Chapter 1

Advances in Polymeric Matrices and Drug Particle Engineering

Sönke Svenson

Dendritic Nanotechnologies, Inc., 2625 Denison Drive, Mt. Pleasant, MI 48858

A n overview is given presenting various strategies to deliver small molecule and macromolecule drugs from polymeric matrices. In addition, drug particle engineering processes are described, including the application of supercritical fluids, controlled precipitation, and co-crystallization of drugs with polymeric templates, with the goal to facilitate drug solubility in water, and therefore, enhance drug bioavailability.

Introduction

There is a considerable interest in the development of drug delivery systems for small molecule and macromolecule drugs that provide stable and controllable blood levels over time to minimize the number of required administrations, and thereby improve patient compliance, and to prevent temporary drug overconcentration with its often adverse side effects. Besides particular drug delivery routes, two major approaches are followed to achieve this goal. (1,2) In the first approach, the drug molecules are embedded into a polymeric matrix or the drug molecules are co-crystallized with a polymeric template. In some cases the drug is chemically linked to the matrix, creating a prodrug delivery system. Drug release is a result of either drug diffusion from the matrix and/or

2 © 2006 American Chemical Society

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degradation of the polymeric matrix. In case of prodrugs, the chemical link to the matrix has to be hydrolyzed as well, triggered either by a change of the local pH or the presence of enzymes. Polymeric matrices utilized in this approach are microspheres, polymer strands, millirods, nanogels, and more 'classical' hydro­gels. Desired routes of application include injection and oral delivery.

The second approach relies on engineering the drug particles through processing protocols. Examples for this approach include (i) drug precipitation in the presence of a supercritical fluid, (ii) spray-freezing of drug particles, (iii) controlled precipitation in the presence of a crystal growth inhibitor, (iv) milling of larger drug particles to the desired particle size, and (v) coating of drug particles with a polymeric layer or coating of the drug molecules onto a crystalline surface.

Several of these systems and their utilization in drug delivery and gene transfection are being highlighted in this overview and some wil l be discussed in detail in the following chapters of this volume.

Results and Discussion

Injectable Polymeric Matrices

Most traditional drug delivery systems involve either suspending a drug into a biodegradable polymer monolith and surgically implanting it into the body, or encapsulating a drug into polymeric microspheres and injecting them subcutaneously. Injectable drug delivery based on polymer solution platforms consist of a biodegradable polymer dissolved in a biocompatible solvent along with the desired drug, usually in the form of suspended particles. A unique feature is that polymer phase inversion occurs on injection of the depot solution into the water-based physiologic environment. Thus the membrane carrier forms in vivo, simultaneous with the onset of the drug release. Consequently, the thermodynamic and mass transfer characteristics associated with the in vivo liquid demixing process play important roles in establishing the membrane morphology and drug release kinetics. Fast phase inverting systems and a mathematical model describing their behavior are presented in chapter 2 of this volume.

Biodegradable, injectable polymeric microsphere formulations have been commercialized for small molecules, peptides, and proteins. A n important principle in developing stable microsphere formulations is minimizing molecular mobility during processing, after administration, and during storage. For example, a process has been developed and commercialized for encapsulating proteins by first suspending a lyophilized form of the drug in a biodegradable polymer solution, spray-freezing the suspension into liquid nitrogen to form nascent microspheres, extracting the polymer solvent into a polymer non-solvent

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at temperatures below 0°C, and finally drying the product under vacuum to minimize residual solvents. The mechanism of drug release from microsphere formulations involves (i) hydration of the microspheres, (ii) fast dissolution of drug or protein molecules located at the surface or with fast surface access via pores (burst), and (iii) slow drug dissolution through additional pores and channels created by degradation of the polymer matrix. The relative size of the drug particles to the microsphere is an important parameter in controlling the initial release or burst. Encapsulating sub-micron protein particles results in substantially lower initial release than encapsulating larger particles. Finally, the duration of drug release can be controlled by polymer degradation characteristics. For example, blending of poly(lactide-co-glycolide) with an amphiphilic polymer is an important tool in tailoring release profiles to meet clinical needs. (3)

