pharma question

[firstaid2012]

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Q During an experiment 500 mg of both drugs X and Y are simultaneously administered to a health human subject by bolus IV infusion. The serum concentrations immediately post bolus of drug X and Y are 5mg/L and 2.5 mg/L respectively. Assuming there is no interaction between drugs X and Y, which of the following findings regarding drug Y is most likely compared with drug X?

A- Decreased clearance
B- Decreased half life
C- Deceased Vd
D- Increased clearance
E- Increased half life
F- Increased Vd

please explain

 

[Myxedema]

"...serum concentrations immediately post bolus..." --> This eliminates clearance and half-life. So the question becomes: Is the volume of distribution higher or lower for drug Y?

If a blood is able to pass from the blood into tissues, Vd will be high and serum concentration will be low. This can be also illustrated with the equation Vd = Dose / C; so concentration and Vd are inversely proportional. Since drug Y has a lower concentration, it has a higher Vd.

 

[Ronin786]

I think the answer is that the Vd for Y is greater than X. Since the concentration was immediately after injection, I think its safe to assume that the drug hasn't had enough time to be cleared or for a 1/2 life to have gone by.

Plus the 1/2 life depends on the CL and Vd and the CL also depends on Vd, so really Vd is the limiting factor in it all, especially at such a short time after administration.

From that you can simply plug into the equation. Vd = amount in body/plasma concentration = dose x bioavailability/plasma concentration. Biovailability here is 1 since its an IV drug.

For X the dose is 500 and the plasma conc. is 5, whereas for Y the only change is the plasma conc. which is 2.5. So 500/5 = 100, and 500/2.5 = 200. So the Vd for Y is higher than the Vd for X.

In addition, they don't give you the rate of elimination for the drug or the elimination constant, so realistically you can only calculate the Vd with the given info.