ALBERT

Description: Inefficient retroviral-mediated gene transfer to human hematopoietic stem cells (HSC) and insufficient gene expression in progeny cells derived from transduced HSC are two major problems associated with HSC-based gene therapy. In this study we evaluated the ability of a murine stem cell virus (MSCV)-based retroviral vector carrying the low-affinity human nerve growth factor receptor (NGFR) gene as reporter to maintain gene expression in transduced human hematopoietic cells. CD34+ cells lacking lineage differentiation markers (CD34+Lin−) isolated from human bone marrow and mobilized peripheral blood were transduced using an optimized clinically applicable protocol. Under the conditions used, greater than 75% of the CD34+ cell population retained the Lin− phenotype after 4 days in culture and at least 30% of these expressed a high level of NGFR (NGFR+) as assessed by fluorescence-activated cell sorter analysis. When these CD34+Lin−NGFR+ cells sorted 2 days posttransduction were assayed in vitro in clonogenic and long-term stromal cultures, sustained reporter expression was observed in differentiated erythroid and myeloid cells derived from transduced progenitors, and in differentiated B-lineage cells after 6 weeks. Moreover, when these transduced CD34+Lin−NGFR+ cells were used to repopulate human bone grafts implanted in severe combined immunodeficient mice, MSCV-directed NGFR expression could be detected on 37% ± 6% (n = 5) of the donor-type human cells recovered 9 weeks postinjection. These findings suggest potential utility of the MSCV retroviral vector in the development of effective therapies involving gene-modified HSC.

Description: Non-Hodgkin’s lymphoma (NHL) is an attractive indication for the use of antibody-drug conjugate (ADC) therapeutics since NHL is responsive to conventional chemotherapeutic and antibody treatments. Additionally, experience with rituximab indicates that depletion of normal B-cells can be tolerated without untoward side effects, suggesting that B-cell specific surface proteins that are also prevalent in NHL are potential ADC targets. ADCs with stable linkers have lower toxicity than ADCs containing the same drugs with less stable linkers, however, for these ADCs to be effective the antibody must be degraded to release the active drug. This limits potential targets to surface antigens that upon antibody binding are highly internalized and subsequently degraded. We hypothesized that components of the B-cell receptor (BCR) would be excellent targets for stable-linker ADCs since, when cross-linked, the BCR is targeted to the lysosomal-like compartment MIIC as part of B-cell antigen presentation. In particular, CD79, the signaling component of the B-cell receptor, comprised of two peptide chains CD79a (Igα, mb-1) and CD79b (Igβ, B29), seemed a promising target for ADC treatment of NHL. CD79’s expression is restricted to the B-cell lineage and is on the surface of almost all NHLs. Unlike surface-Ig targeted therapies, which require a new drug for each cancer clone, CD79 sequences are invariant and a single drug is appropriate for the entire population. We found that ADCs targeted to CD79b were more effective than those targeted to CD79a. Furthermore, excellent efficacy is demonstrated with ADCs containing stable linkers and our data indicate in particular, that both anti-CD79b-MCC-DM1 and anti-CD79b-MC-MMAF are effective at low doses in subcutaneous cell-line xenograft models of follicular lymphoma, mantle cell lymphoma, Burkitt’s lymphoma and disseminated Burkitt’s lymphoma. Mechanism-of-action experiments show that anti-CD79b antibodies down regulate surface BCR expression and internalized antibody accumulates in the lysosomal-like MIIC compartment as hypothesized. We evaluated a number of ADCs targeted to other B-cell antigens and found that they were not effective if the ADC had a stable linker, indicating that CD79 has special properties as a target for ADCs. Our results suggest that anti-CD79b-MCC-DM1 and anti-CD79b-MC-MMAF are particularly promising therapeutics for the treatment of NHL.

