Monitoring Lidocaine Single-Crystal Dissolution by Ultraviolet Imaging

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1 RESEARCH ARTICLE Monitoring Lidocaine Single-Crystal Dissolution by Ultraviolet Imaging JESPER ØSTERGAARD, FENGBIN YE, JUKKA RANTANEN, ANAN YAGHMUR, SUSAN WENG LARSEN, CLAUS LARSEN, HENRIK JENSEN Department of Pharmaceutics and Analytical Chemistry, Faculty of Pharmaceutical Sciences, University of Copenhagen, Universitetsparken 2, DK-2100 Copenhagen, Denmark Received 25 November 2010; revised 13 January 2011; accepted 9 February 2011 Published online in Wiley Online Library (wileyonlinelibrary.com) ABSTRACT: Dissolution critically affects the bioavailability of Biopharmaceutics Classification System class 2 compounds. When unexpected dissolution behaviour occurs, detailed studies using high information content technologies are warranted. In the present study, an evaluation of real-time ultraviolet (UV) imaging for conducting single-crystal dissolution studies was performed. Using lidocaine as a model compound, the aim was to develop a setup capable of monitoring and quantifying the dissolution of lidocaine into a phosphate buffer, ph 7.4, under stagnant conditions. A single crystal of lidocaine was placed in the quartz dissolution cell and UV imaging was performed at 254 nm. Spatially and temporally resolved mapping of lidocaine concentration during the dissolution process was achieved from the recorded images. UV imaging facilitated the monitoring of lidocaine concentrations in the dissolution media adjacent to the single crystals. The concentration maps revealed the effects of natural convection due to density gradients on the dissolution process of lidocaine. UV imaging has great potential for in vitro drug dissolution testing Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci Keywords: crystals; dissolution; dissolution rate; imaging methods; natural convection; UV/Vis spectroscopy; UV imaging INTRODUCTION Dissolution is frequently the limiting or ratecontrolling step in drug absorption of poorly watersoluble drugs. 1 4 The commonly used in vitro dissolution methods including rotating disc, USP (United States Pharmacopeia) paddle and basket require relatively large amounts of drug substance and/or dissolution media. A current trend in drug development is to explore the physical properties of active compounds using high-throughput technologies and to use the identified solid forms (salt, polymorphic, solvate, co-crystal and amorphous) as a central part of product development. 5 Thus, micro-dissolution techniques requiring only a few milligrams of material, which may be of particular interest in early drug development, have been developed to overcome limita- Correspondence to: Jesper Østergaard (Telephone: ; Fax: ; joe@farma.ku.dk) This article contains supplementary material available from the authors upon request or via the Internet at Journal of Pharmaceutical Sciences 2011 Wiley-Liss, Inc. and the American Pharmacists Association tions related to dissolution testing. 6 8 In the abovementioned methods, the concentration of dissolved substance is measured as a function of time in the bulk release media. Alternatively, for example, in the case of unexpected dissolution behaviour, more detailed information might be gained by monitoring the dissolution process immediately next to the surface of a small amount of the dissolving material. Imaging techniques capable of providing spectrally, spatially and temporally resolved information are emerging tools in more detailed studies of drug dissolution due to their potentially high information content Ultraviolet (UV) imaging technology, which has recently become commercially available, may constitute an alternative and complementary technology to, for instance, Fourier transform infrared and magnetic resonance imaging. UV imaging has the ability to generate visual images from simultaneous spectroscopic, spatial and time data. With UV imaging, it is possible to measure the intensity of light in the UV range passing through an area of a quartz tube as a function of position and time. 16 Thus, UV imaging may facilitate quantification of drug substances 1

