Adsorptive stripping voltammetric determination of ketoconazole in pharmaceutical preparations and urine using carbon paste electrodes

Transcription

1 Adsorptive stripping voltammetric determination of ketoconazole in pharmaceutical preparations and urine using carbon paste electrodes Mojtaba Shamsipur* and Khalil Farhadi Department of Chemistry, Razi University, Kermanshah, Iran Received 22nd February 2000, Accepted 21st June 2000 Published on the Web 2nd August 2000 The oxidation of ketoconazole on a bare carbon paste electrode was studied voltammetrically. The results indicated that the process is irreversible and controlled by an adsorption extraction process which allows the accumulation of the drug at the electrode surface. After the optimization of solution ph, accumulation variables and instrumental parameters, sensitive differential pulse and linear sweep voltammetric peaks were obtained whose peak currents were linearly proportional to the ketoconazole concentration over the ranges and M, respectively. Based on these findings, a simple procedure was developed for the determination of ketoconazole in human urine and formulations. Ketoconazole, cis-1-acetyl-4-{4-[2-(2,4-dichlorophenyl)- 2-(1H-imidazol-1-ylmethyl)-1,3-dioxolan-4-yl]methoxy}piperazine (KC), is a highly effective broad spectrum antifugal agent. It is used to treat a wide variety of superficial and systemic mycoses, 1,2 and has the advantage over other imidazole derivatives of producing adequate sustained blood levels following oral administration. 3 Moreover, it has been found that ketoconazole may cause some changes in cyctochrome P-450 dependent monooxygenase activities 4,5 and also in epoxide hydrolase activity. 6 Hence, the determination of ketoconazole in biological specimens and dosage forms has received considerable attention. Owing to the vital importance of ketoconazole in biological fluids and pharmaceutical preparations, several chromatographic 3,7 12 and spectroscopic methods for its quantitative determination have been reported. However, some of these methods need expensive equipment and/or are time consuming, and in many cases, owing to their relatively low sensitivities, they need a preliminary extraction step into an organic medium. Electroanalytical techniques are well known as excellent procedures for the determination of drug dosage forms and drugs in biological fluids. 22,23 In recent years, the increasing use of carbon paste electrodes (CPEs) for electroanalytical measurement of a variety of organic species of biological and pharmaceutical importance has been reported This is due not only to their excellent reproducibility and high analytical sensitivity, but also to the fact that these electrodes are suitable for the determination of substances that undergo oxidation reactions. We have recently reported a new extractive spectrophotometric method for the determination of ketoconazole in pharmaceutical preparations. 21 Owing to the lack of electroanalytical methods for the determination of ketoconazole, we were also interested in studying the electrochemical behaviour of the drug in different solvent media and in designing suitable electroanalytical procedures for its sensitive determination in pharmaceutical preparations and biological fluids. 34 In this paper, we report the development of a sensitive anodic stripping voltammetric method for the determination of ketoconazole. The method was successfully applied to the determination of ketoconazole in tablets and creams and to its recovery from urine samples. Experimental Reagents All of the chemicals used in this study were of the highest purity available and used without further purification except for vacuum drying over P 2 O 5. Doubly distilled, de-ionized water was used throughout. Pure ketoconazole and tablets and creams containing 200 mg and 2% of the drug, respectively, were obtained from Behvazan Pharmaceutical, Rasht, Iran. Spectroscopic grade graphite powder (Fluka, Buchs, Switzerland) and Nujol (Fluka, d = g cm 23 ) as an agglutinant were used. A Britton Robinson (0.05 M in acetic, phosphoric and boric acids) and phosphate buffer solutions were used as the supporting electrolyte. The ph was adjusted with 1 M NaOH solution. N 2 gas with 99.99% purity (Sabalan, Tehran, Iran) was used to de-aerate solutions, if necessary. Apparatus The experiments were carried out in an all-glass cell designed for a three-electrode potentiostat circuit. A Metrohm (Herisau, Switzerland) 694 VA voltammetric stand equipped with a carbon disk (7.07 mm 2 area) and a Metrohm 693 VA processor and a thermal printer was used for the voltammetric measurements. A double-junction Ag/AgCl, 3 M KCl electrode served as the reference electrode and a Pt wire was used as an auxiliary electrode. A Metrohm 692 ph meter was used for ph adjustments. DOI: /b001452o Analyst, 2000, 125, This journal is The Royal Society of Chemistry 2000

