PEGylated Liposomes Incorporated with Nonionic ... - IngentaConnect

Current Nanoscience, 2011, 7, 191-199

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PEGylated Liposomes Incorporated with Nonionic Surfactants as an Apomorphine Delivery System Targeting the Brain: In Vitro Release and In Vivo Real-time Imaging Shu-Hui Hsua, Saleh A. Al-Suwayehb, Chih-Chieh Chenc, Chen-Hsien Chic and Jia-You Fangb,c,d,* a

Department of Pharmacy, Chia Nan University of Pharmacy and Science, Tainan, Taiwan, bDepartment of Pharmaceutics, College of Pharmacy, King Saud University, Riyadh, Saudi Arabia, cPharmaceutics Laboratory, Graduate Institute of Natural Products, Chang Gung University, Kweishan, Taoyuan 333, Taiwan, dDepartment of Cosmetic Science, Chang Gung Institute of Technology, Kweishan, Taoyuan 333, Taiwan Abstract: The clinical application of apomorphine, a dopamine receptor agonist for treating Parkinson’s disease, is limited by its instability and the need for frequent injections. In the present work, apomorphine was encapsulated within liposomes to protect it from degradation and enhance the permeability across the blood-brain barrier (BBB). Stearylamine was used to produce a positive surface charge for the liposomes. The liposomal systems with different compositions were characterized by the mean size, zeta potential, drug encapsulation percentage, stability, and in vitro release characteristics. PEGylated liposomes and liposomes incorporating Brij 78 showed a size of 130~160 nm. When Tween 80 was added to the liposomes, the vesicle size increased to > 260 nm. Apomorphine was successfully entrapped by liposomes with an encapsulation percentage of > 70%, with the systems containing Brij 78 showing the highest level of 99%. The loading of apomorphine into liposomes resulted in slower release behavior compared to the drug in an aqueous solution. In comparison to free drug, apomorphine in PEGylated liposomes exhibited greater stability in plasma. The in vivo brain uptake of PEGylated liposomes after an intravenous bolus injection into rats was monitored by in vivo real-time bioluminescence imaging for 1 h. The results showed that the uptake of PEGylated liposomes into the brain was rapid and prolonged. PEGylated liposomes may offer a promising strategy for targeting apomorphine to the brain. This opens up new opportunities for treating Parkinson’s disease.

Keywords: Apomorphine, PEGylated liposomes, blood-brain barrier, brain targeting, drug delivery systems. 1. INTRODUCTION Parkinson’s disease affects approximately 1% of people over 65 years old and approximately 3% of those over 85 years old. Symptoms of Parkinson’s disease are caused by a marked deficiency in dopamine resulting from a loss of dopaminergic neurons in the substantia nigra, and probably other anatomical areas [1]. Apomorphine is considered to be a classical mixed type dopamine D1 and D2 receptor agonist. It has been used in the therapy of Parkinson’s disease and for treating erectile dysfunction [2]. However, its inherent instability, negligible oral bioavailability, and short half-life (~41 min) complicate its applicability in clinical practice [3,4]. Apomorphine should be targeted to the central nervous system (CNS) to exert its therapeutic activity. The CNS is protected by the blood-brain barrier (BBB). Its primary function is to maintain homeostasis in the brain. Unique features, such as tight junctions, low vesicular transport, and high metabolic activity help achieve this barrier function [5,6]. The tight junctions produce approximately 100 times greater transendothelial electrical resistance than junctions of the peripheral capillary endothelium [7]. Most hydrophilic drugs cannot easily be transported across the BBB to produce significant therapeutic efficiency. The delivery of drugs to the brain has traditionally been approached with medicinal chemistry or barrier disruption and neurosurgically based invasive brain drug delivery. Although some of the methods are promising, no method has yet proven to be efficient, and the invasive procedures are by nature severely limited [8]. One possible way of delivering drugs to the brain noninvasively is by employing nanocarriers such as polymeric nanoparticles and liposomes [9]. These nanocarriers not only mask BBB-limiting characteristics of the drugs, but may also protect the drugs from chemical/enzymatic degradation. Numerous combinations of polymers can be used to optimize BBB penetration by nanoparticles. Toxicity is usually mediated by biodegradation of the polymers *Address correspondence to this author at the Pharmaceutics Laboratory, Graduate Institute of Natural Products, Chang Gung University, 259 WenHwa 1st Road, Kweishan, Taoyuan 333, Taiwan; Tel: +886-3-2118800; Ext. 5521; Fax: +886-3-2118236; E-mail: [email protected] 1573-4137/11 $58.00+.00

