INTERNAL STANDARD VERSUS EXTERNAL STANDARD IN BIOANALYTICAL ASSAY FOR BIOEQUIVALENCE STUDIES

FARMACIA, 2007, Vol.LV, 5 569 INTERNAL STANDARD VERSUS EXTERNAL STANDARD IN BIOANALYTICAL ASSAY FOR BIOEQUIVALENCE STUDIES I. MIRCIOIU 1, A.ALDEA 3, VALENTINA ANUTA 1,2 *, OLIMPIA NEAGU 4, DALIA MIRON 1,2, F. RADULESCU 1,2, F. ENACHE 5 1 Biopharmacy&Pharmacol Res, 23 Pitar Mos Str., Bucharest, Romania 2 UMF Carol Davila, Faculty of Pharmacy, 6 Traian Vuia Str., Bucharest, Romania 3 Pfizer Romania S.R.L., Bucharest, Romania 4 National Medicine Agency, Bucharest, Romania 5 Petru Poni Technical College., Onesti, Romania *corresponding author: office@bpres.ro; vali_anuta@yahoo.com Abstract The paper presents the evaluation of performances for two different quantification methods (using Internal and External Standard, respectively) of cephalexin in plasma samples and of their impact in the evaluation of drugs pharmacokinetics and bioequivalence. Deproteinisation of plasma samples, separation by reversed-phase mechanism and UV detection were performed for determination of cephalexin. Paracetamol was used as Internal Standard. HPLC method used a Kromasil 100-5 C18 column, with an isocratic elution (1 ml/min) at 45 C column temperature. The chromatographic method was applied for evaluation of plasma concentrations in a single-dose (500mg), two-way, cross-over bioequivalence study with 24 healthy volunteers enrolled. Two different quantification methods were applied and evaluated: - Internal Standard Method - in which drug concentrations were calculated by normalizing the drug peak area to the internal standard peak area and interpolation onto A A/ A SI (c) calibration curve; - External Standard Method: in which drug concentrations were calculated by interpolation of the drug peak areas onto A A (c) calibration curves. The calibration curves were linear in the range 0.25-50 μg/ml with a quantification limit of 0.25 µg/ml, for the Internal Standard Method, while for the External Standard Method the calibration curves were linear in the range 2.5-50 μg/ml with a quantification limit of 2.5 µg/ml. The quantification limit (LLOQ) for the External Standard approach couldn t be lowered less than ten times LLOQ of the Internal Standard approach. Pharmacokinetic evaluation of data provided similar estimations for area under curve in the time interval 0-t (AUC 0-t ), plasma maximum concentration (C max ), time of the maximum plasma concentration (T max ) and area under curve in the time interval 0- (AUC 0- ) and dissimilar estimations for plasma concentration half time (t 1/2 ) and area under curve extrapolated after last experimental point (AUC extra ). Estimations concerning 90% confidence intervals for ratios of pharmacokinetic parameters defining bioequivalence were practically independent on the bioanalytical method. The Internal Standard method presented higher accuracy and precision than External Standard method, especially for the low concentrations.

