In the recent years, interest in poly (DL-lactide) (PDLL) and polyglycolide
(PG) for use as controlled-release drug carriers has increased because of their
biodegradability and biocompatibility (Edlund and Albertsson,
2002). They can be degraded via a simple, non-enzymatic hydrolysis mechanism
and gave non-toxic products (Yuehuei, 2000). Copolymers
of DLL and G have all been reported either as random or block copolymers with
partially matched with the requirements of the application such as mechanical
properties and hydrolytic biodegradation (Kister et al.,
1998; Jacobs et al., 1991). Use of these
polyester films in biomedical, pharmaceutical and food packaging have already
received wide attention (Wang et al., 2004; Plackett
et al., 2006; Martino et al., 2006;
Houchin et al., 2007).
The PDLL film is the most widely studied. However, the PDLL film is rigid and
brittle below their glass transition temperature (Tg, 50-60°C)
with low plastic deformation. Flexible PDLL films can be achieved by blending
PDLL with plasticizers. The plasticizers such as poly ethylene glycol, Methoxy
Poly Ethylene Glycol (MPEG), partial fatty acid esters, tributyl citrate, adipates
and branched polylactides have been used for this purpose (Martin
and Averous, 2001; Ljungberg and Wesslen, 2003;
Martino et al., 2006; Ouchi
et al., 2006). Moreover, partial migration of the plasticizers has
been found (Martino et al., 2006). The MPEG plasticizer
molecules which are chemically bonded to PDLLG chains with different MPEG block
lengths and DLL/G ratios were successfully prepared and their film properties
have been reported as previously described (Baimark et
al., 2007; Morakot et al., 2008). The
MPEG block attachment has affected to their mechanical and hydrolytic degradation
properties. Although, the diblock copolymers of MPEG-b-PDLL, MPEG-b-PG,
MPEG-b-PCL, MPEG-b-PDLLG and MPEG-b-PDLLCL have been widely
investigated (Kim et al., 1998; 1999,
2005; Ren et al., 2005;
Beletsi et al., 1999). However, the controlled-release
drug delivery application of MPEG-b-PDLLG films has been scarcely published.
Therefore, this application of the MPEG-b-PDLLG films prepared from solution
casting method is the focus of attention in the present work. It was hypothesized
that drug release rates could be adjusted by varying the MPEG block length and
DLL/G ratio of PDLLG block.
Ibuprofen was used as a poorly-water soluble model drug, in this study. This
drug has been loaded into biodegradable matrices such as monofilaments (Zurita
et al., 2006) and microspheres (Thompson et
al., 2007). In this research, the ibuprofen-loaded MPEG-b-PDLLG
films were prepared by film casting of MPEG-b-PDLLG and ibuprofen homogeneous
blended solution. Influences of MPEG block length and DLL/G ratio on film characteristics
and drug release behaviors were investigated and discussed.
MATERIALS AND METHODS
This research was conducted on June 2008-March 2009 at Mahasarakham University, Mahasarakham, Thailand.
MPEG-b-PDLLGs with different MPEG block lengths and DLL/G ratios were
synthesized by ring-opening polymerization of DLL and G monomers in bulk under
nitrogen atmosphere at 130°C for 24 h (Baimark et
al., 2007). They were designed as MPEG2,000-PDLL, MPEG5,000-PDLL and
MPEG5,000-PDLLG. The 2,000 and 5,000 were molecular weights of MPEG blocks,
whereas the PDLL and the PDLLG contained DLL/G ratios of 100/0 and 85/15 mol%,
The MPEG (Fluka) was used after dried under reduced pressure at 120 °C
for 4 h. The DLL and G monomers were prepared from D,L-lactic acid (85%, Acros
Organics) and glycolic acid (99%, Acros Organics). The MPEG and stannous octoate
(95%, Sigma) was used as the initiating system. The obtained MPEG-b-PDLLGs
were characterized by various methods as previously described (Baimark
et al., 2007) and summarized in Table 1. Ibuprofen
(99.95%) was supplied by the Government Pharmaceutical Organization, Thailand.
Dichloromethane (AR, CALRO ERBA) was used as without further purification.
Preparation of Ibuprofen-Loaded Films
MPEG-b-PDLLG films containing ibuprofen were prepared by solution
casting as following method. Five hundred miligrams of MPEG-b-PDLLG and
5 mg of ibuprofen were completely dissolved in 15 mL of dichloromethane. The
solution was poured on glass Petri dish (5 cm in diameter) and dried at room
temperature for 24 h. Film was lifted off the glass Petri dish and dried in
vacuum oven at room temperature for another week. The MPEG-b-PDLLG films
without ibuprofen loading were also prepared by the same method as control.
Table 2 shows each film formulation.