to toxic compounds [7,10]. Liposomes are microscopic vesicles consisting of membrane-like phospholipid bilayers surrounding an aqueous medium. Liposomes are made up of phospholipids and cholesterol, which has little toxicity to animals and humans [11]. Liposomes have been extensively investigated as potential delivery systems because an enormous diversity of structures and compositions can be achieved [12]. An important prerequisite for the success of applying drugs is site specificity. The aim of the present work was to modify cationic liposomes with polyethylene glycol (PEG)-phospholipids and nonionic surfactants which facilitate the transport of apomorphine into the brain. Different rationales drove these approaches: the use of positively charged liposomes can be explained on the basis of their interaction with negative charges of the BBB, while the use of surfactants and PEG is connected with possible endocytic mechanisms of BBB crossing, with the aim of prolonging the circulation half-life [13]. The feasibility of using apomorphine-loaded liposomes as a parenteral formulation was demonstrated through extensive characterization of the size, charge, appearance, drug release, hemolysis, and drug stability. The liposomes were injected intravenously into rats in order to study the targeting efficiency to the rat brain. A bioluminescence imaging modality was used to monitor liposomes targeted to the brain. 2. MATERIALS AND METHODS 2.1. Materials Apomorphine HCl, stearylamine, polyoxyl 20-stearyl ether (Brij 78), sulforhodamine B, and cholesterol were purchased from Sigma-Aldrich Chemical (St. Louis, MO, USA). Hydrogenated soybean phosphatidylcholine (SPC, Phospholipon® 80H) was supplied by American Lecithin (Oxford, CT, USA). 1,2-Dioleoyl-snglycero-3-trimethylammonium-propane (DOTAP) and the polyethylene glycol (PEG) derivative of distearoylphosphatidylethanolamine (DSPE-PEG) with a mean molecular weight of 5000 were obtained from Nippon Oil (Tokyo, Japan). Polyoxyethylene 20sorbitan monooleate (Tween 80) was from Kanto Chemical (Tokyo, Japan). Soyaethyl morpholinium ethosulfate (Forestall®) was supplied by Croda (East Yorkshire, UK). Cellulose membranes (Cellu-

© 2011 Bentham Science Publishers Ltd.

192 Current Nanoscience, 2011, Vol. 7, No. 2

Sep® T2, with a molecular weight cutoff of 6000~8000) were purchased from Membrane Filtration Products (Seguin, TX, USA). 2.2. Preparation of Liposomes SPC (3%, w/v), cholesterol (1%), and cationic (0.2%) and nonionic surfactants (0.3%) were dissolved in a determined volume of a chloroform: methanol (2: 1) solution. The organic solvent was evaporated in a rotary evaporator at 50 °C, and solvent traces were removed by maintaining the lipid film under a vacuum for 4 h. The film was hydrated with double-distilled water containing 0.1% (w/v) apomorphine using a probe-type sonicator (VCX 600, Sonics and Materials, Danbury, CT, USA) at 25 W for 30 min. The total volume of the final products was 10 ml. 2.3. Mean Size and Zeta Potential The mean vesicle size (z-average) and zeta potential of liposomes were measured by a laser scattering method (Nano ZS® 90, Malvern, Worcestershire, UK) using a helium-neon laser with a wavelength of 630 nm. The liposomes were diluted 100-fold with double-distilled water to achieve the measurement of both size and surface charge. The determination was repeated three times per sample for three independent batches. 2.4. Transmission Electron Microscopy (TEM) The size and morphology of liposomes were observed using a Hitachi H-7500 electron microscope (Tokyo, Japan). One drop of liposomes was deposited on a carbon film-covered copper grid to form a thin-film specimen, which was stained with 1% phosphotungstic acid. The sample was then examined and photographed with a microscope. 2.5. Apomorphine Encapsulation Percentage The encapsulation efficiency of apomorphine entrapped by liposomes was determined by an ultracentrifugation method. The product was centrifuged at 48,000 xg and 4 °C for 30 min in a Beckman Optima MAX® ultracentrifuge (Beckman Coulter, Fullerton, CA, USA) in order to separate the incorporated drug from the free form. The supernatant and precipitate were analyzed by high-performance liquid chromatography (HPLC) to determine the encapsulation percentage (%) of the total drug load. The HPLC method for apomorphine was described previously [14]. 2.6. Erythrocyte Hemolysis Test This test was modified from the method by Thomas et al. [15]. Blood samples were obtained from healthy donors by venipuncture and collected into test tubes containing 124 mM sodium citrate (one volume of sodium citrate solution + nine volumes of blood). This experiment was approved by the Institutional Review Board at Chang Gung Memorial Hospital. Erythrocytes were immediately separated by centrifugation at 2000 xg for 5 min and washed three times with four volumes of a normal saline solution. Erythrocytes collected from 1 ml of blood were resuspended in 10 ml of normal saline. Immediately thereafter, 2.5 ml of 2% (w/v) dispersions of the formulations and mixtures thereof in saline were incubated with 0.1 ml of the erythrocyte suspension. Incubations were carried out at 37 °C while the test tubes were gently tumbled. After 1 h of incubation, the samples were centrifuged for 5 min at 2000 xg. The absorbance of the supernatant was measured at 415 nm to determine the percentage of cells undergoing hemolysis. Hemolysis induced by double-distilled water was taken as 100%. 2.7. In Vitro Apomorphine Release Apomorphine release from the liposomes was measured using a Franz diffusion cell [14]. A cellulose membrane was mounted between the donor and receptor compartments. The donor medium consisted of 0.5 ml of vehicle containing apomorphine. The receptor medium consisted of 5.5 ml of pH 7.4 buffer. The available

