Azin Jahangiri 1, Leila Barghi 1*
1 Department of Pharmaceutics, School of Pharmacy, Urmia University of medical sciences, Urmia, Iran
*Correspondence:
Leila Barghi, Department of Pharmaceutics, School of Pharmacy, Urmia University of medical sciences, Urmia, Iran, Phone: +984432754991, Urmia, Iran. Email: : Leila_barghi@yahoo.com
Abstract
The polymeric nanoparticles are prepared from biocompatible and biodegradable polymers with size ranged between 10 to 1000 nm. Based on their preparation methods, nanospheres or nanocapsules are formed. Nanospheres are matrices system while the nanocapsules are the systems in which the drug is incorporated in the core of system and surrounded by a unique polymeric membrane. Polymeric nanoparticles have been developed in order to deliver drugs in a controlled and targeted manner with reduced side effects. This article reviews preparation methods of polymeric nanoparticles and synthesis methods of biodegradable polymers utilized for development of polymeric nanoparticles. Moreover application of polymeric nanoparticles in development of drug delivery systems will be discussed.
Keywords: Polymeric nanoparticles, drug delivery, nanospheres, nanocapsules
Introduction
The polymeric nanoparticles can be considered as promising carriers for development of drug delivery systems. They are prepared from biocompatible and biodegradable polymers with size ranged between 10 to 1000 nm. Drug can be dissolved, entrapped, encapsulated or attached to a nanoparticles matrix. Nanospheres and nanocapsules are two types of nanoparticles which can be obtained depending on their preparation method [1]. Nanospheres are matrix system in which drug is uniformly dispersed. Nanocapsules are the system in which the drug is incorporated in the core of system and surrounded by a unique polymeric membrane [2]. Either from preformed polymers or by monomeric polymerization, polymeric nanoparticles can be formulated. Emulsion-solvent evaporation, double emulsion and evaporation, emulsions- diffusion, solvent displacement, salting out and dialysis are the most essential methods for preparation of polymeric nanoparticles from polymers already preformed. Monomeric polymerization can be performed via emulsion polymerization or interfacial polymerization. Based on the physicochemical properties of the polymer and the drug to be incorporated, appropriate method for the preparation of nanoparticles is chosen. The abovementioned preparation methods of polymeric nanoparticles will be discussed briefly in this review. Moreover synthesis method of biodegradable polymers utilized for polymeric nanoparticles preparation and finally application of these nanoparticles in drug delivery will be discussed.
Methods for preparation of nanoparticles from preformed polymers
Emulsion-solvent evaporation, double emulsion and evaporation, emulsions- diffusion, solvent displacement, salting out and dialysis are the most essential methods for preparation of polymeric nanoparticles from preformed polymers.
Emulsion-solvent evaporation is defined as a two steps method. In the first step, the organic solution of a polymer containing the dissolved drug requires emulsified into an aqueous phase. During the second step polymer solvent is evaporated either by continuous magnetic stirring at room temperature or under reduced pressure which result in polymer precipitation in the form of nanospheres in which the drug is finely dispersed in the polymer matrix network. This method utilizes high-speed homogenization or ultrasonication. The prepared nanospheres are collected by ultracentrifugation and washed with distilled water to remove stabilizer residue or any free drug. The stir rate, type and amount of stabilizer, viscosity of organic and aqueous phases, and temperature are the most important parameters can affect the size of prepared particles. Application of this method limited to lipophilic drugs, and also the scale-up of the high energy requirements in homogenization is the other limitation for this method [3-4].
Application of emulsion-solvent evaporation method via single emulsion formation is limited to incorporation of the lipophilic drugs. Because loading of hydrophilic drugs using this method is very low. Consequently double emulsion technique is employed, which involves the addition of aqueous drug solutions to organic polymer solution under vigorous stirring to form w/o emulsions. This w/o emulsion is added into second aqueous phase with continuous stirring to form the w/o/w emulsion. Nanoparticles can be formed by solvent evaporation and isolated by centrifugation at high speed. In this method the amount of hydrophilic drug to be incorporated, concentration of stabilizer, polymer concentration and the volume of aqueous phase are the variables that affect the properties of nanoparticles [2]. Polycaprolactone-poly- ethylene oxide block copolymer (PCE) and poly lactide (PLA) nanocapsules containing bovine serum albumin (BSA) were prepared by means of a modified W/O/W double emulsion technology without stabilizer. In this method a mixture of glycerin and water was used instead of the traditional stabilizer [5].
