Formulation and In Vivo Evaluation of Immediate Release Glimepiride Coated Pellets Using 3 2 Full Factorial design by Novel Liquid Layering Technology

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1 Research Article ISSN: Available online through Formulation and In Vivo Evaluation of Immediate Release Glimepiride Coated Pellets Using 3 2 Full Factorial design by Novel Liquid Layering Technology Y. Roopa Rani 1*, P.Vijayalakshmi 2, J. Venkateswara Rao 3 1 Sultan-Ul-Uloom College of Pharmacy, Banjara hills, Hyderabad Andhra Pradesh, India. 2 Sree Dattha Institute of Pharmacy, Sheriguda, Ibrahimpatnam, Hyderabad , Andhra Pradesh, India. 3 Bharat Institute of Pharmacy, Mangalpally, Ibrahimpatnam, Hyderabad , Andhra Pradesh, India. Received on: ; Revised on: ; Accepted on: ABSTRACT The aim of the present study was to develop immediate release liquid coated pellets of glimepiride with hydroxy propyl cellulose-lf as hydrophilic polymer and PVP K30 as binder. In this study, a 3 2 full factorial design was employed to optimize the formulation of pellets. In order to optimize formulations, the amount of HPC LF and the amount of PVP K 30, were chosen as independent variables. The prepared pellets were evaluated for drug content, particle size, subjected to Scanning Electron Microscopy (SEM), FT-IR & Differential Scanning Calorimetry (DSC) and evaluated for in vitro release. The drug content was in the range of 98.78% to 99.72%. The mean particle size of the drug-loaded pellets was in the range µm. Rapid drug release was observed in F9 as using HPC LF and PVP K 30 with 3% and 1% concentration. The optimized formulation (F9) shows highest percent of drug release by the end of 8 min, when compared with the marketed product (DIAPRIDE 2 mg) i.e., within 8 min. SEM photographs confirmed that the prepared formulations were spherical in nature with a smooth surface. The compatibility between drug and polymers in the drug-loaded pellets was confirmed by FT-IR & DSC studies. In vivo bioavailability studies indicated significance difference between Glimepiride coated pellets and pure drug. Therefore, the present Glimepiride coated pellets is considered to be potentially useful for the treatment where improved patient compliance and convenience is expected. Keywords: Glimepiride, pellets, liquid layering technique, HPC-LF, bioavailability studies. *Corresponding author. Y. Roopa Rani Sultan-Ul-Uloom College of Pharmacy, Banjara hills, Hyderabad , Andhra Pradesh, India. 1.0 INTRODUCTION It is estimated that 40% or more of active substances being identified through combinatorial screening programs are poorly soluble in water 1. Poor solubility is not only a problem for the formulation development and clinical testing; it is also an obstacle at the very beginning when screening new compounds for pharmacological activity. There is a definite need for smart technological formulation approaches to make such poorly soluble drugs bioavailable 2. Multiparticulate dosage forms (MPDFs) are receiving an immense attention as alternative drug delivery systems for oral drug delivery even though single-unit dosage forms have been widely used for decades 3. Pelletization is often referred to as a size enlargement process that involves production of agglomerates relatively with a narrow size range of 0.5mm to 2mm 4,5 and they are called pellets. Pellets have free flowing properties and low porosity of about 10%. Since their multiparticulate nature offers many important pharmacological as well as technological advantages over conventional single unit solid dosage forms 6. In the last two decades, pellets have established their position for many reasons 7. Pellets offer a greater flexibility in pharmaceutical solid dosage from design and development. They flow freely and pack easily without significant difficulties, resulting in uniform and reproducible fill weight of capsules and tablets 8, 9. Layering processes are probably the most well controlled and straight forward pelletization techniques. The layering process comprises the deposition of successive layers of drug entities from solution, suspension or dry powder on nuclei which may be crystals or granules of the same material or inert starter seeds. They are classified into two categories: solution/suspension layering and powder layering 10. In solution/suspension layering drug particles and other components are dissolved or suspended in the application medium. The droplets impinge on the started seed or cores and spread evenly as the solution or suspension is sprayed on the cores 11. Experimental designs have long been employed to optimize various industrial products and/or processes such as the factorial designs (FDs) since 1926, the screening designs since , the central composite designs (CCDs) since 1951, (Box GEP. 1951) and mixture designs (SMDs) since The goal of the present study was to develop a multiparticulate dosage form containing Glimepiride drug layered pellets with the objective of enhancing the solubility and bioavailability of the drug by using liquid layering technology. 2.0 MATERIALS AND METHODS: Diapride 2mg conventional tablet was purchased from Micro Labs Ltd, Bangalore. Glimepiride was generous gift sample from Dr. Reddy s

