Question

A prescription requires 1 g of cefazolin to be administered to the patient in 100 mL of normal saline. Cefazolin is an antibiotic that can administered (IV infusion). Cefazolin is a weak acid (MW = 455, pKa1 =2.1 and pKa2 =4.9) and is poorly soluble in aqueous IV infusion solutions such as 5% dextrose (D5W) or 0.9% saline (NS). Because of the solubility problem, we can prepare the formulation using the sodium salt of cefazolin (MW=477). When assayed approximately 75 hours after the formulation has been prepared, the concentration was found to be 8.4 mg/mL. The drug was reported to undergo first-order degradation.

QUESTION:
Why did we decide to use cefazolin sodium for this prescription and how much cefazolin sodium should we use for this prescription?

Answer 1

Blood is mostly water and cefazolin is poorly soluble in aqueous IV infusion solutions . But the sodium salt of cefazolin is soluble and therefore it is suitable for intravenous infusion. Thus, to overcome solubility issues of cefazoline, it is converted to its sodium salt.

Now, prescription requires 1 g cefazolin. Its molecular weight = 455.

The molecular weight of cefazolin sodium salt = 477

Therefore the amount of cefazoline sodium is needed for the prescription = (477 x 1)/455 = 1.048 g