High-Resolution Multispectral Optoacoustic Tomography of the Vascularization and Constitutive Hypoxemia of Cancerous Tumors

Visualizing vasculature and its response to therapy in the tumor microenvironment

Tumor vasculature plays a critical role in the progression and metastasis of tumors, antitumor immunity, drug delivery, and resistance to therapies. The morphological and functional changes of tumor vasculature in response to therapy take place in a spatiotemporal-dependent manner, which can be predictive of treatment outcomes. Dynamic monitoring of intratumor vasculature contributes to an improved understanding of the mechanisms of action of specific therapies or reasons for treatment failure, leading to therapy optimization. There is a rich history of methods used to image the vasculature. This review describes recent advances in imaging technologies to visualize the tumor vasculature, with a focus on enhanced intravital imaging techniques and tumor window models. We summarize new insights on spatial-temporal vascular responses to various therapies, including changes in vascular perfusion and permeability and immune-vascular crosstalk, obtained from intravital imaging. Finally, we briefly discuss the clinical applications of intravital imaging techniques.

Multispectral optoacoustic tomography for in vivo detection of lymph node metastases in oral cancer patients using an EGFR-targeted contrast agent and intrinsic tissue contrast: A proof-of-concept study

Oral cancer patients undergo diagnostic surgeries to detect occult lymph node metastases missed by preoperative structural imaging techniques. Reducing these invasive procedures that are associated with considerable morbidity, requires better preoperative detection. Multispectral optoacoustic tomography (MSOT) is a rapidly evolving imaging technique that may improve preoperative detection of (early-stage) lymph node metastases, enabling the identification of molecular changes that often precede structural changes in tumorigenesis. Here, we characterize the optoacoustic properties of cetuximab-800CW, a tumor-specific fluorescent tracer showing several photophysical properties that benefit optoacoustic signal generation. In this first clinical proof-of-concept study, we explore its use as optoacoustic to differentiate between malignant and benign lymph nodes. We characterize the appearance of malignant lymph nodes and show differences in the distribution of intrinsic chromophores compared to benign lymph nodes. In addition, we suggest several approaches to improve the efficiency of follow-up studies.

The Potential of Photoacoustic Imaging in Radiation Oncology

Radiotherapy is recognized globally as a mainstay of treatment in most solid tumors and is essential in both curative and palliative settings. Ionizing radiation is frequently combined with surgery, either preoperatively or postoperatively, and with systemic chemotherapy. Recent advances in imaging have enabled precise targeting of solid lesions yet substantial intratumoral heterogeneity means that treatment planning and monitoring remains a clinical challenge as therapy response can take weeks to manifest on conventional imaging and early indications of progression can be misleading. Photoacoustic imaging (PAI) is an emerging modality for molecular imaging of cancer, enabling non-invasive assessment of endogenous tissue chromophores with optical contrast at unprecedented spatio-temporal resolution. Preclinical studies in mouse models have shown that PAI could be used to assess response to radiotherapy and chemoradiotherapy based on changes in the tumor vascular architecture and blood oxygen saturation, which are closely linked to tumor hypoxia. Given the strong relationship between hypoxia and radio-resistance, PAI assessment of the tumor microenvironment has the potential to be applied longitudinally during radiotherapy to detect resistance at much earlier time-points than currently achieved by size measurements and tailor treatments based on tumor oxygen availability and vascular heterogeneity. Here, we review the current state-of-the-art in PAI in the context of radiotherapy research. Based on these studies, we identify promising applications of PAI in radiation oncology and discuss the future potential and outstanding challenges in the development of translational PAI biomarkers of early response to radiotherapy.

Imaging methods to evaluate tumor microenvironment factors affecting nanoparticle drug delivery and antitumor response

Standard small molecule and nanoparticulate chemotherapies are used for cancer treatment; however, their effectiveness remains highly variable. One reason for this variable response is hypothesized to be due to nonspecific drug distribution and heterogeneity of the tumor microenvironment, which affect tumor delivery of the agents. Nanoparticle drugs have many theoretical advantages, but due to variability in tumor microenvironment (TME) factors, the overall drug delivery to tumors and associated antitumor response are low. The nanotechnology field would greatly benefit from a thorough analysis of the TME factors that create these physiological barriers to tumor delivery and treatment in preclinical models and in patients. Thus, there is a need to develop methods that can be used to reveal the content of the TME, determine how these TME factors affect drug delivery, and modulate TME factors to increase the tumor delivery and efficacy of nanoparticles. In this review, we will discuss TME factors involved in drug delivery, and how biomedical imaging tools can be used to evaluate tumor barriers and predict drug delivery to tumors and antitumor response.

