AAPS PharmSciTech 2003; 4 (1) Article 1 (http://www.pharmscitech.org).
`Synergistic Effect of PEG-400 and Cyclodextrin to Enhance Solubility of
`Progesterone
`Submitted: November 20, 2002; Accepted: January 9, 2003
`Indranil Nandi1, Michelle Bateson2, Mohammad Bari3, and Hemant N. Joshi4
`1Geneva Pharmaceutical Technology Corp, 2400 Route 130 North, Dayton NJ 08810
`2Galen Limited, Millbrook, Larne BT40 2SH Northern Ireland
`3Forest Laboratories Inc, 330 Prospect Street, Inwood, NY 11096
`4Barr Laboratories, Pharmaceutical Research & Development, 2 Quaker Road, Pomona, NY 10970
`
`
`KEYWORDS: cosolvent, cyclodextrins, PEG-400, pro-
`gesterone, solubilization, synergism
`INTRODUCTION
`The addition of cosolvent to a formulation is a com-
`monly used method for improving the solubility of a
`drug. Polyethylene glycol (PEG)-400 is one of the most
`widely used cosolvents for improving the aqueous
`solubility of hydrophobic drugs. Cyclodextrins (CDs)
`also have been used to improve solubility and stability
`of drug compounds.1 Higher molecular weight PEGs
`have been used in conjunction with various CDs in
`solid dispersion systems.2 Harada3 reported that PEGs
`form complexes with α-CD and γ-CD, but not with β-
`CD. In the current study, therefore, β-CD was chosen to
`investigate the synergistic effects of CD and PEG-400
`on the solubility of a model hydrophobic drug.
`In CD aqueous solutions, the addition of propylene gly-
`col or ethanol has been reported to reduce the solubility
`of testosterone and ibuprofen.4 Hydroxypropyl methyl-
`cellulose (HPMC) was observed to increase the solubi-
`lization effect of CDs. The amount of CD needed in the
`solid dosage form was significantly lower in the pres-
`ence of HPMC.5 Enhancement of solubilization of
`ETH-615 and midazolam was reported in the presence
`of water-soluble polymers.6,7 Zung8 hypothesized that a
`series of alcohols have a synergistic effect on the cosol-
`vency and complexation of pyrene. Zung also sug-
`gested that the cosolvents could act as a space-
`regulating molecule to assist the drug to fit inside the
`CD cavity.
`
`Corresponding Author: Hemant N. Joshi, Barr Labo-
`ratories, Pharmaceutical Research & Development, 2
`Quaker Road, Pomona, NY 10970. Phone: (845) 362-
`7055; Fax: (845) 362-2774; Email:
`hjoshi@barrlabs.com.
`
`In this study, progesterone, a neutral hydrophobic com-
`pound, was selected as a model compound. The goal of
`the project was to test the hypothesis that PEG-400 and
`CD may have a synergistic effect on the solubility of
`progesterone. Captisol® (hepta sulfobutyl ether) and
`Trappsol® HPB (hydroxypropyl beta CD) were used as
`2 different types of CDs. Captisol possesses a negative
`charge, whereas Trappsol HPB is uncharged. The effect
`of polysorbate 80 on the synergism of PEG-CD in
`solubilization of the model compound also was exam-
`ined.
`
`MATERIALS AND METHODS
`Materials
`Progesterone, micronized powder, USP was purchased
`from Gerdina, CA. Captisol and Trappsol HPB were
`purchased from Cydex, Inc, Kansas City, KS, and
`CTD, Inc, High Spring, FL, respectively. PEG-400 and
`polysorbate 80 were obtained from Sigma and Spec-
`trum, respectively. All other chemicals used were of
`analytical grade and were used as is.
