Author's personal copy Biomaterials 31 (2010) 2358–2370 Contents lists available at ScienceDirect Biomaterials journal homepage: www.elsevier.com/locate/biomaterials Self-assembling methoxypoly(ethylene glycol)-b-poly(carbonate-co-L-lactide) block copolymers for drug delivery Michael Danquah a, Tomoko Fujiwara b, Ram I. Mahato a,* a Department of Pharmaceutical Sciences, University of Tennessee Health Science Center, 19 South Manassas (Room 224), Memphis, TN 38103-3308, USA b Department of Chemistry, The University of Memphis, Memphis, TN 38152, USA article info abstract Article history: Bicalutamide is the most widely used non-steroidal antiandrogen for treating early stage prostate Received 11 September 2009 cancer, but suffers variable oral absorption due to its limited aqueous solubility. Thus, our objective was Accepted 24 November 2009 to synthesize novel biodegradable copolymers for the systemic micellar delivery of bicalutamide. Available online 17 December 2009 Flory–Huggins interaction parameter (cFH) was used to assess compatibility between bicalutamide and poly(L-lactide) or poly(carbonate-co-lactide) polymer pairs. Polyethylene glycol-b-poly(carbonate-co- Keywords: lactide) [PEG-b-P(CB-co-LA)] copolymers were synthesized and characterized by NMR and gel Micelles permeation chromatography. These micelles had average diameter of 100 nm and had a smooth surface Copolymer Polyethylene glycol and distinct spherical shape. Drug loading studies revealed that adding the carbonate monomer could Polycarbonate increase bicalutamide loading. Among the series, drug loading of micelles formulated with PEG-b-P(CB- Poly lactic acid co-LA) copolymer containing 20 mol% carbonate was about four-fold higher than PEG-b-PLLA and Bicalutamide aqueous solubility of bicalutamide increased from 5 to 4000 mg/mL. CMC values for PEG-b-P(CB-co-LA) copolymers was up to 10-fold lower than those of PEG-b-PLLA. In vitro release experiments showed PEG-b-P(CB-co-LA) copolymers to be more efficient in sustaining the release of bicalutamide compared to PEG-b-PLLA. Bicalutamide-loaded PEG-b-P(CB-co-LA) micelles showed significant inhibition of LNCaP cell growth in a dose-dependent manner which was similar to the methanol solution of free drug. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction One way of improving the solubility of hydrophobic drugs and in particular bicalutamide is by using polymeric micelles. Micelles are Androgens are known to play a pivotal role in the development attractive drug delivery vehicles primarily because they can solu- and maintenance of the prostate by interacting with the androgen bilize hydrophobic drugs in their core leading to improved receptor (AR) [1]. Consequently, androgen ablation, especially the bioavailability and drug stability. Furthermore, micelles are capable use of antiandrogens, has been used as a standard treatment for of preventing drug degradation, minimizing the adverse effects of men with prostate cancer. Among various antiandrogens, bicaluta- the drug on visceral organs and have the possibility of being made mide (CasodexÔ) is the most widely used non-steroidal anti- site-specific [4]. A number of amphiphilic diblock copolymers androgen for treating early stage prostate cancer due to its relatively composed of polyethylene glycol (PEG) and various biodegradable long half life and tolerable side effects [2]. However, bicalutamide hydrophobic cores capable of forming micelles have been reported exhibits poor aqueous solubility (5 mg/mL), which results in poor in the literature. Examples include: poly(ethylene glycol)-b-poly- and variable drug absorption across the gastro intestinal tract. (aspartic acid) [PEG-b-PAA] [5], poly(ethylene glycol)-b-poly(lac- Furthermore, bicalutamide cannot be administered systemically, as tide-co-glycolic acid) [PEG-b-PLGA][6], poly(ethylene glycol)-b- traditional approaches to increase its aqueous solubility using poly(caprolactone) [PEG-b-PCL] [7] and poly(ethylene glycol)-b- solubilizing agents such as dimethyl sulfoxide (DMSO) and Cre- poly(D,L-lactide) [PEG-b-PDLLA] [8]. Key properties of these micelle mophor EL is not practical in humans due to hemolysis, acute systems such as size, stability, drug release kinetics and drug loading hypertensive reactions and neuropathies [3]. have also been well studied. We have recently demonstrated the feasibility of using PEG-b- PDLLA micelles to increase the aqueous solubility of bicalutamide * Corresponding author. Tel.: þ1 901 448 6929; fax: þ1 901 448 2099. E-mail address: [email protected] (R.I. Mahato). [9]. Although we were able to increase the aqueous solubility of URL: http://www.uthsc.edu/pharmacy/rmahato bicalutamide, we observed only moderate drug loading levels 0142-9612/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.biomaterials.2009.11.081 Author's personal copy M. Danquah et al. / Biomaterials 31 (2010) 2358–2370 2359 which may not be high enough for systemic administration. To 2.2. Computation of solubility