Solubilization of stratum corneum lipid liposomes by Triton X-100. Influence of the level of cholesteryl sulfate in the process

Colloids and Surfaces A: Physicochemical and Engineering Aspects 182 (2001) 15 – 23 www.elsevier.nl/locate/colsurfa

Solubilization of stratum corneum lipid liposomes by Triton X-100. Influence of the level of cholesteryl sulfate in the process M. Co´cera, O. Lo´pez, L. Coderch, J.L. Parra, A. de la Maza * Departamento de Tensioacti6os, Instituto de In6estigaciones Quı´micas y Ambientales de Barcelona, Consejo Superior de In6estigaciones Cientı´ficas (C.S.I.C.), Calle Jorge Girona 18 -26, 08034 Barcelona, Spain Received 21 July 1999; accepted 23 November 2000

Abstract The interaction of Triton X-100 (TX-100) with stratum corneum (SC) lipid liposomes varying the proportion of cholesteryl sulfate (Chol-sulf) was investigated. The surfactant/lipid molar ratios (Re) and the bilayer/aqueous phase surfactant partition coefficients (K) were determined by monitoring the changes in the static light scattering of the system during solubilization. The fact that the free surfactant concentration was always similar to its critical micelle concentration (CMC) indicates that the liposome solubilization was mainly ruled by the formation of mixed micelles. The TX-100 ability to saturate and to solubilize SC liposomes decreased as the proportion of Chol-sulf in bilayers increased until a minimum was reached for a Chol-sulf proportion of 10%. Inversely, the surfactant partitioning into liposomes (or affinity with these bilayers) increased as the proportion of Chol-sulf increased until a maximum was reached at the same Chol-sulf proportion. Hence, when the Chol-sulf proportion in bilayers was 10% (the same that existing in the intercellular SC lipids) the ability of TX-100 molecules to interact with liposomes exhibits a minimum despite their enhanced partitioning into liposomes. These effects may be related to the reported dependencies between the level of Chol-sulf in the intercellular lipids and the abnormalities in the skin properties as the barrier function. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Stratum corneum lipid liposomes; Triton X-100; Stratum corneum liposome solubilization; Influence of cholesteryl sulfate in stratum corneum liposome solubilization; Static light-scattering changes; Surfactant/stratum corneum lipids molar ratios; Surfactant partition coefficients

Abbre6iations: Cer, ceramides type III; Chol, cholesterol; Chol-sulf, cholesteryl sulfate; CMC, critical micellar concentration; K, bilayer/aqueous phase surfactant partition coefficient; KSAT, bilayer/aqueous phase surfactant partition coefficient for liposome saturation; KSOL, bilayer/aqueous phase surfactant partition coefficient for liposome solubilization; PA, palmitic acid; PI, polydispersity index; PIPES, piperazine-1,4 bis(2-ethanesulphonic acid); r 2, regression coefficient; Re, effective surfactant/lipid molar ratio; ReSAT, effective surfactant/lipid molar ratio for liposome saturation; ReSOL, effective surfactant/lipid molar ratio for liposome solubilization; SB, surfactant concentration in the bilayers; SW, surfactant concentration in the aqueous medium; SSAT, surfactant concentration in the aqueous medium for liposome saturation; SSOL, surfactant concentration in the aqueous medium for liposome solubilization; SC, stratum corneum; SLS, static light-scattering; TX-100, Triton X-100. * Corresponding author. Tel.: + 34-93-4006161; fax: +34-93-2045904. E-mail address: [email protected] (A. de la Maza). 0927-7757/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 7 7 5 7 ( 0 0 ) 0 0 8 1 8 - 9

