Before we start

This was an assignment I had to do for pharmaceutics during my pharmacy degree. The purpose of this assignment was to:

• Introduce us on how to use the dissolution methods,

• Understand how to use and make a calibration curve,

• Understand how drugs are released from topical semisolid dosage forms,

• Provide a textural profile and compare over time and,

• Improve ourselves in making a readable and quality-like report with a significant and relevant meaning.

I have made a general how-to-guide which will focus on those area mentioned. The topic we shall be focusing on is the “Comparative and Assessment Study of the Release of Salicylic Acid in Topical Semisolid Dosage Form”. The main focus is on how aspirin (Salicylic Acid) is released from 3 different topical semisolid dosage form. These are: an aqueous cream, an ointment and a gel. So, how are we able to do this? By using the USP dissolution apparatus method and with the help of a calibration curve (which is explained later on during this guide). What is useful is that you can do further testing by comparing the release of aspirin at different sets of time. This means you can do testing at different weeks for a couple of months and compare how aspirin is being released. This is very useful to notice and compare the shelf life or how your topical semisolid dosage form is holding up over a certain time in releasing aspirin successfully.

The main goal is to:

1. Successfully release aspirin from the topical semisolid dosage form,

2. Successfully maintain the release of aspirin being released over a period of time and,

3. To maintain a good textural profile of each topical semisolid dosage form over a certain amount of time.

How this should be ORGANISED

1. Title

   Put a suitable title that describes your pharmaceutics experience. For example, ‘Comparative and assessment study of the release of salicylic acid in topical semisolid dosage form using the dissolution method’. Add the author’s name and with which University or agency this is being done with.

2. Abstract with keywords

   This should summarise all the findings within your assignment, short and sweet.

3. Introduction

   Introduce on the calibration curve, semisolids being used, the main Active Pharmaceutical Ingredient (API), a description of the correct USP Dissolution Apparatus and the reason why we are doing this.

4. Methods and materials

   Includes a detail description of all the excipient and the API, all the material being used, the formulation with their method of manufacture, how the USP Dissolution Apparatus 2 is set up and being used, how the calibration curve is made, the spreadability method and how each formulation is being assessed.

5. Results

   This should show the results of the dissolution, spreadability and physical properties of the formulations.

6. Conclusion

   Identify what went well and what can be improved in terms of accuracy and technique to get the best results. Include a conclusion on the dissolution, absorbance, paddles, sampling method, mean and standard deviation, dissolution medium, air pockets, spreadability and assessment of semi-solid dosage form.

7. Funding and conflicts of interest

   Include a description of the conflict of interest and disclose that this assessment was not paid and that there were no financial interests.

8. Reference

Comparative and assessment study of the release of salicylic acid in topical semisolid dosage form using the dissolution method.

Abstract

Salicylic acid is a non-steroidal anti-inflammatory drug (NSAIDs) and is a metabolite of acetylsalicylic acid (aspirin) which is its prodrug. In topical semi-solid dosage forms such as creams, ointments and gels, it is used to treat skin conditions where the outer layer of the skin is thickened e.g. psoriasis, acne and seborrheic dermatitis. The aim of this study is to determine the release of salicylic acid in a cream, gel and ointment using in-vitro diffusion cell USP apparatus 2 at 50 rates per minute. Distilled water at 25 ̊ C was used as a dissolution medium and sampling was done at 10-minute intervals for 170 minutes. The UV mini 1240 spectrophotometer was used to measure the absorbance at 230 nm of the samples of all semi-solid dosage forms that had different concentrations of salicylic acid. Self-assessment and the spreadability methods were used to see the characteristics of the semi-solid dosage forms after two and five weeks of manufacture. The results of the study show that the rate of drug release from the gel was high, moderately high for the aqueous cream and low for the ointment. Over time, the release of salicylic acid from the semi-solid dosage formulations increased slightly.

Keywords: Dissolution, salicylic acid, drug release, gel, ointment, aqueous cream.

Introduction

A calibration curve is an analytical method employed in analytical analysis to determine the concentration of an unknown sample solution. It is a graph generated experimentally and the best of fit line was drawn through the obtained absorbance points. The graph is constructed using known concentration values against absorbance values obtained. The calibration curve can then be used in various analyses of various pharmaceutical formulations including semi-solid preparations such as creams, ointments and gels. The optimum consistency of such a formulation helps ensure that a suitable dose is applied or delivered to the target site. This is particularly important with formulations of potent drugs. A reduced dose would not deliver the desired effect, and an excessive dose may lead to undesirable side effects. Hence, the release of the API is an important aspect in terms of the amount and its environment it is been released in.

