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#1
04-12-2015
 USMLE Forums Newbie Steps History: 1+CK+CS+3 Posts: 1 Threads: 1 Thanked 0 Times in 0 Posts Reputation: 10
real-life LD and MD question

So I'm a doctor who has long since forgotten how to do math (I know sad). Needless to say, I know the formula for loading dose (LD) is...

LD = VD x Cp / f

where VD is volume of distribution and Cp is target dose and f is bioavailability.

I also know the formula for maintenance dose (MD) is....

MD = Cp x Cl / f

where Cp is the target dose and Cl is clearance rate and f is bioavailability.

With that being said, let me present a real-life example.
The drug Dutasteride which blocks 5-alpha-reductase has the following parameters...
Vd=400L
Cl= 0.02L/kg/hr (1.18L/hr)
T 1/2= 840 hours
Cp= 40 ng/mL
f=60%

I would like to know, what is my loading dose and what is my maintenance dose? I know according to the package insert that patients that have been on 0.5mg for 1 year have a steady state serum concentration of 40 ng/mL. With that being said, I calculate the loading dose to be 26.67 mg. That makes sense to me, but the maintenance dose is .8 mg/hr or 19mg/day, which seems way to large to me, especially considering the fact that people at 1 year achieve the steady state serum concentration on no more than 0.5mg/day. I think I'm missing something, so I thought I could see if someone could help.

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