Matrices based on Polyesters

Poly(orthoesters) (POE) have been under development since the early 1970's and four distinct families have been synthesized. However, long reaction times, scale-up difficulties, and poor control of the molecular weight have so far lowered the commercial potential of POE, despite interesting properties such as excellent ocular biocompatibility. POE hydrolysis proceeds in three consecutive steps. First, the lactic or glycolic acid segment in the polymer backbone hydrolyzes to generate a polymer fragment containing a carboxylic acid end-group that wil l lower the pH and catalyze orthoester hydrolysis. A second cleavage produces free a-hydroxy acid, the major catalyst for the hydrolysis of the orthoester linkages. Further hydrolysis proceeds in two steps to first generate the diol, or mixture of diols, used in the synthesis and pentaerythritol dipropionate, followed by ester hydrolysis to produce pentaerythritol and propionic acid. D etails about synthesis and properties o f this very interesting polymeric matrix are provided in chapter 3 of this volume.

Novel biodegradable poly(phosphoester) copolymers containing both phosphonate and lactide ester linkages in the polymer backbone, which degrade in vivo into nontoxic residues, have been synthesized by bulk or solvent polymerization techniques. Paclitaxel, incorporated into poly(phosphoester) microspheres by standard methods, was continuously released in vitro and in vivo over at least 60 days. The PACLIMER® formulation was tested for safety and toxicity in mice, rats and dogs. A no-adverse effect level for PACLIMER® microsphere administrations could not be determined but tolerability was demonstrated and a maximum tolerated dose was identified. Pilot studies in human ovarian patients have shown good tolerability of the drug, with continuous release of paclitaxel from the microspheres over at least 8 weeks at plasma concentrations far below those causing systemic toxicity.

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Poly(phosphoester) matrices are described in more detail in chapter 4 of this volume.

The processes used to prepare injectable microspheres typically involve the use of solvents. A new class of absorbable polyester waxes for drug delivery has been developed that allow preparation of microspheres in high yields using solvent-free processes. These polymers are made by polycondensation of monoglycerides and diacids or anhydrides, resulting in an aliphatic polyester backbone with pendant fatty acid ester groups. The expected hydrolysis products are glycerol, a dicarboxylic acid and a fatty acid, all of which are known to be biocompatible. For example, microspheres containing risperidone pamoate as model drug were prepared from poly(monostearoyl glyceride-co-succinate) using the spinning disk process. A single dose intramuscular pharmacokinetic study was performed i n Beagle dogs using formulations with 25% and 32% drug. The mean plasma concentration was measured, indicating that the 25% drug formulation did not give burst release and provided 30 days of sustained release at target drug plasma levels (>10 ng/mL), while the 32% drug formulation gave a small burst followed by over 21 days of sustained release at target levels. (4)

Drug molecules chemically linked to poly(ethylene glycol) (PEG prodrugs) have an altered biodistribution compared to the free drug, leading to longer half-lives, enhanced drug levels in the blood (area under the curve, AUG), drug release profiles without or reduced burst, and enhanced drug accumulation in tumors through the Enhanced Permeability and Retention (EPR) effect. For example, optimized delivery was demonstrated for the hydroxyl-containing anticancer drugs paclitaxel and camptothecin, chemically linked through ester bonds to P E G of molecular weights 20-40 kDa. Using appropriate linker molecules, P E G could also be linked to ammo-containing drugs such as the anticancer agents daunorubicin and doxorubicin. Branching of P E G at the distal ends produced highly loaded prodrugs. Drug release from the carrier matrix can be controlled by specific release mechanisms (e.g., 1,6-benzyl elimination). (5)

Matrices based on Polyanhydrides

A newer class of degradable polymeric matrices is composed of hydro-lytically labile anhydride linkages. (6) These building blocks are easily modified with vinyl moieties to create a photopolymerizable and crosslinkable system. The mild conditions of photopolymerization are conducive to in situ formation of the network. Especially appealing is the ability to tailor the crosslinking density to match the requirements of a particular application, e.g., controlled release rates. For example, 2-hydroxyethylmethacrylate (HEMA) has been copolymerized with [l,3-6w(carboxyphenoxy)]propyl dimethacrylate to prepare degradable crosslinked copolymer networks. The kinetics of the copolymerizations as well as mass loss and swelling profiles were elucidated, in

