Sustained release from reverse micellar solutions by phase transformations into lamellar liquid crystals

165

Journalof ControlledRelease, 23 (1993) 165-174 0 1993 Elsevier Science Publishers B.V. All rights reserved

COREL

0168-3659/93/$06.00

00800

Sustained release from reverse micellar solutions by phase transformations into lamellar liquid crystals CC. queller-G~ymann

and H.-J. Hamann

Institut ftir Pharmazeutische Technotogie der Technischen Universitiit Braunschweig, Braunschweig, Germany (Received 3 I July 199 2; accepted in revised form 23 September

1992 )

Reverse micellar solutions of either fen~profen acid (FH) or fenoprofen sodium salt ( FNa) in isopropylmyristate (IPM) were formed in presence of phospholipids (PL), Phosphohpon 100. Solubilization of water within the micelles is also possible. However, the increase of water content causes a change in shape and size and finally a phase transformation from the reverse micellar solution into a lamellar liquid crystal. The colloidal microstructure of the micelles was studied by small angle X-ray scattering (SAXS) and viscosimetry. Solubilization of the drug in its free acid form results in almost spherical micelles, while solubilization of drug in its sodium salt form results in cylindrical micelles. The lamellar liquid crystals which form on contact with aqueous media can be used for sustained release, as the diffusion coefficient of the drug within the liquid crystals is smaller by factor 100 than that within an oily solution. The apparent diffusion coefficient of the drug depends on the thickness of the liquid crystalline interface which is also influenced when either the free acid or the salt is solubilized in the system. Key words: Fenoprofen; Phospholipids; Reverse micellar solution; Liquid crystals; Sustained release

Lamellar liquid crystalline systems have been proposed as semisolid vehicles for topical administration of drugs [ 11. The diffusion coefficient of a drug within a liquid crystalline phase is about one to two orders of magnitude smaller than in solution [ 2 ] because liquid crystals have a highly ordered microstructure and an increased viscosity. For the same reasons, solutions can be handled more easily than semisolids. In order to control release it is sufficient Correspondence to: Prof. Dr. C. Mueller-Goymann, fdr Pharmazeutische sit& B~unschweig, schweig, Germany.

Institut Technologie der Technischen UniverMendelssohnstr. 1, D-3300 Braun-

if the drug solution transforms into a liquid crystalline system on contact with biofluids after application. An appropriate system for such a controlled release mechanism is an oily solution of reverse mixed micelles, consisting of phospholipids and drug which changes its microstructure on contact with aqueous media [ 31. The present ~ntribution deals with the phase transfo~ation of a reverse micellar solution into a liquid crystal, induced by addition of water. In addition the effect on drug release of the phase transformation is studied. As a model drug fenoprofen was chosen in its acid and sodium salt form, because even the drug itself is able to form liquid crystals in presence of water [ 4,5 1.

166

Structure I. Formula of fenoprofen

sodium salt.

Experimental Materials Fenoprofen acid (FH) and fenoprofen sodium salt (FNa) were prepared from fenoprofen calcium salt, which was provided by Eli-Lilly, Giessen, Germany. An aqueous solution of fenoprofen calcium was suspended in lN-HCl. The free acid was extracted with chloroform. After evaporating the organic solvent the oily drug had a refractive index of ng = 1.5742. In order to prepare the sodium salt, the free acid was combined with an equimolar amount of 1 N NaOH solution. During slow evaporation of water, FNa crystallized as dihydrate. As phospholipid (PL) Phospholipon 100 was used, which was provided by Nattermann, Cologne, Germany. Phospholipon 100 is purified chromatographically, and enriched up to a content of 95% phosphatidylcholine. It is suitable even for parenteral application. Isopropylmyristate (IPM) was used as oily vehicle. It was of commercial grade and was obtained from Henkel, Dusseldorf, Germany and Dragoco, Holzminden, Germany. In order to avoid convection the aqueous receptor compartment for the release experiments consisted of a hydrogel of high optical transparency. The gel was prepared with double-distilled water and Natrosol HX 250 as the gelling agent. Natrosol HX 250 is purified hydroxyethylcellulose of pharmaceutical grade and was provided by Hercules Powder, USA-Wilmington.

