Injectable In Situ Forming Microparticles: A Novel Drug Delivery System

Pharmaceutical formulation research has recently been focusing on delivery systems which provide long therapeutic effects and reduced side effects, and involving simplified production stages and facilitated application process. In situ forming microparticle (ISM) systems, one of the latest approach in this field, offer a new encapsulation technique and meet the objectives stated above. Factors such as the carrier used to form the multiparticles, amount and type of drug and the vehicle type can be taken as the main performance criteria for these systems. Ongoing studies have shown that this new multiparticulate drug delivery system is suitable for achieving new implant delivery system with low risk of dose-dumping, capable of being modulated to exhibit varying release patterns, reproducible, easily applicable and welltolerated compared with classically surgical implants. Keywords: Injectable, In situ forming, Microparticle system, Controlled drug delivery.


INTRODUCTION
Various intramuscular or subcutaneous controlled drug delivery systems in the form of implants or microparticles have been developed based on biodegradable polymers. There are different techniques available to form drug-polymer composites as implants or microparticles [1][2]. Implants are generally formed by blending drug, polymer and additives followed by melt extrusion, melt compression or injection molding to obtain the desired implant size or shape [2]. Problems have occurred with these processes include elevated process temperature, poor content uniformity (especially with low-dose drugs) and often the requirement of surgical administration [1]. Additionally preparation of biodegradable implants and especially of microparticles is complicated and involves multiple-step processes and formulation parameters to be controlled, and this has also affect scale-up and cost [3].
In order to avoid inconvenient surgical insertion of large implants, injectable biodegradable and biocompatible polymeric particles (microspheres, microcapsules, nanocapsules, nanospheres etc.) could be employed for controlled drug delivery [3]. Microparticles of size < 250 µm, ideally < 125 µm, are suitable for this purpose [4]. Biodegradable polymers used for these systems are natural or synthetic in origin and are degraded in vivo by suitable biodegradation kinetics, either enzymatically or non-enzymatically or both, to produce biocompatible and toxicologically safe byproducts which are further eliminated by normal metabolic pathways. The polymers selected for these systems must also meet some other requirements such as drug compatibility, suitable mechanical properties and ease of processing [5][6][7].
Drug release from these systems follows Higuchi square root of time relationship [22] with a high burst release in general [23][24][25][26][27]. Potential solvent toxicity and high viscosity of polymer solutions causing injection problems can possibly be occurred to these systems [11][12][13][14][15][16][17][25][26][27]. To overcome these problems a novel in situ forming microparticle system has been developed [28][29]. ISM systems are based on an emulsion of an internal drugcontaining polymer solution and a continuous oil or aqueous phase. After injection, the inner polymeric phase hardens upon contact with body fluids and thus forms in situ microparticles.
ISM systems have significantly reduced initial burst release and viscosity (which is primarily controlled by the external phase). Thus, easier injectability and reduced pain has been achieved, compared to use of the polymer solutions (in situ implant systems). Additionally ISMs are multiparticulates, and thus minimize variations in implant morphology (after solidification) and provide more consistent and reproducible drug release profile [28][29][30][31][32][33][34].

METHODS OF PREPARATION OF ISM SYSTEMS
ISM systems consist of an internal drugcontaining polymer solvent phase (polymer phase) that is dispersed into an external oil or water phase. Following injection of this dispersion, the inner polymer phase hardens upon contact with body fluids and forms in situ microparticles. The preparation process for ISM is simple, when compared with classical techniques for the preparation of various other microparticles such as solvent evaporation or phase separation methods [28][29]34].

