The broad range of tumor types and their varying reactions to treatment make the search for new effective cancer therapies incredibly challenging. The need to offer patients more personalized cancer treatment is driving advances in preclinical positron emission tomography (PET) oncology research. PET provides three-dimensional (3D) functional imaging using radiotracers, showing the spatial distribution of biomolecular activity in the bodies of animal models and humans. Non-invasive in vivo imaging technologies such as PET enable researchers to better understand the course of tumor progression, by visualizing cancer-related processes in real-time. The combination of other imaging modalities, such as magnetic resonance imaging (MRI) and computed tomography (CT) enables both structural and functional imaging in one experiment.
Integrating functional and anatomical information
Multi-modal systems, such as PET/CT, PET/MR and PET/SPECT/CT, are able to provide quantitative 3D tomographic images of radiotracers, bone, and soft tissue, furthering the growing knowledge of cancer biology and treatment. PET/CT is a valuable tool in oncology research due to its ease-of-use, high-throughput capabilities and high resolution for bone and pulmonary applications. It has been used extensively to study cancer therapeutics and tumor biology, and for tracer development, since the mid-1990s.
PET/MR, while less established in preclinical imaging, is gaining ground in oncology due to its ability to image without ionizing radiation and its potential for multiparametric imaging. Another key benefit is the superior anatomical soft tissue contrast, which offers the unique ability to detect tumor margins, and evaluate tracer distribution within individual tumors.
Analyzing tumor biology
Preclinical in vivo imaging methods are helping to extend knowledge of tumor morphology, progression and biomarker expression. PET provides information on the expression of receptors, energy metabolism, and other tumor biomarkers, by imaging an intravenously injected radiotracer – a radioisotope, most commonly fluorine-18 (18F), attached to a molecular probe that targets a specific molecule or metabolic pathway – and monitoring its uptake by tumor cells. A key characteristic of tumor cells is their elevated metabolic turnover, and 18F-fludeoxyglucose (18F-FDG) is often used as a radiolabeled glucose analogue tracer to analyze glucose uptake in tumors and to track their progression and monitor aggressiveness.
“Conventional” PET tracers, such as 18F-FDG or 18F-Fluorothymidine (FLT), are considered the gold standard and monitor universal biomarkers of tumors, including altered metabolism and hypoxia, proliferation, and metastasis. More specific PET agents are capable of targeting the expression of one molecule or gene product, and have the potential to help researchers better understand and assess tumor biology and therapy responses.
For example, the PET radiopharmaceutical gallium-68 prostate specific membrane antigen (68Ga-PSMA) has revolutionized prostate cancer imaging in recent years. The PSMA found in cell membranes is highly expressed in the prostate and in particular prostate cancer cells, making 68Ga-PSMA very effective in imaging. This tracer has the added advantage of using 68Ga, which has comparatively lower production costs.
Advances in immuno-oncology
Imaging, together with a better understanding of cancer genomics and developments in molecular pathology, are key players in achieving personalized cancer treatment. The goal of providing patients with more precise cancer treatment is compounded by inter- and intra-tumoral heterogeneity. The move from pure anatomical imaging to molecular imaging with PET has enabled researchers to visualize tumor heterogeneity, which is particularly important when administering combination therapies for cancer. PET imaging, in parallel with genomic profiling, could allow for visualization of drug-induced changes in a specific biochemical process, and could provide insights into drug target engagement or alterations in tumor phenotype.
Developing new imaging biomarkers
Researchers at the Institute of Cancer Research, London, led by Dr Gabriela Kramer-Marek, are focused on the development and characterization of imaging biomarkers to inform and guide cancer treatment management for individual patients. The group is studying predictive imaging biomarkers, and biomarkers that help to assess drug resistance and tumor response to drugs, with particular interest in the development of theranostic agents against receptors from the epidermal growth factor tyrosine kinase receptor (EGFR) family. Overexpression of human EGFR receptors (HER) have been found in many human malignancies and have facilitated the development of target-specific drugs, some of which are currently in routine clinical practice and others in clinical trials.