Characterization of Ibuprofen-Loaded Films
Thermal properties of the films were characterized by Differential Scanning
Calorimetry (DSC) using a Perkin-Elmer DSC Pyris Diamond. For DSC analysis,
film samples (5-10 mg) were heated at rate of 10°C min-1 under
a helium atmosphere over a temperature range of 20-100°C.
|| Characteristics of MPEG-b-PDLLG
|aMPEG block length was 2,000 g mol-1,
bMPEG block length was 5,000 g mol-1, cCalculated
from 1H-NMR spectra (EO: Ethylene oxide (repeating units of MPEG), DLL:
D,L-lactide and G: Glycolide), dNumber-average molecular weight
Molecular Weight Distribution (MWD) obtained from gel permeation chromatography
Mechanical properties including tensile strength and percent of elongation at break of the films were measured by tensile tester using a Lloyds LRX+ Universal Mechanical Testing Machine. The films with 10x40 mm in size were performed at 25°C and 65% relative humidity with the speed of 20 mm min-1 and 1 kN load cell. The experimental values for mechanical properties represent averages of measurements from the five replicate films.
Mophological characteristics of film surface and cross section were determined by Scanning Electron Microscopy (SEM) using a JEOL JSM-6460LV SEM. The film samples were cut by paper scissors and coated with gold for enhancing conductivity before scan.
In vitro Drug Release Test
For In vitro drug release test, the film with 10x10 mm in size was
incubated in 20 mL of Phosphate Buffer Saline (PBS) with pH 7.4 at 37°C
in a Heto SBD50 shaking water bath at 150 rpm rotation speed. At appropriate
times, the all PBS medium was removed to a separate tube and replaced with 20
mL of fresh PBS. The amount of released ibuprofen was assayed by UV-Vis spectrophotometry
using a Lambda 25 UV-Vis spectrophotometer at 220 nm (Borovac
et al., 2006). All the measurements were carried out in triplicate.
The films after drug release test were dried in vacuum oven at room temperature
for 2 weeks before morphology observation by using SEM.
The data were expressed as Mean±SD. Statistical analysis was performed
using a one-way Analysis of Variance (one-way ANOVA).
RESULTS AND DISCUSSION
The all MPEG-b-PDLLG films with and without ibuprofen entrapment appear as clear transparent and flexible films. The drug-loaded films were thicker than the drug-free films as presented in Table 2.
The MPEG5,000-PDLL, MPEG2,000-PDLL and MPEG5,000-PDLLG were completely amorphous
state. The melting temperature did not observe from their DSC thermograms (Baimark
et al., 2007). Figure 1 shows the DSC thermograms
of ibuprofen and drug-loaded MPEG-b-PDLLG film. The DSC curve of ibuprofen
presents a single melting temperature at 80°C with heat of melting of 58.5
J g-1, while this melting temperature of ibuprofen disappeared in
the DSC curve of drug-loaded MPEG5,000-PDLL film, as shown in Fig.
1 (b). The ibuprofen crystallites were also disappeared when the MPEG2,000-PDLL
and MPEG5,000-PDLLG were used as the film matrices instead of the MPEG5,000-PDLL
(Fig. 1c, d).
The mechanical properties of films were determined from their tensile strength
and elongation at break. These mechanical properties of the both drug-free and
drug-loaded films are shown in Fig. 2a and b.
|| Film formulations and thicknesses of drug-free and drug-loaded
|aPrepared from MPEG5,000-PDLL. bPrepared
from MPEG2,000-PDLL. cPrepared from MPEG5.000-PDLLG, dMeasured
from SEM micrographs
|| DSC thermograms of (a) ibuprofen and Film No. (b) 2, (c)
4 and (d) 6
|| (a) Tensile strengths and (b) Elongations at break of various
For drug-free films (Film No. 1, 3 and 5), the results of mechanical properties
accorded to the literature that the tensile strength at break of the diblock
copolymer films decreased whereas the elongation at break increased when the
MPEG block length was increased and the G units were copolymerized (Morakot
et al., 2008). However, the mechanical properties of the films did
not significantly change after drug entrapment for all MPEG-b-PDLLG films
(Film No. 2, 4 and 6).
The morphology of the films with and without drug entrapment was determined
from SEM micrographs of their film cross sections and surfaces. Figure
3a and b show SEM micrographs of the drug-loaded films
for Film No. 2, 4 and 6. They were smooth and uniform cross sections and surfaces
without phase separation.
||SEM micrographs of (a) cross-sections and (b) surfaces of
drug-loaded films before drug release test
Ibuprofen Release Studies
The influences of MPEG block length and incorporated glycolide on ibuprofen
release behaviors were investigated as in vitro. Figure
4 shows ibuprofen release profiles from the drug-loaded films. The amount
of drug released out from the films increased with the time. The effect of rapid
initial burst release did not occur. The drug release rates were in sequence
order of MPEG5,000-PDLLG>MPEG5,000-PDLL>MPEG2,000-PDLL films.
