LABORATORY 5: Preparation and Evaluation of the Effect of Ointment Bases on in Vitro Drug

Shenyang Pharmaceutical University LAB 5: OINTMENT PHARMACEUTICS I

LABORATORY 5: Preparation and evaluation of the effect of ointment bases on in vitro drug release

1. LABORATORY OBJECTIVES

a) To learn the preparation methods of different types of ointment bases.

b) To learn the methodsfor determining drug release from an ointment and compare the impact of different ointment bases on drug release.

2. INTRODUCTION

Ointments are semisolid preparations intended for external application to the skin or mucous membranes. They are used topically for several purposes such as protectants, antiseptics, emollients, antipruritics, kerotolytics, and astringents. Also, some drugs can be absorbed into the general circulation through the skin and produce systemic therapeutic effects.

In the formulation of ointments, the base forms the main part of the product. An ointment base is of prime importance because itdoes not only serve as the vehicle but also functions as the carrier of the medicinal agents in the product. It affects theoverall quality of an ointment, the release rate of the drug substance, as well as its absorptioninto the body. The commonly used ointment bases are classified into three different groups:

Oleaginous bases: This group includes hydrocarbon bases, fat and fixed oil bases. A simpleointmentwhich is prepared from the fusion of beeswax and vegetable oil or petrolatumalone can be applied as ointment bases.Other oleaginous ingredients suchas liquid paraffin andlanolin are commonly used to adjust the viscosity of the bases.

Emulsion bases: Emulsion bases are composed of semisolid or solid oleaginous components, water soluble ingredients, and emulsifying agents. The commonly used emulsifiers includesoap, high molecular weight fatty alcohols, fatty alcohol sulfate,polybasic alcohol ester such as triethanolaminesoaps, sodium lauryl sulfate, Tween-80, etc.By selecting different types of emulsifying agents, O/W and W/O emulsion based ointments can be prepared. An emulsion based ointment is also called a cream.

Water-soluble bases: Water-soluble bases include natural and synthetic water soluble high molecular substances, commonly used are glycerinated gelatin, cellulose derivatives, polyethylene glycol, and polypropylene acid, etc.

In addition to the solid drug substance which can dissolve in or form eutectic mixture with the ointment base, insoluble drug substances should be pulverized prior to being incorporated in the ointment base. If necessary, penetration enhancers, humectants and preservatives can be added in the ointment. Ointments can be prepared bythe incorporation method, fusion method or emulsification method depending on the properties of the drug substances and bases.

Ointments containing semisolid and liquid ingredients can be prepared using the incorporation method which includes levigating the drug powder with a suitable amount of liquid or a portion of the base followed by incorporating the mixture into the base with geometric dilution to ensure uniform distribution of the active ingredient. When the mixing is completed, the product should not show grittiness when it was applied to the skin. For ointments containing ingredients with different melting points, which cannot be mixed uniformly at room temperature, the fusion method should be applied.In this process, the drug substance which is soluble in the base can be directly incorporated in the molten baseand the insoluble drug substance can be added to the molten or softened base with constant agitation until it congeals. Emulsion based ointment can be prepared by using the emulsification method. The oil phase is first heated to 70~80℃ (if necessary,the oil phase can be filtered when it is hot to get rid of foreign matter) and the ingredientssoluble in water is added to the aqueous phase. The aqueous phase is subsequently heated to the same or a slightly higher temperature compared with that of oil phase. The water phase is added into the oil phase gradually with constant agitation until the final product congeals.

In addition to the determination of melting range, pH, viscosity, drug stability and skin irritation, ointments are also evaluated with respect to the in vitrodrug release rate. Drug release rate and percutaneous absorption mainly depend on the nature of the drug itself.However, the composition and properties ofthe ointment basesalso influence drug release and percutaneous absorption to a certain extent.Furthermore, the different drug incorporation techniques have also been shown to affect the drug release rate. In general, thefastest drug release rate is obtained with water soluble and emulsion ointment bases and the drug release rate from hydrocarbon ointment base is usually the slowest.

Several methods can be used to evaluate the influence of different types ofbases on drug release from ointments. The measurement of drug release rate from an ointment to the release medium can be performed through a semi-permeable membrane. Gel diffusion orexcisedanimal/human skin method also can be employed to study the drug release rate from ointments. Release of the drug from ointments normally followsthe Higuchi equationwhich shows that the cumulative release amounts of drug is proportion to t 1/2 as shown below:

M = kt 1/2

3. METHODS

3.1 Preparation of a simple ointment

3.1.1Formula

Beeswax 6.6g

Vegetable oil 15mL（6.7g）

3.1.2 Procedures

Melt the beeswax in an evaporating dish using a water bath. When the wax has completely melted, add the vegetable oil slowly with constant stirring. When the mixtureis homogeneous, remove the dish from the heat and stir the mixture until it begins to congeal.

3.1.3Notes

Uponthe addition of the vegetable oil, stir the mixture constantly until the mixture is uniform. After removing from heat, stir the mixture continuously until it begins to congeal to prevent stratification.

3.2 Preparation of O/W emulsion bases

3.2.1 Formula

Stearyl Alcohol 1.8g

White petrolatum 2.0g

Liquid paraffin 1.3mL

Sodium lauryl sulfate 0.2g

Ethylparaben 0.02g

Glycerin 0.1g

Distilled water q.s.