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1. Introduction The stratum corneum (SC) forms a continuous sheath of alternating squamae (protein-enriched corneocytes) embedded in an intercellular matrix enriched in nonpolar lipids displayed as lamellar sheets. The proportion of cholesterol and cholesteryl sulfate (Chol-sulf) in these lipids is claimed to play an important role in the stability properties of the SC (cohesion and desquamation) and in the regulation of the skin barrier function [1–5]. Thus, although the Chol-sulf level in vivo is 10% [6], patients with recessive X-linked ichthyosis display a 3-fold increase in this lipid due to steroid sulfatase deficiency [7,8]. Furthermore, tissues with extremely tenacious intercellular cohesion also present higher Chol-sulf proportions than that existing in skin lipids [9]. However, a physico-chemical study of the role played by this lipid on the SC lipid properties using a simplified membrane model such as SC lipid liposomes is still lacking. Wertz and Downing prepared liposomes from lipid mixtures modeling the SC composition and studied their interaction with sodium dodecyl sulfate to determine its effect on human skin [6,10,11]. Stratum corneum lipid liposomes have been also used as membrane models to study the adsorption of enhancer agents and to compare these results with those obtained in skin studies [12 –15]. The interaction of Triton X-100 (TX-100) with liposomes leads to the breakdown of lamellar structures and to the formation of lipid-surfactant mixed micelles [16 – 20]. A significant contribution in this area has been made by Lichtenberg [21], who postulated that the surfactant/lipid molar ratio (Re) producing liposome solubilization depends on the surfactant critical micelle concentration and on the bilayer/aqueous medium partition coefficients (K). The formation of liposomes was studied using a mixture of four lipids modeling the SC composition and the interaction of oxyethylenated octylphenols and nonylphenols with these liposomes [22 –24]. The role played by the ceramides in the interaction of sodium dodecyl sulfate with SC lipid liposomes was also investigated [25].

Here, one seeks to extend these studies by characterizing the influence of Chol-sulf on the resistance of SC lipid liposomes against TX-100. To this end, the Re and K parameters of this interaction was determined at lytic levels, varying the proportion of Chol-sulf from 1 to 25% to know whether the variations of TX-100 susceptibility from these liposomes are significant in relation to the Cholsulf variations found in diseased states. This information may shed light on the possible correlation between the level of Chol-sulf in bilayers and the abnormalities in the skin barrier function and in the SC cohesion.

2. Materials and methods TX-100, octylphenol polyethoxylated with 10 U of ethylene oxide and active matter of 100% was purchased from Rohm and Hass (Lyon France). Piperazine-1,4-bis(2-ethanesulphonic acid) (PIPES) was obtained from Merck (Darmstadt, Germany). PIPES buffer was prepared as 20 mM PIPES containing 110 mM Na2SO4, adjusted to pH 7.20 with NaOH. Reagent grade organic solvents, ceramides type III (Cer), cholesterol (Chol) and palmitic acid (PA) were supplied by Sigma (St Louis, MO). Chol-sulf was prepared by reaction of cholesterol with excess chlorosulphonic acid in pyridine and purified chromatographically. The molecular weight of ceramide type III (Sigma) was determined by low resolution fast atom bombardment mass spectrometry using a Fisons VG Auto Spec Q (Manchester, UK) with a caesium gun operating at 20 Kv. From this analysis a molecular weight of 671 g mol − 1 was obtained for the majority compound used. This value was similar to the molecular weight of ceramide III (667 g) calculated from the structure of this compound reported by Wertz [10], despite the fact that the ceramide type III used was a mixture of ceramides of different chain length (mainly containing stearic and nervonic acids, purity approx. 99%). As a consequence, the molecular weight obtained was used to calculate the molarity of the lipid mixtures investigated. The lipids of the highest purity grade available were stored in chloroform/methanol 2:1 under nitrogen at − 20°C until use.