2 CPE preparation Carbon pastes were prepared by thorough mixing in a mortar of 0.50 g of powdered graphite and 0.24 g of Nujol. The paste was packed into a hole in the Teflon body of the electrode (2 mm deep and 3 mm diameter) and smoothed on paper until it had a shiny appearance. Electrical contact was established with a banana plug, connected with the carbon paste through a copper wire. Voltammetric procedure Appropriate amounts of ketoconazole working standard solution were placed in the cell containing 0.1 M phosphate buffer or 0.05 M Britton Robinson buffer at the selected ph. Stripping voltammetric measurements were carried out after accumulation of the drug on to and into the electrode via stirring (2000 rpm) for a time period ranging from 1 to 20 min. After a 10 s rest period, the voltammogram was recorded using the previously selected voltammetric technique. The accumulation step was performed in the absence of an applied potential (open-circuit) and the solution was de-gassed by passing purified nitrogen prior to the determination step. For experiments involving the medium exchange technique, 35 the electrode was rinsed with water (after preconcentration), then transferred to the electrolytic bulk solution and the voltammogram was recorded between 0.4 and 1.0 V. After each run, the electrode was placed in a blank solution with cyclic scanning of the potential from 0.3 to 0.9 V several times. Using this procedure, not only was excellent reproducibility for the voltammetric signal obtained, but also the same surface of the electrode could be used for ca. 20 assays. Voltammetric parameters In order to establish optimum conditions for the determination of ketoconazole by means of differential pulse voltammetrics (DPV) and linear sweep voltammetric (LSV) techniques, various instrumental variables were studied. In the case of DPV, the peak intensity increased linearly up to 50 mv when the pulse amplitude was varied in the range mv, hence 50 mv was selected. Small differences occurred in the peak current when the step was changed between 2 and 10 mv; 10 mv s 21 was chosen since it causes an increase in scan rate. In LSV, the best results were obtained with a scan rate of 100 mv s 21. At higher scan rates, the charging current reduced the sensitivity of the signal. Analysis of tablets and creams was placed in the solution and the solution was stirred for 5 min (2000 rpm). The electrode accumulated with ketoconazole was then rinsed with water, immersed in 0.1 M phosphate buffer solution of ph 12 and the voltammetric procedure was followed. However, when the volume of urine was larger than 2 ml, the peak height decreased rapidly. The decreased peak height with increasing amount of urine possibly indicates the existence of strong interactions between ketoconazole and the urine matrix, prohibiting the quantitative extraction of the drug from the sample. Such a strong bonding of ketoconazole to plasma proteins has already been reported. 36 In order to avoid the urine interferences, solid liquid extraction was carried out using C 18 cartridges as described. 3,33 The cartridge was first conditioned with 2 ml of 0.5% acetic acid. Subsequently, 2 3 ml of spiked urine were added, then two sequential washings (0.5 ml of 40% methanol in water followed by 0.5 ml of 20% acetone in water) and finally elution of KC with 75% methanol in water (2 ml) were carried out. The collected solution was evaporated nearly to dryness by gentle heating on a warm water-bath. The residue was transferred into a 10 ml volumetric flask and diluted to volume with 0.1 M phosphate buffer solution (ph 12) and the above procedure was followed. Results and discussion Cyclic voltammetry Fig. 1 shows repetitive cyclic voltammograms for M ketoconazole at a CPE, recorded following 5 min of stirring at 2000 rpm. The first scan (designated 1) yields a large current response at 480 mv, while subsequent scans (2 and 3) result in a substantial decrease in the peak current to a nearly constant value. This behaviour is indicative of accumulation of the analyte at the CPE. No peaks are observed in the cathodic branch, emphasizing the occurrence of an irreversible process. Similar irreversible behaviour has been reported for the oxidation of the piperazine rings of doxazosin 33 and trazodone. 37 Fig. 2(A) shows the cyclic voltammograms for M ketoconazole at various accumulation times. The dependence of the voltammetric peak current on the preconcentration time for ketoconazole on the CPE studied and on a graphite electrode (Metrohm) are also shown in Fig. 2(B). It should be noted that the CPE and graphite electrodes used had about the same electroactive areas. As can be seen, whereas the peak current on the graphite electrode is small and levels off only at less than 5 min accumulation, a large, gradual increase in i pa on the CPE is observed which tends to level off at preconcentration times > 20 An accurately weighed amount of KC cream (about 0.5 g) or finely powdered KC tablet (about g) was dissolved in H 2 O containing a few drops of 1 M HCl. The excipients were separated by filtration and the filter-paper was washed three times