Creams are white, semi-solid preparations often medicated, intended for external application to the skin and mucous membrane (1). Creams are generally manufactured as o/w (oil in water phase) emulsion and can also be w/o (water in oil phase) which are less greasy than ointments (1). Creams can be washed off the skin surface and are therefore convenient for the patient (1). Creams can include medicament, such as salicylic acid, which has surface pharmacological activity. Gels are formed from a liquid base that has been thickened with other components (1). Gels may contain a single-phase or double phase system, with the liquid in the gel-forming a continuous phase with the thickening agents that enhance viscosity (1). Gelled solutions allow unhindered diffusion of dissolved molecules throughout the polymer scaffold and hence drug release should be equivalent to that from a simple solution (1). Ointments are fatty preparations that are self-occlusive and are generally used on dry lesions (1). Unmedicated ointments are used as emollients to soothe, smooth and hydrate dry skin conditions. These semi-solid preparations are prepared from soft, hard and liquid paraffin to generate a hydrocarbon base (1). Due to the occlusive nature of the hydrocarbon bases, they prevent trans-epidermal water loss, which results in skin hydration and minimal water loss (1).

Salicylic acid is white or colourless circular crystals that are slightly soluble in water but freely soluble in alcohol (2). Salicylic acid has keratolytic properties and applied topically in skin conditions such as seborrhoeic dermatitis, psoriasis and acne (2). Salicylic acid can possess fungicidal properties and is used topically in the treatment of dermatophyte skin infections (2). Salicylic acid has a log P of 2.26 where active pharmaceutical ingredients (API) with a log P of less than 5.0 are generally well absorbed (3). In this experiment, salicylic acid will be used as the main API.

The United States Pharmacopoeia (USP) dissolution apparatus 2 (paddle apparatus) has a metallic or an inert rigid stirring blade which has been coated and has a rotation speed of 50-75 rotation per minute (4). The chosen dosage form should remain at the bottom centre of the vessel. It is used to test solid dosage forms such as tablets and capsules, as well as particulates such as suspensions and powders (4). The apparatus generates cumulative dissolution results. Cellulose nitrate membrane with 8μm pores (Sartorius Stedim) is a hydrophilic membrane that has a high flow rate with high non-specific adsorption (5). The membrane has a uniform pore structure which ensures homogenous distribution of the particles retained on the filter surface and is effective up to a temperature of 1300C (5). The apparatus is used for control release of APIs from their dosage form. Control release systems are manufactured to maintain drug concentration in the blood or in the tissue at the desired amount for as long as possible (6). They are able to control the drug release rate and duration. Generally, the part of the dose is released to rapidly attain an effective therapeutic concentration of the drug, and then the drug release kinetics follows a well-defined behaviour (6). To attain the desired kinetics, mathematical and statistical modelling are used. These enable prediction of release kinetics and allows for the measurement of important physical parameters. These parameters include the drug diffusion coefficient as per Nernst Brunner Film Theory. Hence, Fick’s law of diffusion is used to establish a relationship between a constant and a diffusion coefficient (6). A theory which assumed that the process at the surface proceeds faster than the transport process and that a linear concentration gradient is confined to the layer of solution adhering to a solid surface (6).

2. Methods and materials

2.1 Excipients and active pharmaceutical product

The API or active pharmaceutical ingredients is the central ingredient that will produce a pharmacological effect (7). Salicylic acid is the active pharmaceutical ingredients in the semisolid formulations in this experiment. Pharmaceutical excipients are substances other than the pharmacologically active drug or prodrug which are included in the manufacturing process or are contained in a finished pharmaceutical product dosage form (8). In addition to transporting the active drug to the site in the body where the drug is intended to exert its action, excipients play an important part in the manufacturing process (8).

Salicylic acid: White or colourless acicular crystals or a white or almost white, crystalline powder. It is slightly soluble in water. Melting point of 158-161 ͦC. Salicylic acid is photosensitive, protect from light (9). This is the active ingredient in the formulation, and it has anti-inflammatory properties, has keratolytic properties and is applied topically in the treatment of hyperkeratotic and scaling skin conditions such as dandruff and seborrhoeic dermatitis, ichthyosis, psoriasis, and acne. Preparations usually contain between 2 and 6% salicylic acid (10).

