Abstract 2517: Optimizing breast cancer therapy by inhibiting the adenosine receptor and oxygen consumption

Immune checkpoint blockade has shown remarkable promise in melanoma and other tumor types. However, a large proportion of patients do not respond or develop acquired resistance. Tumors can activate multiple checkpoints and immunosuppressive pathways to evade immune surveillance, which are likely a cause of treatment failure. Therefore, inhibition of multiple checkpoints may be necessary for maximal efficacy of immunotherapies. The adenosine A2 receptor (A2aR), was shown to function as an immune checkpoint. Its blockade inhibited tumor growth and metastases and synergized with other checkpoint inhibitors. We propose that using AZD4635, a potent and selective A2aR antagonist, will enhance the therapeutic efficacy of anti-PD-1/PD-L1. In addition, we propose that blocking tumor oxygen consumption using deferiprone (DFP), phenformin (Phen) and metformin (Met) will further enhance A2aR and PD-1/PD-L1 blockade efficacy. We evaluated the effects of MET, Phen, DFP, and AZD4635 on tumor cells, T cell, and macrophage proliferation and effector function in vitro. IC50 measurements showed that T cells are the most sensitive to inhibition by these drugs, while 4T1 tumor cells appear to be the least sensitive. Naïve mouse T cells were activated with anti-CD3 and anti-CD28 coated dynabeads in the presence of titrating doses of all four drugs for 72 hours then analyzed by flow cytometry for proliferation and activation status. AZD4635 and DFP increased the expression of the activation marker CD25 (IL2Ra) on CD8 T cells. In addition, AZD4635 also increased expression of Granzyme B on both CD4 and CD8 T cells. Met appears to have little to no effect on T cell proliferation activation. We utilized the Seahorse XF Mito Stress Test assay to measure the mitochondrial respiratory activity of T cells in the presence these drugs. T cells were activated with dynabeads for 72 hours and then incubated with titrating doses of the drugs overnight before running on the Seahorse assay. DFP and Phen had the strongest effect on Maximal Respiration and Spare Respiratory Capacity at dilutions of 1uM followed by 16uM. A similar, albeit less profound effect was observed in groups treated with Met and AZD4635. Lastly, we obtained and generated two mouse breast cancer cell lines (4T1 and E0771) bearing HIF-1 reporter systems. We treated 4T1-HRE and E0771 cells with increasing doses of cobalt chloride (CoCl2) to chemically mimic hypoxia by inducing HIF1a expression. Both 4T1-HRE and E0771-HRE demonstrated increased luciferase activity that correlated with increasing doses of CoCl2 in vitro. Future experiments will focus on characterizing the adenosine pathway in vivo in 4T1-HRE and E0771-HRE tumors and examine how drugs that target the adenosine A2AaR receptor (AZD4635), and oxygen consumption (DFP, Phen and Met) influence tumor oxygen consumption in vivo as well as the activation states of immune cells in the tumor microenvironment.

Citation Format: Sadna Budhu, Mayuresh Mane, Mamadou A. Bah, Juan Zurita, Inna Serganova, Soe Min, Anais Assouvie, Jedd D. Wolchok, Jason Koutcher, Vladimir Ponomarev, Taha Merghoub. Optimizing breast cancer therapy by inhibiting the adenosine receptor and oxygen consumption [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 2517.

Abstract 5873: LDH inhibition boosts effector T cells while destabilizing regulatory T cells and improves responses to CTLA-4 blockade

Tumor reliance on glycolysis is a hallmark of cancer and a mechanism of resistance to immunotherapy. This resistance is due to lactate-mediated immune suppression and competition for glucose between T cells and tumor cells within the tumor microenvironment. We have shown that CTLA-4 blockade is more effective in glycolysis-low tumors, or tumors lacking functional lactate dehydrogenase A (LDH-A), primarily due to functional destabilization of regulatory T cell suppression. LDH inhibitors (LDHi) have been reported to inhibit tumor glucose uptake and slow tumor cell proliferation in pre-clinical models of cancer. However, their effect on immune cells has not been explored in depth. In addition, the optimal conditions for pharmacological inhibition of LDH in combination with immunotherapy to maximize anti-tumor immune and therapeutic responses require further investigation. At baseline, tumor cells express higher levels of ldha and consume more glucose than tumor-infiltrating T cells, creating a therapeutic window for tumor-specific targeting of the glycolysis pathway. In vivo, LDHi relies on the adaptive immune system and the overexpression of tumor LDH to delay B16F10 murine melanoma progression. We found that treatment with LDHi has two effects: 1) reduction of tumor cell glucose uptake and 2) increase in glucose uptake by tumor-infiltrating T cells. Thus, LDH inhibition is an effective, tumor-specific strategy to reduce tumor cell glucose uptake and increase glucose availability within the tumor microenvironment, consequently boosting tumor-infiltrating T cell glucose uptake. In vitro, increased glucose levels improve effector T cell killing of tumor cells while reducing regulatory T cell suppressive ability. Accordingly, inhibiting LDH in combination with CTLA-4 blockade is more effective in controlling tumor progression compared to CTLA-4 blockade alone, and that this combination promotes effector T cell infiltration and activation, while destabilizing regulatory T cell function. Additionally, we observe serum LDH and lactate levels correlate with primary tumor burden as well as tumor LDH levels. Therefore, serum LDH may serve as a biomarker for tumor burden and tumor LDH, as well as clinical response to LDHi. This study provides a comprehensive rationale for combining immune checkpoint blockade with inhibitors of glycolysis for patients with highly glycolytic cancers.

Citation Format: Svena Verma, Inna Serganova, Lauren Dong, Sadna Budhu, Levi Mangarin, Roberta Zappasodi, Taha Merghoub, Jedd D. Wolchok. LDH inhibition boosts effector T cells while destabilizing regulatory T cells and improves responses to CTLA-4 blockade. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 5873.

Facts and Perspectives: Implications of tumor glycolysis on immunotherapy response in triple negative breast cancer

Triple negative breast cancer (TNBC) is an aggressive disease that is difficult to treat and portends a poor prognosis in many patients. Recent efforts to implement immune checkpoint inhibitors into the treatment landscape of TNBC have led to improved outcomes in a subset of patients both in the early stage and metastatic settings. However, a large portion of patients with TNBC remain resistant to immune checkpoint inhibitors and have limited treatment options beyond cytotoxic chemotherapy. The interplay between the anti-tumor immune response and tumor metabolism contributes to immunotherapy response in the preclinical setting, and likely in the clinical setting as well. Specifically, tumor glycolysis and lactate production influence the tumor immune microenvironment through creation of metabolic competition with infiltrating immune cells, which impacts response to immune checkpoint blockade. In this review, we will focus on how glucose metabolism within TNBC tumors influences the response to immune checkpoint blockade and potential ways of harnessing this information to improve clinical outcomes.

Abstract 3537: Pharmacologic inhibition of the glycolytic pathway improves response to immune checkpoint blockade

Preferential engagement in glycolysis is a hallmark of cancer cells and contributes to the progression and metastasis of many tumor types, including melanoma and triple-negative breast cancer (TNBC). Tumor reliance on glycolysis is emerging as a mechanism of resistance to immunotherapy, due in part to lactate-mediated immunosuppression and competition for glucose between effector T cells and tumor cells within the tumor microenvironment. Elevated serum lactate dehydrogenase (LDH) levels are associated with poor outcomes in cancer patients, and we observed that serum lactate and LDH levels correlate with primary tumor burden in mice. We recently demonstrated that genetic dampening of LDH subunit A in 4T1 TNBC results in improved and long-lasting anti-tumor responses to CTLA-4 blockade in mice. Therefore, we hypothesize that combining LDH inhibitors (LDHi) with CTLA-4 blockade will be an effective strategy to combat resistance to anti-CTLA-4 therapy. Since activated T cells rely on glycolysis, we first determined that glycolytic cancers overexpress LDH (compared with immune cells) by analyzing single-cell transcripts from patient melanoma biopsies. LDHA gene expression is significantly higher in malignant cells than infiltrating CD8+ T cells, and we replicated these findings at the protein level in whole cell lysate from B16-F10 melanoma and 4T1 TNBC tumor cells and tumor-antigen specific T cells in vitro. We show that LDHi reduces tumor lactate production and glucose consumption without inhibiting anti-tumor T-cell killing in vitro. The anti-tumor effect of LDH inhibition requires adaptive immunity, as daily treatment with LDHi results in reduced tumor burden in immunocompetent but not immune-deficient mice. Finally, we show that targeting LDH in combination with CTLA-4 blockade is more effective in slowing B16-F10 growth compared with CTLA-4 blockade alone, and that this combination promotes effector T cell activation while destabilizing regulatory T cell function.

Citation Format: Svena Verma, Inna Serganova, Sadna Budhu, Lauren Dong, Myat Ko, Levi Mangarin, Taha Merghoub, Jedd Wolchok, Roberta Zappasodi. Pharmacologic inhibition of the glycolytic pathway improves response to immune checkpoint blockade [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 3537.

