Monitoring vascular normalization induced by antiangiogenic treatment with (18)F-fluoromisonidazole-PET

Current trends in the characterization and monitoring of vascular response to cancer therapy

Tumor vascular physiology is an important determinant of disease progression as well as the therapeutic outcome of cancer treatment. Angiogenesis or the lack of it provides crucial information about the tumor's blood supply and therefore can be used as an index for cancer growth and progression. While standalone anti-angiogenic therapy demonstrated limited therapeutic benefits, its combination with chemotherapeutic agents improved the overall survival of cancer patients. This could be attributed to the effect of vascular normalization, a dynamic process that temporarily reverts abnormal vasculature to the normal phenotype maximizing the delivery and intratumor distribution of chemotherapeutic agents. Longitudinal monitoring of vascular changes following antiangiogenic therapy can indicate an optimal window for drug administration and estimate the potential outcome of treatment. This review primarily focuses on the status of various imaging modalities used for the longitudinal characterization of vascular changes before and after anti-angiogenic therapies and their clinical prospects.

Adaptation to Hypoxia May Promote Therapeutic Resistance to Androgen Receptor Inhibition in Triple-Negative Breast Cancer

Triple-negative breast cancer (TNBC) surpasses other BC subtypes as the most challenging to treat due to its lack of traditional BC biomarkers. Nearly 30% of TNBC patients express the androgen receptor (AR), and the blockade of androgen production and AR signaling have been the cornerstones of therapies for AR-positive TNBC. However, the majority of women are resistant to AR-targeted therapy, which is a major impediment to improving outcomes for the AR-positive TNBC subpopulation. The hypoxia signaling cascade is frequently activated in the tumor microenvironment in response to low oxygen levels; activation of the hypoxia signaling cascade allows tumors to survive despite hypoxia-mediated interference with cellular metabolism. The activation of hypoxia signaling networks in TNBC promotes resistance to most anticancer drugs including AR inhibitors. The activation of hypoxia network signaling occurs more frequently in TNBC compared to other BC subtypes. Herein, we examine the (1) interplay between hypoxia signaling networks and AR and (2) whether hypoxia and hypoxic stress adaptive pathways promote the emergence of resistance to therapies that target AR. We also pose the well-supported question, “Can the efficacy of androgen-/AR-targeted treatments be enhanced by co-targeting hypoxia?” By critically examining the evidence and the complex entwinement of these two oncogenic pathways, we argue that the simultaneous targeting of androgen biosynthesis/AR signaling and hypoxia may enhance the sensitivity of AR-positive TNBCs to AR-targeted treatments, derail the emergence of therapy resistance, and improve patient outcomes.

Vascular Normalization: A New Window Opened for Cancer Therapies

Preclinical and clinical antiangiogenic approaches, with multiple side effects such as resistance, have not been proved to be very successful in treating tumor blood vessels which are important targets for tumor therapy. Meanwhile, restoring aberrant tumor blood vessels, known as tumor vascular normalization, has been shown not only capable of reducing tumor invasion and metastasis but also of enhancing the effectiveness of chemotherapy, radiation therapy, and immunotherapy. In addition to the introduction of such methods of promoting tumor vascular normalization such as maintaining the balance between proangiogenic and antiangiogenic factors and targeting endothelial cell metabolism, microRNAs, and the extracellular matrix, the latest molecular mechanisms and the potential connections between them were primarily explored. In particular, the immunotherapy-induced normalization of blood vessels further promotes infiltration of immune effector cells, which in turn improves immunotherapy, thus forming an enhanced loop. Thus, immunotherapy in combination with antiangiogenic agents is recommended. Finally, we introduce the imaging technologies and serum markers, which can be used to determine the window for tumor vascular normalization.

Normalization in tumor ecosystem: Opportunities and challenges

Current research in cancer therapy aims to exploit efficient strategies to have long-lasting effects on tumors and to reduce or even revoke the chance of recurrence. Within the tumor stroma, O₂ and nutrients are abnormally distributed between various cells (preferentially for supplying cancer cells), the immune contexture is abnormally positioned (permissive essentially for cells exhibiting tumor-promoting capacity), the fibroblast and fibrotic content is abnormally distributed (presence of both extracellular matrix [ECM] stiffening and ECM-degrading factors both for tumor-promoting purposes), and the tumor vasculature is abnormally orchestrated (for hindering drug delivery and increasing the chance of tumor metastasis). Resistance is actually an adaptive response to an imbalance in the tumor ecosystem; thus, the key consideration for effective cancer therapy is to bring back the normal status in this ecosystem so as to reach the desired durable outcome. Vascular normalization, metabolic modulation (glucose delivery in particular), balancing cellular dispersion, and balancing the pH rate and O₂ delivery within the tumor microenvironment are suggested strategies to reverse abnormality within the tumor stroma.