Hsu et al.

diffusion area between cells was 0.785 cm2. The stirring rate and temperature were kept at 600 rpm and 37 °C, respectively. At appropriate intervals, 300-μl aliquots of the receptor medium were withdrawn and immediately replaced with an equal volume of fresh buffer. The amount of drug released was determined by HPLC. 2.8. Apomorphine Stability in Plasma The stability of apomorphine in human plasma was tested in vitro [14]. Blood was taken from healthy human donors (20~32 years old) by venipuncture. A stock solution was prepared by dissolving a weighed amount of apomorphine in water or liposomes. A volume of 0.1 ml of the vehicle was added to 3.9 ml of prewarmed (37 °C) plasma solution (human plasma: 50 mM pH 7.4 phosphate buffer of 4: 1). The resulting dispersion was incubated at 37 °C for 2 or 6 h. The reaction mixture was withdrawn, and acetonitrile was rapidly added to stop the process; then the mixture was centrifuged at 5 °C and 1000 rpm for 10 min. The apomorphine content in the supernatant was measured by HPLC. 2.9. Animals The in vivo experiments were performed with male SpragueDawley rats (230~270 g). The animal experiment protocol was reviewed and approved by the Institutional Animal Care and Use Committee of Chang Gung University. Animals were housed and handled according to institutional guidelines. Food and water were given ad libitum. 2.10. In Vivo Bioluminescence Imaging of Rat Brain In vivo bioluminescence imaging was performed with IVIS® Spectrum imaging system (Xenogen, Alameda, CA, USA) linked to Living Image® 3.1 software (Xenogen). This system provides high signal-to-noise images of fluorescence signals emerging from within living animals [16]. Sulforhodamine B as the fluorescence dye was added to the liposomes at a concentration of 0.92 mg/ml. A control solution or liposomal dispersion at 1 μl/g was given by a tail vein injection, and rats were immediately imaged with the system. The animals were placed prone in a light-tight chamber, and a grayscale reference image was obtained under low-level illumination. The images were monitored every 5 min for 1 h after the injection. Optical excitation was carried out at 605 nm, and the emission wavelength was detected at 680 nm. All experimental results were repeated on at least three different animals and representative pictures are shown. 2.11. Statistical Analysis The statistical analysis of differences among various treatments was performed using unpaired Student’s t-test. A 0.05 level of probability was taken as the level of significance. An analysis of variance (ANOVA) test was also used if necessary. 3. RESULTS 3.1. Physicochemical Characterization of Liposomes The sizes and zeta potentials of the prepared liposomes detected by a Zetasizer are shown in Table 1. Formulations L1 to L3 are liposomes with various cationic surfactants. The results clearly showed that the cationic surfactants in the phospholipid bilayers modified the properties of these liposomes. The mean diameters of L1 to L3 were in the range of 80~160 nm, with stearylamine and Forestall respectively showing the highest and lowest sizes. The zeta potential of these three systems also exhibited the same trend. We further used stearylamine as the standard cation in liposomes to study the effect of PEG and nonionic surfactants on liposome characteristics. The incorporation of DSPE-PEG (L4) did not greatly change the vesicle size, although there was a statistically significant reduction (p < 0.05) after incorporation. There was no significant difference in the size between liposomes with or without Brij 78