Genetic and Drug Inhibition of LDH-A: Effects on Murine Gliomas

The effects of the LDH-A depletion via shRNA knockdown on three murine glioma cell lines and corresponding intracranial (i.c.) tumors were studied and compared to pharmacologic (GNE-R-140) inhibition of the LDH enzyme complex, and to shRNA scrambled control (NC) cell lines. The effects of genetic-shRNA LDH-A knockdown and LDH drug-targeted inhibition (GNE-R-140) on tumor-cell metabolism, tumor growth, and animal survival were similar. LDH-A KD and GNE-R-140 unexpectedly increased the aggressiveness of GL261 intracranial gliomas, but not CT2A and ALTS1C1 i.c. gliomas. Furthermore, the bioenergetic profiles (ECAR and OCR) of GL261 NC and LDH-A KD cells under different nutrient limitations showed that (a) exogenous pyruvate is not a major carbon source for metabolism through the TCA cycle of native GL261 cells; and (b) the unique upregulation of LDH-B that occurs in GL261 LDH-A KD cells results in these cells being better able to: (i) metabolize lactate as a primary carbon source through the TCA cycle, (ii) be a net consumer of lactate, and (iii) showed a significant increase in the proliferation rate following the addition of 10 mM lactate to the glucose-free media (only seen in GL261 KD cells). Our study suggests that inhibition of LDH-A/glycolysis may not be a general strategy to inhibit the i.c. growth of all gliomas, since the level of LDH-A expression and its interplay with LDH-B can lead to complex metabolic interactions between tumor cells and their environment. Metabolic-inhibition treatment strategies need to be carefully assessed, since the inhibition of glycolysis (e.g., inhibition of LDH-A) may lead to the unexpected development and activation of alternative metabolic pathways (e.g., upregulation of lipid metabolism and fatty-acid oxidation pathways), resulting in enhanced tumor-cell survival in a nutrient-limited environment and leading to increased tumor aggressiveness.

LDH-A—Modulation and the Variability of LDH Isoenzyme Profiles in Murine Gliomas: A Link with Metabolic and Growth Responses

Three murine glioma cell lines (GL261, CT2A, and ALTS1C1) were modified to downregulate the expression of the murine LDH-A gene using shRNA, and compared to shRNA scrambled control (NC) cell lines. Differences in the expression of LDH-A and LDH-B mRNA, protein and enzymatic activity, as well as their LDH isoenzyme profiles, were observed in the six cell lines, and confirmed successful LDH-A KD. LDH-A KD (knock-down) resulted in metabolic changes in cells with a reduction in glycolysis (GlycoPER) and an increase in basal respiratory rate (mitoOCR). GL261 cells had a more limited ATP production capacity compared to CT2A and ALTS1C1 cells. An analysis of mRNA expression data indicated that: (i) GL261 LDH-A KD cells may have an improved ability to metabolize lactate into the TCA cycle; and (ii) that GL261 LDH-A KD cells can upregulate lipid metabolism/fatty acid oxidation pathways, whereas the other glioma cell lines do not have this capacity. These two observations suggest that GL261 LDH-A KD cells can develop/activate alternative metabolic pathways for enhanced survival in a nutrient-limited environment, and that specific nutrient limitations have a variable impact on tumor cell metabolism and proliferation. The phenotypic effects of LDH-A KD were compared to those in control (NC) cells and tumors. LDH-A KD prolonged the doubling time of GL261 cells in culture and prevented the formation of subcutaneous flank tumors in immune-competent C57BL/6 mice, whereas GL261 NC tumors had a prolonged growth delay in C57BL/6 mice. In nude mice, both LDH-A KD and NC GL261 tumors grew rapidly (more rapidly than GL261 NC tumors in C57BL/6 mice), demonstrating the impact of an intact immune system on GL261 tumor growth. No differences between NC and KD cell proliferation (in vitro) or tumor growth in C57BL/6 mice (doubling time) were observed for CT2A and ALTS1C1 cells and tumors, despite the small changes to their LDH isoenzyme profiles. These results suggest that GL261 glioma cells (but not CT2A and ALTS1C1 cells) are pre-programmed to have the capacity for activating different metabolic pathways with higher TCA cycle activity, and that this capacity is enhanced by LDH-A depletion. We observed that the combined impact of LDH-A depletion and the immune system had a significant impact on the growth of subcutaneous-located GL261 tumors.

Epigenetic, Metabolic, and Immune Crosstalk in Germinal-Center-Derived B-Cell Lymphomas: Unveiling New Vulnerabilities for Rational Combination Therapies

B-cell non-Hodgkin lymphomas (B-NHLs) are highly heterogenous by genetic, phenotypic, and clinical appearance. Next-generation sequencing technologies and multi-dimensional data analyses have further refined the way these diseases can be more precisely classified by specific genomic, epigenomic, and transcriptomic characteristics. The molecular and genetic heterogeneity of B-NHLs may contribute to the poor outcome of some of these diseases, suggesting that more personalized precision-medicine approaches are needed for improved therapeutic efficacy. The germinal center (GC) B-cell like diffuse large B-cell lymphomas (GCB-DLBCLs) and follicular lymphomas (FLs) share specific epigenetic programs. These diseases often remain difficult to treat and surprisingly do not respond advanced immunotherapies, despite arising in secondary lymphoid organs at sites of antigen recognition. Epigenetic dysregulation is a hallmark of GCB-DLBCLs and FLs, with gain-of-function (GOF) mutations in the histone methyltransferase EZH2, loss-of-function (LOF) mutations in histone acetyl transferases CREBBP and EP300, and the histone methyltransferase KMT2D representing the most prevalent genetic lesions driving these diseases. These mutations have the common effect to disrupt the interactions between lymphoma cells and the immune microenvironment, via decreased antigen presentation and responsiveness to IFN-γ and CD40 signaling pathways. This indicates that immune evasion is a key step in GC B-cell lymphomagenesis. EZH2 inhibitors are now approved for the treatment of FL and selective HDAC3 inhibitors counteracting the effects of CREBBP LOF mutations are under development. These treatments can help restore the immune control of GCB lymphomas, and may represent optimal candidate agents for more effective combination with immunotherapies. Here, we review recent progress in understanding the impact of mutant chromatin modifiers on immune evasion in GCB lymphomas. We provide new insights on how the epigenetic program of these diseases may be regulated at the level of metabolism, discussing the role of metabolic intermediates as cofactors of epigenetic enzymes. In addition, lymphoma metabolic adaptation can negatively influence the immune microenvironment, further contributing to the development of immune cold tumors, poorly infiltrated by effector immune cells. Based on these findings, we discuss relevant candidate epigenetic/metabolic/immune targets for rational combination therapies to investigate as more effective precision-medicine approaches for GCB lymphomas.

Introducing a new reporter gene, membrane-anchored Cypridina luciferase, for multiplex bioluminescence imaging

Bioluminescence reporter gene imaging is a robust, high-throughput imaging modality that is useful for tracking cells and monitoring biological processes, both in cell culture and in small animals. We introduced and characterized a novel bioluminescence reporter—membrane-anchored Cypridina luciferase (maCLuc)—paired with a unique vargulin substrate. This luciferase-substrate pair has no cross-reactivity with established d-luciferin- or coelenterazine-based luciferase reporters. We compare maCLuc with several established luciferase-based reporter systems (firefly, click beetle, Renilla, and Gaussia luciferases), using both in vitro and in vivo models. We demonstrate the different imaging characteristics of these reporter systems, which allow for multiplexed-luciferase imaging of 3 and 4 separate targets concurrently in the same animal within 24 h. The imaging paradigms described here can be directly applied for simultaneous in vivo monitoring of multiple cell populations, the activity of selected signal transduction pathways, or a combination of both constitutive and inducible reporter imaging.

CTLA-4 blockade drives loss of Treg stability in glycolysis-low tumours

Limiting the metabolic competition in the tumor microenvironment (TME) may increase the effectiveness of immunotherapy. Because of its critical role in glucose metabolism of activated T cells, CD28 signaling has been proposed as a T-cell metabolic biosensor¹. Conversely, CTLA-4 engagement has been shown to down-regulate T-cell glycolysis¹. Here, we investigated the impact of CTLA-4 blockade on the metabolic fitness of intra-tumor T cells in relationship to the tumor glycolytic capacity. We found that CTLA-4 blockade promotes immune cell infiltration and metabolic fitness especially in glycolysis-low tumors. Accordingly, anti-CTLA-4 achieved better therapeutic outcomes in mice bearing glycolysis-defective tumors. Intriguingly, tumor-specific CD8⁺ T-cell responses correlated with phenotypic and functional destabilization of tumor-infiltrating regulatory T cells (Tregs) toward IFN-γ- and TNF-α-producing cells in glycolysis-defective tumors. By mimicking the highly and poorly glycolytic TME in vitro, we show that the effect of CTLA-4 blockade to promote Treg destabilization is dependent on Treg glycolysis and CD28 signaling. These findings indicate that decreasing tumor competition for glucose may facilitate the therapeutic activity of CTLA-4 blockade, thus supporting its combination with inhibitors of tumor glycolysis. Moreover, these results reveal a new mechanism through which anti-CTLA-4 interferes with Treg function in the presence of glucose.

Lactate Dehydrogenase A Depletion Alters MyC-CaP Tumor Metabolism, Microenvironment, and CAR T Cell Therapy

To enhance human prostate-specific membrane antigen (hPSMA)-specific chimeric antigen receptor (CAR) T cell therapy in a hPSMA⁺ MyC-CaP tumor model, we studied and imaged the effect of lactate dehydrogenase A (LDH-A) depletion on the tumor microenvironment (TME) and tumor progression. Effective LDH-A short hairpin RNA (shRNA) knockdown (KD) was achieved in MyC-CaP:hPSMA⁺ Renilla luciferase (RLuc)-internal ribosome entry site (IRES)-GFP tumor cells, and changes in tumor cell metabolism and in the TME were monitored. LDH-A downregulation significantly inhibited cell proliferation and subcutaneous tumor growth compared to control cells and tumors. However, total tumor lactate concentration did not differ significantly between LDH-A knockdown and control tumors, reflecting the lower vascularity, blood flow, and clearance of lactate from LDH-A knockdown tumors. Comparing treatment responses of MyC-CaP tumors with LDH-A depletion and/or anti-hPSMA CAR T cells showed that the dominant effect on tumor growth was LDH-A depletion. With anti-hPSMA CAR T cell treatment, tumor growth was significantly slower when combined with tumor LDH-A depletion and compared to control tumor growth (p < 0.0001). The lack of a complete tumor response in our animal model can be explained in part by (1) the lower activity of human CAR T cells against hPSMA-expressing murine tumors in a murine host, and (2) a loss of hPSMA antigen from the tumor cell surface in progressive generations of tumor cells.