Optoacoustic imaging in endocrinology and metabolism

Imaging is an essential tool in research, diagnostics and the management of endocrine disorders. Ultrasonography, nuclear medicine techniques, MRI, CT and optical methods are already used for applications in endocrinology. Optoacoustic imaging, also termed photoacoustic imaging, is emerging as a method for visualizing endocrine physiology and disease at different scales of detail: microscopic, mesoscopic and macroscopic. Optoacoustic contrast arises from endogenous light absorbers, such as oxygenated and deoxygenated haemoglobin, lipids and water, or exogenous contrast agents, and reveals tissue vasculature, perfusion, oxygenation, metabolic activity and inflammation. The development of high-performance optoacoustic scanners for use in humans has given rise to a variety of clinical investigations, which complement the use of the technology in preclinical research. Here, we review key progress with optoacoustic imaging technology as it relates to applications in endocrinology; for example, to visualize thyroid morphology and function, and the microvasculature in diabetes mellitus or adipose tissue metabolism, with particular focus on multispectral optoacoustic tomography and raster-scan optoacoustic mesoscopy. We explain the merits of optoacoustic microscopy and focus on mid-infrared optoacoustic microscopy, which enables label-free imaging of metabolites in cells and tissues. We showcase current optoacoustic applications within endocrinology and discuss the potential of these technologies to advance research and clinical practice.

Full three-dimensional segmentation and quantification of tumor vessels for photoacoustic images

Quantitative analysis of tumor vessels is of great significance for tumor staging and diagnosis. Photoacoustic imaging (PAI) has been proven to be an effective way to visualize comprehensive tumor vascular networks in three-dimensional (3D) volume, while previous studies only quantified the vessels projected in one plane. In this study, tumor vessels were segmented and quantified in a full 3D framework. It had been verified in the phantom experiments that the 3D quantification results have better accuracy than 2D. Furthermore, in vivo vessel images were quantified by 2D and 3D quantification methods respectively. And the difference between these two results is significant. In this study, complete vessel segmentation and quantification method within a 3D framework was implemented, which showed obvious advantage in the analysis accuracy of 3D photoacoustic images, and potentially improve tumor study and diagnosis.

Full-frequency correction of spatial impulse response in back-projection scheme using space-variant filtering for optoacoustic mesoscopy

The fidelity and quality of reconstructed images in optoacoustic mesoscopy (OPAM) can be significantly improved by considering the spatial impulse response (SIR) of the employed focused transducer into reconstruction. However, the traditional method fully taking the SIR into account can hardly meet the data-intensive requirements of high resolution OPAM because of excessive memory and time consumption. Herein, a modified back-projection method using a space-variant filter for full-frequency correction of the SIR is presented, and applied to the OPAM system with a sphere-focused transducer. The proposed method can readily manage the large datasets of the OPAM and effectively reduce the extra time consumption. The performance of the proposed method is showcased by simulations and experiments of phantoms and biological tissue. The results demonstrate that the modified back-projection method exhibits better image fidelity, resolution and contrast compared to the common and weighted back-projection methods that are not or not fully accounting for the SIR.

Oxygen-Enhanced Optoacoustic Tomography Reveals the Effectiveness of Targeting Heme and Oxidative Phosphorylation at Normalizing Tumor Vascular Oxygenation

Multispectral optoacoustic tomography (MSOT) is an emerging noninvasive imaging modality that can detect real-time dynamic information about the tumor microenvironment in humans and animals. Oxygen enhanced (OE)-MSOT can monitor tumor vasculature and oxygenation during disease development or therapy. Here, we used MSOT and OE-MSOT to examine in mice the response of human non-small cell lung cancer (NSCLC) xenografts to a new class of antitumor drugs, heme-targeting agents heme-sequestering peptide 2 (HSP2) and cyclopamine tartrate (CycT). HSP2 inhibits heme uptake, while CycT inhibits heme synthesis in NSCLC cells, where heme is essential for ATP generation via oxidative phosphorylation. HSP2 and CycT can inhibit ATP generation and thereby suppress NSCLC cell tumorigenic functions. MSOT showed that treatment of NSCLC tumors with HSP2 or CycT reduced total hemoglobin, increased oxygen saturation, and enhanced the amplitude of response to oxygen gas breathing challenge. HSP2 and CycT normalized tumor vasculature and improved tumor oxygenation, where levels of several hypoxia markers in NSCLC tumors were reduced by treatment with HSP2 or CycT. Furthermore, treatment with HSP2 or CycT reduced levels of angiogenic factor VEGFA, its receptor VEGFR1, and vascular marker CD34. Together, our data show that heme-targeting drugs HSP2 and CycT elicit multiple tumor-suppressing functions, such as inhibiting angiogenic function, normalizing tumor vasculature, alleviating tumor hypoxia, and inhibiting oxygen consumption and ATP generation. SIGNIFICANCE: Heme-targeting agents HSP2 and CycT effectively normalize tumor vasculature and alleviate tumor hypoxia, raising the possibility of their combination with chemo-, radio-, and immunotherapies to improve antitumor efficacy.See related commentary by Tomaszewski, p. 3461.