Description: Targeting cytotoxic drugs to cancer cells using antibody–drug conjugates (ADCs), particularly those with stable linkers between the drug and the antibody, could be an effective cancer treatment with low toxicity. However, for stable-linker ADCs to be effective, they must be internalized and degraded, limiting potential targets to surface antigens that are trafficked to lysosomes. CD79a and CD79b comprise the hetrodimeric signaling component of the B-cell receptor, and are attractive targets for the use of ADCs because they are B-cell–specific, expressed in non-Hodgkin lymphomas (NHL), and are trafficked to a lysosomal-like compartment as part of antigen presentation. We show here that the stable-linker ADCs anti-CD79b-MCC-DM1 and anti-CD79b-MC-MMAF are capable of target-dependent killing of nonHodgkin lymphoma cell lines in vitro. Further, these 2 ADCs are equally effective as low doses in xenograft models of follicular, mantle cell, and Burkitt lymphomas, even though several of these cell lines express relatively low levels of CD79b in vivo. In addition, we demonstrate that anti-CD79b ADCs were more effective than anti-CD79a ADCs and that, as hypothesized, anti-CD79b antibodies downregulated surface B-cell receptor and were trafficked to the lysosomal-like major histocompatibility complex class II–positive compartment MIIC. These results suggest that anti-CD79b-MCC-DM1 and anti-CD79b-MC-MMAF are promising therapeutics for the treatment of NHL.

Description: Abstract 2721 Poster Board II-697 The role of the CD40 pathway in B-cell cancers is rather enigmatic due to having polar opposite roles in tumor biology. Stimulation of the CD40 receptor can reduce cell viability of NHL cell lines and xenograft tumor models, whereas, in sharp contrast, constitutive CD40 stimulation can actually promote proliferation of NHL cell lines, as well as drive lymphomagenesis and transformation of primary B-cells. Therefore, targeting the CD40 pathway has been challenging due to a knowledge gap in understanding the factors that may dictate a pro- or anti-tumor role. To address this directly, we employed a large panel of NHL cell lines and xenografts models to simulate a patient population and to define the molecular parameters to test in the context of clinical trials. Using baseline gene expression profiling, we found that the in vitro activity of an antibody that stimulates the CD40 pathway, Dacetuzumab (SGN-40), correlated strikingly with a transcriptional profile that was representative of an inactive CD40 pathway. Resistant cells, on the other hand, displayed a transcriptional profile indicative of an activated CD40 pathway. Consistent with these observations, constitutive phosphorylation of ERK, a node downstream of CD40 signaling, was present predominantly in cell lines that were resistant to Dacetuzumab. Fifteen genes, based on their association with in vitro activity and their biology as components of the CD40 network, were selected as a classifier from these pre-clinical data and independently tested in a panel of cell lines and xenografts models. Testing the selected gene classifier by qRT-PCR from baseline fixed tumor tissue of 39 patients treated with Dacetuzumab in phase I and II clinical trials revealed the ability to predict sensitivity in patients, as determined by tumor shrinkage 〉10%, with an overall accuracy of 80%. Consistent with the observed tumor shrinkage, the classifier positive, predicted to respond to Dacetuzumab, patients also exhibited a prolonged progression free survival (PFS) compared to classifier negative patients, predicted not to respond to Dacetuzumab (HR = 0.23, p = 0.001). Finally, using a novel microarray approach to genome-wide expression profiling on fixed tumor tissue revealed that CD40 pathway activation was indeed an important predictor of response. Overall, these data suggest that sensitivity to Dacetuzumab can be predicted by pre-determining the CD40 pathway activation status of tumors. Disclosures: Dornan: Genentech, Inc.: Employment, Equity Ownership. Burington:Genentech, Inc.: Employment, Equity Ownership. Yue:Genentech, Inc.: Employment, Equity Ownership. Shi:Genentech, Inc.: Employment, Equity Ownership. Advani:Seattle Genetics, Inc.: Research Funding. Yu:Genentech, Inc.: Employment, Equity Ownership. Lau:Genentech, Inc.: Employment, Equity Ownership. Yu:Genentech, Inc.: Employment, Equity Ownership. De Vos:Seattle Genetics, Inc.: Research Funding. Forero-Torres:Seattle Genetics, Inc.: Research Funding. Modrusan:Genentech, Inc.: Employment, Equity Ownership. Koeppen:Genentech, Inc.: Employment, Equity Ownership. Yang:Genentech, Inc.: Employment, Equity Ownership. Du:Genentech, Inc.: Employment, Equity Ownership. Elkins:Genentech, Inc.: Employment, Equity Ownership. Polson:Genentech, Inc.: Employment, Equity Ownership. Offringa:Genentech, Inc.: Employment, Equity Ownership. Whiting:Seattle Genetics, Inc.: Employment, Equity Ownership. Ebens:Genentech, Inc.: Employment, Equity Ownership.