with water. The filtrate and washing solutions of the tablet and cream samples were transferred quantitatively into 250 and 100 ml calibrated flasks, respectively, and diluted to volume with water, then the voltammetric procedure was followed. Analysis of urine samples Different volumes of urine samples (0.2 3 ml) were placed in 10 ml calibrated flasks, enough ketoconazole was added to obtain a final concentration of M and the solutions were diluted to volume with 0.1 M phosphate buffer of ph 12. The sample thus prepared was transferred into the cell, a CPE Fig. 1 Repetitive cyclic voltammograms for M ketoconazole solution at ph 12 on the CPE, after a preconcentration step. Scan rate, 100 mv s 21 ; accumulation time (t acc ), 5 min; stirring speed, 2000 rpm Analyst, 2000, 125,

3 min. The observed behaviour is indicative of the adsorptive extractive accumulation of the drug on the CPE, in comparison with that on graphite, where the accumulation takes place solely by adsorption To investigate further the occurrence of adsorptive extractive phenomena, the differential pulse voltammograms of ketoconazole were measured on the CPE after a 15 min preconcentration period, before and after extruding an outer layer of the carbon paste (Fig. 3). As can be seen, after extruding 0.3 mm of the outer layer of the carbon paste accumulated by the drug and placing the electrode in an electrolyte blank solution, the CPE still shows a clear voltammogram (B), although it is considerably diminished in comparison with that obtained at the initial surface of the CPE. However, since in LSV experiments the stripping takes place using a high scan rate of 100 mv s 21, an accumulation within the CPE seems to be of little practical importance, and only the analyte close to the electrode surface will be stripped. Cyclic voltammograms of M KC at various scan rates (v) without any accumulation are shown in Fig. 4(A) and the corresponding log i pa vs. log n plot is shown in Fig. 4(B). As can be seen, log i pa increases with log n, clearly demonstrating two linear regions with different slopes, the first between 10 and 200 mv s 21 (slope = 0.6, r = ) and the second between 200 and 700 mv s 21 (slope = 0.970, r = ) [Fig. 4(B)]. At high scan rates, the slope is close to unity, the value expected for an ideal reaction of surface species, 41 so in this case the process possesses a dominant adsorptive component. A plot of the current vs. the square root of scan rate resulted in a linear graph with the regression equation i p (ma) = n 1/2 (V s 21 ) 1/2 (r = 0.998), supporting the occurrence of an adsorptive extractive accumulation. Such a change from a diffusion-controlled process to an adsorption process with increasing scan rate could be due to the weak adsorption of the reactant. In this case, there is a small difference between the energies required for the oxidation of adsorbed and dissolved ketoconazole. The net effect is an increase in the height of the anodic peak compared with that in the absence of adsorption, because both the adsorbed and diffusing reactants contribute to the current. Similarly to the case of strong adsorption, the relative contribution of the adsorbed reactant increases with increasing scan rate. 42 Effect of ph Cyclic voltammetry was also used to study the effect of the ph of the solution on the peak potential and peak current for the oxidation of M KC at the CPE. The resulting E p vs. ph data are illustrated in Fig. 5. As can be seen, the peak potential shifts to less positive values with increasing ph. The plot shows two linear regions with the following equations: Fig. 2 (A) Cyclic voltammograms for M ketoconazole solution at ph 12 on the CPE, after a preconcentration step, for various accumulation times. Scan rate, 100 mv s 21 ; stirring speed, 2000 rpm. t acc = (1) 0, (2) 30, (3) 60, (4) 90, (5) 120, (6) 150, (7) 180, (8) 240, and (9) 300 s. (B) Current vs. accumulation time graphs on the CPE (1) and a graphite electrode (2). Fig. 3 Differential pulse voltammograms for M ketoconazole after 5 min preconcentration (A) and after extruding 0.3 mm of the outer layer of the carbon paste accumulated by the drug (B). ph, 12; scan rate, 10 mv s 21. Fig. 4 (A) Cyclic voltammograms of M ketoconazole solution at ph 12 on the CPE. t acc, 5 min; scan rate = (1) 20, (2) 50, (3) 100, (4) 200, (5) 300, (6) 400, (7) 500, (8) 600 and (9) 700 mv s 21. (B) The corresponding log Ivs. log n plot. Analyst, 2000, 125,

