1-Aspirin Bioavailability (2016-17) PDF

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Pharmacological treatment of disease...

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PRINCIPLES OF PHARMACOLOGY

Bioavailability of Different Aspirin Formulations INTRODUCTION Pharmacology is, by definition, concerned with the biological activity of chemicals. However, patients take medicines and these are mixtures of biologically active chemicals and excipients, produced to allow the pharmacologically active drug to gain access to the body. The process of making a medicine containing a drug is called formulation. It used to be thought that the process of formulation did not influence the properties of the drug when used in humans but in recent years this has been shown not to be so. Tablets with the same active drug made by different companies in different ways give different levels of the drug in the body. To deal with this situation the concept of bioavailability was developed. The bioavailability of a drug is the amount that enters the circulation after administration of the product. It is measured by comparing the amount of drug entering the body after intravenous injection (100% bioavailability) and the amount entering by the proposed route, say oral. Clearly in a laboratory experiment we can’t properly measure bioavailability but it is possible to look at the rate of entry into the body indirectly. Aspirin (acetylsalicylic acid) is metabolised rapidly to give salicylic acid, which is active, and then salicyluric acid. With normal doses 95% of the material from the drug in the urine is salicylic acid and salicyluric acid. Therefore, if you can measure both of these in the urine it gives a valid indirect measure of bioavailability. Aspirin is an analgesic, antiinflammatory, anti-pyretic drug that is available in a number of dosage forms which give different speeds of action. Three main types of aspirin are available in the UK: 1) Ordinary tablets, 300mg, called aspirin BP, that break up within about 15 minutes when shaken with water at 37°C. 2) Aspirin clear (Beechams and Aspro Clear) that are dissolved in water and swallowed in solution. 3) Enteric coated aspirin (e.g. Caprin and Nu-seals) which have a sugar and wax coating and take about 5 hours to dissolve. These types of preparations are usually used in arthritic conditions where patients have to take large daily doses, up to 1000mg / day, and must therefore avoid the risk of stomach ulceration that would otherwise occur.

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AIM To compare three different aspirin formulations.

METHOD 1)

Firstly you will need to prepare a calibration curve from the salicylate solution provided. Use test tubes to prepare the following solutions. Duplicates will increase your accuracy. Standard Salicylate Distilled water (ml) Solution (ml)

Salicylate (µ µ g)

0 0.2 0.4 0.6 0.8 1.0

0 160 320 480 640 800

1.0 0.8 0.6 0.4 0.2 0

Content

2)

Add 5ml of Trinder’s salicylate reagent (take care, see note under hazard at the end) to each tube and mix the solutions carefully. Then pipette 3ml of this purple mixture into a cuvette (photocell) and read the absorption at 620nm. Do this for the contents of each tube. Use the values you obtain to complete Table 1 in the results section. Plot a calibration graph of salicylate concentration in µg/ml (horizontal axis) against absorption units (vertical axis). You can then use this to read off subsequent unknown salicylate samples.

3)

Now the experiment proper should start but consuming aspirin and waiting for it to pass through your body takes longer than the practical time allows. Therefore, we have provided you with 4 urine samples for analysis. A, B, C, and D. Each represents one of the 3 main types of aspirin used (detailed earlier in the Introduction) plus a non-dosed urine sample.

4)

Assume that each of the aspirin formulations has been taken in a dose of 600mg (2 x tablets) 60 minutes before you obtain the samples. Add 1ml of each urine sample to 4 labelled test tubes. Then, as before, add 5ml of Trinder’s salicylate reagent to; mix the solutions carefully. Pipette 3ml of this purple mixture into a cuvette (photocell) and read the absorption at 620nm. Do this for the contents of each tube. If the reading is off scale, repeat with 0.5ml of urine and 0.5ml of water as required.

5)

Estimate the salicylate content of the urine with Trinder’s reagent comparing your readings with the standard calibration curve prepared earlier. Subtract the 2

blank value from the test results. If the absorption value of a urine sample after the addition of Trinder’s reagent is off scale, dilute the urine and re-assay (remeasure). Do not dilute the coloured solution. 6)

Ensure that you complete the results section by entering your data and comments in the tables and boxes provided.

PRACTICAL HAZARD ASSESSMENT TABLE (COSHH). Note: Trinder’s reagent is very acidic and therefore all of your final concentrations will also be acidic. Wear gloves and eye protection and take care not to spill fluids onto clothing. MATERIAL

T

Salicylate

Y

E

F

I

C

H

O E S

M E L

PRECAUTIONS

Do not mouth pipette or inject.

Trinder’s salicylate reagent Y

Y

Do not mouth pipette or inject. Avoid contact with skin, use laboratory gloves & eye protection.

t=toxic, e=explosive, f=flammable, i=irritant, c=corrosive, h=harmful. Oes=occupational exposure limits, mel=maximum exposure limit. Y=yes.

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MECHANISMS OF DRUG ACTION Bioavailability of Different Aspirin Formulations RESULTS

Table 1: Data for standard curve Tube Number

Standard Solution (ml)

Distilled water (ml)

Salicylate ( )

Absorbance ( )

1 2 3 4 5 6

0.0 0.2 0.4 0.6 0.8 1.0

1.0 0.8 0.6 0.4 0.2 0.0

0 160 320 480 640 800

14 14 14 14 14 14

Table 2: Absorbance of urine samples Urine Sample

A B C D

Urine sample ( )

Trinder’s salicylate reagent ( )

1.0 1.0 1.0 1.0

5.0 5.0 5.0 5.0

Absorbance ( )

14 14 14 14

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Table 3: Salicylate content of urine samples

Urine Sample

Urine sample – non-dosed urine sample

A B C D

Absorbance ( )

-

Salicylate ( )

14 14 14 14

Table 4: Sample identity

Urine Sample

Identity

Rationale for your conclusion

A

B

C

D

5

How might the different aspirin formulations be used clinically?

Aspirin Formulation

Clinical Use

.

How would you estimate aspirin bioavailability for each aspirin formulation from your experimental results?

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