In this method, polymer is dissolved in a partially water-miscible solvent such as ethyl acetate or propylene carbonate and saturated with water to ensure the initial thermodynamic equilibrium of both liquids. Afterward, the polymer-water saturated solvent phase is emulsified in an aqueous solution containing stabilizer. Therefore solvent diffusion to the external phase leads to the formation of nanospheres or nanocapsules. Finally, the solvent is removed by evaporation or filtration, according to its boiling point. Advantages of this method are including high encapsulation efficiencies, narrow size distribution, no need for high energy requirements, high batch-to-batch reproducibility, ease of scale up and simplicity. In this method high volumes of water to be removed from the suspension which can be consider as its disadvantage. Also the leakage of water-soluble drug into the saturated-aqueous external phase during emulsification results in reducing encapsulation efficiency. Thus this technique is an appropriate method for encapsulation of lipophilic drugs [2-3, 6-7].
In solvent displacement (nanoprecipitation) method, solution of a polymer in a water-miscible solvent with intermediate polarity is introduced to the stirred aqueous medium in the presence or absence of a surfactant. Fast diffusion of organic solvent leads to precipitation of polymer on the interface between the water and the organic solvent and nanospheres formation [3]. Preparation of nanocapsules by this method is possible when a small volume of nontoxic oil is incorporated in the organic phase. Considering the oil-based central cavities of the nanocapsules, high loading efficiencies are generally reported for lipophilic drugs when incorporated in nanocapsules [6]. Since the diffusion rate of water-miscible solvents must be enough to produce spontaneous emulsification, Solvent selection is the most important step in this method. Spontaneous emulsification is not observed if the coalescence rate of the formed droplets is sufficiently high [3]. Acetone and dichloromethane are used as suitable water-miscible solvent. However, dichloromethane as a water-miscible solvent results in particle size increasing of formed nanoparticles [8]. Application of this method is limited to preparation of nanoparticles and nanocapsules of lipophilic drugs [9]. Various polymeric nanoparticles containing different polymers such as polylactide- co- glycolide (PLGA) [9-10], PLA [11] and poly caprolactone (PCL) [12] were prepared by this method.
Salting Out Method
In this method polymer and drug are dissolved in a water miscible solvent such as acetone. Afterward this solution emulsified into an aqueous gel containing the salting-out agent (electrolytes, such as magnesium chloride, calcium chloride, and magnesium acetate, or non- electrolytes such as sucrose) and a colloidal stabilizer such as polyvinyl pyrrolidone (PVP) or hydroxyethylcellulose (HEC), polyvinyl alcohol (PVA) followed by dilution with a sufficient volume of water to enhance the diffusion of organic solvent into the aqueous phase, thus inducing the formation of nanospheres. Both the solvent and the salting out agent are then eliminated by cross-flow filtration [3, 13-14]. It is a selective method in processing of thermally instable substances for the reason that this method does not require an increase of temperature for solvent removal [15]. The selection of the salting out agent is important, because it can play an important role in the encapsulation efficiency of the drug [6].
Polymer is dissolved in an organic solvent such as Dimethylformamide (DMF) or acetone and placed inside a dialysis tube with appropriate molecular weight cut off. The organic phase was dialyzed against the water, with changes of the aqueous phase every 2-3 hours until the organic solvent had been completely removed by diffusing out of the dialysis membrane. The organic solvent diffusion out of the membrane leads to precipitation of polymer due to a loss of solubility and formation of nanoparticles can occurred. Then, the sample solution in the dialysis tube was collected and centrifuged to achieve nanoparticles. PLGA nanoparticles of testosterone, Paclitaxel and coumarin were prepared by means of this method [3, 16-17].
Monomeric polymerization can be performed via emulsion polymerization or interfacial polymerization.