2 Laboratories Limited, (Hyderabad, India). Polyvinylpyrrolidone (PVP) K 30, Hydroxy Propyl Cellulose (HPC) LF and MCC were obtained from MSN laboratories, Hyderabad. All other polymers and solvents used were of analytical grade. 2.1 Procedure for the preparation of core pellets: Step 1: Preparation of core pellets: Weigh 44g of microcrystalline cellulose (MCC) allow passing through #30/#40 mesh, so that the powder passes through #30 mesh and retains on #40 mesh. The binder solution is prepared by taking 0.05g of PVP K 30 and dissolved in water to make a solution. Now add the binder solution to the MCC and mix well, then the mixed residue is passed through the sieves and sieved sugar (5g) is sprayed on to the prepared beads. Then the residue is kept in the oven for about an hour at 45 o C, and they are collected and used as core pellets. Then the pellets are passed through the sieves of 20 mesh and 24 mesh so that the pellets passed through the 20 mesh and retained on the 24 mesh are collected and used for seal coating. Step 2: Coating of polymer solution: Prepare polymer solutions of 3%, 4% and 5% by mixing 1.5g, 2g and 2.5g of HPC-LF in 10 ml of methanol and stirred, till the polymer is dissolved. Now take 50g weighed core pellets. Then the prepared coating solution was sprayed on to the core pellets, which are present in the coating pan rotating at a speed of 45rpm and hot air (70 o C) is passed simultaneously through the pan to dry the coated pellets. Step 3: Coating of drug solution on to sub-coated pellets: Dissolve 0.5g, 1g, 1.5g of PVP K 30 in water so that 1%, 2%, 3% solutions are prepared. Take 1g of glimepiride pure drug and PVP K 30 in a beaker with methanol and stir it well till mixing is completed. Now the prepared solution is sprayed on to the sub coated pellets through spray gun at a rate of a 2ml/min. Pellets are located in the coating pan, rotating at a speed of 45rpm and they are dried simultaneously through the usage of hot air at a temperature of 70 o C. The drug coated pellets are dried for about half-an-hour, and then they are collected from the coating pan and packed in an appropriate manner to prevent them from contamination. The compositions of the glimepiride solution with polymers are shown in Table 1. Table 1: Composition of Glimepiride solution with polymers coated on core pellets S.No Formu- Core Glimepiride HPC-LF PVP K30 Methanol lation Pellets (g) (g) (g) (ml) (g) 1 F F F F F F F F F F Experimental design and statistical analysis: In this study, a 3 2 full factorial design was employed to optimize the formulation of pellets. In order to optimize formulations, the amount of HPC LF and the amount of PVP K 30, were chosen as independent variables. HPC LF, being hydrophilic is more permeable to water so it promotes release of drug. PVP K 30 is also hydrophilic and acts as a binder and viscosity enhancer. Hence the combination of a release promoting and binder was used to obtain immediate release of drug. Selection of response variables was crucial. The target was to obtain the drug release immediately, but simultaneously to achieve the maximum release. Therefore the response variables selected for evaluation of immediate release were percent of drug release within 2min and 8min was selected as dependent variables. 2.3 Analysis of data: The data obtained by experimental design was processed using Design expert software. 