INTRODUCTION:

Targeting active drug molecule to colon is highly desirable in the treatment of colonic disorders like ulcerative colitis, crohn's disease, infectious diseases and colon cancer whereby high local concentration can be achieved with reduced side effects. To achieve successful colonic delivery, a drug needs to be protected from absorption and/or the environment of the upper gastrointestinal tract (GIT) and then be abruptly released into the proximal colon, which is considered the optimum site for colon-targeted delivery of drugs. There are various approaches have been developed and studied in order to achieve colonic drug release. These approaches rely on gastrointestinal pH, transit time, microbial flora and luminal pressure for site-specific delivery.

At present, Time-dependent formulations are designed to release drug load after a predetermined lag time (5-6 hrs), which is based on transit time of drug through stomach and small intestine. A nominal lag time of 5 or 6 hours is incorporated into the delivery system on the assumption that this is the time required for the dosage form to reach the colon.^{1, 2}.

Prednisolone, a typical glucocorticoid, acts specifically on the inflammatory sites and decrease inflammation. Glucocorticoids are naturally occurring hormones that prevent or suppress inflammation and immune responses when administered at pharmacological doses. They induce the lipocortin-1 synthesis (Phospholipase A2 inhibitory proteins collectively called lipocortins), which then binds to cell membranes, preventing the phospholipase A2 from coming into contact with its substrate arachidonic acid. This leads to diminished eicosanoid production. The cyclooxygenase (both COX-1 and COX-2) expression is also suppressed, potentiating the effect. It is drug of choice for inflammatory conditions like ulcerative colitis and Crohn’s disease. Prednisolone has many side effects in upper gastrointestinal tract. So there is need to develop drug delivery system for Prednisolone, which can release the drug in colon, and to achieve highest local concentration and minimize systemic side effects. Prednisolone is a poorly water soluble drug having pH-independent solubility. So it is required to enhance its solubility.

Press coating is an accepted pharmaceutical manufacturing technique because of its many advantages such as low labor and energy requirements, and use of nonsolvent processes. Recently, this technique has been applied to prepare different solid dosage forms, such as fast-disintegrating tablets or controlled-release preparations. Time-controlled release preparations have been extensively developed to achieve time- and/or site-specific release. An attempt has been made to develop colon targeted press coated Prednisolone tablets capable of releasing drug only after a well lag time dependent on the outer coating composition and thickness of polymer.

MATERIALS AND METHODS:

Materials:

Prednisolone was obtained from Naprod Life Sciences Pvt. Ltd., Mumbai as a gift. Natrosol M, Natrosol L, Natrosol HHX were supplied by Signet Chemical Corporation, Mumbai as free sample. Croscarmellose Sodium (Ac-Di-Sol^®), microcrystalline cellulose (Avicel PH 101^®), magnesium stearate and Hydroxypropyl β-cyclodextrin (HP-β-CD) were denoted by Cadila Healthcare Ltd., Ahmedabad, India.

Methods:

Phase solubility studies^{3, 4}:

Drug-hydroxypropyl β-cyclodextrin (HP β-CD) interactions in solution were investigated by the phase solubility analysis. An excess of drug was added to 10 ml of HP β-CD aqueous solutions (0.00-0.009 mol L^–1) in conical flasks and shaken at 37۫⁰C for 72 hours in orbital shaking incubator. At equilibrium after 3 days, aliquots were withdrawn, filtered through Whatman filter paper (N0. 41) and absorbances were measured at wavelength 248.0 nm spectrophotometrically. The apparent stability constant of the prednisolone- hydroxypropyl β-cyclodextrin (HP β-CD) complex was calculated from the phase-solubility diagram,

Slope

Kc = -----------------

S₀ (1 – Slope)

Where, Kc is the stability constant (binding constant) (M^-1), slope is obtained from the linear relationship between the concentration of prednisolone and hydroxypropyl β-cyclodextrin (HP β-CD) and S₀ is intrinsic solubility of prednisolone. Each experiment was carried out in triplicate.

Preparation of core tablets:

The core tablet was prepared by direct compression method. Prednisolone (5 mg/tablet) & Hydroxypropyl β-Cyclodextrin (HP β-CD) (18.56 mg/tablet) were dry blended first for 15 minutes followed by the addition of Avicel PH 101 ^®, Ac-Di-Sol^®and magnesium stearate (0.75 mg).The all ingredients were blended further for 10 minutes. The resulting powder mixtures were then compressed into tablets (75 mg/tablet weight) using a rotary tablet machine equipped with 6 mm concave punch. The table 1 gives the details of core tablets formulations. The core tablets were evaluated for thickness, hardness, weight variation, friability and drug content or physical properties.