Emulsion polymerization is one of the nanoparticles preparation methods based on monomer polymerization. This method is divided into two categories: A. Polymerization by using of an organic continuous phase; this procedure involves the dispersion of monomer into an emulsion or inverse microemulsion, or into nonsolvent. Surfactants or protective soluble polymers were used to prevent aggregation in the early stages of polymerization. Utilizing toxic organic solvents, surfactants, monomers and initiator, which are subsequently eliminated from the formed particles, are considered as disadvantages of this procedure. B. Polymerization by using of aqueous continuous phase; the monomer is dissolved in a continuous aqueous phase without any surfactants or emulsifiers. The polymerization process can be initiated in the continuous phase collides which contain ion or a free radical as an initiator. On the other hand, the monomer molecule can be transformed into an initiating radical by high-energy radiation, including g-radiation, or ultraviolet or strong visible light. Chain growth starts when initiated monomer ions or monomer radicals collide with other monomer molecules according to an anionic polymerization mechanism. Phase separation and formation of solid particles can take place before or after termination of the polymerization reaction [1].
In this method two lipophilic and hydrophilic monomers are dissolved in two continuous and dispersed phase and the polymerization takes place at the interface of the two phases. Hollow polymeric nanoparticles were synthesized by interfacial cross-linking reactions via polyaddition, polycondensation and radical polymerization [1, 18-20].
The most essential properties of polymers for biomedical applications such as drug delivery systems are biodegradability, biocompatibility, suitable solubility and appropriate mechanical properties. Biodegradable polymers are degraded and catabolized by microorganism and finally converted to carbon dioxide and water. Among hydrolytically degradable polymers, aliphatic polyesters represent appropriate properties for biomedical application. Poly glycolide, Poly L-lactide (PLLA) and polycaprolactone (PCL) are the most extensively investigated polyesters [21].
Polycondensation of diols and dicarboxylic acids, self-polycondensation of hydroxyl acids, ring-opening polymerization (ROP) of lactones and lactides are the main synthesis methods of polyesters. Polycondensation which can be applicable for variety of combinations of diols and diacids requires higher temperature and longer reaction time to obtain high molecular weight polymers. Moreover syntheses of polymers by controlled chain lengths are not possible by means of this method. ROP of cyclic lactones are recognized as a one pot polymerization method to synthesis high molecular weight homo- and co- polyesters. The advantages of ROP compared to polycondensation are: milder reaction conditions (lower temperature), shorter reaction times and the absence of reaction by-products. In ROP, using specific initiator molecules such as hydroxyl containing molecules lead to the molecular weight control of the polymers. The rate of polymerization can be controlled by the application of a wide-range of biocompatible catalytic systems, such as Tin (II) 2-ethylhexanoic acid (stannous octoate). The most extensively studied monomers for synthesis of aliphatic polyester used for biomedical applications are lactide, glycolide and caprolactone [22-23].
Polyglycolide
Polyglycolide is a highly crystalline polymer (45–55% crystallinity) and therefore exhibits excellent mechanical properties such as high tensile modulus. Its solubility in organic solvents was very low. The glass transition temperature of this polymer ranges from 35 to 40 °C with melting point greater than 200 °C. The biomedical applications of polyglycolide are limited by its disadvantages such as high rate degradation, acidic degradation products and low solubility. Therefore to overcome these drawbacks, several copolymers containing glycolide blocks are being synthesized [23-24].
PLLA
PLLA is also a crystalline polymer (37% crystallinity) and the degree of crystallinity depends on the molecular weight and polymer processing parameters. The glass transition temperature of that are ranged between 60 to 65 °C with a melting temperature of 175 °C. PLLA is a slow-degrading polymer compared to polyglycolide. It exhibits appropriate mechanical properties including good tensile strength, low extension and high modulus [22].