Methods The reverse micellar solutions were prepared by dissolution of a weighed amount of PL in IPM of 60°C. After cooling to room temperature the solutions remained clear and transparent up to a PL content of 60% (w/w). For water-containing

reverse micellar solutions a small amount of water was pipetted by a Hamilton-syringe to the oily solution of PL. Solubilization of the water took place immediately without stirring. For drug-loaded reverse micellar solutions either water-free or water-containing oily solutions were combined with either FH or FNa. Both the free acid and the salt were easily solubilized within the reverse micellar solutions. The systems were studied by polarized light microscopy (PLM) using a photomicroscope (Zeiss, type III, Oberkochen, Germany), crossed polarizers and a 1 plate in between. The colloidal microstructure and the structural parameters of the mesophases were determined by small angle X-ray scattering (SAXS) and small angle X-ray diffraction (SAXD). The equipment consisted of a Kratky camera KCLC (Paar, Graz, Austria), an X-ray generator Philips PW 1730 (Kassel, Germany ) , an X-ray detector OED-50 (Braun, Munich, Germany) and a multi-channel analyser MCA 8 100 of Canberra Electronics (Munich, Germany). The amount of fenoprofen released was analysed photometrically at 272 nm with a Shimadzu Spectrophotometer UV 160 (Duisburg, Germany) or in lower concentration with an isocratic reverse-phase HPLC method according to Levine and Caplane [ 61. Pump Spectroflow 400 and UV-detector Spectroflow 757 were from Kratos ( Weiterstadt, Germany). The viscosity of the reverse micellar solutions was determined in a Haake viscosimeter RV 100 with CV 100 measurement equipment (Karlsruhe, Germany ) . Release experiments of FH alone in IPM, FNa alone in IPM, FH in reverse micellar solution and FNa in reverse micellar solution were carried out in two different ways: i. A small donor chamber of 10 ml volume was separated by a membrane (cellulose filter) from an acceptor compartment containing either 577 ml or 628 ml of phosphate buffer solution of pH 7.4 as perfect sink. For each experiment between 3.7 and 4.2 g of the drug-loaded oily system was used. Only the acceptor compartment was stirred

167

at 400 rpm, as stirring of the donor compartment would have disturbed the formation of liquid crystals at the interface between membrane and donor. Samples of 500 ~1 volume were drawn from the acceptor at different times and replaced by fresh buffer solution. The drug content was determined by an HPLC method. ii. 1 ml of the drug-loaded oily system was placed on the surface of 2 ml of a hydrogel of 3.5% Natrosol in a 4 ml cuvette. The diffusion of the drug into the hydrogel was monitored spectrophotometrically at different times and at different distances from the interface between donor and acceptor compartment. For this purpose, the Shimadzu Spectrophotometer UV 160 was slightly modified by us in order to draw the cuvette through the UV-beam vertically. Also a special collimation slit was used in order to measure exactly the whole width of the cuvette with the gel. Thus, the mean drug concentration per volume element at different distances from the interface could be determined. Results and Discussion Reverse micellar solutions PL was dissolved in IPM by stirring and slight heating. The oily systems were transparent and isotropic up to a content of 60% PL. From 65% onwards, liquid crystalline and crystalline textures could be proved by PLM. At a low content of PL the viscosity of the oily systems did not differ significantly from pure IPM. From 20% of PL onwards, the viscosity increased considerably. Finally the systems transformed into isotropic oily gels [ 5 1. The oily solutions of low viscosity, e.g. 5% of PL, showed a considerable X-ray scattering in the small angle region [ 51, which is an indication of colloidal aggregates having a distinct electron density difference in comparison with the dispersion medium. Classical solutions of statistically distributed molecules would not show an Xray scattering, and solubilization phenomena would not occur either. Oily solutions of reverse micelles, however, have a high capacity for solu-