Molecular Imaging with Reporter Genes: Has Its Promise Been Delivered?

The first reporter systems were developed in the early 1980s and were based on measuring the activity of an enzyme-as a surrogate measure of promoter-driven transcriptional activity-which is now known as a reporter gene system. The initial objective and application of reporter techniques was to analyze the activity of a specific promoter (namely, the expression of a gene that is under the regulation of the specific promoter that is linked to the reporter gene). This system allows visualization of specific promoter activity with great sensitivity. In general, there are 2 classes of reporter systems: constitutively expressed (always-on) reporter constructs used for cell tracking, and inducible reporter systems sensitive to endogenous signaling molecules and transcription factors that characterize specific tissues, tumors, or signaling pathways.This review traces the development of different reporter systems, using fluorescent and bioluminescent proteins as well as radionuclide-based reporter systems. The development and application of radionuclide-based reporter systems is the focus of this review. The question at the end of the review is whether the "promise" of reporter gene imaging has been realized. What is required for moving forward with radionuclide-based reporter systems, and what is required for successful translation to clinical applications?

LDH-A regulates the tumor microenvironment via HIF-signaling and modulates the immune response

Previous studies show that LDH-A knockdown reduces orthotopic 4T1 breast tumor lactate and delays tumor growth and the development of metastases in nude mice. Here, we report significant changes in the tumor microenvironment (TME) and a more robust anti-tumor response in immune competent BALB/c mice. 4T1 murine breast cancer cells were transfected with shRNA plasmids directed against LDH-A (KD) or a scrambled control plasmid (NC). Cells were also transduced with dual luciferase-based reporter systems to monitor HIF-1 activity and the development of metastases by bioluminescence imaging, using HRE-sensitive and constitutive promoters, respectively. The growth and metastatic profile of orthotopic 4T1 tumors developed from these cell lines were compared and a primary tumor resection model was studied to simulate the clinical management of breast cancer. Primary tumor growth, metastasis formation and TME phenotype were significantly different in LDH-A KD tumors compared with controls. In LDH-A KD cells, HIF-1 activity, hexokinase 1 and 2 expression and VEGF secretion were reduced. Differences in the TME included lower HIF-1α expression that correlated with lower vascularity and pimonidazole staining, higher infiltration of CD3⁺ and CD4⁺ T cells and less infiltration of TAMs. These changes resulted in a greater delay in metastases formation and 40% long-term survivors (>20 weeks) in the LDH-A KD cohort following surgical resection of the primary tumor. We show for the first time that LDH-depletion inhibits the formation of metastases and prolongs survival of mice through changes in tumor microenvironment that modulate the immune response. We attribute these effects to diminished HIF-1 activity, vascularization, necrosis formation and immune suppression in immune competent animals. Gene-expression analyses from four human breast cancer datasets are consistent with these results, and further demonstrate the link between glycolysis and immune suppression in breast cancer.

Enhancement of PSMA-Directed CAR Adoptive Immunotherapy by PD-1/PD-L1 Blockade

Chimeric antigen receptor (CAR) T cell therapy in hematologic malignancies has shown remarkable responses, but the same level of success has not been observed in solid tumors. A new prostate cancer model (Myc-CaP:PSMA(+)) and a second-generation anti-hPSMA human CAR T cells expressing a Click Beetle Red luciferase reporter) were used to study hPSMA targeting and assess CAR T cell trafficking and persistence by bioluminescence imaging (BLI). We investigated the antitumor efficacy of human CAR T cells targeting human prostate-specific membrane antigen (hPSMA), in the presence and absence of the target antigen; first alone and then combined with a monoclonal antibody targeting the human programmed death receptor 1 (anti-hPD1 mAb). PDL-1 expression was detected in Myc-CaP murine prostate tumors growing in immune competent FVB/N and immune-deficient SCID mice. Endogenous CD3⁺ T cells were restricted from the centers of Myc-CaP tumor nodules growing in FVB/N mice. Following anti-programmed cell death protein 1 (PD-1) treatment, the restriction of CD3⁺ T cells was reversed, and a tumor-treatment response was observed. Adoptive hPSMA-CAR T cell immunotherapy was enhanced when combined with PD-1 blockade, but the treatment response was of comparatively short duration, suggesting other immune modulation mechanisms exist and restrict CAR T cell targeting, function, and persistence in hPSMA expressing Myc-CaP tumors. Interestingly, an “inverse pattern” of CAR T cell BLI intensity was observed in control and test tumors, which suggests CAR T cells undergo changes leading to a loss of signal and/or number following hPSMA-specific activation. The lower BLI signal intensity in the hPSMA test tumors (compared with controls) is due in part to a decrease in T cell mitochondrial function following T cell activation, which may limit the intensity of the ATP-dependent Luciferin-luciferase bioluminescence signal.

Abstract PR06: Overcoming intratumor T-cell exclusion by modulation of lactate metabolism to improve immune checkpoint therapies in aggressive breast cancer

Background: Breast cancer (BC) has historically been considered immunologically silent; however, several observations indicate that potentiation of immune functions can benefit BC patients. Intratumor T-cell infiltration has prognostic significance in patients with BC across different molecular and histological categories. In addition, PD-L1 can be overexpressed in BC, in particular in the highly aggressive triple negative BC (TNBC) subtype. This is associated with poor prognosis specifically in patients with luminal B and basal-like phenotypes, thus making these subtypes rational targets of PD-1/PD-L1 axis blockade treatments. However, initial results from early-phase clinical trials show a modest activity of immune checkpoint blockade monotherapy against BC, with 19% TNBC and 3-12% hormone receptor-positive patients achieving a clinical response to anti-PD-1/PD-L1 therapies. Lack of inflammation and T-cell infiltration at the tumor site are characteristic features of tumors that do not respond to checkpoint blockade. Since BC is typically poorly infiltrated by T cells, having strategies that reverse this immune exclusion as well as non-invasive modalities to predict this effect are crucial to improve the clinical efficacy of immune checkpoint blockade.

Experimental Design: Our working model is that tumor glycolysis (lactate) and T-cell infiltration are mechanistically interdependent, since we reasoned that a highly glycolytic tumor microenvironment (due to lactate production) could hamper survival, expansion and effector functions of T cells in BC lesions, thus explaining T-cell exclusion. Therefore, we tested whether genetic and/or pharmacologic inhibition of lactate production/consumption could restore intratumor T-cell infiltration and immune function in BC models, thus favoring tumor responsiveness to checkpoint blockade.

Results: By interrogating a compendium of 4 BC patient gene expression datasets, we found that patients harboring tumors with high expression of lactate dehydrogenase A (LDH-A) or the lactate transporters MCT-1/4 have a significantly higher risk to develop metastases (p&lt;10−16), whereas those having tumors with increased expression of CD3 and CD8 transcripts experience a better prognosis (p&lt;10−16). Gene expression data from MSKCC's cBio Portal showed an inverse correlation between glycolysis- and immune-related gene expression signatures in BC patients, which was in agreement with our hypothesis. To mechanistically demonstrate the impact of lactate metabolism on intratumor T-cell infiltration in BC, we generated an LDH-A-knocked-down variant (LDH-A KD) of the metastatic TNBC murine model 4T1, and studied its growth and immune infiltrate in vivo in comparison with the control 4T1 tumor. Animals implanted with LDH-A KD 4T1 showed a 5-fold increase in tumor-infiltrating CD3+ T cells and a 4-fold reduction in tumor-associated macrophages, and experienced a significantly prolonged survival. LDH-A KD tumors could be completely eradicated in immunocompetent but not in immunodeficient mice, further supporting the immunologic basis of the antitumor effects of LDH-A inhibition. In vitro analyses showed that proliferation, activation and pro-inflammatory cytokine release of activated CD8 T cells were significantly hampered when they were co-cultured with 4T1 cells. Of note, blocking lactate transport with two new MCT-1/4 small-molecule inhibitors promoted immune functions of CD8 T cells cultured with 4T1, without showing significant toxicity in cultures of T cells alone.

Citation Format: Roberta Zappasodi, Arnab Ghosh, Inna Serganova, Ivan Cohen, Yasin Senbabaoglu, Masahiro Shindo, Mayuresh M. Mane, Avigdor Leftin, Ellen Ackerstaff, Jason A. Koutcher, Jedd D. Wolchok, Ronald G. Blasberg, Taha Merghoub. Overcoming intratumor T-cell exclusion by modulation of lactate metabolism to improve immune checkpoint therapies in aggressive breast cancer [abstract]. In: Proceedings of the Second CRI-CIMT-EATI-AACR International Cancer Immunotherapy Conference: Translating Science into Survival; 2016 Sept 25-28; New York, NY. Philadelphia (PA): AACR; Cancer Immunol Res 2016;4(11 Suppl):Abstract nr PR06.

Synthesis and evaluation of an (18)F-labeled pyrimidine-pyridine amine for targeting CXCR4 receptors in gliomas

INTRODUCTION

Chemokine receptor-4 (CXCR4, fusin, CD184) is expressed on several tissues involved in immune regulation and is upregulated in many diseases including malignant gliomas. A radiolabeled small molecule that readily crosses the blood-brain barrier can aid in identifying CXCR4-expressing gliomas and monitoring CXCR4-targeted therapy. In the current work, we have synthesized and evaluated an [(18)F]-labeled small molecule based on a pyrimidine-pyridine amine for its ability to target CXCR4.