Total: 20g

3.2.2 Procedures

a) Melt the oleaginous phase (stearyl alcohol, white petrolatum and liquid paraffin) in an evaporating dish using a water bath at the temperature of 70~80℃.

b) Dissolve the water-soluble ingredients (sodium lauryl sulfate,ethylparaben, glycerin ) inthe calculated amount of water in anevaporating dish or a beaker and heat the solution to 70～80℃.

c) Add the aqueous phase to the oleaginous phase slowly andheat the mixture on the water bath for additional several minutes under constantstirring.

d) Remove the dish/beaker from the heat and stir the mixture at room temperature until it congeals.

3.3 Preparation of water soluble bases

3.3.1Formula

Glycerin 24.4g

Water 24.7g

1%(w/v) sodium benzoate 1 mL

Carbopol940 0.25g

Triethanolamine 0.25g

3.3.2 Procedures

a) Disperse Carbopol 940 uniformlyin water under vigorous stirring.

b) Add glycerin to the mixture prepared above and stir until uniform. Add triethanolamine and 1% (w/v) sodium benzoate and stir until a homogenous solution is formed.

3.3.3 Notes

Preparation of 1% (w/v) sodium benzoate solution: Add 1gof sodium benzoate in water with the final volume of 100mL.

3.4 Preparation of 5%Diclofenac Potassium ointments

3.4.1 Procedures

a) Preparation of diclofenac potassiumointment with a simple oleaginousointment base:Place the preweighed amount of diclofenac potassium powder (0.5g) in a mortar andthen triturate the drug powder with9.5 g of the ointment baseusingthe geometric dilution method.

b) Preparation of diclofenac potassium ointment with a paraffin base: Heat and melt 9.5gof paraffin in an evaporating dish using a water bath.Add 0.5 g of diclofenac potassium powder in the liquefied base and stir constantly.Remove the dish from the water bath and stir until the mixture begins to congeal.

c) Preparation of diclofenac potassium ointment with anO/W emulsion base: Place0.5g of diclofenac potassium powder in a mortar.Triturate the drug powder with 9.5 g of emulsion base using the geometric dilution method.

d) Preparation of diclofenac potassium ointment with a water soluble base: Place 0.5 gof diclofenac potassium powder in a mortar.Triturate the drug powder with 9.5 g of water soluble base using the geometric dilution method.

3.4.2 Notes

Diclofenac potassium should be milled into a fine powder (passing through a100- mesh sieve according to the China Pharmacopeia) before being used for preparing different ointments.

3.5 Comparison of drug release rate from ointments prepared with different bases

3.5.1. Procedures

a) Each of the four products is filled into a short glass tube with an internal diameter of 2cmand 2cm in length. Thelength of the filled ointment is about 1.5cm. The opening is covered carefully with a piece of glassinepaperand a rubber band. Make sure that there are no air bubbles underneath the surface of the glassine paper.

b) Add the tube with the sample prepared in a) in a beaker containing100mL of 37℃distilled water, which is placed in a thermostatic water bath of 37℃±1oC. Immerse the sealed tube in the beaker and make sure that the dialysis glassine paper is approximately 1 mm below the water level. Record the time as time zero. Stir the solution gently but continuously with a stirring bar on a magnetic stirring plate. At specified time points of 15, 30, 45, 60, 90, 120, 150 min, withdraw approximately 5 mL and replace with distilled water of the same volume. Measure the content of diclofenac potassiumin the release medium using the method as described in the following part c).

c) Drug content measurement: Accurately weigh diclofenac potassium reference standard and prepare the stock solution in a 100mL volumetric flask. Dilute with water to prepare solutions with different concentrations of 5, 7.5, 10, 12.5, 15 and 17.5μg/mL. Measure the absorbance of the standard solutions at 275nm (CP 2005 version appendices IVA). Usingthe drug concentration as the X axis and the absorbance as the Y axis,astandard curve can be established. Dilute 5 mL of the drug release samples collected at different time points with water to 50 mL in a volumetric flask and measure the absorbance at 275 nm. Calculate the concentration of diclofenac potassium in the medium using the standard curve. "Zero" the machine before each reading using water as the blank.

3.5.2. Notes

If the absorbance value is higher than 0.8, dilute the solution with water and take the measurement again.In this case, be sure tocalculate the concentration with thecorrect dilution factor.

1. RESULTS AND DISCUSSION
2. Apply all four types of diclofenac potassium ointments on your skin.Check if they are smooth in texture, compare theirviscosity and spreadability, and discuss the function of each ingredient in these four formulations.
3. Record the amount of drug released at different timepoints and recordthe results in Table 1.

Table 1.Amount of diclofenac potassiumreleased from four different ointments as a function of time

Plot the cumulative amount of diclofenac potassium released as a function of time for the four ointment preparations and discuss the difference in drug release rate for these four ointment bases.

5. QUESTIONS

a) How many methods can beused to incorporate drug substances during ointment preparation?

b)Whichare the factors influencing the drug release behavior from ointments?

References

[1] Leon Lachman, Herbert A. Lieberman, Joseph L. Kanig.Theory and Practice of Industrial Pharmacy, 3rd ed., Lea & Febiger, 1986.

[2] H. C. Ansel, N. G. Popovich and L. V.Allen, Jr. Pharmaceutical Dosage Forms and Drug Delivery Systems, 8th ed., Williams and Wilkins, Baltimore, 2005.

(Cuifang CAI)

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