Bioactive-Based Poly(anhydride-esters) and Blends for Controlled

Chapter 3

Bioactive-Based Poly(anhydride-esters) and Blends for Controlled Drug Delivery Downloaded by UNIV OF PITTSBURGH on July 20, 2013 | http://pubs.acs.org Publication Date (Web): July 8, 2013 | doi: 10.1021/bk-2013-1135.ch003

R. Fogaça,1,2 M. A. Ouimet,1 L. H. Catalani,2 and K. E. Uhrich*,1 1Department

of Chemistry and Chemical Biology, Rutgers University, Piscataway, New Jersey 08854, United States 2Instituto de Química, Universidade de São Paulo, São Paulo, SP 05513-970, Brasil *E-mail: [email protected].

Biodegradable polymers exhibit several potential therapeutic advantages for controlled release due to their biologically relevant (i.e., bioactive) molecules. Bioactive-based poly(anhydride-esters) are one such example. Compared to conventional drug formulations, these polymers offer improved efficacy and reduced toxicity. Utilizing specific synthetic methods, various bioactive agents can be chemically incorporated within the polymer backbone and then released in a sustained manner, thus overcoming current issues of drug delivery, such as poor patient compliance due to repeated administration. Moreover, the polymers can be formulated into hydrogels, microspheres, or electrospun fibers to increase their potential use and applications.

Introduction Biodegradable polymers have been extensively studied in recent years for biomaterials, gene delivery, drug delivery, and tissue engineering (1). Polyanhydrides are of particular interest as advantageous biomedical delivery systems because of their surface erosion behavior. This property enables a near zero-order drug release profile for bioactives physically entrapped within the polymer matrix (2–5). Poly(anhydride-esters) (PAEs), in particular, have been investigated over the past decade as drug delivery systems for a wide variety of bioactive molecules. Typically bioactive molecules are physically incorporated © 2013 American Chemical Society In Tailored Polymer Architectures for Pharmaceutical and Biomedical Applications; Scholz, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.

Downloaded by UNIV OF PITTSBURGH on July 20, 2013 | http://pubs.acs.org Publication Date (Web): July 8, 2013 | doi: 10.1021/bk-2013-1135.ch003

within PAE matrices and released through a combination of matrix degradation and diffusion. Unfortunately, this method of drug incorporation can suffer from low drug loading, poor mechanical properties, and limited control over drug release rates. To overcome issues associated with physical incorporation, chemical incorporation of bioactives into PAEs has been investigated. By chemically incorporating the bioactives within a polymer backbone, drug loading can be increased without compromising the material’s mechanical properties and the bioactives can be released in a controlled, sustained fashion during subsequent polymer degradation. Furthermore, the degradation rate and drug release rate can be manipulated based on the polymer’s chemical structure (2, 6). Many prevalent bioactive molecules have been chemically incorporated within PAE backbones for controlled release. PAEs based on morphine, a potent narcotic analgesic, have shown sustained release of the opioid leading to extended analgesia in the treatment of chronic pain – extending analgesia from 3 hours for free morphine to 3 days (7). Hydroxycinnamic acid (HC) derivatives such as ferulic, sinapic, and coumaric acids – which exhibit antioxidant, antibacterial, and anti-inflammatory activities (8) – have also been incorporated within PAE backbones. In addition to providing sustained release, incorporation of ferulic acid into a polymer backbone overcomes its chemical instability, which results in decomposition and thus limited bioactivity. Salicylic acid (SA), a nonsteroidal anti-inflammatory drugs (NSAIDs) well known for its anti-inflammatory and analgesic properties (9, 10), has the most extensive research to date as a bioactive incorporated within PAEs.

Figure 1. Hydrolytic degradation of polymer 1 to therapeutic SA and compatible linker acid. The incorporation of SA into PAEs and subsequent formulation has been a major focus of the Uhrich laboratory. The SA-containing PAEs (SAPAEs) are polymeric prodrugs in which SA has been incorporated into a PAE backbone using linker molecules, and upon hydrolytic cleavage of the labile anhydride and ester bonds, the drug (e.g., SA (2)) and the biocompatible “linker” molecule (3) are released (Figure 1). The degradation rate of the polymer (1) can be altered by changing the chemical structure of the linker (6) to vary polymer hydrophilicity or by copolymerizing the bioactive-based monomer with a non-bioactive monomer with appropriate hydrophilic characteristics. As noted above, these polymers are unique in their composition. In addition, the SAPAEs are biocompatible (11–14), stable under storage conditions (15), and can be exposed to ionizing radiation for sterilization without changing their 28 In Tailored Polymer Architectures for Pharmaceutical and Biomedical Applications; Scholz, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.

physicochemical properties (16). Further, SAPAEs are effective for controlling inflammation (11, 14), promoting bone growth (11, 17), and preventing biofilm formation (14, 18) and can be fabricated into different geometries including disks (6), fibers (19), microspheres (20, 21), coatings (18), and stents (22).


