In vitro release of a water-soluble agent from low viscosity biodegradable, injectable oligomers

Visible light degradable micelles for intraocular corticosteroid delivery

Visible light responsive micellar drug delivery formulations are of notable interest for the treatment of ocular diseases, as their successful development would enable controlled drug release at the back of the eye, improving efficacy and reducing side-effects when compared to existing approaches. In this work, an aliphatic polycarbonate-based visible light responsive micelle formulation based on mPEG-b-poly(5-hydroxy-trimethylene carbonate) (PHTMC) was prepared wherein the pendant hydroxyl groups of the PHTMC repeating units were protected by blue light-labile [7-(diethylamino)coumarin-4-yl]methyl (DEACM). The photo-labile DEACM provided a photo-triggered release profile, as, upon the removal of these protecting groups by photo-irradiation, the micelles underwent structural disruption, leading to the release of the payload. The removal of DEACM also deprotected the pendant hydroxyl groups of PHTMC, leading to PHTMC backbone degradation via intramolecular cyclization.

Polymer micelles for the protection and delivery of specialized pro-resolving mediators

Specialized pro-resolving mediators (SPMs) are being considered for the treatment of chronic inflammatory diseases. However, these polyunsaturated fatty acids are prone to oxidation and as a result have a short biological half-life. It was reasoned that a micelle formulation would provide sustained delivery of SPMs while providing protection from oxidation. Thus, micelle formulations were prepared with poly(ethylene glycol) (PEG) as the hydrophilic block and poly(trimethylene carbonate) (PT) containing unsaturated pendant groups, specifically benzyloxy (BT) and sorbate (ST) groups, as the hydrophobic block. The potential of these micelles was assessed using linoleic acid as a model SPM. Loading into a micelle core reduced the extent of oxidation of the model SPM and a sustained release of non-oxidized model drug was achieved for up to 20 days in vitro from the PEG-P(T-BT) micelles. These micelles were also non-cytotoxic over a wide concentration range, demonstrating the potential of this formulation for effective SPM release in vivo.

In Vivo Degradation Mechanisms of Aliphatic Polycarbonates and Functionalized Aliphatic Polycarbonates

Aliphatic polycarbonates (APCs) have been studied for decades but have not been as utilized as aliphatic polyesters in biomaterial applications such as drug delivery and tissue engineering. With the recognition that functionalized aliphatic polymers can be readily synthesized, increased attention is being paid to these materials. A frequently provided reason for utilizing these polymers is that they degrade to form diols and carbon dioxide. However, depending on the structure and molecular weight of the APC, degradation may not occur. In this review, the mechanisms by which APCs and functionalized APCs have been found to degrade in vivo are examined with the objective of providing guidance in the continued development of these polymers as biomaterials.

In Vivo Degradation Mechanism and Biocompatibility of a Biodegradable Aliphatic Polycarbonate: Poly(Trimethylene Carbonate-co-5-Hydroxy Trimethylene Carbonate)

A recently developed viscous liquid aliphatic polycarbonate, poly(trimethylene carbonate-co-5-hydroxy trimethylene carbonate), has advantageous properties for the delivery of acid-sensitive drugs such as proteins and peptides. This copolymer degrades in vitro via an alkaline-catalyzed intramolecular cyclization reaction yielding oligo (trimethylene carbonate), glycerol, and carbon dioxide, but its in vivo degradation mechanisms are presently unknown. The in vivo degradation mechanism and tissue response to this copolymer were investigated following subcutaneous implantation in Wistar rats. The molecular weight and composition of the copolymer varied in the same manner following subcutaneous implantation as observed in vitro. These findings suggest that the copolymer also degraded in vivo principally via intramolecular cyclization. The tissue response in terms of the inflammatory zone cell density, fibrous capsule thickness, and macrophage response was intermediate to that of two clinically used biodegradable sutures, Vicryl and Monocryl, indicating that the copolymer can be considered biotolerable. Collectively, the data show that further development of this copolymer as a drug delivery material is warranted.

In vivo degradation behavior of enzyme-degradable poly(trimethylene carbonate)-based biohybrid networks of varying water content

Polymeric biohybrid networks have significant potential as supportive materials for soft connective tissue regeneration. Their success in this regard is determined by their initial mechanical properties, which are dependent on their water content, as well as the rate at which these properties change with time due to cell mediated degradation. In this study the in vivo degradation and tissue response following implantation of matrix metalloproteinase (MMP)-degradable poly(trimethylene carbonate) (PTMC)-based biohybrid networks were assessed in a Wistar rat model. The networks examined varied in equilibrium water content from circa 20% to 70% w/w. The networks degraded through MMP secretion by inflammatory cells at the tissue-material interface, generating a mass loss profile consistent with surface erosion but modulus and sol content changes consistent with a bulk erosion process. This degradation profile was explained in terms of a population gradient in MMP concentration from the surface to the bulk of the networks due to diffusion restrictions. A histological analysis of the tissue surrounding the implants confirmed a moderate tissue response comparable to that observed towards a Vicryl^TM suture, suggesting that these new materials can be considered biocompatible.

Injectable thermosensitive hydrogel systems based on functional PEG/PCL block polymer for local drug delivery

Injectable in situ thermosensitive hydrogels have potential applications in tissue engineering and drug delivery. The hydrogel formulations exist as aqueous solutions at room temperature but rapidly solidify into gels at 37 °C in situ, making them highly suitable for administering drugs in a minimally invasive manner to the target organ(s). The hydrogel formed with nanoparticles assembled with amphiphilic polymer blocks of polyethyleneglycol (PEG) and biodegradable polycaprolactone (PCL) have been tested as platforms for targeted and sustained drug delivery, and have shown encouraging results. In this review, we summarize the influence of the molecular weight, PEG/PCL ratio and functional structure of hydrophobic PCL blocks on the critical gelation temperature, gelling behavior and drug release kinetics of the hydrogels. The current studies on the biomedical applications of thermosensitive PEG/PCL hydrogels have also been discussed.

Corelease of bioactive VEGF and HGF from viscous liquid poly(5-ethylene ketal ε-caprolactone-co-D,L-lactide)

The potential of a viscous liquid injectable delivery system composed of poly(5-ethylene ketal ε-caprolactone-co-D,L-lactide) (PEKCDLLA) to release bioactive vascular endothelial growth factor (VEGF) and hepatocyte growth factor (HGF) using an osmotic pressure release mechanism for the purpose of treating critical limb ischemia was investigated. VEGF and HGF were lyophilized separately with trehalose and bovine serum albumin (BSA) and incorporated into the polymer by simple mixing. VEGF and HGF were released by convective flow through superhydrated regions formed within the polymer as a result of the osmotic activity generated upon dissolution of the particles, along with the contributions of polymer degradation at later time points. A sustained release of highly bioactive VEGF and HGF for over 40 days with minimal burst was achieved under conditions of multidirectional delivery. The solubility of the growth factors in the concentrated trehalose solution formed upon dissolution of the particle within the polymer was determined to be a key parameter governing the rate and extent of growth factor release. This formulation approach, of using a low viscosity polymer delivery vehicle, is potentially useful for localized delivery of acid and temperature sensitive proteins, such as VEGF and HGF. This system may also serve as a platform for controlled and predictable delivery patterns for other therapeutic proteins in other clinical settings.