ENHANCEMENT OF DISSOLUTION RATE AND FORMULATION DEVELOPMENT OF RITONAVIR TABLETS EMPLOYING STARCH PHOSPHATE- A NEW MODIFIED STARCH

ENHANCEMENT OF DISSOLUTION RATE AND FORMULATION DEVELOPMENT OF RITONAVIR TABLETS EMPLOYING STARCH PHOSPHATE- A NEW MODIFIED STARCH

P. R. Chowdary and Veeraiah Enturi

University College of Pharmaceutical Sciences, Andhra University, Visakhapatnam-530003, Andhra Pradesh, India

ABSTRACT

The objective of the study is to prepare, characterize and evaluate starch phosphate, a new modified starch as a carrier in solid dispersions for enhancing the dissolution rate of ritonavir. The feasibility of formulating solid dispersions of ritonavir in starch phosphate into compressed tablets with enhanced dissolution rate was also investigated.   Starch phosphate was prepared by reacting starch with di-sodium hydrogen orthophosphate anhydrous at elevated temperatures. It was insoluble in water and has good swelling (400%) property without pasting or gelling when heated in water. Solid dispersions of ritonavir in starch phosphate were prepared by solvent evaporation method employing various weight ratios of drug: starch phosphate such as 2:1(SD-1), 1:1(SD-2), 1:2(SD-3), 1:3(SD-4) and 1:9(SD-5) and were evaluated for dissolution rate and efficiency. All the solid dispersions prepared gave rapid and higher dissolution of ritonavir when compared to pure drug. A 58.34 and 94.41 fold increase in the dissolution rate (K₁) of ritonavir was observed with solid dispersions SD-4 and SD-5 respectively. The DE₃₀ was also increased from 6.80% in the case of ritonavir pure drug to 76.25% and 84.05% in the case of these solid dispersions. Ritonavir (50 mg) tablets were prepared employing ritonavir alone and its solid dispersions SD-3 and SD-4 by wet granulation method and were evaluated. Ritonavir tablets formulated employing its solid dispersions in starch phosphate gave rapid and higher dissolution rate and DE₃₀ when compared to plain and commercial tablets.  A 9.95 and 28.14 fold increase in the dissolution rate (K₁) was observed with tablet formulations containing solid dispersions SD-3 and SD-4 respectively when compared to plain tablets

Formulation Development

INTRODUCTION: Ritonavir, a widely prescribed antiretroviral protease inhibitor drug belong to Class II under BCS and exhibit low and variable oral bioavailability due to its poor aqueous solubility. Its oral absorption is dissolution rate limited and it requires enhancement in solubility and dissolution rate for increasing its oral bioavailability. Several techniques ¹ such as micronization, cyclodextrin complexation, use of surfactants and solubilizers, solid dispersion in water soluble and dispersible carriers, use of salts, prodrugs and polymorphs  which exhibit high solubility , microemulsions and self emulsifying micro and nano disperse systems have been used to enhance the solubility, dissolution rate and bioavailability of poorly soluble drugs. Among the various approaches, solid dispersions in water dispersible excipients are simple, industrially useful approach for enhancing the solubility, dissolution rate and bioavailability of poorly soluble drugs.

Starch phosphate is one of the modified starches used in the frozen food industry ^{2, 3}. It is produced by phosphorification of free hydroxyl groups of anhydroglucose units of starch molecule. They are esterified with phosphate reagents. Phosphate reagents for starch phosphate monoester are orthophosphate salts ⁴. No reports are available on its use as pharmaceutical excipient. We have earlier reported starch phosphate as an efficient disintegrant in tablet formulations ⁵.

The objective of the present study is to prepare, characterize and evaluate starch phosphate as a carrier in solid dispersions for enhancing the dissolution rate of ritonavir. The feasibility of formulating solid dispersions of ritonavir in starch phosphate into compressed tablets with enhanced dissolution rate was also investigated.

MATERIALS AND METHODS:

Materials: Ritonavir was gift sample from M/s Hetero Drugs Ltd., Hyderabad, starch phosphate was prepared in the laboratory, Dichloromethane (Qualigens), potato starch (S.D Fine Chemicals), citric acid (Qualigens) Methanol (S.D Fine Chemicals), lactose, talc, magnesium stearate , acacia  were procured from commercial sources.