Gelatine: Widely used in a variety of pharmaceutical formulations, including its use as a biodegradable matrix material in an implantable delivery system. It is commonly used as a gelling agent in pharmaceuticals (11). Gelatine is soluble in warm water. Gelatine occurs as a light amber to faintly yellow coloured, vitreous, brittle solid. It is practically odourless and tasteless and is available as translucent sheets, flakes, and granules, or as a coarse powder (11).

Glycerol: In topical pharmaceutical formulations and cosmetics, glycerine is used primarily for its humectant and emollient properties. Glycerine is used as a solvent or cosolvent in creams and emulsions. Glycerine is additionally used in aqueous and non-aqueous gels and also as an additive in patch applications. Glycerine is a clear, colourless, odourless, viscous, hygroscopic liquid; melting point 17.8 degrees Celsius (11).

Wool fat/alcohol: A purified, anhydrous, waxy material obtained from the wool of the sheep. It is a yellow, unctuous substance. When melted, it is a clear or almost clear, yellow liquid. Melting point 38-42 ͦC. Store it at a temperature not exceeding 25 degrees (12). Wool fat is used in the formulation of water-in-oil creams and ointments. When mixed with suitable vegetable oil or with soft paraffin it gives emollient creams that penetrate the skin. It can absorb about 30% of water. It is commonly used as an absorption base and used in water-in-oil emulsions; a lubricant, emollient and can easily penetrate the skin (12).

White soft paraffin: Purified and wholly or a nearly decolourized mixture of semi-solid hydrocarbons, obtained from petroleum. Soft paraffin is used as an ointment basis and as an emollient in the management of skin disorders. It is not readily absorbed by the skin (13). White soft paraffin has been bleached with hydrogen peroxide to remove the normal yellow colour leaving it white or translucent. The melting point range for it is 38-56 degrees Celsius and it must be protected from light (13).

Hard paraffin: A purified mixture of solid saturated hydrocarbons having the general formula CnH2n+2. Used for ointment base; stiffening agent. Melting point 50-57 ͦC. Paraffin is mainly used in topical pharmaceutical formulations as a component of creams and ointments. In ointments, it may be used to increase the melting point of a formulation or to add stiffness (11). Paraffin is an odourless and tasteless, translucent, colourless, or white solid. It feels slightly greasy to the touch and may show a brittle fracture (11).

Liquid paraffin: A colourless, transparent and oily liquid. It is practically insoluble in water, miscible with hydrocarbons and is used as a faecal softener (14). Liquid paraffin may be used as an ingredient of ointment bases, as an emollient and cleanser in certain skin conditions. Relative density 0.827 to 0.890 (14).

Aqueous cream: Is a light, hydrocarbon-based emulsion, which is officially registered in the British Pharmacopoeia and categorised by the British National Formulary as a non-proprietary emollient preparation (15). It is used as a topical, external medicine, emollient moisturiser and general-purpose substitute for toiletries such as soap, shower gel, shaving cream and lip salve. The common ingredients are: liquid hydrocarbons, white soft paraffin, wax, purified water, emulsifying wax containing sodium lauryl sulphate, cetostearyl alcohol and chlorocresol (15,16).

2.2 Material

Bunsen burner(s), spatula(s), beaker(s), mortar and pestle, evaporating dish, tile for levigation, 100 mL volumetric flasks (6), volumetric flasks 1000 mL, UV mini 1240 spectrophotometer, cellulose nitrate filter (8μm, Sartorius Stedim.), 10 mL syringe (3), stopwatch, stirring rod, tweezer, glass plate (2), 100g weights (2), sonicator, dissolution jars (3), 4 quartz cuvettes, top loader balances, analytical scale, USP dissolution apparatus 2 (USP2).

2.3 Formulations and method of manufacture

2.3.1 Salicylic acid gel

Table 1. Formulation of salicylic acid gel and ingredients melting points

Formulation of salicylic acid gel

1. 39mL of water was heated to boiling point and was transferred to the evaporating dish where 6,5 g of gelatine was dissolved in aliquots by stirring slowly to prevent the formation of bubbles.

2. 147g glycerol was added and stirred to form a homogeneous mixture.

3. The evaporating dish was then removed from the Bunsen burner and 6g salicylic acid was added after particle size reduction with a mortar and pestle.

4. The 200 g formulation was left to sit until a gel formed and then transferred into a labelled jar.

2.3.2 Salicylic acid ointment

Table 2. Formulation of salicylic acid ointment and ingredients melting points

Formulation of salicylic acid ointment

1. All equipment had to be fully washed and dried out before use, these included: spatulas, evaporating dish, 300ml beaker and the stirring rod.