bilizing both polar and amphiphilic molecules. The effect of water solubilization within the reverse micelles of PL on viscosity is shown in Fig. la. Up to a content of 400 pm01 water/g of reverse micellar solution consisting of 30% PL and 70% IPM (both w/w), there was only a slight change of viscosity from 0.2 Pas to 3 Pas. Further addition of water increased the viscosity considerably and finally resulted in gel-like systems. The rheograms belonging to these systems are shown in Fig. 1b. By means of the yield value and plastic flow behaviour the rheograms reveal the gel character of the systems containing 680 and 720 pmol water per gram. Further addition of water beyond 720 pmol/g induced a phase transformation into an oily vesicle dispersion of reduced viscosity. This only occurred if the systems were stirred. In a contact preparation where water was carefully placed on top of the oily system, a lamellar liquid crystalline phase arose at the interface. The transformation from reverse micelles into a lamellar liquid crystal can be explained by a different degree of hydration of the phospholipid molecules and simultaneously by a change of shape of the hydrated molecules characterized by their packing parameter [ 7-91. The latter also determines the form of the micelles. From SAXS measurements the electron distance distribution was calculated by means of the method of indirect Fourier transformation developed by Glatter [ 10 1. Fig. 2a represents an electron distance distribution P(r) of two oily phospholipid soluT200- (a)

& ET150. lOO-

Fig. 1. (a) Dynamic viscosity of an oily solution of 30% PL and 70°h IPM (both w/w) at room temperature versus increasing amount of solubilized water per gram oily solution.

168

22.5. 20.

2.5 -, 4

6

16

12

20

24

26

32

S6

*4

(b) Rheograms of ternary systems of Fig. la: shear stress r versus shear rate D.

25,Opmollg 62,Spmollg

0

10

20

30

40

LECITHIN LECITHIN

SO

60

IN IPM IN IPM

70

80

’

PI

___

MI.4 : 25,O pmollg

LECITHIN

SO,0 pmol/g

WATER

IN IPM

r PI Fig. 2. (a) Electron distance distribution p(r) of an oily solution of 2% lecithin (PL) - and 5% lecithin (PL) ---- in IPM. (b) Electron distance distribution p(r) of a ternary system consisting of 25 pmol lecithin (PL) and 50 pmol water in IPM. The ternary system is a reverse micellar solution of rodlike associates.

tions of different phospholipid contents [ 11,121. The maximum of the distribution curve gives the most frequent electron distance r. For the reverse micellar solution of PL in IPM the most frequent r is 19 A. For an ideal sphere this would be the radius. The asymmetric shape of the electron distance distribution reveals an anisometric shape of the micelles. The length of the micelles can be determined from the intercept of the distribution curve with the abscissa; in the case of Fig. 2a this is 78 A. From this finding can be concluded that non-hydrated phospholipid molecules associate into ellipsoidal micelles of which the long axis (78 A) is about twice that of the shortaxis (38A). The micelles grow into long rods with increasing water content (Fig. 2b). The length of the micelles increases to 170 A, if 50 pmol/g water is solubilized within the reverse micelles. At the same time the viscosity increases because rods disturb the flow behaviour more than do spheres (Fig. la). The type of flow also changes from Newtonian to pseudoplastic (Fig. 1b ) . When the hydration of the phospholipid molecules is at maximum the shape of the phospholipid molecules is almost cylindrical and resembles palisades. These hydrated cylinders are stacked in

169

LECITHIN m

FENOPROFEN

-NA

Fig. 3. Molecular model of the association of phospholipid molecules in an oily IPM system. From left to right, top: reverse micelles of ellipsoidal shape transform into rodlike ones with an increasing amount of solubilized water! they finally transform into lamellar liquid crystals. Bottom: drug solubilization either in its free acid or sodium salt form causes a change in shape of the colloidal aggregates - with fenoprofen acid rodlike micelles transform into more spherical ones, with fenoprofen salt rodlike micelles transform into extremely long rods.

’ b!lr; 800

200

0

I ?‘_ ! l. ‘2 . .I . . l: .,, ..a :.:... *:. : $. * . . zp’ * . . _ . *_ .. * .e.; ., .+tj ._a.’ r, . a.. ::>q:;:‘. ,*: 1‘ l ::: . .‘I* n _>‘?y:“& .. ** * ‘_“ie l’.*$... * .. **.c”** .._:*-5, _ * ;i ,, .:~~.?.~“‘.~::.:;;-.;-‘;:r.. .*:‘:.” q:1.&.‘y.; a:. s .a.. .: ~++..y,.,‘:“:.* is : *.* ,,:.-. : 2 -. ,.. I :.... a. .. “.:, , . . . . .* . . .*.%