Poly(anhydride-ester) Polymerization Methods Several polymerization methods have been used to synthesize polyanhydrides including melt-condensation, solution, inter-facial, and ring-opening approaches. For SAPAEs, specifically, melt-condensation and solution methods have been successfully employed (6, 12, 23, 24). In melt-polymerization, a diacid (i.e., polymer precursor) is first activated with an excess of acetic anhydride to yield the monomer. The monomer is then melted under high temperature (e.g., 160 – 180 °C) and vacuum (e.g., > 2 mm Hg) to initiate polymerization (Figure 2). Although this method is highly reproducible and allows for easy scale-up, it may not be suitable for thermally sensitive bioactives, especially if the difference between a monomer’s melting temperature and decomposition temperature is narrow.

Figure 2. Synthesis of SAPAEs via melt-condensation polymerization by acetylating the diacid (A) and polymerizing monomer under vacuum at high temperatures (B). Solution polymerization (C) is an alternative method. For these thermally sensitive monomers, solution polymerization at low or ambient temperatures is necessary. In solution polymerization, where strict control of stoichiometry is required, the diacid is reacted with triphosgene in the presence of a base, forming phosgene in situ as a coupling agent. As compared to meltpolymerization, polymers with lower molecular weights are obtained and scale-up can be difficult. 29 In Tailored Polymer Architectures for Pharmaceutical and Biomedical Applications; Scholz, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.


Poly(anhydride-esters) as Delivery Systems As mentioned previously, the SAPAE physicochemical properties can be altered by changing the chemical structure of the linker (6, 12, 23, 24). Uhrich et al. demonstrated that the glass transition temperature, hydrophobicity, and release rates were altered as the previously described linker molecules were changed. Several linkers were evaluated including linear and branched aliphatic, heteroatomic, and aromatic structures (6, 23). Glass transition temperatures decreased with increasing alkyl chain length and polymers containing aromatic linkers exhibited the highest values. Hydrolytic degradation of the polymer to release SA was also found to be a function of linker structure. The general trends demonstrated that more hydrophilic linkers (leading to a more hydrophilic polymer overall) exhibited faster in vitro degradation than did more hydrophobic structures (23, 25). In addition to their linker-guided tunability, drug delivery devices based on the SAPAEs can be formulated into microspheres, hydrogels, and electrospun fibers, for example, depending upon their intended application. These devices could be introduced in vivo via numerous delivery routes including topical (e.g., subcutaneous and epicutaneous), enteral (e.g., oral and rectal), and parenteral (e.g., intravenous and intramuscular) administration. The formulation of SAPAE microspheres, hydrogels, and fibers for controlled release applications are detailed below.

Poly(anhydride-ester) Microspheres

Microsphere-based drug delivery systems have garnered significant attention in recent years as they can be easily administered via non-surgical methods (e.g., by injection) (26). As a result, compounds with pharmaceutical activity are routinely encapsulated within polymer microspheres (27–30). Microspheres exhibit increased release rates compared to other systems such as polymer discs due to the larger surface area-to-volume ratio offered by the microspheres (3, 29). For example, three different SAPAE compositions with a heteroatomic (4), linear aliphatic (5), and branched aliphatic linkers (6) were evaluated as microsphere formulations (Figure 3A) (21). SAPAE microspheres were produced using an oil-in-water single emulsion, solvent evaporation technique. As shown in representative scanning electron microscopy (SEM) images (Figure 3B-D), the resulting microspheres exhibited smooth morphologies and did not aggregate, which is important as aggregated microspheres could potentially hinder efficient injection (31). In addition, the microspheres showed a relatively narrow size distribution with diameters ranging from 2 to 34 µm (21), allowing for uniform microsphere degradation and thus more predictable drug release profiles (31).

