Validation of a novel imaging technique to detect early osteoarthritis using an antibody specific for post-translationally modified collagen type II

As the age of the world's population increases, osteoarthritis (OA) is becoming by far the most common joint condition and is a major cause of chronic pain and disability. OA is a disease of the whole joint but one major characteristic is the breakdown of the articular cartilage, a smooth layer of tissue that protects the ends of bones and enables frictionless movement of the joint.

Currently there are no treatments that prevent the breakdown of cartilage and patients must depend upon simple pain killers, physiotherapy and ultimately surgical joint replacement. In recent years new therapies for treating OA have been identified (such as inhibitors of the key enzymes that break down the tissue), but the introduction of such treatments, has been hampered by the lack of specific tests capable of detecting early disease (where treatment is most likely to be successful), and discerning modest changes in disease progression following treatment (in order to conduct clinical trials). The current gold standard of OA assessment is by plain X-ray which is insensitive, and MRI which is a more sensitive and specific radiographic tool but costly and harder to access. The bad news is that although large pharmaceutical companies have developed 'safe' and effective drugs to use in individuals with OA, they have chosen not to test these compounds in clinical trials at the current time. There is therefore an urgent unmet need to develop a simple and sensitive tool to detect cartilage damage in man.

We have developed a human antibody that binds strongly and specifically to damaged cartilage and we believe that this antibody could be developed as an imaging marker for early OA. We have labelled our antibody with a fluorescent marker and shown that it homes to the joints of mice with inflammatory arthritis (where there is also cartilage damage). Our current proposal is to test this antibody in a mouse model of OA in which OA has been induced by surgically cutting the medial meniscus (leading to joint destabilisation). Two refinements will be made to improve the intensity of staining. Firstly we plan to use a near infra red (NIR) fluorescent probe, which has better tissue penetration and which also reduces background signal. Secondly, we will fuse two antibody fragments together which will improve binding of the antibody to the damaged cartilage and increase the amount of fluorescence that the antibody fragment will carry. If successful we will test this imaging technique in patients (in a subsequent application). The ability to translate this project to humans is tractable especially as the antibody was originally raised against modified human cartilage, and stains damaged tissues from human as well as murine arthritic joints. Translation of this project to the human is likely to have a significant impact on the development, validation and introduction of new treatments for OA.

Given the huge economic and personal burden of OA, there is an urgent unmet need to develop disease modifying OA drugs (DMOAD) that can reduce or stop the progression of the disease. The introduction of potential DMOADs, has so far been hampered by the lack of specific and sensitive serum and/or radiographic biomarkers capable of detecting early disease, and discerning modest changes in disease progression. The development of specific molecular methods for detection of early disease and for assessment of disease response to treatment is an important and tractable area for development in preclinical OA models. Moreover, these techniques can be easily translated for use in human disease.
The aim of this proposal is to use an optical imaging technology in conjunction with near-infrared (NIR) fluorescence labeled antibody fragment that is specific to damaged cartilage to visualise the development and progression of murine OA (induced by destabilisation of the medial meniscus (DMM)), and to validate this technique as an early marker for OA. The labelled antibody fragment to modified type II collagen was originally identified from a phage display library and has been used to stain damaged cartilage from both OA and rheumatoid arthritis joints. We have previously shown that it can be used as an in vivo marker of inflammatory arthritis by fluorescently labelling the fragment. We now wish to label the antibody with NIR fluorescence to improve tissue penetration and reduce the problems associated with autofluorescence. We will also modify the fragment by expressing it as a fusion protein, a diabody, to improve signal intensity in the diseased tissue. Ultimately, this targeted approach will be tested in humans with early (cartilage loss on MRI but with preserved joint space on X-Ray) disease.

The objective of this study is to develop a non invasive imaging technology, which is sensitive, specific, fast and cheap for monitoring the progression of OA. It will initially be developed in a preclinical setting using a murine model of OA, but ultimately it will be validated in human disease. The first to benefit from this study are the researchers and industry that are identifying targets for treatment, and validating new DMOADs. By using near-infrared (NIR) fluorescence labeled antibody that is able to detect small changes in the cartilage breakdown response to treatment can be monitored longitudinally and subtle changes in response to treatments can be measured. This has been a major problem up until now as the current X-ray and MRI detect late stage disease well, but fail to detect early changes occuring in the articular cartilage. Ultimately, this targeted approach will be tested in humans with early (cartilage loss on MRI but with preserved joint space on X-Ray) disease. If successful, this will have great advantages in detecting those suitable for early intervention and for monitoring the success of new DMOADs.