In vitro inflammatory response evaluation of pre-degraded bioresorbable polymers

Student thesis: Doctoral ThesisDoctor of Philosophy

Abstract

Bioresorbable polymers composed of poly(lactide), poly(glycolide) and their related copolymers have become increasingly popular in the biomedical field, and especially for the preparation of bone substitute constructs. However, there are reports of a delayed inflammatory reaction occurring months or years after implantation. Despite clinical studies demonstrating adverse reactions to bioresorbable polymers, there are few in vitro studies that corroborate these results. Due to the long polymer degradation times, in vitro tests carried out at physiological temperature, 37°C, tend to assess the short-term biocompatibility of these materials, with the long-term biocompatibility assessed using in vivo trials. The aim of this thesis is to develop an in vitro protocol that can be used to assess the delayed inflammatory response of bioresorbable polymers, in an accelerated timeframe.

In this thesis, an increased temperature accelerated degradation methodology, that simulates the long-term degradation of poly(D,L-lactide-co-glycolide) (PDLLGA) and poly(L-lactide-co-glycolide) (PLLGA), was validated. Polymer samples were degraded in phosphate-buffered saline (PBS), under sterile conditions. Degradation temperatures of 47°C, 57°C and 70°C were selected and compared to physiological temperature, 37°C. At predetermined time intervals, samples were retrieved and evaluated for changes in mass, swelling, molecular weight, crystallinity, and thermal properties. The results from this thesis suggest that the degradation mechanism at elevated temperatures is similar to that observed at 37°C.

The accelerated degradation methodology developed was then used to prepare PDLLGA and PLLGA samples at increasing levels of degradation. Cytotoxicity analysis demonstrated that PDLLGA samples showed increasing levels of cytotoxicity at the later stages of degradation, with PLLGA samples demonstrating significantly less cytotoxic behaviour. A new dynamic flow culture methodology for testing the cytotoxicity of these degradable materials was also developed, following concern that accumulation of acidic degradation products in a closed multi-well culture environment could overestimate cytotoxicity. In addition to cytotoxicity testing, the inflammatory cytokines expressed by cells in response to degraded PDLLGA and PLLGA were analysed, and a suggested mechanism by which lactide-based bioresorbable materials could module the inflammatory response through the G-protein coupled receptor (GPCR), hydroxycarboxylic acid receptor 1 (HCA1), has been proposed.

This research has contributed to an improved understanding of the delayed inflammatory response and a novel understanding of the anti-inflammatory and immunosuppressive role of the degradation product, lactate/lactic acid. It is recommended that the methodology developed in this thesis should be used for the preclinical in vitro assessment of other bioresorbable polymers to evaluate their long-term biocompatibility.
Date of AwardJul 2021
Original languageEnglish
Awarding Institution
  • Queen's University Belfast
SponsorsNorthern Ireland Department for the Economy
SupervisorFraser Buchanan (Supervisor), Louise Carson (Supervisor) & Efrosyni Themistou (Supervisor)

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