3-D response surface curves were constructed to study the effect of two independent variables alone and in combination of percent drug release at 2min and 8min. All the responses observed were simultaneously fitted to quadratic models and were evaluated in terms of statistical parameters (Table 2). Grid search was conducted over the experimental domain to find the compositions of the optimized formulations. The resultant experimental values of the responses were quantitatively compared with that of the predicted values by calculating residuals and linear plots. Table 2: Independent variables and their selected levels for pellet formulation Coded factors Level Factor 1 Factor 2 HPC LF PVP K 30-1 Low high Evaluation parameters Particle size analysis: The particle size of the pellets was measured by using projected area diameter. It is the diameter of a circle having the same area as the particle viewed normally to the plane surface on which the particle is at rest in a stable position Drug content: Weigh 110mg of glimepiride drug layered pellets (equivalent to 2mg) and dissolve them in a mixture of methanol & phosphate buffer in the ratio 1:4. After mixing of the pellets in the desired solvent, now 2ml solution is collected from that above stock solution, and transferred into the 10ml volumetric flask and makeup the volume with phosphate buffer ph 7.4 and the absorbance was measured at 225nm In vitro drug release studies: Accurately weighed pellets equivalent to 2 mg of Glimepiride was placed in 900 ml of dissolution media (7.4 phosphate buffer) contained in USP dissolution apparatus II and stirred at a speed of 50 rpm at 37±0.5 C. 5ml aliquots were withdrawn at 1, 2, 4, 6, 8 minute and replaced by 5 ml of fresh dissolution media. The collected samples

3 were analyzed after suitable dilution at 225 nm using UV-visible spectrophotometer against the blank Drug Excipient Compatibility Studies: The drug excipient compatibility studies were carried out by Fourier Transmission Infrared Spectroscopy (FTIR) method and Differential Scanning Colorimetry (DSC) method Differential Scanning Calorimetry (DSC) Differential Scanning Calorimetry (DSC) studies were carried out using DSC 60, having TA60 software, Shimadzu, Japan. Samples were accurately weighed and heated in sealed aluminium pans at a rate of 10 C/ min between 25 and 350 C temperature rang under nitrogen atmosphere. Empty aluminium pan was used as a reference Fourier Transform Infrared Spectroscopy (FTIR) FTIR spectra for pure drug, physical mixture and optimized formulations were recorded using a Fourier transform Infrared spectrophotometer. The analysis was carried out in Shimadzu-IR Affinity 1 Spectrophotometer. The samples were dispersed in KBr and compressed into disc/pellet by application of pressure. The pellets were placed in the light path for recording the IR spectra. The scanning range was cm -1 and the resolution was 1 cm SEM studies The surface and shape characteristics of pellets were determined by scanning electron microscopy (SEM). Photographs were taken and recorded at suitable magnification. 2.5 In vivo studies Animal Preparation Twelve male Wistar rats were (weighing grms) selected for this study, all the animals were healthy during the period of the experiment. All efforts were made to maintain the animals under controlled environmental conditions (Temperature 25 0 C, Relative Humidity 45% and 12 h alternate light and dark cycle) with 100 % fresh air exchange in animal rooms, uninterrupted power and water supply and rats were fed with standard diet and water ad libitum. The protocol of animal study was approved by the institutional animal ethics committee (IAEC/SUCP/07/2013). The rats were fasted overnight before administration of the formulation (Glimepiride coated pellets) and pure drug (Glimepiride). The rats were randomly divided into two groups each group contains six animals. The Group A rats were received Glimepiride coated pellets coated pellets and Group B received pure drugs Glimepiride coated pellets administered orally (Dissolved in distilled water). Blood samples (Vollala V. Rajesham, 2009) for pharmacokinetic analysis were obtained at different time intervals 0, 15, 30, 60, 120, 240, 360min and 24hrs after dosing. Blood samples were collected in heparinized tubes and were centrifuged for 10min at 3,000 rpm at room temperature Preparation of Plasma Samples for HPLC Analysis 14 : Rat plasma (0.5 ml) was prepared for chromatography by precipitating proteins with 2.5 ml of ice-cold absolute ethanol for each 0.5 ml of plasma. After centrifugation the ethanol was transferred into a clean tube. The precipitate was resuspended with 1 ml of acetonitrile by vortexing for 1 min. After centrifugation ( rpm for 10 min), the acetonitrile was added to the ethanol and the organic mixture was taken to near dryness by a steam of nitrogen at room temperature. Internal standard Gliclazide was added to the samples. Samples were reconstituted in 200 µ1 of Mobile phase was injected for HPLC analysis. The chromatographic conditions are depicted in Table 3. Table 3: Chromatographic conditions: Column C18 Mobile Phase Acetonitrile : Phosphate buffer (P H 3.0) (40:60) Flow rate 0.8ml/min Injection volume 20µl Retention times Glimepiride , Gliclazide (Internal standard) RESULTS AND DISCUSSION: 3.1 Particle size analysis: The