Inhibition of tumor lactate oxidation: consequences for the tumor microenvironment

Feedback loop between hypoxia and energy metabolic reprogramming aggravates the radioresistance of cancer cells

Radiotherapy is one of the mainstream approaches for cancer treatment, although the clinical outcomes are limited due to the radioresistance of tumor cells. Hypoxia and metabolic reprogramming are the hallmarks of tumor initiation and progression and are closely linked to radioresistance. Inside a tumor, the rate of angiogenesis lags behind cell proliferation, and the underdevelopment and abnormal functions of blood vessels in some loci result in oxygen deficiency in cancer cells, i.e., hypoxia. This prevents radiation from effectively eliminating the hypoxic cancer cells. Cancer cells switch to glycolysis as the main source of energy, a phenomenon known as the Warburg effect, to sustain their rapid proliferation rates. Therefore, pathways involved in metabolic reprogramming and hypoxia-induced radioresistance are promising intervention targets for cancer treatment. In this review, we discussed the mechanisms and pathways underlying radioresistance due to hypoxia and metabolic reprogramming in detail, including DNA repair, role of cancer stem cells, oxidative stress relief, autophagy regulation, angiogenesis and immune escape. In addition, we proposed the existence of a feedback loop between energy metabolic reprogramming and hypoxia, which is associated with the development and exacerbation of radioresistance in tumors. Simultaneous blockade of this feedback loop and other tumor-specific targets can be an effective approach to overcome radioresistance of cancer cells. This comprehensive overview provides new insights into the mechanisms underlying tumor radiosensitivity and progression.

Targeting immune-onco-metabolism for precision cancer therapy

Immune cells play a key role in host defence against infection and cancer. Unlike infection, cancer is a multidimensional disease where cancer cells require continuous activation of certain pathways to sustain their growth and survival. The tumour milieu plays an important role in defining the metabolic reprogramming to support this growth and evasion from the immune system. Cancer and stromal cells modulate each other's metabolism during cancer progression or regression. The mechanism related to change in the metabolism and its role in the crosstalk between tumour and immune cells is still an area of immense importance. Current treatment modalities can be immensely complemented and benefited by targeting the immuno-oncology metabolism, that can improve patient prognosis. This emerging aspect of immune-oncology metabolism is reviewed here, discussing therapeutic possibilities within various metabolic pathways and their effect on immune and cancer cell metabolism.

Microenvironmental influences on T cell immunity in cancer and inflammation

T cell metabolism is dynamic and highly regulated. While the intrinsic metabolic programs of T cell subsets are integral to their distinct differentiation and functional patterns, the ability of cells to acquire nutrients and cope with hostile microenvironments can limit these pathways. T cells must function in a wide variety of tissue settings, and how T cells interpret these signals to maintain an appropriate metabolic program for their demands or if metabolic mechanisms of immune suppression restrain immunity is an area of growing importance. Both in inflamed and cancer tissues, a wide range of changes in physical conditions and nutrient availability are now acknowledged to shape immunity. These include fever and increased temperatures, depletion of critical micro and macro-nutrients, and accumulation of inhibitory waste products. Here we review several of these factors and how the tissue microenvironment both shapes and constrains immunity.

Imaging the Rewired Metabolism in Lung Cancer in Relation to Immune Therapy

Metabolic reprogramming is recognized as one of the hallmarks of cancer. Alterations in the micro-environmental metabolic characteristics are recognized as important tools for cancer cells to interact with the resident and infiltrating T-cells within this tumor microenvironment. Cancer-induced metabolic changes in the micro-environment also affect treatment outcomes. In particular, immune therapy efficacy might be blunted because of somatic mutation-driven metabolic determinants of lung cancer such as acidity and oxygenation status. Based on these observations, new onco-immunological treatment strategies increasingly include drugs that interfere with metabolic pathways that consequently affect the composition of the lung cancer tumor microenvironment (TME). Positron emission tomography (PET) imaging has developed a wide array of tracers targeting metabolic pathways, originally intended to improve cancer detection and staging. Paralleling the developments in understanding metabolic reprogramming in cancer cells, as well as its effects on stromal, immune, and endothelial cells, a wave of studies with additional imaging tracers has been published. These tracers are yet underexploited in the perspective of immune therapy. In this review, we provide an overview of currently available PET tracers for clinical studies and discuss their potential roles in the development of effective immune therapeutic strategies, with a focus on lung cancer. We report on ongoing efforts that include PET/CT to understand the outcomes of interactions between cancer cells and T-cells in the lung cancer microenvironment, and we identify areas of research which are yet unchartered. Thereby, we aim to provide a starting point for molecular imaging driven studies to understand and exploit metabolic features of lung cancer to optimize immune therapy.