2 2 ØSTERGAARD ET AL. in solution immediately adjacent to the solid material and recording of concentration maps, spatially and temporarily resolved. Detection in the UV range is suitable for most drug substances; UV imaging will therefore be a widely applicable format for conducting advanced drug dissolution studies in drug development. For instance, monitoring dissolution at the aqueous solid interface of a crystal may provide information on the rate-limiting steps of the dissolution process. 17 It has been shown that detailed insights into the dissolution processes, for example, face-specific/dependent dissolution, may be attained from dissolution studies performed on single crystals. Such studies are, however, at present technically challenging. In addition, measurements of concentration gradients in the vicinity of the solid surface may lead to knowledge on the relative importance of diffusion and convective currents to dissolution rates. To this end, the influence of natural convection caused by density gradients on drug dissolution has been demonstrated Furthermore, dissolution studies conducted under conditions with little or no convective currents due to flow or agitation may also be of relevance, for example, crystal suspensions intended for parenteral administration, that is, subcutaneous, intra-muscular or intra-articular administration routes. The objective of the present investigation was to perform an evaluation of the applicability of a commercially available UV imaging detector for conducting dissolution studies. Using lidocaine as a model compound, the aim was to develop a simple setup for measuring the dissolution of lidocaine from a single crystal into aqueous buffer. Dissolution of lidocaine into M phosphate buffer (ph 7.40) was selected as an initial model system for UV imaging due to previous experiences related to intra-articular drug delivery, 25 the high aqueous solubility and associated rapid dissolution allowing short experimental times. MATERIALS AND METHODS Chemicals and Reagents Lidocaine (Ph Eur (European Pharmacopoeia) 6th ed.) was obtained from Unikem, Copenhagen, Denmark. Lidocaine single crystals for the dissolution experiments were obtained from the recrystallisation of lidocaine in n-hexane. 26 Both the starting material and recrystallised lidocaine were identified to be monoclinic (P2 1 /c) lidocaine, using X-ray powder diffractometry. 27 Sodium dihydrogenphosphate monohydrate was obtained from Merck (Darmstadt, Germany). The dissolution medium used was M sodium phosphate, ph Purified water from a Milli-Q deionisation unit (Millipore, Bedford, Massachusetts) was used throughout the study. Figure 1. crystals. Scanning electron micrographs of lidocaine UV Imaging Setup UV imaging was performed using an Actipix SDI300 dissolution imaging system (Paraytec Ltd., York, United Kingdom) with Actipix flow-through type dissolution cartridges (supporting information Fig. 1). A syringe pump was used for the infusion of the dissolution medium and the lidocaine standard solutions. The total detection area of the UV imaging system is 9 7mm 2 ( pixels); however, the selected imaging area was mm 2. The pixels (7 7 :m 2 ) were binned 4 4. The light source is a pulsed Xe lamp, and imaging was performed at 254 nm. The quartz dissolution cell [ mm 3 (height width length)] contained approximately 0.56 ml of dissolution media with the flow-through inserts in place. Images were recorded ( images per second) and analysed using Actipix D100 software version 1.3 (Paraytec Ltd. York, United Kingdom). Pixel intensities were converted into absorbance using the Actipix software, 16 allowing the concentration of lidocaine within the imaging area as a function of position and time to be determined by the use of a calibration curve. The procedures for constructing calibration curves were similar to those reported previously. 16

3 MONITORING LIDOCAINE SINGLE-CRYSTAL DISSOLUTION BY UV IMAGING 3 Dissolution Studies The procedure for UV imaging was as follows: dark images (lamp turned off; 10 s) and reference images (10 s) were recorded with the dissolution cell filled with the M sodium phosphate buffer (ph 7.40). After 60 s, data collection was paused and the lidocaine single crystal was mounted in the dissolution cell. Then, data collection was resumed and the dissolution cell with the crystal in place was filled with the dissolution buffer delivered by the syringe pump at a flow rate of 0.5 ml/min. The flow was arrested upon complete filling of the cell, whereas recording of UV images was continued for 5 15 min. The single crystals were oriented differently (horizontally or vertically) by turning the entire UV imaging sensor head because the crystals could only be mounted in one defined manner inside the dissolution cell. Experiments were conducted at ambient temperature (19 C 23 C). Calibration curves