Chapter 8, Problem 8.6P

(Pharmacokinetics) Tarzlon is a liquid antibiotic that is taken orally to treat infections of the spleen. It is effective only if it can maintain a concentration in the bloodstream (based on volume of body fluid) above 0.4 mg per dm³ of body fluid. Ideally, a concentration of 1.0 mg/dm³ in the blood should be realized. However, if the concentration in the blood exceeds 1.5 mg/dm³, harmful side effects can occur. Once the Tarzlon reaches the stomach, it can proceed in two pathways, both of which are first order: (1) It can be absorbed into the blood-stream through the stomach walls; (2) it can pass out through the gastrointestinal tract and not be absorbed into the blood. Both these processes are first order in Tarzlon’s concentration in the stomach. Once in the bloodstream, Tarzlon attacks bacterial cells and is subsequently degraded by a zero-order process. Tarzlon can also be removed from the blood and excreted in urine through a first-order process within the kidneys. In the stomach:

Absorption into blood            k₁ = 0.15 h⁻¹

Elimination through gastrointestinal    k₂ = 0.6 h⁻¹

In the bloodstream:

Degradation of Tarzlon        k₃ = 0.1 mg/dm³·h

Elimination through urine        k₄ = 0.2 h⁻¹

One dose of Tarzlon is 250 mg in liquid form: Volume of body fluid = 40 dm³.

1. (a) Plot and analyze the concentration of Tarzlon in the blood as a function of time when 1 dose (i.e., one liquid capsule) of Tarzlon is taken.
2. (b) How should the Tarzlon be administered (dosage and frequency) over a 48-h period to be most effective? (Hint: Recall what it says on many antibiotic prescriptions regarding the first dose.)
3. (c) Comment on the dose concentrations and potential hazards.
4. (d) How would your answers change if the drug were taken on a full or empty stomach?