International Journal of Pharmaceutics 379 (2009) 32–40
`
`Contents lists available at ScienceDirect
`
`International Journal of Pharmaceutics
`
`j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / i j p h a r m
`
`CDs as solubilizers: Effects of excipients and competing drugs
`Phatsawee Jansook, Thorsteinn Loftsson∗
`
`Faculty of Pharmaceutical Sciences, University of Iceland, Hofsvallagata 53, IS-107 Reykjavik, Iceland
`
`a r t i c l e
`
`i n f o
`
`a b s t r a c t
`
`Article history:
`Received 24 February 2009
`Received in revised form 2 June 2009
`Accepted 3 June 2009
`Available online 12 June 2009
`
`Keywords:
`Cyclodextrins
`Formulation
`Solubility
`Aggregates
`Permeation
`Availability
`
`In recent years cyclodextrins (CDs) have been acknowledged by the pharmaceutical industry as very
`useful enabling excipients for solubilization and stabilization of drugs in aqueous formulations. Their
`effect is however strongly influenced by other commonly used excipients. The purpose of this investiga-
`tion was to examine the effects of excipients and drug combinations on the effects of CD solubilization
`of drugs and drug availability. The model drug was dexamethasone, the competing drugs tested were
`hydrocortisone, indomethacin and amphotericin B, and the sample CDs were ␥-cyclodextrin (␥CD) and
`2-hydroxypropyl-␥-cyclodextrin (HP␥CD). Benzalkonium chloride and hydroxypropyl methylcellulose
`enhance the solubilizing effect of the CDs whereas in general EDTA decreased the effect. The effect of
`second drug present in the aqueous formulation did depend on the affinity of that drug for the CD. Drugs
`which readily formed complexes with the CDs (e.g. hydrocortisone) decreased their ability to solubilize
`dexamethasone. Drugs that have little affinity for CDs (e.g. amphotericin B) did in some cases improve
`the CD solubilization of dexamethasone. Flux diagrams obtained through semi-permeable cellophane
`membrane indicated that drug/CD complexes self-assemble to form aggregates, especially at CD concen-
`trations above 5% (w/v). This aggregate formation was affected by the excipients and did influence drug
`availability from the formulations.
`
`© 2009 Elsevier B.V. All rights reserved.
`
`1. Introduction
`
`Cyclodextrins (CDs) are cyclic torus-shaped molecules, consist-
`ing of 6–8 d-(+)-glucopyranose units with hydrophilic outer surface
`and lipophilic central cavity. During the past two decades CDs
`have received growing attention, mainly due to their ability to
`increase aqueous solubility and stability of poorly water-soluble
`drugs through formation of inclusion complexes (Brewster and
`Loftsson, 2007; Loftsson and Duchêne, 2007). Furthermore CDs
`can act as permeation enhancers by keeping hydrophobic drug
`molecules in solution and deliver them to the surface of a biologi-
`cal membrane, thus leading to improve transepithelial permeation
`and bioavailability of drug (Loftsson et al., 2007). Pharmaceutical
`excipients that are present in a given drug formulation can enhance
`or decrease the solubilizing effect of CDs (Loftsson and Brewster,
`1996; Loftsson et al., 1999). Polymers can enhance the CD complex-
`ation of drugs and they can enhance the drug permeation through
`biological membranes, possibly through formation of ternary com-
`plexes or co-complexes (Jarho et al., 1998; Kristinsson et al., 1996;
`Loftsson, 1998; Mura et al., 2001; Chowdary and Srinivas, 2006).
`Frequently dosage forms contain more than one active ingredient.
`Dexamethasone can, for example, be found in various combination
`eye drops such as eye drops containing dexamethasone, neomycin
`
`and polymyxin B, and eye drops containing dexamethasone and
`tobramycin. However, combination products containing CD have
`not been marketed.