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2.1. Liposome preparation and characterization Liposomes formed by mixtures of SC lipids varying the percentage of Chol-sulf from 1 to 25%, the relative proportions of the other lipids remaining constant, were prepared following the method described by Wertz [6,22]. The lipid compositions investigated are given in Table 1 (final lipid concentration ranging from 0.5 to 5.0 mM). The experiment 3 corresponded to the composition of the intercellular lipids. The lipid composition and concentration of liposomes after preparation were determined by thin-layer chromatography coupled to an automated flame ionization detection system (Iatroscan MK-5, Iatron Lab., Tokyo, Japan) [22,26]. In order to find out whether all the lipid mixture components formed liposomes, vesicular dispersions were analyzed for these lipids [26]. The dispersions were then spun at 140 000×g at 25°C for 4 h to remove the vesicles [27]. The supernatants were tested again for these components. No lipids were detected in any of the supernatants. The phase transition temperatures of the lipid mixtures forming liposomes were determined by proton magnetic resonance (1H NMR), showing values ranging from 55 to 59°C [22]. The size distribution and polydispersity index (PI) of liposomes after preparation were determined with dynamic light-scattering measurements using a photon correlator spectrometer (Malvern AutoTable 1 Liposome lipid composition corresponding to the six experiments, in which the percentage of cholesteryl sulfate varied from 1 to 25% and the relative proportions of the other lipids remained constant Exp no.

1 2 3 4 5 6

Liposome lipid composition (%) Cer

Chol

PA

Chol-sulf

44.0 42.2 40.0 37.8 35.6 33.4

27.5 26.4 25.0 23.6 22.2 20.8

27.5 26.4 25.0 23.6 22.2 20.8

1.0 5.0 10.0 15.0 20.0 25.0

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sizer 4700c PS/MV). Samples were adjusted to the appropriate concentration range with PIPES buffer. Measurements were taken at 25°C at a scattering angle of 90°.

2.2. Parameters in6ol6ed in the interaction of TX-100 with SC lipid liposomes In the analysis of the equilibrium partition model proposed by Schurtenberger et al. [28] for bile salt/lecithin systems, Lichtenberg et al. and Almog et al. [21,27] have shown that for a mixing of lipids (at a lipid concentration L (mM)) and surfactant (at a concentration ST (mM)), in dilute aqueous media, the distribution of surfactant between lipid bilayers and aqueous media obeys a partition coefficient K, given (in mM − 1) by K= SB/[(L + SB) · SW]

(1)

where SB is the concentration of surfactant in the bilayers (mM) and SW is the surfactant concentration in the aqueous medium (mM). For LSB, the definition of K, as given by Schurtenberger, applies: K= SB/(L · SW)= Re/SW

(2)

where Re is the effective molar ratio of surfactant to lipid in the bilayers (Re=SB/L). Under any other conditions, Eq. (2) has to be employed to define K; this yields: K= Re/SW [1+ Re]

(3)

This approach is consistent with the experimental data offered by Lichtenberg et al. and Almog et al. [21,27] for different surfactant lipid mixtures over wide ranges of Re values. The validity of this model for the surfactant/lipid system investigated has been studied in Section 3. The solubilization of liposomes was characterized by two parameters termed ReSAT and ReSOL (according to the nomenclature adopted by Lichtenberg et al. [21]) corresponding to the Re ratios at which static light-scattering (SLS) starts to decrease with respect to the original value and shows no further decrease. These parameters corresponded to the surfactant/lipid molar ratios at which the surfactant: (i) saturated liposomes; and (ii) led to a complete liposome solubilization.

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Equal volumes of TX-100 solutions were added to the liposome suspensions and the resulting mixtures were left to equilibrate for 24 h at 25°C. The final surfactant concentration (mM) was calculated from each mixture. This time was chosen as the optimum period needed to achieve a complete equilibrium surfactant/liposome in the lipid concentration range used [17,29]. The temperature of 25°C was selected for the following reasons; (i) the reasonable stability of the SC liposomes under these conditions; (ii) similar experimental conditions to those used to study the interaction of this surfactant with PC liposomes; (iii) these experimental conditions are generally used in ‘in vivo’ tests to study the interaction of surfactants with skin [30 –32]. SLS measurements were made at 25°C using a spectrofluorophotometer Shimadzu RF-540 (Kioto, Japan) with both monochromators adjusted to 500 nm [33]. The assays were carried out in triplicate and the results given are the average of those obtained. The determination of Re and SW parameters was carried out on the basis of the linear dependence existing between the TX-100 concentrations required to saturate and to solubilize liposomes and the lipid concentration (L), which can be described by the equations: SSAT = SW,SAT +ReSAT · [L]

(4)

SSOL = SW,SOL +ReSOL · [L]

(5)

where SSAT and SSOL are the total surfactant concentrations. The surfactant/lipid molar ratios ReSAT and ReSOL and the aqueous concentration of surfactant SW,SAT and SW,SOL are in each curve respectively the slope and the ordinate at the origin (zero lipid concentration). The KSAT and KSOL parameters (bilayer/aqueous phase surfactant partition coefficient for saturation and complete liposome solubilization) were determined from the Eq. (3).