Treatment Strategies and Metabolic Pathway Regulation in Urothelial Cell Carcinoma: A Comprehensive Review

For a long time, cisplatin-based chemotherapy had been viewed as first-line chemotherapy for advanced and metastatic urothelial carcinoma (UC). However, many patients with UC had been classified as cisplatin-ineligible who can only receive alternative chemotherapy with poor treatment response, and the vast majority of the cisplatin-eligible patients eventually progressed, even those with objective response with cisplatin-based chemotherapy initially. By understanding tumor immunology in UC, immune checkpoint inhibitors, targeting on programmed death 1 (PD-1) and cytotoxic T lymphocyte-associated antigen 4 (CTLA-4) pathways, had been proven as first-line treatment for cisplatin-ineligible metastatic UC and as second-line treatment for patients with platinum-refractory metastatic UC by the U.S Food and Drug Administration (FDA). In 2020, JAVEIN bladder 100 further reported that PD-L1 inhibitors showed benefits on prolonged survival and progression-free survival as maintenance therapy. Besides targeting on immune checkpoint, manipulation of the tumor microenvironment by metabolic pathways intervention, including inhibition on tumor glycolysis, lactate accumulation and exogenous glutamine uptake, had been investigated in the past few years. In this comprehensive review, we start by introducing traditional chemotherapy of UC, and then we summarize current evidences supporting the use of immune checkpoint inhibitors and highlight ongoing clinical trials. Lastly, we reviewed the tumor metabolic characteristic and the anti-tumor treatments targeting on metabolic pathways.

Dissecting the Proton Transport Pathway in Oral Squamous Cell Carcinoma: State of the Art and Theranostics Implications

Cancer cells overexpress proton exchangers at the plasma membrane in order acidify the extracellular matrix and maintain the optimal pH for sustaining cancer growth. Among the families of proton exchangers implicated in carcinogenesis, carbonic anhydrases (CAs), monocarboxylate transporters (MCTs), Na⁺/H⁺ exchangers (NHEs), sodium bicarbonate cotransporters (NBCs), and vacuolar ATPases (V-ATPases) are highlighted. Considerable research has been carried out into the utility of the understanding of these machineries in the diagnosis and prognosis of several solid tumors. In addition, as therapeutic targets, the interference of their functions has contributed to the discovery or optimization of cancer therapies. According to recent reports, the study of these mechanisms seems promising in the particular case of oral squamous cell carcinoma (OSCC). In the present review, the latest advances in these fields are summarized, in particular, the usefulness of proton exchangers as potential prognostic biomarkers and therapeutic targets in OSCC.

Lactic Acid Accumulation in the Tumor Microenvironment Suppresses 18F-FDG Uptake

The process by which tumor cells take up 2-[¹⁸F]fluoro-2-deoxy-D-glucose (FDG) is heterogeneous and influenced by a multitude of factors. In mouse tumor grafts, the core of the tumor often presents lower FDG uptake than the periphery. Whether this pattern is caused by the intrinsic avidity of individual cells for FDG, the density of viable cells in the tumor, or the perfusion of the radiotracer remains unknown. In this study, we used radioluminescence microscopy to measure FDG uptake in single cells isolated from the core and periphery of the tumor and found that differences in FDG uptake persist on the level of single cells. Single cells from the core of 4T1 and MDA-MB-231 tumors grafts took up 26% to 84% less FDG than those from the periphery. These differences were observed in mice with large tumors (>8 mm diameter) but not in those with smaller tumors. To explain the origin of these differences, we examined the influence of three microenvironmental factors on FDG uptake. Hypoxia was ruled out as a possible explanation because its presence in the core would increase and not decrease FDG uptake. Higher cell proliferation in the periphery was consistent with higher FDG uptake, but there was no evidence of a causal relationship. Finally, lactate was higher in the core of the tumor, and it suppressed FDG uptake in a dose-dependent fashion. We therefore conclude that lactic acidosis-the combination of lactate ion buildup and acidic pH-can increase the heterogeneity of FDG uptake in MDA-MB-231 and 4T1 tumor grafts. SIGNIFICANCE: Analysis of single cells from heterogeneous tumors reveals the role played by the tumor microenvironment, lactic acidosis in particular, on the uptake by tumor cells of ¹⁸F-FDG, a PET imaging agent.