Methods:

Preparation of Starch Phosphate: Starch phosphate was prepared based on the method of Choi et al., ⁶ with some modifications. Potato starch (100 g) and di-sodium hydrogen orthophosphate anhydrous (30 g) were suspended in 100 ml of water and continuously stirred for 20 min. This starch slurry was then filtered and the wet starch mixture was conditioned for 12 h at room temperature (28°C). To enhance phosphorylation, this mixture was heated in a forced air oven at 130°C for 3 h. The product obtained was ground and sized.

Characterization of Starch Phosphate: The starch phosphate prepared was evaluated for following

Solubility: Solubility of starch phosphate was tested in water, aqueous buffers of pH 1.2, 4.5, and 7.4 and organic solvents such as alcohol, dichloromethane, chloroform, acetone and petroleum ether.

pH: The pH of a 1% w/v slurry was measured.

Melting Point: Melting point was determined by using melting point apparatus as well as by DSC spectra.

Viscosity: Viscosity of 1% dispersion in water was measured using Ostwald Viscometer.

Swelling Index: Starch phosphate (200 mg) was added to 10 ml of water and light liquid paraffin taken in two different graduated test tubes and mixed. The dispersion in the tubes was allowed to stand for 12 h. The volumes of the sediment in the tubes were recorded. The swelling index of the material was calculated as follows.

Volume of sediment in water – Volume of sediment in light liquid paraffin 

S.I (%) =                                                                                                  X 100                                                                                                      

Volume of sediment in light liquid paraffin

Test for gelling property: The gelling property (gelatinization) of the starch and starch phosphate prepared was evaluated by heating a 7% w/v dispersion of each in water at 100⁰C for 30 min.

Moisture absorption: The hygroscopic nature of starch phosphate was evaluated by moisture absorption studies in a closed desiccator at 84% relative humidity and room temperature.

Particle size: Particle size analysis was done by sieving using standard sieves.

Density: Density (g/cc) was determined by liquid displacement method using benzene as liquid.

Bulk density: Bulk density (g/cc) was determined by three tap method in a graduated cylinder.

Angle of repose: Angle of repose was measured by fixed funnel method.

Compressibility index: Compressibility index (CI) was determined by measuring the initial volume (V_o) and final volume (V) after hundred tappings of a sample of starch phosphate in a measuring cylinder. CI was calculated using equation

Compressibility index (CI) =  V_o – V   X 100

V_o

Estimation of Ritonavir: An UV spectrophotometric method based on the measurement of absorbance at 210 nm in 0.1N hydrochloric acid was used for estimation of ritonavir. The method obeyed Beer- Lambert’s law in the concentration range of 1-10 µm/mL. When the standard drug solution was assayed repeatedly (n=6), the relative error (accuracy) and coefficient of variation (precision) were found to be 0.60% and 1.0% respectively. No interference from excipients used was observed.

Preparation of Solid Dispersions of Ritonavir in Starch Phosphate: Solid dispersions of ritonavir and starch phosphate were prepared in 2:1 (SD-1), 1:1 (SD-2), 1:2 (SD-3), 1:3 (SD-4) and 1:9 (SD-5) ratios of drug: carrier by solvent evaporation method. Ritonavir (1 g) was dissolved in dichloromethane (10 ml) in a dry mortar to get a clear solution. Starch phosphate (1 g) was then added and mixed. The thick slurry was triturated for 15 min for complete evaporation of dichloromethane and then dried at 55^oC until dry. The dried mass was pulverized and sieved through mesh no. 100.

Preparation of Ritonavir-SD Tablets: Compressed tablets each containing 50 mg of ritonavir were prepared by wet granulation method employing ritonavir alone and its solid dispersions (SD-3 and SD-4) in starch phosphate. Lactose was used as diluent to adjust the weight of the tablet to 220 mg. acacia (2%), talc (2%) and magnesium stearate (2%) were incorporated respectively as binder and lubricants.

The tablet granules were prepared by wet granulation method and were compressed into tablets on a Cadmach 16-station rotary tablet punching machine (M/s Cadmach Engineering Co. Pvt. Ltd., Mumbai) using 9 mm concave punches. All the tablets prepared were evaluated for content of active ingredients, hardness, friability, disintegration time and dissolution rate as per official (IP) methods.