2. The Bunsen burner apparatus was set up first and water left to heat up in a water bath.

3. The evaporating dish was placed on top of the water bath. The weighing paper was placed on the scale and tarred.

4. 7,5 g of salicylic acid was then weighed out using a spatula and was triturated into a fine powder.

5. Another weighing paper was placed on the scale, tarred and 25 g of hard paraffin was measured.

6. 35 g of white soft paraffin and 15g of wool alcohols/fat was weighed out the same way.

7. For the weighing of the liquid paraffin, a 300ml beaker was placed on the scale, tarred and the liquid paraffin was poured in it until the scale read 167,5 g.

8. Once the water had heated up to a suitable temperature (subjectively estimated), the weighed out hard paraffin was put into the evaporating dish until it dissolved as it had the highest melting point, followed by wool alcohols/fat, white soft paraffin then liquid paraffin as it had the lowest melting point.

9. Finally, the evaporating dish with the contents was removed off the Bunsen burner and salicylic acid was then added in small portions until dissolved. The 250 g formulation was left to set and then transferred into a labelled jar.

2.3.3 Salicylic acid cream

Table 3. Formulation of salicylic acid cream and ingredients melting points

Formulation of salicylic acid cream

1. 7.5g of salicylic acid was placed on a weighing paper and weighed out on a scale.

2. The aqueous cream was placed on a weighing paper and then on a weighing scale.

3. 242,5 g of the aqueous cream was weighed out.

4. The salicylic acid powder was triturated into a fine powder and then the levigation method was employed in mixing the salicylic acid and the aqueous cream until it formed a homogenous mixture.

5. The 250 g formulation was transferred into a labelled jar.

2.4 Release of salicylic acid from the topical vehicles

The release of salicylic acid from topical semisolids in the form of a gel, cream and ointment was measured using an in vitro diffusion cell apparatus. The USP dissolution apparatus (USP2) was used: The paddle apparatus consists of a cylindrical glass vessel with a hemispherical bottom and a volume of 1000mLs. The vessel was immersed in a water bath device that allows holding the temperature inside the vessel at 25 degrees Celsius, which maintained optimum receptor conditions during the test. A shaft is positioned so that the axis was not more than 2 mm from the vertical axis of the vessel at any point and should rotate smoothly without significant disturbances that could affect the results. The paddle needed to be coated with a suitable coating to ensure that it was inert so that it does not interfere with the dissolution fluid used. The jar, with the membrane and lid, was weighed on a top-loading balance, and the weight was recorded. The formulation was then packed into the glass jar such that, air pockets were not formed inside the jar. Using tweezers, the membrane was soaked in distilled water then blotted dry with flitter paper and spread evenly over the formulation making sure not to form any wrinkles or air pockets. The lid was then screwed on, and the jar was measured and the weight was recorded. The diameter of the exposed membrane was measured for all three formulations. 500mLs of distilled water, maintained at 25 degrees Celsius, was placed in the vessel. The formulation jars were immersed into each of the vessels, with the jars resting on the base of the receptor vessels, with the membrane facing upwards. The agitating paddles were lowered to 1-2cm below the solvent surface. The paddle drive mechanism was engaged to agitate at a rate of 50 rates per minutes (rpm) to maintain near-sink conditions at the membrane surface. At each sampling time interval, withdraw a specimen of 5mL aliquots from a zone midway between the surface of the medium and the top of the paddle, not less than 1 cm from the vessel wall. A 10 mL syringe was used at 10-minute intervals over 170 minutes. This was assayed by UV mini 1240 spectrophotometer at 230 nm wavelength. The absorbance readings were recorded and the aliquots were returned to the bulk solution in the receptor beaker to maintain the volume of 500mL, hence not altering the concentration from removing volume.