The melting transition of ibuprofen in DSC curve (Fig. 1a)
indicated its crystalline state. However, the crystallizability of the ibuprofen
was suppressed when it was entrapped in the MPEG-b-PDLLG films suggested
the molecules of MPEG-b-PDLLG film matrix and loaded drug are well mixed
together. Then the MPEG-b-PDLLG molecules can inhibit ibuprofen crystallization.
The results suggest that the ibuprofen molecules are well distributed throughout
the MPEG-b-PDLLG film matrices. This due to the dichloromethane is a
good solvent for dissolving the both MPEG-b-PDLLG and ibuprofen to prepare
well homogeneous solution before film casting.
The mechanical properties of the drug-free MPEG-b-PDLLG films (Film
No. 1, 3 and 5) depended upon the MPEG block length and copolymerized G units.
|| Ibuprofen release from drug-loaded films
It can be expected that the tensile strength at break of films decreased,
while the percentage elongation at break increased as the decreasing glass transition
temperature (Tg) of the MPEG-b-PDLLG. The Tgs of
MPEG2,000-PDLL, MPEG5,000-PDLL and MPEG5,000-PDLLG were 48, 37 and 28°C,
respectively (Baimark et al., 2007).
The film mechanical properties were not affected from the drug loading suggested that the dispersed drug and MPEG-b-PDLLG film matrices with MPEG-b-PDLLG/ibuprofen ratio of 100/1 (w/w) were homogeneous blends for the all MPEG-b-PDLLG films (Film No. 2, 4 and 6). Thus, the mechanical properties of MPEG-b-PDLL/drug blended films did not differ from the pure MPEG-b-PDLLG films.
The homogeneous cross sections and surfaces of drug-loaded films were clearly
observed from their SEM micrographs (Fig. 3). This can be
explained that the blended films were prepared from the MPEG-b-PDLLG/ibuprofen
miscible blended solution in dichloromethane. The results of morphological characteristics
supported that the ibuprofen was uniformly dispersed and distributed throughout
the film matrices on the molecular level as previously described in DSC results.
Ibuprofen Release Studies
The ibuprofen release profiles in Fig. 4 show as sustained-release
patterns suggested that the film matrices can control the drug releasing. The
drug release rates decreased as the MPEG block length decreased from 5,000 to
2,000 g mol-1. This may be explained that the drug release rate from
the film was directly related to film swelling. The swelling of MPEG-b-PDLL
film decreased with the hydrophilic MPEG block length (or MPEG molecular weight)
(Morakot et al., 2008). Then drug release from
the MPEG2,000-PDLL film showed slower than from the MPEG5,000-PDLL film. Moreover,
the drug release rate also increased when the G units were incorporated into
the PDLL block. Because of the G units had higher hydrophilic than the DLL units.
It is important to note that the ibuprofen-loaded films prepared in this work
showed more nearly zero-order sustained-release of ibuprofen in comparison with
the both ibuprofen-loaded monofilaments and microspheres as previously reported
(Zurita et al., 2006; Thompson
et al., 2007).
||SEM micrographs of cross-sections (left column) and surfaces
(right column) of drug-loaded films after drug release test
In addition, the film opaque had appeared since the first 3 h of released time due to the water molecules diffused in the film matrices. However, the film transparency was recovered when they were dried to remove imbibed water molecules. This may be suggested that the ibuprofen molecules may release out through the intermolecular spaces that the water molecules had diffused to the film matrices. Then it can be proposed that releasing of ibuprofen from the film matrices occurred by diffusion process but did not induce surface erosion of film matrix. This is clearly confirmed by film morphological study after drug release test from the SEM micrographs as shown in Fig. 5a and b. The film cross sections and surfaces were still smooth and uniform after drug release test.
The ibuprofen-loaded MPEG-b-PDLLG films with uniform and transparent morphology were successfully prepared by film casting of MPEG-b-PDLLG and ibuprofen solution in dichloromethane. The ibuprofen crystallites had disappeared when it was entrapped in the films. The amount of drug released from the films was increased when the MPEG-b-PDLLG with longer MPEG block length and G copolymerization was used as the film matrix.
These biodegradable MPEG-b-PDLLG films are very interesting for use as controlled-release drug delivery systems, especially poorly water-insoluble drugs because the drug release rate can be adjusted by varying the MPEG block length and the DLL/G ratio.
The authors would like to acknowledge the Center of Excellence for Innovation in Chemistry (PERCH-CIC), Commission on Higher Education, Ministry of Education, Thailand for financial support.