Dissolution Rate Study: Dissolution rate of ritonavir as such and from its solid dispersions and tablets prepared was studied in 0.1N hydrochloric acid (900 ml) employing USP 8 station Dissolution Rate Test Apparatus (M/s Labindia Disso 8000) with a paddle stirrer at 50 rpm. Ritonavir or its solid dispersions equivalent of 100 mg of ritonavir and one tablet containing 50 mg of ritonavir was used in each test. A temperature 37+1^oC was maintained in each test. Samples of dissolution medium (5 ml) were withdrawn through a filter (0.45μ) at different time intervals and assayed for ritonavir at 210 nm. For comparison, dissolution of ritonavir from one commercial brand was also studied.  All the dissolution experiments were conducted in triplicate (n=3).

RESULTS AND DISCUSSION: Starch phosphate was prepared by reacting starch with di-sodium hydrogen orthophosphate anhydrous at elevated temperatures. The reactions involved are shown in Fig. 1.

FIG. 1: PHOSPHORIFICATION OF POTATO STARCH TO PRODUCE STARCH PHOSPHATE

Starch phosphate prepared was found to be white, crystalline, non hygroscopic powder and can easily be ground to different sizes. Powder which passes through mesh no.80 and retained on mesh no.120 was collected. This powder has an average particle size of 152 µm. The starch phosphate prepared was characterised by determining various physical properties. The properties of starch phosphate prepared are summarised in Table 1.

TABLE 1: PHYSICAL PROPERTIES OF THE STARCH PHOSPHATE PREPARED

When tested for m.p., it was charred at 210⁰C. Starch phosphate prepared was insoluble in water, aqueous fluids of acidic and alkaline pH and several organic solvents tested. In water it exhibited good swelling (400%). No gelling/pasting was observed with starch phosphate when its aqueous dispersion was heated at 100⁰C for 30 min, where as potato starch formed a paste/gel during the above heat treatment. In the micromeritic evaluation, the angle of repose and compressibility index values revealed the excellent flow characteristic of starch phosphate prepared.

As starch phosphate, a chemically modified starch was found to be insoluble in water and has good swelling property without pasting or gelling when heated in water it is considered as a promising carrier for solid dispersions for enhancing the dissolution rate of poorly soluble drugs. Solid dispersions of ritonavir in starch phosphate were prepared by solvent evaporation method employing various weight ratios of drug: starch phosphate. All the solid dispersions prepared were found to be fine and free flowing powders with an angle of repose in the range 18^o - 20^o_.  Low C.V. (< 1.0%) in the percent drug content indicated uniformity of drug content in each batch of solid dispersions prepared.

The dissolution rate of ritonavir alone and from its solid dispersions was studied in 0.1N hydrochloric acid. All the solid dispersions prepared gave rapid and higher dissolution of ritonavir when compared to pure drug. The dissolution data were analyzed as per zero order and first order kinetics in each case. The R² values were higher in the first order model than in the zero order model indicating that the dissolution of ritonavir as such and from its solid dispersions followed first order kinetics. The corresponding dissolution rate (K₁) values of various products were estimated. Dissolution Efficiency (DE₃₀) values were calculated as described by Khan et al ⁷. The dissolution parameters of ritonavir and its solid dispersions are given in Table 2.

TABLE 2: DISSOLUTION PARAMETERS OF THE SOLID DISPERSIONS OF RITONAVIR PREPARED EMPLOYING STARCH PHOSPHATE AS A CARRIER

Ratio of drug: starch phosphate in solid dispersions: SD-1 (2:1); SD-2 (1:1); SD-3 (1:2); SD-4 (1:3); SD-5 (1:9); PD₁₀: percent dissolved in 10 min; T₅₀: time for 50 % dissolution; DE₃₀: dissolution efficiency upto 30 min; K₁: first order dissolution rate. Solid dispersions of ritonavir showed superior dissolution properties when compared to ritonavir pure drug. Both dissolution rate (K₁) and DE₃₀ values were much higher in the case of solid dispersions when compared to ritonavir pure drug. The dissolution rate (K₁) and DE₃₀ values increased as the proportion of starch phosphate was increased. The number of folds of increase in dissolution rate (K₁) and DE₃₀ observed with various solid dispersions are shown in Table 2. A 58.34 and 94.41 fold increase in the dissolution rate (K₁) of ritonavir was observed with solid dispersions SD-4 and SD-5 respectively.