2.5 Calibration curve

A standard stock solution was made up of 1g/1L using an appropriate volumetric flask on the day of the dissolution. 1 g of pure salicylic acid was weighed on the analytical scale, powdered by using the mortar and pestle and placed into a 1000 ml volumetric flask. Distilled water was added up to the 1000 ml mark. The solution was placed in a sonicator to ensure rapid dissolving of the salicylic acid. Once the stock solution was ready 5 dilutions were prepared. First dilution: 1 ml of stock solution was placed into a 100 ml volumetric flask using a 1 mL pipette and distilled water was added up to the 100 ml mark. Second: 2 ml of stock solution using 1 mL pipette was transferred into a 100 ml volumetric flask and made up to 100 ml with distilled water. Third: 4 ml of stock solution was transferred using a 10 mL graduated cylinder into a 100 ml volumetric flask and made up to 100 ml with distilled water. Fourth: 6 ml of stock solution was transferred using a 10 mL graduated cylinder into a 100 ml volumetric flask and made up to 100 ml with distilled water. Fifth: 8 ml of stock solution was transferred using a 10 mL graduated cylinder into a 100 ml volumetric flask and made up to 100 ml with distilled water. See Table 4. The calibration curve standard stock solution made on the day of the semi-solid manufacture followed these methods accordingly. Each dilution was placed into a clean cuvette using a pipette and the absorbance of each of the diluted concentrations was measured using a UV mini 1240 spectrophotometer at 230 nm wavelength and recorded. A calibration curve was created and plotted graphically using the absorbance and concentration (mcg/mL) values. The regression line was calculated by using the Microsoft Excel 2016 program.

2.6 Spreadability

Spreadability was tested by placing 3g of each semi-solid formulation on a glass plate. Another glass plate was placed on top of the formulation, and two 100 g weights were placed onto the cover glass plate. After 5 minutes, the diameter of the spread of the semisolid formulation was measured repeatedly at different diameters and recorded. A mean in millimetres and a standard deviation were produced. See Figure 4.

2.7 Assessments of the formulation

The three formulations were tested subjectively for emolliency, viscosity, aesthetic appeal in terms of colour and smell; and texture such as smoothness and grittiness. See Table 12.

3. Calibration curve construction

3.1 Calibration curve constructed on the day of dissolution testing

Stock concentration:

1 gram (1000 mg): 1 litre (1000 mL)

1000 mcg : 1 mL

1000 mcg/mL

Example: 40 mcg/mL, 100 mL volumetric flask were used

: C1V1 = C2V2

: (1000) (V1) = (40) (100)

: V1 = 4ml

Table 4. Concentrations (mcg/mL) used for the calibration curve with allocated stock volume (mL) and deionised volume (mL)

Table 5. Calibration curve data of the day of the dissolution with concentration (mcg/mL) versus average absorbance and its standard deviation using UV mini 1240 at wavelength 230 nm

Equation of the linear regression line: y = 0,0115x + 0,0660

R2 value = 0.98529

Linear regression calculates an equation that minimizes the distance between the fitted line and all of the data points. This was automatically calculated by Microsoft Excel 2016 program. The r2 value tells us how close the data is to the fitted regression line. An r2 value of 0.0 means that knowing the absorbance does not help to predict the concentration (mcg/mL). When r2 equals 1.0, all points lie exactly on a straight line with no scatter. 0% indicates that the model explains none of the variability of the response data around its mean. 100% indicates that the model explains all the variability of the response data around its mean. Therefore, the higher the r2 value, the better the model fits the data. In this instance, our r2 which is 0.98529 (98,529%), is close to 1 (or 100%) hence it means that our model fits the data precisely and accordingly. We are confident that our data properly depict a reliable relationship between the absorbance and the concentration (mcg/mL).

4. Results

4.1 Dissolution

Table 6. Data of weights of dissolution containers with lids, weights of dissolution container with lids and compact semi-solid, the weight of the semi-solid, amount of API (salicylic acid) allocated in each formulation and calculated surface area in respective to their membrane length

Table 7. Dissolution data of salicylic acid gel and the working out data of amount of API (mg per cm^2) and percentage of API per cm^2

Table 8. Dissolution data of salicylic acid ointment and the working out data of amount of API (mg per cm^2) and percentage of API per cm^2

Table 9. Dissolution data of salicylic acid aqueous cream and the working out data of amount of API (mg per cm^2) and percentage of API per cm^2

4.2 Spreadability

Table 10. Spreadability data two weeks after manufacture of the distance (mm) with the mean (mm) and its standard deviation in relation to their semi-solid dosage form

Table 11. Spreadability data five weeks after manufacture of the distance (mm) with the mean (mm) and its standard deviation in relation to their semi-solid dosage form

4.3 Physical properties of formulations

Table 12. Semi-solid dosage form 2 weeks after manufacture are subjectively assessed in spreadability, texture, viscosity and emoliency

5. Conclusion

5.1 Dissolution

Dissolution tests are employed to establish drug API release characteristics of semi-solid formulations. The rationale for conducting these tests is that for a product to be therapeutically effective, the drug must be released from the product (17). A dissolution test may be considered as a critical step for the development and assessment of the quality of products linking to their safety and efficacy attributes (17). The results of the study show that the rate of drug release, over 170 minutes, from a gel, was high, 0.042% per cm^2, moderately high for the cream, 0.033% per cm^2, and low for the ointment, 0.014% per cm^2. See Figure 3. Over time, the release of salicylic acid from the different semi-solid dosage formulations increased slightly. See Figure 2 and 3.