Table 1: Composition of core tablets with varying contents of Ac-Di-Sol^®and Avicel PH 101^®.

Ingredients	Quantity (mg) in each Tablet
Ingredients	G1	G2	G3	G4	G5
Prednisolone	5	5	5	5	5
Hydroxypropyl β-Cyclodextrin (HP β-CD)	-	18.56	18.56	18.56	18.56
Avicel PH 101^®	69.25	49.94	49.19	48.44	47.69
Ac-Di-Sol^®	0	0.75	1.5	2.25	3
Magnesium stearate (1%)	0.75	0.75	0.75	0.75	0.75
Total weight (mg)	75	75	75	75	75

Preparation of press coated tablets:

All excipients were previously passed through the sieve no. 44 and 75 mg of coating was used for the upper and lower shell. In Formulation R4 and R5 have 50 and 100 mg coating was used respectively. The press coating of tablet was performed using a rotary tablet machine. A half amount of the powder was filled into the die to make a powder bed, on the center of which the core tablet was placed manually. Then, the remaining half of the coating material filled in the die, and the contents were compressed under a sufficient compression force, using a concave punch 8 mm in diameter to get the hardness of 7-8 kg/cm² for press coated tablet^5,6,7. The formulations of tablets with their codes are listed in Table 2, 3, 4 and 5.

Table 2: Coating with various grades of Natrosol.

Formulations		Coating Material		Amount used in upper and lower shell (mg)
Code	Core Tablet	Grade	Viscosity ( cps )	Amount used in upper and lower shell (mg)
R1	G5	Natrosol M	1500-2500 At 1%	75
R2	G5	Natrosol L	75 – 150 At 5 %	75
R3	G5	Natrosol HHX	3500-5500 At 1 %	75

Table 3: Coating with different amounts of Natrosol-M.

Formulations		Coating Material	Amount used in upper and lower shell (mg)
Code	Core Tablet	Coating Material	Amount used in upper and lower shell (mg)
R4	G5	Natrosol M	50
R1	G5	Natrosol M	75
R5	G5	Natrosol M	100

Evaluation of press coated tablets:

The Press coated tablets were evaluated for thickness, hardness, weight variation, friability (Indian Pharmacopoeia)^8,9 and drug content (British Pharmacopoeia)¹⁰.

Table 4: Coating with different combinations of hydrophilic polymer (Natrosol M) and high viscosity polymer (Natrosol-HHX).

Formulations		Coating Material (75 mg)	Ratio %
Code	Core Tablet	Coating Material (75 mg)	Ratio %
R₁	G5	Natrosol M: Natrosol HHX	100 : 0
R6	G5	Natrosol M : Natrosol HHX	75 : 25
R7	G5	Natrosol M: Natrosol HHX	50 : 50
R8	G5	Natrosol M: Natrosol HHX	25 : 75
R3	G5	Natrosol M: Natrosol HHX	0 : 100

Table 5: Coating with different combinations of hydrophilic polymer (Natrosol M) and low viscosity polymer (Natrosol-L).

Formulations		Coating Material (75 mg)	Ratio %
Code	Core Tablet	Coating Material (75 mg)	Ratio %
R₁	G5	Natrosol M: Natrosol L	100 : 0
R9	G5	Natrosol M: Natrosol L	75 : 25
R10	G5	Natrosol M: Natrosol L	50 : 50
R11	G5	Natrosol M: Natrosol L	25 : 75
R2	G5	Natrosol M: Natrosol L	0 : 100

In vitro release Study:

The test was carried out in a USP dissolution basket apparatus (Model-DT 60, Veego, India) at 100 rpm and 37° ± 0.5 °C. In vitro drug release studies of press coated tablets were done with changing media (i.e. in 0.1N HCl for 2 hrs, pH 6.8 phosphate buffer for next 4 hrs and finally in pH 7.4 phosphate buffer) in order to relate it to mouth to colon transit time and change in pH conditions. Aliquots (10 ml) of dissolution fluid were removed every hour, filtered and assayed for the amount of prednisolone released by a spectrophotometer at a wavelength of 248.0 nm. All the dissolution studies were performed in triplicate to obtain mean and standard deviation.

RESULTS AND DISCUSSION:

Phase solubility analysis of Prednisolone with HP β-CD:

Prednisolone is a very slightly soluble drug or poorly water soluble drug. The intrinsic solubility of prednisolone was found to be 0.49.This is similar to the reported solubility (0.49 mg/ml).The intrinsic solubility of Prednisolone was 0.49 mg/ml at 37^oC in distilled water ⁵.The solubility increased in presence of HP β-CD.