Monitoring vascular normalization: new opportunities for mitochondrial inhibitors in breast cancer

Preclinical evidence indicates the potential of targeting mitochondrial respiration as a therapeutic strategy. We previously demonstrated that mitochondrial inhibitors’ efficacy was restricted to a metabolic context in which mitochondrial respiration was the predominant energy source, a situation achievable by inducing vascular normalization/hypoxia correction with antiangiogenics. Using molecular imaging, we showed how the same antiangiogenic agent may display different normalizing properties in patients with the same tumor type. This is of key importance, since patients experiencing normalization seem to get more benefit from standard chemotherapy combinations, and also could be eligible for combination with antimitochondrial agents. This scenario emphasizes the need for monitoring vascular normalization in order to optimize the use of antiangiogenics. We have also proposed a method to evaluate anti-mitochondrial agents’ pharmacodynamics; despite promising accuracy in animal studies the clinical results were inconclusive, highlighting the need for research in this field. Regarding patients that respond to antiangiogenics increasing vessel abnormality, in this case an immunosuppressive tumor microenvironment is generated. Whether anti-mitochondrial agents can positively modulate the activity of T effector cell subpopulations remains an area of active research. Our research sheds light on the importance of refining the use of antiangiogenics, highlighting the relevance of tracing vascular normalization as a potential biomarker for antiangiogenics to assist patient-tailored medicine and exploring the role of mitochondrial inhibitors in the context of vascular normalization and correction of hypoxia.

[18F]FMISO PET/CT imaging of hypoxia as a non-invasive biomarker of disease progression and therapy efficacy in a preclinical model of pulmonary fibrosis: comparison with the [18F]FDG PET/CT approach

Purpose

Idiopathic pulmonary fibrosis (IPF) is a progressive disease with poor outcome and limited therapeutic options. Imaging of IPF is limited to high-resolution computed tomography (HRCT) which is often not sufficient for a definite diagnosis and has a limited impact on therapeutic decision and patient management. Hypoxia of the lung is a significant feature of IPF but its role on disease progression remains elusive. Thus, the aim of our study was to evaluate hypoxia imaging with [¹⁸F]FMISO as a predictive biomarker of disease progression and therapy efficacy in preclinical models of lung fibrosis in comparison with [¹⁸F]FDG.

Methods

Eight-week-old C57/BL6 mice received an intratracheal administration of bleomycin (BLM) at day (D) 0 to initiate lung fibrosis. Mice received pirfenidone (300 mg/kg) or nintedanib (60 mg/kg) by daily gavage from D9 to D23. Mice underwent successive PET/CT imaging at several stages of the disease (baseline, D8/D9, D15/D16, D22/D23) with [¹⁸F]FDG and [¹⁸F]FMISO. Histological determination of the lung expression of HIF-1α and GLUT-1 was performed at D23.

Results

We demonstrate that mean lung density on CT as well as [¹⁸F]FDG and [¹⁸F]FMISO uptakes are upregulated in established lung fibrosis (1.4-, 2.6- and 3.2-fold increase respectively). At early stages, lung areas with [¹⁸F]FMISO uptake are still appearing normal on CT scans and correspond to areas which will deteriorate towards fibrotic lesions at later timepoints. Nintedanib and pirfenidone dramatically and rapidly decreased mean lung density on CT as well as [¹⁸F]FDG and [¹⁸F]FMISO lung uptakes (pirfenidone: 1.2-, 2.9- and 2.6-fold decrease; nintedanib: 1.2-, 2.3- and 2.5-fold decrease respectively). Early [¹⁸F]FMISO lung uptake was correlated with aggressive disease progression and better nintedanib efficacy.

Conclusion

[¹⁸F]FMISO PET imaging is a promising tool to early detect and monitor lung fibrosis progression and therapy efficacy.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00259-021-05209-2.

Tumor Hypoxia Regulates Immune Escape/Invasion: Influence on Angiogenesis and Potential Impact of Hypoxic Biomarkers on Cancer Therapies

The environmental and metabolic pressures in the tumor microenvironment (TME) play a key role in molding tumor development by impacting the stromal and immune cell fractions, TME composition and activation. Hypoxia triggers a cascade of events that promote tumor growth, enhance resistance to the anti-tumor immune response and instigate tumor angiogenesis. During growth, the developing angiogenesis is pathological and gives rise to a haphazardly shaped and leaky tumor vasculature with abnormal properties. Accordingly, aberrantly vascularized TME induces immunosuppression and maintains a continuous hypoxic state. Normalizing the tumor vasculature to restore its vascular integrity, should hence enhance tumor perfusion, relieving hypoxia, and reshaping anti-tumor immunity. Emerging vascular normalization strategies have a great potential in achieving a stable normalization, resulting in mature and functional blood vessels that alleviate tumor hypoxia. Biomarkers enabling the detection and monitoring of tumor hypoxia could be highly advantageous in aiding the translation of novel normalization strategies to clinical application, alone, or in combination with other treatment modalities, such as immunotherapy.