30 In Tailored Polymer Architectures for Pharmaceutical and Biomedical Applications; Scholz, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.


Figure 3. SAPAE chemical structures with varying linkers 4-6 used to formulate microspheres (A), SEM images of the SAPAE microspheres according to linker structure 4-6 (B-D), and SA release profile from the SAPAE microspheres (E). Adapted with permission from reference (1). Copyright 2012 Springer Science.

According to the release profiles presented in Figure 3E, the three different linker molecules demonstrate short- and long-term SA release. This can be explained by the varying hydrophobicity of each polymer used for microsphere formulation. SA release was a direct function of the linker structure; polymer microspheres comprised of the heteroatomic linker released 100% SA faster in vitro (3 days) than did the linear aliphatic linker (21 days). The polymer microspheres with the branched aliphatic linker have a projected 100% SA release over 3.5 months (21). Thus, SA can be delivered over days or months providing potential use for a variety of applications. Poly(anhydride-ester) Hydrogels Hydrogels are comprised of hydrophilic polymeric networks that can absorb substantial amounts of water without dissolving (32). If covalent bonds are formed between adjacent polymer chains to cause crosslinking, the resulting hydrogels are considered “chemically produced”. The covalent bonds can be produced though different pathways (e.g., chemical reactions) (33–35). In the case of chemical crosslinking, residual toxic crosslinking agents may remain within the hydrogel structure, making these systems less desirable for in vivo applications. To circumvent this limitation, hydrogels can be produced with 31 In Tailored Polymer Architectures for Pharmaceutical and Biomedical Applications; Scholz, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.


physical crosslinks formed through hydrogen bonds, crystallized domains, or hydrophobic interactions between adjacent polymer chains (36). These physical hydrogels are reversible and offer safer alternatives for drug delivery. Due to their high water content, hydrogels are soft materials that are suitable for a wide variety of applications such as contact lenses, controlled delivery systems, scaffolds, regenerative medicine, skin grafts, and wound dressings (32, 37). In wound healing, hydrogels can provide an ideal moisturizing environment (with unobstructed visibility of the wound bed) which can minimize pain and enhance healing. Moreover, an ideal dressing would be capable of delivering bioactive molecules to improve the healing process. Poly(N-vinyl-2-pyrrolidone) (PVP) (Figure 4A, left) is a common hydrogel material due to its mechanical and water-absorption properties when chemically crosslinked (35, 38). One major drawback, however, is that PVP alone does not have inherent bioactivity. In contrast, SAPAEs have inherent bioactivity but can be brittle and lack the swelling properties necessary for hydrogels. Consequently, Uhrich et al. (39) have combined these two polymer systems to produce physically crosslinked hydrogels with appropriate physical characteristics and drug delivery behavior as potential wound dressing materials. These PVP:SAPAE-based hydrogels would be ideal wound dressings as water absorption by the hydrogel would initiate the PAE degradation and trigger bioactive release. As the bioactives are not physically admixed within the hydrogel matrix, immediate and uncontrolled release of the bioactives can be avoided.

Figure 4. Chemical structures of PVP (A, left) and SAPAE (A, right), representative image of the PVP:SAPAE hydrogel (B), and SEM image of the PVP:SAPAE (70:30) hydrogel porous structure at 4080X magnification (C).

PVP:SAPAE hydrogels (Figure 4A-B) were prepared at various ratios. Water uptake, as indicated by swelling ratio (Q) (see Table 1), changed in relation to the PVP:SAPAE ratio, with higher PVP content correlating to larger swelling values. 32 In Tailored Polymer Architectures for Pharmaceutical and Biomedical Applications; Scholz, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.

Furthermore, water uptake and swelling resulted in pore formation throughout the hydrogel structure (Figure 4C) and promoted SA release. The SA release profiles are currently being investigated to ascertain the time frame over which all SA is released; this outcome will be the subject of a future publication.


Table 1. Swelling ratio (Q) variation according to PVP content (39) PVP:SAPAE ratio

Swelling ratio (Q)

70:30

14.2 ± 1.4

60:40

7.2 ± 0.8

50:50

4.9 ± 0.1

PAEs containing bioactives other than SA have been successfully blended with PVP to form hydrogels as well. Uhrich et. al. (40) have prepared PAEs based on HC derivatives (ferulic, sinapic, and coumaric acids) that could be beneficial for wound dressing applications due to the therapeutic properties of the HC (released via PAE hydrolysis). The chemical structures of the HC-based PAEs (HCPAEs) are presented in Figure 5.

