. Although mutations in the PI3K/AKT/mTOR pathway are highly characterized in cancer, recent evidence indicates that alterations in the proliferative signals are major drivers of other diseases such as overgrowth disorders and polycystic kidney disease. In this review, we briefly summarize the role of the PI3K/AKT/mTOR pathway in cell proliferation by comparing the effect of alterations in PI3K enzymes in different tissues. In particular, we discuss the most recent findings on how the same pathway may lead to different biological effects, due to the convergence and cooperation of different signaling cascades.

Optimization of the phenylurea moiety in a phosphoinositide 3-kinase (PI3K) inhibitor to improve water solubility and the PK profile by introducing a solubilizing group and ortho substituents

Phosphoinositide 3-kinase (PI3K) is a promising anti-cancer target, because various mutations and amplifications are observed in human tumors isolated from cancer patients. Our dihydropyrrolopyrimidine derivative with a phenylurea moiety showed strong PI3K enzyme inhibitory activity, but its pharmacokinetic property was poor because of lack of solubility. Herein, we report how we improved the solubility of our PI3K inhibitors by introducing a solubilizing group and ortho substituents to break molecular planarity.

Anti-tumour-promoting and thermal-induced protein denaturation inhibitory activities of β-sitosterol and lupeol isolated from Diospyros lotus L.

In this study, the anti-tumour-promoting and thermal-induced protein denaturation inhibitory activities of β-sitosterol (1) and lupeol (2), isolated from Diospyros lotus L., were explored. Compound 1 showed a marked concentration-dependent inhibition against 12-O-tetradecanoylphorbol-13-acetate (20 ng/32 pmol)-induced Epstein–Barr virus early antigen activation in Raji cells with IC50 of 270 μg/ml, without significant toxicity (70% viability). Compound 2 showed significant anti-tumour-promoting effect with IC50 of 412 μg/ml, without significant toxicity (60% viability). In heat-induced protein denaturation assay, compound 1 exhibited a concentration-dependent attenuation with a maximum effect of 73.5% at 500 μg/ml with EC50 of 117 μg/ml, while compound 2 exhibited a maximum effect of 59.2% at 500 μg/ml with EC50 of 355 μg/ml. Moreover, in silico docking studies against the phosphoinositide 3-kinase enzyme also show the inhibitory potency of these compounds. In short, both the compounds exhibited a marked anti-tumour-promoting and potent inhibitory effect on thermal-induced protein denaturation.

Heterozygous expression of the oncogenic Pik3caH1047R mutation during murine development results in fatal embryonic and extraembryonic defects

The phosphoinositide 3-kinase (PI3K)/AKT signalling pathway regulates many cellular functions including proliferation, migration, survival and protein synthesis. Somatic mutations in PIK3CA, the gene encoding the p110α catalytic subunit of PI3K enzyme, are commonly associated with many human cancers as well as recently being implicated in human overgrowth syndromes. However, it is not clear if such mutations can be inherited through the germline. We have used a novel mouse model with Cre recombinase (Cre)-conditional knock-in of the common H1047R mutation into the endogenous Pik3ca gene. Heterozygous expression of the Pik3caH1047R mutation in the developing mouse embryo resulted in failed ‘turning’ of the embryo and disrupted vascular remodelling within the embryonic and extraembryonic tissues, leading to lethality prior to E10. As vascular endothelial growth factor A (VEGF-A) signalling was disrupted in these embryos, we used Cre under the control of the Tie2 promoter to target the Pik3caH1047R mutation specifically to endothelial cells. In these embryos turning occurred normally but the vascular remodelling defects and embryonic lethality remained, likely as a result of endothelial hyperproliferation. Our results confirm the lethality associated with heterozygous expression of the Pik3caH1047R mutation during development and likely explain the lack of inherited germline PIK3CA mutations in humans.