570 FARMACIA, 2007, Vol.LV, 5 The evaluated methods led to similar results in the evaluation of bioequivalence due to the fact that low concentrations did not influence significantly the main pharmacokinetic parameters. Consequently, from the biopharmaceutical point of view, the methods are equivalent. Rezumat Studiul prezintă evaluarea performanţelor a două metode diferite de cuantificare a cefalexinei din probele de plasmă (metoda utilizând Standard Extern şi metoda utilizând Standard Intern) şi impactul acestora în evaluarea farmacocineticii şi bioechivalenţei substanţelor medicamentoase. Pentru determinarea cefalexinei din probele de plasmă s-a ales un mecanism de separare cu fază inversă şi detecţie UV, prepararea probelor fiind făcută prin deproteinizare. Standardul intern ales a fost paracetamolul. Separarea s-a realizat pe o coloana cromatografica de tip C18, (Kromasil 100-5 C18) cu eluţie izocratică, la temperatura de 45 C. Metoda analitică a fost aplicată în cadrul unui studiu de bioechivalenţă de tip încrucişat cu două perioade şi două secvenţe, la care au participat 24 de voluntari sănătoşi, cărora li s-a administrat o doză unică de Ceafalexină (500mg). Două metode diferite de cuantificare au fost evaluate: - Metoda Standardului Intern: în care concentraţiile de medicament s-au calculat prin raportarea ariei de analit la aria standardului intern; - Metoda Standardului Extern: în care concentraţiile de substanţă activă s-au calculat prin interpolarea ariilor de analit pe curbele de calibrare. Curbele de calibrare au fost lineare în domeniul 0.25-50 μg/ml cu o limită de cuantificare de 0.25 µg/ ml pentru metoda standardului intern, în timp ce la aplicarea metodei standardului extern acestea au fost lineare în domeniul 2.5-50 μg/ml cu o limită de cuantificare de 2.5 µg/ ml. Limita de cuantificare a fost de zece ori mai mare în cazul metodei standardului intern. Evaluarea farmacocinetică a datelor a dus la rezultate similare pentru parametrii farmacocinetici AUC 0-t, C max, T max şi AUC 0- şi valori diferite pentru t 1/2 şi AUC extra. Estimările intervalului de încredere 90% pentru raportul mediilor parametrilor farmacocinetici definitorii ai bioechivalenţei au fost independente de metodele de cuantificare folosite. Din punct de vedere analitic, metoda standardului intern prezintă o acurateţe şi o precizie mai bună decât metoda standardului extern, în special la valorile mici ale concentraţiei. Din punct de vedere farmacocinetic însă, metodele evaluate au condus la rezultate similare privind evaluarea bioechivalenţei, datorită faptului că în cazul cefalexinei nivelele mici de concentraţie de la ultimele recoltări nu au influenţat semnificativ parametrii farmacocinetici. Internal Standard External Standard Bioanalytical assay Bioequivalence INTRODUCTION Internal standard usage in bioanalytical studies is recommended by guidelines [1] since offers the advantage of a better precision and accuracy

FARMACIA, 2007, Vol.LV, 5 571 of the results. The evaluation of internal standard provides corrections for changes in sample size or concentration due to the variations in the pretreatment of samples prior to analysis or due to instrumental variations. With the internal standard method, a calibration plot is obtained by preparing and analyzing calibration solutions containing different concentrations of the compound of interest with a fixed concentration of the internal standard added. The calibration curve is constructed by plotting the ratio of peak area of the analyte to the internal standard peak area versus the nominal concentration of the analyte. Further, the advantages of internal standard method are significantly reduced by the efforts to find an appropriate substance to fulfil the task of an internal standard. It is not to forget that utilization of internal standard includes the implicit hypothesis that its extent and rate of extraction are very close to that of analyte. However, any structural analogues (excluding radiolabeled isotopes) are not chemically identical to target analytes. Even deuterium labeled internal standards may demonstrate unexpected behavior which can compromise an accurate quantification of analyte concentration due to unexpected matrix effect [2] Thus, if these standards are added to initial samples, their losses during sample preparation could be different from losses of target compounds leading to lowering of accuracy in quantitation by this method. On the other hand, good correlation coefficients of the calibration curves are false friends being insensitive to lack of precision and accuracy at lower concentrations.[3] Consequently, though nobody contested the advantages of the utilization of internal standard, even in the last years not few methods renouncing to this advantages were published in literature [4,5,6]. The impact of quantification method on the cephalexin pharmacokinetic parameters was studied in order to compare the two method performances in bioequivalence studies. Reagents HPLC grade methanol, acetonitrile, phosphoric acid, as well as analytical grade potassium dihydrogen phosphate and perchloric acid were purchased from Merck (Darmstadt, Germany) Ultra-high quality water (obtained using a Milli TKA-Genpure UV system) was used for chromatography. HPLC conditions The analyses were carried out using a Waters 600 Multisolvent Delivery System, a Waters 717 plus autosampler and a Waters 486 Tunable Absorbance Detector (Waters, Milford, MA, USA). The detector was set at 261 nm.