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addition to the release kinetics of various encapsulated materials, all determined as a function of the crosslinking density. It was found that the mass loss followed a surface degradation mechanism for a network composed of 70 wt% crosslinker, but resembled a bulk eroding system more when the crosslinker content was reduced to 30 wt%. (7)

In an alternative approach to crosslinking the polymeric matrix via photo-polymerization, crosslinked amino acid-containing poly(anhydride-co-imides) have been synthesized and characterized. These polymers are crosslinked exclusively via anhydride linkages, and therefore, can degrade into water-soluble molecules such as amino acid and natural fatty acid. This alternative crosslinking approach avoids the formation of poly(methacrylic acid) as a hydrolysis by-product, a nondegradable macromolecule with poor bio­compatibility. Preliminary in vitro results indicated that these polymers can be used as bioerodible supports in controlled drug delivery applications for up to 70 hours. (8)

Matrices based on Polyacetals

The prevalence of carbohydrates in naturally occurring interface structures raises the question of whether a biomaterial built of common acyclic carbo­hydrates would have the features necessary for advanced pharmacological engineering, i.e., inertness in vivo (non-bioadhesiveness or 'stealth' properties), biodegradability of the main chain, and low toxicity. These interface carbo­hydrates have common non-signaling substructures, i.e., acetal and ketal groups. Hydrophilic polymers (polyals) consisting of these carbohydrate substructures should, therefore, be highly biocompatible. Acyclic hydrophilic polyals can be prepared via either polymerization of suitable monomers or lateral cleavage of cyclic polyals (e.g., polysaccharides). Polyals of various types have been prepared and characterized in vitro and in vivo as model components of bioconjugates. For example, matrices assembled from 20 kDa poly(L-lysine) as backbone and 10 kDa polyals as protective graft showed strong correlation of blood half-life in rats with the number of polyal chains per backbone. Matrices modified with 10 and 20 polyal grafts per backbone had half-lives of 9.8 and 25.3 hours, while the half-life of unprotected polylysine was ca. 20 seconds. (9)

Nanogel, Hydrogel, and Electrogel Matrices

There is a vital need for efficient small (<100 nm), biocompatible vehicles for the delivery of drugs, proteins (enzymes, insulin, antigen protein), and nucleic acids (DNA plasmids) or the extraction of toxics from body fluids. Appropriately modified nanogels meet this need as they are porous in nature. Nanogels are crosslinked macromolecular networks made from water-dispersible, biocompatible polymers that can be designed to either deliver or

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rapidly absorb large quantity of desired molecules with a response time in seconds versus days for 'classical' hydrogels. Nanogels can be formed via chemical crosslinking or physical self-assembly of polymers. Both approaches are discussed in detail in chapters 5 and 6 of this volume.

Matrices based on triblock copolymers made from poly(L-lactide)-poly(ethylene glycol)-poly(L-lactide) (PLL-PEG-PLL) form elastic hydrogels with potential applications in drug delivery. Rheology studies on these gels, formed with varying lengths of the hydrophobic P L L A blocks, revealed strong dependence of the hydrogel strength on the P L L A block length, thus offering a mechanism to control its mechanical properties. The gel strength is dependent upon the network structure, which in turn governs the degradation behavior of the gels and hence the release rate of bioactive molecules. Equilibrium properties of hydrogels such as drying/swelling isotherm, osmotic pressure and water activity were subject of another study, using poly(ethylene oxide)-poly(propylene oxide) diblock copolymers. It was found that swelling of these hydrogels is a diffusion-limited process, while drying is evaporation-limited. A diffusion model to fit water loss or gain as a function of time has been applied to extract useful parameters such as water diffusion coefficients in these hydrogels. In another study, PEO hydrogels were synthesized directly in water or physiological medium by free radical homopolymerization of telechelic PEO macromonomers. Their ability to serve as semi-permeable, biocompatible membranes for an artificial pancreas was examined. In vitro tests confirmed their good biocompatibility. Both glucose and insulin diffused through these hydrogels but the behavior o f the latter was more complex. T he crosslinking reaction could be extended to include direct encapsulation of biologically active materials such as hepatocytes. Details of these studies are discussed in chapters 7 to9 .