Manipulation of immune‒vascular crosstalk: new strategies towards cancer treatment

Tumor vasculature is characterized by aberrant structure and function, resulting in immune suppressive profiles of tumor microenvironment through limiting immune cell infiltration into tumors, endogenous immune surveillance and immune cell function. Vascular normalization as a novel therapeutic strategy tends to prune some of the immature blood vessels and fortify the structure and function of the remaining vessels, thus improving immune stimulation and the efficacy of immunotherapy. Interestingly, the presence of "immune‒vascular crosstalk" enables the formation of a positive feedback loop between vascular normalization and immune reprogramming, providing the possibility to develop new cancer therapeutic strategies. The applications of nanomedicine in vascular-targeting therapy in cancer have gained increasing attention due to its specific physical and chemical properties. Here, we reviewed the recent advances of effective routes, especially nanomedicine, for normalizing tumor vasculature. We also summarized the development of enhancing nanoparticle-based anticancer drug delivery via the employment of transcytosis and mimicking immune cell extravasation. This review explores the potential to optimize nanomedicine-based therapeutic strategies as an alternative option for cancer treatment.

Spatial Characterization of Tumor Perfusion Properties from 3D DCE-US Perfusion Maps are Early Predictors of Cancer Treatment Response

There is a need for noninvasive repeatable biomarkers to detect early cancer treatment response and spare non-responders unnecessary morbidities and costs. Here, we introduce three-dimensional (3D) dynamic contrast enhanced ultrasound (DCE-US) perfusion map characterization as inexpensive, bedside and longitudinal indicator of tumor perfusion for prediction of vascular changes and therapy response. More specifically, we developed computational tools to generate perfusion maps in 3D of tumor blood flow, and identified repeatable quantitative features to use in machine-learning models to capture subtle multi-parametric perfusion properties, including heterogeneity. Models were developed and trained in mice data and tested in a separate mouse cohort, as well as early validation clinical data consisting of patients receiving therapy for liver metastases. Models had excellent (ROC-AUC > 0.9) prediction of response in pre-clinical data, as well as proof-of-concept clinical data. Significant correlations with histological assessments of tumor vasculature were noted (Spearman R > 0.70) in pre-clinical data. Our approach can identify responders based on early perfusion changes, using perfusion properties correlated to gold-standard vascular properties.

High-resolution optoacoustic imaging of tissue responses to vascular-targeted therapies

The monitoring of vascular-targeted therapies via magnetic resonance imaging, computed omography or ultrasound is limited by their insufficient spatial resolution. By taking advantage of the intrinsic optical properties of haemoglobin, here we show that raster-scanning optoacoustic mesoscopy (RSOM) provides high-resolution images of the tumour vasculature and of the surrounding tissue, and that the detection of a wide range of ultrasound bandwidths enables the distinction of vessels of differing size, allowing for detailed insights into vascular responses to vascular-targeted therapy. By using RSOM to examine the responses to vascular-targeted photodynamic therapy in mice with subcutaneous xenografts, we observed a significant and immediate occlusion of the tumour vessels, followed by haemorrhage within the tissue and the eventual collapse of the entire vasculature. By using dual-wavelength RSOM, which distinguishes oxyhaemoglobin from deoxyhaemoglobin, we observed an increase in oxygenation of the entire tumour volume immediately after the application of the therapy, and a second wave of oxygen reperfusion approximately 24 h thereafter. We also show that RSOM allows for the quantification of differences in neo-angiogenesis that predict treatment efficacy.

Liposome-Encapsulated Zoledronate Favors Tumor Vascular Normalization and Enhances Anticancer Efficacy of Cisplatin

The aim of this study was to examine the effectiveness of alanine-proline-arginine-proline-glycine (APRPG) peptide-conjugated PEGylated cationic liposomes-encapsulated zoledronic acid (ZOL) (APRPG-PEG-ZOL-CLPs) in achieving vascular normalization. Cisplatin (diamminedichloroplatinum, DDP) was used to improve anticancer efficacy. The present study showed that APRPG-PEG-ZOL-CLPs increased anticancer efficacy, which was regarded as vascular normalization. Our results demonstrated that the viability, migration, and tube formation of human umbilical vein endothelial cells (HUVECs) were evidently repressed by APRPG-PEG-ZOL-CLPs. Moreover, APRPG-PEG-ZOL-CLPs could decrease vessel density, as well as hypoxia-inducible factor 1α (HIF-1α), and increase thrombospondin 1 (TSP-1) expression of tumors. Therefore, the anticancer efficacy of APRPG-PEG-ZOL-CLPs combined with DDP was superior to that of PEG-ZOL-CLP or ZOL treatment combined with DDP schemes, as demonstrated by the obviously evident reduction in tumor volume. These results indicated that APRPG-PEG-ZOL-CLPs were most effective in normalizing tumor vasculature to elevate the therapeutic effect of antitumor drugs.