Poly (d,l-lactic-co-glycolic acid) (PLGA)
Copolymers of lactic and glycolic acids have received a considerable attention in the area of biodegradable polymers as drug delivery carriers. Low toxicity, constant biodegradation rate, mechanical resistance, and regular individual chain geometry are the desirable properties of these copolymers. The PLGA copolymers can basically synthesized via two methods: A. Through a direct polycondensation of lactic acid and glycolic acid; B. Through an opening polymerization of cyclic dimers of lactic acid (lactide) and glycolic acid (glycolide). The second method are resulting in copolymers with high molecular weight and therefore with better mechanical properties than the ones from the second method. Lactide is more hydrophobic than glycolide, thus PLGA copolymers with high ratio of lactide to glycolide are less hydrophilic and demonstrates low degradation rate. The rate of hydration and hydrolysis of PLGA is high due to low crystallinity of PLGA in comparison to PGA and PLLA [25].
PCL is a semi-crystalline, hydrophobic polymer with a glass transition temperature (Tg) of −60 ◦C and melting point ranging from 59 to 64 ◦C [23]. PCL is synthesized via ring-opening polymerization method with using ε-caprolactone monomer and a variety of anionic, cationic and co-ordination catalysts [21]. Suitable properties of PCL including excellent biocompatibility, good solubility and low melting point, make it as an attractive carrier for development of controlled drug delivery systems. However biodegradation of PCL is slow which restricts its clinical application. Thus preparation of PCL copolymers is proposed [26-28]. Block and random copolymers of PCL can be synthesized by using monomers such as ethyleneoxide, polyvinylchloride, chloroprene, polyethylene glycol (PEG), polystyrene, diisocyanates (urethanes), tetrahydrofuran (THF), diglycolide, dilactide, δ-valerlactone, substituted caprolactones, 4-vinyl anisole, styrene, methyl methacrylate and vinyl acetate [21, 29]. Among these monomers, PEG is suitable to construct caprolactone block copolymers because of its hydrophilicity, nontoxicity and absence of antigenicity and immunogenicity [30].
PCL–PEG diblock copolymers can be synthesized by ROP from monomethoxy-PEG (MPEG) and ε-CL in presence of a catalyst [31-32]. MPEG was first distilled with dried toluene to remove residual water in the solvent. Then Ɛ-caprolactone (Ɛ-CL) and stannous octoate were added and the mixture was refluxed for several hours at appropriate temperature with mechanical stirring. Copolymers with different PCL block lengths can be obtained by varying the concentration of Ɛ-CL monomer. Yield and molecular weight of synthesized copolymers are affected by the solvent moisture content and the catalyst concentration respectively. Therefore, MPEG and Ɛ-CL monomers should be dried to reach to constant weight before use and the polymerization must be carried out under dry nitrogen or vacuum. Stannous octoate has become the most widely used catalyst because it is commercially available and soluble in common organic solvents and cyclic ester monomers [30, 33].
There are two types of PCL/PEG triblock copolymers: PCL–PEG–PCL (PCEC) and PEG–PCL–PEG (PECE) copolymers. Preparation of PCEC is similar to the synthesis of diblock copolymers, with the exception of using dihydroxy PEG as initiator instead of MPEG [34-36].
Two steps methods were employed for obtaining PECE copolymers. These copolymers were synthesized by coupling reaction with using PCL diol and PEG in presence of l-lysine methyl ester diisocyanate (LDI) as the chain extender [37-39]. Preparation of PECE from MPEG–PCL diblock copolymer by using isophorone diisocyanate (IPDI) and hexamethylene diisocyanate as coupling agent is possible too [30].