Spatial heterogeneity of oxygenation and haemodynamics in breast cancer resolved in vivo by conical multispectral optoacoustic mesoscopy

The characteristics of tumour development and metastasis relate not only to genomic heterogeneity but also to spatial heterogeneity, associated with variations in the intratumoural arrangement of cell populations, vascular morphology and oxygen and nutrient supply. While optical (photonic) microscopy is commonly employed to visualize the tumour microenvironment, it assesses only a few hundred cubic microns of tissue. Therefore, it is not suitable for investigating biological processes at the level of the entire tumour, which can be at least four orders of magnitude larger. In this study, we aimed to extend optical visualization and resolve spatial heterogeneity throughout the entire tumour volume. We developed an optoacoustic (photoacoustic) mesoscope adapted to solid tumour imaging and, in a pilot study, offer the first insights into cancer optical contrast heterogeneity in vivo at an unprecedented resolution of <50 μm throughout the entire tumour mass. Using spectral methods, we resolve unknown patterns of oxygenation, vasculature and perfusion in three types of breast cancer and showcase different levels of structural and functional organization. To our knowledge, these results are the most detailed insights of optical signatures reported throughout entire tumours in vivo, and they position optoacoustic mesoscopy as a unique investigational tool linking microscopic and macroscopic observations.

Advances in Optoacoustic Neurotomography of Animal Models

Unlike traditional optical methods, optoacoustic imaging is less sensitive to scattering of ballistic photons, so it is capable of high-resolution interrogation at a greater depth. By integrating video-rate visualization with multiplexing and sensing a range of endogenous and exogenous chromophores, optoacoustic imaging has matured into a versatile noninvasive investigation modality with rapidly expanding use in biomedical research. We review the principal features of the technology and discuss recent advances it has enabled in structural, functional, and molecular neuroimaging in small-animal models. In extending the boundaries of noninvasive observation beyond the reach of customary photonic methods, the latest developments in optoacoustics have substantially advanced neuroimaging inquiry, with promising implications for basic and translational studies.

Optical-resolution photoacoustic microscopy for monitoring vascular normalization during anti-angiogenic therapy

Monitoring the changes in tumor vascularity is important for anti-angiogenic therapy assessment with therapeutic implications. However, monitoring vascularity is quite challenging due to the lack of appropriate imaging techniques. Here, we describe a non-invasive imaging technique using optical-resolution photoacoustic microscopy (OR-PAM) to track vascular changes in prostate cancer treated with an anti-angiogenic agent, DC101, on a mouse ear xenograft model. Approximately 1-3 days after the initial therapy, OR-PAM imaging detected tumor vascular changes such as reduced vessel tortuosity, decreased vessel diameter and homogenized intratumoral vessel distribution. These observations indicated vessel normalization, which was pathologically validated as increased fractional pericyte coverage, functional perfusion and drug delivery of the vessels. After four DC101 interventions, OR-PAM imaging eventually revealed intratumoral vessel regression. Therefore, OR-PAM imaging of the vasculature offers a promising method to study anti-angiogenic drug mechanisms of action in vivo and holds potential in monitoring and guiding anti-angiogenic therapy.

Photoacoustic imaging as a tool to probe the tumour microenvironment

The tumour microenvironment (TME) is a complex cellular ecosystem subjected to chemical and physical signals that play a role in shaping tumour heterogeneity, invasion and metastasis. Studying the roles of the TME in cancer progression would strongly benefit from non-invasive visualisation of the tumour as a whole organ in vivo, both preclinically in mouse models of the disease, as well as in patient tumours. Although imaging techniques exist that can probe different facets of the TME, they face several limitations, including limited spatial resolution, extended scan times and poor specificity from confounding signals. Photoacoustic imaging (PAI) is an emerging modality, currently in clinical trials, that has the potential to overcome these limitations. Here, we review the biological properties of the TME and potential of existing imaging methods that have been developed to analyse these properties non-invasively. We then introduce PAI and explore the preclinical and clinical evidence that support its use in probing multiple features of the TME simultaneously, including blood vessel architecture, blood oxygenation, acidity, extracellular matrix deposition, lipid concentration and immune cell infiltration. Finally, we highlight the future prospects and outstanding challenges in the application of PAI as a tool in cancer research and as part of a clinical oncologist's arsenal.