`The purpose of the present investigation is to study the influ-
`ence of common pharmaceutical excipients, as well as that of
`competing drugs, on the CD solubilization of drugs that possess
`poor solubility in water and their availability. Aqueous eye drop
`solution was used as a sample formulation with dexamethasone
`as a model drug. The tested competing drugs were hydrocorti-
`sone, which has similar steroidal structure as dexamethasone,
`indomethacin, which is a carboxylic acid fully ionized at physiologic
`pH, and amphotericin B, which is a water-insoluble polyene antibi-
`otic that has low affinity for the CD central cavity. The sample CDs
`were ␥-cyclodextrin (␥CD), which has limited solubility in water,
`and its water-soluble derivative 2-hydroxypropyl-␥-cyclodextrin
`(HP␥CD). Previous studies have shown that mixtures of ␥CD and
`HP␥CD can be more potent solubilizers than the individual CDs
`(Jansook and Loftsson, 2008). Thus, mixtures of ␥CD and HP␥CD
`were also included in this study. The physicochemical properties of
`the excipients and sample compounds are shown in Table 1.
`
`2. Materials and methods
`
`2.1. Materials
`
`∗ Corresponding author. Tel.: +354 525 4464; fax: +354 525 4071.
`E-mail address: thorstlo@hi.is (T. Loftsson).
`
`Dexamethasone (Dx) was purchased from Fagron group (Ams-
`terdam, Netherlands), hydrocortisone (HC) from ICN Biomedicals
`
`0378-5173/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
`doi:10.1016/j.ijpharm.2009.06.005
`
`APOTEX EX1017
`
`Page 1
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`P. Jansook, T. Loftsson / International Journal of Pharmaceutics 379 (2009) 32–40
`
`33
`
`Table 1
`Physicochemical properties of the excipients and sample compounds (Lavasanifar et al., 2002; Moffat et al., 2004; Nokhodchi et al., 2005; Brewster and Loftsson, 2007).
`
`Physicochemical properties
`
`(A)
`
`Dexamethasone
`
`Hydrocortisone
`
`Indomethacin
`
`Amphotericin B
`
`␥CD
`
`HP␥CD a
`
`Chemical structure
`
`Molecular weight
`Melting point (◦C)b
`pKa
`Octanol/water partition coefficient
`S0 (mg/ml) in water (at RT)
`
`392.5
`270 (dec.)
`–
`1.8
`0.08
`
`Physicochemical properties
`
`(B)
`
`362.5
`214 (dec.)
`–
`1.6
`0.4
`
`357.8
`158
`4.5
`−1.0 (at pH 7.4)
`0.8 (at pH 7.2)
`
`924.1
`>170 (dec.)
`5.5; 10
`0.8
`0.001
`
`1297.1
`≥200 (dec.)
`–
`−12
`249
`
`1576
`≥200 (dec.)
`–
`<−10
`>500
`
`Edetate disodium
`
`Benzalkonium chloride
`
`Hydroxypropylmethylcellulose
`
`Chemical structure
`
`Molecular weight
`Melting point (◦C)b
`pKa
`Octanol/water partition coefficient
`S0 (mg/ml) in water (at RT)
`
`336.2
`252 (dec.)
`2.0; 2.7; 6.2; 10.3
`–
`96
`
`a Representative structure.
`b Dec.: decomposition upon heating.
`c Varies with the alkyl chain length of the homolo.
`
`360 (average)
`≈40
`–
`9.98 for C12; 32.9 for C14; 82.5 for C16
`Very soluble
`
`c
`
`10,000–1,500,000 (approximately)
`190–200 (browns)
`–
`–
`Varies with the viscosity
`
`Page 2
`
`

`
`34
`
`P. Jansook, T. Loftsson / International Journal of Pharmaceutics 379 (2009) 32–40
`
`Fig. 1. The effect of additives in aqueous CD solution on the solubility and the flux of dexamethasone through semi-permeable cellophane membrane MWCO 3500; (A) ␥CD;
`(B) ␥CD/HP␥CD (ratio 80:20); (C) ␥CD/HP␥CD (ratio 20:80); (D) HP␥CD; ((cid:2)) EDTA (0.1%, w/v); (♦) BAC (0.02%, w/v); ((cid:4)) HPMC (0.1%, w/v); ((cid:5)) all excipients (i.e. in the aqueous
`eye drop formulation).