Immunometabolism in cancer at a glance

The scientific knowledge about tumor metabolism has grown at a fascinating rate in recent decades. We now know that tumors are highly active both in their metabolism of available nutrients and in the secretion of metabolic by-products. However, cancer cells can modulate metabolic pathways and thus adapt to specific nutrients. Unlike tumor cells, immune cells are not subject to a ‘micro-evolution’ that would allow them to adapt to progressing tumors that continuously develop new mechanisms of immune escape. Consequently, immune cells are often irreversibly affected and may allow or even support cancer progression. The mechanisms of how tumors change immune cell function are not sufficiently explored. It is, however, clear that commonly shared features of tumor metabolism, such as local nutrient depletion or production of metabolic ‘waste’ can broadly affect immune cells and contribute to immune evasion. Moreover, immune cells utilize different metabolic programs based on their subtype and function, and these immunometabolic pathways can be modified in the tumor microenvironment. In this review and accompanying poster, we identify and describe the common mechanisms by which tumors metabolically affect the tumor-infiltrating cells of native and adaptive immunity, and discuss how these mechanisms may lead to novel therapeutic opportunities.

Summary: This ‘At a Glance’ review and accompanying poster address how tumors can negatively affect immune cells through depletion of critical nutrients or through production of toxic metabolic products.

Dysfunctional T cell metabolism in the tumor microenvironment

Metabolic and signaling pathways are integrated to determine T cell fate and function. As stimulated T cells gain distinct effector functions, specific metabolic programs and demands are also adopted. These changes are essential for T cell effector function, and alterations or dysregulation of metabolic pathways can modulate T cell function. One physiological setting that impacts T cell metabolism is the tumor microenvironment. The metabolism of cancer cells themselves can limit nutrients and accumulate waste products. In addition to the expression of inhibitory ligands that directly modify T cell physiology, T cell metabolism may be strongly inhibited in the tumor microenvironment. This suppression of T cell metabolism may inhibit effector T cell activity while promoting suppressive regulatory T cells, and act as a barrier to effective immunotherapies. A thorough understanding of the effect of the tumor microenvironment on the immune system will support the continued improvement of immune based therapies for cancer patients.

MACC1 mediates chemotherapy sensitivity of 5-FU and cisplatin via regulating MCT1 expression in gastric cancer

Chemotherapeutic insensitivity is a main obstacle for effective treatment of gastric cancer (GC), the underlying mechanism remains to be investigated. Metastasis-associated in colon cancer-1 (MACC1), a transcription factor highly expressed in GC, is found to be related to chemotherapy sensitivity. Monocarboxylate transporter 1 (MCT1), a plasma membrane protein co-transporting lactate and H⁺, mediates drug sensitivity by regulating lactate metabolism. Targeting MCT1 has recently been regarded as a promising way to treat cancers and MCT1 inhibitor has entered the clinical trial for GC treatment. However, the correlation of these two genes and their combined effects on chemotherapy sensitivity has not been clarified. In this study, we found that MACC1 and MCT1 were both highly expressed in GC and exhibited a positive correlation in clinical samples. Further, we demonstrated that MACC1 could mediate sensitivity of 5-FU and cisplatin in GC cells, and MACC1 mediated MCT1 regulation was closely related to this sensitivity. A MCT1 inhibitor AZD3965 recovered the sensitivity of 5-FU and cisplatin in GC cells which overexpressed MACC1. These results suggested that MACC1 could influence the chemotherapy sensitivity by regulating MCT1 expression, providing new ideas and strategy for GC treatment.