EXPERIMENTAL

The nonradioactive standards and the nitro precursor used in this study were prepared using established methods. An HPLC method was developed to separate the nitro-precursor from the nonradioactive standard and radioactive product. The nitro-precursor was radiolabeled with (18)F under inert, anhydrous conditions using the [(18)F]-kryptofix 2.2.2 complex to form the desired N-(4-(((6-[(18)F]fluoropyridin-2-yl)amino)methyl)benzyl)pyrimidin-2-amine ([(18)F]-3). The purified radiolabeled compound was used in serum stability, partition coefficient, cellular uptake, and in vivo cancer targeting studies.

RESULTS

[(18)F]-3 was synthesized in 4-10% decay-corrected yield (to start of synthesis). [(18)F]-3 (tR ≈ 27 min) was separated from the precursor (tR ≈ 30 min) using a pentafluorophenyl column with an isocratic solvent system. [(18)F]-3 displayed acceptable serum stability over 2 h. The amount of [(18)F]-3 bound to the plasma proteins was determined to be > 97%. The partition coefficient (LogD7.4) is 1.4 ± 0.5. Competitive in vitro inhibition indicated 3 does not inhibit uptake of (67)Ga-pentixafor. Cell culture media incubation and ex vivo urine analysis indicate rapid metabolism of [(18)F]-3 into hydrophilic metabolites. Thus, in vitro uptake of [(18)F]-3 in CXCR4 overexpressing U87 cells (U87 CXCR4) and U87 WT indicated no specific binding. In vivo studies in mice bearing U87 CXCR4 and U87 WT tumors on the left and right shoulders were carried out using [(18)F]-3 and (68)Ga-pentixafor on consecutive days. The CXCR4 positive tumor was clearly visualized in the PET study using (68)Ga-pentixafor, but not with [(18)F]-3.

CONCLUSIONS

We have successfully synthesized both a radiolabeled analog to previously reported CXCR4-targeting molecules and a nitro precursor. Our in vitro and in vivo studies indicate that [(18)F]-3 is rapidly metabolized and, therefore, does not target CXCR4-expressing tumors. Optimization of the structure to improve the in vivo (and in vitro) stability, binding, and solubility could lead to an appropriate CXCR4-targeted radiodiagnositic molecule.

Comparative Analysis of T Cell Imaging with Human Nuclear Reporter Genes

Monitoring genetically altered T cells is an important component of adoptive T cell therapy in patients, and the ability to visualize their trafficking/targeting, proliferation/expansion, and retention/death using highly sensitive reporter systems that do not induce an immunologic response would provide useful information. Therefore, we focused on human reporter gene systems that have the potential for translation to clinical studies. The objective of the in vivo imaging studies was to determine the minimum number of T cells that could be visualized with the different nuclear reporter systems. We determined the imaging sensitivity (lower limit of T cell detection) of each reporter using appropriate radiolabeled probes for PET or SPECT imaging.

METHODS

Human T cells were transduced with retroviral vectors encoding for the human norepinephrine transporter (hNET), human sodium-iodide symporter (hNIS), a human deoxycytidine kinase double mutant (hdCKDM), and herpes simplex virus type 1 thymidine kinase (hsvTK) reporter genes. After viability and growth were assessed, 10(5) to 3 × 10(6) reporter T cells were injected subcutaneously on the shoulder area. The corresponding radiolabeled probe was injected intravenously 30 min later, followed by sequential PET or SPECT imaging. Radioactivity at the T cell injection sites and in the thigh (background) was measured.

RESULTS

The viability and growth of experimental cells were unaffected by transduction. The hNET/meta-(18)F-fluorobenzylguanidine ((18)F-MFBG) reporter system could detect less than 1 × 10(5) T cells because of its high uptake in the transduced T cells and low background activity. The hNIS/(124)I-iodide reporter system could detect approximately 1 × 10(6) T cells; (124)I-iodide uptake at the T cell injection site was time-dependent and associated with high background. The hdCKDM/2'-(18)F-fluoro-5-ethyl-1-β-d-arabinofuranosyluracil ((18)F-FEAU) and hsvTK/(18)F-FEAU reporter systems detected approximately 3 × 10(5) T cells, respectively. (18)F-FEAU was a more efficient probe (higher uptake, lower background) than (124)I-1-(2-deoxy-2-fluoro-1-d-arabinofuranosyl)-5-iodouracil for both hdCKDM and hsvTK.

CONCLUSION

A comparison of different reporter gene-reporter probe systems for imaging of T cell number was performed, and the hNET/(18)F-MFBG PET reporter system was found to be the most sensitive and capable of detecting approximately 35-40 × 10(3) T cells at the site of T cell injection in the animal model.

Abstract 1120: Bioenergetics of T cells in the context of adoptive immunotherapy

Tumor-specific T cell therapy has been used as an experimental approach for an anti-tumor therapy (Sadelain M 2003, Gattinoni L 2006). Adoptive T cell transfer therapies rely on ex vivo T cell isolation, transduction, activation and expansion of autologous tumor-reactive T cell populations prior to patient administration (Yee C 2002, Dudley ME 2003, Bollard CM 2004).

The metabolic changes occurring in modified T-cells are the object of growing interest. For example, it has been shown that quiescent T cells display low energetic and biosynthetic demands, generating ATP through the TCA cycle and OXPHOS (oxidative phosphorylation). Activated T cells undergo conversion from a resting to an active state and ATP production switches from OXPHOS to high glycolysis. Our objective was to assess T cell metabolism and evaluate changes in glycolytic and mitochnodrial function that occur during transduction and expansion, prior to administration of T-cells in adoptive immunotherapy protocols.

Methods. Human T-cells were isolated from buffy coat using ficoll separation. 48 hours after phytohemagglutinin (PHA) stimulation, cells were transduced with the retroviral vector bearing chimeric antigen receptor (CAR) targeting PSMA (Maher, Brentjens et al. 2002), and transduction efficacy was assessed by FACS. T-cells further were stimulated by exposure to antigen-presenting cells, APC. The metabolic profiles were determined at different steps of T cell stimulation and transduction. A Seahorse XF96 Analyzer was used to measure glycolysis (extracellular acidification rate, ECAR) and the oxygen consumption rate (OCR), a measure of oxidative phosphorylation (OXPHOS).

Results: Glycolysis and oxygen consumption was low in resting non-stimulated cells. Both CAR-transduced and non-transduced T cells increased glycolysis (5-fold), oxygen consumption (3-fold) and ATP-linked OCR following PHA stimulation. No significant metabolic differences were observed between CAR-transduced and non-transduced T cells. High glycolytic activity was maintained by T cells over the 25 day course of T cell transduction and expansion. Mitochondrial function (oxygen consumption) declined during this period. Restimulation by exposure to antigen-presenting cells resulted in a small mitochondrial response in transduced T cells, whereas glycolysis remained high and showed no additional response.

Conclusion: Glycolysis (ECAR) remains high, whereas oxygen consumption (OCR and OXPHOS) declines during T cell preparation for adoptive T cell therapy. Whether the specific changes in glucose metabolism and mitochondria respiration can influence cytotoxic function remains to be elucidated.

Citation Format: Ekaterina Moroz, Maxim Moroz, Inna Serganova, Juan Zurita, Jason Lee, Nisargbhai Shah, Vladimir Ponomarev, Ronald Blasberg. Bioenergetics of T cells in the context of adoptive immunotherapy. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 1120. doi:10.1158/1538-7445.AM2014-1120

Relationships between LDH-A, lactate, and metastases in 4T1 breast tumors

PURPOSE

To investigate the relationship between lactate dehydrogenase A (LDH-A) expression, lactate concentration, cell metabolism, and metastases in murine 4T1 breast tumors.

EXPERIMENTAL DESIGN

Inhibition of LDH-A expression and protein levels were achieved in a metastatic breast cancer cell line (4T1) using short hairpin RNA (shRNA) technology. The relationship between tumor LDH-A protein levels and lactate concentration (measured by magnetic resonance spectroscopic imaging, MRSI) and metastases was assessed.

RESULTS

LDH-A knockdown cells (KD9) showed a significant reduction in LDH-A protein and LDH activity, less acid production, decreased transwell migration and invasion, lower proliferation, reduced glucose consumption and glycolysis, and increase in oxygen consumption, reactive oxygen species (ROS), and cellular ATP levels, compared with control (NC) cells cultured in 25 mmol/L glucose. In vivo studies showed lower lactate levels in KD9, KD5, and KD317 tumors than in NC or 4T1 wild-type tumors (P < 0.01), and a linear relationship between tumor LDH-A protein expression and lactate concentration. Metastases were delayed and primary tumor growth rate decreased.

CONCLUSIONS

We show for the first time that LDH-A knockdown inhibited the formation of metastases, and was accompanied by in vivo changes in tumor cell metabolism. Lactate MRSI can be used as a surrogate to monitor targeted inhibition of LDH-A in a preclinical setting and provides a noninvasive imaging strategy to monitor LDH-A-targeted therapy. This imaging strategy can be translated to the clinic to identify and monitor patients who are at high risk of developing metastatic disease.

Molecular imaging of expression of vascular endothelial growth factor a (VEGF a) in femoral bone grafts transplanted into living mice

The biology of cells transplanted with bone grafts is incompletely understood. Focusing on the early angiogenic response postgrafting, we report a mouse femur graft model in which grafts were derived from mice transgenic for a firefly luciferase (FLuc) bioluminescence reporter gene driven by a promoter for the angiogenic signaling molecule vascular endothelial growth factor (VEGF). Upon transplantation into wild-type (wt) mice, in vivo bioluminescence imaging (BLI) permitted longitudinal visualization and measurements of VEGF promoter activity in the transplanted graft cells and demonstrated a lag period of 7 days posttransplantation prior to robust induction of the promoter. To determine cellular mediators of VEGF induction in graft bone, primary graft-derived osteoblastic cells (GDOsts) were characterized. In vitro BLI on GDOsts showed hypoxia-induced VEGF expression and that this induction depended on PI3K signaling and, to a lesser degree, on the MEK pathway. This transcriptional regulation correlated with VEGF protein production and was validated in GDOsts seeded on demineralized bone matrix (DBM), a bone graft substitute material. Together, combined imaging of VEGF expression in living animals and in live cells provided clues about the regulation of VEGF in cells post-bone grafting. These data are particularly significant toward the development of future smart bone graft substitutes.