`Methods
`Solubility Experiments
`In the solubility experiments, progesterone was sus-
`pended in various solvents, namely: water, polysorbate
`80, PEG-400, and 3% and 6% aqueous solutions of
`Captisol and Trappsol HPB. The samples were shaken
`at 25°C ± 2°C for 24 hours and filtered through a 0.45-
`µm syringe filter. The drug concentration in the filtered
`solution was determined using high performance liquid
`chromatography (HPLC) after appropriate dilution with
`methanol. The cosolvent systems of PEG-400/water
`and PEG-400/water/polysorbate 80 were prepared by
`weight. The percentage of PEG-400 in the PEG-
`400/water cosolvent system was varied from 0.2% to
`90%. The solubilization capacity of cosolvent systems
`
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`AAPS PharmSciTech 2003; 4 (1) Article 1 (http://www.pharmscitech.org).
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`Table 1. Solubility of Progesterone in Selected Vehicles at 25ºC ± 2°C
`
`Vehicles
`
`PEG-400
`
`Polysorbate 80
`
`3% Captisol® aqueous solution
`
`3% Trappsol® HPB aqueous solution
`
`6% Captisol®aqueous solution
`
`6% Trappsol® HPB aqueous solution
`
`Water
`
`Solubility of Progesterone (mg/mL)*
`
`15.3 ± 0.03
`
`11.9 ± 2.31
`
`1.6 ± 0.05
`
`1.1 ± 0.02
`
`5.0 ± 0.08
`
`1.3 ± 0.05
`
`0.007
`
`
`
`*Expressed as mean ± SD (n = 3)
`
`
`was investigated in a similar fashion as mentioned in
`this section. In another set of solubility experiments,
`the solubility of progesterone was determined in a 50%
`PEG-400/water system containing 3% Captisol or 3%
`Trappsol HPB in the presence of polysorbate 80.
`Chromatographic Analysis
`A simple chromatographic system was developed in-
`house to examine progesterone in the solubility sam-
`ples. The mobile phase consisted of acetonitrile and
`0.5% acetic acid solution (50:50, vol/vol). A 10-cm
`C18 column with a particle diameter of 5 µm was used.
`The flow rate was 1.2 mL/min and the wavelength of
`detection was 254 nm. The observed retention time for
`progesterone in this system was 5.9 minutes.
`RESULTS AND DISCUSSION
`Table 1 lists the solubility values of progesterone in
`selected vehicles. Progesterone has very poor water
`solubility (0.007 mg/mL) at 25°C ± 2°C. The solubility
`values of progesterone in PEG-400 and polysorbate 80
`were observed to be approximately 15.3 mg/mL and
`11.9 mg/mL, respectively. In solutions containing 3%
`Captisol or 3% Trappsol HPB, the solubility values
`were approximately 1.6 mg/mL and 1.1 mg/mL,
`respectively. The solubility increased in both systems
`with further addition of the CDs, although not to the
`same degree in each. It became obvious that PEG-400,
`polysorbate 80 and CDs help solubilization of proges-
`terone.
`The solubility of progesterone in PEG-400 decreased
`significantly with a small addition of water—from ap-
`proximately 15.3 mg/mL in 100% PEG-400 to 1.45
`
` 2
`
`mg/mL in 90% PEG-400 system. The solubility in a
`50% PEG-400/water system was found to be only 0.2
`mg/mL, showing a nonlinear decline. Table 2 lists the
`theoretical and observed solubility values of progester-
`one in the PEG-400/water systems containing 3%
`Trappsol HPB or 3% Captisol at 25°C ± 2°C. The data
`for the system containing 3% Trappsol HPB are de-
`picted in Figure 1. The theoretical solubility for PEG-
`400/water/3% CD systems were calculated by the addi-
`tion of solubilities in PEG-400/water systems and those
`observed in 3% CD solutions. In solutions containing
`Trappsol HPB, at lower PEG-400 concentrations (less
`than 50%), the observed solubility was significantly
`greater than the expected solubility. For example, the
`theoretical value in 5% PEG-400/water system contain-
`ing 3% Trappsol HPB was 1.11 mg/mL. The observed
`solubility of progesterone in the same system was 2.18
`mg/mL, indicating approximately a 96% increase com-
`pared to the theoretical value. In general, the improve-
`ment in solubility due to synergism was observed in
`samples containing 5% to 50% PEG-400 and 3%
`Trappsol HPB. In systems containing PEG-400 concen-
`trations greater than 60%, the synergistic effect de-
`creased, yielding observed solubilities close to the theo-
`retical values. Overall, PEG-400 and Trappsol HPB
`showed a synergistic effect in improving progesterone
`solubility in water. In the case of systems containing
`Captisol, no synergism was observed (Table 2) in im-
`proving the solubility of progesterone. The observed
`solubility was less than the theoretical solubility.
