Recent advances in fluorine MR (19F MR) foreshadow some of the potential benefits to be expected for diagnosis and treatment of inflammation and arthritis. In this plenary lecture we survey the emerging technologies and opportunities for discovery that accompany theranostics with 19F MR, providing examples of current strategies and frontier applications, and considering potential directions for the future. The goal of such predictions - which in the past have typically underestimated the potential and value of new imaging technologies - is to provide an exciting destination to aim for and push the capabilities of MR-guided theranostics of inflammation and arthritis.
Rheumatoid arthritis: A health, social and economic challenge
Rheumatoid arthritis (RA) is one of the most common disabling conditions, affecting approximately 1% of the population worldwide 1, 2. Similar to other inflammatory rheumatic diseases, RA is associated with persistent systemic inflammation, which triggers a broad spectrum of clinical features. Autoimmunity, chronic inflammation and joint destruction are key features of the disease 3. Patients with RA generally have a poor prognosis, and higher mortality rates compared to the general population 4. Patients with RA usually experience a progressive deterioration of their condition; over one quarter of patients who have suffered RA for up to five years require assistance with activities of daily living, and four percent need extensive long-term care 5. These restrictions limit the ability of RA patients to cope independently with everyday routines 5, and severely impact on their ability to sustain employment. Indeed, this contributes to make RA one of the most challenging diseases both to health care systems and to society at large, due to direct costs associated with patient care, and indirect costs due to lost productivity of RA patients. In addition to the major impact of disease duration on an increasing need for care, aging has a very strong impact on the incidence of RA. The prevalence of RA increases to nearly 6% in males over the age of 70 and to nearly 5% in females over the age of 60 6. Multi-morbidities and polypharmacy typical in elderly patients further complicates the already very difficult clinical scenario, and imposes substantial burdens on health care systems. Furthermore, changes in metabolic function resulting from old age might exacerbate unavoidable adverse drug reactions and interactions. Despite significant advances in early detection, diagnostics, and therapy, RA — like most inflammatory rheumatic diseases — primarily follows a chronic progressive course, leading to poor patient outcomes, and underscoring the need for novel therapeutic and diagnostic strategies.
A case for biomedical imaging
The complexity of inflammatory diseases such as RA requires more sophisticated therapeutic approaches to achieve beneficial outcomes. The concept of personalized or individualized medicine - now interchangeably referred to as precision medicine 7 - has developed in recent years as a means to improve clinical outcomes for individual patients. Many aspects of personalized medicine including drug dosing and bioavailability studies would clearly benefit from further developments. While a certain degree of individualized medicine is common practice for some therapeutic regimes in the clinic, this typically involves measurements of drug levels in blood, and does not necessarily reflect the situation in the target organ of pathological importance. Monitoring drug distribution at the site of pathology should be one of the cornerstones of individualized medicine. Current bioavailability studies are usually performed in early stage human studies during the exploration of investigational new drugs. Drugs acting primarily within one particular organ system such as the brain or musculoskeletal may be investigated during preclinical studies (healthy individuals or animals) via administration of radiolabeled pharmaceuticals in combination with nuclear methods such as PET or SPECT. A serious drawback to this approach is the exposure to ionizing radiation. An alternative with the potential to safely monitor drug distribution at the site of pathology routinely in patients is the use of fluorine (19F) magnetic resonance (MR) methods.
Theranostics with Fluorine Magnetic Resonance (19F MR)
We have used 19F MR methods to study fluorine-containing exogenous agents in vivo 8-10. The virtual absence of organic-forms of 19F in body tissue yields background-free images with complete signal selectivity and specificity, making the study of 19F-containing drugs by 19F MR methods in vivo an appealing prospect. Fluorine is an important element for lead optimization during drug discovery, because of its influence on pharmacological properties. Introduction of 19F to lead compounds affects their physicochemical characteristics and as a result their pharmacokinetic and pharmacodynamic properties 11. It is estimated that one third of the top 30 blockbuster drugs contain at least one 19F atom12. This has led to a beneficial side effect — the potential of using 19F as a nucleus for studying structural and functional information by 19F MR 13. Furthermore, another favorable characteristic of 19F is the relative large chemical shift for organic 19F compounds 14, meaning that resonances of multiple 19F nuclei - such as those of the drugs metabolites — can be easily separated 15. Apart from the exposure to ionizing radiation, another drawback of using radiolabeled pharmaceuticals for studying drug distribution is that no distinction can made between the administered drug and its metabolites. 19F MR has already become important for small animal imaging in multiple fields of pre-clinical research. Due to the MR characteristics, 19F MR methods will be highly valuable for studying the pharmacokinetics of 19F-containing drugs.