Description: Abstract 4084 Related work from our group has shown the therapeutic utility of PIM inhibition in multiple myeloma cell lines, xenografts, and primary patient samples (Ebens A. et al., ASH 2010 submitted abstr.). In this study we provide detailed mechanistic findings to show that PIM kinase inhibition co-regulates several important elements of the PI3K/AKT/mTOR pathway, resulting in significant synergy for combination drug treatments. The PIM kinases are a family of 3 ser/thr growth factor- & cytokine-induced proteins hypothesized to have redundant survival and growth functions. GNE-652 is a pan-PIM kinase inhibitor with picomolar biochemical potencies and an excellent kinase selectivity profile. Myeloma cell lines exhibit sensitivity to single agent PIM inhibition and a striking synergy in combination with the PI3K inhibitor GDC-0941. Cells respond to this combination with cell cycle arrest and marked apoptosis in vitro. We tested a panel of selective PI3K/AKT/mTOR inhibitors and found PI3K and AKT inhibitors showed the greatest extent of synergy with GNE-652, whereas mTOR inhibitors were synergistic to a lesser extent. These results suggest that PIM signaling converges on both TORC1 and AKT to generate these differential synergies. BAD is a negative regulator of both Bcl-2 and Bcl-XL, and we were able to confirm previous reports that AKT and PIM cooperate to inactivate BAD (Datt et al., 1997; Yan et al., 2003). Pim has been shown to potentially inactivate PRAS40, a negative regulator of TORC1 (Zhang et al., 2009). We demonstrate that PIM or PI3K inhibition caused a loss of phosphorylation on PRAS40 and results in a physical association of PRAS40 and TORC1 and a decrease in phosphorylated p70S6K and S6RP. These reductions were apparent in 7 of 7 cell lines assayed and enhanced by the combination of PI3K and PIM inhibition in these cell lines. Consistent with prior reports (Hammerman et al., 2005), we show that a second node of convergence between PIM and TORC1 is 4E-BP1. Both GDC-0941 and GNE-652 treatments reduced phosphorylation of 4E-BP1 in 7 of 7 myeloma cell lines. Since dephosphorylated 4E-BP1 competes with eIF4G for the mRNA cap binding factor eIF4E, we assayed immunoprecipitates of eIF4E for the presence of eIF4G and 4E-BP1 and observed increased BP1 and decreased 4G. The combination treatment significantly enhanced the loss of 4G relative to either single agent, and importantly, even at 5× the IC50 concentrations for single agents, combination drug treatment achieved greater extent of effect than single agent treatment. Thus PI3K and PIM pathways are redundant at the level of cap-dependent translational initiation mediated by eIF4E. It has been hypothesized a subset of mRNAs are particularly sensitive to inhibition of cap-dependent translation, and that this includes a number of oncogenes such as cyclin D1. We assayed global protein synthesis in MM1.s cells using 35S-methionine and as expected we observed only a modest ≂∼f20% decrease caused by either GNE-652 or GDC-0941 and this decrease was not enhanced by combination treatment. However, we noted across 7 different myeloma cell lines, strong decreases in levels of cyclin D1 that were enhanced by combination treatment. In summary, we have identified several points at which PIM and PI3K/AKT/mTOR converge to provide synergistic apoptosis in multiple myeloma cell lines. These results provide the rationale for further preclinical development of PIM inhibitors and provide the basis for a possible clinical development plan in multiple myeloma. Disclosures: Munugalavadla: Genentech: Employment, Equity