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drug entrapment of liposomes by DOTAP and Forestall (L2 and L3) was very low. The incorporation of DSPE-PEG and Brij 78 (L4, L5, and L7) resulted in higher or similar drug encapsulation compared to standard liposomes (L1). However, enhanced trapping efficiency was not observed for Tween 80-containing liposomes (L6 and L8). The encapsulation percentage dropped from 86% (L1) to 69% after adding Tween 80 to the systems.

(L1 vs. L5, p > 0.05). Substitution of Brij 78 (L5) by Tween 80 (L6) resulted in an increase in size from 164 to 269 nm (p < 0.05). The further addition of Brij 78 to liposomes containing DSPEPEG (L7) significantly reduced (p < 0.05) the size of the vesicles. The biggest vesicles among all formulations tested were liposomes with both DSPE-PEG and Tween 80 (L8, 291 nm). Representative TEM micrographs of liposomes L1 and L4 is shown in Fig. (1A and 1B), respectively, indicating that the liposomes were approximately 200 nm in diameter. This value was greater than that determined by the light-scattering method, which may have been due to the artificial manipulation for preparing and drying the samples in the TEM process. The liposomes showed an almost spherical ultrafine morphology. Stearylamine, DOTAP, and Forestall all produced a positive charge on the liposomes, with the stearylamine-containing systems showing the highest zeta potential as depicted in Table 1. The PEGylated formulations and liposomes with nonionic surfactants had lower positive charges compared to the related liposomes (L1). Differences in the zeta potential among liposomes (L4 to L8) were negligible.

3.3. Erythrocyte Hemolysis Test To evaluate the safety of the liposomes themselves, hemolytic activity was determined as shown in Table 2. Apomorphine molecules were not loaded in the formulations used in this experiment. Systems with Forestall (L3) caused more-significant hemolysis compared to the others (p < 0.05). The other liposomes showed < 3% hemolysis of erythrocytes. All systems with stearylamine exhibited tolerable hemolysis of erythrocytes. 3.4. In Vitro Apomorphine Release The levels of in vitro apomorphine release from liposomes of different compositions are given in Fig. (2). The amount of apomorphine released from each liposomal system was plotted as a function of time. The aqueous solution of double-distilled water was used as the control vehicle. The free control showed quick release of apomorphine. The drug released from the control

3.2. Apomorphine Encapsulation Percentage As shown in Table 1, apomorphine demonstrated a high rate of encapsulation of 86% in liposomes with stearylamine (L1). The

1A

1B

Fig. (1). Transmission electron microscopic micrograph of the basic liposomes (L1) (A) ans PEGylated liposomes (L4) (B). Original magnification 80,000x. The scale of the bar is 115 nm.

Table 1.

a

The Characterization of Apomorphine-Loaded Liposomes with Cationic Additives by Mean Diameter and Zeta Potential, and Encapsulation

Code

Cationic Additive

Surfactant

Mean Diameter (nm)

Zeta Potential (mV)

Encapsulation (%)

L1

stearylamine

—

162.7±1.4

60.6±2.2

86.1±0.6

L2

DOTAPa

—

109.3±2.4

37.9±2.4

11.9±3.3

L3

Forestall

—

82.7±2.6

23.6±0.4

19.0±7.6

L4

stearylamine

DSPE-PEGb

150.9±3.1

33.9±1.0

85.0±1.1

L5

stearylamine

Brij78

164.1±2.2

36.7±0.6

99.8±2.1

L6

stearylamine

Tween80

268.8±3.7

32.2±6.1

69.2±4.5

L7

stearylamine

DSPE-PEG+Brij78

132.1±3.0

31.4±1.5

82.7±6.8

L8

stearylamine

DSPE-PEG+Tween80

290.6±5.7

26.0±2.1

69.5±2.9

DOTAP, 1,2-dioleoyl-sn-glycero-3-trimethylammonium-propane. b DSPE-PEG, distearoylphosphatidylethanolamine with covalently linked polyethylene glycol. Each value represents the mean±SD (n=3).


