Original Article

Statistical Design and Optimization of Sustained Release Formulations of Pravastatin

10.4274/tjps.galenos.2019.70048

  • Raghavendra Kumar GUNDA
  • Prasada Rao MANCHINENI

Received Date: 03.12.2018 Accepted Date: 07.02.2019 Turk J Pharm Sci 2020;17(2):221-227

Objectives:

The objective of the current study was to formulate a sustained release (SR) formulation for pravastatin. Pravastatin is a lipid lowering, biopharmaceutical classification class-III agent.

Materials and Methods:

SR tablets of pravastatin were prepared using variable amounts of hydroxy methyl propyl cellulose (HPMC) K4M and sodium carboxy methyl cellulose in various proportions by direct compression in a 32 factorial design. The amounts of the polymers HPMC K4M and sodium carboxy methyl cellulose required to obtain prolonged release of drug were chosen as independent variables, X1 and X2, respectively, whereas times taken for 10%, 50%, 75%, and 90% drug release were chosen as dependent variables.

Results:

Nine formulations were developed and were checked using pharmacopoeial tests. The results showed that all the factorial batches were within the standard limits. The dissolution parameters of all formulations were subjected to kinetic fitting and various statistical parameters were determined. Polynomial equations were developed and verified for dependent variables. Formulation F5, containing 25 mg of HPMC K4M and 25 mg of sodium carboxy methyl cellulose, was the formulation most similar (similarity factor f2=89.559, difference factor f1=1.546) to the marketed product (Pravachol).

Conclusion:

The best formulation (F5) follows Higuchi’s kinetics and non-Fickian diffusion zero order kinetics (n=1.083).

Keywords: Pravastatin, sustained release tablet, HPMC K4M, 32 factorial design, zero order kinetics, non-Fickian diffusion mechanism

INTRODUCTION

Enteral delivery is an effective, popularly used mode of administration for both immediate and new drug delivery systems. In the case of chronic therapy, immediate release dosage forms are administered in a repetitive manner, resulting in more problems.1 The majority of these drugs undergo the first pass effect or presystemic elimination, which results in poor bioavailability and shorter activity.

Sustained release (SR) formulations show constant Css levels for a prolonged period, decreased dosing frequency, and patient compliance.2 Zero-order drug release from the formulation will aid the Css constantly for a longer period. Zero-order kinetics is one of the aims of SR forms.2,3

Polymers were utilized for achieving sustained drug release. The literature reveals that utilization of polymers plays a key role in pharmaceutical product development.4

Natural polymers remain preferred due to their numerous advantages. Extensively used natural gums include xanthan gum, guar gum, tragacanth gum, and alginates. Cellulosics like hydroxy methyl propyl cellulose (HPMC), hydroxy propyl cellulose, carboxy methyl cellulose (CMC), and sodium (S) CMC belong to the semisynthetic category and have been extensively studied in SR tablet formulations.5

Direct compression is a widely used manufacturing method for the preparation of tablets.6 The current research experimentation focuses on the design of a SR formulation for pravastatin.

Pravastatin, a potent hypolipidemic agent, belongs to biopharmaceutical classification class-III. It is a specific inhibitor (competitive) of HMG CoA. Pravastatin is useful for the effective management of atherosclerotic vascular disease. It undergoes an extensive first pass effect in the liver. Its bioavailable fraction is 0.17, about 50% of protein binding (plasma proteins). The elimination half-life for pravastatin is 1.5-2 h and it is eliminated from the body via feces and urine. Hence, research work was planned to formulate and evaluate SR tablets for pravastatin as a model drug and had the objective that the optimized formulation trial should show desired SR of the drug by means of an enhanced dissolution rate.7,8,9,10,11,12,13,14

Response surface methodology (RSM) with a polynomial equation has been extensively applied in the design and development of pharmaceutical products. Variations of RSM include 32 factorial design, central composite design, and Box-Behnken design. RSM is applied when only a few significant factors are involved in the optimization procedure. The advantage of this method is less experimentation and time, the results are more effective, and it is more cost effective than tradition experimentation models.15,16,17,18

Hence an attempt was made in the present research work to formulate SR tablets of pravastatin using HPMC K4M and SCMC. Instead of a heuristic method, a standard statistical tool design of experiments was used to study the effect of formulation variables on the release properties.

A 32 factorial design was used to study the effect of polymers on the drug release profile (effect of independent variables or factors), i.e. the quantity of HPMC K4M and SCMC, on the dependent variables (t10%, t50%, t75%, t90%).19


MATERIALS AND METHODS

The materials used in the research were procured from various sources. Pravastatin was a gift sample from Konis Pharma Ltd, Baddi, India. HPMC K4M, SCMC, and lactose were obtained from Meditech Pharma Ltd, Solan. Magnesium stearate, talc, and lactose obtained from Loba Chemie Pvt. Ltd, Bombay.