Modelling the transport of fluid through heterogeneous, whole tumours in silico

Cancers exhibit spatially heterogeneous, unique vascular architectures across individual samples, cell-lines and patients. This inherently disorganised collection of leaky blood vessels contribute significantly to suboptimal treatment efficacy. Preclinical tools are urgently required which incorporate the inherent variability and heterogeneity of tumours to optimise and engineer anti-cancer therapies. In this study, we present a novel computational framework which incorporates whole, realistic tumours extracted ex vivo to efficiently simulate vascular blood flow and interstitial fluid transport in silico for validation against in vivo biomedical imaging. Our model couples Poiseuille and Darcy descriptions of vascular and interstitial flow, respectively, and incorporates spatially heterogeneous blood vessel lumen and interstitial permeabilities to generate accurate predictions of tumour fluid dynamics. Our platform enables highly-controlled experiments to be performed which provide insight into how tumour vascular heterogeneity contributes to tumour fluid transport. We detail the application of our framework to an orthotopic murine glioma (GL261) and a human colorectal carcinoma (LS147T), and perform sensitivity analysis to gain an understanding of the key biological mechanisms which determine tumour fluid transport. Finally we mimic vascular normalization by modifying parameters, such as vascular and interstitial permeabilities, and show that incorporating realistic vasculatures is key to modelling the contrasting fluid dynamic response between tumour samples. Contrary to literature, we show that reducing tumour interstitial fluid pressure is not essential to increase interstitial perfusion and that therapies should seek to develop an interstitial fluid pressure gradient. We also hypothesise that stabilising vessel diameters and permeabilities are not key responses following vascular normalization and that therapy may alter interstitial hydraulic conductivity. Consequently, we suggest that normalizing the interstitial microenvironment may provide a more effective means to increase interstitial perfusion within tumours.

The structure of tumours varies widely, with dense and chaotically-formed networks of blood vessels that differ between each individual tumour and even between different regions of the same tumour. This atypical environment can inhibit the delivery of anti-cancer therapies. Computational tools are urgently required which facilitate a deeper understanding of the relationship between blood vessel architectures and therapeutic response. We have developed a computational framework which integrates the complex tumour vascular architecture to predict fluid transport across all lengths scales in whole tumours. We apply our model to two tumour cell-lines and show that differences in their inherent vascular structures influence flow through cancerous tissue. We also use our platform to predict the fluid dynamic response following vascular normalization therapy in realistic, static tumour networks and show that the response is dependent on tumour vascular architecture. We hypothesise that therapy may alter the permeability of interstitial tissue to fluid transport and show that lowering interstitial fluid pressure is not a necessary therapeutic outcome to increase tumour perfusion.

Limitations of Anti-Angiogenic Treatment of Tumors

Clinical trials using anti-vascular endothelial growth factor /(VEGF) molecules induce a modest improvement in overall survival, measurable in weeks to just a few months, and tumors respond differently to these agents. In this review article, we have exposed some tumor characteristics and processes that may impair the effectiveness of anti-angiogenic approaches, including genotypic changes on endothelial cells, the vascular normalization phenomenon, and the vasculogenic mimicry. The usage of anti-angiogenic molecules leads to hypoxic tumor microenvironment which enhances tumor invasiveness. The role of tumor-infiltrating cells, including tumor associated macrophages and fibroblasts (TAMs and TAFs) in the therapeutic response to anti-angiogenic settings was also highlighted. Finally, among the new therapeutic approaches to target tumor vasculature, anti-PD-1 or anti-PD-L1 therapy sensitizing and prolonging the efficacy of anti-angiogenic therapy, have been discussed.

Monitoring the Process of Endostar-Induced Tumor Vascular Normalization by Non-contrast Intravoxel Incoherent Motion Diffusion-Weighted MRI

Tumor vascular normalization has been proposed as a new concept in anti-tumor angiogenesis, and the normalization window is considered as an opportunity to increase the effect of chemoradiotherapy. However, there is still a lack of a non-invasive method for monitoring the process of tumor vascular normalization. Intravoxel incoherent motion diffusion-weighted magnetic resonance imaging (IVIM DW-MRI) is an emerging approach which can effectively assess microperfusion in tumors, without the need for exogenous contrast agents. However, its role in monitoring tumor vascular normalization still needs further study. In this study, we established a tumor vascular normalization model of CT26 colon-carcinoma-bearing mice by means of Endostar treatment. We then employed IVIM DW-MRI and immunofluorescence to detect the process of tumor vascular normalization at different times after treatment. We found that the D* values of the Endostar group were significantly higher than those of the control group on days 4, 6, 8, and 10 after treatment, and the f values of the Endostar group were significantly higher than those of the control group on days 6 and 8. Furthermore, we confirmed through analysis of histologic parameters that Endostar treatment induced the CT26 tumor vascular normalization window starting from day 4 after treatment, and this window lasted for 6 days. Moreover, we found that D* and f values were well correlated with pericyte coverage (r = 0.469 and 0.504, respectively; P < 0.001, both) and relative perfusion (r = 0.424 and 0.457, respectively; P < 0.001, both). Taken together, our findings suggest that IVIM DW-MRI has the potential to serve as a non-invasive approach for monitoring Endostar-induced tumor vascular normalization.

Clinical evaluation of vascular normalization induced by recombinant human endostatin in nasopharyngeal carcinoma via dynamic contrast-enhanced ultrasonography

BACKGROUND

The present study confirmed the presence and exact range of "vascular normalization window" induced by recombinant human endostatin (RHES) in patients with nasopharyngeal carcinoma (NPC) by analyzing the variation of dynamic contrast-enhanced ultrasonography (DCE-US) quantitative parameters. Also, the clinical application of DCE-US in the evaluation of vascular normalization was assessed.