`
`Page 3
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`P. Jansook, T. Loftsson / International Journal of Pharmaceutics 379 (2009) 32–40
`
`35
`
`Table 2
`HPLC conditions.
`
`Drugs
`
`Mobile phasea
`
`Flow rate (ml/min)
`
`Wavelength (nm)
`
`Retention time (min)
`
`Dexamethasone
`Dexamethasone Hydrocortisone
`Dexamethasone Indomethacin
`Dexamethasone amphotericin B
`
`ACN:THF:water (33:1:66)
`ACN:THF:water (33:1:66)
`ACN:0.5% acetic acid (50:50)
`ACN:0.25 mM EDTA (37:63)
`
`1.5
`1.4
`1.5
`1.0
`
`241
`241 and 254
`240 and 240
`241 and 403
`
`5.1
`5.1 and 3.0
`7.2 and 1.9
`4.4 and 3.1
`
`a Volume ratios. ACN: acetonitrile; THF: tetrahydrofuran; acetic acid: aqueous acetic acid solution; EDTA: aqueous disodium edetate dehydrate solution.
`
`(Aurora, OH), amphotericin B (AmB) and indomethacin (IDM) from
`Sigma (St. Louis, MO), ␥-cyclodextrin (␥CD) and 2-hydroxypropyl-
`␥-cyclodextrin (HP␥CD) MS 0.6 (MW 1576 Da) from Wacker
`Chemie (Munich, Germany), disodium edetate dehydrate (EDTA)
`and sodium chloride (NaCl) from Merck (Darmstadt, Germany),
`benzalkonium chloride (BAC) and hydroxypropyl methylcellulose
`4000 (HPMC) from Sigma (St. Louis, MO), semi-permeable cello-
`phane membranes (SpectaPor®, molecular weight cut-off (MWCO)
`3500) from Spectrum Europe (Breda, Netherlands). All other chemi-
`cals used were of analytical reagent grade purity. Milli-Q (Millipore,
`Bedford, MA) water was used for the preparation of all solutions.
`
`2.2. Solubility determinations
`
`Solubility of dexamethasone and in water or aqueous CD solu-
`tions was determined by heating in autoclave (121 ◦C for 20 min)
`(Loftsson and Hreinsdóttir, 2006). Excess amount of dexametha-
`sone was added to an aqueous solution containing 0–20% (w/v)
`CD (pure ␥CD, pure HP␥CD, or a mixture of ␥CD and HP␥CD),
`benzalkonium chloride (0.02%, w/v), EDTA (0.1%, w/v) and/or
`hydroxypropyl methylcellulose (HPMC) (0.1% w/v), individual com-
`pounds or mixtures thereof. The effect of HPMC on the solubility
`of dexamethasone was determined in 0.10–0.75% (w/v) HPMC
`solutions in pure water. The suspensions formed were heated in
`autoclave at 121 ◦C for 20 min in sealed glass vials and then allowed
`to cool to room temperature. Then small amount of solid drug was
`added to the suspensions, pH adjusted to 7.4 with concentrated
`aqueous hydroxide solution, and the suspension allowed to equi-
`librate in the resealed vials at room temperature (22–23 ◦C) for 7
`days under constant agitation. Many drugs such as indomethacin
`are known to exist in more than one polymorphic form and thus it
`is essential to add small amount of the solid drug to the test media
`after heating. After equilibrium was attained, the suspensions were
`filter through 0.45 ␮m membrane filters, the filtrates diluted with
`mobile phase and analyzed by HPLC. The phase-solubility profiles
`were determined according to Higuchi and Connors (1965).
