EFFECT OF RHEOLOGICAL PARAMETERS ON PHARMACEUTICAL AVAILABILITY OF KETOPROFEN FROM HYDROGEL PRODUCTS MADE ON CARBOPOL BASE Marian Mikołaj Zgoda, Justyna Kołodziejska Department of drug Form Technology, Applied Pharmacy Faculty Medical University of Lodz Summary Rheological parameters were tested in market hydrogel products of anti-inflammatory activity. Their prescription was worked out on Carbopol base. The rate of volatile components loss from pharmaceutical preparations was estimated with gravimetric method, while their viscosity parameters (structural viscosity, yield stress, the area of hysteresis loop) were determined with cone-plate digital rheometer. The kinetics of the therapeutic agent (ketoprofen) release through a standard Viscing membrane to external compartment was tested in hydrogels in vitro. The aim of the study was to determine the relationship between rheological parameters dependent on the prescription of the tested hydrogels and pharmaceutical availability of ketoprofen. Pharmaceutical preparations of anti-inflammatory activity made on Carbopol base are non-newtonian liquids diluted with shear. Due to ethanol content in their prescription, after application on skin, the increase of structural viscosity and the decrease of ketoprofen pharmaceutical availability accompany the process of volatile components loss from the surface layer of the preparation. Ketoprom hydrogel, having propylene glycol in its composition, is characterized by higher values of viscosity, hysteresis pool area and yield stress than Fastum preparation. Insignificant differences between experimentally determined in vitro areas under the curves of ketoprofen release are associated with rheological parameters of the investigated hydrogels, which was anticipated on the basis of Einstein-Smoluchowski s equation. Ketoprofen penetration from the tested pharmaceutical products on Carbopol base to the external compartment is in accordance with kinetics of 0 order and is diffusion-controlled.
Key words: Carbopol, ketoprofen, Fastum, Ketoprom, volatile components, viscosity, pharmaceutical availability INTRODUCTION Polyacrylic acid (carboxypolymethylene, carboxyvinyl polymer, carbomer), marketing name: Carbopol, is a universal adjuvant substance applied in current pharmaceutical practice. The direction of application of particular market products under the name Carbopol is conditioned by physicochemical properties formed by mean molecular weight dependent on the number of side chains in polyacrylic acid structure [1]. Carbopols are used for condensing solutions, stabilizing suspensions and producing tablets and capsules. Due to adhesive properties they can be applied in working out nasal, subbucal, intra-uterine and intrarectal forms of drugs [2-4]. In recent years, particularly popular direction of Carbopols application is their introduction into the prescription of pharmaceutical preparations in the form of hydrogel of water or water-ethanol dispersion [4-6]. The aim of this study is to estimate rheological parameters (viscosity, kinetics of volatile components loss) of market hydrogel products of anti-inflammatory activity, the prescription of which was worked out on the base of Karbomer 940 (Carbopol 940 or Carbopol 980). Furthermore, this study aims at determining the relationship between physicochemical properties resulting from the prescription of the tested preparations and pharmaceutical availability of the anti-inflammatory therapeutic agent (ketoprofen). MATERIAL AND METHODS Reagents: -Fastum, batch: 44418, exp. date:10. 2009, (Fastum-1), -Fastum, batch: 51007, exp. date: 12. 2009, (Fastum-2), -Fastum, batch: 51009, exp. date: 12. 2009, (Fastum-3), -Ketoprom, batch: 4004, exp. date: 04. 2007, (Ketoprom-1), - Ketoprom, batch: 4005, exp. date: 04 2007, (Ketoprom-2), - Ketoprom, batch: 4006, exp. date: 04. 2007, (Ketoprom-3). 2
Apparatus: - cone-plate digital rheometer DEV-III-Brookfield, version 3.1 with Rheocalc for Windows software; - bath thermostat PGWE1, Medingen; - spectrophotometer Nicolet Evolution 300, version 1.0, Spectro-Lab; - technical balance, Precision Engineering Plant Radwag. Determination of viscosity of the tested hydrogels [7, 8]. Viscosity determinations of hydrogels were performed at 37 0 C with cone-plate digital rheometer, Brookfield DV-III, version 3,0 connected with bath theromostat PGWE Medingen. Determination of the rate of volatile components loss from hydrogels containing ketoprofen The determination of the rate of volatile components loss was performed from the glass plates (Petri dish) of 55 mm diameter, which were covered with uniform layer of hydrogel. The plates were placed in a theromostat at 37±0,1 0 C with gravitational air circulation and the sample mass was determined after every 30 min. Testing of the kinetics of therapeutic agent (ketoprofen) release from hydrogels through a dialysis membrane to model dialysis fluid [9] The rate of ketoprofen diffusion from hydrogel preparations was tested with the technique applied for transdermal therapeutic systems (TTS) according to the requirements of European Pharmacopoeia, USP XXIV and British Pharmacopoeia. The way of determination The niche of a dialysis container of the volume V = πr2h = 19,625 cm 3 (2r = 5,0 cm, h = 1,0 cm) and the area of exchange Pw = πr 2 = 19,625 cm 2 was filled full, using a dispenser with the tested preparation. Then, the surface of the preparation was covered with a prepared dialysis membrane (Viscing Dialysis Tybing C/150 of wall thickness 0,1 mm and declared pores diameter 25Å). The container cover was plugged by screwing in to perceptible resistance of the nut. The prepared in this way dialysis container was placed 3