MATERIALS AND METHODS

A total of 30 previously untreated patients with stage III-IVA NPC were enrolled in the present study and were randomly but equally divided into RHES (endostar [ES]) and normal saline (NS) groups. The patients in the ES group were administered RHES intravenously, while the patients in the NS group were administered normal saline daily for 5 days prior to intensity modulated radiotherapy coupled with concurrent chemotherapy. All patients underwent DCE-US on the day before the administration and on days 3 and 5 subsequently. The Audio Video Interleave of each DCE-US examination was analyzed quantitatively using the CHI-Q software. Several parameters were investigated, such as peak intensity (PI), time to peak (TTP), and mean transit time (MTT).

RESULTS

The PI, TTP, and MTT differed significantly at the three time points in the ES group (all P<0.001) but not in the NS group (all P>0.05). In the ES group, PI increased and subsequently decreased, whereas TTP, as well as MTT, lessened initially and then increased within the 5 days after administration of RHES. The maximum value of PI and the minimum value of TTP, as well as MTT, occurred on day 3 (all P<0.05). Furthermore, the values of PI, TTP, and MTT were similar prior to the administration of RHES in both groups (all P>0.05). However, the PI of the ES group was significantly higher (P<0.05), whereas the TTP and the MTT were significantly lower following administration of RHES (all P<0.05) compared with the corresponding parameters of the NS group.

CONCLUSION

DCE-US is a suitable method for the clinical evaluation of vascular normalization induced by antiangiogenic agents. The "vascular normalization window" induced by RHES occurs in patients with NPC, and the exact range is within about 5 days post-administration, which contributes towards optimizing the modality of RHES combined with radiotherapy and chemotherapy for NPC patients.

How to Modulate Tumor Hypoxia for Preclinical In Vivo Imaging Research

Tumor hypoxia is related with tumor aggressiveness, chemo- and radiotherapy resistance, and thus a poor clinical outcome. Therefore, over the past decades, every effort has been made to develop strategies to battle the negative prognostic influence of tumor hypoxia. For appropriate patient selection and follow-up, noninvasive imaging biomarkers such as positron emission tomography (PET) radiolabeled ligands are unprecedentedly needed. Importantly, before being able to implement these new therapies and potential biomarkers into the clinical setting, preclinical in vivo validation in adequate animal models is indispensable. In this review, we provide an overview of the different attempts that have been made to create differential hypoxic in vivo cancer models with a particular focus on their applicability in PET imaging studies.

Monitoring of tumor vascular normalization: the key points from basic research to clinical application

Tumor vascular normalization alleviates hypoxia in the tumor microenvironment, reduces the degree of malignancy, and increases the efficacy of traditional therapy. However, the time window for vascular normalization is narrow; therefore, how to determine the initial and final points of the time window accurately is a key factor in combination therapy. At present, the gold standard for detecting the normalization of tumor blood vessels is histological staining, including tumor perfusion, microvessel density (MVD), vascular morphology, and permeability. However, this detection method is almost unrepeatable in the same individual and does not dynamically monitor the trend of the time window; therefore, finding a relatively simple and specific monitoring index has important clinical significance. Imaging has long been used to assess changes in tumor blood vessels and tumor changes caused by the oxygen environment in clinical practice; some preclinical and clinical research studies demonstrate the feasibility to assess vascular changes, and some new methods were in preclinical research. In this review, we update the most recent insights of evaluating tumor vascular normalization.

Anterior gradient 2 as a supervisory marker for tumor vessel normalization induced by anti-angiogenic treatment

Anti-angiogenic therapy provides transient tumor vascular normalization, which results in a window of opportunity for improvement of radio- or chemotherapy. Biomarkers indicating this window are required for rationalizing anti-angiogenesis. Anterior gradient 2 (AGR2), the majority of which is secreted from tumor cells, is an easily detected plasma protein. In the present study, it was demonstrated that AGR2 could be applied as a biomarker for the supervision of vascular normalization during anti-angiogenic treatment with gold nanoparticles (AuNPs). Nude mice inoculated with SW620 human colorectal cancer cells were treated with AuNPs. Vessel density, pericyte coverage, vessel permeability, tumor hypoxia, tumor growth and AGR2 secretion were detected following treatment with AuNPs at days 0, 4, 6, 9 and 14. Tumor volume and vessel density were reduced, whereas pericyte coverage was increased, and hypoxia and vessel permeability were improved between days 6–9; however, these improvements decreased by day 14, revealing a time frame for tumor vascular normalization, namely days 4–9, during treatment with AuNPs in mice. AGR2 levels in tumor tissues and plasma were significantly low at day 9, along with vascular normalization; therefore, AGR2 can be used as a potential marker for monitoring tumor vascular normalization during anti-angiogenic treatment.

