Discovery of 2-{3-[2-(1-isopropyl-3-methyl-1H-1,2-4-triazol-5-yl)-5,6-dihydrobenzo[f]imidazo[1,2-d][1,4]oxazepin-9-yl]-1H-pyrazol-1-yl}-2-methylpropanamide (GDC-0032): a β-sparing phosphoinositide 3-kinase inhibitor with high unbound exposure and robust in vivo antitumor activity

Dysfunctional signaling through the phosphoinositide 3-kinase (PI3K)/AKT/mTOR pathway leads to uncontrolled tumor proliferation. In the course of the discovery of novel benzoxepin PI3K inhibitors, we observed a strong dependency of in vivo antitumor activity on the free-drug exposure. By lowering the intrinsic clearance, we derived a set of imidazobenzoxazepin compounds that showed improved unbound drug exposure and effectively suppressed growth of tumors in a mouse xenograft model at low drug dose levels. One of these compounds, GDC-0032 (11l), was progressed to clinical trials and is currently under phase I evaluation as a potential treatment for human malignancies.

Abstract P4-07-01: The Class I Selective PI3K Inhibitor GDC-0941 Enhances the Efficacy of Docetaxel in Human Breast Cancer Models by Increasing the Rate of Apoptosis

Docetaxel (DTX) is commonly used as a front-line treatment option for breast cancer but many patients ultimately relapse and succumb to disease progression. Phosphatidylinositol 3-kinases (PI3K) are lipid kinases that regulate breast tumor cell growth, migration and survival. Given that phosphatidylinositol 3-kinase (PI3K) is frequently activated in breast cancer and induces chemo-resistance, it is an attractive target for combination therapy with standard of care drugs such as DTX. The current study was intended to determine if GDC-0941, an orally bioavailable class I selective PI3K inhibitor, enhances the anti-tumor activity of DTX in human breast cancer models in vitro and in vivo. A panel of 25 breast tumor cell lines representing luminal, HER2+ and basal sub-types were treated with GDC-0941, DTX or the combination of both drugs and assayed for cellular viability, modulation of PI3K pathway markers and apoptosis induction. The combination of GDC-0941 and DTX decreased the cellular viability of breast tumor cell lines in vitro but with variable drug synergy. The addition of both drugs resulted in stronger synergistic effects in a sub-set of tumor cell lines that was not predicted by breast cancer sub-type or mutations within the PI3K pathway. Human xenografts of breast cancer cell lines and patient-derived tumors were utilized to assess efficacy of GDC-0941 and DTX in vivo. In these models, the best combination efficacy was detected when the two drugs were dosed within 1 hour of each other. We also observed that GDC-0941 increased the rate of apoptosis in cells arrested in mitosis upon co-treatment with DTX. Our data provides a preclinical rationale for evaluating GDC-0941 in combination with DTX for breast cancer treatment.

Citation Information: Cancer Res 2012;72(24 Suppl):Abstract nr P4-07-01.

Enhancement of CD8+ T cell responses by ICOS/B7h costimulation

Although the recently identified ICOS/B7h costimulatory counterreceptors are critical regulators of CD4(+) T cell responses, their ability to regulate CD8(+) responses is unclear. Here we report using a tumor-rejection model that ectopic B7h expression can costimulate rejection by CD8(+) T cells in the absence of CD4(+) T cells. Although responses of naive T cells were significantly augmented by priming with B7h, B7h was surprisingly effective in mobilizing recall responses of adoptively transferred T cells. To explore why secondary responses of CD8(+) T cells were particularly enhanced by B7h, kinetics of ICOS up-regulation, proliferative responses, and cytokine production were compared from both naive and rechallenged 2C-transgenic T cells costimulated in vitro. Although B7h costimulated proliferative responses from both CD8(+) populations, rechallenged cells were preferentially costimulated for IL-2 and IFN-gamma production. These results indicate that ICOS/B7h counterreceptors likely function in vivo to enhance secondary responses by CD8(+) T cells.

B7h, a novel costimulatory homolog of B7.1 and B7.2, is induced by TNFalpha

In a screen to identify genes induced by NF-kappaB/Rel transcription factors, we cloned a novel gene, b7h, that is a close homolog of B7 costimulatory ligands expressed on antigen-presenting cells. B7h can costimulate proliferation of purified T cells through a receptor on T cells distinct from CD28 or CTLA-4. Surprisingly, although B7h is expressed in unstimulated B cells, its expression is induced in both 3T3 cells and embryonic fibroblasts treated with TNFalpha, and it is upregulated in nonlymphoid tissues of mice treated with LPS, a potent activator of TNFalpha. These data define a novel costimulatory ligand for T cells and suggest that induction of B7h by TNFalpha may function as a mechanism to directly augment recognition of self during inflammation.