Figure 5. HCPAE polymers (7a-c) used to prepare physically crosslinked hydrogels. PVP:HCPAE hydrogels exhibited similar swelling behaviors as compared to PVP:SAPAE hydrogels. As with the SAPAEs, higher PVP content yielded higher swelling ratios. A wide range of PVP-to-HCPAE ratios were investigated with many showing appreciable swelling values relevant for wound dressing applications. A notable exception, however, was the 90:10 PVP:HCPAE system which did not swell, but completely dissolved when exposed to water. 33 In Tailored Polymer Architectures for Pharmaceutical and Biomedical Applications; Scholz, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.


Poly(anhydride-ester) Electrospun Fibers Electrospinning is a well-established technique for producing polymeric fibers with diameters ranging from nanometers to micrometers. Polymer fibers possess large surface areas and can be useful in a wide variety of applications including controlled drug release systems, scaffolds for tissue engineering, and in regenerative medicine (41). Due to their low molecular weight and poor mechanical properties, the SAPAEs previously discussed are not suitable for electrospinning when used alone. However, highly uniform SAPAE micro- and nanofibers have been successfully prepared when blended with poly(lactic-co-glycolic acid) (PLGA) or PVP (42, 43). Notably, highly aligned PLGA:SAPAE (70:30) electrospun nanofibers were prepared to promote cell alignment for nerve regeneration applications. The environment provided by the nanofibers mimics that found in vivo for cell alignment, promoting nerve cell differentiation and directing the attachment and morphology of nerve cells. Specifically, Schwann cells showed directed cell elongation and proliferation in directions parallel to the oriented nanofibers along with aligned neurite outgrowth. In addition, the fibers degraded over 42 days to locally release SA and reduce inflammation during the neurite outgrowth and regeneration processes. As another example, PVP:HCPAE blended electrospun microfibers (PVP:HCPAE 70:30) were produced for bioactive wound dressing applications. Figure 6A-B shows the microstructure of PVP:HCPAE electrospun fibers exhibiting diameters of ca. 10 µm. The fibers present a ribbon-like structure, which can be related to both solvent evaporation during the electrospinning process (44); electrospinning parameters must be adjusted to optimize the fiber morphology. These electrospun systems are suitable materials to produce hydrogels given their physicochemical properties. Electrospun PVP-based hydrogels have been described as appropriate materials for wound healing; in additional to the benefits of using hydrogels as a wound dressing, an electrospun material can control the rate of debriding protein release to ultimately enhance the healing process (45). Therefore, these electrospun fibers have the capabilities of a hydrogel, the ability to remove non-viable tissue, and exhibit controlled drug release from the bioactive-based PAEs.

Figure 6. SEM images of PVP:HCPAE (ferulic acid 70:30) (A) and PVP:HCPAE (sinapic acid 70:30) electrospun fibers (B). 34 In Tailored Polymer Architectures for Pharmaceutical and Biomedical Applications; Scholz, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.


Summary Biodegradable polymers play an integral role in drug delivery, particularly in the realm of biomaterials. Bioactive-based PAEs are a unique class of polymers that allows for controlled drug delivery, high drug loading, and can be fabricated into various devices depending on the end use. The bioactive-based PAEs described here have been fabricated into microspheres, hydrogels, and fibers, yielding novel biomaterials capable of sustained bioactive release upon polymer hydrolytic degradation. SAPAE formulation into microspheres provides a non-invasive method for drug delivery and provides the potential to be used as a carrier device. Innovative physical hydrogels and electrospun fibers utilizing SAPAEs and HCPAEs blended with PVP and/or PLGA have significant potential as enhanced wound dressings. The polymer design allows for more control current delivery systems which can improve upon current treatments. The drug release tunability, ease of fabrication, and ability to change the bioactive molecule allows for these polymers to be employed in a broad range of applications within the biomaterial field.

Acknowledgments The authors acknowledge the many contributions of Ray Ottenbrite, who has been a leader and inspiration to the polymer field, specifically for biomedical applications. Dr. Bryan Langowski and Uhrich group members are thanked for intellectual contributions to this work.

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Bioactive-Based Poly(anhydride-esters) and Blends for Controlled

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