3. Results and discussion The critical micelle concentration (CMC) of TX-100 in the working medium was previously reported, which was 0.15 mM [34]. The character-

ization of the geometric properties of liposomes used in the present study was previously reported [22]. This study demonstrated that these liposomes were formed by unilamellar vesicles in all cases. Furthermore, the vesicle size distribution of liposomes after preparation varied very little showing in all cases a similar value of about 100 nm (PI lower than 0.1), thereby indicating that the size distribution was very homogeneous. The size of vesicles after addition of equal volumes of PIPES buffer and equilibration for 24 h showed always values similar to those obtained after preparation with a slight polydispersity index increase (between 0.11 and 0.13). Hence, the SC lipid liposomes investigated were reasonably stable in the absence of surfactant under the experimental conditions used.

3.1. Influence of Chol-sulf in the interaction of TX-100 with SC lipid liposomes A systematic study based on SLS variations of liposomes varying the level of Chol-sulf and due to the action of TX-100 was performed to shed light on the possible dependencies between the level of Chol-sulf in skin lipids and the function barrier abnormalities. To this end, the percentage of Chol-sulf varied from 1 to 25% (lower and higher values than that in SC lipids, which was 10%), the relative proportion of the other lipids remained constant (Table 1). To determine the partition coefficients of TX-100 between bilayers and the aqueous phase, the validity of the equilibrium partition model proposed by Lichtenberg et al. and Almog et al. [21,27] based on Eq. (1) for the systems investigated was first studied. This equation may be expressed by: L/SB = (1/K)(1/SW)− 1. Hence, this validity requires a linear dependence between L/ SB and 1/SW; this line should have a slope of 1/K, intersect with the L/SB axis at − 1 and intersect with the 1/SW at K. To test the validity of this model for the system investigated, liposomes were mixed with varying sublytic TX-100 concentrations (ST). The resultant surfactant-containing vesicles were then spun at 140 000× g at 25°C for 4 h to remove the vesicles [27]. No lipids were detected in the supernatants

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Fig. 1. Percentage changes in static light-scattering of SC liposomes, (lipid composition for the experiment no. 3, Table 1) the lipid concentration ranging from 0.5 to 5.0 mM, induced by the presence of increasing amounts of TX-100. Symbols: Lipid concentrations: 0.5 mM (), 1.0 mM ( ), 2.0 mM ( ), 3.0 mM ( ), 4.0 mM (), 5.0 mM ().

[26]. The TX-100 concentration in the supernatants (SW) was determined by HPLC [35] and its concentration in the lipid bilayers was calculated (SB =ST −SW). The results of the experiments in which SB and SW were measured (at the same range of lipid and surfactant concentrations used to determine K) were plotted in terms of the dependence of L/SB on 1/SW. Straight lines were obtained for each lipid mixture tested (r 2 = 0.992, 0.992, 0.993, 0.991, 0.991 and 0.992 for the experiments 1, 2, 3, 4, 5 and 6, respectively, Table 1). These straight lines were dependent on L and intersected with the L/SB axis always at − 0.969 0.12. Both the linearity of these dependences and the proximity of the intercept to −1 support the validity of this model to determine K for these surfactant/liposome systems. To determine the Re and SW parameters the SLS variations in SC liposomes due to the action of TX-100 were studied varying the lipid composition (Table 1), the lipid concentration ranging from 0.5 to 5.0 mM. The SLS curves obtained for the experiment 3 are given in Fig. 1. The addition of surfactant led to an initial increase and a subsequent fall in the SLS intensity of the system until it achieved a low constant value. The curves