Lactate promotes glutamine uptake and metabolism in oxidative cancer cells

Oxygenated cancer cells have a high metabolic plasticity as they can use glucose, glutamine and lactate as main substrates to support their bioenergetic and biosynthetic activities. Metabolic optimization requires integration. While glycolysis and glutaminolysis can cooperate to support cellular proliferation, oxidative lactate metabolism opposes glycolysis in oxidative cancer cells engaged in a symbiotic relation with their hypoxic/glycolytic neighbors. However, little is known concerning the relationship between oxidative lactate metabolism and glutamine metabolism. Using SiHa and HeLa human cancer cells, this study reports that intracellular lactate signaling promotes glutamine uptake and metabolism in oxidative cancer cells. It depends on the uptake of extracellular lactate by monocarboxylate transporter 1 (MCT1). Lactate first stabilizes hypoxia-inducible factor-2α (HIF-2α), and HIF-2α then transactivates c-Myc in a pathway that mimics a response to hypoxia. Consequently, lactate-induced c-Myc activation triggers the expression of glutamine transporter ASCT2 and of glutaminase 1 (GLS1), resulting in improved glutamine uptake and catabolism. Elucidation of this metabolic dependence could be of therapeutic interest. First, inhibitors of lactate uptake targeting MCT1 are currently entering clinical trials. They have the potential to indirectly repress glutaminolysis. Second, in oxidative cancer cells, resistance to glutaminolysis inhibition could arise from compensation by oxidative lactate metabolism and increased lactate signaling.

Thyroid Cancer Metabolism: A Review

Metabolic dysregulation within the tumor microenvironment (TME) is critical to the process of tumorigenesis in various cancer types. Thyrocyte metabolism in papillary and anaplastic thyroid cancer, however, remains poorly characterized, and studies analyzing the role of multicompartment metabolism in thyrocyte oncogenesis are sparse. We present a review of the current knowledge on cellular metabolism in non-cancerous and cancerous thyroid tissues, focusing on the monocarboxylate transporters MCT1 and MCT4, and on a transporter of the outer mitochondrial membrane TOMM20. Understanding the metabolic phenotype of tumor cells and associated stromal cells in thyroid cancer can have profound implications on the use of biomarker staining in detecting subclinical cancer, imaging as it relates to expression of various transport proteins, and therapeutic interventions that manipulate this dysregulated tumor metabolism to halt tumorigenesis and eradicate the cancer. Future studies are required to confirm the prognostic significance of these biomarkers and their correlation with existing staging schemas such as the AGES, AMES, ATA and MACIS scoring systems.

Quantitative constraint-based computational model of tumor-to-stroma coupling via lactate shuttle

Cancer cells utilize large amounts of ATP to sustain growth, relying primarily on non-oxidative, fermentative pathways for its production. In many types of cancers this leads, even in the presence of oxygen, to the secretion of carbon equivalents (usually in the form of lactate) in the cell’s surroundings, a feature known as the Warburg effect. While the molecular basis of this phenomenon are still to be elucidated, it is clear that the spilling of energy resources contributes to creating a peculiar microenvironment for tumors, possibly characterized by a degree of toxicity. This suggests that mechanisms for recycling the fermentation products (e.g. a lactate shuttle) may be active, effectively inducing a mutually beneficial metabolic coupling between aberrant and non-aberrant cells. Here we analyze this scenario through a large-scale in silico metabolic model of interacting human cells. By going beyond the cell-autonomous description, we show that elementary physico-chemical constraints indeed favor the establishment of such a coupling under very broad conditions. The characterization we obtained by tuning the aberrant cell’s demand for ATP, amino-acids and fatty acids and/or the imbalance in nutrient partitioning provides quantitative support to the idea that synergistic multi-cell effects play a central role in cancer sustainment.

Lactate, a putative survival factor for myeloma cells, is incorporated by myeloma cells through monocarboxylate transporters 1

Background

Lactate levels within tumors are correlated with metastases, tumor recurrence, and radioresistance, thus apparently contributing to poor outcomes in patients with various cancers. We previously reported that high-level production of lactate by multiple myeloma (MM) cell lines is associated with high-level LDH activity within such MM cells. However, the kinetics of lactate remains to be studied. In the present study, we attempted to elucidate the mechanism of lactate incorporation into MM cells.

Methods

Six MM cell lines and stromal cells obtained through long-term culture of bone marrow samples from MM patients were employed. Incorporation of lactate was quantified using C¹⁴-labeled lactate. The role of MCT1, a member of the monocarboxylate transporters (MCTs), expressed on MM cells, was examined in the presence of its inhibitor (α-cyano-4-hydroxycinnamic acid: CHC) and by using gene-silencing technique.