`Co-complexation of two different drugs was also investigated. In
`that case excess of both dexamethasone and a second drug (hydro-
`cortisone, indomethacin or amphotericin B) were simultaneously
`added to the aqueous complexation media and the solubility of
`both drugs determined as previously described, except when the
`chemically instable amphotericin B was present then heating in an
`autoclave was replaced by heating in an ultrasonic bath at 60 ◦C for
`
`Fig. 2. The phase solubility diagrams of dexamethasone and the determined ␥CD
`content in aqueous ␥CD solution containing no additives and with additives; (䊉)
`no additive; ((cid:2)) EDTA (0.1% w/v); (♦) BAC (0.02%, w/v); ((cid:4)) HPMC (0.1%, w/v); ((cid:5))
`all excipients (i.e. in the aqueous eye drop formulation). The aqueous cyclodextrin
`solutions were in all cases saturated with dexamethasone.
`
`Table 3
`Effect of additives on dexamethasone CE using the CE obtained in aqueous complexation medium without additives as a reference.
`
`The additivesa
`
`␥CDb
`
`CE
`
`Ratio
`
`HP␥CD
`
`CE
`
`␥CD/HP␥CD
`
`Ratio
`
`(80/20)b
`
`Ratio
`
`1.00
`0.85
`1.16
`1.13
`1.42
`
`(20/80)
`
`CE
`1.44 ± 0.02
`1.16 ± 0.03
`1.23 ± 0.03
`1.02 ± 0.01
`1.41 ± 0.01
`
`Ratio
`
`1.00
`0.81
`0.85
`0.71
`0.98
`
`No additive
`EDTA
`BAC
`HPMC
`EDTA + BAC + HPMC
`
`1.00
`0.88
`1.57
`1.25
`1.95
`
`1.11 ± 0.05
`0.14 ± 0.01
`1.28 ± 0.02
`0.12 ± 0.01
`1.26 ± 0.01
`0.21 ± 0.02
`1.24 ± 0.03
`0.17 ± 0.02
`1.31 ± 0.02
`0.27 ± 0.02
`a Concentration of additives: EDTA 0.1% (w/v); BAC 0.02% (w/v); HPMC 0.1% (w/v).
`b Calculated from the initial slope of the Bs-type phase–solubility diagram (cyclodextrin concentration 7–23 mM).
`
`1.00
`1.15
`1.14
`1.11
`1.18
`
`CE
`0.62 ± 0.03
`0.52 ± 0.03
`0.71 ± 0.04
`0.69 ± 0.01
`0.87 ± 0.02
`
`Page 4
`
`

`
`36
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`P. Jansook, T. Loftsson / International Journal of Pharmaceutics 379 (2009) 32–40
`
`Fig. 3. The effect of a second drug in aqueous CD solution on phase solubility profiles of dexamethasone and its second drug in pure and different ratios of ␥CD/HP␥CD
`mixtures in the aqueous eye drop formulation: (A) dexamethasone (DX); (B) hydrocortisone (HC); (C) indomethacin (IDM); (D) amphotericin B (AmB). ␥CD ((cid:2)); ␥CD/HP␥CD
`ratio (80/20) (♦); ((cid:5)) ␥CD/HP␥CD ratio (20/80); ((cid:4)) HP␥CD.
`
`30 min. The complexation efficiency (CE) was determined from the
`linear phase-solubility diagrams (plots of the total drug solubility
`([drug]t) versus total CD concentration ([CD]t) in moles per liter)
`(Loftsson et al., 2005):
`CE = Slope
`= [drug/CD complex]
`1 − Slope
`[CD]
`where K1:1 is the stability constant of the drug/CD 1:1 complex and
`S0 is the intrinsic solubility of the drug.