An understanding between the interaction of hydrophilic like molecule and lipophilic like molecule is essential in the drug release from the semi-solid formulation into its environment. Hence, the formulation of aqueous creams are generally manufactured as o/w (oil in water) and the formulation gels are formed from a liquid hydrophilic base that has been thickened with other components (1). Gelled solutions allow unhindered diffusion of dissolved molecules throughout the polymer scaffold where the drug release should be equivalent to that from a simple solution (1). The type of formulation of the aqueous cream and gel is a hydrophilic base, where the API was released into the water dissolution medium (a hydrophilic medium) easily. This was supported by the moderately high to high percentage and amount of drug release seen in the data. See figure 2 and 3. Therefore the hydrophilic base semi-solid formulation will present quick and effective therapeutically effect if it reached the adequate level of API release. Likewise, the formulation of the ointments is a fatty preparations that are self-occlusive (1). Unmedicated ointments are used as emollients to soothe, smooth and hydrate dry skin conditions. The formulation of the ointment preparation is prepared from soft, hard and liquid paraffin to generate a hydrocarbon base (1). Due to the occlusive nature of the hydrocarbon bases, they prevent transepidermal water loss, which results in skin hydration and minimal water loss (1). Hence, the type of formulation of the ointment is the reason why there was such a low percentage of API released in the water dissolution medium. This is supported by the low percentage and amount of drug release seen in the data. See Figure 2 and 3.

Therefore, an oil base or hydrophobic semi-solid preparation will release a small amount of API in water or hydrophilic environment. This could indicate that the ointment formulation could act as a long term release dosage form of the API, a useful advantage on the hydrophilic base formulation, but it could also mean under-dosing since the little amount of the API is released producing no desire effects at all. Furthermore, the skin is a fatty and lipophilic base (18), which could demonstrate the ointment formulation, a hydrocarbon base, to be more effective in the release of the API than the aqueous cream and gel formulation. This is seen from the like-base dissolves and release the API in the like-environment. However, due to its occlusive nature of the ointment formulation, the amount of water will increase as the skin rehydrates which will evidently slow down the release of the API as demonstrated in the dissolution studies. The optimum consistency of such a formulation helps ensure that a suitable dose is applied or delivered to the target site. This is particularly important with formulations of potent drugs. A reduced dose would not deliver the desired effect, and an excessive dose may lead to undesirable side effects.

5.2 Absorbance

The UV mini 1240 spectrophotometer, was used to obtain the absorbance values of the different semi-solid formulations (19). These absorbance values were obtained at 10 minute time intervals and were then converted to concentrations of the drug (3% salicylic acid) that were in the dissolution fluid (2), see Table 7-9. The different semi-solid formulations released the 3% salicylic acid at different rates based on the nature of the formulation. All the formulations contained 3% salicylic acid (20), but it was evident that the gel released the most amount of API. This is due to its hydrophilic base, which allows it to release the hydrophobic drug easily into the hydrophilic dissolution fluid. The cream had the second quickest release rate followed by the ointment. Therefore, the release rate decreased with increasing hydrophobicity/lipophilicity of the base of the formulation in a hydrophilic environment such as the water dissolution medium.

5.3 Paddles

The distance between the formulation and the paddle in the different vessels containing the semi-solid formulation was different. Since this distance influences the distribution of the salicylic acid in the dissolution fluid, the varying release rates could have also been influenced by this distance. This difference in distance, however, influences the dissolution rate that is negligible in comparison to the influence of the nature of the formulation.

5.4 Sampling method

Due to the difference in the distribution of the 3% salicylic acid formulation in the dissolution fluid from the influence of the paddles, the sampling of the dissolution fluid containing the 3% salicylic acid formulation would have varied in the different formulations. This means the distribution of the 3% salicylic acid formulation was not consistent throughout the dissolution fluid even though a complete homogenous solution should have been met. Therefore, sampling at the surface of the dissolution fluid would give a different absorbance value as compared to sampling at the centre of the dissolution fluid. Since different individuals were sampling the different formulations, the area and depth of sampling in the formulation could have varied. This was made to be as constant as possible by everyone sampling at the same area and depth. I.e. at each sampling time interval, withdraw a specimen of 5mL aliquots from a zone midway between the surface of the medium and the top of the paddle, not less than 1 cm from the vessel wall. The aliquot was returned whenever sampling to not change the volume which would change the concentration.