were constructed by flowing lidocaine standard solutions (concentration range: M) through the dissolution cell at a flow rate of 1 ml/min. The amount of lidocaine dissolved was determined by averaging absorbance values over the imaging area. From the increase in average absorbance, the molar absorptivity and the volume element (area light path), the cumulated amount of lidocaine dissolved was calculated. In the calculation of the amount of dissolved lidocaine, it is a premise that the UV absorbance for each pixel read is within the linear range (Beer s law is valid) and that lidocaine does not diffuse out of the imaged volume element. Scanning Electron Microscopy The lidocaine crystal samples were analysed using a scanning electron microscope (JSM 5200; JEOL, Tokyo, Japan). An acceleration voltage of 10 kv and a working distance of 20 mm were used. The samples were mounted on a double-faced adhesive tape and sputter coated for 120 s, with a thin gold palladium layer in an Auto sputter coater (E5200; Bio-Rad, Watford, England). RESULTS AND DISCUSSION Figure 1 shows scanning electron micrographs of the lidocaine crystals. Needle-shaped single lidocaine crystals, mm in length, were placed in the insert and mounted in the dissolution cell for UV imaging (supporting information Fig. 1). The lidocaine crystals were oriented horizontally or vertically in the dissolution cell (0.56 ml) and surrounded completely by dissolution medium except at the base. Upon insertion of the lidocaine crystal, the dissolution cell was filled (0.5 ml/min) with M phosphate buffer, ph 7.4, at ambient temperature (19 C 23 C). The flow was stopped while recording the UV images. Figure 2 shows (in false colour) dissolution of lidocaine into the cell volume in the form of a UV absorbance map obtained at 254 nm. Figure 2a shows the cell immediately after filling with phosphate buffer. The lidocaine crystal positioned vertically is apparent as the light yellow zone. In Figures 2a 2c, the dissolution of lidocaine is apparent as an increase in absorbance (increasing the intensity of the yellow and red colour). Figure 2 shows that the increase in absorbance is most predominant in the lower part of the cell. Finally, dissolved lidocaine was removed by applying a flow of buffer (0.5 ml/min), leaving only the signal due to the presence of the crystal (Fig. 2d). A video clip of the lidocaine dissolution process can be found in the supporting information. The obtained UV observations reveal that the maximum absorbance is not detected at the position of the lidocaine crystal as might have been expected. This is due to the fact that light coming from the fibre optics is striking the sensor surface behind the crystal. The lidocaine crystal is positioned at approximately 3 mm in front of the sensor chip. The left-to-right linear arrangement of the optical fibres allows light, emitted at different angles, to reach the sensor chip behind the crystal. Figure 3 depicts a UV image from the dissolution experiment (after 5 min) in which the lidocaine crystal was oriented horizontally. From inspection of Figures 2 and 3, it is readily apparent that the dissolved lidocaine distributes primarily to the lower part of the dissolution cell. A small edge effect is observed in Figures 2 and 3 (at the base of the crystal). This is due to mounting of the crystal that was performed by simply inserting the crystal into a small hole drilled in the support. The relatively large increase in absorbance at the base is due to the lidocaine dissolving in the (excess) buffer found in this hole when this space was not fully taken up by the crystal. Additional singlecrystal dissolution experiments with the crystal and cell oriented differently confirmed that the highest absorbance (concentration) always occurred in the lower part of the cell. Diffusion of dissolved lidocaine away from the crystal surface is expected to provide a symmetrical concentration distribution around the crystal in the stagnant liquid. However, convection due to density gradients may, in addition to diffusion, affect the concentration distribution upon dissolution of lidocaine from the crystal. The importance of natural convection and density gradients on dissolution behaviour can, thus, be spatially imaged in real time using the present technology. Using a densitometer, the density of the pure phosphate buffer and a saturated solution of lidocaine in phosphate buffer at 25 C was determined to be and g/ cm 3 (Standard deviations were < g/cm 3 ; n = 3), respectively.

