need help with this gen. pharm question!!!

3/12/2014 8:31:09 PM
A 35 year old woman is given 500 mg of drug X intravenously. Several minutes later, the serum concentration of drug X is 12.5 mg/L. 2 hours later the serum concentration is 10mg/L. Assuming the drug X has first order elimination kinetics which of the following will be the serum concentration of drug X (in mg/L) in the next 2 hours?

A.) 6
B.) 6.5
C.) 7
D.) 7.5
E.) 8
F.) 8.5

I chose 7.5 but it was wrong. I know that in first order kinetics, a fraction (1/2) is lost per unit of time. However, half of 12.5 is 5.5 but that choice is not present. Can anyone help me? Whaaaaa?

3/14/2014 9:23:57 AM
i dont get it either...if anyone out there knows how to solve this please show how its done step by step

3/14/2014 12:16:24 PM
First order kinetics:fixed proportion of drug is excreted per unit time.

here 1/5 th of the drug is excreted in 2hrs, ((12.5-10)/12.5)/l =1/5 or say (500-400)/500mg=1/5
so in next 2 hrs 1/5th of the remaining would be excreted

after 2 hrs remaining drug is 10mg/l or 400mg
so in next two hours 1/5th i.e. 2 mg/l or 80 mg would be excreted
leaving 320 mg or 8mg/l..i hope it's right.
(--here Vd=500/12.5=40L, which can give amount at each level though it can be done even without calculating Vd)

3/17/2014 1:18:56 PM
OK I see what you did there. Your reasoning makes sense. Thanks a lot!!

9/20/2014 6:17:07 PM
shubz123 wrote:
OK I see what you did there. Your reasoning makes sense. Thanks a lot!!

simple way is 12.5-10/12.5 = 1/5

10-x/10 = 1/5 --> x = 8.

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