From "Bench to Bedside": In-Silico Modelling to Inform the Assessment of Intervertebral Disc Cell-Based Therapies

Abstract

Lower back pain is a major global socio-economic concern, with the primary cause believed to be degeneration of the intervertebral disc (IVD). While the spine field is filled with a growing body of basic research and many promising regenerative therapies, the avascular nature of the IVD and its limited nutrient supply remain the greatest obstacles on the path towards clinical adoption of cell-based therapies. Subsequently, this thesis focuses on the importance of the nutrient microenvironment throughout the full development and assessment process, in order to accelerate the translation of therapies all the way from ?bench to bedside?. Firstly, this work sought to consolidate early metabolite measurements and employ in-silico models, underpinned by more recent experimental parameters of degeneration and nutrient transport, with the objective of re-evaluating current knowledge in terms of grade-specific stages of degeneration. Secondly, this work aimed to characterise the local nutrient microenvironment of 2D cell monolayers, commonly used 3D in vitro culture systems and ex vivo disc organ culture systems in order to highlight the effect of culturing parameters and to place ?standard practice? culturing conditions into context in terms of physiological relevance. The results revealed that large variations and gradients in metabolite concentrations are easily established without the careful consideration of key parameters and that this diversity currently exists across the research field and may explain heterogeneous results on the regenerative potential of cell therapies. Moreover, every culture system is unique and while one external concentration may be suitable for one culture configuration, it may not be appropriate for another. It is believed that with careful deliberation of the external boundary and in vitro culture design, harmony and standardisation of physiologically relevant local microenvironments will push towards greater reproducibility across the field and accelerate innovative therapeutics for clinical translation. Even after demonstrating safety and efficacy in animals, many of these promising therapies appear to have stunted success in humans. This work presents in-silico models which correlate favourably to the pre-clinical literature in terms of the capabilities of animal regeneration and predict that compromised nutrition is not a significant challenge in small animal discs. On the contrary, this work highlights a very fine clinical balance between an adequate cell dose for sufficient repair, through de novo matrix deposition, without exacerbating the human microenvironmental niche. While these findings help to explain the failed translation of promising pre-clinical data and the limited results emerging from clinical trials at present, they also enable the research field and clinicians to manage expectations on cell-based regeneration. Moreover, this work provides a platform to inform the design of clinical trials, and as computing power and software capabilities increase, it is conceivable that the future holds the generation of patient-specific models which could be used for patient assessment, as well as pre- and intraoperative planning.