B cell-specific activator protein prevents two activator factors from binding to the immunoglobulin J chain promoter until the antigen-driven stages of B cell development

The immunoglobulin J chain gene is inducibly transcribed in mature B cells upon antigen recognition and a signal from interleukin-2 (IL-2). B cell-specific activator protein (BSAP), a transcription factor that silences J chain transcription, has been identified as a nuclear target of the IL-2 signal. The levels of BSAP progressively decrease in response to IL-2 and this change correlates with the differentiation of B cells into antibody secreting plasma cells. Here we report the binding of the upstream stimulatory factor (USF) to an E-box motif immediately upstream from the BSAP site on the J chain promoter. Mutations in the USF binding motif significantly decrease J chain promoter activity in J chain expressing B cell lines. We also show that a functional relationship exists between USF and a second J chain positive-regulating factor, B-MEF2, using co-immunoprecipitation assays and transfections. Finally, we provide evidence that the binding of BSAP prevents USF and B-MEF2 from interacting with the J chain promoter during the antigen-independent stages of B cell development. It is not until the levels of BSAP decrease during the antigen-driven stages of B cell development that both USF and B-MEF2 are able to bind to their respective promoter elements and activate J chain transcription.

The transcription factor NF-kappaB/p50 interacts with the blk gene during B cell activation

The B cell-specific transcription factor Pax-5 has been shown previously to interact with the promoter of the blk gene in vitro. blk encodes a tyrosine kinase associated with the B cell receptor, which is expressed during the early but not the final stages of B cell development. To investigate whether Pax-5 regulates expression of the blk gene in vivo during B cell development and/or activation, Pax-5a was overexpressed in B cell lines. Increases in blk promoter activity using a chloramphenicol acetyltransferase reporter gene system suggested a role for Pax-5a as a transcriptional activator. Subsequent site-specific mutagenesis studies showed that mutations of the Pax-5 binding site on blk significantly alter promoter activity, although results suggested that other factors could bind to this region as well. Using mobility shift assays, we detected an inducible transcription factor that interacts strongly with a sequence overlapping the Pax-5 site on the blk promoter and identified this as a homodimer of NF-kappaB/p50, a member of the NF-kappaB/Rel family of transcription factors. This factor was present at high levels in lipopolysaccharide-activated normal B cells and in plasma cell lines but either at low levels or undetectable levels in resting normal B cells or pre-B or mature B cell lines. In contrast, lipopolysaccharide induction of a pre-B cell line (703/Z) induced a complex that contained both NF-kappaB/p50 and p65. These studies suggest that different NF-kappaB complexes are able to interact with a sequence overlapping the Pax-5 site on the blk promoter and that the relative levels of "bound" factor influence levels of blk expression. Since p50 homodimers and p50/p65 heterodimers of the NF-kappaB complex should have opposing effects on blk transcription, this could provide a mechanism to differentially regulate blk expression during B cell development and activation.

Dependence of BSAP repressor and activator functions on BSAP concentration

During a B cell immune response, the transcription factor BSAP maintains its activator functions but is relieved of its repressor functions. This selective targeting of BSAP activities was shown to be regulated by a concentration-dependent mechanism whereby activator motifs for BSAP had a 20-fold higher binding affinity than repressor motifs. An exchange of activator and repressor motifs, however, showed that the context of the motif, rather than the affinity, determined whether BSAP operated as an activator or repressor.

An interleukin-2 signal relieves BSAP (Pax5)-mediated repression of the immunoglobulin J chain gene

Cytokine regulation of B cell development was analyzed using interleukin-2 (IL-2)-induced transcription of the J chain gene as a model system. A nuclear target of the IL-2 signal was identified as the Pax5 transcription factor, BSAP, which recognizes a negative regulatory motif in the J chain promoter. Functional assays showed that BSAP mediates the silencing of the J chain gene during the early stages of B cell development, but repression is relieved during the antigen-driven stages in a concentration-dependent manner by an IL-2-induced down-regulation of BSAP RNA expression. At the low levels present in J chain-expressing plasma cells, BSAP repression could be overridden by positive-acting factors binding to down-stream J chain promoter elements. Overexpression of BSAP in these cells reversed the positive regulation and inhibited J chain gene transcription. Thus, IL-2 regulation of BSAP concentration may provide a mechanism for controlling both repressor and activator functions of BSAP during a B cell immune response.