`
`= K1:1 · S0
`
`(1)
`
`2.3. Quantitative determinations
`
`Quantitative determinations of the individual drugs were
`performed on a reversed-phase high performance liquid chromato-
`graphic (HPLC) component system consisting of Hewlett Packard
`Series 1100, consisting of a G132A binary pump with a G1379A sol-
`vent degasser, a G13658 multiple wavelength detector, a G1313A
`
`auto sampler, and Phenomenex Luna 5 ␮ C18 reverse-phase col-
`umn (150 mm× 4.6 mm). The HPLC chromatographic conditions
`are shown in Table 2. Quantitative analysis of ␥CD content was
`determined by HPLC (Dionex UltiMate 3000, USA). The liquid
`chromatograph comprised of an UltiMate 3000 and a differential
`refractive index detector (Shodex RI-101, Japan) with a sensitivity
`of 600 ␮RIU. Data integration was done using CHROMELEON® soft-
`ware version 6.80 for LC integration. The column used was Luna NH2
`100A (10 ␮m, 250 mm× 4.6 mm) (Phenomenex, USA). The HPLC
`conditions were as follows. Mobile phase: 67% (v/v) acetonitrile
`in pure water; flow rate: 1 ml/min; injection volume: 20 ␮l; and
`column oven temperature: 25 ◦C.
`
`2.4. Permeation studies
`
`The permeability studies of dexamethasone, hydrocortisone,
`indomethacin and amphotericin B from eye drop preparations (the
`
`Page 5
`
`

`
`37
`P. Jansook, T. Loftsson / International Journal of Pharmaceutics 379 (2009) 32–40
`by heating in an autoclave (121 ◦C for 20 min). Before further test-
`ing the solutions were allowed to cool to room temperature and
`equilibrate for 7 days under constant agitation.
`
`donor phase) were carried out using Franz diffusion cell system
`(FDC 400 15FF, Vangard International, Neptune, NJ). The donor
`chamber and the receptor chamber were separated with a single
`semi-permeable cellophane membrane. The membrane was soaked
`overnight in the receptor phase that consisted of aqueous pH 7.4
`phosphate buffer saline solution containing 5% (w/v) ␥CD/HP␥CD
`(1:1 weight ratio) mixture. CD was added to the receptor phase
`to ensure sufficient drug solubility. The receptor phase was soni-
`cated under vacuum to remove dissolved air before it was placed
`in the receptor chamber. The study was carried out at ambient
`temperature (22–23 ◦C) under continuous stirring of the receptor
`phase by magnetic stirring bar rotating at 300 rpm. A 150 ␮l sam-
`ple of receptor medium was withdrawn at 30, 60, 120, 180, 240, and
`360 min and replaced immediately with an equal volume of fresh
`receptor phase. The drug concentration in the receptor sample was
`determined by HPLC. The steady state flux was calculated as the
`slope (dq/dt) of linear section of the amount of drug in the receptor
`chamber (q) versus time (t) profiles, and the apparent permeability
`coefficient (Papp) was calculated from the flux (J) according to Eq.
`(2):
`J = dq
`= Papp Cd
`A dt
`where A is the surface area of the mounted membrane (1.77 cm2)
`and Cd is the initial concentration of the drug in the donor phase.
`
`(2)
`
`3. Results and discussions
`
`Phase-solubility profiles of dexamethasone in aqueous solu-
`tions containing ␥CD or 80/20 mixture of ␥CD and HP␥CD were
`of Bs-type, with an initial linear increase followed by a decrease in
`dexamethasone concentration (Fig. 1A and B), indicating that the
`complexes formed had limited solubility in the aqueous complex-
`ation media. In contrast, AL type was observed in the complexation
`media containing pure HP␥CD or 20/80 mixture of ␥CD and HP␥CD
`(Fig. 1C and D), indicating formation of formation of water-soluble
`complexes (Higuchi and Connors, 1965). HP␥CD consists of mixture
`of numerous structurally related isomers and thus HP␥CD and its
`complexes do not in general form crystalline precipitates (Loftsson
`and Brewster, 1996). Table 3 shows the effects of the individual
`excipients on the CE. In general, EDTA decreases the solubilizing
`effect of the CDs where as the surface active BAC increases the effect.