Results

MM cell lines as well as stromal cells were found to produce lactate. Incorporation of C¹⁴-labeled lactate into MM cells occurred in all 6 MM cell lines analyzed. Inhibition of MCT1 by using CHC or MCT1-targeting siRNA reduced lactate incorporation and caused apoptosis in MM cells. This apoptosis was enhanced when the activity of pyruvate dehydrogenase kinase was blocked by dichroloacetate. Survival of normal peripheral blood mononuclear cells was not influenced by MCT1 inhibition.

Conclusions

The present data suggest that lactate is produced by MM cell lines and stromal cells, and contributes to the survival of such MM cells in autocrine or paracrine manners. Suppression of lactate incorporation by targeting MCT1 may provide a novel therapeutic strategy for MM which may be applicable for other B-cell neoplasms.

Inhibition of monocarboxylate transporter-1 (MCT1) by AZD3965 enhances radiosensitivity by reducing lactate transport

Inhibition of the monocarboxylate transporter MCT1 by AZD3965 results in an increase in glycolysis in human tumor cell lines and xenografts. This is indicated by changes in the levels of specific glycolytic metabolites and in changes in glycolytic enzyme kinetics. These drug-induced metabolic changes translate into an inhibition of tumor growth in vivo. Thus, we combined AZD3965 with fractionated radiation to treat small cell lung cancer (SCLC) xenografts and showed that the combination provided a significantly greater therapeutic effect than the use of either modality alone. These results strongly support the notion of combining MCT1 inhibition with radiotherapy in the treatment of SCLC and other solid tumors.

Proton channels and exchangers in cancer

Although cancer is characterized by an intratumoral genetic heterogeneity, a totally deranged pH control is a common feature of most cancer histotypes. Major determinants of aberrant pH gradient in cancer are proton exchangers and transporters, including V-ATPase, Na+/H+ exchanger (NHE), monocarboxylate transporters (MCTs) and carbonic anhydrases (CAs). Thanks to the activity of these proton transporters and exchangers, cancer becomes isolated and/or protected not only from the body reaction against the growing tumor, but also from the vast majority of drugs that when protonated into the acidic tumor microenvironment do not enter into cancer cells. Proton transporters and exchangers represent a key feature tumor cells use to survive in the very hostile microenvironmental conditions that they create and maintain. Detoxifying mechanisms may thus represent both a key survival option and a selection outcome for cells that behave as unicellular microorganisms rather than belonging to an organ, compartment or body. It is, in fact, typical of malignant tumors that, after a clinically measurable yet transient initial response to a therapy, resistant tumor clones emerge and proliferate, thus bursting a more malignant behavior and rapid tumor progression. This review critically presents the background of a novel and efficient approach that aims to fight cancer through blocking or inhibiting well characterized proton exchangers and transporters active in human cancer cells. This article is part of a Special Issue entitled: Membrane channels and transporters in cancers.

Glucose-lactate metabolic cooperation in cancer: insights from a spatial mathematical model and implications for targeted therapy

A recent study has hypothesised a glucose-lactate metabolic symbiosis between adjacent hypoxic and oxygenated regions of a developing tumour, and proposed a treatment strategy to target this symbiosis. However, in vivo experimental support remains inconclusive. Here we develop a minimal spatial mathematical model of glucose-lactate metabolism to examine, in principle, whether metabolic symbiosis is plausible in human tumours, and to assess the potential impact of inhibiting it. We find that symbiosis is a robust feature of our model system-although on the length scale at which oxygen supply is diffusion-limited, its occurrence requires very high cellular metabolic activity-and that necrosis in the tumour core is reduced in the presence of symbiosis. Upon simulating therapeutic inhibition of lactate uptake, we predict that targeted treatment increases the extent of tissue oxygenation without increasing core necrosis. The oxygenation effect is correlated strongly with the extent of wild-type hypoxia and only weakly with wild-type symbiotic behaviour, and therefore may be promising for radiosensitisation of hypoxic, lactate-consuming tumours even if they do not exhibit a spatially well-defined symbiosis. Finally, we conduct in vitro experiments on the U87 glioblastoma cell line to facilitate preliminary speculation as to where highly malignant tumours might fall in our parameter space, and find that these experiments suggest a weakly symbiotic regime for U87 cells, thus raising the new question of what relationship might exist between symbiosis and tumour malignancy.

