Dose required to maintain a therapeutic plasma level

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Hiya all.

I'm sorry if this is the wrong place to post, so if so, please accept my apologies.

I'm totally stuck on a question so any help would be very gratefully received.

The therapeutic plasma level for the drug is a concentration between 27-80mg/litre. The drug has a half-life of 4 hours (assume the patient has 5 litres of blood) and a bio-availability of 90%.

After giving an initial loading dose of 400mg the patient will be given a regular dose of 200mg. How many hours would you suggest between doses to maintain therapeutic plasma level?

Please someone help - thankyou

What is the patient's elimination rate constant for the medication?

Or at least what is the population based elim. rate constant?

Originally Posted by SmallPharma

What is the patient's elimination rate constant for the medication?

Or at least what is the population based elim. rate constant?

If we have the half-life, we can find the elimination rate constant (k)

t1/2 = 0.693/k
so, k = 0.693/4 hours = 0.17ish

Let's say we want to keep the steady state concentration at an average of 50mg/L

avg steady state concentration= (Bioavailability x Dose (po) )
[space space space space space] Clearance x dosing interval

You can find the clearance with the volume and half life you were given. Solve for dosing interval. There are other equations to make sure your min and max fall within the parameters, but I don't have those in front of me.
I have to go to class otherwise I'd finish it for you