Preparation of oil-in-oil (O/O)-ISM system
The O/O-ISM system comprises of polymer solution phase and external oil phase. Polymer solution phase can be prepared by dissolving a biodegradable polymer such as PLA or PLGA in a water-miscible, biocompatible solvent (this may also act as a plasticizer for the polymer) such as NMP, 2pyrrolidone, dimethyl sulphoxide (DMSO), triacetin and/or low molecular weight grade of polyethylene glycol {such as polyethylene glycol (PEG) 200 or 400}, which are able to form highly concentrated polymer solutions in combination with surfactants such as polyethylene glycol sorbitan monooleate (Tween 80) or polyoxyethylene-polyoxypropylene copolymer (Pluronic F 68). Peanut oil and sesame oil (oil for injection) can be used as a biocompatible external oil phase with surfactants such as sorbitan monooleate (Span 80) or triglyceride (Miglyol 812) with/without aluminum stearate or aluminummonostearate. Subsequently, accurately weighed internal and external phases (in different ratios) are loaded preferably into polypropylene syringes A and B, respectively. The two syringes are coupled with a connector and emulsification can be achieved by pushing the content of syringe A into syringe B to and fro for 50 mixing cycles. At the end of this, the contents are pushed into one syringe, the connector is removed and a needle is attached, and is ready for use [19][20][31][32][33][34][35][36][37][38].

Preparation of oil in-water (O/W)-ISM system
The O/W-ISM system is able to prepare similar with O/O-ISM system, except some differences in the following aspects. This system consists of an internal polymer solution phase in a hydrophobic solvent such as ethyl acetate and an external aqueous phase containing aqueous surfactant like Poloxamer 188 (Lutrol F 68). Emulsification of O/W-ISM can be achieved by 25 mixing cycles in a two-syringe system as described above for O/O-ISM system [30,39].