Monocarboxylate transporters 1-4 in NSCLC: MCT1 is an independent prognostic marker for survival

Introduction

Monocarboxylate transporters (MCTs) 1–4 are lactate transporters crucial for cancers cells adaption to upregulated glycolysis. Herein, we aimed to explore their prognostic impact on disease-specific survival (DSS) in both cancer and tumor stromal cells in NSCLC.

Methods

Tissue micro arrays (TMAs) were constructed, representing both cancer and stromal tumor tissue from 335 unselected patients diagnosed with stage I–IIIA NSCLC. Immunohistochemistry was used to evaluate the expression of MCT1-4.

Results

In univariate analyses; ↓MCT1 (P = 0.021) and ↑MCT4 (P = 0.027) expression in cancer cells, and ↑MCT1 (P = 0.003), ↓MCT2 (P = 0.006), ↓MCT3 (P = 0.020) expression in stromal cells correlated significantly with a poor DSS. In multivariate analyses; ↓MCT1 expression in cancer cells (HR: 1.9, CI 95%: 1.3–2.8, P = 0.001), ↓MCT2 (HR: 2.4, CI 95%: 1.5–3.9, P<0.001), ↓MCT3 (HR: 1.9, CI 95%: 1.1–3.5, P = 0.031) and ↑MCT1 expression in stromal cells (HR: 1.7, CI 95%: 1.1–2.7, P = 0.016) were significant independent poor prognostic markers for DSS.

Conclusions

We provide novel information of MCT1 as a candidate marker for prognostic stratification in NSCLC. Interestingly, MCT1 shows diverging, independent prognostic impact in the cancer cell and stromal cell compartments.

Tumor microenvironment in head and neck squamous cell carcinoma

The tumor microenvironment (TME) of head and neck squamous cell carcinoma (HNSCC) is comprised of cancer-associated fibroblasts (CAFs), immune cells, and other supporting cells. Genetic changes in the carcinoma cells, such as alterations to TP53, NOTCH1, and specific gene expression profiles, contribute to derangements in cancer and microenvironment cells such as increased ROS, overproduction of cytokines, and epithelial to mesenchymal transition (EMT). CAFs are among the most critical elements of the TME contributing to proliferation, invasion, and metastasis. The adaptive immune response is suppressed in HNSCC through overexpression of cytokines, triggered apoptosis of T cells, and alterations in antigen processing machinery. Overexpression of critical cytokines, such as transforming growth factor-β (TGF-β), contributes to EMT, immune suppression, and evolution of CAFs. Inflammation and hypoxia are driving forces in angiogenesis and altered metabolism. HNSCC utilizes glycolytic and oxidative metabolism to fuel tumorigenesis via coupled mechanisms between cancer cell regions and cells of the TME. Increased understanding of the TME in HNSCC illustrates that the long-held notion of "condemned mucosa" reflects a process that extends beyond the epithelial cells to the entire tissue comprised of each of these elements.

Targeting lactate metabolism for cancer therapeutics

Lactate, once considered a waste product of glycolysis, has emerged as a critical regulator of cancer development, maintenance, and metastasis. Indeed, tumor lactate levels correlate with increased metastasis, tumor recurrence, and poor outcome. Lactate mediates cancer cell intrinsic effects on metabolism and has additional non-tumor cell autonomous effects that drive tumorigenesis. Tumor cells can metabolize lactate as an energy source and shuttle lactate to neighboring cancer cells, adjacent stroma, and vascular endothelial cells, which induces metabolic reprogramming. Lactate also plays roles in promoting tumor inflammation and in functioning as a signaling molecule that stimulates tumor angiogenesis. Here we review the mechanisms of lactate production and transport and highlight emerging evidence indicating that targeting lactate metabolism is a promising approach for cancer therapeutics.

Simultaneous PLK1 inhibition improves local tumour control after fractionated irradiation

PURPOSE

Polo-like kinase 1 (PLK1) plays an important role in mitotic progression, is frequently overexpressed and associated with a poor prognosis of cancer patients, thus providing a promising target in anticancer treatment. Aim of the current project was to evaluate the effect of the novel PLK1 inhibitor BI 6727 in combination with irradiation.

MATERIAL AND METHODS

In vitro proliferation and radiation cell survival assays as well as in vivo local tumour control assays after single treatment and combined radiation and drug application were carried out using the squamous cell carcinoma models A431 and FaDu. In addition, cell cycle phases were monitored in vitro and in vivo.