Editorial: Lipid Kinases and Bone Homeostasis: Lessons Learned From Phosphoinositide 3‐Kinase Isoform–Specific Knockouts

Bone homeostasis is maintained throughout life by coordinated osteocyte regulation of bone resorption by osteoclasts and bone formation by osteoblasts. Many pathologic bone states such as osteoporosis, inflammatory arthritis, and bone metastasis have increased numbers, survival, or function of osteoclasts resulting in enhanced bone resorption outpacing bone formation. Differentiation, survival, recruitment, and resorptive activity of osteoclasts and their precursors are highly regulated by intracellular signals stimulated by soluble mediators, extracellular matrix, and adjacent cells. In this issue of Arthritis & Rheumatology, Gyoőri and colleagues describe a specific role for the enzyme phosphoinositide 3-kinase β (PI3Kβ) in the regulation of osteoclast-mediated bone resorption (1), suggesting that a specific inhibitor of this enzyme isoform may be a novel antiresorptive agent.

The tandem PH domain-containing protein 2 regulates chemokine-induced cytoskeletal reorganization and malignant B cell migration (P5194)

Abstract The tandem PH domain-containing protein 2 (TAPP2), along with its homologue TAPP1, is best known for high-specificity binding to PI(3,4)P2, a lipid product of phosphoinositide 3-kinases (PI3K). While PI3K enzymes have a number of functions in cell biology, including cell migration, the functions of PI(3,4)P2 and its binding proteins are not well understood. Previously we found that TAPP2 is highly expressed in leukemic B cells with strong migratory capacity. Here we show that the migration of human malignant B cells is significantly inhibited by TAPP2 knockdown (KD), manifested as reduction in percentage of migrating cells, migration velocity and directionality. TAPP2 KD led to alterations in chemokine-induced rearrangement of the actin cytoskeleton and failure to form polarized morphology. TAPP2 co-localized with the stable F-actin-binding protein utrophin, with both molecules reciprocally localizing against F-actin accumulated at the leading edge. In TAPP2 KD cells, Rac was over-activated and localized to multiple membrane protrusions, suggesting that TAPP2 may act in concert with utrophin and stable F-actin to spatially restrict Rac activation and reduce formation of multiple membrane protrusions. In summary, this study identified TAPP2 as a novel regulator of malignant B cell migration and a potential therapeutic intervention target.

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The Tandem PH Domain-Containing Protein 2 (TAPP2) Regulates Chemokine-Induced Cytoskeletal Reorganization and Malignant B Cell Migration

The intracellular signaling processes controlling malignant B cell migration and tissue localization remain largely undefined. Tandem PH domain-containing proteins TAPP1 and TAPP2 are adaptor proteins that specifically bind to phosphatidylinositol-3,4-bisphosphate, or PI(3,4)P2, a product of phosphoinositide 3-kinases (PI3K). While PI3K enzymes have a number of functions in cell biology, including cell migration, the functions of PI(3,4)P2 and its binding proteins are not well understood. Previously we found that TAPP2 is highly expressed in primary leukemic B cells that have strong migratory capacity. Here we find that SDF-1-dependent migration of human malignant B cells requires both PI3K signaling and TAPP2. Migration in a transwell assay is significantly impaired by pan-PI3K and isoform-selective PI3K inhibitors, or by TAPP2 shRNA knockdown (KD). Strikingly, TAPP2 KD in combination with PI3K inhibitor treatment nearly abolished the migration response, suggesting that TAPP2 may contribute some functions independent of the PI3K pathway. In microfluidic chamber cell tracking assays, TAPP2 KD cells show reduction in percentage of migrating cells, migration velocity and directionality. TAPP2 KD led to alterations in chemokine-induced rearrangement of the actin cytoskeleton and failure to form polarized morphology. TAPP2 co-localized with the stable F-actin-binding protein utrophin, with both molecules reciprocally localizing against F-actin accumulated at the leading edge upon SDF-1 stimulation. In TAPP2 KD cells, Rac was over-activated and localized to multiple membrane protrusions, suggesting that TAPP2 may act in concert with utrophin and stable F-actin to spatially restrict Rac activation and reduce formation of multiple membrane protrusions. TAPP2 function in cell migration is also apparent in the more complex context of B cell migration into stromal cell layers – a process that is only partially dependent on PI3K and SDF-1. In summary, this study identified TAPP2 as a novel regulator of malignant B cell migration and a potential therapeutic intervention target.

