Enzyme- and cell-mediated degradation of poly(ethylene carbonate) by surface erosion

Rational design of poly(peptide-ester) block copolymers for enzyme-specific surface resorption

Tissue resorption and remodeling are pivotal steps in successful healing and regeneration, and it is important to design biomaterials that are responsive to regenerative processes in native tissue. The cell types responsible for remodeling, such as macrophages in the soft tissue wound environment and osteoclasts in the bone environment, utilize a class of enzymes called proteases to degrade the organic matrix. Many hydrophobic thermoplastics used in tissue regeneration are designed to degrade and resorb passively through hydrolytic mechanisms, leaving the potential of proteolytic-guided degradation underutilized. Here, we report the design and synthesis of a tyrosol-derived peptide-polyester block copolymer where protease-mediated resorption is tuned through changing the chemistry of the base polymer backbone and protease specificity is imparted through incorporation of specific peptide sequences. Quartz crystal microbalance was used to quantify polymer surface resorption upon exposure to various enzymes. Aqueous solubility of the diacids and the thermal properties of the resulting polymer had a significant effect on enzyme-mediated polymer resorption. While peptide incorporation at 2 mol% had little effect on the final thermal and physical properties of the block copolymers, its incorporation improved polymer resorption significantly in a peptide sequence- and protease-specific manner. To our knowledge, this is the first example of a peptide-incorporated linear thermoplastic with protease-specific sensitivity reported in the literature. The product is a modular system for engineering specificity in how polyesters can resorb under physiological conditions, thus providing a potential framework for improving vascularization and integration of biomaterials used in tissue engineering.

Biodegradation Behavior Control for Shape Memory Polyester Poly(Glycerol-Dodecanoate): An In Vivo and In Vitro Study

Poly(glycerol-dodecanoate) (PGD) has garnered increasing attention in biomedical engineering for its degradability, shape memory, and rubber-like mechanical properties. Adjustable degradation is important for biodegradable implants and is affected by various aspects, including material properties, mechanical environments, temperature, pH, and enzyme catalysis. The crosslinking and chain length characteristics of poly(lactic acid) and poly(caprolactone) have been widely used to adjust the in vivo degradation rate. The PGD degradation rate is affected by its crosslink density in in vitro hydrolysis; however, there is no difference in vivo. We believe that this phenomenon is caused by the differences in enzymatic conditions in vitro and in vivo. In this study, it is found that the degradation products of PGD with different molar ratios of hydroxyl and carboxyl (MR_H/C) exhibit varied pH values, affecting the enzyme activity and thus achieving different degradation rates. The in vivo degradation of PGD is characterized by surface erosion, and its mass decreases linearly with degradation duration. The degradation duration of PGD is linearly extrapolated from 9-18 weeks when MR_H/C is in the range of 2.00-0.75, providing a protocol for adjusting the degradation durations of subsequent implants made by PGD.

Investigating the Biodegradation Mechanism of Poly(trimethylene carbonate): Macrophage-Mediated Erosion by Secreting Lipase

Poly(trimethylene carbonate) (PTMC), as one of the representatives of biodegradable aliphatic polycarbonates, has been found to degrade in vivo via surface erosion. This unique degradation behavior and the resulting nonacidic products make it more competitive with aliphatic polyesters (e.g., polylactide) in clinical practice. However, this surface degradation mechanism is complicated and not fully understood to date despite the findings that several reactive oxygen species and enzymes can specifically degrade PTMC in vitro. Herein, the biodegradation mechanism of PTMC was investigated by using possible degradation factors, distinct cell lines, and the inhibitors of these factors. The results demonstrate that PTMC undergoes a specific macrophage-mediated erosion. Macrophages tend to fuse into giant cells and elicit a typical inflammatory response by releasing proinflammatory cytokines. In addition, macrophages are suggested to primarily secrete enzymes (lipase specifically) to erode the PTMC bulk extracellularly as inhibiting their activity effectively prevented this eroding process. The clarification of the biodegradation mechanism in this work suggests that the degradation of PTMC highly depends on the foreign body response. Thus, it reminds the researchers to consider the effect of the microenvironment on the degradation and drug release of PTMC-based implantation devices and localized drug delivery systems.

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.

Antioxidant-mediated control of degradation and drug release from surface-eroding poly(ethylene carbonate)

Surface-eroding polymers are of significant interest for various applications in the field of controlled drug delivery. Poly(ethylene carbonate), as an example, offers little control over the rate of degradation and, thus, drug release, which usually conflicts with the requirements for long-acting medications. Here, we challenged an option to decelerate the degradation of poly(ethylene carbonate) in vitro and in vivo. When polymer films loaded with distinct antioxidants (vitamins) along with the model drugs leuprorelin and risperidone were incubated in superoxide radical solution and phagocyte culture, the mass loss and drug release from the delivery vehicle was a function of the type and dose of the utilized antioxidant. Once the polymer surface was "attacked" by reactive oxygen species, the antioxidants were released on demand quenching the polymer-degrading radicals. Accordingly, specific combinations of polymer and radical scavengers resulted in controlled release medications with an extended "life-time" of one month or longer, which is difficult to achieve for poly(ethylene carbonate) in the absence of antioxidants. A comparable degradation and drug release behavior was observed when antioxidant-loaded poly(ethylene carbonate) films were implanted in rats. Furthermore, linear correlations were obtained between the mass loss of the polymer films and the released fraction of drug (with slopes close to 1), a clear indication for the surface erosion of poly(ethylene carbonate) in vitro and in vivo. Overall, an addition of antioxidants to poly(ethylene carbonate)-based controlled drug delivery vehicles represents a reasonable approach to modify the performance of long-acting medications, especially when a "life time" of weeks to months needs to be achieved. STATEMENT OF SIGNIFICANCE: Surface-eroding poly(ethylene carbonate) (PEC) is of significant interest for long-acting injectable formulations. However, PEC offers only little control over the rate of degradation and, thus, drug release kinetics. We describe an option to decelerate the degradation rate of PEC in vitro and in vivo. When polymer films loaded with distinct antioxidants along with model drugs were incubated in superoxide radical solution, phagocyte culture and implanted in rats, their mass loss and drug release was a function of the type and dose of the utilized antioxidant. Accordingly, specific combinations of polymer and radical scavengers resulted in controlled release medications with an extended "life-time" of one month or longer, which is difficult to achieve for PEC in the absence of antioxidants.

Hard Block Degradable Polycarbonate Urethanes: Promising Biomaterials for Electrospun Vascular Prostheses

We report biodegradable thermoplastic polyurethanes for soft tissue engineering applications, where frequently used carboxylic acid ester degradation motifs were substituted with carbonate moieties to achieve superior degradation properties. While the use of carbonates in soft blocks has been reported, their use in hard blocks of thermoplastic polyurethanes is unprecedented. Soft blocks consist of poly(hexamethylene carbonate), and hard blocks combine hexamethylene diisocyanate with the newly synthesized cleavable carbonate chain extender bis(3-hydroxypropylene)carbonate (BHPC), mimicking the motif of poly(trimethylene carbonate) with highly regarded degradation properties. Simultaneously, the mechanical benefits of segmented polyurethanes are exploited. A lower hard block concentration in BHPC-based polymers was more suitable for vascular grafts. Nonacidic degradation products and hard block dependent degradation rates were found. Implantation of BHPC-based electrospun degradable vascular prostheses in a small animal model revealed high patency rates and no signs of aneurysm formations. Specific vascular graft remodeling and only minimal signs of inflammatory reactions were observed.