Data Handling Coursework

Data Handling Coursework

 

1. Phenytoin is an anti-epileptic drug. It is a weak acid with a pKa of 8.1. The unionised form is insoluble, and will form a precipitate. Consequently when infusing intravenously (IV) it is important to avoid this from happening.
A pharmacist prepares a phenytoin solution for IV infusion; commonly the pH of infusion solutions is pH 5.0 A colleague suggests they should consider changing the pH of the solution to avoid precipitation.
Using the Henderson-Hasselbalch equation, determine the best way for the pharmacists to proceed.
50 marks
2. During a clinical trial, 60 mg of a drug X is given intravenously to a volunteer, and plasma samples taken every two hours are analysed. The data obtained are shown in the Table

Table 1.Plasma concentrations of drug X in volunteer
Time /h Plasma Concentration (Cp) /mg L-1
2 10
4 6.7
6 4.5

a) On semi-log (i.e. log Y axis) graph paper plot the data above, and label accordingly.
10 marks
b) Estimate the initial plasma concentration Cp(0) at time zero.
c) …and using this, calculate the volume of distribution for drug X.

d) What does this tell us about the physicochemical nature of Drug X?

e) Why can Vd be a useful parameter for a prescriber?
f) Estimate graphically the half-life of the drug in this person.
g) Calculate the elimination rate constant Kel .
h) Now calculate the clearance of drug X in this person.
i) Explain in words what your answer to (h) means.

 
Formulae for Q2:
Vd = Q/Cp(0) Kel = CL / Vd t½=0.693/Kel
Where:
Vd = volume of distribution
Q = dose
Cp(0) = plasma concentration at time zero.
Kel = elimination rate constant
CL = clearance
t½ = half-life

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