Number of Pages: 4 (Double Spaced) Language Style: English (U.K.) Writing Style: APA Number of sources: 4 Answer both questions. Total marks: 100 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.
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Number of Pages: 4 (Double Spaced) Language Style: English (U.K.) Writing Style: APA Number of sources: 4 Answer both questions. Total marks: 100 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 Table 1. 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. 5 marks c) ?and using this, calculate the volume of distribution for drug X. 5 marks d) What does this tell us about the physicochemical nature of Drug X? 5 marks e) Why can Vd be a useful parameter for a prescriber? 5 marks f) Estimate graphically the half-life of the drug in this person. 5 marks g) Calculate the elimination rate constant Kel . 5 marks h) Now calculate the clearance of drug X in this person. 5 marks i) Explain in words what your answer to (h) means. 5 marks Formulae for Q2: Vd = Q/Cp(0) Kel = CL / Vd t?=0.693/Kel Where: Vd = volume of distribution Q = dose Cp(0) = plasma concentration at time zero. Kel = elimination rate constant CL = clearance t? = half-life
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