19F MR monitoring of fluorinated drugs and cells
Non-steroidal anti-inflammatory drugs (NSAIDs) have been shown to have anti-inflammatory, analgesic, and antipyretic effects and when applied topically they are believed to reduce inflammation in muskuloskeletal disorders. These compounds have been shown to penetrate the human skin but no human in vivo studies have been carried out so far to study the distribution, let alone quantification, of active compound at the site of injury where the compound is acting. In a recent study we demonstrated the feasibility of tracking 19F-containing NSAID in vivo at 7.0 T, which encourages further studies in patients suffering from inflammatory conditions 16. It is conceivable, that the concentration of drug ultimately penetrating the skin to reach its site of action (in the case of NSAIDs, areas of inflammation) might be close to or even lower than the detection limit. In biological tissue, the NMR properties of 19F-containing drugs are also expected to change as has been previously reported for T1 17 and T2 18 relaxation times. This is due to several factors such as changes in the environment but also pharmacokinetic (e.g. degradation of parent compound) as well as pharmacodynamic processes (e.g. protein binding). In contrast to our feasibility study in normal subjects, the 19F-containing topical NSAID is commonly applied up to three times daily in patients and over longer periods of time; an accumulation of active compound at its site of action is to be expected. This potentially increases the locally available quantities of drugs in vivo. However, it is still likely that the concentration of drug at the site of action to be too low for detection. The low availability of drug and low 19F signal sensitivity are indeed main limitations for drug targeting studies at the moment. Although, accessing in vivo distribution and quantification of 19F-contained drugs presents several challenges, the preliminary findings of our 19F MR study at 7.0 T are encouraging and point towards the prospect of applying 19F MR and NSAID therapy to the field of theranostics with the ultimate goal to visualize and measure the concentration along with providing dosage guiding of a therapeutically-active compound reaching the inflammatory site in rheumatoid arthritis patients. While this is, for the moment, merely a gedankenexperiment, it continues to motivate new research and encourages further studies with patients suffering from inflammatory conditions. These pioneering efforts include 19F MR visualization (i) of collagen induced arthritic inflammation in small rodents upon invasive administration of a perfluorcarbon contrast agent as a means of studying therapeutic response 19 and (ii) of arthritic rabbit knees following intra-articular injection of a perfluoro-15-crown-5-ether emulsion 20. In a recent proof-of-concept study, dendritic cells (DCs) labeled with a clinical-grade 19F label (perfluoro dialkyl ether, CS-1000, Celsense) DCs were visualized at their site of application using combined 19F and 1H MR imaging at 3.0 T 21. Although DCs could be clearly visualized at their site of application, there was no MRI evidence of cell accumulation in the draining lymph nodes 21, possibly due to cell densities in these regions being on the order of, or below, the cell detection limit threshold for the experimental configuration employed 21.
Are we out of the woods yet?
A crucial limitation concerning 19F MR methods is signal sensitivity. This could be of major concern when imaging anatomical regions where the concentration of the 19F signal source is expected to be below the detection limit for conventional MR systems. One solution is to enhance the availability of 19F signal. We previously showed that we could increase the uptake of 19F nanoparticles in inflammatory cells, in order to enhance the 19F signal and improve detection 9, 22. The sensitivity of the RF coil also determines the signal-to-noise ratio (SNR). We constructed a room temperature dual-tunable RF volume resonator coil tailored for mouse brain imaging, and could achieve a highly homogeneous 19F and 1H signal within the target organ. We used this to image inflammation in the brain and the adjacent draining lymph nodes in a mouse model of neuroinflammatory disease 10. In 19F MRI it is common practice to overcome the signal constraints by resorting to imaging with low spatial resolution. However, this reduces imaging detail and the precision with which the 19F signal source can be localized within specific anatomical structures. To overcome these constraints, we recently applied the concept of cryogenically-cooling RF hardware to reduce thermal noise and thereby improve the SNR. By boosting SNR we can achieve reduced scan time or increased spatial resolutions 23, 24. We characterized the first ever quadrature-driven 19F transceive cryogenically-cooled RF probe (19F-CRP) and reported an SNR gain of ~15-fold 25 compared to the room temperature coil we used in our previous studies. This SNR gain could be attributed to multiple factors in addition to cryogenic cooling, including differences in coil design, and sample loading and RF pulse power adjustments. Importantly, the considerable SNR gain realized with the 19F-CRP enabled the acquisition of 19F MR images with an unprecedented spatial resolution 25.
What will the foreseeable future bring?
19F MR methods have an enormous potential to form an integral part of personalized medicine, not only for RA, but in many scenarios with an inflammatory component. By developing the necessary tools to characterize the 19F MR properties of therapeutic compounds, novel diagnostic techniques to study inflammatory pathologies such as RA may be harmonized with standard methods of therapy monitoring. Nevertheless, several challenges lie ahead. The limited number of reports referring to 19F MR in humans is testament to the practical obstacles inherent in in vivo 19F MR 21, 26-29. This is especially true for magnetic field strengths below 3.0 T, where 19F MR is challenging, if not unfeasible. These limitations notwithstanding, it will be conceptually appealing to pursue in vivo 19F MR at ultrahigh field strengths greater than 7.0 T, where the intrinsic sensitivity gains at higher magnetic field strengths will make 19F Theranostic MR not just a theoretical possibility, but a practical reality 30-41.
Pushing the field strength boundaries in MRI
Physicists, engineers, and pioneers from related disciplines have already taken further steps into the future, theoretically, with something they are calling Extreme Field MR (EF-MR) 42. This envisions human MR at 14.0 Tesla and at 20.0 Tesla 43, 44, and is an important conceptual leap; these fields will span even more of the crucial "resolution gap" in our understanding of human biology and inflammatory diseases. The sensitivity gain at 20.0 T is expected to reduce scan times for 19F MR by a factor of 8 versus today’s 7.0 T capabilities. While this is, for the moment, merely a vision, it promises fluorine MR with a sub-millimeter spatial resolution in 5-10 min scan time. While the first 20 Tesla class MR instruments will likely be devoted to discovery and to proofs-of-principle, findings should guide the best use of lower-resolution imaging techniques at lower magnetic fields. The only thing that could keep the dream of human MR at 14.0 T or 20.0 T from becoming reality would be a lack of conviction to follow the path and see what develops. Failing to do so for fear that the clinics will be left behind is to ignore the entire recent history of MR imaging discovery. When an entire community of experts devotes its creative efforts to the task, the gain of knowledge alone will lead to answers for questions that we don't yet even know we should ask.
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