`Polysorbate 80 often is used in the PEG-400/water
`system as a surfactant. Different percentages of poly
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`AAPS PharmSciTech 2003; 4 (1) Article 1 (http://www.pharmscitech.org).
`Table 2. Solubility of Progesterone in PEG-400/Water Systems with 3% Trappsol ® HPB or 3% Captisol ® at
`25°C ± 2 °C
`
`Observed Value
`(with 3% Captisol®)
`
`% of PEG-400 in
`PEG-400/Wwater
`System
`
`Solubility of Progesterone (mg/mL)
`Observed Value
`Theoretical Value
`Theoretical Value
`with 3% Trappsol®
`(with 3% Captisol®)*
`(with 3% Trappsol®
`HPB (% increase+)
`HPB)*
`1.45
`3.05
`2.79 (9.4)
`2.55
`90
`1.06
`2.40
`2.09 (10.0)
`1.90
`70
`1.10
`1.80
`1.77 (36.1)
`1.30
`50
`1.11
`1.65
`1.91 (66.1)
`1.15
`30
`1.21
`1.61
`2.09 (88.3)
`1.11
`10
`ND
`1.61
`2.18 (96.4)
`1.11
`5
`1.18
`1.61
`1.61 (45.0)
`1.11
`1
`*The theoretical solubility is the solubility of progesterone in a PEG-400/water system without CD plus the solubility of
`progesterone in 3% CD aqueous solution.
`+% increase = (Observed solubility - theoretical solubility)*100/theoretical solubility
`Values in italics indicate that the observed values were greater than theoretical values.
`ND = not determined
`n=1 only few data points were listed in the table Figure 1 has all the points
`
`
`
`
`sorbate 80 (0%-12%) were added to a 50% PEG-
`400/water system containing 3% Trappsol HPB or 3%
`Captisol. The solubility of progesterone in a 50%
`PEG-400/water system without polysorbate 80 was
`observed to be 0.19 mg/mL. By adding polysorbate
`80, the solubility increased sequentially to 1.03
`mg/mL with 12% polysorbate 80. The observed solu-
`bility in a system containing Trappsol HPB were
`greater than the theoretical solubility for a system con-
`taining up to 6% polysorbate 80. The increase in solu-
`bility was 37% and 12% for samples containing 0%
`and 6% polysorbate 80 in the PEG-400/water system.
`Higher amounts of polysorbate 80 nullified the syner-
`gistic effect of PEG-400 and Trappsol HPB. In the
`case of Captisol, no synergistic effect was observed to
`improve the solubility.
`In early 1990s, it was believed that cosolvents reduced
`the solubilization capacity of CDs. The solubility of
`testosterone with hydroxypropyl-β-CD was reported to
`be lower in the presence of 80% ethanol.9 However, in
`recent years, polymers have been reported to improve
`the solubilization capacity of CDs. A synergism be-
`tween CDs and water-soluble polymers in solubilizing
`naproxen was observed.10 A mathematical model was
`developed to describe the combined effect of cosol-
`vency and complexation on fluasterone solubilization.11
`The most fundamental model for solubilization of a
`solute in a solvent involves liberation of a solute mole-
`
`cule, creation of a hole in the solvent, and accommoda-
`tion of the solute molecule in the solvent cavity.12 Work
`must be done to overcome the intermolecular forces of
`attraction in dissolving a solute. Four types of interac-
`tions, namely solute-solvent, ion-dipole, dipole-dipole,
`and hydrogen bonding-hydrophobic, have been re-
`ported. In addition, if the system involves a polymer,
`the conformation of polymer chains also plays a role in
`solute-solvent interactions.