Table 2.

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Hemolysis Percentage (%) After 1 h of Incubation at 37 °C with Liposomes

Code

Cationic Additive

Surfactant

Hemolysis (%)

L1

stearylamine

—

1.61±0.33

L2

DOTAPa

—

0.42±0.30

L3

Forestall

—

6.28±1.03

L4

stearylamine

DSPE-PEGb

2.59±1.01

L5

stearylamine

Brij78

0.91±0.16

L6

stearylamine

Tween80

1.00±0.48

L7

stearylamine

DSPE-PEG+Brij78

0.97±0.08

L8

stearylamine

DSPE-PEG+Tween80

1.69±2.55

3.5. Apomorphine Stability in Plasma Table 3 summarizes the amounts of apomorphine remaining after incubation in plasma for 2 and 6 h. Marked degradation of apomorphine occurred in the aqueous solution with no intact apomorphine molecules shown at 2 or 6 h. Once apomorphine was loaded into the liposomes, the stability of apomorphine in plasma increased. The results indicate that the addition of DSPE-PEG increased apomorphine stability (L1 vs. L4, p < 0.05) at both 2 and 6 h. The Brij 78 incorporation increased the stability at 6 h but not at 2 h (L1 vs. L5). Almost no protective effect on apomorphine stability was found for liposomes with Tween 80 alone (L6). The combination of PEG and nonionic surfactants (L7 and L8) resulted in a higher protective effect than for systems with single additives at 2 h (p < 0.05). However, this effect did not last to 6 h.

a

DOTAP, 1,2-dioleoyl-sn-glycero-3-trimethylammonium-propane. b DSPE-PEG, distearoylphosphatidylethanolamine with covalently linked polyethylene glycol Each value represents the mean±SD (n=4).

Control L1 L4 L5 L6 L7 L8 60

Release percentage (%)

tween that of liposomes with DSPE-PEG and nonionic surfactants alone. It was found that liposomes with the combined addition exhibited a biphasic release pattern. The initial burst effect occurred within 1 h, after which the drug was found to be released in a sustained manner over a period of 8 h.

Table 3. Percentage of Apomorphine Remaining in the Stability Experiment by Incubating Liposomes with Plasma at 37 °C for 2 and 6 h Code

Drug Remaining at 2 h (%)

Drug Remaining at 6 h (%)

Control

0

0

L1

8.44±5.74

0.91±1.01

L4

13.21±1.15

4.79±1.05

L5

7.28±5.95

5.83±3.22

L6

1.86±2.64

0

L7

103.48±19.17

0.80±1.60

L8

21.37±15.75

0.66±1.32

40

Each value represents the mean±SD (n=4).

20

0 0

2

4

6

8

Time (h)

Fig. (2). In vitro release-time profiles of apomorphine across a cellulose membrane from an aqueous solution (control) and liposomes. Each value is presented as the mean and S.D. (n = 4).

exhibited an initial burst, then gradually leveled off after 2 h. The inclusion of the drug into liposomes significantly reduced (p < 0.05) the release. The cumulative percentage release of apomorphine from liposomes varied from 14% to 33% for 8 h depending upon the compositions added. The standard formulation with stearylamine (L1) showed the highest drug release among all liposomes. The addition of DSPEPEG to the standard liposomes (L4) reduced apomorphine release; this reduction, however, was not significant (p > 0.05). Liposomes incorporating Brij 78 (L5) or Tween 80 (L6) further slowed down apomorphine release to the same level. Burst release of the drug from systems with PEG or nonionic surfactants alone (L1, L4, L5, and L6) occurred within the first 2 h, after which it reached a plateau. The combined addition of DSPE-PEG and nonionic surfactants (L7 and L8) contributed to an apomorphine release level be-

