Pharmaceutical Calculations

a) You need 70% alcohol. In what proportion should 95%, 60%, and 50% alcohol be mixed to prepare a solvent of the desired concentration?

(3 marks)

b) Prepare twelve 1 gram suppositories. Each suppository contains 50 mg of drug X and 300 mg of drug Y. Theobroma oil is used as the base for the suppositories. Quantities are calculated for an excess of 25%. Mould calibration is 1. Displacement values of drug X and drug Y are 0.7 and 1.5, respectively. How many grams of theobroma oil base are needed in this preparation?

(3 marks)

c) You receive a prescription for 10 ml of isotonic 0.5% w/v chloramphenicol eye drops. How many grams of sodium chloride are required to make this product? (A 1% w/v solution of chloramphenicol has a freezing point depression of 0.06˚C. 1% w/v sodium chloride produces a freezing point depression of 0.576˚C).

(4 marks)

d) Patient requires intravenous (“IV”) potassium supplementation. The prescription calls for potassium chloride ("KCl”) 60 mEq IV STAT. Your pharmacy stocks CONCENTRATED KCl for injection 14.9% 20mL vials. (Molecular weight of KCl - 74.5 g/mol.) Patient only has peripheral IV access.
Your hospital policy states that KCl IV infusion must have an osmolarity of less than 600 mOsm/L, a potassium concentration of not more than 100 mEq/L and infusion rate cannot exceed 10 mmol potassium per hour for peripheral administration. 

i) What is volume of diluent needed? What diluent would you use?

ii) What is the volume of CONCENTRATED KCl stock solution you would use to prepare your infusion bag(s) in the above?

iii) What is an appropriate infusion rate, in mL/h, for the above?

(3 marks)

e) Patient requires vancomycin for treating a serious infection. Patient is a 78-year-old female, her current body weight is 117 lbs and her current serum creatinine is 75 µmol/L.

i) Calculate her creatinine clearance (“CLcr”) using the Cockcroft- Gault equation.

ii) Vancomycin clearance (“CLvanco”) and volume of distribution (‘ Vd”) can estimated by Vd = 0.7 L/kg and CLvanco (mL/min/kg) = 0.695 * CLcr (mL/min/kg) + 0.05. What is the expected half-life in this patient?

iii) Recommend a dosing regimen that will have an expected 24-hour area-under-the-concentration-vs-time-curve ("AUC24") between 400 to 600 mgh/L and an expected steady-state trough between 10 to 15 mg/L. Give a complete order including drug, dose, route and frequency.

iv) Calculate the expected steady-state concentration if vancomycin is given as a constant-rate continuous intravenous infusion at 50 mg/h.

(5 marks)

f) A lady presents a prescription for mercaptopurine 75 mg/m² orally daily for 2 weeks.

i) The prescription is for her 3-year-old daughter. Her daughter is 105 cm tall and weighs 15 kg. What is her body surface area?

ii) Your pharmacy stocks mercaptopurine 50mg oral tablets and 20mg/mL oral suspension. How much would you dispense?

(2 marks)

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