572 FARMACIA, 2007, Vol.LV, 5 The separation was performed on a Kromasil 100-5-C18, Azko Nobel, 150*4.6 mm column at 45 C. The mobile phase consisted of acetonitrile: methanol: 0.025M potassium phosphate buffer ph=-2.8 (85:7.5:7.5 v/v/v). The flow-rate was 1mL/min, and the injection volume was 100 μl. Standard and plasma solutions Stock solutions of cephalexin were made by dissolving 25 mg of cephalexin reference substance in 100 ml of water (250 μg/ml). Separate solutions were prepared for the calibration curve samples and quality control ones. Plasma solutions (calibration standards and quality control samples) were prepared by spiking various volumes of the diluted stock solution of cephalexin into blank plasma. The internal standard solution (paracetamol 100 μg/ml) was prepared in water and stored at 4 C during the study. Preparation of spiked calibration standards and QC (quality control) samples The plasma samples were stored in the freezer at 20 C and allowed to thaw at room temperature before processing. To 0.5 ml of plasma sample 50 μl Internal Standard (Paracetamol 100 μg/ml in water) and 100 μl perchloric acid 12% solution were added. The resulting sample was centrifuged for 10 min at 4000 rpm. An aliquot of 100 µl from the supernatant solution was injected into the chromatographic system. Clinical Study The study was projected as a single-dose, randomized, twotreatments, two-periods, two-sequences cross-over study under fasting conditions, comparing equal doses of test and reference products. The study protocol was approved by the National Medicine Agency and an Independent Ethics Committee. 24 male and female healthy volunteers were enrolled in the study. A single dose, 500 mg of cephalexin, was applied in each of the two study periods. Venous blood samples (approximately 5mL) were collected through a catheter inserted in an antecubital vein, pre-dose (0 hours) and at 0.25, 0.5, 0.75, 1, 1.25, 1.5, 2, 2.5, 3, 4, 5, 6 and 8 hours after the drug administration. Blood samples were centrifuged at 5 C for 6 minutes at approximately 3000 rpm, and the plasma collected was separated in 2 aliquots, transferred to labelled 1.5 ml polypropylene tubes (Eppendorff type) and immediately frozen and stored at a temperature under -20 C until delivery to analytical facility.

FARMACIA, 2007, Vol.LV, 5 573 RESULTS AND DISCUSSION 1. Bioanalytical study HPLC method: Linearity was tested in the range 0.25-50 μg/ml for both internal and external standard methods. Although calibration curve proved to be linear for both methods (r=0.999 for Internal standard method and r=0.998 for the External Standard one) (figure 1), the correlation coefficient proved to be insensitive to lack of accuracy of the samples at lower concentrations (0.25, 0.5 and 1 μg/ml) for the External Standard method (Table 1). Deviation of calibration standards from nominal concentration did not exceeded 5% for all points in case of the Internal Standard method, while for the External Standard method deviation was within the requested limits ( ±15% from nominal concentration for samples other than LLOQ and ± 20% for the LLOQ sample) [1] only in the range 2.5-5- μg/ml. A. B. Figure 1 Calibration curves for cephalexin: A- Internal Standard method; B External Standard method In case of the External Standard method the precision was good (CV<10%) for all samples but the deviation of back-calculated individual points for concentrations lower than 2.5 μg/ml was far outside the limits. (Table I).