PREVIOUS STUDIES ON ISM SYSTEMS
In vitro studies  Drug release decreased with increase in molecular weight of the encapsulated drug. Substitution of triacetin with triethyl citrate and Miglyol 812 with soybean oil resulted in variation in release pattern of drug. Soybean oil (mixed triglyceride of unsaturated and saturated fatty acids) which is relatively more hydrophobic than Miglyol 812 (mixed C 8 -C 10 triglyceride of saturated fatty acids) produced relatively more hydrophobic PLGA microglobule dispersion, and thereby delayed the dissolution of released myoglobin [35].
The above novel microencapsulation process overcomes some of the disadvantages associated with other methods by (a) excluding the use of unacceptable organic solvents like dichloromethane or NMP and using acceptable vehicle mixture instead to prepare biodegradable PLGA microspheres, (b Luan and Bodmeier investigated the influence of PLGA type (molecular weight and end-group functionality) on leuprolide release from ISM systems which were based on an emulsion of PLGA solution in NMP dispersed in an oil phase (sesame oil containing 2 % Span 80 and 2.5 % aluminum stearate). The results of in vitro drug release were in contrast to those for microparticles prepared by classical solvent evaporation method. The use of lower molecular weight PLGA resulted in lower initial release from ISMs than when higher molecular weight PLGA was used. ISMs prepared with PLGA combinations showed a decrease in initial release when low-molecular-weight PLGA content was increased. Slow solvent diffusion from low-molecular-weight PLGA droplets into release medium led to a less porous structure of solidified microparticles, which explains the lower initial release. Moreover PLGA with free carboxylic acid end groups led to a lower drug release compared to PLGA with esterified end groups [32].
The influence of various formulation and processing parameters on drug release from ISM systems has been investigated in another study by Luan and Bodmeier. The systems consisted of leuprolide acetatecontaining internal polymer phase {PLGA (RG 503H)/PLA (R 202H) and NMP} and external oil phase (peanut/sesame oil, Span 80 and aluminum monostearate). In vitro solidification and dissolution studies releaved that in situ forming microparticles prepared by using PLGA were spherical and had a smooth surface but a porous inner structure as seen in Figure 2A. Drug release from these ISMs [31] was occurred in two phases with a high initial release followed by an almost constant and slower release, as shown in Figure 3A. Drug release profiles obtained from ISMs prepared by using PLA were almost monophasic ( Figure 3B) compared to ISMs that contained PLGA ( Figure 3A). In particular, ISMs with 10 % drug loading and 40 % polymer concentration showed lower initial and almost linear drug release from days 2 to 150, and also exhibited a smooth surface with a less porous inner structure, which explains the slower drug release compared to others, as shown in Figures 2B and 3B. In contrast to RG 503H, R 202H led to a much lower initial leuprolide release {Figs 3A and 3B); this could possibly be explained by the higher carboxylic acid content of lower molecular weight PLA (R 202H acidic number is 10 mg KOH/g versus 4 mg KOH/g for RG 503H)}. In both systems, leuprolide acetate and PLGA or PLA were dissolved in polar solvent NMP. Therefore existence of ionic interactions between polymer and drug was also possible and a stronger interaction in the presence of R 202H could impede drug loss from polymer solution and thus lead to a lower initial release [31].
Kranz and Bodmeier investigated in vitro drug (diltiazem hydrochloride and buserelin acetate) release from ISM systems (PLA or PLGA in 2-pyrrolidone, NMP or DMSO dispersed into peanut oil phase) as a function of type of solvent and polymer, concentration of polymer and ratio of internal polymer phase:external oil phase.
According to the data obtained, initial drug release from ISM systems decreased with increasing polymer concentration and decreasing polymer phase:external oil phase ratio. Decrease in the release of the drug was in the rank order of DMSO > NMP > 2pyrrolidone, for the biocompatible solvent. In contrast to the release of low molecular weight diltiazem hydrochloride, peptide release (buserelin acetate) was strongly dependent on polymer degradation/erosion. The results confirmed that ISM system significantly reduced burst effect due to the presence of an external oil phase compared to in situ implant systems [33].
In another study by Kranz and Bodmeier [36], the effects of key formulation variables on the release of bupivacaine hydrochloride from ISM systems and in situ forming implants were investigated. The ISM systems were prepared as a combination of PLA-solvent phase dispersed into an external peanut oil phase. The solvent release from ISMs and in situ forming implants into phosphate buffer which influences polymer precipitation rate was investigated as a function of the type of Figure 3: (A) Influence of drug loading ( =10%; = 5%; ▲ = 2.5% ) on leuprolide release from ISMs containing %30 w/w RG 503H; and (B) Leuprolide release from ISMs at different drug loadings and polymer concentrations (x = 15% drug, 30% R 202H; = 10% drug, 30% R 202; = 10% drug, 40% R 202H; all % are given w/w) [31]. solvent (NMP, DMSO and 2-pyrrolidone), polymer concentration and polymer:oil phase ratio. Upon contact with the release medium, solvent release into buffer medium from ISMs was significantly slower compared to in situ forming implant solutions due to the presence of an external oil phase. The solvent release of ISM systems into phosphate buffer decreased with increasing polymer concentration and decreasing polymer:oil phase ratio. The type of solvent used decreased solvent release rate in the rank order: DMSO > NMP >> 2-pyrrolidone. A slower solvent release into the aqueous medium resulted in less porous microparticles, thus explaining the reduced initial drug release from ISM systems compared to in situ forming implant solutions [36]. Incorporation of a model protein hen egg white lysozyme with ISM system was also investigated by Körber and Bodmeier. Ternary solvent blends of DMSO, ethyl acetate and water were used to adjust the protein solubility in order to facilitate the incorporation of either dispersed or dissolved protein into the polymer solution to achieve a good entrapment and to get a low initial release. The ISM systems consisted of lysozyme-containing polymer solutions (40 % PLGA-RG 502H) in sesame oil (external phase). The release of lysozyme from ISMs was only slightly affected by the investigated DMSO/ethyl acetate/water ratios (100/0/0, 75/25/0 and 70.5/23.5/6) and was incomplete. This result could be attributed to the protein, which is located in the polymer rich phase following solidification of ISMs and adsorption of the positively charged lysozyme to the negatively charged PLGA [44]. . These studies indicate that the best temperature of storage was 4 ºC for both systems which retained their stability for more than 12 months at 4 ºC.