obtained for the different experiments (Table 1) showed similar trends to those exhibited by experiment 3 (results not shown). This SLS behavior is similar to that reported for the interaction of the same surfactant with PC liposomes [34] although showing in all cases a more pronounced initial SLS increase. The surfactant concentrations producing 100% (SSAT) and 0% (SSOL) of SLS were obtained for each lipid concentration by graphical methods. The arrows A and B (curve for 5.0 mM lipid conc., Fig. 1) correspond to these parameters. When plotting the SSAT and SSOL values thus obtained for each experiment versus the lipid concentration the curves shown in Fig. 2(A) and (B) were respectively obtained, in which an acceptable linear relationship was established in all cases. The straight lines obtained corresponded to the Eqs. (4) and (5) from which the Re and SW were determined. The results obtained for each experiment including the regression coefficients of the straight lines (r 2) are given in Table 2. The free surfactant concentrations (SW,SAT, SW,SOL) were always comparable to the TX-100 CMC (0.15 mM), also showing similar values to those reported for the interaction of this surfac-

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M. Co´cera et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 182 (2001) 15–23

tant with PC liposomes [34]. These findings extent to SC liposomes the observation made with PC liposomes, that the free surfactant concentration must reach the CMC for solubilization to commence and indicate that liposome solubilization was mainly ruled by formation of mixed micelles [21,34]. Furthermore, the rise in the Chol-sulf

percent in liposomes resulted in a slight increase in both SW,SAT and SW,SOL. The variations of Re and K versus the proportion of Chol-sulf in bilayers are plotted in Fig. 3(A) and (B), respectively. The increase in the Chol-sulf proportion resulted in a clear increase in the Re parameters until a maximum was reached

Fig. 2. (A) Surfactant concentrations resulting in saturation (100% SLS) of SC liposomes vs. lipid concentration and due to the action of TX-100 for the experiments of Table 1. Symbols: experiment no. 1 (), 2 ( ), 3 ( ), 4 ( ), 5 () and 6 (). (B) Surfactant concentrations resulting in complete solubilization (0% SLS) of SC liposomes vs. lipid concentration and due to the action of TX-100 for the experiments of Table 1. Symbols: experiment no. 1 (), 2 ( ), 3 ( ), 4 ( ), 5 () and 6 ().

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Table 2 Surfactant to lipid molar ratios (Re), partition coefficients (K) and surfactant concentrations in the aqueous medium (SW) resulting in the interaction of TX-100 with SC liposomes varying the proportion of Chol-sulf from 1 to 25%, the proportions of the other lipids remaining constant Exp. no.

SW,SAT (mM) SW,SOL (mM) ReSAT (mol mol−1)

ReSOL (mol mol−1)

KSAT (mM−1)

KSOL (mM−1)

r 2 (SAT)

r 2 (SOL)

1 2 3 4 5 6

0.14 0.15 0.15 0.16 0.16 0.16

2.9 3.7 4.0 3.8 3.6 3.2

1.19 2.62 2.90 2.67 2.21 1.44

4.63 4.92 5.0 4.94 4.60 4.48

0.993 0.995 0.992 0.998 0.992 0.996

0.996 0.994 0.997 0.994 0.992 0.995

0.16 0.16 0.16 0.16 0.17 0.17

0.20 0.65 0.77 0.75 0.55 0.30

for a Chol-sulf proportion of 10% (Fig. 3(A)). This increase is more pronounced for ReSOL. The rise in the proportion of this lipid also resulted in a rise in the surfactant partitioning between these bilayers and water (Fig. 3(B)) until a maximum was achieved at the same Chol-sulf proportion. Given that the surfactant capacity to saturate or solubilize liposomes is inversely related to the Re parameters, these capacities decreased as the proportion of Chol-sulf increased showing a minimum at the Chol-sulf proportion of 10%. As a consequence, at this proportion liposomes exhibited the highest resistance to be saturated and solubilized by TX-100. Inversely, the surfactant affinity with SC liposomes (TX-100 partitioning between bilayers and water) exhibited a maximum at the same Chol-sulf proportion. Thus, at lower and higher Chol-sulf proportions than that existing in the SC (experiment 3, Table 1) the ability of the surfactant molecules to saturate or solubilize SC liposomes increased despite their reduced partitioning into liposomes. This effect, which was specially pronounced at low Chol-sulf proportions emphasizes the fragility of these bilayers against the action of TX-100. The increased surfactant partitioning and the increased Re values in liposomes containing a Chol-sulf similar to that of the SC lipids underlines the stability of these bilayers against TX-100 in spite of the enhanced number of surfactant molecules incorporated into liposomes. From these findings it may be assumed that the proportion of Chol-sulf in SC lipid liposomes plays an important role both in the resistance of