-

* loo

200

300

400

500

500

700

‘4 WO

channel

, Boo

number

Fig- 4. S*XD of an Oily gel consistingof 55’h PL and 45Oh IPM (both w/w) - intensity I of the scattered beam in counts per second versus channel number.

layers which form the lamellar liquid crystals (Fig. 3, top). The oily gel which forms in binary systems of

PL and IPM consists of close-packed reverse micelles of slightly anisometric shape. Fig. 4 represents a SAXD measurement of a binary system

170

‘;:

90. (a) 10,0X FENOPROFEN-HA

0

00.

70 .

5.0% FENOPROFEN-NA 60.

2.5% FENOPROFEN-NA

VEHICLE

TIME 77 0

[H]

(b)

40.

-----Q-_--O

02x 0

0

20

40

80

80

100

120

140

160

180

ycLR’R’oFEN

110% FENOPROFEN

200

220

TIME

[Ii]

Fig. 5. Thickness D of the lamellar liquid crystal at the interface between the reverse micellar solution and the hydrogel versus time for different concentrations of (a) fenoprofen sodium salt and (b) fenoprofen acid in the reverse micellar solution.

of 55% PL (w/w), showing that there is a long range order between the micelles. Whether the microstructure is cubic or not cannot be decided from the single interference in SAXD. Influence of drug solubilization on the phase transformation of reverse micellar solutions As described in a previous paper [ 5 1, ternary systems of FH, FNa and water form lamellar liq-

uid crystals at room temperature which mix homogeneously - at least from a microscopic point of view - with lamellar liquid crystals consisting of PL, IPM and water. From this finding solubilization, both of FH and FNa within reverse micellar solutions, was expected. Although the solubilization of either FH or FNa was possible, the resulting micellar systems differed in various aspects. Solubilization of the free acid within re-

171

yl.20 .

(b)

-

B

E 1.00 IAl . -

3OMlN

-

QOMIN

0

0.40

90MIN

-

160

YIN

-

1110

MIN

-

0.60

16OMIN

D

30 MIN

-4-

.

0.60

0.60

-

1200MIN

0,20

0 0

2

4

6

8

10

12

14

16 x

C-1 lyl1.20 E : g ~

1.00

Y

”

0.60

0.60

0

0 2

4

6

6

10

12

14

16

0

2

4

6

6

10

x Cm4

12

14 x

16

Cd

Fig. 6. Drug release into a hydrogel at different times - drug concentration versus increasing distance from the interface between hydrogel and preparation: (a) 0.248% fenoprofen sodium salt; Top: release from a reverse micellar solution, bottom: release from an oily suspension; (b) 0.2% fenoprofen acid, Top: release from a reverse micellar solution, bottom: release from an oily solution.

TABLE 1 Diffusion coeffkients 15)

Diffusion coeffkient

of the drug in oily IPM solution, in reverse micellar solutions and in lamellar liquid crystals (from refs. 4,

. 10’ cm2 s-’

Reverse micellar solution

Oily solution

FH

FNa

FH 1%

FH 10%

4.7

1.9

160

44

verse micelles of partially hydrated phospholipids resulted in a transformation of rod-like micelles into spheres, whilst the solubilization of the salt elongated the rod-like micelles drasti-

Lamellar liquid crystals FNa

1.8

tally (Fig. 3, bottom). An explanation could be the different degree of dissociation of the drug and in consequence a different degree of hydration, influencing the shape of the hydrated mol-

172

I

rmg1.

104

FENOPROFEN

0

10%

FENOPROFEN-NA

100

IN

REV

MIC

IN REV

SOL MIC

SOL

so60.