Small Molecule PET Tracers in Drug Discovery

The process of discovering and developing a new pharmaceutical is a long, difficult, and risky process that requires numerous resources. Molecular imaging techniques such as PET have recently become a useful tool for making decisions along a drug candidate's development timeline. PET is a translational, noninvasive imaging technique that provides quantitative information about a potential drug candidate and its target at the molecular level. Using this technique provides decisional information to ensure that the right drug candidate is being chosen, for the right target, at the right dose within the right patient population. This review will focus on small molecule PET tracers and how they are used within the drug discovery process. PET provides key information about a drug candidate's pharmacokinetic and pharmacodynamic properties in both preclinical and clinical studies. PET is being used in all phases of the drug discovery and development process, and the goal of these studies are to accelerate the process in which drugs are developed.

Fatty acid synthase (FASN) as a therapeutic target in breast cancer

INTRODUCTION

Ten years ago, we put forward the metabolo-oncogenic nature of fatty acid synthase (FASN) in breast cancer. Since the conception of this hypothesis, which provided a model to explain how FASN is intertwined with various signaling networks to cell-autonomously regulate breast cancer initiation and progression, FASN has received considerable attention as a therapeutic target. However, despite the ever-growing evidence demonstrating the involvement of FASN as part of the cancer-associated metabolic reprogramming, translation of the basic science-discovery aspects of FASN blockade to the clinical arena remains a challenge. Areas covered: Ten years later, we herein review the preclinical lessons learned from the pharmaceutical liabilities of the first generation of FASN inhibitors. We provide an updated view of the current development and clinical testing of next generation FASN-targeted drugs. We also discuss new clinico-molecular approaches that should help us to convert roadblocks into roadways that will propel forward our therapeutic understanding of FASN. Expert opinion: With the recent demonstration of target engagement and early signs of clinical activity with the first orally available, selective, potent and reversible FASN inhibitor, we can expect Big pharma to revitalize their interest in lipogenic enzymes as well-credentialed targets for oncology drug development in breast cancer.

The role of radionuclide probes for monitoring anti-tumor drugs efficacy: A brief review

Despite recent advances in the development of new therapeutic agents and diagnostic imaging modalities, cancer is still one of the main causes of death worldwide. A better understanding of the molecular signature of cancer has promoted the development of a new generation of anti-cancer drugs and diagnostic agents that specifically target molecular components such as genes, ligands, receptors and signaling pathways. However, intrinsic heterogeneity of tumors has hampered the overall success of target therapies even among patients with similar tumor types but unpredictable different responses to therapy. In this sense, post-treatment response monitoring becomes indispensable and nuclear medicine imaging modalities could provide the tools for an early indication of therapeutic efficacy. Herein, we briefly discuss the current role of PET and SPECT imaging in monitoring cancer therapy together with an update on the current radiolabeled probes that are currently investigated for tumor therapy response assessment.

18F-Fluoromisonidazole Kinetic Modeling for Characterization of Tumor Perfusion and Hypoxia in Response to Antiangiogenic Therapy

Multiparametric imaging of tumor perfusion and hypoxia with dynamic ¹⁸F-fluoromisonidazole (¹⁸F-FMISO) PET may allow for an improved response assessment to antiangiogenic therapies. Cediranib (AZD2171) is a potent inhibitor of tyrosine kinase activity associated with vascular endothelial growth factor receptors 1, 2, and 3, currently in phase II/III clinical trials. Serial dynamic ¹⁸F-FMISO PET was performed to investigate changes in tumor biomarkers of perfusion and hypoxia after cediranib treatment. Methods: Twenty-one rats bearing HT29 colorectal xenograft tumors were randomized into a vehicle-treated control group (0.5% methylcellulose daily for 2 d [5 rats] or 7 d [4 rats]) and a cediranib-treated test group (3 mg/kg daily for 2 or 7 d; 6 rats in both groups). All rats were imaged before and after treatment, using a 90-min dynamic PET acquisition after administration of 42.1 ± 3.9 MBq of ¹⁸F-FMISO by tail vein injection. Tumor volumes were delineated manually, and the input function was image-derived (abdominal aorta). Kinetic modeling was performed using an irreversible 1-plasma 2-tissue compartmental model to estimate the kinetic rate constants K₁, K₁/k₂, and k₃-surrogates for perfusion, ¹⁸F-FMISO distribution volume, and hypoxia-mediated entrapment, respectively. Tumor-to-blood ratios (TBRs) were calculated on the last dynamic frame (80-90 min). Tumors were assessed ex vivo by digital autoradiography and immunofluorescence for microscopic visualization of perfusion (pimonidazole) and hypoxia (Hoechst 33342). Results: Cediranib treatment resulted in significant reduction of mean voxelwise ¹⁸F-FMISO TBR, K₁, and K₁/k₂ in both the 2-d and the 7-d groups (P < 0.05). The k₃ parameter was increased in both groups but reached significance only in the 2-d group. In the vehicle-treated groups, no significant change in TBR, K₁, K₁/k₂, or k₃ was observed (P > 0.2). Ex vivo tumor analysis confirmed the presence of hypoxic tumor regions that nevertheless exhibited relatively lower ¹⁸F-FMISO uptake. Conclusion:¹⁸F-FMISO kinetic modeling reveals a more detailed response to antiangiogenic treatment than a single static image is able to reveal. The reduced mean K₁ reflects a reduction in tumor vascular perfusion, whereas the increased k₃ reflects a rise in hypoxia-mediated entrapment of the radiotracer. However, if only late static images are analyzed, the observed reduction in ¹⁸F-FMISO uptake after treatment with cediranib may be mistakenly interpreted as a global decrease, rather than an increase, in tumor hypoxia. These findings support the use of ¹⁸F-FMISO kinetic modeling to more accurately characterize the response to treatments that have a direct effect on tumor vascularization and perfusion.