The polymeric constituents of hydrogels can be engineered following patterns found in nature. F or e xample, s ilk-elastinlike p rotein p olymers a re a class of genetically engineered block copolymers composed of repeated silk-like (GAGAGS) and elastin-like (GVGVP) peptide blocks. Some of these polymers self-assemble in water and have been extensively studied for drug delivery and gene transfection applications. The thermal characterization, in vitro and in vivo delivery of plasmid D N A , and the potential of controlled adenoviral delivery from one of these polymers is being reviewed in chapter 10.

Burst release of drugs from a polymeric matrix, often an undesired side effect of matrix-based delivery, can be utilized for a dual-release approach i n anticancer therapy. Biodegradable polymer millirods, c omposed o f p oly(D,L-lactide-co-glycolide (PLGA) and impregnated with anticancer drugs have been designed to be implanted in tumors after image-guided radiofrequency ablation, an alternative therapy to surgery for patients with unresectable tumors. First, burst release wil l rapidly raise the drug concentration in the surrounding tissue,

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followed by sustained release to maintain this drug concentration for an extended period of time. This dual-release approach is described in detail in chapter 11.

One major challenge of drug delivery from polymeric matrices is predicting the drug solubility in the respective matrix to optimize permeation flux and meet the therapeutic dose requirement. Although a critical parameter, there is no easy quantitative measurement method available. Therefore, a drug solvation para­meter model has been developed for drug delivery from adhesive matrices that replaces the hard to measure quantity by two easily accessible parameters. Details of this model are described in chapter 12 of this volume.

In most delivery systems, drug release is controlled by the interactions between drug and carrier. Weak interactions often result in initial burst release while strong interactions delay or even prevent complete release of the drug. Controlled storage and delivery of drugs from a hydrogel-based system would therefore represent a significant advance i n pharmaceutical delivery. A novel redox-activated hydrogel (electrogel) has been developed that achieves trapping and release of drugs by a single mechanism, the reversible switching of the gel from a reduced hydrophobic (storage) state to an oxidized hydrophilic (release) state. The electrogel, derived from a self-doped electroactive polythiophene, is compatible with small molecule drugs in all charge states and has the ability to deliver drugs in molecular or particulate (crystalline, powder, excipient-formulated) forms. Highly potent but extremely toxic anticancer drugs can thus be c hemically i solated, s equestered, a nd d elivered 1 ocally t o t umor s ites using the electrogel. (10)

Besides using polymeric matrices for drug delivery applications, these materials, when loaded with silver particles, can be utilized in antimicrobial coatings. Loading a polymer coating with a small amount of silver nanoparticles (10 ug/cm2) has shown to completely inhibit growth of the Gram-positive bacterium Staphylococcus aureus. This study is presented in chapter 14 of this volume.

Visualization of processes occurring within matrix formulations during drug release such as water uptake, drug diffusion, and polymer dissolution or erosion, are crucial for understanding the mechanism of drug release upon contact with dissolution media. A new e xperimental a pproach, F ourier T ransform i nfrared (FTIR) spectroscopic imaging, utilizing the macro attenuated total reflection (ATR) IR methology, has been developed to obtain more detailed insight into these processes. The application of this new method to formulations of ibuprofen in P E G matrices is being described in chapter 13.

Effect of Coating and Processing on Polymer Matrices

For some applications, i.e., dosing to elderly and children having problems with swallowing tablets, or in cases where immediate drug release is essential, the availability of fast disintegrating tablets is very important. The key

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properties of fast dissolving matrices are fast absorption of water into the matrix core and immediate break-up of the matrix to release its load. The use of a highly water-soluble carbohydrate, D-mannose, as the main constituent of a fast disintegrating tablet is being described in chapter 23.