`The water-soluble polymer HPMC results in a small increase in the
`solubility, especially when pure ␥CD is used as a solubilizer. The
`greatest increase in solubility is obtained when the eye drop for-
`mulation contains mixture EDTA, BAC and HPMC, and especially in
`eye drop formulations where large fraction of the drug is in the form
`of solid complex particles, i.e. the eye drop formulation containing
`␥CD (CE ratio 1.95) and 80/20 mixture of ␥CD and HP␥CD (CE ratio
`1.42). Enhanced CE results in increased drug solubility in the aque-
`ous eye drop formulation and consequent increased availability of
`the drug as can be seen in greater drug flux through the MWCO
`3500 semipermeable cellophane membrane (Fig. 1). The flux dia-
`grams are obtained by plotting the drug flux through the membrane
`as a function of the CD concentration in the donor phase which was
`saturated with the drug. Since the molecular weight of the individ-
`ual dexamethasone/CD complexes (1690–1969 Da) are much less
`than the molecular weight cutoff of the membrane (3500 Da) the
`flux (J) should be proportional to the total amount of dissolved
`
`CE (combination)
`
`Dexamethasone
`
`CE value
`
`CE ratio a
`
`Second drug
`
`CE value
`
`CE ratiob
`
`–
`–
`–
`–
`
`0.07
`0.24
`0.48
`0.45
`
`0.18
`0.69
`1.11
`1.21
`
`0.30
`1.71
`1.32
`1.40
`
`–
`–
`–
`–
`
`0.28
`0.35
`0.38
`0.42
`
`0.70
`1.00
`0.88
`1.13
`
`1.17
`1.76
`1.05
`1.31
`
`–
`–
`–
`–
`
`0.11
`0.32
`0.74
`0.68
`
`0.005
`0.003
`0.006
`0.006
`
`0.0007
`0.0006
`0.0007
`0.0007
`
`–
`–
`–
`–
`
`0.35
`0.17
`0.46
`0.56
`
`0.06
`0.05
`0.09
`0.08
`
`0.01
`0.01
`0.01
`0.02
`
`2.5. Aqueous eye drop sample formulations
`
`The aqueous dexamethasone eye drop solutions were prepared
`by dissolving dexamethasone in 9 ml of an aqueous solution con-
`taining benzalkonium chloride (2 mg), EDTA (10 mg) and HPMC
`(10 mg) and various types and amounts of CD (pure ␥CD, pure
`HP␥CD or mixtures of ␥CD/HP␥CD). The amount of CD needed
`to solubilize given amount of dexamethasone (1.5–15 mg/ml) was
`determined from the phase-solubility profiles. The pH was adjusted
`to 7.4 with concentrated aqueous sodium hydroxide solution and
`the osmolality was adjusted to 260–330 mOsm/kg with NaCl. The
`final volume was adjusted to 10.0 ml and the solutions sterilized
`
`Table 4
`The apparent complexation constant and CE of various drugs in eye drop preparations.
`
`Cyclodextrin
`
`␥CD/HP␥CD ratio
`
`CE (individual)
`
`Dexamethasone
`␥CD
`␥CD/HP␥CD
`␥CD/HP␥CD
`HP␥CD
`
`Hydrocortisone
`␥CD
`␥CD/HP␥CD
`␥CD/HP␥CD
`HP␥CD
`
`Indomethacin
`␥CD
`␥CD/HP␥CD
`␥CD/HP␥CD
`HP␥CD
`
`Amphotericin B
`␥CD
`␥CD/HP␥CD
`␥CD/HP␥CD
`HP␥CD
`
`–
`80/20
`20/80
`–
`
`–
`80/20
`20/80
`–
`
`–
`80/20
`20/80
`–
`
`–
`80/20
`20/80
`–
`
`0.26
`0.69
`1.26
`1.07
`
`0.31
`1.90
`1.61
`1.21
`
`0.09
`0.06
`0.07
`0.07
`
`0.07
`0.07
`0.05
`0.04
`
`a CE of dexamethasone when the second drug is also present in the complexation medium/CE of dexamethasone in the aqueous complexation medium when no other
`drug is present.
`b CE of the drug in presence of dexamethasone/CE of the drug when no dexamethasone is present.