Abstract P6-01-01: Metastatic and non-metastatic isogenic breast cancer cells lines show different metabolic signatures in response to intermittent hypoxia and transient glutamine/glucose deprivation

INTRODUCTION. Cancer cells quickly adapt to their microenvironment by reprogramming their metabolism. Specific metabolic responses to defined stress conditions, mimicking tumor microenvironmental changes, may therefore unveil signatures of cancer phenotype, not detected in standard tissue culture.

PURPOSE. To identify distinct metabolic signatures associated with metastatic ability, we investigated in the two isogenic breast cancer cell lines, 4T1 (highly metastatic) and 67NR (non-metastatic), how glutamine supply/deprivation affects cellular glucose metabolism, and vice-versa, in aerobic or hypoxic conditions, and the effects on cell growth.

METHODS. 4T1 and 67NR cells were cultured in medium A, i.e. DME containing 1% P/S, 10% FBS, 25 mM glucose (Glc), and 6 mM glutamine (Gln). Cell growth was determined in tissue culture from cells cultured for 48 h in various media A-D: A, all nutrients available; B: A but no Gln; C: A but 2 mM Gln; D: A but no Glc. Cellular mitochondrial function was also assessed in both 4T1 and 67NR cells using an XF96 Analyzer (Sea Horse Bioscience, Billerica, MA) and the specific inhibitors: oligomycin, FCCP, antimycin and rotenone. Metabolism was studied in cells grown on microcarriers in the various media A-D under aerobic (∼21% O2) and hypoxic conditions (∼1% O2), using our magnetic resonance (MR)-compatible cell perfusion system on a 500 MHz spectrometer. For the cell perfusion studies, Glc or Gln were exchanged for 1–13C-Glc or 3–13C-Gln respectively. The metabolic fate of 13C-labeled nutrients was followed under the various environmental conditions by 13C MR spectroscopy (MRS), while energy metabolism was observed by 31P MRS.

RESULTS. Deprivation of either Glc or Gln for 48 h reduced significantly the cell growth of 4T1 and 67NR cells, resulting in a similar growth rate for the nutrient-deprived 4T1 and 67NR cells. As determined from the MR experiments, each cell line adopted different metabolic strategies to cope when exposed to specific metabolite stress conditions, including on the level of oxygen availability. Unlike in 67NR cells, glycolysis is glutamine-dependent in the more aggressive 4T1 cell line (∼2-fold higher glucose-derived lactate synthesis rate was observed in the presence of glutamine), which decreases when oxygen becomes available to fuel TCA cycle activity, as monitored by glucose-derived glutamate synthesis. The studies further indicate an impaired TCA cycle activity in 67NR cells, reflected in a high accumulation rate of glucose- or glutamine-derived succinate (significantly higher compared to 4T1 cells). This is consistent with the mitochondrial functional analysis, showing higher mitochondrial TCA activity in 4T1 cells than in 67NR, most likely due to impairment of succinate dehydrogenase (SDH, respiratory Complex II) in the latter.

CONCLUSIONS. Our results support the association between increased mitochondrial metabolism and metastatic potential, observed recently. We are investigating the SDH status in these cells lines and in additional metastatic and non-metastatic breast cancer cell lines, to establish whether SDH, and glutamine metabolism, could be potential targets for therapy.

Citation Information: Cancer Res 2012;72(24 Suppl):Abstract nr P6-01-01.

Abstract 1003: A link between lactate dehydrogenase A, lactate and tumor phenotype identified by imaging

Metabolic changes in primary tumors have a significant impact on tumor progression and on the development of the metastatic phenotype. In cancer patients, serum total lactate dehydrogenase (LDH) levels are often increased, and the gene for LDH-A protein, is frequently upregulated in tumors (Koukourakis, Giatromanolaki et al. 2003). These features have been linked to poor prognosis, and a greater metastatic potential has been reported in patients with high LDH serum levels. Since LDH-A protein is required for the maintenance and progression of many tumors (Fantin, St-Pierre et al. 2006), it is also becoming a potential target for cancer therapy (Le, Cooper et al. 2010; Seth, Grant et al. 2011). We investigated the relationship between LDH-A expression and lactate production in two isogenic breast cancer lines (67NR and 4T1) (Serganova, Rizwan et al. 2011). These two cell lines were derived from a single mammary tumor that developed spontaneously in a BALB/c mouse. Each subclone was shown to have different phenotypic properties (Aslakson and Miller 1992). LDH-A expression, lactate concentration, glucose utilization and oxygen consumption were measured in cells, and the potential relationship between tumor lactate levels (measured by magnetic resonance spectroscopic imaging (MRSI)) and tumor glucose utilization (measured by [18F] 2-deoxy-2-fluoro-D-glucose positron emission tomography ([18F]FDG-PET)) was assessed in orthotopic breast tumors derived from these cell lines. We show a substantial difference in LDH-A expression between 67NR and 4T1 cells under normoxia and hypoxia. We also show that small orthotopic 4T1 tumors generate ten-fold more lactate than corresponding 67NR tumors. The high lactate levels in small primary 4T1 tumors are associated with intense pimonidazole staining (a hypoxia indicator). Less intense hypoxia staining was observed in the larger 67NR tumors, and is consistent with the gradual increase and plateau of lactate concentration in enlarging 67NR tumors. We show that lactate-MRSI has a greater dynamic range than [18F]FDG-PET and may be a more sensitive measure to evaluate the aggressive and metastatic potential of primary breast tumors. We hypothesized that tumor lactate levels monitored by MRSI will reflect LDH-A enzymatic activity and tumor phenotype. We also suggest that lactate-MRSI is a more sensitive measure than [18F]FDG-PET, and could be used in the clinic to evaluate the aggressive potential of primary breast tumors. Aslakson, C. J. and F. R. Miller (1992). Cancer Res 52(6): 1399-1405. Fantin, V. R., J. St-Pierre, et al. (2006). Cancer Cell 9(6): 425-434. Koukourakis, M. I., A. Giatromanolaki, et al. (2003). Br J Cancer 89(5): 877-885.Le, A., C. R. Cooper, et al. (2010). Proc Natl Acad Sci U S A 107(5): 2037-2042. Serganova, I., A. Rizwan, et al. (2011). Clin Cancer Res 17(19): 6250-6261. Seth, P., A. Grant, et al. (2011). Neoplasia 13(1): 60-71.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 1003. doi:1538-7445.AM2012-1003

Abstract 3223: Magnetic resonance spectroscopic imaging of lactate in LDH-A silenced metastatic breast tumors

Glucose and glutamine are the major carbon sources for rapidly proliferating tumors. They provide the precursors for proteins, lipids, and nucleic acids, and as well as metabolic-reducing capability (NADPH). Pyruvate is mainly derived from glucose and glutamine metabolism and can be converted to lactate by the lactate dehydrogenase (LDH) complex or enter the TCA cycle for conversion to CO2 and ATP. Recently, a link between tumor lactate levels (monitored by MRSI), LDH-A expression and tumor phenotype has been demonstrated (Serganova I. et al., 2011). We hypothesize that tumor lactate levels monitored by MRSI will reflect LDH-A enzymatic activity, tumor “aggressiveness” and the propensity for developing metastases. The current study employs 4T1 cells bearing the stable produced LDH-A shRNA to down regulate the level of LDH-A expression. LDH-A silenced and control 4T1 breast cancer cells were injected into the mammary fat pad of athymic nu/nu female mice (5 mice for each group). The in vivo growth rate of the LDH-A silenced tumors was significantly slower than that of control tumors; the average doubling time of the LDH-A silenced tumors was 1.5 fold higher than control tumors (p&lt;0.01). The lactate signal was acquired using a selective multiple-quantum coherence transfer (SelMQC) editing sequence in combination with chemical shift imaging (CSI) (He Q. et al., 1995). The tumors were scanned at small (∼100 mm3) and larger (∼300 mm3) volumes and in vivo lactate concentration was measured. The LDH-A silenced tumors produce less lactate, ∼ 6 mM at small volumes and ∼ 3 mM at larger volumes, whereas the control tumors produce ∼ 11 mM at small, and ∼7 mM at larger volumes. Preliminary results suggest that high tumor lactate concentrations are linked with a greater propensity to develop metastases. Studies evaluating metastatic burden are ongoing. We suggest that LDH-A expression and tumor lactate levels may be potential markers of elevated metastatic potential. In addition, MRSI might offer a noninvasive approach for assessing tumor progression and metastatic potential.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 3223. doi:1538-7445.AM2012-3223

Monitoring the induction of heat shock factor 1/heat shock protein 70 expression following 17-allylamino-demethoxygeldanamycin treatment by positron emission tomography and optical reporter gene imaging

The cell response to proteotoxic cell stresses is mediated primarily through activation of heat shock factor 1 (HSF1). This transcription factor plays a major role in the regulation of the heat shock proteins (HSPs), including HSP70. We demonstrate that an [124I]iodide-pQHNIG70 positron emission tomography (PET) reporter system that includes an inducible HSP70 promoter can be used to image and monitor the activation of the HSF1/HSP70 transcription factor in response to drug treatment (17-allylamino-demethoxygeldanamycin [17-AAG]). We developed a dual imaging reporter (pQHNIG70) for noninvasive imaging of the heat shock response in cell culture and living animals previously and now study HSF1/HSP70 reporter activation in both cell culture and tumor-bearing animals following exposure to 17-AAG. 17-AAG (10-1,000 nM) induced reporter expression; a 23-fold increase was observed by 60 hours. Good correspondence between reporter expression and HSP70 protein levels were observed. MicroPET imaging based on [124I]iodide accumulation in pQHNIG70-transduced RG2 xenografts showed a significant 6.2-fold reporter response to 17-AAG, with a corresponding increase in tumor HSP70 and in tumor human sodium iodide symporter and green fluorescent protein reporter proteins. The HSF1 reporter system can be used to screen anticancer drugs for induction of cytotoxic stress and HSF1 activation both in vitro and in vivo.