3.6. In Vivo Bioluminescence Imaging of the Rat Brain Brain uptake was evaluated by bioluminescence imaging following injection of sulforhodamine B-labeled liposomes. Fig. (3) shows the time courses of the bioluminescence images of liposomes of the representative animals evaluated from 0 to 1 h (left to right, top to bottom). The fluorescence signals were shown in the brain after deducting the auto-fluorescence of the animal itself. Immediately following injection of the control solution, the signal was clearly visualized in the brain region (Fig. 3A). Free sulforhodamine B in aqueous solution was rapidly eliminated from the brain. Brain accumulations of liposomes with PEG alone (L4) and the PEG/Brij 78 combination (L7) are shown in Fig. (3B and 3C), respectively. A significantly higher brain uptake was observed following the administration of liposomes than the free form. Intravenous administration of PEGylated liposomes prolonged the duration in the rat brain. The fluorescence gradient in liposome-treated rats decreased following an increase in time. However, this decay was not significant. The fluorescence kinetics of both liposomes were similar to each other. Fig. (4) shows another method to eliminate the tissue autofluorescence by applying spectral unmixing to multi-spectral fluorescence images. The spectral filters of the imaging system were respectively scanned at 415~760 and 490~850 nm for excitation and emission. Images with the highest transmission of the specified excitation and emission wavelengths were superimposed with the



3A

3B

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3C Fig. (3). Bioluminescence imaging of representative animals at different time points from 0 to 1 h following an intravenous injection of sulforhodamine B in an aqueous solution (control) (A), PEGylated liposomes without Brij 78 (B), and PEGylated liposomes with Brij 78 (C).

Fig. (4). Composite images of the highest transmission of the specified excitation and emission wavelengths superimposed with the auto-fluorescence algorithm of representative animals at 5 min following an intravenous injection of sulforhodamine B in an aqueous solution (control) (A), PEGylated liposomes without Brij 78 (B), and PEGylated liposomes with Brij 78 (C).


auto-fluorescence algorithm by Living Image 3.1 software. Merging these images taken at 5 min post-injection confirms the stronger fluorescence of the brain by liposomal administration than by the aqueous solution. Representative images exhibit that Brij 78 incorporation into PEGylated liposomes (L7) strengthened the signal in the brain region as observed in Fig. (4C). DISCUSSION Clinical failures of most potentially effective therapeutics for treating CNS disorders are often not due to a lack of drug potency but rather shortcomings in the method by which the drug is delivered. The efficient BBB transport of apomorphine is an important step for the onset of pharmacological activity of this drug. In the present study, cationic liposomes with PEG and/or nonionic surfactants were investigated for their effectiveness at enhancing the brain delivery of apomorphine. Improvements in clinical outcomes can be achieved by modifying the physicochemical properties and release of drug-loaded liposomes. Using in vivo imaging, we demonstrated enhanced and sustained brain delivery of systemically injected liposomes. It is important to characterize the physicochemical properties of liposomes since the removal of nanocarriers from the bloodstream depends on their size, charge, and surface properties [13]. Nanocarriers with a hydrophobic surface and negative charges promote protein adsorption and activate the complement system [17]. Moreover, cationic nanocarriers were found to efficiently target the brain because they bind to the paracellular area of the BBB, which is an area rich in anionic sites [18]. Stearylamine (L1) but not DOTAP (L2) or Forestall (L3) was used to create positive charges on the liposomes because it showed the highest zeta potential and apomorphine encapsulation. Liposomes were PEGylated to form a hydrated and stealthy shell that protects the vesicles from destruction by proteins [19]. PEG can also inhibit the P-glycoprotein (Pgp) efflux pump, thus increasing the residence of nanocarriers in the brain [20]. DSPE-PEG used in this work has both DSPE groups and PEG spacers. Although PEG may reside on the vesicles to produce the larger size, the increased hydrophilicity of the SPC films may favor interfacial film curvature [21], thus reducing the size of the liposomes with DSPE-PEG (L4 and L7). Furthermore, DSPE may improve the insufficient cohesion of SPC, thus minimizing the vesicle size [22]. Different approaches have been developed to target nanocarriers to the CNS. Brij 78 is helpful in increasing brain uptake by opening tight junctions of the BBB [13,23]. Several mechanisms were proposed for the transport of nanocarriers coated with Tween 80 across the BBB. Tween 80 acts as an anchor for apolipoproteincoated nanoparticles, which thus mimic lipoprotein particles and interact with and then are taken up by brain capillary endothelial cells [24]. Other mechanisms such as tight junction modulation or P-gp inhibition may also occur [25]. Brij 78 and Tween 80 produced different vesicle sizes. Brij 78 did not largely influence the size. Tween 80 incorporation (L6 and L8) greatly increased the mean size of liposomes to > 260 nm. Nanoparticles of > 200 nm are rapidly filtered by the spleen and removed by reticuloendothelial cells [17]. This is because Tween 80 has a spacious head group which adds to the bulkiness of the SPC head group [26]. The addition of PEG and other surfactants to liposomes led to an initial decrease in the zeta potential. It is clear that the presence of PEG or nonionic surfactants on the shell can shield the positive charges on the liposomes and nanoparticles [27,28]. A physically stable nanocarrier solely stabilized by electrostatic repulsion will have a minimum zeta potential of ±30 mV [29]. The positive charge of most of the liposomes tested showed a value of > 30 mV, which indicates the stability of the prepared systems. With stearylamine incorporation, a higher encapsulation load of apomorphine was achieved compared to DOTAP and Forestall. Stearylamine is thought to yield mixed films with phospholipids