`For a reaction (in this case, the solubilization of a solute
`in a solvent), free energy (∆F) is defined as ∆F = ∆H –
`T∆S, where terms H and S are enthalpy and entropy,
`respectively. For a spontaneous reaction to occur, the
`associated ∆F must decrease or ∆F has to be negative.
`The dissolution of a solute involves the breaking of
`solid-state bonds in the solute, which is normally an
`endothermic process. The incorporation of the liberated
`solute molecules in the solvent cage is normally an exo-
`thermic process. One has to consider such enthalpic and
`entropic contributions in understanding the mechanism
`of solubilization.
`Water as a solvent has some unique properties: a high
`level of hydrogen bonding, a sizable dielectric constant
`(80 at 20°C), and large surface tension (71 dynes/cm).
`The structure of PEG-400 is H-(O-CH2-CH2)n-OH,
`where n is approximately 8 to 9. This peculiar structure
`makes PEG miscible with water through hydrogen
`bonding. The hydrophobic hydrocarbon region helps to
`
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`AAPS PharmSciTech 2003; 4 (1) Article 1 (http://www.pharmscitech.org).
`
`Figure 1. Effect of 3% Trappsol ® HPB on the solubility of progesterone in PEG-
`400/water systems.
`
`
`
`
`break the hydrogen bonding between water molecules,
`thus reducing overall intermolecular interactions.13 In
`other words, PEG may assist to reduce the dipole mo-
`ment of water and allow hydrophobic compounds to fit
`in.
`The solubilization of solute molecules because of the
`inclusion complex in CD has been demonstrated in
`numerous cases. Although a variety of other factors,
`such as Van der Waals, hydrogen bonding, and hydro-
`phobic forces, play important roles in forming a stable
`complex, it mainly depends on the CD cavity and the
`accessibility of the drug molecule to the CD cavity. The
`type, length, and degree of substitution also affect the
`solubilization effect of CD. In the case of Captisol, the
`negative charge on the molecule also helps ion pairing
`with the cationic molecules.
`In the current study, the system examined is very com-
`plex with 4 components: progesterone, water, PEG-
`400, and CD. In the quaternary system in this study,
`the ∆F value must be more negative than the ∆F
`values of
`the PEG-400/water/progesterone or
`CD/water/progesterone systems. It is possible that ∆H
`might have a negative value and the entropic term
`
`must be positive (∆S > 0). The entropic term may in-
`dicate spontaneity or ease of preparation. Faucci and
`Mura10 studied synergism between CD and water-
`soluble polymers on naproxen solubility. They re-
`ported that water-soluble polymers increased the com-
`plexation efficacy of CDs toward naproxen. No previ-
`ous
`sonication or heating
`treatments of
`the
`drug/CD/polymer suspensions was necessary to obtain
`this favorable effect. The synergistic effect of CD and
`PEG-400 in the current study could be attributed to
`additional breaking of hydrogen bonds in water's
`structure and a decrease in the dipole moment. At
`PEG-400 concentrations of 50% and higher, the syn-
`ergistic effect diminished. It must be because of a shift
`from a predominantly aqueous environment to a PEG-
`based environment. In that case, water would be act-
`ing as a cosolvent instead of the main solvent. As a
`result, the polymer conformation and the effect of CD
`on the polymer conformation would affect the solubil-
`ity of the model drug.
`An evaluation may indicate that the negative charge on
`Captisol may have a role to play in this lack of syner-
`gism. The ionic charge of Captisol must have intro-
`duced some kind of orderliness in the solvent structure,
`
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`AAPS PharmSciTech 2003; 4 (1) Article 1 (http://www.pharmscitech.org).
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