5.5 Mean, standard deviation

In our experiment, we could not obtain more than one reading of the absorbance at a specific time for the same formulation due to time constraints and only one UV mini 1240 spectrophotometer being available. A suggestion came up to use our colleague’s group practical results but the suggestion was not accepted as there would be too many variables due to the constant human error i.e. their formulation, dissolution method, calibration method and sampling method would have been different to ours causing an even higher inaccuracy and a larger standard deviation than if we did not use their data. If it were possible, a t-test statistical analysis should have been in place to compare each cream between each group. Our way forward would be using two UV mini 1240 spectrophotometer and hence sampling at the same time all three semi-solid formulations or sampling only one semi-solid formulation at a time, for 180 minutes with one UV mini 1240 spectrophotometer. This is more beneficial as there can be more than 3 samplings (including blank) as there is space within the UV mini 1240 spectrophotometer. However, this would implement 540 minutes practical increasing possible human errors due to fatigue.

5.6 Dissolution medium

The dissolution medium used in our experiment was water (20). Water is not a good dissolution medium due to not having a good buffering capacity since its pH is very sensitive to the addition of any acidic or basic species (21). A buffer is an aqueous solution consisting of a mixture of a weak acid and its conjugate base, or vice versa. Its pH changes very little when a small amount of strong acid or base is added to it and thus it is used to prevent changes in the pH of a solution (22). If we add a strong acid or strong base to water, the pH will change. For instance, if an acid is added, the proton (H+) from the acid binds to neutral water molecules to form H3O+ raising the concentration of H+ (21,22). Any excipients or API in the semisolid formulation could change the pH of the dissolution medium i.e. salicylic acid, a weak acid (9), could potentially vary the dissolution medium pH. A varying pH will change the dissolution rate of the formulation introducing errors in the experiment (17). Hence, keeping a constant and unchanged pH is an important aspect in a dissolution medium.

5.7 Air pocket

The cream and ointment had no air pockets but when the gel was packed into the jar, air pockets became difficult to eliminate due to the elasticity of the gel. Air pockets can bring about variation in the results due to the present gases. Air pockets misrepresent the area and volume occupied by the gel inside the jar, which incorrectly represents the results. If there is an air pocket that is in contact with the membrane, it would alter results due to the surface area measured on the membrane would misrepresent the surface area that is directly in contact with the formulation. Hence, if there is an air pocket, the formulation is not in contact with the membrane to diffuse through the membrane and into the dissolution fluid to create a concentration for the absorbance which will cause inaccuracy and false results. If air pockets are present at the membrane, a decreased concentration should be seen compared to what it should be.

5.8 Spreadability

The efficacy of a topical therapy depends on the patient spreading the drug formulation in an even layer to administer a standard dose (23). Spreadability is, therefore, an important characteristic of these formulations and is responsible for correct dosage transfer to the target site which is highly dependent on the spreadability of the formulation, ease of application on the substrate, and the consumer preference (23). Semi-solid preparations common property is the ability to cling to the application surface for a reasonable period before they are washed off or worn off (23). The semi-solid formulations went under the spreadability method at two and five weeks after manufacture, see Figure 4 for results. After 2 weeks from the manufacture of the semi-solid preparations, the ointment formulation was the highest in terms of spreading the furthest, 93,00 mm, and the gel was the lowest, 72,67 mm. The aqueous cream was in the middle of the two formulations with 84,67 mm. After 5 weeks from the manufacture of the semi-solid preparations, the ointment formulation was the highest in terms of spreading the furthest, 104,00 mm, and the gel was the lowest, 44,00 mm. The aqueous cream was in the middle of the two formulations with 72,67 mm. The gel did not go as further than the other two formulations due to the elasticity nature of the gelatine (11). The ointment mainly made out of liquid paraffin and other paraffin including wool fat (11-14), produced a thin, cool and unctuous formulation that could be spread easily and furthest.