Selective depletion of myelin-reactive T cells with the anti-OX-40 antibody ameliorates autoimmune encephalomyelitis

The OX-40 protein was selectively upregulated on encephalitogenic myelin basic protein (MBP)-specific T cells at the site of inflammation during the onset of experimental autoimmune encephalomyelitis (EAE). An OX-40 immunotoxin was used to target and eliminate MBP-specific T cells within the central nervous system without affecting peripheral T cells. When injected in vivo, the OX-40 immunotoxin bound exclusively to myelin-reactive T cells isolated from the CNS, which resulted in amelioration of EAE. Expression of the human OX-40 antigen was also found in peripheral blood of patients with acute graft-versus-host disease and the synovia of patients with rheumatoid arthritis during active disease. The unique expression of the OX-40 molecule may provide a novel therapeutic strategy for eliminating autoreactive CD4+T cells that does not require prior knowledge of the pathogenic autoantigen.

Target organ-specific up-regulation of the MRC OX-40 marker and selective production of Th1 lymphokine mRNA by encephalitogenic T helper cells isolated from the spinal cord of rats with experimental autoimmune encephalomyelitis

Lewis x Buffalo F1 rat lymphocytes express both forms of the allelic marker RT7.1 (Lewis) and RT7.2 (Buffalo). We generated myelin basic protein (MBP)-specific encephalitogenic F1 T helper cell lines and adoptively transferred them into naive irradiated Lewis recipients, which enabled us to detect and isolate donor T cells (with RT7.2) within the recipients. The spinal cord and cerebrospinal fluid (CSF) were highly enriched for the donor T cells compared with the blood and spleen. The donor cell number peaked on the first day of disease in the spinal cord and CSF and decreased as the disease progressed. A high percentage of the donor T cells isolated from the spinal cord were positive for the T helper cell activation marker OX-40, whereas a (lower) percentage of CSF donor cells expressed OX-40. Donor cells isolated from blood or spleen were negative for OX-40 expression. In contrast, the IL-2 receptor (CD25) was positive on all the transferred T cells in all tissue sites examined. Cell-sorting experiments showed that the MBP-specific donor cells were enriched for IFN-gamma, IL-2, TNF-alpha, and IL-3 mRNA when compared with the host-recruited spinal cord cells, whereas similar amounts of IL-10 mRNA were produced by both populations. Lymphokine mRNA production was also enriched in donor T cells isolated from the spinal cord compared with donor T cells isolated from the spleen. The spinal cord donor cells produced higher levels of IL-2, IFN-gamma, and IL-3 mRNA, whereas similar amounts of IL-10 and TNF-alpha mRNA were produced from donor cells isolated from the spleen and the spinal cord. Our data suggest that the amount/percentage, activation state, and enhanced lymphokine production at the site of inflammation are all important factors in determining the autoimmune potential of Ag-specific effector T helper cells.

Target organ-specific up-regulation of the MRC OX-40 marker and selective production of Th1 lymphokine mRNA by encephalitogenic T helper cells isolated from the spinal cord of rats with experimental autoimmune encephalomyelitis

Lewis x Buffalo F1 rat lymphocytes express both forms of the allelic marker RT7.1 (Lewis) and RT7.2 (Buffalo). We generated myelin basic protein (MBP)-specific encephalitogenic F1 T helper cell lines and adoptively transferred them into naive irradiated Lewis recipients, which enabled us to detect and isolate donor T cells (with RT7.2) within the recipients. The spinal cord and cerebrospinal fluid (CSF) were highly enriched for the donor T cells compared with the blood and spleen. The donor cell number peaked on the first day of disease in the spinal cord and CSF and decreased as the disease progressed. A high percentage of the donor T cells isolated from the spinal cord were positive for the T helper cell activation marker OX-40, whereas a (lower) percentage of CSF donor cells expressed OX-40. Donor cells isolated from blood or spleen were negative for OX-40 expression. In contrast, the IL-2 receptor (CD25) was positive on all the transferred T cells in all tissue sites examined. Cell-sorting experiments showed that the MBP-specific donor cells were enriched for IFN-gamma, IL-2, TNF-alpha, and IL-3 mRNA when compared with the host-recruited spinal cord cells, whereas similar amounts of IL-10 mRNA were produced by both populations. Lymphokine mRNA production was also enriched in donor T cells isolated from the spinal cord compared with donor T cells isolated from the spleen. The spinal cord donor cells produced higher levels of IL-2, IFN-gamma, and IL-3 mRNA, whereas similar amounts of IL-10 and TNF-alpha mRNA were produced from donor cells isolated from the spleen and the spinal cord. Our data suggest that the amount/percentage, activation state, and enhanced lymphokine production at the site of inflammation are all important factors in determining the autoimmune potential of Ag-specific effector T helper cells.