Metabolic imaging: a link between lactate dehydrogenase A, lactate, and tumor phenotype

PURPOSE

We compared the metabolic profiles and the association between LDH-A expression and lactate production in two isogenic murine breast cancer cell lines and tumors (67NR and 4T1). These cell lines were derived from a single mammary tumor and have different growth and metabolic phenotypes.

EXPERIMENTAL DESIGN

LDH-A expression, lactate concentration, glucose utilization, and oxygen consumption were measured in cells, and the potential relationship between tumor lactate levels [measured by magnetic resonance spectroscopic imaging (MRSI)] and tumor glucose utilization [measured by [(18)F]2-deoxy-2-fluoro-D-glucose positron emission tomography ([(18)F]FDG-PET)] was assessed in orthotopic breast tumors derived from these cell lines.

RESULTS

We show a substantial difference in LDH-A expression between 67NR and 4T1 cells under normoxia and hypoxia. We also show that small orthotopic 4T1 tumors generate 10-fold more lactate than corresponding 67NR tumors. The high lactate levels in small primary 4T1 tumors are associated with intense pimonidazole staining (a hypoxia indicator). Less-intense hypoxia staining was observed in the larger 67NR tumors and is consistent with the gradual increase and plateau of lactate concentration in enlarging 67NR tumors.

CONCLUSIONS

Lactate-MRSI has a greater dynamic range than [(18)F]FDG-PET and may be a more sensitive measure with which to evaluate the aggressive and metastatic potential of primary breast tumors.

ATP-binding cassette transporters modulate both coelenterazine- and D-luciferin-based bioluminescence imaging

Bioluminescence imaging (BLI) of luciferase reporters provides a cost-effective and sensitive means to image biological processes. However, transport of luciferase substrates across the cell membrane does affect BLI readout intensity from intact living cells. To investigate the effect of ATP-binding cassette (ABC) transporters on BLI readout, we generated click beetle (cLuc), firefly (fLuc), Renilla (rLuc), and Gaussia (gLuc) luciferase HEK-293 reporter cells that overexpressed different ABC transporters (ABCB1, ABCC1, and ABCG2). In vitro studies showed a significant BLI intensity decrease in intact cells compared to cell lysates, when ABCG2 was overexpressed in HEK-293/cLuc, fLuc, and rLuc cells. Selective ABC transporter inhibitors were also applied. Inhibition of ABCG2 activity increased the BLI intensity more than two-fold in HEK-293/cLuc, fLuc, and rLuc cells; inhibition of ABCB1 elevated the BLI intensity two-fold only in HEK-293/rLuc cells. BLI of xenografts derived from HEK-293/ABC transporter/luciferase reporter cells confirmed the results of inhibitor treatment in vivo. These findings demonstrate that coelenterazine-based rLuc-BLI intensity can be modulated by ABCB1 and ABCG2. ABCG2 modulates d-luciferin-based BLI in a luciferase type-independent manner. Little ABC transporter effect on gLuc-BLI intensity is observed because a large fraction of gLuc is secreted. The expression level of ABC transporters is one key factor affecting BLI intensity, and this may be particularly important in luciferase-based applications in stem cell research.

Imaging colon cancer response following treatment with AZD1152: a preclinical analysis of [18F]fluoro-2-deoxyglucose and 3'-deoxy-3'-[18F]fluorothymidine imaging

PURPOSE

To determine whether treatment response to the Aurora B kinase inhibitor, AZD1152, could be monitored early in the course of therapy by noninvasive [(18)F]-labeled fluoro-2-deoxyglucose, [(18)F]FDG, and/or 3'-deoxy-3'-[(18)F]fluorothymidine, [(18)F]FLT, PET imaging.

EXPERIMENTAL DESIGN

AZD1152-treated and control HCT116 and SW620 xenograft-bearing animals were monitored for tumor size and by [(18)F]FDG, and [(18)F]FLT PET imaging. Additional studies assessed the endogenous and exogenous contributions of thymidine synthesis in the two cell lines.

RESULTS

Both xenografts showed a significant volume-reduction to AZD1152. In contrast, [(18)F]FDG uptake did not demonstrate a treatment response. [(18)F]FLT uptake decreased to less than 20% of control values in AZD1152-treated HCT116 xenografts, whereas [(18)F]FLT uptake was near background levels in both treated and untreated SW620 xenografts. The EC(50) for AZD1152-HQPA was approximately 10 nmol/L in both SW620 and HCT116 cells; in contrast, SW620 cells were much more sensitive to methotrexate (MTX) and 5-Fluorouracil (5FU) than HCT116 cells. Immunoblot analysis demonstrated marginally lower expression of thymidine kinase in SW620 compared with HCT116 cells. The aforementioned results suggest that SW620 xenografts have a higher dependency on the de novo pathway of thymidine utilization than HCT116 xenografts.

CONCLUSIONS

AZD1152 treatment showed antitumor efficacy in both colon cancer xenografts. Although [(18)F]FDG PET was inadequate in monitoring treatment response, [(18)F]FLT PET was very effective in monitoring response in HCT116 xenografts, but not in SW620 xenografts. These observations suggest that de novo thymidine synthesis could be a limitation and confounding factor for [(18)F]FLT PET imaging and quantification of tumor proliferation, and this may apply to some clinical studies as well.

Different strategies for reducing intestinal background radioactivity associated with imaging HSV1-tk expression using established radionucleoside probes

One limitation of HSV1-tk reporter positron emission tomography (PET) with nucleoside analogues is the high background radioactivity in the intestine. We hypothesized that endogenous expression of thymidine kinase in bacterial flora could phosphorylate and trap such radiotracers, contributing to the high radioactivity levels in the bowel, and therefore explored different strategies to increase fecal elimination of radiotracer. Intestinal radioactivity was assessed by in vivo microPET imaging and ex vivo tissue sampling following intravenous injection of 18F-FEAU, 124I-FIAU, or 18F-FHBG in a germ-free mouse strain. We also explored the use of an osmotic laxative agent and/or a 100% enzymatically hydrolyzed liquid diet. No significant differences in intestinal radioactivity were observed between germ-free and normal mice. 18F-FHBG-derived intestinal radioactivity levels were higher than those of 18F-FEAU and 124I-FIAU; the intestine to blood ratio was more than 20-fold higher for 18F-FHBG than for 18F-FEAU and 124I-FIAU. The combination of Peptamen and Nulytely lowered intestinal radioactivity levels and increased (2.2-fold) the HSV1-tk transduced xenograft to intestine ratio for 18F-FEAU. Intestinal bacteria in germ-free mice do not contribute to the high intestinal levels of radioactivity following injection of radionucleoside analogues. The combination of Peptamen and Nulytely increased radiotracer elimination by increasing bowel motility without inducing dehydration.

Imaging a Genetically Engineered Oncolytic Vaccinia Virus (GLV-1h99) Using a Human Norepinephrine Transporter Reporter Gene

PURPOSE

Oncolytic viral therapy continues to be investigated for the treatment of cancer, and future studies in patients would benefit greatly from a noninvasive modality for assessing virus dissemination, targeting, and persistence. The purpose of this study was to determine if a genetically modified vaccinia virus, GLV-1h99, containing a human norepinephrine transporter (hNET) reporter gene, could be sequentially monitored by [(123)I]metaiodobenzylguanidine (MIBG) gamma-camera and [(124)I]MIBG positron emission tomography (PET) imaging.

EXPERIMENTAL DESIGN

GLV-1h99 was tested in human malignant mesothelioma and pancreatic cancer cell lines for cytotoxicity, expression of the hNET protein using immunoblot analysis, and [(123)I]MIBG uptake in cell culture assays. In vivo [(123)I]MIBG gamma-camera and serial [(124)I]MIBG PET imaging was done in MSTO-211H orthotopic pleural mesothelioma tumors.

RESULTS

GLV-1h99 successfully infected and provided dose-dependent levels of transgene hNET expression in human malignant mesothelioma and pancreatic cancer cells. The time course of [(123)I]MIBG accumulation showed a peak of radiotracer uptake at 48 hours after virus infection in vitro. In vivo hNET expression in MSTO-211H pleural tumors could be imaged by [(123)I]MIBG scintigraphy and [(124)I]MIBG PET 48 and 72 hours after GLV-1h99 virus administration. Histologic analysis confirmed the presence of GLV-1h99 in tumors.

CONCLUSION

GLV-1h99 shows high mesothelioma tumor cell infectivity and cytotoxic efficacy. The feasibility of imaging virus-targeted tumor using the hNET reporter system with [(123)I]MIBG gamma-camera and [(124)I]MIBG PET was shown in an orthotopic pleural mesothelioma tumor model. The inclusion of human reporter genes into recombinant oncolytic viruses enhances the potential for translation to clinical monitoring of oncolytic viral therapy.