each pellet size was measured by using electron microscope and found to be within the Indian Pharmacopoel range. The particle size range is in between The results are summarized in Table 4. Table 4: Size of core pellets S.No Pellet Size (µm) 3.2 Drug content/content uniformity: All the formulations were found to contain an almost uniform quantity of the drug, as per content uniformity studies indicating reproducibility of the technique. Drug content in the all formulations were evaluated and the values were found to be between ± 0.35% to ± 0.51% (Table 5). As per the USP requirements, the formulations were found to meet the criteria for content uniformity. No significant difference in the drug content among the formulations indicated good content uniformity. Table 5: Percentage drug content of different formulations of Glimepiride coated pellets Formulation (%) Drug content F ± 019 F ± 0.31 F ± 0.35 F ± 0.32 F ± 0.24 F ± 0.29 F ± 0.36 F ± 0.51 F ± 0.33 F ± 0.41

4 3.3 In vitro drug release studies: The results obtained in the in vitro drug release for the formulations F1 to F10 and innovator product DIAPRIDE 2 mg immediate release tablet was tabulated in Table 6. The graphs are depicted in Figure 1. Formulation F1 to F8 & F10 shows % drug release of 88.78, 79.59, 76.71, 83.29, 94.98, 65.12, 68.23, & respectively at the end of 8 min. Rapid drug dissolution was observed in F9 as using HPC LF and PVP K 30 with 3% and 1% concentration. The optimized formulation (F9) shows highest percent of drug release by the end of 8 min, when compared with the marketed product i.e., within 8 min (Figure 2). The enhanced dissolution rates of drug layered pellets may be due to many factors such as the pellets can disperse freely throughout GIT after administration. Table 6: Cumulative % drug release of pellets coated with Glimepiride Formulation 0 min 1min 2min 4min 6min 8min F ± ± ± ± ± 0.62 F ± ± ± ± ± 0.78 F ± ± ± ± ± 0.89 F ± ± ± ± ± 0.73 F ± ± ± ± ± 0.42 F ± ± ± ± ± 0.42 F ± ± ± ± ± 0.67 F ± ± ± ± ± 0.61 F ± ± ± ± ± 0.79 F ± ± ± ± ± 0.63 Diapride 2mg ± ± ± ± ± Interpretation of FTIR data The FTIR spectra of pure Glimepiride (Figure 3) displayed bands at cm -1 due to N-H stretch, at cm -1 due to C-H Aromatic stretching, at cm -1 due to C=O Amide stretching, at 1673 cm - 1 due to Methylene Cyclohexame bending. The spectra also showed bands at 1079 cm -1 due to Sulfoxides. The FTIR spectrum of HPC-LF and PVP K 30 were shown in Figure 4 & 5 respectively. The FTIR spectrum of optimized formulation (Figure 6) also exhibited the same characteristic bands. Thus, the presence of characteristic absorption bands of Glimepiride and the optimized formulation F9 suggest that there is no interaction takes place between the drug and excipients used in the formulation. Figure 1: In-vitro release of Glimepiride liquid coated pellets and marketed product Figure 3: FTIR Spectra of Glimepiride Figure 2: Comparision of cumulative % drug release of Glimepiride of optimized formulation (F5) with marketed product DIAPRIDE 2mg Figure 4: FTIR Spectra of HPC-LF Pure Drug

5 Figure 8: DSC Thermogram of Pure Drug HPC-LF Figure 5: FTIR Spectra of PVP K30 Figure 9: DSC thermogram of PVP K30 Figure 6: FTIR spectra of optimized 3.5 Drug Excipient Compatibility Studies by DSC DSC thermograms revealed that there is no considerable change observed in melting endotherm of pure drug (212) (Figure 7) and drug in optimized formulation (208) (Figure 10). The thermograms of HPC-LF & PVP K 30 were seen in Figure 8 & 9 respectively. It indicates that there is no interaction takes place between drug and other excipients used in the formulation. Figure 10: DSC thermogram of optimized Formulation (F9) 3.6 Scanning Electron Microscopy: The Glimepiride optimized pellet formulation (F9) was characterized by SEM analysis to understand the pellet shape and surface morphology. The pellets prepared were having spherical in shape and smooth surface with minimal pores, indicating the uniform coating of the pellets which shown in Figure 11 & 12. Figure 7: DSC Thermogram of Glimepiride Figure 11: SEM images of Glimepiride coated Glimepiride pellets of optimized formulation (F9)