into a dish with 1000 cm 3 of distilled water. The solution over the container was stirred mechanically at the rate not greater than 35-40 rpm. The rate of the process of mass exchange was investigated determining the amount of ketoprofen diffusing onto dialysis fluid at the same time intervals by spectrophotometric method. Approximation equation at p=0,05 and r 0,9965 : A = 0,6411c + 4,2789. 10-2, with which the dependence between absorbance (A) and therapeutic agent concentration (c) was described, transformed to the form: c = A 4,2789. 10-2 /0,6411 enabled to determine the amount of ketoprofen diffusing through the phase boundary in the time function t (min). RESULTS AND DISCUSSION Flow curves (dependence of shear stress on shear rate) of Fastum and ketoprom determined at 37 0 C are presented in figure 1. Viscosity measurements demonstrated that the tested hydrogels containing ketoprofen are non-newtonian systems flow curves of which are not straight lines and do not cross the start of co-ordinate system (Fig. 1). Shear stress of these systems increases more slowly than linearly together with the increase of the shear rate (liquids diluted with shear) [10, 11]. All the tested preparations are viscoelastic liquids having experimentally determined yield stress. They demonstrate a tendency to flow after exceeding a certain boundary shear stress and at lower stress they behave like elastic solid bodies. Yield stress was determined describing the shear stress-shear rate dependence in the form of a flow curve by Casson s mathematical model (with Rheocalc for Windows program). It is a rheological model recommended for description of flow curves of non-linear viscoelastic liquids [12]. Casson s model is described by the following formula: τ = τ o + ηγ, where: τ shear stress, τ o yield stress, η plastic viscosity, γ shear rate. Parameters of this model for Fastum hydrogel are presented in table 2, while for Ketoprom hydrogel in table 3. 4
In the case of Ketoprom, the values of boundary shear stress, at which the preparation starts to flow are slightly higher (62,4-63,9 N/m) than for hydrogel Fastum (50,2-61,3 N/m). This proves more rigid structure of Ketoprom, which is associated with the content of propylene glycol in its prescription (transferase cross-linking of Carbopol hydrogel structures). However, on the other hand, lower values of Fastum hydrogel yield stress facilitate spreading of the preparation on pathologically changed tissue. Gel does not remain long on the surface of its application and the use of slight strength of shear causes exceeding of the boundary value and flowing of the preparation [8, 12]. Interpretation of viscosity parameters at 37 0 C leads to anticipation of pharmaceutical availability of the therapeutic agent (ketoprofen) from the tested hydrogels. High pharmaceutical availability of the therapeutic agent is explained by relatively low viscosity of systems (37 0 C) with preservation of hysteresis loop of not large area [13, 14]. On the basis of viscosity measurements carried out at 37 0 C, theoretical value of ketoprofen diffusion coefficient was calculated from the tested gels to the external compartment. Einstein-Smoluchowski s equation was used in the form: kt D =, 6πrη where: D diffusion coefficient, k Boltzman constant, T temperature in Kelvin scale r observed radius of ketoprofen particle, η viscosity [15]. Viscosity determined at three freely selected shear rates and calculated values of diffusion coefficients of ketoprofen for Fastum gel are presented in table 4, while for Ketoprom gel in table 5. 5
At shear rate 0,4 l/s Fastum viscosity is within the range 147112-186375 mpa. s and is lower than Ketoprom viscosity: 190351-212716 mpa. s. Thus, theoretically calculated ketoprofen diffusion coefficients from Fastum hydrogel are higher than those from Ketoprom. At shear rate 0,4 l/s for Fastum they are: 6,1580-7,8015. 10-22 m 2 /s, while for Ketoprom: 5,3954-6,0293. 10-22 m 2 /s. Similar dependences were also observed at other shear rates. With very much alike prescription compositions of the tested hydrogels, which is particularly associated with the application of Carbopols of similar physicochemical parameter, insignificant differences in the values of viscosity probably result from the use of propylene glycol in Ketoprom prescription. The above also finds confirmation in the results of a viscosity test determined as hysteresis loop test. It is a traditional qualitative test for the presence of thixotrophy [14]. It is performed by increasing continuously shear rate to a certain maximal value and then immediately decreasing it. Due to destruction of the preparation structure, which takes place in the course of the experiment, specific flow curves are obtained with traditional hysteresis loop. A hypothetical hysteresis loop obtained for Ketoprom-1 preparation is presented in figure 2. Positive thixotrophy was observed for all the tested hydrogels. In the conditions of isothermal flow of the liquid, which previously was for a longer time at rest, at constant shear rate, the shear stress decreased for these systems reversibly to the passing time. Hysteresis loop ascending and descending curves were described with correlation equation and the areas under these curves as well as the area drawn by hysteresis loop expressed in conventional units (c.u.) were calculated with integration method. The obtained results for Fastum preparations all tested series are demonstrated in table 6, while for Ketoprom all tested series in table 7. Hysteresis pools determined at 37 0 C for Fastum preparations are characterised by smaller areas than hysteresis loops obtained for Ketoprom preparations. The values of the areas drawn by Fastum hysteresis loops are within the limits: 2,1412-4,6510 c.u., while those for Ketoprom are: 4,3213-7,2896 c.u. The kinetics of the rate of volatile components loss by Fastum hygrogels are presented in figure 3, while by Ketoprom hydrogels in figure 4. 6