Bioluminescence evaluation of efficacy of oncolytic treatment with Newcastle disease virus (NDV) in malignant pleural mesothelioma

e14536

Background: Malignant pleural mesothelioma (MPM) is a highly aggressive tumor that arises from multipotent cells of the pleura. Chemotherapy and radiation have very limited therapeutic effects, and average survival time after diagnosis varies between 10 and 16 months. Immunotherapy, gene therapy (oncolytic viral therapy), and photodynamic therapy offer alternative treatment options, with promising results in animal studies. In the following study, the oncolytic efficacy of Newcastle disease virus (NDV(F3aa)-GFP) on MPM is tested and investigated by bioluminescence imaging. Methods: NDV(F3aa)-GFP was tested for viral cytotoxicity at different multiplicities of infection (MOI) against several mesothelioma cell lines in vitro by analyzing release of intracellular lactate dehydrogenase. For in vivo studies, MSTO 211H cells were transduced with firefly (Photinus pyralis) luciferase (FLuc)- encoding cDNAs (MSTO td 211H). Tumor-bearing animals (1e7 cells injected intrapleurally) were treated with either single or multiple doses of NDV(F3aa)-GFP (1e7 plaque-forming units) at different time points (days 1, 3, and 10) and followed by bioluminescence imaging. Results: Mesothelioma cell lines exhibited susceptibility to NDV lysis in the following order of sensitivity: MSTO 211H>MSTO td 211H>H-2452>VAMT>JMN. The cell lines H-2052, H-2373, and HMESO were not sensitive to viral treatment. In vivo studies with MSTO td 211H cells showed complete response to viral therapy in >75% of the animals, resulting in eradication of tumor detected by bioluminescence imaging at day 10 after treatment. Control animals were sacrificed after 23 days due to tumor burden, while >72% of the virally treated animals survived >50 days after tumor injection. No signs of toxicity were observed in the treatment group. In addition, multiple treatment showed a significantly better response compared with single treatment (p=0.005). Conclusions: NDV appears to be an efficient viral oncolytic agent in therapy of malignant pleural mesothelioma in a murine model, and warrants further investigation as a potential therapeutic agent.

No significant financial relationships to disclose.

Multimodality imaging of TGFbeta signaling in breast cancer metastases

The skeleton is a preferred site for breast cancer metastasis. We have developed a multimodality imaging approach to monitor the transforming growth factor beta (TGFbeta) signaling pathway in bone metastases, sequentially over time in the same animal. As model systems, two MDA-MB-231 breast cancer cells lines with different metastatic tropisms, SCP2 and SCP3, were transduced with constitutive and TGFbeta-inducible reporter genes and were tested in vitro and in living animals. The sites and expansion of metastases were visualized by bioluminescence imaging using a constitutive firefly luciferase reporter, while TGFbeta signaling in metastases was monitored by microPET imaging of HSV1-TK/GFP expression with [(18)F]FEAU and by a more sensitive and cost-effective bioluminescence reporter, based on nonsecreted Gaussia luciferase. Concurrent and sequential imaging of metastases in the same animals provided insight into the location and progression of metastases, and the timing and course of TGFbeta signaling. The anticipated and newly observed differences in the imaging of tumors from two related cell lines have demonstrated that TGFbeta signal transduction pathway activity can be noninvasively imaged with high sensitivity and reproducibility, thereby providing the opportunity for an assessment of novel treatments that target TGFbeta signaling.

A novel recombinant vaccinia virus expressing the human norepinephrine transporter retains oncolytic potential and facilitates deep-tissue imaging

Noninvasive and repetitive monitoring of a virus in target tissues and/or specific organs of the body is highly desirable for the development of safe and efficient cancer virotherapeutics. We have previously shown that the oncolytic vaccinia virus GLV-1h68 can target and eradicate human tumors in mice and that its therapeutic effects can be monitored by using optical imaging. Here, we report on the development of a derivative of GLV-1h68, a novel recombinant vaccinia virus (VACV) GLV-1h99, which was constructed to carry the human norepinephrine transporter gene (hNET) under the VACV synthetic early promoter placed at the F14.5L locus for deep-tissue imaging. The hNET protein was expressed at high levels on the membranes of cells infected with this virus. Expression of the hNET protein did not negatively affect virus replication, cytolytic activity in cell culture, or in vivo virotherpeutic efficacy. GLV-1h99-mediated expression of the hNET protein in infected cells resulted in specific uptake of the radiotracer [131I]-meta-iodobenzylguanidine (MIBG). In mice, GLV-1h99-infected tumors were readily imaged by [124I]-MIBG positron emission tomography. To our knowledge, GLV-1h99 is the first oncolytic virus expressing the hNET protein that can efficiently eliminate tumors and simultaneously allow deep-tissue imaging of infected tumors.

Imaging of HSV-tk Reporter gene expression: comparison between [18F]FEAU, [18F]FFEAU, and other imaging probes

Herpes virus type 1 thymidine kinase (HSV1-tk) and the mutant HSV1-sr39tk are the 2 most widely used "reporter genes" for radiotracer-based imaging. Two pyrimidine nucleoside analogs, [18F]FEAU (1-(2'-deoxy-2'-fluoro-beta-d-arabinofuranosyl)-5-ethyluridine) and [18F]FFEAU (1-(2'-deoxy-2'-fluoro-beta-d-arabinofuranosyl)-5-(2-fluoroethyl)uridine), have generated recent interest as potential new probes for imaging HSV1-tk and HSV1-sr39tk gene expression.

METHODS

We compared [18F]FEAU and [18F]FFEAU with a series of other pyrimidine nucleoside derivatives (including 1-(2'-deoxy-2'-fluoro-beta-d-arabinofuranosyl)-5-iodouridine [FIAU]) and with acycloguanosine analogs using a stable HSV1-tk transduced cell line (RG2TK+) and wild-type RG2 cells.

RESULTS

The in vitro accumulation data and the calculated and normalized clearance constant, nKi, as well as sensitivity and selectivity indices indicated that 2 pyrimidine nucleoside probes, [18F]FEAU and [18F]FFEAU, had the best uptake characteristics. These probes were selected for further dynamic PET studies in nude rats bearing subcutaneous RG2TK+ and RG2 tumors. The 2-h postinjection [18F]FEAU uptake levels were 3.3% +/- 1.0% and 0.28% +/- 0.07% dose/cm3 in subcutaneous RG2TK+ and RG2 tumors, respectively, and 2.3% +/- 0.2% and 0.19% +/- 0.01% dose/cm3, respectively, for [18F]FFEAU. The corresponding RG2TK+/RG2 uptake ratios were 11.5 +/- 1.5 and 12.2 +/- 1.4, respectively. The inherent problem of comparing different radiolabeled pyrimidine nucleoside and guanosine-based probes for imaging HSV1-tk expression using different transduced cell lines and assay systems in the absence of an independent thymidine kinase-enzyme assay is discussed.

CONCLUSION

For HSV1-tk reporter systems that require a 1- to 4-h PET paradigm, HSV1-tk-[18F]FEAU is the current top contender.

Molecular imaging: reporter gene imaging

Non-invasive in-vivo molecular genetic imaging developed over the past decade and predominantly utilises radiotracer (PET, gamma camera, autoradiography), magnetic resonance and optical imaging technology. Molecular genetic imaging has its roots in both molecular biology and cell biology. The convergence of these disciplines and imaging modalities has provided the opportunity to address new research questions, including oncogenesis, tumour maintenance and progression, as well as responses to molecular-targeted therapy. Three different imaging strategies are described: (1) "bio-marker" or "surrogate" imaging; (2) "direct" imaging of specific molecules and pathway activity; (3) "indirect" reporter gene imaging. Examples of each imaging strategy are presented and discussed. Several applications of PET- and optical-based reporter imaging are demonstrated, including signal transduction pathway monitoring, oncogenesis in genetic mouse models, endogenous molecular genetic/biological processes and the response to therapy in animal models of human disease. Molecular imaging studies will compliment established ex-vivo molecular-biological assays that require tissue sampling by providing a spatial and a temporal dimension to our understanding of disease development and progression, as well as response to treatment. Although molecular imaging studies are currently being performed primarily in experimental animals, we optimistically expect they will be translated to human subjects with cancer and other diseases in the near future.

Imaging of hypoxia-driven gene expression in an orthotopic liver tumor model

The purpose of this study was to monitor hypoxia in an orthotopic liver tumor model using a hypoxia-sensitive reporter imaging system and to image enhanced gene expression after clamping the hepatic artery. C6 and RH7777 Morris hepatoma cells were transduced with a triple reporter gene (HSV1-tk/green fluorescent protein/firefly luciferase-triple fusion), placed under the control of a HIF-1-inducible hypoxia responsive element (HRE). The cells showed inducible luciferase activity and green fluorescent protein expression in vitro. Isolated reporter-transduced Morris hepatoma cells were used to produce tumors in livers of nude rats, and the effect of hepatic artery clamping was evaluated. Tumor hypoxia was shown by immunofluorescence microscopy with the hypoxia marker EF5 [2-(2-nitro-1H-imidazol-1-yl)-N-(2,2,3,3,3-pentafluoropropyl acetamide)] and the fluorescent perfusion marker Hoechst 33342, and by pO(2) electrode measurements. For tumor hypoxia imaging with the HRE-responsive reporter, both luciferase bioluminescence and [(18)F]2'-fluoro-2'-deoxyarabinofuranosyl-5-ethyluracil positron emission tomography was done, and the presence of hypoxia in Morris hepatoma tumors were successfully imaged by both techniques. Transient clamping of the hepatic artery caused cessation of tumor perfusion and severe hypoxia in liver tumors, but not in adjacent liver tissue. These results show that the orthotopic reporter-transduced RH7777 Morris hepatomas are natively hypoxic and poorly perfused in this animal model, and that the magnitude of hypoxia can be monitored using a HRE-responsive reporter system for both bioluminescence and positron emission tomography imaging. However, the severity of tumor ischemia after permanent ligation of the hepatic artery limits our ability to image severe hypoxia in this animal model.