70 -

60-

50-

40-

30-

20-

0

20

40

60

60

100

120

140

160 bT

[s”‘l

Fig. 7. Amount of drug released from a reverse micellar solution versus square root of time. Drug diffusion takes place via lamellar liquid crystals at the interface between donor and acceptor compartment.

ecules. Hence the packing parameter within the micelles would also differ and the resulting shape of the micelles vary from long rods to almost spherical aggregates. While FNa seems to increase liquid crystal formation, FH has a destabilizing effect. In both cases further addition of water leads to a transformation into lamellar liquid crystals. However, the rate and the amount of liquid crystals arising was different, as shown in Figs. 5a and 5b for the layer thickness of the lamellar liquid crystals which formed at the interface between the reverse micellar solution and a hydrogel. With FH the formation of the lamellar liquid crystal was slower and occurred to a lesser extent than with FNa. Release experiments from reverse micellar solutions Figs. 6a and 6b show drug release from a reverse micellar solution into a hydrogel in comparison with drug release from an oily solution

of FH as well as drug release from an oily suspension of FNa into a hydrogel. It is obvious at each specific time and distance from the interface that drug release from the reverse micellar solution via the lamellar liquid crystal at the interface is slower than from pure IPM solution or suspension. As for drug release of FH and FNa from the reverse micellar solution, the diffusion of FNa is slower than that of FH. There are several reasons for this phenomenon. 1. The liquid crystalline layer, which slows down diffusion, grows faster and to a higher extent with FNa than with FH. Assuming a dependence of diffusion on layer thickness, the diffusion rate must be minimal with FNA because of the enlarged thickness of the lamellar liquid crystal. 2. The solubilization of the drug within the lamellar liquid crystal takes place via Van der Waals-London interactions for both FH and FNa and in addition via polar interactions in case of FNa. Since polar interactions are stronger than hydrophilic ones, diffusion of FH is faster than that of FNa. Since the calculation of diffusion coefficients was not available from drug diffusion experiments into the hydrogel, another set of release experiments was performed. For that purpose the reverse micellar solution was separated from an aqueous buffer of pH 7.4 by a simple cellulose filter. This filter allowed diffusion of water from the acceptor compartment into the donor compartment, and thus transformation of the reverse micellar solution into a lamellar liquid crystal. Fig. 7 shows the amount of drug released from the reverse micellar solution versus square root of time. According to Higuchi the diffusion coefficients were calculated from the slopes [ 13 ] (Table 1). While the diffusion coefficient of FH (4.7.lo-* cm2 s-l ) is slightly higher than that of FNa ( 1.9*1O-8 cm2 SK’), both are lower by factor 10 at least than the diffusion coefficient of FH in oily solution, which is 4.4. lo-’ cm2 s-l for 10% FH and 1.6*10-6 cm2 s-l for 1% FH. Carbonic acids tend to associate into dimers in lipophilic solvents [ 141, hence diffusion rate decreases with increasing concentration of drug. Since the diffusion coefficient of FNa from the

173

c

pB]

(4

150

-

3h

____

6h

DONATOR

ACCEPTOR

I REV. MIC.

SOL

HVDR

so-

SOL

I

- 2.0

- 1.2

-1.6

- 0.6

(b)

-

3h

e-s.

6,,

- 0,4

0

DONATOR

ACCEPTOR

SINK

50

REV. WC.

SOL

a

0

-0s

3

____

3h 6h

DONATOR

ACCEPTOR

ioa

50

SINK

REV WC.

HVDR

SOL

1

1

wI

- 1.6

SOL

-1.2

- 0.8

- 0.4

0 x

bml

Fig. 8. Apparent concentration gradient of the drug in a reverse micellar solution after 3 and 6 h on contact with an aqueous medium and liquid crystals growing at the interface: (a) 1O”Yo fenoprofen sodium salt, (b) 10% fenoprofen acid, (c ) lo/afenoprofen acid.

174

reverse micellar solution is of the same order of magnitude as the diffusion coefficient of FNa within a pure lamellar liquid crystal (1.8. lo-’ cm2 s- ’ ) we conclude that diffusion is controlled by the lamellar liquid crystal at the interface alone. We propose the model of Fig. 8a simulating both the apparent concentration gradient of FNa within the reverse micellar solution and the liquid crystalline interface at different times. As can be seen in Fig. 8a, the region of decreasing FNa content is limited to the liquid crystalline layer. However, the same situation does not hold for FH, as can be seen in Fig. 8b. The lamellar liquid crystal does not grow that far into the reverse micellar solution. Hence the region of decreasing FH content is much broader and reaches into the reverse micellar solution. This means that the diffusion rate is not only limited to the concentration gradient within the liquid crystal but is also influenced by the concentration gradient within the micellar solution. Since the latter is higher than the former, the overall apparent diffusion coefficient of FH should exceed that of the drug within pure lamellar liquid crystals. The same holds for 1% FH although the liquid crystalline interface is thicker than for 10% FH (Fig. 8~). In general, sustained release by a liquid crystalline interface can be verified. It must be kept in mind that the solubilized drug also influences the degree of sustained release.