574 FARMACIA, 2007, Vol.LV, 5 Table I Back-calculated concentrations for the calibration standards (S1-S8) for Cephalexin in the range 0.25-50 µg/ml (Mean for three determinations) Internal Standard Method External Standard Method Theoretical Concentration Calculated Calculated conc level conc. Deviation(%) conc. Deviation(%) (µg/ml) (μg/mll) (μg/ml) S8 0.25 0.26 3.19 0.55 120.58 S7 0.5 0.48-3.39 0.79 58.02 S6 1 0.96-4.05 1.22 22.43 S5 2.5 2.47-1.35 2.58 3.26 S4 5 4.89-2.24 4.79-4.12 S3 10 9.90-1.05 9.50-5.04 S2 25 25.53 2.12 24.39-2.45 S1 50 49.77-0.46 50.41 0.83 The accuracy and precision studies confirmed the previous results. For the Internal Standard method both intra-sequence and intersequence accuracy and precision were situated within the accepted limits. The precision was better than 5 % and the bias did not exceed 4.25 % at all concentration levels tested (0.75, 7.5 and 30 μg/ml) (QC1-QC3). For the External Standard method, precision was good but accuracy at the lowest quality control level (0.75 μg/ml) was very poor (approximately 145%). (Table II) Table II Accuracy for the determinations of cephalexin in plasma samples Internal Stamdard Method External Standard Method Concentration level Theoretical conc (µg/ml) Calculated conc. (μg/ml) Deviation(%) Calculated conc. (μg/ml) Deviation(%) QC3 0.75 0.7592 1.23 1.1562 156.93 0.75 0.7541 0.54 1.0885 141.88 0.75 0.7445-0.73 1.1321 151.57 0.75 0.7375-1.67 1.1000 144.45 0.75 0.7551 0.68 1.0841 140.90 QC2 7.5 7.7643 3.52 7.5957 1.28 7.5 7.6137 1.52 7.2376-3.50 7.5 7.5406 0.54 7.4853-0.20 7.5 7.8189 4.25 7.9427 6.30 7.5 7.6920 2.56 7.9706 6.27 QC1 30 29.4183-1.94 29.2607-2.46 30 29.8297-0.57 27.6162-7.95 30 30.6439 2.15 28.1721-6.09 30 28.8694-3.77 29.0675-3.11 30 30.6172 2.06 29.3853-2.05

FARMACIA, 2007, Vol.LV, 5 575 Because of the lack of accuracy at lower concentrations, the lower limit of quantification (LLOQ) for the External Standard method had to be raised at 2.5 μg/ml. The new calibration curve was linear in the range 2.5-50 μg/ml (r=0.9986) (Figure 2), with accuracy requirements within the accepted limits for all concentration levels (Table III). Figure 2 Calibration curve for the External Standard method, in the range 2.5-50 μg/ml Table III Back-calculated concentrations for the calibration standards for Cephalexin in the range 2.5-50 µg/ml External Stadard Method Concentration Theoretical Calculated conc. level conc (µg/ml) (μg/ml) Deviation(%) S5 2.5 2.793 11.725 2.5 2.809 12.360 2.5 2.986 19.445 S4 5 5.626 12.519 5 4.821-3.576 5 5.134 2.678 S3 10 9.531-4.692 10 10.629 6.294 10 10.806 8.058 S2 25 23.620-5.520 25 22.377-10.491 25 24.343-2.626 S1 50 50.884 1.769 50 50.324 0.648 50 50.815 1.631