and Flory–Huggins interaction parameters (cFH)of solve this problem, the focus of the present study was to specifically bicalutamide with core of PEG-b-PLLA/PEG-poly(carbonate-co-lactide) micelles design and develop a new family of biodegradable amphiphilic The Flory–Huggins interaction parameter (cFH) which characterizes polymer– copolymers to improve the aqueous solubility of bicalutamide by drug compatibility was calculated using equations (1) and (2): enhancing its loading levels. Our strategy involves modifying the hÀ Á À Á À Á i1 polyester component of the well established PEG-b-polyester 2 2 2 2 D ¼ dd À dp polarityþ dd þ dp dispersionþ dd À dp hydrogen (1) copolymer into polyester/polycarbonate copolymer system. The semicrystalline poly(L-lactide) was chosen as the polyester block D2V c ¼ d (2) because it is FDA approved, possesses good mechanical properties FH RT which may provide adequate stability to the micelle system and is where D2 is the solubility difference between the drug (d) and the core of the known for its application as a drug delivery material [10–12]. For polymeric micelle (p). Vd is the molar volume of the drug, T is the temperature in the carbonate block we selected the cyclic 5-methyl-5-benzylox- Kelvin and R is the gas constant. The Hansen partial solubility parameters [(d ) ,(d ) ,(d ) ] for the drug (bica- ycarbonyl-1,3-dioxane-2-one carbonate monomer. 5-methyl-5- x d x p x h lutamide) and the hydrophobic block of PEG-b-PLLA and PEG-b-poly(carbonate-co- benzyloxycarbonyl-1,3-dioxane-2-one is a modification of 5-ben- lactide) [PEG-b-P(CB-co-LA)] copolymers used in equation (1) were estimated using zyloxycarbonyl-1,3-dioxane-2-one which is an intermediate in the the Molecular Modeling Pro software from ChemSW (Fairfield, CA). This software synthesis of numerous antiviral compounds [13]. This carbonate approximates solubility parameters using the Hansen theory of solubility group monomer was chosen since polycarbonates are biodegradable, contribution method. (dx)d,(dx)p and (dx)h refers to the partial solubility parameters accounting for Van der Waals forces of dispersion between atoms, permanent exhibit low toxicity and possess tunable mechanical properties dipole–dipole forces between molecules and the proclivity of molecules hydrogen [14–16]. Furthermore, polycarbonate degrades into carbon dioxide bonding, respectively. Here, subscript x refers to the drug or polymer core. and benzyl alcohol, which unlike the degradation products of poly(L-lactide) [e.g. lactic acid] are less acidic, has less effect on 2.3. Synthesis of 5-methyl-5-benzyloxycarbonyl-1,3-dioxane-2-one microenvironment pH and as such will not result in local inflam- A mixture of 2,2-bis(hydroxymethyl)propionic acid (22.5 g, 0.168 mol), potas- mation. We hypothesize that the introduction of the carbonate sium hydroxide (88% assay; 10.75 g, 0.169 mol), and dimethylformamide (DMF) monomer would provide additional degrees of freedom to tailor (125 mL) was heated to 100 C for 1 h with stirring at which point homogenous a micelle delivery system that is relatively stable, exhibits improved potassium salt solution was formed. Benzyl bromide (34.5 g, 0.202 mol) was added sustained release and has a hydrophobic core that is more dropwise to the warm solution, and stirring was continued at 100 C for 15 h. Upon completion of the reaction, the mixture was cooled and the solvent was removed compatible with bicalutamide leading to enhanced drug loading. under vacuum. The residue was dissolved in ethyl acetate (150 mL), hexanes Improvement in the extent of compatibility between a drug and (150 mL), and water (100 mL). The organic layer was retained, washed with water the core-forming block of the micelle may translate into superior (100 mL), dried (Na2SO4), and evaporated. The resulting solid was recrystallized encapsulation efficiency [17–20]. A number of groups have from toluene to give pure benzyl 2,2-bis(methylol)propionate, as white crystals explored the possibility of predicting drug solubilization in micelles (20 g, 58%). Benzyl 2,2-bis(methylol)propionate (11.2 g, 0.05 mol) was dissolved in pyridine based on thermodynamics and found their predictions to closely (25 mL, 0.3 mol) and CH2Cl2 (150 mL), and the solution was chilled to À78 C under approximate experimental results [18,21–23]. Polymer/drug N2. A solution of triphosgene (7.5 g, 25.0 mmol) in CH2Cl2 was added dropwise over compatibility may be characterized by the Flory–Huggins interac- 1 h, after which the reaction mixture was stirred at room temperature for 2 h. The reaction was quenched by the addition of saturated aqueous NH4Cl (75 mL). Subse- tion parameter (cFH) which accounts for the forces of interaction quently, the organic layer was washed with 1 M aqueous HCl (3 Â 100 mL), saturated between the polymer and the drug; and low cFH values suggest that aqueous NaHCO3 (1 Â 100 mL), dried (Na2SO4), filtered and evaporated to give 5- the polymer is thermodynamically a good solvent for the drug.