Figure 1: Phase solubility diagram of Prednisolone in 0.00-0.009 mol L^–1 HP β-CD aqueous solution.

The phase solubility diagram is shown in figure 1.The phase solubility diagram can be classified as A_Ltype. The linear regression analysis gave slope=0.553, intercept=0.0013 and r²=0.9965.The slope (0.553) indicates formation of 1:2 (Drug: HP β-CD) complex. Solubility constant obtained from the data is 958.68 M^–1. This is similar to reported solubility constant (960 M^–1)³. Prednisolone-HP β-CD interactions in solution were concluded by taking Infrared spectra of Prednisolone, HP β-CD and physical mixture. Infrared spectra of Prednisolone, HP β-CD and physical mixture are given in Figure No.2, 3 and 4 respectively.

Figure 2: IR Spectrum of Prednisolone.

Figure 3: Infrared spectrum of HP β-CD.

Figure 4: Infrared spectrum of Prednisolone-HP β-CD Physical Mixture.

It was observed that FT-IR spectra of HP β-CD and physical mixture have similarity, physical mixture gives two new peaks at 1650(-C=O stretching (Diketone)).so the finding indicates that that prednisolone having C=O (Diketone) may be interaction with HP β-CD.

Evaluation of core and press tablets:

All core and press coated tablets showed uniform thickness. Percent of drug content was more than 97 %. The percent friability values for all the formulations were below 1% indicating that the friability was within the prescribed limits. All core and press coated tablets showed acceptable hardness.

Dissolution of core tablets:

The Prednisolone core tablets were designed for faster disintegration and dissolution. Effect of Ac-Di-Sol level on drug release from core tablets (formulations G1 to G5) was determined. Drug release profile is given in Figure 5.

Figure 5: Effect of Ac-Di-Sol^® level on drug release profile from core tablets (G1 to G5).

Formulation G1 showed delayed drug release as it did not contain Ac-Di-Sol and Hydroxypropyl β-Cyclodextrin (Figure). Formulations G2 to G5 showed faster drug release because of presence of HP β-CD and Ac-Di-Sol. Ac-Di-Sol helps in disintegration of tablets and HP β-CD helps in solubilization of prednisolone. Formulations G3, G4 and G5 showed complete release of drug in nine minutes. Formulation G5 was chosen for further studies because G5 showed faster initial drug release as compared to initial drug release from G3 and G4.

v Effect of Natrosol (hydroxyethylcellulose) viscosity grades on drug release from the press-coated tablet:

Press-coated tablets were prepared by altering the Natrosol viscosity grades of the outer layer and their dissolution behaviors were determined. Dissolution data for effect of different hydroxyethylcellulose viscosity grades (Natrosol L, M, and HHX) on drug release from the press-coated tablet is given in Figure 6. The lag time increased in the order of Natrosol HHX > Natrosol M > Natrosol L as increasing viscosity grades of Natrosol ¹¹.

Figure 6: Effect of polymer viscosity on drug release from the press coated tablet.

R1- Natrosol-M (75 mg)

R2- Natrosol-L (75 mg)

R3- Natrosol-HHX (75 mg)

Formulation R2 (Natrosol-L coated tablet) showed lag time of 2 hrs because NATROSOL-L is low viscosity (75 cps) erodible polymer. When tablet comes in contact with dissolution medium surface erosion take place. So dissolution medium penetrates into core composition very easily. This lead to rapid drug release after 2 hrs.

Formulation R1 (Natrosol-M coated tablet) showed lag time of 5.5 hrs because NATROSOL-M is medium viscosity (1500cps) polymer. When tablet comes in contact with dissolution medium, loose gel structure is formed. Dissolution medium penetrates into core composition and hydrates inner core. When internal pressure builds up to break the outer coating layer, rapid drug release is observed. So after lag time of 5.5 hrs, maximum drug release occurred within next 3 hrs.

Formulation R3 (Natrosol-HHX coated tablet) showed lag time of 9.5 hrs because Natrosol-HHX is higher viscosity (3500cps) swellable polymer. When tablet comes in contact with dissolution medium, tight gelled structure of the outer coating layer is formed. So penetration of dissolution medium into core composition requires more time to hydrate core composition. This lead to higher lag time of 9.5 hrs and very slow drug release after 9.5 hrs.

v Effect of outer layer thickness on drug release from the press coated tablet.

