Question

2. Calculate free drug concentration for drug A whose unbound percentage is 10 and total drug concentration is 10 mg/L. 3. Calculate half life of a drug where CL= 101ml/min and V= 17.5 L.

Answer 1

2) calculation of free drug concentration of a drug A ?

Many drugs are bound to serum proteins, and it is important to remember that it is the free (unbound)drug which is responsible for pharmacological activity of the drug. Typically, the total drug concentration(bound drug + free drug) is measured for the purpose of TDM.

So total drug concentration = bound drug concentration + unbound drug concentration (free drug concentration)

• As mentioned in the question total drug concentration=10mg/L   
• As mentioned in the question Unbound drug percentage (free drug concentration)= 10%  

Total drug concentration= percentage of bound drug concentration+ percentage of free drug concentration

Substituting values ,

10mg/L (total drug concentration) = 10%( percentage of free drug concentration)+90% (percentage of bound drug concentration)

So to get free drug concentration we need to find 10%of total drug concentration that is 10% of 10 mg/L  

Free drug concentration=10% of 10 mg /L

Using basic method for percentage calculation we get (10×10)÷100=1 ,so 10% of 10 mg/L= 1 mg/L

So the free drug concentration for A = 1 mg/L

  

3) calculation of half life a drug

• As mentioned in the question clearance value of drug= 101ml/min
• As mentioned in the question the volume of distribution= 17.5L

To calculate half life we have the formula

Half life = 0.693 ×volume of distribution/clearance value

• In this question volume of distribution (L) and clearance value(ml) are in different units .so converting liters to ml ,

Volume of distribution=17.5L when converted to ml it is multiplied by 1000 = 17.5×1000=17,500 ml

Substituting values in the formula ,

half life = 0.693×17,500/101

= 120.07 minutes

Which is approximately equal to 2 hours

Half life of the drug = 2 hrs