RESULTS

BI 6727 showed a dose-dependent antiproliferative effect and an increase in the mitotic fraction. BI 6727 alone reduced clonogenic cell survival, while radiosensitivity in vitro (SF2) and in vivo (single-dose TCD(50) under clamped hypoxia) was not affected. In contrast, local tumour control was significantly improved after application of BI 6727 simultaneously to fractionated irradiation (A431: TCD(50) = 60.5 Gy [95% C.I. 57; 63] after IR alone and <30 Gy after combined treatment; FaDu: 49.5 Gy [43; 56 Gy] versus 32.9 Gy [26; 40]).

CONCLUSIONS

Despite the lack of direct cellular radiosensitisation, PLK1 inhibition with BI 6727 during fractionated irradiation significantly improves local tumour control when compared to irradiation alone. This result is likely explained by a considerable effect on cell cycle and an independent cytotoxic potential of BI 6727.

PET hypoxia imaging with FAZA: reproducibility at baseline and during fractionated radiotherapy in tumour-bearing mice

PURPOSE

Tumour hypoxia is linked to treatment resistance. Positron emission tomography (PET) using hypoxia tracers such as fluoroazomycin arabinoside (FAZA) may allow identification of patients with hypoxic tumours and the monitoring of the efficacy of hypoxia-targeting treatment. Since hypoxia PET is characterized by poor image contrast, and tumour hypoxia undergoes spontaneous changes and is affected by therapy, it remains unclear to what extent PET scans are reproducible. Tumour-bearing mice are valuable in the validation of hypoxia PET, but identification of a reliable reference tissue value (blood sample or image-derived muscle value) for repeated scans may be difficult due to the small size of the animal or absence of anatomical information (pure PET). Here tumour hypoxia was monitored over time using repeated PET scans in individual tumour-bearing mice before and during fractionated radiotherapy.

METHODS

Mice bearing human SiHa cervix tumour xenografts underwent a PET scan 3 h following injection of FAZA on two consecutive days before initiation of treatment (baseline) and again following irradiation with four and ten fractions of 2.5 Gy. On the last scan day, mice were given an intraperitoneal injection of pimonidazole (hypoxia marker), tumours were collected and the intratumoral distribution of FAZA (autoradiography) and hypoxia (pimonidazole immunohistology) were determined in cryosections.

RESULTS

Tissue section analysis revealed that the intratumoral distribution of FAZA was strongly correlated with the regional density of hypoxic (pimonidazole-positive) cells, even when necrosis was present, suggesting that FAZA PET provides a reliable measure of tumour hypoxia at the time of the scan. PET-based quantification of tumour tracer uptake relative to injected dose showed excellent reproducibility at baseline, whereas normalization using an image-derived nonhypoxic reference tissue (muscle) proved highly unreliable since a valid and reliable reference value could not be determined. The intratumoral distribution of tracer was stable at baseline as shown by a voxel-by-voxel comparison of the two scans (R = 0.82, range 0.72-0.90). During treatment, overall tracer retention changed in individual mice, but there was no evidence of general reoxygenation.

CONCLUSION

Hypoxia PET scans are quantitatively correct and highly reproducible in tumour-bearing mice. Preclinical hypoxia PET is therefore a valuable and reliable tool for the development of strategies that target or modify hypoxia.

Small-molecule inhibitors of the HIF pathway and synthetic lethal interactions

INTRODUCTION

Activation of the hypoxia response pathway is a feature of many tumours and is one of the key mechanisms associated with tumour growth, chemoresistance and radioresistance. The major component of the hypoxia response pathway is the heterodimeric transcription factor, hypoxia-inducible factor (HIF), which is upregulated in many human cancers. Therefore, HIF is an attractive therapeutic target and several strategies have been developed to target it.

AREAS COVERED

Approaches used in targeting the hypoxia response pathway are discussed. Reviewed are agents that target upstream, directly and downstream of HIF, as well as some of the challenges in HIF-targeted therapy.

EXPERT OPINION

Many of the therapeutic agents that are in clinical use inhibit downstream HIF target genes, but ideally a molecule specific to HIF will have a more potent effect in inhibiting multiple HIF pathways. However, many anti-HIF molecules have multiple targets, which may increase non-specific cytotoxicity. In addition, many anti-HIF agents cannot discriminate between the different isoforms of HIF-α. So, it is important to assess whether targeting both HIF-1α and HIF-2α or each subunit selectively will provide better therapeutic effects.