576 FARMACIA, 2007, Vol.LV, 5 The mean recovery of the analytes from plasma samples was 82% (range 76-90% for three different concentration levels) for cephalexin and 84% for the internal standard. The stock solutions of analyte and internal standard were stable for at least 20 days at +4 C. The stability of unprocessed plasma samples was studied for 3 months at the storage temperature (-20 C), for 24 hours at room temperature, and after three freeze and thaw cycles. The concentration changes relatively to the nominal concentration were less than 15%, indicating no significant substance loss during the study. The processed plasma samples proved to be stable for at least 24 hours. 2. Pharmacokinetic analysis The obtained maximum plasma levels - C max, were in range of 11-42 µg/ml, with a mean of 23.8 μg/ml for both Internal and External Standard methods. These results are consistent with the results reported in literature [7, 8]. For the Internal Standard method all plasma levels, until the last blood sampling (8 hours) could be quantified. Main difficulty to be managed to in the case of the External Standard Method was the missing points problem. For the reference formulation, concentrations at 5, 6 and 8 hours proved to be under LLOQ. For the samples collected at 5 and 6 hours only one of the enrolled subjects presented cephalexin concentrations higher than LLOQ, and at 8 hours no subject presented plasma samples concentration above LLOQ in the case of tested formulation. In this case, a mean value for the 5 or 6-hours concentrations is represented by a single value, and has no real significance to the mean pharmacokinetic profile. (figure 3 A, C). The percent extrapolated area under curve was in range 0.80-4.61 for the internal standard approach and 4.99-32.88 for the external standard approach (table IV). In the case of 4 out of the 48 pharmacokinetic profiles for the External Standard method the percent extrapolated area under curve exceeded 20%. A reason for this situation was the lack of sensitivity (LLOQ of only 2.5 μg/ml) of the External Standard method, which provided insufficient data on the end points of the pharmacokinetic profiles. The mean elimination half-life was approximately 1 hour but the range was quite larger for the External Standard method (table IV). The methods provided similar results for the other pharmacokinetic parameters AUC 0-t, C max, T max and AUC 0-.

FARMACIA, 2007, Vol.LV, 5 577 C(µg/mL) 40 30 20 10 Pharmacokinetic profile -T 0 0 2 4 6 8 10 T(h) A 01-T 02-T 03-T 04-T 05-T 06-T 07-T 08-T 09-T 10-T 11-T 12-T 14-T 15-T 16-T 17-T 18-T 19-T 20-T 21-T 22-T 23-T 24-T Undetectable (<2.5 μg/ml) C(µg/mL) 40 30 20 10 Pharmacokinetic profile -T 0 0 2 4 6 8 10 T(h) B 01-T 02-T 03-T 04-T 05-T 06-T 07-T 08-T 09-T 10-T 11-T 12-T 14-T 15-T 16-T 17-T 18-T 19-T 20-T 21-T 22-T 23-T 24-T 24 Mean Curve 24 Mean Curve Mean of Plasma.C() 18 12 6 Mean R Mean T mean value obtained from a singular value Mean of Plasma.C() 18 12 6 Mean R Mean T 0 0 2 4 6 Plasma.T() 0 0 2 4 6 8 Plasma.T() C Figure 3 Individual (A, B) and mean pharmacokinetic profiles (C, D) using the External (A,C) and Internal Standard (B,D) method Table IV Main pharmacokinetic parameters obtained after oral administration of 500 mg cephalexin (Reference drug) Variable Unit Internal Standard External Standard Mean RSD Range Mean RSD Range C max µg/ml 23.84 35.29 11.063-45.521 23.85 35.29 11.063-45.521 AUC 0-τ µg/ml*h 39.14 25.52 25.241-68.449 34.69 28.883 19.844-64.439 AUC 0- µg/ml*h 39.941 25.52 25.901-70.000 39.23 27.354 28.357-69.728 T max h 0.98 20.84 0.75-1.25 0.99 20.84 0.75-1.25 t half h 1.06 17.76 0.84-1.66 0.83 30.99 0.55-1.68 V ss L 25.134 29.96 14.16-45.23 23.81 30.28 14.64-46.45 AUC extra µg/ml*h 0.801 50.75 0.28-1.98 4.53 58.71 2.02-13.40 AUC extra % 2.00 43.49 0.80-4.61 11.80 55.67 4.99-32.88 MRT h 1.89 14.99 1.49-2.67 1.78 20.23 1.30-2.75 Cl L/h 13.21 22.60 7.14-19.30 13.49 22.03 7.17-19.30 D