Formulation and development of SR pravastatin tablets

Quantities required for the HPMC K4M and SCMC for the preparation of SR pravastatin tablets were selected as independent variables. t10%, t50%, t75%, and t90% were selected as dependent variables. Polynomial equations were developed for dependent variables as per backward stepwise linear regression analysis.20,21

The 3 levels of X1 (HPMC K4M) were 7.5%, 12.5%, and 17.5%. The 3 levels of X2 (SCMC) were 7.5%, 12.5%, and 17.5% (% with respect to average weight of tablet). Nine SR pravastatin tablet formulations were designed using selected combinations of X1 and X2 and checked for the selection of the optimum composition required to meet the primary objective of the study.


Preparation of SR pravastatin tablets

All the ingredients were procured and weighed accurately. They were mixed uniformly in a poly bag for 10-15 min. The resulting mix was subjected to screening (#44). Lubricant was added, followed by mixing well and then compression using a tablet compressor. The resulting tablets were checked in terms of pharmacopoeial limits. The tablets were packed in well-closed air-tight containers.


Experimental design

The experimental design used in the current research was a 32 factorial design; the quantity of HPMC K4M was labeled X1 and the quantity of SCMC was labeled X2 and they are presented in Table 1. The 3 levels chosen for both X1 and X2 were coded as -1=7.5%, 0=12.5%, and +1=17.5%. The formulations for the factorial trials are presented in Table 2.


Evaluation of SR pravastatin tablets


Hardness

This test was performed with the help of a Monsanto hardness tester.


Friability

This test was carried out in a Roche friabilator. The initial weight (W0) of 20 tablets was noted and then they were dedusted in a drum with a speed of 25 rpm for 4 min and weighed (W) again. Percentage friability was calculated using the following equation. The weight loss should not be more than 0.8%.

Friability (%)=[(wo-w)/w]×100


Assay

This test was carried out by taking a fixed number of samples (20) and subjecting them to pulverization. From that above resultant mixture powder   equivalent to 100 mg was dissolved in 100 mL of solvent (6.8 buffer) and sonicated if necessary followed by filtration. The absorbance of the resultant solution was measured using a ultraviolet (UV)-Visible spectrophotometer at 239 nm.15


Thickness

This test was performed with the help of vernier calipers.


In vitro dissolution study

Dissolution tests were performed using the USP Apparatus 2. The specifications were followed as per official methods such as dissolution medium for initial 2 h is 900 mL of pH 1.2 buffer followed by pH 6.8, at 50 rpm and 37±0.5°C. Samples were collected at fixed time intervals by a pre-filter connected syringe and replacement of fresh fluid was done simultaneously. The absorbance of samples was measured at 239 nm using a Labindia UV-3200 UV-Visible spectrophotometer (n=3).9,12,14


Kinetic modeling of drug release

The kinetic data were subjected to statistical modeling, i.e. zero order, first order, Higuchi, and Korsmeyer-Peppas kinetics.22,23

The study did not require ethics committee approval or patient informed consent because it did not focus on any clinical parameter and did not utilize any humans/animals for the processing of work.


RESULTS AND DISCUSSION

SR tablets of pravastatin were formulated with the help of a 32 factorial design for identifying the optimized composition of polymers (HPMC K4M and SCMC) and to obtain prolonged/sustained drug release from the formulation. The experimental design is presented in Table 1. The 2 factors involved in the design of formulations are quantity of HPMC K4M and SCMC, which were labeled as independent variables (X1, X2), while kinetic parameters were labeled as dependent variables (t10%, t50%, t75%, t90%). Nine factorial batches were designed and all trials had 40 mg of pravastatin as a SR tablet dosage form by direct compression technique as per the formulae given in Table 2.