Ownership. Berry:Genentech: Employment, Equity Ownership. Chen:Genentech: Employment, Equity Ownership. Deshmukh:Genentech: Employment, Equity Ownership. Drummond:Genentech: Employment, Equity Ownership. Du:Genentech: Employment, Equity Ownership. Eby:Genentech: Employment, Equity Ownership. Fitzgerald:Genentech: Employment, Equity Ownership. S.Friedman:Genentech: Employment, Equity Ownership. E.Gould:Genentech: Employment, Equity Ownership. Kenny:Genentech: Employment, Equity Ownership. Maecker:Genentech: Employment, Equity Ownership. Moffat:Genentech: Employment, Equity Ownership. Moskalenko:Genentech: Employment, Equity Ownership. Pacheco:Genentech: Employment, Equity Ownership. Saadat:Genentech: Employment, Equity Ownership. Slaga:Genentech: Employment, Equity Ownership. Sun:Genentech: Employment, Equity Ownership. Wang:Genentech: Employment, Equity Ownership. Yang:Genentech: Employment, Equity Ownership. Ebens:Genentech Inc: Employment, Equity Ownership.

Description: Abstract 3001 PIM kinases co-regulate several important elements of the PI3K/AKT/mTOR pathway in myeloma cells (Munugalavadla V. et al., ASH 2010 submitted abstr.). In this study we show that pan-PIM inhibition suppresses growth in myeloma cell lines, xenografts, and primary patient samples, both as a single-agent as well acting synergistically in combination with PI3K, AKT, and mTOR inhibition. The PIM kinases are a family of 3 ser/thr growth factor- & cytokine-induced proteins hypothesized to have redundant survival and growth functions. Although PIM-1,2 have been noted as highly expressed in myeloma (Claudio et al., 2002), there are few data to support potential therapeutic utility of PIM inhibition in this indication. We show myeloma cell lines express all 6 PIM protein isoforms to varying extents, and we describe the properties of a novel pan-PIM inhibitor GNE-652 with picomolar biochemical potency, an excellent selectivity profile, and favorable ADME properties. Myeloma cell lines exhibit a striking prevalence of response to GNE-652 (23 of 25 lines with IC50 〈 1 micromolar, median 〈 0.1 micromolar) and synergy in combination with the PI3K inhibitor GDC-0941 (mean combination index values ~0.2 (n=25)). We used an unrelated compound GNE-568 which has a PIM-1,3 selective profile to test the hypothesis that PIM-2 may have a non-redundant role in myeloma cells. GNE-568 while having cellular potency against PIM-1 and PIM-3, did not have single agent activity in myeloma cell lines nor did it act synergistically with GDC-0941 (n=10 cell lines). Interestingly, PI3K and AKT inhibitors showed the greatest extent of synergy with GNE-652, whereas mTOR-selective inhibitors were synergistic to a lesser extent. Standard of care agents dexamethasone, revlimid, velcade, and melphalan also combined well with GNE-652, but to a lesser extent and not as broadly. The synergistic anti-tumor activity of GNE-652 and PI3K inhibitor GDC-0941 on cell lines or on primary myeloma bone marrow aspirates in vitro was associated with cell-cycle arrest and marked apoptosis. In addition, we found 4 of 4 myeloma xenograft mouse models tested with GNE-562 and GDC-0941 showed excellent combination efficacy that correlated with modulation of the expected pharmacodynamic markers. These results provide the rationale for further preclinical development of PIM inhibitors and provide the basis for a possible clinical development plan in multiple myeloma. Disclosures: Ebens: Genentech: Employment, Equity Ownership. Berry:Genentech: Employment, Equity Ownership. Chen:Genentech: Employment, Equity Ownership. Deshmukh:Genentech: Employment, Equity