Activation of the neutrophil NADPH oxidase by Aspergillus fumigatus

Upon infection of the respiratory system with the fungus Aspergillus fumigatus various leukoctytes, in particular neutrophils, are recruited to the lung to mount an immune response. Neutrophils respond by both phagocytosing conidia and mediating extracellular killing of germinated, invasive hyphae. Of paramount importance to an appropriate immune response is the neutrophil NADPH oxidase enzyme, which mediates the production of various reactive oxygen species (ROS). This is evidenced by the acute sensitivity of both oxidase-deficient humans and mice to invasive aspergillosis. Herein we briefly review the mechanisms and functions of oxidase activation and discuss our recent work identifying at least some of the important players in hyphal-induced oxidase activation and neutrophil function. Among these we define the phosphoinositide 3-kinase enzyme and the regulatory protein Vav to be of critical importance and allude to a kinase-independent role for Syk.

Phosphoinositide 3'-Kinase (PI3K) Delta Inhibitor, GS-9820, Enhances Anti-Tumor Efficacy When Combined with Conventional Chemotherapeutic Agents in Primary Samples and Human Tumor Xenograft Model of Multiple Myeloma

AbstractAbstract 4017Phosphatidylinositide 3-kinases (PI3K) are a family of lipid kinases that are involved in signaling events which control a diverse number of cellular processes. The class I PI3Ks contain 4 isoforms designated p110a, b, d, g, and are activated by cell surface receptors. Aberrant regulation of the PI3K signaling pathway is frequently observed in a wide range of human malignancies including those of hematological origin and has been shown to play an important role in tumor progression. GS-9820 is a highly selective and potent p110d inhibitor (IC50 of 14 nM) with 114- to 400-fold selectivity over the other class I PI3K enzymes and no activity against Class II and III PI3K family members or other PI3K-related proteins including mTOR and DNA-PK. Although a role for PI3Kd has been shown in multiple myeloma (MM), GS-9820 has not been previously evaluated in combination with anti-MM agents in primary MM tumor cells. Hence, we determined the effects of GS-9820 as a single agent and in combination with melphalan, dexamethasone, doxorubicin, or bortezomib on primary MM cells. GS9820 alone inhibited MM tumor cell proliferation in a concentration-dependent fashion with a 50% growth inhibition (IC50) of primary MM cells between 1–10 μM. Notably, combinations of GS-9820 (100nM-1μM) and melphalan (10 μM) or dexamethasone (1 μM) significantly decreased cellular proliferation (P < 0.05), compared to either drug alone in all 6 MM BMMC samples. GS-9820 was synergistic when combined with doxorubicin on primary MM cells from two of these patients’ samples. Similarly, this PI3K inhibitor synergized when it was combined with bortezomib on primary MM cells from two of these patients. Using our LAGk-2 human tumor xenograft model, the combination of GS-9820 and melphalan provided greater growth inhibition than the individual drugs alone. Specifically on day 42 post-tumor implantation, LAGk-2-bearing mice treated with single agent GS-9820 did not show a significant reduction in tumor growth compared to vehicle-treated mice. In contrast, melphalan alone resulted in a significant decrease in tumor size when compared to vehicle-treated mice at this same time point (P = 0.0302). Importantly, the combination of GS-9820 plus melphalan resulted in statistically significantly smaller tumors when compared to melphalan alone (P = 0.0495), GS-9820 alone (P = 0.0061) and vehicle-control (P = 0.0025). In this study, we demonstrate that the combination of GS-9820 with melphalan or dexamethasone markedly inhibits primary MM cell proliferation in vitro. Furthermore, the combination of GS-9820 and melphalan shows enhanced anti-MM effects compared to single agent treatment in vivo using theLAGk-2 human xenograft MM model. The results from these studies suggest that the combination of the PI3K inhibitor, GS-9820, and other anti-MM drugs may be an effective treatment for MM patients. Disclosures:Johnson:Gilead Sciences: Employment. Lannutti:Gilead Sciences: Employment.