PLGA, PCL and their copolymers are the most extensively investigated polyesters for biomedical applications. Examples of various polymers and preparation methods for development of polymeric nanoparticles are shown in table 1. PLGA based drug delivery systems have been developed in order to deliver drugs in a controlled and targeted manner. Oral bioavailability of some drugs such as nuciferine, curcumin and cyclosporine were improved through incorporating to PLGA nanoparticles [40-42]. Protein loaded PLGA nanoparticles demonstrated superior oral bioavailability. Insulin loaded PLGA nanoparticles were prepared via w/o/w emulsion solvent evaporation method. Their physicochemical properties were evaluated and finally invivo studies were carried out in diabetic rats in order to evaluate their oral bioavailability. The results demonstrated that the serum glucose level was considerably reduced after oral administration of insulin nanoparticles [43]. In another study folate coupled pegylated PLGA nanoparticles containing insulin were prepares via double-emulsion solvent evaporation method. These nanoparticles exhibited a twofold increase in the oral bioavailability of insulin in diabetic rats [44]. PCL and its copolymers were utilized to develop nanoparticles containing various drugs. For instance tamoxifen loaded Poly ethylene oxide-modified poly caprolactone nanoparticles via solvent displacement method were prepared by Shenoy et al. This particles with the mean size between 150 to 250 nm, demonstrated tumor-selective biodistribution [45]. In another study tetradrine loaded core-shell structure PEGylated PCL nanoparticles was prepared from different two and tree block copolymers by single o/w emulsion-solvent evaporation method. They showed that prepared drug loaded nanoparticles exhibited more antitumor effect than free tetradrine [46]. Erlotinib loaded PCEC nanoparticles were prepared via solvent displacement method by Barghi et al. They concluded that the prepared PCEC nanoparticles might have the potential to be considered as delivery system for erlotinib [36, 47]. The core-shell type nanoparticles of PCEC triblock copolymers containing clonazepam through dialysis method were prepared by Ryu et al. They investigated effect of organic solvent type in physicochemical properties of obtained nanoparticles. Using 1,4- dioxane and acetone as organic solvent lead to preparation of particles with mean particle size less than 100 nanometer, whereas Use of THF, DMF, DMSO, and DMAc resulted in an increased particle size. Among them, 1,4-dioxane was resulted relatively high drug loading with small particle size [48]. Utilizing PCEC copolymers in preparation of nimodipine micelles via nanoprecipitation method was reported by Ge et al [49]. In a study carried out by Lu et al. bovine serum albumin (BSA) nanocapsules by means of PCE diblock copolymers were prepared using a modified W/O/W double emulsion technology. A mixture of glycerin and water was used instead of the traditional stabilizer system in the preparation of these polymeric nanocapsules. They reported that the formation of the nanocapsules was facilitated by the high viscosity of the mixture and the hydroxyl group of the glycerin [5]. In addition methoxy polyethylene glycol Polycaprolactone (MePEG/PCL) diblock copolymer was used as a carrier for development of paclitaxel loaded nanospheres by dialysis method [50]. A nanocarrier system for octreotide as a somatostatin analogue was prepared by Dubey et al. They prepared Octreotide loaded PCL/PEG nanoparticles via solvent evaporation method. In vivo biodistribution studies exhibited major accumulation of octreotide nanoparticles in tumor compared to free octreotide. Consequently Octreotide nanoparticles exhibited more anti-tumoral effect and fewer side effects in comparison to free octreotide [51].
Table 1. Examples of various polymers and preparation methods for development of polymeric nanoparticles
| Polymer | Preparation method | Drug |
| PLGA | Double emulsion solvent evaporation | Nuciferine |
| PLGA | Emulsion- diffusion- evaporation | Cyclosporine |
| PLGA | Double emulsion solvent evaporation | Insulin |
| pegylated PLGA | Double emulsion solvent evaporation | Insulin |
| Poly ethylene oxide caprolactone | Solvent displacement | Tamoxifen |
| PCEC | Solvent displacement | Erlotinib |
| PCE | Double emulsion solvent evaporation | Bovin Serum Albumin |
| Me PEG/PCL | Dialysis | Taxol |
| PCE | Solvent displacement | Octreotide |
| PCEC | Dialysis | Clonazepam |
Conclusion
Various preparation techniques are available for production of polymeric nanoparticles. Appropriate technique for preparing drug loaded nanoparticles is selected based on drug characterization. Furthermore, the most essential properties of polymers used for nanoparticles preparation are biodegradability, biocompatibility, suitable solubility and appropriate mechanical properties. Among hydrolytically degradable polymers, aliphatic polyesters such as PLGA, PCL and its copolymers represent appropriate properties for biomedical applications. Therefore they are extensively used in development of nanoparticles as delivery systems for various drugs.
Conflict of interest
The authors report no conflict of interest.
References