HSP70-inducible hNIS-IRES-eGFP reporter imaging: response to heat shock

A retroviral vector pQHSP70/hNIS-IRES-eGFP (pQHNIG70) was constructed containing the hNIS-IRES-eGFP dual-reporter genes under the control of an inducible human heat shock protein (HSP)70 promoter and RG2-pQHSP70/hNIS-IRES-eGFP (RG2-pQHNIG70) transduced cells were generated. Heat-induced expression of both reporter genes in RG2-pQHNIG70 cells was validated by enhanced green fluorescent protein (eGFP) fluorescence-activated cell sorter, in vitro radiotracer assays, and immunoblot and immunocytochemistry. A 2.2- to 6.1-fold ((131)I(-)), a 6.1- to 14.4-fold ((99m)TcO(4)(-)), and a 5.1- to 39-fold (fluorescence) increase above baseline was observed in response to graded hyperthermia (39-43 degrees C). Increases in eGFP fluorescence and radiotracer uptake were first noted at 6 hours, reached a maximum at 24 hours, and fell toward baseline at 72 hours. A stable ratio of radiotracer uptake to eGFP fluorescence and to heat shock protein (HSP)70 protein was demonstrated over a wide range of expression levels, induced by different levels of heating. We also demonstrate that the local application of heat on RG2-pQHNIG70 xenografts can effectively induce hNIS and eGFP gene expression in vivo and that this expression can be efficiently visualized by fluorescence, scintigraphic, and micro-positron emission tomography imaging. Endogenous HSP70 protein and reporter expression was confirmed by postmortem tissue evaluations (immunoblot and immunohistochemistry). The pQHNIG70 reporter system can be used to study stress and drug responses in transduced cells and tissues.

Imaging hNET Reporter Gene Expression with 124I-MIBG

The norepinephrine transporter (NET) has recently been suggested as a useful reporter gene. We have extended this effort by constructing an internal ribosomal entry site (IRES)-linked hNET-green fluorescent protein (GFP) hybrid reporter gene for both nuclear and optical imaging.

METHODS

A retroviral vector pQCXhNET-IRES-GFP was constructed and used to generate several reporter cell lines and xenografts. Transduced cells were sorted by fluorescence-activated cell sorting based on GFP expression and used for both in vitro and in vivo imaging studies.

RESULTS

The transduced reporter cells accumulated (123)I- or (124)I-labeled metaiodobenzylguanidine (MIBG) to high levels compared with the wild-type parent cell lines. Differences in MIBG accumulation between cell lines were primarily due to differences in influx (K(1)) rather than efflux (k(2)). The estimated MIBG distribution volumes (V(d)) for transduced Jurkat, C6, and COS-7 cells were 572 +/- 13, 754 +/- 25, and 1,556 +/- 38 mL/g, respectively. A correlation between radiotracer accumulation (K(1)) and GFP fluorescence intensity was also demonstrated. Sequential imaging studies of mice bearing pQCXhNET-IRES-GFP transduced and wild-type C6 xenografts demonstrated several advantages of (124)I-MIBG small-animal PET compared with (123)I-MIBG gamma-camera/SPECT. This was primarily due to the longer half-life of (124)I and to the retention and slow clearance (half-time, 63 +/- 6 h) of MIBG from transduced xenografts compared with that from wild-type xenografts (half-time, 12 +/- 1 h) and other organs (half-time, 2.6-21 h). Very high radioactivity ratios were observed at later imaging times; at 73 h after (124)I-MIBG injection, the C6/hNET-IRES-GFP xenograft-to-muscle ratio was 293 +/- 48 whereas the C6 xenograft-to-muscle ratio was 0.71 +/- 0.19.

CONCLUSION

These studies demonstrate the potential for a wider application of hNET reporter imaging and the future translation to patient studies using radiopharmaceuticals that are currently available for both SPECT and PET.

A human-derived reporter gene for noninvasive imaging in humans: mitochondrial thymidine kinase type 2

A human-derived intrinsically nonimmunogenic reporter gene was tested for PET imaging of different molecular-genetic processes for potential clinical use.

METHODS

The human mitochondrial thymidine kinase type 2 (hTK2) reporter gene truncated at the N terminus (DeltahTK2), alone or fused with green fluorescent protein (GFP), was used for preclinical evaluation in a mouse model. The levels of enzymatic activity of DeltahTK2 and DeltahTK2 GFP proteins were assessed using radiotracer accumulation and prodrug activation assays in vitro and in subcutaneous tumors grown from the corresponding cell lines in nude mice. Kinetic analyses of (124)I-2'-fluoro-2'-deoxy-1-beta-D-beta-arabinofuranosyl-5-iodouracil (FIAU), (18)F-2'-fluoro-2'-deoxy-1-beta-D-beta-arabinofuranosyl-5-ethyluracil (FEAU), or (18)F-9-(4-(18)F-fluoro-3-hydroxymethylbutyl)guanine (FHBG) uptake in tumors and biodistribution studies were performed.

RESULTS

DeltahTK2 was successfully expressed in the cytoplasm of transduced cells. A new anti-hTK2 monoclonal antibody 8G2 was developed. The levels of FIAU and FEAU accumulation in cells expressing DeltahTK2 and DeltahTK2 GFP were at least 10-fold higher than in wild-type cells in vitro and about 6 times higher in vivo. We determined that FEAU is a more specific reporter substrate for DeltahTK2 than FIAU, whereas FHBG is not phosphorylated by this enzyme. In addition, we showed that DeltahTK2 transduced cells can be eliminated by treatment with d-arabinofuranosyl-cytosine.

CONCLUSION

We have tested a human-derived reporter gene that is likely to be nonimmunogenic and potentially allows for long-term monitoring of different molecular-genetic processes by nuclear imaging techniques in humans. Using (124)I-FIAU, (18)F-FIAU, or (18)F-FEAU, it should be possible to image DeltahTK2 reporter gene expression with PET in preclinical and clinical studies.

Multi-Modality Molecular Imaging of Tumors

Noninvasive in vivo molecular-genetic imaging uses nuclear, magnetic resonance, and optical imaging techniques. Described and discussed are "direct" imaging of specific molecules and pathway activity, "indirect" reporter gene imaging, and "bio-marker" or "surrogate" imaging. Applications of PET- and optical-based reporter imaging are demonstrated, including imaging of oncogenesis in genetic mouse models, endogenous molecular-genetic-biological properties, and response to therapy in animal models of human disease. Molecular imaging studies complement established ex vivo molecular-biological assays that require tissue sampling by providing a spatial as well as temporal dimension to our understanding of oncogenesis, and the progression and treatment of cancer. Molecular imaging studies being performed in experimental animals will be translated to animals in the near future.

Reporter gene imaging: potential impact on therapy

Positron emission tomography (PET)-based molecular-genetic imaging in living organisms has enjoyed exceptional growth over the past 5 years; this is particularly striking since it has been identified as a new discipline only within the past decade. Positron emission tomography is one of three imaging technologies (nuclear, magnetic resonance and optical) that has begun to incorporate methods that are established in molecular and cell biology research. The convergence of these disciplines and the wider application of multi-modality imaging are at the heart of this success story. Most current molecular-genetic imaging strategies are "indirect," coupling a "reporter gene" with a complimentary "reporter probe." Reporter gene constructs can be driven by constitutive promoter elements and used to monitor gene therapy vectors and the efficacy of trans gene targeting and transduction, as well as to monitor adoptive cell-based therapies. Inducible promoters can be used as "sensors" to regulate the magnitude of reporter gene expression and can be used to provide information about endogenous cell processes. Reporter systems can also be constructed to monitor mRNA stabilization and specific protein-protein interactions. Promoters can be cell specific and restrict transgene expression to certain tissue and organs. The translation of reporter gene imaging to specific clinical applications is discussed. Several examples that have potential for patient imaging studies in the near future include monitoring adenoviral-based gene therapy, oncolytic herpes virus therapy, adoptive cell-based therapies and Salmonella-based tumor-targeted cancer therapy and imaging. The primary translational applications of noninvasive in vivo reporter gene imaging are likely to be (a) quantitative monitoring of the gene therapy vector and the efficacy of transduction in clinical protocols, by imaging the location, extent and duration of transgene expression; (b) monitoring cell trafficking, targeting, replication and activation in adoptive therapies, involving ex vivo transduction of harvested immune-competent cells and stem/progenitor cells; (c) assessments of endogenous molecular events using different reporter gene imaging technologies following the development of safe, efficient and target-specific vectors for "diagnostic transductions."

Breast cancer bone metastasis mediated by the Smad tumor suppressor pathway

TGF-beta can signal by means of Smad transcription factors, which are quintessential tumor suppressors that inhibit cell proliferation, and by means of Smad-independent mechanisms, which have been implicated in tumor progression. Although Smad mutations disable this tumor-suppressive pathway in certain cancers, breast cancer cells frequently evade the cytostatic action of TGF-beta while retaining Smad function. Through immunohistochemical analysis of human breast cancer bone metastases and functional imaging of the Smad pathway in a mouse xenograft model, we provide evidence for active Smad signaling in human and mouse bone-metastatic lesions. Genetic depletion experiments further demonstrate that Smad4 contributes to the formation of osteolytic bone metastases and is essential for the induction of IL-11, a gene implicated in bone metastasis in this mouse model system. Activator protein-1 is a key participant in Smad-dependent transcriptional activation of IL-11 and its overexpression in bone-metastatic cells. Our findings provide functional evidence for a switch of the Smad pathway, from tumor-suppressor to prometastatic, in the development of breast cancer bone metastasis.