Personalising and targeting antiangiogenic resistance: a complex and multifactorial approach

Pathological angiogenesis involves complex and dynamic interactions between tumour cells and other lineages existing in the microenvironment of the tumour. Preclinical and clinical data suggest that tumours can show dual, different adaptive responses against antiangiogenic agents: one successful adaptation is vascular normalisation, whereas the second adaptation is elicited through vascular trimming and increased hypoxia. These phenomena depend on the type of tumour and the type of agent. The classical approach for investigating acquired resistance against antiangiogenic agents is to identify compensatory signalling pathways emerging in response to VEGF blockade, which has led to the development of highly effective drugs; however, ultimately these drugs fail. Here we review how the dual stromal adaptive patterns determine the mechanisms of escape that go beyond the reprogramming of signal transduction pathways, which obliges us to investigate the tumour as an ecosystem and to develop uni- and multicompartmental models that explain drug resistance involving metabolic and immune reprogramming. We also propose a method for facilitating personalised therapeutic decisions, which uses 18F-fluoromisonidazole-positron emission tomography to monitor the dual stromal response in tumours of individual patients.

Anti-angiogenic Therapy in Cancer: Downsides and New Pivots for Precision Medicine

Primary solid tumors originate close to pre-existing tissue vasculature, initially growing along such tissue blood vessels, and this phenomenon is important for the metastatic potential which frequently occurs in highly vascularized tissues. Unfortunately, preclinic and clinic anti-angiogenic approaches have not been very successful, and multiple factors have been found to contribute to toxicity and tumor resistance. Moreover, tumors can highlight intrinsic or acquired resistances, or show adaptation to the VEGF-targeted therapies. Furthermore, different mechanisms of vascularization, activation of alternative signaling pathways, and increased tumor aggressiveness make this context even more complex. On the other hand, it has to be considered that the transitional restoration of normal, not fenestrated, microvessels allows the drug to reach the tumor and act with the maximum efficiency. However, these effects are time-limited and different, depending on the various types of cancer, and clearly define a specific “normalization window.” So, new horizons in the therapeutic approaches consist on the treatment of the tumor with pro- (instead of anti-) angiogenic therapies, which could strengthen a network of well-structured blood vessels that facilitate the transport of the drug.

Antiangiogenic cancer treatment: The great discovery and greater complexity (Review)

The discovery of tumor angiogenesis opened a new path in fighting cancer. The approval of different antiangiogenic agents, most targeting vascular endothelial growth factor (VEGF) signaling, has either increased the effectiveness of standard chemotherapy or even replaced it by offering better patient outcomes. However, an increasing number of preclinical and clinical observations have shown that the process of angiogenesis is far from clearly understood. Apart from targeting the VEGF pathway, novel strategies aim to influence other molecular factors that are involved in tumor angiogenesis. In addition, naturally occurring compounds seem to offer additional agents for influencing angiogenesis. The first concept of antiangiogenic therapy aimed to destroy tumor vessels, while it turned out that, paradoxically, antiangiogenic drugs normalized vasculature and as a result offered an improvement in chemotherapeutic delivery. In order to design an effective treatment schedule, methods for detecting the time window of normalization and biomarkers predicting patient response are needed. The initial idea that antiangiogenic therapy would be resistance-free failed to materialize and currently we still face the obstacle of resistance to antiangiogenic therapy.

Normoxic or hypoxic adaptation in response to antiangiogenic therapy: Clinical implications

In a recent article in Cell Reports, we described a novel mechanism for acquired resistance against new small-molecule antiangiogenic tyrosine-kinase inhibitors (TKIs). Vascular normalization-inducing TKIs block glycolysis and trigger a nutritional stress response in the tumor compartment that induces a (targetable) switch to mitochondrial metabolism. We discuss the implications for clinical/translational studies and suggest areas for future research.

Targeting Tumor Mitochondrial Metabolism Overcomes Resistance to Antiangiogenics

Epithelial malignancies are effectively treated by antiangiogenics; however, acquired resistance is a major problem in cancer therapeutics. Epithelial tumors commonly have mutations in the MAPK/Pi3K-AKT pathways, which leads to high-rate aerobic glycolysis. Here, we show how multikinase inhibitor antiangiogenics (TKIs) induce hypoxia correction in spontaneous breast and lung tumor models. When this happens, the tumors downregulate glycolysis and switch to long-term reliance on mitochondrial respiration. A transcriptomic, metabolomic, and phosphoproteomic study revealed that this metabolic switch is mediated by downregulation of HIF1α and AKT and upregulation of AMPK, allowing uptake and degradation of fatty acids and ketone bodies. The switch renders mitochondrial respiration necessary for tumor survival. Agents like phenformin or ME344 induce synergistic tumor control when combined with TKIs, leading to metabolic synthetic lethality. Our study uncovers mechanistic insights in the process of tumor resistance to TKIs and may have clinical applicability.

