The treatment options for rheumatoid arthritis (RA) have come a long way in the last several decades progressing from managing acute inflammation with non-steroidal anti-inflammatory drugs (NSAIDs) to disease-modifying anti-rheumatic drugs (DMARDs) like methotrexate and anti-tumor necrosis factor (TNF) antibodies, which can decrease disease activity (1). These new treatment options have improved quality of life in a substantial amount of RA patients, however the largest factor affecting the outcomes of RA treatment is disease duration (2). Put another way, patients who begin treatment in the early stages of RA show a better prognosis than patients who begin treatment in the late stages. Thus, RA patient outcomes can be improved by correctly diagnosing RA early in the disease process and starting an appropriate treatment.
There is a preclinical phase of RA where IgM-rheumatoid factor (RF) and antibodies to citrullinated protein antigens (ACPA) can be detected, without clinical signs of inflammatory (3-5). However, conducting IgM-RF and ACPA screening tests for all patients who do not have any symptoms of RA would not be feasible. Additionally, serum markers are only approximately 60-80 % accurate for RA diagnosis since a significant number of patients are seronegative for these markers. Realistically, unless a patient is aware of a genetic predisposition to developing RA, the first time that a patient would be checked for these autoantibodies is when they experience arthralgia, a common symptom of preclinical RA.
Assessing structural changes of joints can also be used for diagnosis of RA. Conventional radiography or ultrasound has been traditionally used to identify bone erosions seen in the late stages of RA, however they lack the sensitivity to detect soft tissue changes (eg. synovitis) seen in early RA. While magnetic resonance imaging (MRI) or computed tomography (CT) has the sensitivity to detect subclinical inflammation in the soft tissue of joints, the time and cost considerations of using MRI preclude it as a RA screening test for patients with arthralgia (6). Thus, there is a current need for a cost-effective imaging technique that can be used for early diagnosis of RA.
Optical molecular imaging (OMI) is gaining popularity in the clinical setting due to its excellent spatial and temporal resolution and cost of operation. This imaging technique allows for in vivo visualization of cellular and subcellular processes using target-specific fluorescent probes and visible or near-visible light. OMI is currently used to visualize tumors for diagnostic and surgical applications (7), but research into adapting this technology for diagnosing and treating other diseases is currently underway. For instance, researchers at the University of Michigan reported using a near-infrared optical molecular imaging compound to visualize joint inflammation in the Mouse Collagen Antibody-Induced Arthritis model (8).
In this study, the Arthrogen-CIA 5-Clone Cocktail Kit was used to induce the Collagen Antibody-Induced Arthritis Model in Balb/c mice. The mice were subcutaneously or orally treated with one of two near-infrared imaging agents: IRDye800CW or AF680 (9). These compounds are negatively charged imaging agents containing a targeting ligand specific for αVβ3 integrins on activated macrophages that infiltrate the synovial space in during RA and image any existing inflammation.
The IRDye800CW fluorophore, whether subcutaneously and orally administered, exhibited high contrast between inflamed and non-inflamed joints in CAIA mice, while AF680 did not have an obvious difference between inflamed and non-inflamed joints. This contrast peaked at 48 hours after administration, as the signal measured in non-inflamed joints continued to decrease. Ex vivo histological analysis confirmed macrophage infiltration in inflamed ankles that coincided with high signal from IRDye800CW. Additionally, the researchers developed a low affinity stereoisomer of IRDye800CW to confirm the specificity of IRDye800CW. The isomer showed low signals in inflamed joints compared to IRDye800CW.
The researchers also performed analyses using a 3D COMSOL model of a human hand to determine if IRDye800CW can be used in imaging the joints of human patients with a greater tissue depth than seen in the mouse experiments. Using data from their mouse experiments, as well as available clinical data from prior imaging studies, the researchers determined that a positive result in the COMSOL model must have a tissue to background signal ratio greater than 5.5:1. IRDye800CW did achieve the 5.5:1 contrast ratio needed to be considered a positive result, however clinical studies would be needed to confirm the in silica analysis.
These published findings emphasize the importance of developing a quick and efficient screening tool for RA. By applying new molecular tools to a robust and well-characterized disease model such as CAIA, the researchers have developed a novel method that can aid in the early diagnosis of RA in a cost-effective way. Since OMI diagnosis protocols would be more widely available and cheaper than MRI, a wider range of health care providers could use it to screen arthralgia patients for RA risk. OMI also requires a shorter testing time than MRIs, making it more suitable as a screening test. Thus, OMI using an imaging probe specific to RA biomarkers, like IRDye800CW, could provide a cost-effective way to enhance the ability of rheumatologists to diagnose RA in its early stages and improve patient outcomes.
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