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possessing close, ordered packing [30]. Drug leakage was thus limited from highly packed films. This phenomenon might not have been observed in the case of DOTAP and Forestall. It was also reported that DSPE-PEG and Brij 78, when incorporated in appropriate concentrations, increase the lipid packing order and reducing the leakage of encapsulated hydrophilic substances [31-33]. The high drug loading of liposomes was diminished by Tween 80 incorporation. The alkyl chain of Tween 80 is unsaturated. It was reported that unsaturated alkyl chains produce a looser packing of liposomal membranes than saturated ones, which results in increased permeability of molecules [28,34]. Safety is an important prerequisite for developing injections. One practical limitation of liposomes is the hemolysis caused by the interaction between erythrocytes and phospholipids [35]. The hemolytic potential was generally found to be correlated with the severity of lesions. In vitro hemolysis of liposomes interacting with erythrocytes was carried out as a preliminary test of the formulation safety. All stearylamine-containing liposomes showed tolerable hemolysis. This result suggests the potential for the therapeutic application of liposomes for apomorphine delivery. The PEGylated liposomes (L4) showed a higher hemolysis compared to the basic liposomes (L1) without PEG although there is no significant difference between them. The higher hemolysis by PEG may be due to the possibly cosolvent role of PEG in the system described previously [36]. Further investigation is needed to elucidate the actual mechanisms. To develop liposomes with controlled delivery, it is most important to optimize the ability of the drug to be released from the vesicles. The amount of apomorphine released from the aqueous solution was limited, with 60% of the drug being released over 8 h. This may have been due to the use of the Franz assembly. Since a drug is released to a definitive space in the receptor (5.5 ml) and diffusion area (0.785 cm2), drug loading in the receptor compartment is limited. There might be little concentration gradient between the donor and receptor compartments. Nevertheless, this setup is still useful for differentiating the relative release capabilities of various formulations [14,37]. The inclusion of apomorphine into liposomes decreased drug release. In general, a higher load of the drug into liposomes allows slower, more-prolonged release [27,38]. However, that was not the case in the present study. For example, although the incorporation of DSPE-PEG (L4) showed higher entrapment of apomorphine compared to that with Tween 80 (L6), PEG-containing liposomes produced higher release from vesicles compared to Tween 80-containing liposomes. Although PEG provided a rigid structure of bilayers, it is possible that the presence of PEG may have decreased the stability of some liposomal systems [39,40]. The drug may largely have escaped from these vesicles during the in vitro release period. The continuous apomorphine release from liposomes with both DSPE-PEG and nonionic surfactants (L7 and L8) at the later stage of release (1~8 h) confirmed this hypothesis about stability. This phenomenon was not detected in liposomes with Tween 80 alone (L6). This suggests that apomorphine was stably retained in Tween 80-containing systems for a longer time once the drug was already included in the vesicles. The release profiles demonstrate that by altering the composition, apomorphine release can be well controlled. This is important when developing a system for use as a drug carrier for parenteral use. The sustained release of the incorporated drug is a feature quite often correlated with improved pharmacokinetics and efficacy. The clinical relevance of this is that the frequency of apomorphine administration can be reduced. This would be beneficial for patients requiring up to 10~15 injections a day [41]. The key feature of apomorphine responsible for its activity is illustrated by major issues of stability [4]. Apomorphine is metabolized to apomorphine orthoquinone and apocodeine in circulation. Non-enzymatic oxidation is another potent factor in apomorphine metabolism [3,42]. Apomorphine was rather stable when liposomes