A major challenge for matrix-based delivery is the distribution of drug molecules within the matrix. The formation of drug aggregates, crystallization and changes of the crystalline morphology will affect the dissolution rate, and therefore, the bioavailability of drugs released from the matrix. To obtain formulations containing molecularly dispersed drugs, melt extrusion is gaining more attention in the pharmaceutical industry. Several formulations of the drug fenofibrate have been prepared by hot melt extrusion, and the absence of crystalline drug material was established using differential scanning calorimetry (DSC) and X-ray diffraction. (11)

Drug Particle Engineering Using Supercritical Fluids or Liquefied Gases

Supercritical fluids (SCF), i.e., fluids above their critical pressure and temperature, have been the focus of technical and commercial interest in drug particle engineering. Supercritical solutions of drugs and excipients can be rapidly expanded to precipitate the drugs as particles (Rapid Expansion of Supercritical Solutions, RESS). Several variations to this process have been developed. Drugs and excipients that are insoluble in SCF can be dissolved in organic solvents and expanded by an SCF, triggering precipitation of the drug as fine particles. The organic solvent is completely extracted by the SCF anti-solvent (Gas anti-solvent process, GAS). A specific mode of this method is to atomize/spray the solution inside an SCF vessel. Such processes are termed Supercritical Anti-solvent process (SAS), Precipitation under Compressed Anti-solvent (PCA) or Aerosol Solvent Extraction Systems(ASES). (12) Several processes to produce drug particles using SCF are described in chapters 15 to 17 of this volume.

Liquid C 0 2 as the anti-solvent has been utilized to prepare formulations containing a drug (e.g., griseofulvin) and a crosslinked polymer by swelling the polymer/drug mixture in an organic solvent and removing the solvent with the anti-solvent. (13) In a similar way, fine particles of drugs and excipients have been prepared by spraying the organic solution of both under pressure into a cryogenic liquid such as liquid nitrogen (Spray-Freezing into Liquid, SFL) and collecting the particle through lyophilization, and by spraying an alcohol solution of drug and excipient onto a frozen surface, resulting in ultra-rapid freezing, which prevents particle growth during precipitation. Both methods are described in chapters 20 and 21.

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Controlled Precipitation in the Presence of a Crystal Growth Inhibitor

Controlled precipitation can be carried out by adding the solution of a poorly water-soluble drug in a water-miscible solvent into water in the presence of a polymeric crystal growth inhibitor. The anti-solvent water triggers drug crystallization, while the excipient prevents crystal growth. In a similar way small molecules, peptides, proteins, antibodies, D N A and other oligonucleotides have been precipitated in the presence of water-soluble polymers in form of microspheres with controllable size distribution ( P R O M A X X formulation process). In a variation of these approaches, hydrophobic drugs and amphiphilic diblock copolymers dissolved in an organic solvent have been rapidly mixed with a miscible anti-solvent, using a confined impinging jets mixer (Flash Nano Precipitation Process). These approaches are described in chapters 18, 19 and 22. Another way of controlling the crystalline phase and morphology of drug crystals utilizes organized organic assemblies, i.e., self-assembled or Langmuir monolayers on water, as a template. In a proof of concept study, the poly­morphism of calcium carbonate crystals has been successfully controlled. (14)

Conclusion

A n impressive 'toolbox' has been developed to deliver drug molecules from polymeric matrices or to engineer drug particles and crystals to achieve constant blood level concentrations of these drugs and control their release from a matrix or their dissolution behavior from their respective formulations.

References

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8. Cheng, G.; Aponte, M . A . ; Ramirez, C.A. Synthesis and Characteri­zation of Crosslinked Amino Acid-containing Polyanhydrides for Controlled Drug Release Applications. PMSE Preprints, 226th National Meeting of the American Chemical Society, New York, NY, September 2003.

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11. Rosenberg, J.; Degenhardt, M.; Mägerlein, M.; Fastnacht, K.; Berndl, G.; Breitenbach, J. Melt Extrusion: Assessing the Potential of Poorly Soluble Drugs to Form Glass Solutions. PMSE Preprints, 226th National Meeting of the American Chemical Society, New York, N Y , September 2003.

12. Muthukumaran, P.; Chordia, L . Supercritical Fluid Particle Engi­neering: Microparticles to Nanoparticles. Benefits and Limitations. PMSE Preprints, 226th National Meeting of the American Chemical Society, New York, N Y , September 2003.

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14. Couzis, A.; Maldarelli, C.; Chi, C.; Green, D.A. ; Fan, F. Polymorph Selective Crystallization Using Organized Organic Assemblies as Templates. PMSE Preprints, 226th National Meeting of the American Chemical Society, New York, N Y , September 2003.