The spreadability nature of semi-solid preparation is an important characteristic of dermatological conditions (23). A semi-solid preparation that can be spread easily over the wanted area and not over/under spread, is desired by patients which increases patient’s satisfaction of the semi-solid preparation. If over time the viscosity and spread changes from its original spread characteristics, it will influence the patient’s perception of the semisolid preparation which may be negative. A change in the distance of the semi-solid preparation over time could indicate a change of the rheological properties such as the viscosity, elasticity, thixotropy, and flowability which had been affected in some way within the semi-solid preparation (23). This insinuates instability characteristics of the semi-solid preparation due to the compound(s) changing, and hence the interaction within the semi-solid preparation is also changing from the normal and original interactions (23). It is clear in mind that if the distance did not change, it would indicate no changes of rheological properties within the semi-solid preparation, and hence no instability as the product remained the same. In terms of mean distance in millimetres, there was a difference in the semi-solid preparations between the two and five weeks after the manufacture of the creams. The gel was the highest in terms of difference, where the week five preparation had 28,67 mm less than the week-two preparation. The ointment and aqueous cream preparation difference were 11.0 mm and 12.0 mm respectively.

Interestingly enough, the week-five ointment preparation increased and the week-five aqueous cream decreased in terms of distance. These changes in the distance could indicate instability, due to change in the rheology properties. Statistical analysis must be done to identify when the semi-solid preparations were significantly or were insignificantly changed in terms of spreadability and stability. A t-test analysis was done, and we conclude that there were significant changes between the mean distance in spreadability of all semisolid preparations between week 2 and week 5. We speculate that the gel, ointment and aqueous cream preparation showed changes in rheology properties and hence indicate a change instability. Further analysis and tests must be done to demonstrate which compound and which interactions had changed to render the gel more rigid and aqueous cream as it spread less and to understand why the ointment became more viscous. The results could have been more accurate if the semi-solid preparation were tested on spreadability on the day of the manufacturing and not 2 weeks later as we could have seen the distance initially and compared to 2 and 5 weeks later. Another incident was that the aqueous cream was left in the sunlight for 2 weeks which, with no doubt, changed the stability of the preparation especially salicylic acid which is photosensitive (9) hence altering the results. This whole experiment should be redone to increase a more accurate result.

5.9 Assessment of semi-solid dosage form

Consumer preference for semi-solid products depends on various properties of the preparation, collectively known as the textural profile, which includes appearance, odour, extrudability (when applicable), initial sensations upon contact with the skin, spreading properties, tackiness, and residual greasiness after application (24,25). Unfortunately, the aqueous cream was exposed to direct sunlight for two weeks which resulted in the cream to crack and hindered salicylic acid as it is photosensitive (9), which altered the assessment results. The results of each semi-solid preparation, see Table 12, shows each characteristic properties marked out of 10, where 0 represent the lowest and 10 represent the highest. These results are not accurate as it is assessed subjectively. However, it does give us a general idea if the semi-solid preparation is performing in the textural profile and properties that could be worked on or improved to produce a better semi-solid formulation.

To increase reliable results, the assessment test should have been done on the day of the manufacturing of the formulation and not 2 weeks later as we could have seen the semisolid properties initially and compared to 2 and 5 weeks later. Having two or more sets of data could have shown if the textural profile and rheology properties stayed the same or changed. Hence, over time the semi-solid preparation could have changed in some properties showing instability, i.e. is it still aesthetically appealing such as change in texture like odour and colour can show contamination and as such, the introduction of a preservative is needed or does the viscosity and emoliency overtime still meets the requirements for that particular semi-solid preparation. These are important tests as the patient(s) will have these semi-solid preparations for days or up to months i.e. corticoid steroids aqueous cream can be up to two months for usage in eczema. Hence, a semi-solid preparation needs to stay stable and aesthetically appealing till the end of usage and if it is not, it shows an error in manufacturing, packaging and/or storage of the semi-solid preparation. These must be corrected and improved.

6. Funding and conflicts of interest

“Conflicts of interest exist when an investigator, or his/her institution, has financial or personal relationships that may compromise or present the appearance of compromising an individual’s, group’s or institution’s judgment in conducting, reviewing, or reporting research. These relationships vary from those with negligible potential to those with great potential to influence judgment, and not all relationships represent true conflicts of interest. Full informed consent is one in which all related financial interests are disclosed” (26).

This study was not paid by sponsors and there was no financial interest in its outcome. We do not own the drug or device being tested and thus we have no financial interest in the outcome of the study. No payments were made, the funds used were from Rhodes University to cover the expenses of the study and related academic and research activities. The investigators do not have any financial interest in the outcome of the study.

Published 20th November 2019. Last reviewed 1st December 2021.

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