By increasing the outer shell thickness, the lag time was prolonged, since the time required for complete the dissolution of the outer shell would be longer^{11, 12}. Therefore, press-coated tablets were prepared with different amounts of Natrosol-M in the outer layer. Effect of outer layer weight on drug release from the press coated tablet is shown in Figure 7.

Figure 7: Effect of outer layer thickness on drug release from the press coated tablet.

R1 - Natrosol-M (75 mg)

R4- Natrosol-M (50 mg)

R5 - Natrosol-M (100 mg)

Figure 4 indicates that R4 (50mg), R1 (75mg), and R5 (100 mg) with increasing coating thickness showed increased lag time of 3, 5.5 and 6 hrs respectively. Higher coating weight lead to increased lag time. A large amount of coating material produced thicker tablets. This means longer path for water to penetrate into the core composition and drug to diffuse across the swelled layer.

v Effect of high viscosity gellable polymer (Natrosol-HHX) combined with hydrophilic polymer (Natrosol-M) in the outer coating layer.

The lag time and drug release profiles of Prednisolone from press-coated tablets using different weight ratio of Natrosol-HHX: Natrosol-M is given in Figure 8.

Figure 8: Effect of high viscosity gellable polymer (Natrosol-HHX) combined with hydrophilic polymer (Natrosol-M) in the outer coating layer.

R6- N-M: N-HHX (75:25)

R7- N-M: N-HHX (50:50)

R8-N- M: N-HHX (25:75)

R3-N- M: N-HHX (0:100)

Figure 8 indicated that as the percentage of high viscosity polymer (Natrosol HHX) increased from 0 % to 100%, lag time increased from 5.5 hrs to 9.5 hrs. It was also observed that rate of drug release after lag time was also decreased with increase in proportion of high viscosity polymer. This behavior may be because of tight gel formation with high viscosity polymer (Natrosol-HHX).

v Effect of low viscosity polymer (Natrosol-L) combined with hydrophilic polymer (Natrosol-M) in the outer coating layer.

The lag time and drug release profiles of Prednisolone from press-coated tablets using different proportions of Natrosol-L: Natrosol-M is given in Figure 9.

Figure 9: Effect of low viscosity polymer (Natrosol-L) combined with hydrophilic polymer (Natrosol-M) in the outer coating layer.

R9- N- M: N-L (75:25)

R10- N- M: N-L (50:50)

R11- N- M: N-L (25:75)

R2- N-M: N-L (0:100)

Figure 9 indicated that formulations R9, R10, R11 and R2 showed decrease in lag time and increase in drug release with increase in concentration of Natrosol-L, Natrosol-M. Natrosol-L being low viscosity (75Cps) erodible polymer, when tablet containing Natrosol-L comes in contact with dissolution medium surface erosion may take place. This may lead to rapid breaking of coating layer. So, penetration of dissolution medium into core composition may be faster. So, enough internal pressure is produced to break the coating layer rapidly. Therefore, rapid drug release was observed.

CONCLUSION:

Press-coated tablets utilized various viscosity grades of hydroxyethylcellulose (i.e.Natrosol-L, M and HHX) in coat. Dissolution of press coated tablets showed lag phase followed by drug release. Observed lag times were 2.5 hrs, 5.5 hrs and 9.5 hrs for Natrosol-L, Natrosol-M and Natrosol-HHX respectively. So by combining different viscosity grades of Natrosol, it is possible to obtain a time-lags between 2.5 to 9.5 hrs. Increase in weight of coating material (Natrosol-M), increased thickness of tablet and increased lag time. Combining of various viscosity grades of HEC (Natrosol-L, M and HHX) in coat gave range of lag times proportional to viscosity i.e. lower viscosity gave less time and higher viscosity gave more lag time. Formulations R1 gave lag time of 5.5 hrs and complete drug release within 2.5 hrs after lag time. This formulation is more suitable for transit (5.5-6.5 hrs) through stomach and small intestine and faster release of drug in colon.

ACKNOWLEDGEMENTS:

Authors greatly acknowledge Naprod Life sciences Pvt.Ltd, Mumbai for providing gift sample of Prednisolone.They also acknowledge Signet chemical corporation, Mumbai for providing gift samples of Natrosol, Croscarmellose Sodium (Ac-Di-Sol^®), microcrystalline cellulose (Avicel PH 101^®) and magnesium stearate and Cadila Healthcare Ltd., Ahmedabad for providing the gift sample of Hydroxepropyl β-cyclodextrin.

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Received on 15.07.2010 Modified on 04.08.2010

Research J. Pharm. and Tech. 4(3): March 2011; Page 405-410