`
`Page 6
`
`

`
`38
`
`P. Jansook, T. Loftsson / International Journal of Pharmaceutics 379 (2009) 32–40
`
`Fig. 4. The effect of a second drug in aqueous CD solution on the Papp of dexamethasone and its second drug in pure and different ratios of ␥CD/HP␥CD mixtures in the
`aqueous eye drop formulation through semi-permeable cellophane membrane MWCO 3500: (A) dexamethasone (Dx); (B) hydrocortisone (HC); (C) indomethacin(IDM); (D)
`amphotericin B (AmB). ((cid:2)) ␥CD; (♦) ␥CD/HP␥CD ratio (80/20); ((cid:5)) ␥CD/HP␥CD ratio (20/80); ((cid:4)) HP␥CD.
`
`drug (Cd) in the donor phase (Eq. (2)). This is indeed observed in
`Fig. 1A and B. However, the flux diagrams obtained from donor
`phases containing either HP␥CD or ␥CD/HP␥CD 20/80 mixture with
`other excipients (Fig. 1C and D), showed a negative deviation from
`linearity, while their phase-solubility profiles are linear. The main
`reason of such negative deviations is self-assemble of the drug/CD
`complexes to form water-soluble aggregates that are too large to
`be able to permeate the membranes but too small to display static
`light scattering and thus the aqueous solutions appear clear to the
`naked eye (Loftsson et al., 2002, 2004; Jansook et al., in press).
`Both BAC and HPMC enhance dexamethasone solubility at CD
`concentrations below 5% (w/v) but decrease the solubility at higher
`CD concentrations (Fig. 2). The concentration of dissolved ␥CD
`was also determined in this same complexation media that had
`been saturated with dexamethasone. The excipients, EDTA, BAC
`and HPMC, resulted in about 25% reduction in the ␥CD solubility
`at ␥CD concentrations above 5%. Furthermore, when the solubility
`profile of dexamethasone displayed decreased solubility the sol-
`ubility profile of ␥CD displayed increased solubility. Since excess
`dexamethasone was added to all ␥CD solutions one would expect
`that excess dexamethasone should precipitate all ␥CD until its con-
`
`centration corresponded to solution that had been saturated with
`the dexamethasone/␥CD complex. This appears to be the case at
`␥CD concentrations equal or less than 5% but not at higher concen-
`trations. These observations indicate that the complex formation at
`elevated ␥CD concentrations does not follow the simple stoichiom-
`etry that is generally observed in ideal solutions.
`Frequently pharmaceutical formulations contain combination of
`two or more drugs that possess different physicochemical prop-
`erties, including different affinities for the formulation excipients
`such as CDs. Fig. 3 shows the effect of competing drugs on the
`phase-solubility profiles and Table 4 shows the CE calculated from
`the initial linear portion of the phase-solubility profiles. Hydro-
`cortisone is a steroid with similar structure and physicochemical
`properties as dexamethasone (Table 1). The affinity of the two drugs
`to both ␥CD and HP␥CD is of same order of magnitude, as observed
`by the CE values in Table 4, although on the average hydrocortisone
`has greater affinity for the CDs than dexamethasone. Clearly the CE
`is reduced when both drugs are present in the same complexation
`media resulting in decreased dexamethasone and hydrocortisone
`solubilization (Fig. 3 and Table 4). At pH 7.4 indomethacin is much
`less lipophilic and about 10 times more water-soluble than dex-
`
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`
`39
`
`ethasone. The concentration of dissolved dexamethasone in ␥CD
`and ␥CD/HP␥CD (80/20) containing media was determined to be
`between 1.5 and 3 mg/ml while it was determined to be 1.5 and
`15 mg/ml in media containing either HP␥CD or ␥CD/HP␥CD (20/80).
`As previously observed (Fig. 4) the value of Papp decreased with
`increasing concentration of dissolved dexamethasone, possibly due
`to formation of water-soluble aggregates. In all cases, the mixtures
`of ␥CD/HP␥CD resulted in higher Papp values than the pure CDs,
`especially when the ␥CD/HP␥CD ratio was 80/20, which gave the
`highest Papp value (5.23± 0.21 cm/sec; mean± SD). This indicates
`that this mixture is the optimum CD vehicle for a dexamethasone
`eye drop formulation.