All final batches were subjected to various final product quality assurance tests like mean hardness, mean thickness, friability, weight variation, and drug content, and the results are summarized in Table 3. Hardness for finished batches was in the range of 3.47±0.3-4.10±0.5 kg/cm2. Thickness for finished batches was in the range of 2.45±0.15-2.86±0.14 mm. Results for the friability test were less than 0.51%. Drug content for finished batches met the acceptance criterion. Drug release studies were performed for finished batches using pH 1.2 buffer for an initial 2 hour followed by phosphate buffer pH 6.8 as operated under a standard set of conditions at 50 rpm (paddle), 37±0.5°C. Dissolution plots are presented in Figures 1-2-3-4 (kinetic plots) and the statistical parameters are summarized in Table 4. % percentage cumulative drug release  for finished batches F1-F9 at 12 hour was 88.88-99.61%. The result revealed that the release rate of drug was inversely proportional to the quantity of polymers. Hence the desired drug release was achieved by manipulating values of independent variables. A difference was seen in dependent variables due to change in proportions of X1 and X2. Formulation coded F5 containing 25 mg of HPMC K4M and 25 mg of SCMC produced desirable release characteristics (t10%=0.459 h, t50%=3.025 h, t75%=6.040 h, t90%=10.045 h), which was probably due to variation in the viscosity of the polymer matrix. An increase in the viscosity of the stagnant layer results in a corresponding decrease in drug release (due to thicker gel layer formation).24 The dissolution profiles of SR pravastatin tablets were subjected to kinetic modeling. The results are presented in Table 4 and Figures 1-2-3-4. The results reveal that all formulation batches best fitted zero order kinetics and r2 was in the range of 0.995-0.999. They also fitted Higuchi’s kinetics; r2 was in the range of 0.941-0.968. The Peppas treatment revealed that all batches follow a non-Fickian diffusion path (n values 1.046-1.397). Polynomial equations were developed for all dependent variables by linear stepwise backward regression analysis with the help of PCP Disso software and response morphological plots were constructed using SigmaPlot V13. The response morphological plots are presented in Figures 5-6-7-8 for t10%, t50%, t75%, and t90% using X1 and X2 on both axes to show the effects of independent variables on the dependent variables. Kinetic parameters for the trials (F1-F9) are presented in Table 5.

The polynomial equation for the 3² full factorial design was as follows:

Y=b0+b1 X1+b2 X2+b12 X1X2+b11 X1²+b22 X2²…

Y- dependent variable, b0- mean response of 9 trials, b1- estimated coefficient for X1, b2 -estimated coefficient for X2, b12- interaction term, X1² and X2²- coefficients for nonlinearity. Validity of the derived equations was evaluated by formulating 2 counter check batches of intermediate quantities (C1, C2).

The equations for dependant variables developed as mentioned below,

Y1=0.514-0.012 X1-0.094 X2-0.038X1X2+0.055 X12+0.0171 X22 (for t10%)

Y2=3.393-0.078 X1-0.612 X2-0.250 X1X2+0.363 X12+0.112 X22 (for t50%)

Y3=6.79-0.155 X1-1.222 X2-0.507 X1X2+0.722 X12-0.225 X22 (for t75%)

Y4=11.280-0.260 X1-2.01 X2-0.840 X1X2+1.21 X12+0.371 X22 (for t90%)

Batch (F5) is the identical product

The +ve sign for the coefficient of X1 in Y1, Y2, Y3, and Y4 signifies that as the amount of X1 increases all independent variable values also increase. In other words the data demonstrate that both X1 and X2 affect t10%, t50%, t75%, and t90%. From the results it can be concluded that an increase in the amount of polymer leads to a decrease in release rate of the drug and the drug release pattern may be altered by changing the quantities of X1 and X2 to appropriate levels. The dissolution parameters predicted from the polynomial equations and those actually observed from the experimental results are summarized in Table 6. Closeness of results was seen between actual values and predicted values. This proves that the polynomial equation developed was valid and confirms the validity of the derived equations. The response surface/surface morphological plots were presented to show the effects of X1 and X2 on dependent variables. The final best (optimized, based on desirability factor above 0.999) formulation (F5) is an identical product showing a similarity factor (f2) of 89.559, difference factor (f1) of 1.546, and tcal is <0.05 when compared with the marketed product (Pravachol).


CONCLUSION

The current research work focused on the utility of macromolecules (polymers) such as HPMC K4M and SCMC in the formulation of SR tablets for pravastatin using a 32 factorial design. The results revealed that the amount of polymers was inversely proportional to the rate of drug release from the formulation. Utilization of polymers in the formulation was beneficial for obtaining prolonged release of the active moiety. Formulation F5 follows zero order release and a non-Fickian diffusion mechanism. F5 may be administered for the effective management of hypercholesterolemia and atherosclerotic vascular disease and to reduce the risk of cardiovascular disease. The best formulation shows good retaining characteristics. It also avoids the first pass effect, which will ultimately improve the clinical response.


ACKNOWLEDGEMENTS

The author would like to thank the Management & Staff of MAM College of Pharmacy, Kesanupalli, Narasaraopet, Guntur, Andhra Pradesh, India for providing support for successful completion of the research study.

Conflicts of interest: No conflict of interest was declared by the authors. The authors alone are responsible for the content and writing of the paper.

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