4 KCH K a + + " KC + H ph 4 7: E p (mv) = ph (r = ); KCH + domination ph 7 12: E P (mv) = ph (r = ); KC domination It is noteworthy that the intersection of the two linear portions of the curve is located around ph 7.5, close to the pk a value of the piperazine moiety. 43 We found that the exact value of the pk a of ketoconazole, as determined ph-metrically, is 6.95 ± Thus, in the ph range 4 7, the drug is mainly in its protonated form with a more difficult oxidation, whereas in the ph range 8 12, the unprotonated form of the drug which seems to undergo easier oxidation, dominates. Moreover, the oxidation peak current reaches its maximum value at ph 12, so this value was selected as the optimum ph for further qualitative and quantitative studies. Both 0.05 M Britton Robinson buffer and a 0.1 M phosphate buffer were employed to keep the ph of the ketoconazole solutions at ph 12. The use of phosphate buffer resulted in improved sensitivity and repeatability. the electrode behaviour was investigated. For the modified CPEs, the results obtained for variables affecting the adsorptive process were about the same as for the corresponding bare CPEs, except for some diminished response peak. Therefore, a bare CPE with an optimized carbon-to-nujol ratio of (w/w) was used for the ketoconazole determination experiments. Calibration graphs and detection limit After the optimization of all the variables of DPV and LSV for the determination of KC, the variation of peak current with KC concentration was studied. Some of the resulting linear sweep voltammograms and differential pulse voltammograms obtained at various ketoconazole concentrations, and the corresponding current concentration plots, are shown in Figs. 6 and 7, respectively. It is noteworthy that, in both cases, two linear ranges were obtained. The following equations correspond to the two linear ranges for DPV and LSV using the CPE: Optimization of experimental conditions for LSV and DPV Since the adsorption of ketoconazole on the surface of a CPE was used as a suitable preconcentration step prior to its voltammetric determination, it was necessary to optimize the variables affecting the adsorption process. The efficiency of the preconcentration step, performed under open-circuit conditions, was compared with that obtained with a few different applied potentials (i.e., 20.1, 0.0 and 0.2 V). It was found that, under the applied potentials used, the response peak intensity decreased in comparison with the case of opencircuit operation. The influence of stirring speed on the voltammetric response was studied. It was found that the peak current increased steadily in the range rpm, whereas at higher speeds the peak decreased. Hence 2000 rpm was selected. On performing LSV of a M ketoconazole solution, the initial sweep potential was found to have no measurable effect on the peak stripping current; a value of V was chosen for this parameter. The influence of the carbon paste composition in the presence and absence of modifiers (e.g., fatty acids and surfactants) on Fig. 6 Linear sweep voltammograms for different concentrations of ketoconazole at ph 12. Scan rate, 100 mv s 21 ; t acc, 5 min; concentration: (1) , (2) , (3) , (4) , (5) and (6) M. The inset shows the corresponding calibration graph. Fig. 5 Variation of peak potential (A) and peak current (B) of the cyclic voltammograms with ph for M ketoconazole solution at ph 12. Scan rate, 100 mv s 21 ; t acc, 5 min. Fig. 7 Differential pulse voltammograms for different concentrations of ketoconazole at ph 12. Scan rate, 10 mv s 21 ; DE, 50 mv; t acc, 5 min; concentration: (1) , (2) , (3) , (4) , (5) , (6) , (7) , (8) and (9) M. The inset shows the corresponding calibration graph Analyst, 2000, 125,

5 Table 1 DPV: I (ma) = C KC (M) (r = ); KC concentration: M I (ma) = C KC (M) (r = ); KC concentration: M LSV: I (ma) = C M (M) (r = ); KC concentration: M I (ma) = C M (M) (r = ); KC concentration: M The detection limit, calculated following the expression a + 3s yx, 44 where a = intercept and s yx = error standard deviation, was M for DPV and for LSV. Determination of ketoconazole in real samples The procedures used for the determination of ketoconazole in pharmaceutical preparations and in urine were described in the Experimental section. The amount of ketoconazole in tablets and creams was determined by the standard addition method in four aliquots of the corresponding solutions. The results obtained by the proposed and the standard 45 methods are summarized in Table 1. As can be seen, there is good agreement between the ketoconazole contents determined by the proposed and official methods and the declared amounts of the drug in the preparations used. This is indicative of non-interference of the other ingredients and the excipients present in the formulations. However, Table 1 shows much larger errors for the cream sample compared with the tablet (i.e., 5 10% compared with < 1%). This could be due to some possible difficulties with complete dissolution of the cream sample in the HCl solution used. This problem could also be responsible for the difference in the recoveries of the proposed and the official method. 45 In order to investigate the applicability of the proposed method to the determination of ketoconazole in biological fluids, it was applied to the recovery of ketoconazole from a urine sample. The LSV determination of the drug in the urine sample was carried out by the standard addition method at the M level. The mean recovery of ketoconazole from the urine sample was found to be 98.5 ± 2.2%. 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