these liposomes to be saturated and solubilized by TX-100 and in the surfactant affinity with these bilayer structures. These findings may be related to the reported dependencies between the alterations in the level of Chol-sulf in the intercellular lipids and the abnormalities in the SC barrier function [1,2,7,8]. In fact, insufficient or excessive Chol-sulf content would alter the liquid-crystalline ‘melting point’ of these lipids, thereby producing non-physiologic phase transitions that would affect the skin barrier function. Taking into account the nonionic characteristics of TX-100 the ability of this surfactant to be incorporated into liposomes and to solubilize these bilayer structures appears to be related to these fluidity changes. In conclusion, the present study based on the model proposed by Lichtenberg et al. and Almog et al. [21,27] has been shown to be useful in establishing a correlation between the proportion of Chol-sulf in bilayers and the stability of these structures against TX-100. Liposomes modeling the lipid composition of the intercellular lipids exhibited the highest resistance to the action of TX-100 and variations in the proportion of Chol-sulf increased the surfactant ability to saturate or solubilize these bilayer structures. As a consequence, some direct correlation may be envisaged between the present data and the reported abnormalities in the barrier function and in the SC cohesion due to insufficient or excessive proportions of Chol-sulf in skin lipids.

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Acknowledgements We are grateful to Mr G. von Knorring for expert technical assistance. This work was supported by funds from D.G.I.C.Y.T. (Direccio´n General de Investigacio´n Cientı´fica y Te´cnica) (Prog. 3n° PB94-0043), Spain. References

Fig. 3. (A) Effective surfactant to lipid molar ratios (ReSAT and ReSOL) in SC liposomes for TX-100 vs. the percentage of Chol-sulf in liposomes. Symbols: ReSAT () and ReSOL ( ). (B) Partition coefficients (KSAT and KSOL) in SC liposomes for TX-100 vs. the percentage of Chol-sulf in liposomes. Symbols: KSAT () and KSOL ( ).

