Tracking hydrogel therapeutics using quantitative whole-body imaging

Tissue engineering is a new and exciting discipline which aims to create human tissue for transplantation and regeneration of damaged organs. Although it is likely to be several decades before this advanced form of tissue engineering becomes routine, more simplistic forms are already being used in the clinic for repair of bone, cartilage and skin. One of the materials most widely used in tissue engineering is alginate - a natural polymer extracted from seaweed. The polymer strands of alginate can be crosslinked together by addition of positively charged ions that form bridges between strands and make the fluid polymers stiffen into a gel. The stiffness of the gel can be easily modified by adding more crosslinker. This gives alginates the versatile properties that make them perfect for many applications, from stiff bone reconstruction, to soft wound repairs. Additionally, stem cells and drugs can be encapsulated within the gel. This can support the stem cells' ability to regenerate organs, and control the location and delivery rate of drugs.
Given the versatility of alginates, it is not surprising that they are being actively investigated for an extensive range of biomedical applications. However, one beneficial property of alginate is actually hindering the development of the technology. The high water content of alginate gels gives them properties similar to the tissues of the body, making them almost invisible to most medical imaging techniques, including MRI, CT and ultrasound. Once transplanted, it is difficult to visualise their location, properties and degradation, preventing optimisation of the best alginate formulation and hindering accurate assessment of efficacy and safety of therapies. An imaging method which could directly visualise the alginate gel within living animals and patients could be used to speed these novel therapies into effective treatments.
We propose a simple, yet novel method for directly incorporating an imaging agent into the structure of the gel, making it visible to medical imaging methods PET and SPECT. These imaging methods can detect the location of radioactive agents after they have been injected into patients, and are routinely used to identify tumours and heart defects. We have recently shown that these imaging agents can also be used to directly crosslink our alginate gels, thus becoming an integral component of gel structure and making the alginate visible to PET and SPECT. The levels of radioactivity incorporated into the gel are lower than those used in medical scans, making this technique safe for patients. Given that these alginates and radioactive agents are already approved for clinical use, the simplicity of our method could allow it to be used in hospitals within the near future and make a major impact of the development and safety of new therapies.
However, we first need to fully characterise how the radioactive agent interacts with the gel, and then test the technology in relevant mouse models of human disease. Chemical analysis of gel properties will determine stiffness and structure, as well as how long the radioactive agent remains bound to the gel. Stem cells will be grown within the gel and their growth and function measured. Finally, radioactive gels will be injected into mice to 1) monitor delivery of drugs to the brain; 2) graft stem cells into the heart. We have already tried these experiments and the data look highly promising, but further repeats are necessary before our results can be confirmed.
These experiments will validate an important method that could assist development of tissue engineering across multiple applications. Although highly complex tissue engineering approaches frequently make the headlines and could be the medicine of the future, it is the simpler methods which are currently poised to make a difference to patients' lives, and our simple and clinically applicable imaging tool will be able to speed the development of these exciting therapies.

Hydrogels such as alginate are cross-linked polymers with high water content, tuneable porosity, elasticity, and degradation rate. This suits their use in tissue engineering and drug delivery, with several formulations in widespread clinical use, and emerging therapies for various diseases in preclinical development and clinical trials.
Due to its similarity to soft tissue, alginate hydrogel cannot be detected with conventional imaging (MRI, CT, ultrasound), posing a challenge for assessing its biodistribution once in the body. We propose a labelling and imaging method to address this challenge. Our tool will allow serial assessment of delivery, retention and function of alginate hydrogels in vivo so that the safety and efficacy of therapies can be assesed non-invasively.
Alginate hydrogels are formed via Ca2+ ion crosslinking with negative carboxylates across separate alginate polymers. We have shown that certain radioisotopes,commonly used with the medical imaging modalities SPECT and PET, can also be used alongside Ca2+ to form the alginate. This allows hydrogel location to be quantitatively monitored within the body, informing on retention and degradation in the preclincal and clinical setting.
We will characterise chemical and mechanical properties of hydrogels crosslinked with our novel method. We will then demonstrate the use of SPECT and PET imaging in tracking transplantated human mesenchymal stem cells in labelled a hydrogel scaffold for cardiac repair, and nose-to-brain drug delivery using a hydrogel vector containing a radiolabelled small molecule drug.
The information provided by this labelling/imaging tool will accelerate the development of many emerging alginate-based therapeutics, providing non-invasive, quantitative information on their delivery and retention. Our labelling technique can be achieved by combining materials already in clinical use without the need for further chemical modification, making rapid translation highly feasible.

Who: Preclinical researchers developing tissue engineering and/or drug delivery therapies with hydrogels:
How: The tools currently available for researchers developing hydrogel-based therapies have several limitations, including lack of a means to produce quantitative, high temporal and spatial resolution data on hydrogel location post implantation. Our radiolabelling and imaging approach provides a tool to generate this data rapidly on the whole-body scale, giving the hydrogel research community a means to accelerate development and answer fundamental concerns. For example, it enables rapid visualisation of delivery and retention in specific organs, allowing screening of multiple hydrogel formulations of varying material properties (size, shape, elasticity etc.), and delivery routes. This integrated platform between the materials science, regenerative medicine, drug delivery and in vivo research communities will facilitate therapy development more efficiently than current technologies, supporting the MRC Molecular and Cellular Medicine Remit areas of Regenerative Medicine, Pharmacology, and New Technologies.

Who: Preclinical researchers in disease specific fields (diabetes, cardiac repair, neurodegeneration etc.) who are developing hydrogel-based therapeutics:
How: Our approach provides a tool to rapidly generate quantitative data on hydrogel therapeutic behaviour on the whole-body scale in specialised preclinical disease models. This will allow the safety and efficacy of hydrogel therapies to be established non-invasively in relevant pathologies, revealing interactions between disease state and hydrogel behaviour that would otherwise be unobtainable. This will feed forward into adapting hydrogel formulations for increased safety and efficacy for specific diseases and translation for clinical use.

Who: Translational researchers (industry and academia) validating hydrogel-based therapeutics in the clinic:
How: The patient-specific data provided by this hydrogel tracking tool will increase the statistical power of clinical trials by revealing inter-patient variability of material behaviour, thereby increasing cost effectiveness and the chances of successful clinical approval. This supports the MRC remit of New Technology, by providing an advanced technology for clinical trials. It also supports the Pharmacology remit by providing an understanding of "the mechanisms of drug action to improve efficacy and targeting (including through stratification), and to minimise adverse / off-target effects." It will do this by enabling high precision, minimally invasive surgical intervention, such as implantation of hydrogels for tissue engineering, by giving non-invasive feedback on successful hydrogel implantation and its location and retention.
How: Techniques that can provide a predictive measurement of drug or cell-therapy delivery with therapeutic outcomes are of great interest to pharmaceutical companies as they will generate a translatable pipeline for assessing novel therapeutic systems to identify poor or ineffective therapies at an early stage to reduce financial outlays

Who: Patients in disease areas that will benefit from hydrogel-based therapies:
How: Effective treatments do not currently exist for many of the conditions for which alginate-based therapies are in current development. As this includes a large number of the most widespread disease areas (cardiac, diabetes, neurodegeneration), the impact of our technology could be extensive. Our technique will accelerate development of these treatments, potentially bringing better clinical outcomes for patients. This will improve quality of life in a range of disease areas, as well as extending healthy life-span. As our technology can also provide patient-specific information on the success or otherwise of delivery and retention of hydrogel therapeutics, this could inform on treatment decisions such as repeat dosing, thereby providing a route to personalised medicine