Support by the Deutsche Forschungsgemeinschaft (Mu 684/2-3 ) is gratefully acknowledged. References 1

2

3

4

5

6

7

8

9

Conclusion

10

Drug release can be slowed down by lamellar liquid crystals, growing at the interface on contact with aqueous media. Sufficient access of water however has to be guaranteed. This is the reason why the application of these systems on dermis does not make sense. Intramuscular application or application as prolonged released eye-drops, however, is a promising possibility. Further studies are in progress to prove the suitability of this way of application.

11

Acknowledgements We would like to thank the Institut of Botanics for the availability of the TEM.

12

13 14 15

S. Walgren, A.L. Lindstrom and S. Friberg, Liquid crystals as a potential ointment vehicle, J. Pharm. Sci. 73 (1984) 1484. C.C. Mueller-Goymann and S.G. Frank, Interaction of lidocaine and lidocaine-HCl with the liquid crystal structure of topical preparation, Int. J. Pharm. 29 ( 1986) 147. H.-J. Hamann and C.C. Mueller-Goymann, Lyotroper Mesomorphismus von Arzneistoffen unter besonderer Beriicksichtigung der Profene, Acta Pharm. Technol. 33 (1987) 67. H.-J. Hamann, Wechselwirkungen mesogener Arzneistoffe mit kolloidalen Lecithinassoziaten am Beispiel des Fenoprofens, Thesis TU Braunschweig ( 1990). H.-J. Hamann and C.C. Mueller-Goymann, Wechselwirkungen des mesogenen Arzneistoffs Fenoprofen mit einem fltissig-kristallinen Lecithinvehikel, Acta Pharm. Technol. 35 (1989) 121. B.S. Levine and Y.H. Caplan, Simultaneous liquidchromatographic determination of live nonsteroidal anti-inflammatory drugs in plasma or blood, Clin. Chem. 31 (1985) 346. J.N. Israelachvili, D.J. Mitchell and B.W. Ninham, Theory of Self-Assembly of Hydrocarbon Amphiphiles into Micelles and Bilayers, J. Chem. Sot. Far. Trans. II 72 (1976) 1525. J.N. Israelachvili, D.J. Mitchell and B.W. Ninham, Theory of self-assembly of lipid bilayers and vesicles, Biochim. Biophys. Acta 470 (1977) 185. J.N. Israelachvili, S. Marcelja and R.G. Horn, Physical Principles of Membrane Organization, Q. Rev. Biophys. 13 (1980) 121. 0. Glatter, In: Small angle X-ray scattering, Academic Press, London, 1982, 167. (eds): Glatter, O., Kratky, 0. H.-J. Hamann, C. Bubel, CC. Mueller-Goymann and C. Fuehrer, Poster at APV-Kurs Nr. 477: Der Norden dreht auf: Die norddeutschen Universitiiten prlsentieren ihre industrie-relevante galenische Forschung, Kiel, 9.6.1989 EinfluB amphiphiler Arzneistoffmoleklile auf Struktur und Viskositat lecithinhaltiger Organogele. C. Bubel and C. Fuehrer, Rontgenkleinwinkeluntersuchungen an inversmizellaren Systemen, Acta Pharm. Technol. 36 ( 1990) 16s. W.I. Higuchi, Analysis of data on the medicament release from ointments, J. Pharm. Sci. 51 ( 1962) 802. R.T. Morrison and R.N. Boyd, Lehrbuch der organischen Chemie, Verlag Weinheim 1978, p. 638. H.-J. Hamann and C.C. Mueller-Goymann, 5th Int. Conf. Pharmaceut. Technol., Paris 1989, vol. 1, p. 99109. Incorporation and diffusion of mesogenic drug molecules in a liquid crystalline lecithin vehicle as demonstrated with fenoprofen.