The course of the above dependences was described by correlation equation, which enabled to use integration method to calculate the areas under the curves of the rate of volatile components loss by the tested hygrogels, expressed in c.u. The results of the calculation are presented in tables 8 and 9.. It results from the above comparison that Fastum and Ketoprom lose effectively their volatile components during exposure (P(c.u.)/m o = 13,38-19,66), which is associated with ethanol content in the prescription [15]. After application on skin, the process of volatile components loss from the surface layer of the preparation is accompanied by the increase of hydrogel structural viscosity (D=kT/6πrη). During exposure, the rate of ketoprofen diffusion (penetration) into external compartment decreases and thus, the therapeutic agent pharmaceutical availability decreases. The results of the performed in vitro studies on the rate of therapeutic agent release from the tested hydrogels to dialysis fluid are presented in figure 5, as the dependence between the quantity of the released ketoprofen (mg) and the time of release (min) and in figure 6 as the dependence between the quantity of the released ketoprofen (mg) and the square root of the time of release t 1/2 (min 1/2 ). Rectilinear dependence between the quantity of the released substance (M) and the square root of the release time (t 1/2 ) resulting from Higuchi s equation: M = K t proves that the release of ketoprofen from the tested hydrogels is diffusion controlled [16]. The straight lines presented in figure 6 cross the axis of the time square root in a certain distance from the start of co-ordinate system, which points to the existence of the time of delayed release t o depending on the prescription components and physiochemical properties of the tested hydrogels [17]. The course of dependences between the quantity of the released ketoprofen (mg) and the time of release t (min) and between the quantity of the released ketoprofen (mg) and the square root of the release time t 1/2 were described with correlation equations which enabled to use integration method to calculate the areas under the release curves in conventional units (c.u.). The results of the calculations are presented in table 10. 7
It results from approximation equations, presented in table 10, that the course of ketoprofen penetration from the tested pharmaceutical products (Fastum, Ketoprom) through a standard Viscing membrane to the external compartment is in accordance with the kinetics of 0 order. High correlation coefficients r obtained for the equation of the type: y = ax + b at p = 0,05 prove that. The calculated values of the areas under ketoprofen release curves (c.u.) point to slight differences in its pharmaceutical availability from the tested hydrogels. The area under the curve of ketoprofen release from Fastum preparation (dependence in the function of release time) is 6544,86 c.u. and is insignificantly higher than the value obtained for Ketoprom hydrogel (6291,76 c.u.) having higher viscosity. CONCLUSIONS 1. The tested pharmaceutical preparations of anti-inflammatory activity, produced on Carbopol base, are non-newtonian liquids diluted with shear for which shear stress increases more slowly than linearly together with the increase of the shear rate. Ketoprom hydrogel has higher values of viscosity, the area of hysteresis loop and yield stress than Fastum, which results from the content of propylene glycol in Ketoprom prescription. 2. The tested hydrogels lose effectively the volatile components in the course of exposure (P(c.u./m o = 13,38-19,66), which is associated with the content of ethanol in their prescription. After application on skin, the process of volatile components loss from the surface layer of the preparation is accompanied by the increase of hydrogel structural viscosity (D = kt/6πrη) and ketoprofen pharmaceutical availability decreases. 3. Ketoprofen penetration from the tested pharmaceutical products (Fastum, Ketoprom) through a standard Viscing membrane to the external compartment is in accordance with kinetics of 0 order. The course of the dependence between the quantity of the released ketoprofen and the square root of the time of release (Fig. 6) points to diffusion-controlled process. 8
4. The theoretical coefficient of ketoprofen diffusion, calculated with the use of Einstein-Smoluchowski s equation found confirmation in the carried out in vitro studies on the kinetics of therapeutic agent penetration into external compartment through a Viscing type membrane. Insignificant differences between the experimentally determined areas under the curves of ketoprofen release from Fastum and Ketoprom are associated with rheological parameters of the tested hydrogels (structural viscosity, yield stress, area of hysteresis loop, rate of volatile components loss). 9