The group is aiming to develop HER-specific molecular imaging probes that could guide innovative therapy plans and rapidly identify potential responders. Affibody® molecules have been radiolabeled for PET and SPECT imaging, to address the challenges of conventional antibodies, such as large molecular size and slow clearance, and the rapid dissociation from receptors associated with small molecule probes.
In one recent study, Dr Kramer-Marek’s group used EGFR-specific radioligands to measure EGFR expression in mice with head and neck squamous cell cancer (HNSCC), with the aim of defining a predictive biomarker to stratify patients for treatment.1 Cetuximab is currently the only approved anti-EGFR monoclonal antibody (mAb) used for the treatment of HNSCC, and the ability to monitor and assess the drug’s efficacy and any cetuximab-mediated changes in receptor expression could help inform appropriate dosing with anti-EGFR antibodies.
The group used a radiolabeled Affibody molecule (ZEGFR:03115) to non-invasively measure differences in EGFR expression, using a zirconium-89 (89Zr)-labeled conjugate to assess tumor-to-organ ratios at different time points, and a 18F-labeled analog to measure the response to cetuximab treatment in vivo. To evaluate whether 89Zr-deferoxamine (DFO)-ZEGFR:03115 could distinguish between tumors with varying levels of EGFR expression, mice bearing CAL27 (EGFR +++), Detroit562 (EGFR ++), and MCF7 (EGFR +) xenografts received the radiotracer and were imaged three hours after injection using PET/CT. The quantified PET imaging data indicated that the highest levels of radioconjugate accumulation were in CAL27 tumors (Figure 1), which correlated with receptor expression measured ex vivo by Western Blot and IHC staining.
Figure 1: Radioconjugate uptake in xenografts with varying EGFR expression. Representative whole-body sagittal PET/CT images acquired 3 hours after injection. This research was originally published in JNM. Burley et al. J Nucl Med. 2019;60:353-361. © SNMMI and is reproduced in accordance with the Creative Commons Attribution License https://creativecommons.org/licenses/by/4.0/.
To monitor response to cetruximab, 18F-aluminium fluoride (AlF)-NOTA-ZEGFR:03115 was administered intravenously to mice bearing HN5 tumors (EGFR ++++), and the group observed significantly lower uptake in cetuximab-treated mice than in control HN5 tumors (Figure 2).
Figure 2: 18F-AlF-NOTA-ZEGFR:03115 uptake assessed 1 h after injection. Representative sagittal whole-body PET/CT images of mice bearing HN5 tumors (outlined on image) with or without treatment with cetuximab. This research was originally published in JNM. Burley et al. J Nucl Med. 2019;60:353-361. © SNMMI and is reproduced in accordance with the Creative Commons Attribution License https://creativecommons.org/licenses/by/4.0/.
These results, together with an insignificant change in tumor volume during treatment, highlight the potential for using EGFR imaging as a tool for assessing cetruximab efficacy based on receptor level, rather than relying purely on anatomical imaging, and provide image-guided therapeutic strategies for the clinic.
Preclinical oncology in the future
The ongoing development of multi-modal PET technology will continue to drive preclinical oncology research. The benefits of PET, PET/MR, PET/CT and PET/SPECT/CT for tracer development, therapy monitoring and studying tumor biology are changing the way cancer is treated, moving towards a personalized medicine approach. The growing importance of immuno-oncology is augmented by the sophisticated PET imaging systems available on the market. Cutting-edge research, such as that at the Institute of Cancer Research, is bringing the field one step closer to personalized treatment by using Affibody molecules as novel PET agents to target specific molecular pathways in tumor progression. Such studies are vital to achieve the ultimate aim of optimizing cancer treatment and patient care.
1. Burley TA, Pieve CD, Martins CD, Ciobota DM, Allott L, O WJG, Harrington KJ, Smith G and Kramer-Marek G (2019) Affibody-Based PET Imaging to Guide EGFR-Targeted Cancer Therapy in Head and Neck Squamous Cell Cancer Models, J Nucl Med, 60:353-361.