1. Pharmaceutical Calculations
a) A child weighing 50 pounds requires a dose of 7 mg/kg of drug X. Given that a 5 mL dose is to be given each time, what volume of water must be added when the drug X powder is reconstituted? Instruction on the label indicates that dilution to 120 mL (by adding 95 mL of water) gives 200 mg drug X per 5 mL.
b) An intravenous solution of sodium chloride 0.5% requires to be made isotonic by the addition of dextrose. What weight of dextrose is required in the preparation of 500 mL of the intravenous solution? (A 1% solution of sodium chloride depresses the freezing point of water by 0.576°C. A 1% solution of dextrose depresses the freezing point of water by 0.091°C.)
c) You have 20%, 15% and 5% zinc oxide ointment. In what proportion should they be mixed together in order to make a 10% zinc oxide ointment? All these 3 ointments must be used.
d) Patient MMK is a 56-year-old, 154 pounds and 5 feet 4 inches female. She is starting her first dose of rituximab for Non-Hodgkin's Lymphoma.
(i) The regimen is 375 mg/m2 IV once weekly for 4 doses. Calculate the dose for this patient.
(ii) Rituximab is available in 100mg/10mL and 500mg/50mL single-dose vials. You are going to procure the medication for her entire rituximab regimen. How many vials do you need to order for this patient?
(iii) The instruction for the first infusion is as follows:
“Initiate infusion at a rate of 50 mg/hr. In the absence of infusion toxicity, increase infusion rate by 50 mg/hr increments every 30 minutes, to a maximum of 400 mg/hr. "
What is the minimum infusion time for her first infusion, assuming she does not have any infusion toxicity?
(e) Subsequently, patient MMK from part (d) presents with urinary tract infection. (3 marks)
Her latest serum creatinine is 105 μmol/L. The physician prescribed gentamicin 420 mg IV Q24H infused over 1 hour.
(i) Gentamicin volume of distribution is estimated to be 0.25 L/kg, and clearance is expected to 100% of creatinine clearance. What is the expected half-life of gentamicin in this patient?
(ii) Your pharmacy stocks gentamicin 80 mg/2 mL vials. Your hospital policy states that extended-interval aminoglycosides should be diluted in 100 mL normal saline ("NS"). What is the appropriate infusion rate?
(iii) Therapeutic drug monitoring of extended-interval aminoglycoside typically involves taking a single blood level in the middle of the dosing interval. What is the expected steady state concentration at 12 hours after start of infusion?
f) After receiving the gentamicin as in part (e), patient MMK develops mild hypokalemia and the doctor orders potassium chloride ("KCI") 8 mmol to be added to 500 mL of Dextrose 5% + NaCl 0.45% (“D5-1⁄2NS”), and to be infused as a continuous infusion.
(i) The KCl is available in 14.9% concentrated solution. What is the volume required to prepare the infusion solution (molecular weight of KCl is 74.5 g/mol)?
(ii) What is the approximate concentration of potassium (in mEq/L) in the infusion solution?
(iii) What is the approximate osmolarity (in mOsmol/L) of the infusion solution?
g) Eventually patient MMK in part (f) is discharged home with an oral antibiotic. Because she suffers painful mouth sore after her chemotherapy, she requests to have oral solution instead of solid dosage forms.
(i) The doctor orders co-trimoxazole 960 mg PO BD for 3 days. Your pharmacy stocks co-trimoxazole 480 mg oral tablets and 240 mg/5 mL oral suspension in 60 mL bottles. What is the total quantity to be dispensed for this prescription?