Comparative Solubility of Nasal Decongestants and Antihistamines

Research Questions:

• What is an drug solubility? What is drug absorption?
• How can you mimic conditions in the gut to measure solubility of drugs?
• How much variability is there in the solubility of different over-the-counter drugs?

Most students are familiar with taking decongestants and antihistamines for colds and allergies. When taken in pill form, these pills must first dissolve and get absorbed by tissue membranes in the gut before they can exert their effect. The rate of absorption is one element of bioavailability and determines how soon the patient can anticipate relief from their symptoms. Conditions in the stomach can be mimicked by combining a solution of hydrochloric acid with stomach enzymes. Normal peristalsis of the gut can be mimicked using an automatic stirrer.

Materials:

• 4 liters of 0.01 molar HCl with 0.01 g/liter amylase and 0.05 g/ l pepsin (stomach liquid solution)
•  At least one mechanical stirrer with stirring magnets
• A digital timer
• A thermometer that accurately measures the temperature of liquids in the range of body temperature (98.6 F or 37 C)
• 5 tablets of pseudoephedrine (Sudafed)
• 5 tablets of diphenhydramine HCl 25 M (Benadryl)
• 5 tablets of chlorpheniramine maleate (generic allergy medication)
• Several 500 mg beakers

Some materials are available in the home. The drugs can be found in large discount stores, groceries and pharmacies. The solutions that mimic stomach acid and enzymes can be prepared from ingredients purchase from lab supply houses. Talk to your science teacher because they may be able to direct you to the very small quantities of the materials needed here. 0.01 M HCl is a standard solution. For the sake of consistency, students are advised to use this, instead of making their own solutions from stronger concentrations.

Experimental Procedure:

1. Set up your stirrer and plug it in a place where you can use it undisturbed. Put the magnetic stirring bar into your 500 ml beaker. Very carefully add 250 mg of your HCl and enzyme solution into the beaker. Measure the temperature of the HCl solution. Turn on the heater. You are shooting for a temperature of 37 degrees Celsius as this is body temperature.
2. Once your solution is 37 degrees Celsius, drop in one pseudoephedrine tablet, Start your timer and turn on the stirrer. Once the tablet is completely dissolved, note the time and write it down. Repeat 4 more times with the same drug, using clean glassware every time. Average your times together.
3. Repeat step #2, using 5 tablets of chlorpheniramine maleate and diphenhydramine.
4. Prepare a graph or chart comparing the average time it took for the drugs to dissolve.

Consider repeating the experiment, using aspirin, ibuprofen and acetaminophen. If you started with four liters of the HCl-enzyme solution, you should have enough left to repeat the experiment one more time.   

Terms/Concepts: Decongestant, Antihistamine, Hydrochloric acid, Pepsin, Bioavailability, Peristalsis, Amylase, Solubility, Drug absorption                                      

References:

Merck Manuals: Absorption

http://merckmanuals.com/home/sec02/ch011/ch011c.html

The Journal of Pharmacology: Predicting Drug Absorption: How Nature Made It a Difficult Problem.

http://jpet.aspetjournals.org/content/303/3/889.abstract

Biology Innovation: The Digestive System

http://www.biology-innovation.co.uk/digestive_system.php