were used as the delivery systems, especially the systems containing DSPE-PEG. The combined addition of both DSPE-PEG and nonionic surfactants (L7 and L8) seemed to exhibit a synergistic effect in protecting apomorphine. Further studies are needed to explore this mechanism. Apomorphine molecules in formulations with DSPE-PEG and Brij 78 (L7) remained completely intact in plasma after a 2-h incubation. PEG can form hydrophilic and steric barriers against the attachment of plasma proteins to liposomes. PEGylated liposomes thus can effectively prevent apomorphine degradation, which may prolong the residence time in the circulation. However, this protective effect might not last for a long period, since the experimental results indicated a poor ability to stabilize apomorphine by liposomes when the incubation time exceeded 2 h. The presence of the BBB leads to a poor in vitro/in vivo correlation when the targeted delivery systems are tested in receptorbearing cells in vitro and fail in vivo [43]. Thus, one would need to examine the in vivo targeting efficiency rather than the in vitro status. Bioluminescence imaging is a noninvasive and real-time technique for performing in vivo diagnostic studies on animal subjects. It is a high-throughput and sensitive imaging modality [44]. The excitation and emission wavelengths used in this study to detect brain uptake were > 600 nm. The longer wavelengths of bioluminescence in the red and near-infrared regions of the spectrum are transmitted through mammalian tissues more efficiently than are shorter wavelengths of light [45]. Liposomes with DSPE-PEG without (L4) or with Brij 78 (L7) were selected for the in vivo study because of the appropriate size of < 200 nm, since nanocarriers of > 200 nm are rapidly eliminated by the reticuloendothelial system [13,46]. Both formulations also revealed high drug entrapment, acceptable hemolysis, and a protective effect on drug stability. Brain uptake was measured over a short time during which significant uptake was observed for both the aqueous solution and liposomes. Therefore, it is possible that passive permeability may have played a role in transport across the BBB in our case [47]. This result also suggests that the free or liposome-loaded fluorescence dye could be transported from the circulating blood to the brain through the BBB. The total brain level in the control solution steadily dropped thereafter, indicating the metabolism in the plasma and/or brain tissues. Another possibility is P-gp-mediated efflux from the BBB [48]. The results showed that PEGylated liposomes were able to deliver the drug to the brain in a more-prolonged behavior compared to the free form. For in vivo applications of PEGylated liposomes, the entrapped molecules are protected against degradation in serum. The drug encapsulated into liposomes was protected and also was slowly released, thus was effective in providing targeted and sustained delivery. In addition, by using liposomes, it is possible to circumvent the efflux of P-gp [5,13], which is an advantage when targeting drugs to the brain. PEG-coated nanocarriers also display a specific affinity for brain endothelial cells [23]. The most likely mechanism of nanocarrier-mediated transport across the BBB is endocytosis by endothelial cells lining the brain's capillaries [24]. It is triggered by binding of the liposomes to negatively charged sites on the surface of endothelial cells, so they can fuse with the cells to be transported across the biomembrane [11]. The structure of liposomes is similar to that of cells, so they have better affinities with endothelial cells on the BBB to increase transport [49]. Brij 78 was reported to have the ability to open up tight junctions [13,23]. The incorporation of Brij 78 into the systems did not seem to largely influence the kinetics of liposomes according to the bioluminescence imaging (L4 vs. L7), although there was moreintense fluorescence for PEGylated liposomes with Brij 78 in the composite image modality. The mechanisms underlying the brain uptake of liposomes are not yet fully understood. Additional experiments are needed and are underway to elucidate the mechanisms of transport. Although we were not able to determine the

Hsu et al.

exact pathways of PEGylated liposomes, the efficiency of liposomes for brain targeting was confirmed. CONCLUSIONS The purpose of this study was to assess the feasibility of using liposomes to promote the accumulation of apomorphine in the brain. PEGylated liposomes were shown to be promising carriers for apomorphine delivery due to the potential both in encapsulating the drug, thus protecting apomorphine from metabolism, and in delivering the drug across the BBB. The results showed that the compositions of liposomes influenced their physicochemical characteristics, drug release, and stability, which may affect the in vivo performance of the liposomes. PEGylated liposomes provided sustained drug release and protection of apomorphine against degradation, thus increasing the amount of apomorphine reaching the brain. Real-time in vivo imaging of sulforhodamine B-labeled liposomes in the rat brain revealed that nanocarriers can be targeted to brain regions in a prolonged manner. The present data indicate that liposomes represent a promising strategy to overcome the BBB. These results warrant further efforts to clarify the uptake mechanisms in greater detail. REFERENCES [1] [2]

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Received: January 14, 2010

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Revised: August 25, 2010

Accepted: September 22, 2010