NJMS Pharmacology Homework Problems

Basic Principles Problems

E. J. Flynn, Ph.D.

These problems are designed to assist students in knowing whether they are getting the information presented in the text and lectures and , in addition, to provide the faculty with feedback so that potential difficulties can be cleared up prior to the unit examination. If you have questions you can reach me at (973) 972-5451 or e-mail: eflynn@umdnj.edu. It is my intention to provide answers so that you can check your work individually. In addition, student questions will be discussed during the regularly scheduled class time.

1. Phenobarbital (a weak organic acid, pKa=7.2) given orally will be well absorbed from the stomach (pH=1.4) into the plasma (pH=7.4), but if given systemically (i.v.) will be poorly excreted into the stomach. Why is this the case? Defend you argument mathematically.

2. Would you expect morphine (weak base, pKa=8.8) to behave in a similar manner to phenobarbital when given orally or when given systemically? If not, why? Defend you answer mathematically.

3. What dose of lidocaine would a patient receive if given a 0.5 mL injection of a 2% lidocaine solution?

4. What dose of epinephrine would a patient receive if given a 1.0 mL injection of a 1:100,000 epinephrine solution?

5. A single i.v. dose of thiopental is associated with a duration of action of 20-30 minutes. A second i.v. dose given immediately upon awakening is associated with a duration of 2-4 hours. Explain why the duration of action is prolonged with the second dose. (Include a description of how thiopental's pharmacologic action is normally terminated.)

6. If a steady state plasma level of sulfisoxazole of 80 ng/mL is reached during a constant intravenous infusion, how long will it take for the plasma level of drug to fall to 2 ng/mL when the infusion is stopped? (sulfisoxazole's t1/2 = 69.3 min.)

Patient R.G. is to be infused with tetracycline for rickettsial infection. The half-life of the drug in this patient is 11 hours and its volume of distribution is 100 liters. (Use this information for questions 7-16)

7. If you decide to infuse the drug at a rate of 500 microg/min., what would be the steady state plasma concentration of tetracycline?

8. What would the steady state plasma level be if you double the infusion rate? Triple the infusion rate?

9. For an infusion rate of 500 microg/min., how long would it take to reach 50% of the final plateau level?

10. What infusion rate would be required if you wanted to establish a tetracycline concentration of 10 microg/mL in the plasma at steady state?

11. How long would it take to reach 90% of the plateau concentration?

12. If you stop infusing tetracycline when the plasma concentration is 6 microg/mL how long would it take for the plasma level of tetracycline to fall to 0.75 microg/mL?

13. How long would it take to fall from 6 microg/mL to 0.45 microg/mL?

Suppose you decide to switch patient R.G to orally administered tetracycline at a dose of 750 mg/day. The drug is only 77% absorbed from the GI tract.)

14. What would be the plasma concentration of tetracycline at steady state?

15. What would be the plasma concentration of tetracycline at steady state if the same dose was given every 6 hrs? Every 12 hrs? Every 48 hrs?

16. What would be steady state level if the dose was changed from 750 mg/day to 375 mg/day? 1500 mg/day?

17. How long does it take for a drug with a ke = 0.0693/hrs to decline from a steady state plasma level (Css) of 100 microg/mL to 6.25 microg/mL?

18. How long would it take for a drug that saturates its elimination system to fall from 200 microg/mL to 100 microg/mL? (ke = 10 microg/mL/hr)

19. A patient is to receive the antibiotic, tobramycin. The clearance and Vd of tobramycin in this patient are 80 mL/min and 40 L, respectively. What maintenance dose must be administered intravenously, every 6 hours to eventually obtain a steady state plasma concentration of 4 mg/L?

20. Drug X has a narrow therapeutic index: the minimum toxic plasma concentration is 150 mg/L while the minimum therapeutic plasma concentration is 100 mg/L. The half-life is 6 hours. It is essential to maintain the plasma concentration above the minimum therapeutic level. The most appropriate dosing regimen would be how many time a day?

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