Ownership. Drummond:Genentech: Employment, Equity Ownership. Du:Genentech: Employment, Equity Ownership. Eby:Genentech: Employment, Equity Ownership. Fitzgerald:Genentech: Employment, Equity Ownership. S. Friedman:Genentech: Employment, Equity Ownership. E. Gould:Genentech: Employment, Equity Ownership. Kenny:Genentech: Employment, Equity Ownership. Maecker:Genentech: Employment, Equity Ownership. Moffat:Genentech: Employment, Equity Ownership. Moskalenko:Genentech: Employment, Equity Ownership. Pacheco:Genentech: Employment, Equity Ownership. Saadat:Genentech: Employment, Equity Ownership. Slaga:Genentech: Employment, Equity Ownership. Sun:Genentech: Employment, Equity Ownership. Wang:Genentech: Employment, Equity Ownership. Yang:Genentech: Employment, Equity Ownership. Munugalavadla:Genentech: Employment, Equity Ownership.

Description: Abstract 1057 Poster Board I-79 The PI3K-Akt signal transduction pathway plays a key role in the pathogenesis of many human cancers. In AML malignancy, deregulation of upstream receptor tyrosine kinases such as FLT3, c-Kit, and c-FMS or mutations in K-Ras, N-Ras, B-Raf and CEBPA genes lead to activation of PI3K-Akt signaling to promote cell survival and cell growth. A highly selective Class I PI3K inhibitor, GDC-0941, provides exciting therapeutic opportunities for targeting this pathway in AML. Here we show that GDC-0941 significantly inhibits the viability of the majority of a large panel of AML cell lines tested in vitro (80% or 19/24) at a concentration of 〈 1 uM. Because not all AML cell lines responded to GDC-0941, we show that PI3K-Akt pathway activation, evidenced by basal pAkt level, can serve as a potential predictive biomarker for GDC-0941 in AML in that sensitive cell lines displayed higher level of pAkt relative to resistant cell lines. Consistently, GDC-0941 treatment leads to decreased pAkt, and therefore the down-regulation of this important pro-survival signaling. Our further analysis shows that GDC-0941 treatment can induce apoptosis and/or cell cycle arrest. We also obtained fresh AML tumor samples to test whether GDC-0941 can similarly induces apoptosis in blast cells and showed that GDC-0941 treatment results in a down-regulation of pAkt level and increased apoptosis. Other PD biomarkers such as phospho-BAD level and Bim expression are both consistent with the observed apoptotic responses. Furthermore, the mammalian target of rapamycin complex 1 (mTORC1) inhibitor, rapamycin, synergizes with GDC-0941 to produce an increased amount of apoptosis in several AML cell lines tested. This is likely due to the fact that long-term treatment with rapamycin induces the sensitivity of the PI3K –Akt signaling pathway by releasing the negative feedback loop of mTORC1-S6K-IRS1/2 module. Importantly, in some AML cell lines we observe synergies between GDC-0941 and AraC. Interestingly, while AraC alone does not induce apoptosis in AML cell lines with PTEN loss or mutation, the synergy between AraC and GDC-0941 comes from increased apoptotic response, suggesting that GDC-0941 can synergize with chemo agents that induces S/G2 cell cycle arrest. Together, our preclinical data suggest that GDC-0941 may be used as a targeted therapy in AML patients as a single agent or in combination with other chemotherapies in clinic. Disclosures: Ma: Genentech Inc.: Employment. Du:Genentech, Inc.: Employment, Equity Ownership. Sun:Genentech Inc.: Employment. Shi:Genentech, Inc.: Employment, Equity Ownership. Friedman:Genentech Inc.: Employment. Dornan:Genentech, Inc.: Employment, Equity Ownership. Ebens:Genentech, Inc.: Employment, Equity Ownership, Patents & Royalties.