6 table 6. A substantial high drug release is achieved in F9 formulation which is very much faster than the marketed product (DIAPRIDE 2mg). From figure 13, the plots were found to be linear for 60% and 70% dissolution rate. In figure 15, the plot shows linear nature at 70%, 80%, 90% dissolution rates. Figure 12: SEM images of coated pellets of optimized formulation (F9) 3.7 Experimental design and statistical analysis: The experiments were designed to study effect of two independent variables at three levels on response variables such as percent drug release at 2min and 8min. The coefficients of the polynomial equations generated for % drug release at 2min and 8min are given below. The counter plot will help in understanding of the design space. In all the above counter plots the design space is available in the red colour region which is indicated in the plot as shaded region from 3-3.5% of HPC LF concentration, so this is the space we should focus on, to obtain the formulation with accurate results. The design space observed in the plot is helpful for further development of the process by giving us the clear confirmation regarding the usage of the ingredients which have direct effect on the release of drug. The immediate release of glimepiride was observed at the 3% concentration of HPC and 1% of PVP K Statistical models: T2 = *A 4.03*B T8 = *A 4.01*B For % release response at 2min, the model F-value was to be , which imply the model is significant. P-value which is less than indicates model terms are significant. Adequate precision shows signal to noise ratio. For % release response at 8min, the model F-value was to be , which imply the model is significant. P-value which is less than indicates model terms are significant. Adequate precision shows signal to noise ratio. The ratio of indicates an adequate signal thus the proposed model can be used to navigate the design space. Since the r 2 values are quite high for both the responses i.e and at 2min and 8min, the polynomial equations will excellently fits to the experimental data and are statistically valid. Response surface plots are very helpful in learning about both the main and interaction effects of the independent variables. From the above 3D-surface graphs (figure 14 and figure 16), the observation noted is that the linear relationship is observed because drug release from the pellets is only due to the HPC LF influence, but very minor effect of PVP K 30 is observed in the release profile of drug. The equations also gave us the information regarding the usage of the polymer and the binder in an appropriate way to get the dosage form prepared in an accurate and optimal manner by educating us about the amount of ingredients having direct influence on the release property of drug, should be added or to be deduced to obtain the formulation with better solubility and the bioavailability. The F9 formulation is checked for its drug release in 6 trials and each trial is performed for 3 times to avoid the variance in the release profile of the drug. The variance is very less as and the critical F value (2.2718) is greater than calculated F value (2.2336), so it is confirmed that the F9 formulation is the best formulation among the ten prepared formulations. Figure 13: Counter plot showing the influence of amount of polymer and amount of binder on the release profile of Glimepiride after 2min 3.9 Response surface analysis: By using 3 2 factorial design, ten batches of glimepiride drug layered pellets were prepared by liquid layering technology varying two independent variables, polymer (HPC-LF) and the binder (PVP K 30). The amount of drug release at 2min and 8min were taken as the dependent variables were determined and the results are summarized in Figure 14: Response surface plot showing the influence of amount of polymer and amount of binder on the release profile of Glimepiride after 2min

7 Figure 15: Counter plot showing the influence of amount of polymer and amount of binder on the release profile of Glimepiride after 8min The chromatogram of glimepiride and internal standard Gliclazide was shown in Figure 17. The mean Glimepiride plasma concentrations - time profiles for the prepared Glimepiride coated pellets and the pure drug Glimepiride are shown in Table 9. The bioavailability parameters for the both formulations are summarized in Table 10. The statistical comparison of C max (ng/ml) (840, 760), AUC0 8 and AUC0 t (689473±135, ±322) and (609270±228, ±492). The prepared Glimepiride coated pellets indicated significant difference between the two treatments, there is significant difference for the period effect was observed in this study. Based on the statistical inferences it was concluded that the two formulations exhibited significantly different plasma level-time profiles. Figure 17: Chromatograph for Glimepiride and Gliclazide as internal standard Table 9: Plasma levels of Glimepiride coated pellets and Glimepiride Pure drug at different time intervals (Mean ± SD, n = 6) Figure 16: Response surface plot showing the influence of amount of polymer and amount of binder on the release profile of Glimepiride after 8min 3.10 Validation