Challenges for chemistry in molecular imaging

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 41), 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.

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.

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.

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.

There are thousands of “molecular imaging” papers published each year, but the number of new molecular probes approved for clinical use in the last decade can be counted on one hand.  There are a number of barriers, both economic and technical, to overcome to achieve clinical adoption of a new probe.  There are shared development challenges among MR, PET, SPECT, and optical probes, as well as challenges that are unique to the molecule and imaging modality.  Drawing on examples from the MR and nuclear imaging fields, I will discuss various barriers facing probe development and translation:  1) unmet medical need and willingness to pay; 2) preclinical efficacy, study design, bias, and reproducibility; 3) safety and efficacy; 4) clinical trial risk and imperfect truth standards; 5) regulatory risk; 6) market factors - size, competition, reimbursement.  

Many of these factors may seem irrelevant to the basic scientist working on the synthesis and early application of a novel molecular probe.  However, an understanding of the different barriers to human molecular imaging, and ultimate clinical adoption, enables better design of molecular probes and better preclinical studies.  While it is quite difficult to invent a molecular probe that will achieve clinical adoption, it is easy to create new probes that will not overcome some of these obvious barriers. Consideration of the ultimate goal (clinical adoption, clinical research tool, or preclinical research tool) and the challenges to meet that end goal are best performed very early in the development process in order to achieve success.

Over the last 18 years, our laboratory has focused on the designed chemical synthesis, assembly and applications of uniform-sized nanocrystals. In particular, we developed a novel generalized procedure called as the “heat-up process” for the direct synthesis of uniform-sized nanocrystals of many metals, oxides, and chalcogenides. For the last 10 years, our group has been focused on medical applications of various uniform-sized nanoparticles. Using 3 nm-sized iron oxide nanoparticles, non-toxic T1 MRI contrast agent was realized for high resolution MRI of blood vessels down to 0.2 mm. Very recently, we report on the biocompatibility evaluation and MR imaging of extremely small and uniform-sized iron oxide nanoparticles in large animal models including most clinically-relevant non-human primates. These biocompatible iron oxide nanoparticles are successfully used as T1 MR contrast agent for high-resolution MR angiography of macaque monkeys. Furthermore, dynamic MR imaging enabled the detection of cerebral ischemia in beagle dogs and monkeys. These current pilot studies on nonhuman primates clearly demonstrate great promise for these iron oxide nanoclusters as a strong potential candidate for next-generation MR contrast agent. We report the first successful practical application of biocompatible TaOx/SiO2 core/shell nanoparticles (TSNs) as a hemostatic adhesive for minimally invasive procedures as well as an immobilized marker for image-guided procedures. TSNs are highly biocompatible as evidenced by little cytotoxicity and less inflammation reaction than the synthetic polymer adhesive mixture, CA-Lp. We also demonstrate that TSNs can work as a tissue adhesive with the assistance of multimodal imaging in a liver puncture model to stop internal bleeding. Moreover, owing to their adhesive property and multimodal imaging capability, TSNs were utilized as an injectable immobilized anatomical marker to aid image-guided surgery. 

We reported the first successful demonstration of high-resolution in vivo three-photon imaging using ultra-bright Mn2+ doped ZnS nanocrystals. The large three-photon cross-section of the nanocrystals enabled targeted cellular imaging under high spatial resolution, approaching the theoretical limit of three-photon excitation. Owing to the enhanced Stokes shift achieved through nanocrystal doping, the three-photon process was successfully applied to high-resolution in vivo tumor-targeted imaging.

Molecular imaging (MI), as its name implies, is a field that lies squarely at the intersection of molecular biology and traditional medical imaging. Molecular imaging originated from the need to better understand the fundamental molecular pathways inside organisms in a non-invasive manner. Over the past two decades, two factors have acted in concert to fuel the ascent of molecular imaging in both the laboratory and the clinic:  an increased understanding of the molecular mechanisms of disease and the continued development of in vivo imaging technologies, ranging from improved detectors to novel labelling methodologies. The advent of molecular imaging has, in turn, prompted a paradigm shift in medical imaging as a whole, from its foundations in purely anatomical imaging towards techniques aimed at probing tissue phenotype and function. Taking cancer as an example, both the cellular expression of disease biomarkers and fluctuations in tissue metabolism and microenvironment have emerged as extremely promising targets for imaging. Indeed, the field has produced effective molecularly-targeted agents applied with a wide variety of imaging modalities, from fluorescence and luminescence to magnetic resonance. Molecular imaging also contributes to improving the treatment of disease by optimizing the pre-clinical and clinical tests of new medications.