Description: Antibody-drug conjugates (ADCs), potent cytotoxic drugs linked to antibodies via specialized chemical linkers, provide a means to increase the effectiveness of chemotherapy by targeting the drug to neoplastic cells while reducing side effects. Our research has focused on ADCs that contain the maytansinoid DM1, monomethylauristatin E (MMAE), or monomethylauristatin F (MMAF), all of which are potent inhibitors of microtubule polymerization. These drugs are attached to the antibody via several different linker formats (Table 1). Table 1 Linker-drug Linker Mechanism Linked via Drug Cell Permeable PAB-vc-MC-MMAE Cleavage of peptide bond Cysteine Yes MC-MMAF Uncleavable Cysteine No SPP-DM1 Reduction of disulfide bond Lysine Yes MCC-DM1 Uncleavable Lysine Yes We identified seven cell surface proteins expressed in non-Hodgkin lymphoma (NHL) with limited expression patterns in normal tissue (CD19, CD20, CD21, CD22, CD72, CD79, and CD180), as potential therapeutic targets for ADCs. ADCs directed to any of these seven targets are effective in xenograft models when containing linkers that can be cleaved in the tumor microenvironment or when internalized by tumor cells. However, ADCs with uncleavable linkers, i.e., linkers that are active when antibody is internalized and degraded within cells, are effective only when targeted to CD22 or CD79b. This suggests that these antigens provide the desired intracellular targeting of the cytotoxic drug. Further, we demonstrated that the uncleavable-linker anti-CD22 and anti-CD79b ADCs result in complete tumor regression with no recurrence in some xenograft models of NHL. In vitro experiments revealed that sensitivity of a given cell line to the ADCs anti-CD22-MCC-DM1 and anti-CD79b-MCC-DM1, correlated more with sensitivity of the cell line to the corresponding free drug, than to the amount of surface expression of the target or the amount of internalized ADC. We conducted pilot safety studies in rats (non-binding species) and cynomolgus monkeys (binding species). In rats, ADCs with uncleavable linkers caused less target-independent hepatic and hematologic toxicity than their counter parts with cleavable linkers, likely due to their decreased systemic release of free drug. Because of these results, we evaluated three ADCs with uncleavable linkers (anti-CD22-MCC-DM1, anti-cynomolgus monkey CD79b-MCC-DM1, and anti-CD22-MC-MMAF) at ∼30 mg/kg q3 weeks for 2 doses in cynomolgus monkeys. Normal B-cells were depleted in circulation and in tissue, particularly in germinal centers of lymphoid tissues; an expected pharmacological effect. Other findings included clinically tolerated and reversible elevations in liver enzymes (MCC-DM1 and MC-MMAF conjugates) and decreases in platelet counts (MCC-DM1 conjugates); and minimal to mild sciatic nerve degeneration (MCC-DM1 conjugates). The data demonstrate that anti-CD22 and anti-CD79b ADCs with uncleavable linkers have potent efficacy in xenograft models of NHL, favorable safety profiles in pilot animal studies, and suggest that for specific targets, uncleavable linkers provide a promising mechanism to improve margins of safety in humans.

