PET in cancer imaging
Professor Carolyn Anderson, University of Pittsburgh Bridgeside
Over the past few decades, there have been major advances towards radiometal-labeled PET tracers, particularly agents labelled with Ga-68 (T1/2 = 68 min), Cu-64 (T1/2 = 12.7 h) and Zr-89 (T1/2 = 3.3 d). Our lab developed 68Ga- and 64Cu-labeled LLP2A, a peptidomimetic with picomolar affinity for very late antigen 4 (also called integrin 41), which is involved in tumour and immune cell adhesion and migration. We have found optimal chelator conjugates of LLP2A for Ga-68 and Cu-64, which are being applied to PET imaging of metastatic melanoma and monitoring the response to targeted radiotherapy with beta emitting Lu-177-labeled LLP2A in mouse models. We are also investigating imaging in mouse models post-treatment with radiation and immunotherapy. There is considerable effort by many groups towards developing PET tracers to image the programmed death (PD)-1/programmed death ligand 1 (PD-L1) interaction between tumor/myeloid cells and T-cells. Blockade of PD-1/PD-L1 is clinically effective, and is now being tested in combination with radiation therapy (RT). PD-L1 upregulation during RT may serve as a predictive biomarker, but monitoring expression by immunohistochemistry is limited by tissue-sampling artifact. Non-invasive PET imaging of PD-L1 expression would provide real-time information of PD-L1 levels in the tumor and its microenvironment. We have labeled an anti-mouse PD-L1 antibody with Zr-89 for PET imaging in two mouse models (melanoma and head and neck cancer) after treatment with RT and anti-PD-1 mAb therapy. PET imaging data, with confirmation by flow cytometry, show that RT upregulates PD-L1 expression, primarily in myeloid cells in the tumour microenvironment.
Challenges in development of chemical probes in vivo fluorescence and magnetic resonance imaging
Professor Kazuya Kikuchi, Osaka University
One of the great challenges in the post-genome era is to clarify the biological significance of intracellular molecules directly in living cells. If we can visualize a molecule in action, it is possible to acquire biological information, which is unavailable if we deal with cell homogenates. One possible approach is to design and synthesize chemical probes that can convert biological information to chemical output. In this talk, molecular design strategies for MR and fluorescence imaging probes are introduced.
MRI (Magnetic Resonance Imaging) is an imaging technique using nuclear magnetic resonance phenomenon. MRI has been clinically used since it yields highly spatial resolution images of deep regions in living animal bodies. 19F MRI is suitable for monitoring particular signals concerning biological phenomena because 19F MRI shows little endogenous background signals. We have also developed the 19F MRI probes to detect protease activity and gene expression on the basis of paramagnetic resonance enhancement (PRE) effect. However, 19F MRI probes have faced two challenges. First, 19F MRI has the low sensitivity. Second, the suppression of molecular mobility induced by the increase in molecular size shortens the transverse relaxation time (T2), which is a crucial factor affecting the MRI contrast, resulting in attenuation of the MRI signals. To solve these challenges, we developed a novel 19F MRI contrast agent, fluorine accumulated silica nanoparticle for MRI contrast enhancement (FLAME), which is composed of a perfluorocarbon core and a robust silica shell. FLAME has advantages such as high sensitivity, stability, modification of the surface, and biocompatibility. The activatable derivative of FLAME will also be introduced
Intravital imaging by two-photon excitation microscopy (TPEM) has been widely utilized to visualize cell functions. However, small molecular probes (SMPs) commonly used for cell imaging cannot be simply applied to intravital imaging because of the challenge of delivering them into target tissues, as well as their undesirable physicochemical properties for TPEM imaging. Here, we designed and developed a functional SMP with an active-targeting moiety, higher photostability, and fluorescence switch, and imaged target cell activity by injecting the SMP into living animals. The SMPs are based on BODIPY structure which is optimized for photostability and for fluorescence wavelenghth overlap for multicolor imaging. The combination of the rationally designed SMP with a fluorescent protein as a reporter of cell localization enabled quantitation of osteoclast activity and time-lapse imaging of its in vivo function associated with changes in cell deformation and membrane fluctuations.
Challenges in magnetic resonance imaging
Professor Silvio Aime, University of Turnin
The possibility of carrying out Functional and Molecular Imaging protocols by means of MRI is very attractive for the superb anatomical resolution that is attainable by this technique. However, MRI suffers from an intrinsic insensitivity (with respect to the competing imaging modalities) that has to be overcome by designing suitable amplification procedures involving the use of properly designed chemicals. This approach relies first on the development of paramagnetic contrast agents endowed with an enhanced sensitivity and on the identification of efficient routes of accumulation of the imaging probes at the sites of interest. In this context much attention has been devoted to the design and use of self-assembled systems based on amphiphilic molecules as well on the use of whole cells, where the imaging reporters are represented by highly stable paramagnetic Gd(III) complexes
Besides relaxation agents much attention is currently devoted also to the use of CEST agents (CEST= Chemical Exchange Saturation Transfer). Upon applying a second rf field at the absorption frequency of an exchangeable protons pool, a net saturation effect is detected on the water signal. These are frequency encoding systems that allow multiple agents detection in the same anatomical region as well as they offer the possibility of designing innovative responsive probes that report on specific parameters of the microenvironment in which they distribute. To overcome sensitivity issues, also for this class of agents, the use of Liposomes (LipoCEST) and RBCs (ErythroCEST) appear to offer valuable solutions.
Finally the access to hyperpolarized molecules has opened new horizons providing the possibility of investigating in vivo metabolic processes. It will be shown how hyperpolarization of molecules like pyruvate and lactate can be attained by procedures base on the use of para-Hydrogen and magnetic field cycling.
Challenges in radiolabelling methodologies
Professor Veronique Gouverneur, University of Oxford
The success of Positron Emission Tomography (PET) and renewed interest in [18F]radiochemistry led to creative methods to incorporate 18F into molecules of increasing complexity. Despite these advances, clinically useful radiotracers lie within a narrow accessible space with [18F]fluoroalkanes and [18F]fluoroarenes at the forefront. Many potentially high value PET 18F-labeled tracers and drugs lie outside this radiochemical space, and the ability to test tracers not amenable to traditional or newly developed 18F-labeling intervention would be a major boost for PET imaging. A more diverse range of 18F-tags could immediately serve medicinal chemists by informing the selection of lead compounds much earlier in the drug discovery pipeline. This lecture will present our general approach to late stage 18F-fluorination and the recent contribution we have made to this field of research with the labeling of a range of high value 18F-tags for PET.