578 FARMACIA, 2007, Vol.LV, 5 C max plasma maximum concentration; AUC 0-t - area under curve in the time interval 0-t; AUC 0- - area under curve in the time interval 0- ; T max time of the maximum plasma concentration; t half plasma concentration half time; V ss distribution volume at steady state; AUC extra - area under curve extrapolated after last experimental point; MRT mean residence time; Cl clearence Estimations concerning 90% confidence intervals for ratios of pharmacokinetic parameters defining bioequivalence [9] were practically similar for both bioanalytical methods (Table V). Table V Estimations concerning 90% confidence intervals for ratios of pharmacokinetic parameters defining bioequivalence for cephalexin Internal standard 0.96-1.08 90% Confidence interval AUC 0- External standard 0.98-1.09 Internal standard 0.97-1.08 External standard 0.98-1.09 Internal standard 0.96-1.15 C max External standard 0.96-1.15 AUC 0-n CONCLUSIONS The performances of two different quantification methods (using Internal and External Standard respectively) for cephalexin in plasma samples were validated, applied and evaluated. The methods were linear in the range 0.25-50 μg/ml for Internal Standard method and 2.5-50 μg/ml for External Standard method. The Internal Standard method presented higher accuracy and precision than External Standard method, especially for the low concentration levels. The evaluated methods led to similar results regarding evaluation of bioequivalence due to the fact that low drug concentrations, in case of cephalexin did not significantly influence the main pharmacokinetic parameters. REFERENCES 1. *** FDA Food and Drug Administration, Center for Drug Evaluation and Research (CDER) Bioanalytical Method Validation- Guidance for Industry, 2001; 2. Sherry Wang, Matthew Cyronak, Eric Yang - Does a stable isotopically labeled internal standard always correct analyte

FARMACIA, 2007, Vol.LV, 5 579 response? A matrix effect study on a LC/MS/MS method for the determination of carvedilol enantiomers in human plasma-journal of Pharmaceutical and Biomedical Analysis 2007, 43, 701 707 3. C. Mircioiu, D. Miron, I. Mircioiu, F. Radulescu - Outliers and pharmacokinetic validation of data in bioequivalence studies 8-th International Symposium of Pharmaceutical Sciences, Ankara, 13-18 june 2006 4. Alaa El-Gindy, Shehab Sallam, Randa A. Abdel-Salam-High performance liquid chromatographic determination of 3- methylflavone-8-carboxylic acid, the main active metabolite of flavoxate hydrochloride in human urine-journal of Pharmaceutical and Biomedical Analysis 2007, 44, 274 278 5. D. Grotto, L.D. Santa Maria, S. Boeira, J. Valentini, M.F. Charao, A.M. Moro, P.C. Nascimento, V.J. Pomblum, S.C. Garcia-Rapid quantification of malondialdehyde in plasma by high performance liquid chromatography visible detection-journal of Pharmaceutical and Biomedical Analysis 2007, 43, 619 624 6. Joanna Karpinska, Bozena Mikołuc, Radosław Motkowski, Janina Piotrowska-Jastrzebska-HPLC method for simultaneous determination of retinol, α-tocopherol and coenzyme Q10 in human plasma-journal of Pharmaceutical and Biomedical Analysis 2006, 42, 232 236 7. *** Goodman & Gilman's The Pharmacological Basis Of Therapeutics, Mcgill Edition, 10th Edition, 2001 8. Yoshiaki Akimoto, Akio Uda, Hiroaki Omata, Jun Shibutani, Hitoshi Nishimura, Masamichi Komiya, Tomonori Kawana, Kenji Kaneko, Akira Fujii, Takashi Kaneda, Hirotsugu Yamamoto -Cephalexin Concentrations in Radicular Granuloma Following a Single Oral Administration of 250- or 500-mg Cephalexin-Gen. Pharmac., 1994, 25(8) 1563-1566 9. *** FDA Food and Drug Administration, Center for Drug Evaluation and Research (CDER) Bioavailability and Bioequivalence Studies for Orally Administered Drug Products General Considerations, Guidance for Industry, 2002