`
`4. Conclusions
`
`Common pharmaceutical excipients like various salts, preser-
`vatives and water-soluble polymers can have significant effect on
`the solubilizing effects of CDs and the drug availability from aque-
`ous drug formulations. The CD solubilization is also affected in
`combination formulations containing more than one drug. Thus,
`CD formulation studies should always be performed in media that
`closely resembles the final drug formulation.
`
`Acknowledgement
`
`This work was financially supported by Eimskip grant 2007, Ice-
`land
`
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`Fig. 5. Apparent permeation coefficients of dexamethasone (mean± SD) in eye
`drop formulations containing different dexamethasone concentrations through
`semi-permeable membrane MWCO 3500; ((cid:4)) ␥CD; (
`) ␥CD/HP␥CD 80/20; (
`)
`␥CD/HP␥CD 20/80; ((cid:5)) HP␥CD.
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`amethasone (Table 1) and indomethacin has much lower affinity
`for the CDs. Indomethacin (pKa 4.5) is in its ionized form at phys-
`iologic pH (pH 7.4) and able to form ion pair with benzalkonium
`in the aqueous complexation media. In addition, indomethacin
`exists in several different polymorphic forms that possess differ-
`ent intrinsic solubilities in water (Kaneniwa et al., 1985; Iohara et
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`has in most cases insignificant effect on the CE of dexamethasone
`and its solubilization, but as expected dexamethasone results in
`10–20-fold decrease in the CE of indomethacin (Table 4). Ampho-
`tericin B is a lipophilic compound but its aqueous solubility is only
`about 1 ␮g/ml (Table 1). Amphotericin B has very little affinity for
`the CDs with CE between 0.04 and 0.07. Consequently, CD solubi-
`lization of amphotericin B is significantly reduced by the presence
`of dexamethasone in the aqueous complexation media resulting
`in up to 100-fold reduction in the CE (Fig. 3 and Table 4). Inter-
`estingly, presence of amphotericin B results in 5–76% increase in
`the CE of dexamethasone. It is possible that this increase in CE
`is due to formation of amphotericin B/dexamethasone/CD ternary
`complexes.
`The permeability profiles in Fig. 4 show how the permeabil-
`ity coefficient is affected by the increasing CD concentration as
`well as that of competing drugs. The permeability coefficient is
`obtained by dividing the total concentration of dissolved drug (i.e.
`both free drug and drug in CD complexes and complex aggre-
`gates) into the flux (see Eq. (2)) and thus a decrease in the flux
`can either be due to actual decrease in the permeability coef-
`ficient or due to formation of water-soluble drug/CD aggregates
`that are unable to permeate the semi-permeable cellophane mem-
`brane. The general observation is that the drug/␥CD complexes
`permeate the membrane more rapidly (i.e. have larger perme-
`ability coefficients) than the drug/HP␥CD complexes and that the
`permeability coefficients of the individual drugs in a mixture are
`independent of each other. Also, the permeability coefficient of
`the drugs decreases with increasing CD concentration. However,
`amphotericin B appears to increase the permeability of dexam-
`ethasone and vice versa from complexation media containing ␥CD
`(Fig. 4A and D). At low CD concentration (<10%, w/v), the per-
`meability coefficients of indomethacin are insignificantly higher
`when dexamethasone is present compared to those from pure
`indomethacin donor phases.
`According to Table 3 the highest CE was obtained in the com-
`plete eye drop formulation that contained in addition to CD and
`NaCl 0.1% EDTA, 0.02% BAC and 0.1% HPMC (all %, w/v). The per-
`meation coefficients of dexamethasone from eye drop solutions
`containing different types and amounts of CD are shown in Fig. 5.
`All the eye drop solutions tested were saturated with dexam-
`
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`P. Jansook, T. Loftsson / International Journal of Pharmaceutics 379 (2009) 32–40
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