In vitro -in vivo studies
The influence of various preparation and formulation parameters on in vitro and in vivo release of bupivacaine hydrochloride from ISM systems, which were prepared by emulsifying drug-containing polymer solutions (PLGA in 2-pyrrolidone) into a peanut oil phase at a polymer to oil phase ratio of 1:1, 0.5:1, 0.25:1 and 0.1:1, was investigated by Kranz et al [37]. Ready-to-inject formulations were prepared by probe sonication. Alternatively, emulsion formation prior to injection was carried out using two syringes filled with polymer phase in the first and peanut oil phase in the second container. In vitro drug release was highly influenced by the polymer to oil phase ratio and initial drug release was reduced with decreasing polymer phase/external oil phase ratio. A hot plate model has been used in order to quantify the pharmacodynamic effect of bupivacaine release from ISM systems in male Sprague-Dawley rats. ISM with a polymer/oil phase ratio of 0.25:1 and 0.5:1 were selected for in vivo studies due to reduced initial in vitro release of drug. The in vivo drug release studies were in good agreement with in vitro drug release data. With ISM system, the analgesic effect of bupivacaine hydrochloride was prolonged when compared to the injection of a drug solution or drug-polymer solution [37].
Rungseevijitprapaa and Bodmeier [30] evaluated the forces required to inject biodegradable emulsions of the ISM systems via different sizes of syringe and needle into a newly developed chicken meat model using the compression mode of a texture analyzer (Figure 4). The injection forces obtained were finally correlated with the injectability of formulations into living rats. The results indicate that the flow of ISM formulations through the needle could be described by the well-known Poiseuille equation. The injection forces obtained were directly proportional to the form ulations' viscosity, injection speed Figure 4: Illustration of the setup for injection force measurement with chicken meat model [30]. and the second power of the syringe plunger radius, and inversely proportional to the fourth power of the needle radius. This test method could differentiate very well the required forces from various formulations and injection conditions as well as different injection sites (muscle, subcutaneous tissues or air). In comparison to polymer solutions that form implants in situ, the ISM systems were more easily injectable with smaller needle size, and thus it was expected to be less painful and give better patient compliance [30].

Myotoxicity studies on ISM systems
The potential in vitro and in vivo myotoxicity of ISM and in situ implant systems was investigated by Kranz et al [34]. Recent studies by a firm (Rovi) have enabled it to develop commercial formulations of "in situ microparticles" (ISM™) for new therapeutic applications. Extended release formulations based on ISM™ technology are currently being developed for psychiatric and oncologic drugs due to their industrial potential, as well as commercial and sanitary interest. In September 2010, the experimental stage has begun for the first Phase I trial of Risperidone-ISM™ on healthy volunteers, which is the first candidate for this drug delivery system. This first trial aims mainly to evaluate the pharmacokinetics and tolerability of a single intramuscular administration of Risperidone in an ISM™ formulation. This trial will serve not only to confirm the pharmacokinetic profile of this innovative depot formulation for the monthly administration of an approved anti-psychotic, but it will also serve as a proof of concept for validating ISM technology as a base platform for other developments. In this regard, other new formulations with ISM™, for monthly administration of another widely used antipsychotic, and for quarterly administration of an approved aromatase inhibitor that is currently used extensively in the treatment of hormone-dependent breast cancer, are already in a pre-clinical phase [47].

CONCLUSION
In situ forming microparticle (ISM) systems offer a new encapsulation technique that provides prolonged release of drug along with much greater ease of preparation and administration than conventional microparticles and surgically implanted systems. ISMs are an attractive alternative to parenteral drug delivery, especially those prepared by existing complicated microencapsulation methods. Moreover, recent developments in this field such as the first Phase I trial of Risperidone-ISM™ suggest there is a possibility for prolonged release of bioactive molecules applied as intramuscular injection.