[1] P.M. Elias, M.L. Williams, M.E. Maloney, J.A. Bonifas, B.E. Brown, S. Grayson, E.H. Epstein, J. Clin. Invest. 74 (1984) 1414. [2] M.L. Williams, Am. J. Dermatol. 6 (1984) 381. [3] N.Y. Schu¨rer, G. Plewig, P.M. Elias, Dermatologica 183 (1991) 77. [4] K. Harada, T. Murakami, N. Yata, S. Yamamoto, J. Invest. Dermatol. 99 (1992) 278. [5] A.P.M. Lavrijsen, J.A. Bouwstra, G.S. Gooris, A. Weerheim, H.E. Bodde´, M. Ponec, J. Invest. Dermatol. 105 (1995) 619. [6] P.W. Wertz, W. Abrahamm, L. Landmann, D.T. Downing, J. Invest. Dermatol. 87 (1986) 582. [7] L.J. Shapiro, R. Weiss, D. Webster, J.T. France, Lancet I (1978) 70. [8] E. Zettersten, M.Q. Man, J. Sato, M. Denda, A. Farrell, R. Ghadially, M.L. Williams, K.R. Feingold, P.M. Elias, J. Invest. Dermatol. 111 (1998) 784. [9] P.W. Wertz, D.T. Downing, J. Lipid Res. 25 (1984) 1320. [10] P.W. Wertz, in: O. Braun-Falco, H.C. Korting, H. Maibach (Eds.), Liposome Dermatics (Griesbach Conference), Springer, Heidelberg, 1992, pp. 38 – 43. [11] D.T. Downing, W. Abraham, B.K. Wegner, K.W. Willman, J.L. Marshall, Arch. Dermatol. Res. 285 (1993) 151. [12] K. Miyajima, S. Tanikawa, M. Asano, K. Matsuzaki, Chem. Pharm. Bull. 42 (1994) 1345. [13] K. Yoneto, S.K. Li, A.H. Ghanem, D.A.J. Crommelim, W.I. Higuchi, J. Pharm. Sci. 84 (1995) 853. [14] K. Yoneto, S.K. Li, W.I. Higuchi, W. Jiskoot, J.N. Herron, J. Pharm. Sci. 85 (1996) 511. [15] T.M. Suhonen, L. Pirskanen, M. Raı¨sa¨nen, K. Kosonen, J.H. Rytting, P. Paronen, A. Urtti, J. Control. Release 43 (1997) 251. [16] N. Kamenda, M. El- Amrani, J. Appell, M. Lindheimer, J. Colloid Interface Sci. 143 (1991) 463. [17] M.A. Partearroyo, A. Alonso, F.M. Gon˜i, M. Tribout, S. Paredes, J. Colloid Interface Sci. 178 (1996) 156. [18] T. Inoue, Interaction of surfactants with phospholipid vesicles, in: M. Rosoff (Ed.), Vesicles, Marcel Dekker, New York, 1996, pp. 151– 195. [19] O. Lopez, A. de la Maza, L. Coderch, C. Lopez-Iglesias, E. Wehrli, J.L. Parra, FEBS Lett. 426 (1998) 314. [20] O. Lopez, M. Co´cera, R. Pons, N. Azemar, C. Lo´pezIglesias, E. Wehrli, J.L. Parra, A. de la Maza, Langmuir 15 (1999) 4678.

M. Co´cera et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 182 (2001) 15–23 [21] D. Lichtenberg, J. Robson, E.A. Dennis, Biochim. Biophys. Acta 821 (1985) 470. [22] A. de la Maza, M.A. Manich, L. Coderch, P. Bosch, J.L. Parra, Colloids Surf. A 101 (1995) 9. [23] A. de la Maza, J. Baucells, P. Gonzalez Ensen˜at, J.L. Parra, J. Colloid Interface Sci. 184 (1996) 155. [24] A. de la Maza, O. Lopez, L. Coderch, J.L. Parra, Colloids Surf. A 145 (1998) 83. [25] A. de la Maza, O. Lopez, M. Co´cera, L. Coderch, J.L. Parra, Chem. Phys. Lipids 94 (1998) 181. [26] R.G. Ackman, C.A. Mc Leod, A.K. Banerjee, J. Planar Chrom. 3 (1990) 450. [27] S. Almog, B.J. Litman, W. Wimley, J. Cohen, E.J. Wachtel, Y. Barenholz, A. Ben-Shaul, D. Lichtenberg, Biochemistry 29 (1990) 4582.

.

23

[28] P. Schurtenberger, N. Mazer, W. Ka¨nzig, J. Phys. Chem. 89 (1985) 1042. [29] J. Ruiz, F.M. Gon˜i, A. Alonso, Biochim. Biophys. Acta 937 (1988) 127. [30] K.P. Wilheim, C. Surber, H.I. Maibach, J. Invest. Dermatol. 97 (1991) 927. [31] K.P. Wilheim, C. Surber, H.I. Maibach, J. Invest. Dermatol. 96 (1991) 963. [32] M. Scha¨fer-Korting, in: O. Braun-Falco, H.C. Korting, H. Maibach (Eds.), Liposome Dermatics (Griesbach Conference), Springer, Berlin, 1992, pp. 299– 307. [33] A. de la Maza, J.L. Parra, Biophys. J. 72 (1997) 1668. [34] A. de la Maza, J.L. Parra, Biochem. J. 303 (1994) 907. [35] M.S. Holt, E.H. McKerrell, J. Perry, R.J. Watkinson, J. Chromatogr. 362 (1986) 419.