The DE₃₀ was also increased from 6.80% in the case of ritonavir pure drug to 76.25% and 84.05% in the case of SD-4 and SD-5 respectively. Thus solid dispersions of ritonavir prepared employing starch phosphate as carrier showed marked enhancement in the dissolution rate (K₁) and DE₃₀ of ritonavir.

The feasibility of formulating ritonavir solid dispersions in starch phosphate into tablets retaining their rapid and higher dissolution rates was also investigated. Ritonavir (50 mg) tablets were prepared employing ritonavir alone and its solid dispersions SD-3 and SD-4 by wet granulation method and were evaluated. All the ritonavir tablets prepared were found to contain the ritonavir with in 100±3% of the labelled claim. Hardness of the tablets was in the range 5-8 Kg/sq.cm. Percentage weight loss in the friability test was less than 0.55% in all the cases.

Tablets formulated employing solid dispersions disintegrated rapidly with in 1.0 min. Tablets formulated employing ritonavir pure drug disintegrated within 5-8 min. As such all the ritonavir tablets prepared were of good quality with regard to drug content, friability, hardness and disintegration time and fulfilled the official (IP) specifications of uncoated tablets.

The dissolution parameters of the prepared tablets are given in Table 3. Dissolution of ritonavir from all the tablets prepared followed first order kinetics with correlation coefficient ‘R²’ values > 0.925.

Ritonavir tablets formulated employing its solid dispersions in starch phosphate (TF2 and TF3) gave rapid and higher dissolution rate and DE₃₀ when compared to plain (TF1) and commercial tablets.  A 9.95 and 28.14 fold increase in the dissolution rate (K₁) was observed with formulations TF2 and TF3 when compared to formulation TF1. A 2.23 and 6.29 fold increase in the dissolution rate (K₁) was observed with formulations TF2 and TF3 when compared to commercial formulation. Thus solid dispersions of ritonavir in starch phosphate could be formulated into compressed tablets retaining their fast dissolution characteristics and fulfilling official standards.

TABLE 3: DISSOLUTION PARAMETERS OF RITONAVIR TABLETS FORMULATED EMPLOYING RITONAVIR ALONE AND ITS SOLID DISPERSIONS IN STARCH PHOSPHATE

TF1: tablets formulated employing ritonavir alone and using lactose as diluent; TF2: tablets formulated employing ritonavir solid dispersion SD-3; TF3: tablets formulated employing ritonavir solid dispersion SD-4 PD₁₀: percent dissolved in 10 min; T₅₀: time for 50 % dissolution; DE₃₀: dissolution efficiency upto 30 min; K₁: first order dissolution rate.

CONCLUSION: Starch phosphate prepared by reacting starch with di-sodium hydrogen orthophosphate anhydrous at elevated temperatures was insoluble in water and has good swelling (400%) property without pasting or gelling when heated in water. Solid dispersions of ritonavir in starch phosphate prepared by solvent evaporation method employing various weight ratios of drug: starch phosphate gave rapid and higher dissolution of ritonavir when compared to pure drug. Dissolution followed first order kinetics. A 58.34 and 94.41 fold increase in the dissolution rate (K₁) of ritonavir was observed with solid dispersions prepared at 1:3 and 1:9 ratios of drug; starch phosphate respectively. The DE₃₀ was also increased from 6.80% in the case of ritonavir pure drug to 76.25% and 84.05% in the case of these solid dispersions. Ritonavir tablets formulated employing its solid dispersions in starch phosphate also gave rapid and higher dissolution rate and DE₃₀ when compared to plain and commercial tablets.  A 9.95 and 28.14 fold increase in the dissolution rate (K₁) was observed with tablet formulations containing solid dispersions prepared at 1:2 and 1:3 ratios respectively when compared to plain tablets. Solid dispersions of ritonavir prepared employing starch phosphate as carrier showed marked enhancement in the dissolution rate (K₁) and DE₃₀ of ritonavir. These solid dispersions could be formulated into compressed tablets retaining their fast dissolution characteristics and fulfilling official (I.P.) standards.

ACKNOWLEDGEMENTS: Authors are thankful to University Grants Commission, New Delhi for providing financial assistance in the form of UGC JRF to Veeraiah Enturi.

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