ACRIN 6684: Assessment of Tumor Hypoxia in Newly Diagnosed Glioblastoma Using 18F-FMISO PET and MRI

PURPOSE

Structural and functional alterations in tumor vasculature are thought to contribute to tumor hypoxia which is a primary driver of malignancy through its negative impact on the efficacy of radiation, immune surveillance, apoptosis, genomic stability, and accelerated angiogenesis. We performed a prospective, multicenter study to test the hypothesis that abnormal tumor vasculature and hypoxia, as measured with MRI and PET, will negatively impact survival in patients with newly diagnosed glioblastoma.

EXPERIMENTAL DESIGN

Prior to the start of chemoradiation, patients with glioblastoma underwent MRI scans that included dynamic contrast enhanced and dynamic susceptibility contrast perfusion sequences to quantitate tumor cerebral blood volume/flow (CBV/CBF) and vascular permeability (k^trans) as well as ¹⁸F-Fluoromisonidazole (¹⁸F-FMISO) PET to quantitate tumor hypoxia. ROC analysis and Cox regression models were used to determine the association of imaging variables with progression-free and overall survival.

RESULTS

Fifty patients were enrolled of which 42 had evaluable imaging data. Higher pretreatment ¹⁸F-FMISO SUV_peak (P = 0.048), mean k^trans (P = 0.024), and median k^trans (P = 0.045) were significantly associated with shorter overall survival. Higher pretreatment median k^trans (P = 0.021), normalized RCBV (P = 0.0096), and nCBF (P = 0.038) were significantly associated with shorter progression-free survival. SUV_peak [AUC = 0.75; 95% confidence interval (CI), 0.59-0.91], nRCBV (AUC = 0.72; 95% CI, 0.56-0.89), and nCBF (AUC = 0.72; 95% CI, 0.56-0.89) were predictive of survival at 1 year.

CONCLUSIONS

Increased tumor perfusion, vascular volume, vascular permeability, and hypoxia are negative prognostic markers in newly diagnosed patients with gioblastoma, and these important physiologic markers can be measured safely and reliably using MRI and ¹⁸F-FMISO PET. Clin Cancer Res; 22(20); 5079-86. ©2016 AACR.

Monitoring vascular normalization induced by antiangiogenic treatment with (18)F-fluoromisonidazole-PET

Background: Rationalization of antiangiogenics requires biomarkers. Vascular re‐normalization is one widely accepted mechanism of action for this drug class. The interstitium of tumors with abnormal vasculature is hypoxic. We sought to track vascular normalization with 18F‐misonidazole ([18F]‐FMISO, a probe that detects hypoxia) PET, in response to window‐of‐opportunity (WoO) treatment with the antiangiogenic dovitinib.

Methods: Two patient‐derived pancreas xenografts (PDXs; Panc215 and Panc286) and the spontaneous breast cancer model MMTV‐PyMT were used. Animals were treated during 1 week of WoO treatment with vehicle or dovitinib, preceded and followed by [18F]‐FMISO‐PET, [18F]‐FDG‐PET, and histologic assessment (dextran extravasation, hypoxia and microvessel staining, and necrosis, cleaved caspase‐3 and Ki67 measurements). After WoO treatment, gemcitabine (pancreas)/adriamycin (breast) or vehicle was added and animals were treated until the humane endpoint. Tumor growth inhibition (TGI) and survival were the parameters studied.

Results: [18F]‐FMISO SUV did not change after dovitinib‐WoO treatment compared to vehicle‐WoO (0.54 vs. 0.6) treatment in Panc215, but it decreased significantly in Panc286 (0.58 vs. 1.18; P < 0.05). In parallel, 10‐KDa perivascular dextran extravasation was not reduced with dovitinib or vehicle‐WoO treatment in Panc215, but it was reduced in Panc286. Whereas the addition of dovitinib to gemcitabine was indifferent in Panc215, it increased TGI in Panc286 (TGI switched from −59% to +49%). [18F]‐FMISO SUV changes were accompanied by an almost 100% increase in interstitial gemcitabine delivery (665–1260 ng/mL). The results were validated in the PyMT model.

Conclusions: [18F]‐FMISO accurately monitored vascular re‐normalization and improved interstitial chemotherapy delivery.

We describe a new technique to monitor the efficacy of antiangiogenics.

This drug class currently lacks biomarkers.

Antiangiogenics exert their positive effect by inducing stromal normalization.

18F‐fluoromisonidazole PET is able to monitor stromal normalization.

Tumors showing stromal normalization by PET experience benefit from antiangiogenics.