Summary: This Review details the potential of photoacoustic imaging to visualise features of the tumour microenvironment such as blood vessels, hypoxia, fibrosis and immune infiltrate to provide unprecedented insight into tumour biology.

Optoacoustic mesoscopy for biomedicine

Fuelled by innovation, optical microscopy plays a critical role in the life sciences and medicine, from basic discovery to clinical diagnostics. However, optical microscopy is limited by typical penetration depths of a few hundred micrometres for in vivo interrogations in the visible spectrum. Optoacoustic microscopy complements optical microscopy by imaging the absorption of light, but it is similarly limited by penetration depth. In this Review, we summarize progress in the development and applicability of optoacoustic mesoscopy (OPAM); that is, optoacoustic imaging with acoustic resolution and wide-bandwidth ultrasound detection. OPAM extends the capabilities of optical imaging beyond the depths accessible to optical and optoacoustic microscopy, and thus enables new applications. We explain the operational principles of OPAM, its placement as a bridge between optoacoustic microscopy and optoacoustic macroscopy, and its performance in the label-free visualization of tissue pathophysiology, such as inflammation, oxygenation, vascularization and angiogenesis. We also review emerging applications of OPAM in clinical and biological imaging.

Towards clinical application of non-invasive imaging to detect bacterial infections

In vivo imaging technologies offer a great potential for the diagnosis of difficult-to-treat bacterial infections. A major limitation of conventional imaging modalities is the lack of specificity to distinguish the site of bacterial infection from sterile inflammation. Targeted approaches like antibiotics linked to imaging tracers for detection of various bacterial pathogens or species-specific antibodies combined with anatomical imaging modalities are currently being evaluated to overcome this problem. Considering the recent progress in optical and targeted imaging that may accelerate preclinical development programs, clinical implementation of in vivo imaging modalities to detect bacterial infection foci becomes realistic in the future.

Vascular patterning of subcutaneous mouse fibrosarcomas expressing individual VEGF isoforms can be differentiated using angiographic optical coherence tomography

Subcutaneously implanted experimental tumors in mice are commonly used in cancer research. Despite their superficial location, they remain a challenge to image non-invasively at sufficient spatial resolution for microvascular studies. Here we evaluate the capabilities of optical coherence tomography (OCT) angiography for imaging such tumors directly through the murine skin in-vivo. Data sets were collected from mouse tumors derived from fibrosarcoma cells genetically engineered to express only single splice variant isoforms of vascular endothelial growth factor A (VEGF); either VEGF120 or VEGF188 (fs120 and fs188 tumors respectively). Measured vessel diameter was found to be significantly (p<0.001) higher for fs120 tumors (60.7 ± 4.9μm) compared to fs188 tumors (45.0 ± 4.0μm). The fs120 tumors also displayed significantly higher vessel tortuosity, fractal dimension and density. The ability to differentiate between tumor types with OCT suggests that the visible abnormal vasculature is representative of the tumor microcirculation, providing a robust, non-invasive method for observing the longitudinal dynamics of the subcutaneous tumor microcirculation.

The protective role of sphingosine-1-phosphate against the action of the vascular disrupting agent combretastatin A-4 3-O-phosphate

Solid tumours vary in sensitivity to the vascular disrupting agent combretastatin A-4 3-O-phosphate (CA4P), but underlying factors are poorly understood. The signaling sphingolipid, sphingosine-1-phosphate (S1P), promotes vascular barrier integrity by promoting assembly of VE-cadherin/β-catenin complexes. We tested the hypothesis that tumour pre-treatment with S1P would render tumours less susceptible to CA4P. S1P (1μM) pretreatment attenuated an increase in endothelial cell (HUVEC) monolayer permeability induced by 10μM CA4P. Intravenously administered S1P (8mg/kg/hr for 20 minutes then 2mg/kg/hr for 40 minutes), reduced CA4P-induced (30mg/kg) blood flow shut-down in fibrosarcoma tumours in SCID mice (n≥7 per group), as measured by tumour retention of an intravenously administered fluorescent lectin. A trend towards in vivo protection was also found using laser Doppler flowmetry. Immunohistochemical staining of tumours ex vivo revealed disrupted patterns of VE-cadherin in vasculature of mice treated with CA4P, which were decreased by pretreatment with S1P. S1P treatment also stabilized N-cadherin junctions between endothelial cells and smooth muscle cells in culture, and stabilized tubulin filaments in HUVEC monolayers. We conclude that the rapid shutdown of tumour microvasculature by CA4P is due in part to disruption of adherens junctions and that S1P has a protective effect on both adherens junctions and the endothelial cell cytoskeleton.

Molecular imaging probes for multi-spectral optoacoustic tomography

In this review, we discuss recent progress in emerging optoacoustic probes, their mechanisms, applications and challenges for biological imaging using MSOT.