question about drug's shelf life
A solution of a new cardiotonic was found to lose 15% of its potency within 106 weeks of preparation when formulated as a solution and stored at 4degrees C. What is the shelf life of this medicine under these conditions? (Assume first order kinetics, logC=logC0-kt/2.303)
a. 34.3 weeks
b. 68.5 weeks
c. 68.5 days
d. 34.35 days
e. None of the above
very nice,now it is much easier,u will find t1/2 ,and then by multiplying with 0.152.the answer is 68.68 weeks
by the way such question which needs complicated formula i didnt focus on the theories,i feel they r not very important,but as i saw from ur side (when u mentioned about different temperature)u know these concepts very well.
Do i have really to study them?
l'optimisme de la volonte et la pessimisme de l'intelligence
Hi Husseinmr,
Yes that is the correct answer and a good way to get to it.
I would think it very unlikely that the PEBC exams would ask in-depth questions about pharmacokinetic and thermodynamic theory. However, as you can see from the above question it can help you to know where the equations are coming from and what they mean. You do not have to derive the formula or explain it, just have a good overview of the subject area and know how you can rearrange equations to your benefit.
Mat :)
Ok,
So they have given you the equation for first order kinetics. Usually with these types of questions they give you everything you need. Certainly on the PEBC paper they provide you with all such equations at the beginning of the paper. Remember these questions often look scarier than they are and they may give you information you do not really need (e.g. 4 degrees C in this question is not used in the calculation but is telling you that you do not have to find the rate constant for a different temperature).
One of the things you are required to know here is what is considered to be a standard shelf life. The answer to that is t90 or the time taken for a drug to lose 10% of its potency. So what they are saying is, we know how long it takes the drug to lose 15% potency, how long will it take to lose only 10% potency (and therefore be past its shelf life).
Looking at the equation provided you also need to know what the terms mean:
So what we are looking for is the rate constant k. Now, this is usually calculated with the Arrhenius equation but we don't need that here is they have told us we are looking for the shelf life under the same conditions they used to calculate the t85 (time taken to for drug to degrade to 85% of original concentration). So this means our rate constant k can be found using the data and equation provided.
I will give you a head start on the calculation and see how you get on.
Now solve for k. Once you have k, use it to find the time (t) taken to get to a concentration of 90%(0.9).
If you still have any difficulty let me know.
Good Luck
Mat