of optimum formulation: The observed values during the response variables are compared with the predicted values. From the results and ANOVA (Table 7), it was found that mathematical models generated were statistically significant and valid for predicting values of response parameters (Table 8) at selected levels of formulation variables. Table 7: ANOVA of dependant variables for Glimepiride Response F value P value R square Adeq. model Precision 2min < min < Table 8: Actual and predicted values of Glimepiride at 2min and 8min Run order T2 T8 Actual Predicted Actual Predicted value value value value Time (min) Glimepiride Glimepiride coated pellets Glimepiride Pure drug Table 10: Comparison of pharmacokinetic parameters of Glimepiride coated pellets and Glimepiride Pure drug in rats (Mean ± SD, n = 6). Parameters Glimepiride Glimepiride coated pellets Pure drug Dose (mg/kg) 2 2 Cmax (ng/ml) AUC 0-t (µg.h/ml) ± ±492 AUC 0-inf (µg.hr/ml) ± ±322 Tmax (h- 1 ) t 1/2 (h) 0.908± ±0.213 K el (h- 1 ) ± ± CONCLUSION: In this present work, systematic efforts were made to prepare core pellets and then coated with the drug solution of Glimepiride by liquid layering technology. The core pellets were evaluated for physical properties and found to be within the limits. The drug layered pellets were also evaluated for drug content and in vitro drug release studies and found to be within Indian Pharmacopoeal limits. This work has demonstrated the use of 3 2 factorial design, derived reduced polyno-

8 mial equation, two dimensional contour plots and response surface plots in optimizing formulation variables in the preparation of glimepiride layered pellets by liquid layer technology. Based on the physicochemical properties, evaluation parameters and dissolution studies, it was concluded that F9 finalized as optimized formulation. The optimized formulation (F9) shows highest percent of drug release by the end of 8 min, when compared with the marketed product. The optimized pellet formulation (F9) of Glimepiride was characterized by SEM analysis to understand the pellet shape and surface morphology and found to be spherical in shape and smooth surface with minimal pores, indicating the uniform coating of the pellets. DSC and FTIR data revealed that no interactions takes place between the drug and polymers used in the optimized formulation. In vivo study indicated significance difference between Glimepiride coated pellets and pure drug, both exhibited significantly different drug plasma level - time profiles. Therefore, the present Glimepiride coated pellets is considered to be potentially useful for the treatment where improved patient compliance and convenience is expected. 5.0 REFERENCES: 1. Lipinski C, Poor aqueous solubility- an industry wide problem in drug discovery, Am. Pharm. Rev, 5, 2002, Suman Katteboinaa, V S R Chandrasekhar P, Balaji S, International Journal of PharmTech Research, 1(3), 2009, Mikkilineni Bhanu Prasad, Suryadevara Vidyadhara and Pavuluri Trilochani, Development and evaluation of diltiazem hydrochloride controlled-release pellets by fluid bed coating process, Journal of Pharmaceutical Technology and Research, 4(2), 2013, Gajdos B, Rotary granulators - Evaluation of process technology for pellet production using factorial design, Drugs Made Ger, 27, 1984, Kristensen, H.G. and Schaefer T, Granulation. A review of pharmaceutical wet granulation, Drug Dev. Ind. Pharm, 1987, 13, Ghebre-Sellassie I, Pellets: A general overview. In Ghebre- Sellassie Pharmaceutical Pelletization Technology. I (ed.), Vol. 37, Marcel Dekker, Inc., New York, USA, 1989, Ghebre SI, Gordon R, Fawzi MB, Nesbitt RU, Evaluation of a high-speed pelletization process and equipment, Drug Dev Ind Pharm, 11, 1985, Lyne CW, Johnston HG, The selection of pelletisers, Powder Technol, 29, 1981, Vuppala MK, Parikh DM, Bhagat HR. Application of powder-layering technology and film coating for manufacture of sustained-release pellets using a rotary fluid bed processor. Drug Dev Ind pharm, 23, 1997, Marcel Dekker Inc., 1989 Devices GSI. Pharmaceutical Pelletization Technology. Vol. 37, pp Zimm KR, Schwartz JB, Connor RE, Drug release from multi particulate pellet system, Pharma Dev Technol, 1, 1996, Plackett RL, Burman JP. The design of optimum multifactorial experiments. Biometrika, 33, 1946, Scheff E H. Experiments with mixtures. J Royal Stat Soc Ser B, 20, 1958, Venkata Rajesham V, Rajyalaxmi G and Anbu J. Simultaneous determination of Simvastatin and nifedipine in rat plasma by high performance liquid chromatography and pharmacokinetic studies, pharmacology online, 3, 2008, Source of support: Nil, Conflict of interest: None Declared