Description: Abstract 3788 Poster Board III-724 Multiple myeloma (MM) is a malignancy characterized by clonal expansion and accumulation of long-lived plasma cells within the bone marrow. Phosphatidylinositol 3' kinase (PI3K) -mediated signaling is frequently dysregulated in cancer and controls fundamental cellular functions such as cell migration, growth, survival and development of drug resistance in many cancers, including MM, and therefore represents an attractive therapeutic target. Here, we demonstrate in vitro, that a potent and selective pan-isoform PI3Kinhibitor, GDC-0941, modulates the expected pharmacodynamic markers, inhibits cell-cycle progression and induces apoptosis; overcomes resistance to apoptosis in MM cells conferred by IGF-1 and IL-6; and is additive or synergistic with current standard of care drugs including dexamethasone, melphalan, lenolidamide and bortezomib. In cell lines we find sensitivity to GDC-0941 is positively correlated with pathway activation as determined by phospho-AKT-specific flow-cytometry and Western-blot analysis. Preliminary results indicate apoptosis of MM cells is correlated with increased expression of the proapoptotic BH3-only protein BIM; mechanisms of increased apoptosis in MM will be further explored and an update presented. We further extend these in vitro findings to show that GDC-0941 has activity as a single agent in vivo and combines well with standard of care agents in several murine xenograft models to delay tumor progression and prolong survival. Our results suggest that GDC-0941 may combine well with existing therapies, providing a framework for the clinical use of this agent, and a rational approach to improving the efficacy of myeloma treatment. Disclosures: Munugalavadla: Genentech: Employment, Patents & Royalties. Berry:Genentech: Employment, Patents & Royalties. Du:Genentech, Inc.: Employment, Equity Ownership. Mariathasan:Genentech: Employment, Patents & Royalties. Slaga:Genentech: Employment, Patents & Royalties. Sun:Genentech Inc.: Employment. Chesi:Genentech, Inc.: Consultancy; Amgen: Consultancy; Celgene: Consultancy; Merck: Research Funding. Bergsagel:Genentech: Consultancy; Amgen: Consultancy; Celgene: Consultancy; Merck: Research Funding. Ebens:Genentech, Inc.: Employment, Equity Ownership, Patents & Royalties.

Description: Abstract 3787 Poster Board III-723 Acute myeloid leukemia (AML) is a biologically and molecularly heterogeneous malignancy characterized by an accumulation of myeloid-lineage cells in the blood and bone marrow. Phosphatidylinositol 3' kinase (PI3K) -mediated signaling is frequently dysregulated in cancer and controls fundamental cellular functions such as cell migration, growth, survival and development of drug resistance in many cancers, including AML, and therefore represents an attractive therapeutic target, even against the backdrop of AML disease heterogeneity. In AML, for example, activating mutations in Ras and Flt3 are common and a number of autocrine and paracrine signaling mechanisms have been proposed as important disease mediators; all converge on PI3K as an important signal transduction node to provide growth- and survival-promoting effects. Here, we demonstrate that PI3K p110-a,b,d catalytic subunits are all prevalent across a panel of ∼30 cell lines and that the majority of cell lines as well as primary patient samples show evidence of PI3K pathway activation. We demonstrate in vitro with both cell lines and patient cell isolates, that a potent and selective pan-isoform PI3K inhibitor, GDC-0941, modulates pharmacodynamic markers such as p-Akt, p-4E-BP1, and p-FOXO-3a, and that treated cells show a G0/G1 cell-cycle arrest. A strong induction of apoptosis is observed by annexin/PI staining with upregulation of cleaved caspases and PARP. Preliminary observations suggest significant induction of p27 and Bim as direct transcriptional targets of FOXO-3a, as well as downregulation of c-Myc levels; these targets may represent key mediators of observed cellular effects and are the subject of ongoing work. Additional studies are currently underway to identify clinical standard-of-care agents and new molecular entities (pipeline NMEs) that combine favorably with PI3k-inhibition in vitro, and the results of several in-vivo xenograft studies will also be presented. Disclosures: Sun: Genentech Inc.: Employment. Berry:Genentech: Employment, Patents & Royalties. Du:Genentech, Inc.: Employment, Equity Ownership. Ma:Genentech: Employment, Patents & Royalties. Shi:Genentech: Employment, Patents & Royalties. Medeiros:Genentech: Research Funding. Ebens:Genentech, Inc.: Employment, Equity Ownership, Patents & Royalties.