calculation help please

[vytorin2007]

How do calculate the T1/2 when given the peak and trough values?

thanks for your help

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[keke]

can you post up the prob?

 

[vytorin2007]

Hi,

I can't remember the exact problem but it gave drug amount ,admin IV then gave the drugs peak value and then a trough value.

thanks

 

[keke]

I think this is how you solve it

Example peak: 20 mcg/ml
12 hrs later trough is 1.25 mcg/ml
t1/2=?

Since it takes 4 half lives from from peak to trough, the example here it takes 12 hrs from peak to trough. Then 12 hrs / 4 half lives gives 3 hrs per half live therefore t1/2= 3hrs

Ex. peak at time X = 20 mcg/ml
1 half live = 10 mcg/ml
2 half live = 5 mcg/ml
3 half live = 2.5 mcg/ml
4 half live = 1.25 mcg/ml
12 hrs later

 

[npage148]

C2=C1e^(-kt)

T1/2=k/0.693

 

[sdn1977]

You need more info than we are being given.

t(1/2) = 0.693 Vd
Cl

This is assuming first order elimination rate kinetics where plasma decay is described by Cp = Cp(o) x e (raised to the- kdt)....my computer won't let me do those math upper & lower scripts..

This is where Cp(o) & Cp are the plama conc @ the beginning & end of th time interval t & e (raised to the -kdt) is the fraction of Cp(o) remaining at time t.

If Cp is equal to 1/2Cp(o) then, by definition, t = t1/2

The relationship between kd & t1/2 can then be derived....

But - this only works for first order kinetics. You should also know the Vd of your drug & the elimination rate constant.

Good luck - I think you might need to go back to your kinetics book & redo some problems.....

 

[vytorin2007]

thanks guys for the info