1. Objective: Explain the variation in dosing intervals for medications 1)Introduce Half-Life (t 1/2 ) 2)Relate Half-Life to Dosing intervals of aspirin and Aleve pain relievers Longevity of a Single Dose Time to Therapeutic Steady State

2. The time it takes a substance to decrease to half of a previous value. 2H 2 O 2 O 2 + 2H 2 O Hydrogen Peroxide t 1/2 = 654 minutes, appx 11 hours Half-Life-t 1/2 √

3. Hydrogen Peroxide Half-Life 43 2 1 Half-Life is a good predictor of how long a substance will exist 50%> 25%> 12.5%> <6.25%

4. Drug Half-Lives Peak Amounts The time required for the total amount of a drug in blood plasma to diminish by half, once the drug reaches its peak concentration. Time to Peak: 1.5hrs

5. In general, next dose is given once the previous dose reaches its half-life Dosing Intervals Half-Life Half-Life (PDR)Dosage Interval Aspirin6 hours4 Hours Aleve12-17 hours12 Hours Normal Dose Interval is based on one Half-Life PDR: Physicians Desk Reference √

6. Draw Half-Life curves for Aspirin and Aleve Estimate the Amount and Percent of Drug remaining after two Half-Lives Half-Life (PDR)Time to Peak (PDR) Aspirin6 hours1-2 hours Aleve12-17 hours1-2 hours

7. 50% 25%

8. 50% 25%

9. Pharmacokinetics Absorption Distribution Metabolism Excretion Molecular Structure Bond Polarity Why do Drugs Have Different Half-Lives? Sodium O - Na + √

10. Half-Life and Longevity of a Single Dose Rule of thumb: For a single dose, a drug is eliminated from the body after five half-lives. Half-Life (PDR) Elimination time Aspirin6 hours30 hours Aleve12 hours60 hours

11. Rule of thumb: It takes five dose intervals for a drug to reach a therapeutic steady state level. Half-Life and Therapeutic Steady State Level About How Many Half-Lives? 5 √

12. A: A loading dose is administered followed by maintenance doses Q: What if a drug has too long of a half life to relieve a condition?

13. Digoxin: Treat arrhythmia t 1/2 = 1.6 days. Examples of Drugs administered with Loading + Maintenance doses To reach therapeutic steady state would take __?__ days or five half lives. 8 Medrol: Steroid to reduce inflammation How much time would pass to attain a therapeutic steady state amount? t 1/2 = 18-26 hours About 5 days

14. What to Know… Half-Life: The time it takes a substance to decrease to half of a previous value. Half-Life is a good predictor of how long a substance will exist. Drug Half-Life: The time required for the total amount of a drug in blood to diminish by half, once the drug reaches its peak concentration. See PDR How to construct a half-life curve for a drug. A dose interval is approximately one Half-Life. Pharmacokinetics is the study of reaction rates of drugs in biological systems. Rule of thumb: For a single dose, a drug is eliminated from the body after five half-lives. Rule of thumb: It takes five dose intervals or five half-lives for a drug to reach a therapeutic steady state amount.