4 4 ØSTERGAARD ET AL. Figure 2. Ultraviolet (UV) imaging of lidocaine dissolution at 254 nm from single crystal positioned vertically in M phosphate buffer (ph 7.4). UV images (absorbance) obtained during (a) immediately upon filling of the cell (t = 0 min), (b) 2 min upon arresting flow, (c) 5 min upon arresting flow and (d) during flushing the cell with phosphate buffer. Figure 3. Ultraviolet (UV) imaging of lidocaine dissolution at 254 nm from single crystal positioned horizontally in M phosphate buffer (ph 7.4). UV image (absorbance) obtained 5 min after initiation of the dissolution experiment (stagnant conditions). Figure 4 shows isoabsorbance contours for a selected image during single-crystal dissolution which was readily converted into isoconcentration profiles using the molar absorptivity g 254 of 436 M 1 cm 1 determined from the calibration curve. Construction of the calibration curve providing the molar absorptivity was performed as described previously. 16 The conversion of light intensity into absorbance is shown in supporting information, Figures 2 and 3, for a lidocaine standard solution, indicating the satisfactory performance of the UV imaging system. It has to be realised that the concentrations represented by the isoabsorbance contours correspond to values averaged over the light path (3 mm) and thus, most likely, deviate significantly from local concentrations that will be encountered inside the cell. In comparison, the solubility of lidocaine in M phosphate buffer (ph 7.4) has previously been determined to be 44.5 mm at 37 C. 25 From the increase in absorbance with time, the amount of lidocaine dissolved from the crystal as a function of time can be determined when the imaging area, length of the light path and the molar absorptivity are known. Because attempts to control the dimensions of the crystals investigated were not made, crystal-to-crystal repeatable dissolution profiles

5 MONITORING LIDOCAINE SINGLE-CRYSTAL DISSOLUTION BY UV IMAGING 5 Figure 4. Isoabsorbance (isoconcentration) profiles obtained by UV imaging of lidocaine single crystal positioned vertically in M phosphate buffer (ph 7.4). UV images (absorbance) obtained 4 min after initiation of the dissolution experiment (stagnant conditions). The numbers represent the concentrations in millimolar. cannot be expected. For the crystal shown in Figure 2, two dissolution time profiles are depicted in Figure 5. Consecutive dissolution experiments with the selected crystal were made under stagnant conditions. Between each experiment, the cell was flushed with phosphate buffer. The dissolution rate, indicated from the slope of the profiles, decreased slightly from experiment to experiment. Linear regression analysis provided slope (dissolution rates) of 18.8 and 15.4 nmol/ min for the first and second dissolution profile, respectively. For comparison, the relative standard deviation for the molar absorptivity was 2.1% (n = 7), indicating that the observed change in slope for the dissolution profiles is significant. This appears reasonable because the crystal surface area available for dissolution is smaller in the second dissolution experiment as compared with the first profile. For the vertically oriented crystal, approximately 23 : g of lidocaine was dissolved after 6 min under the natural convection conditions prevailing. Prolonged quantitative dissolution experiments were not possible for lidocaine under the stagnant conditions applied due to deviation from Beer s law. The images obtained for lidocaine indicate that UV imaging will also be suitable for less soluble compounds. The present setup allowed for quantitative measurements of the amount of drug dissolved in the aqueous medium. Refinements have to be made with respect to controlling crystal size and dimensions, crystal positioning and hydrodynamics in order to take full advantage of the opportunities that UV imaging offers for single-crystal dissolution studies. Work along these lines is in progress in our laboratory. The setup with natural convection only may be of interest in relation to, for instance, in situ Figure 5. Lidocaine dissolution profiles obtained in M phosphate buffer (ph 7.4) for the crystal shown in Figure 2 under stagnant conditions. Two consecutive profiles obtained with the same crystal oriented vertically. suspension forming systems, stents and implants for parenteral administration. CONCLUSION The current report constitutes the first attempt to apply UV imaging for monitoring single-crystal dissolution of drug substances. The UV imaging detector facilitates monitoring of drug substance concentrations in dissolution media immediately adjacent to a single crystal of lidocaine in a spatially and temporally resolved format. The ability to attain two-dimensional images of the dissolution process is likely to

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