| Date post: | 31-Dec-2015 |
| Category: |
Documents |
| Upload: | wesley-malone |
| View: | 223 times |
| Download: | 0 times |
Objective: Explain the variation in dosing intervals for medications
1) Introduce Half-Life (t1/2)2) Relate Half-Life to • Dosing intervals of aspirin and Aleve pain relievers • Longevity of a Single Dose• Time to Therapeutic Steady State
The time it takes a substance to decreaseto half of a previous value.
2H2O2 O2 + 2H2O
Hydrogen Peroxide
t1/2 = 654 minutes, appx 11 hours
Half-Life-t1/2
√
Hydrogen Peroxide Half-Life
4321
Half-Life is a good predictor of how long a substance will exist
50%>
25%>
12.5%><6.25%
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
In general, next dose is given once the previous dose reaches its half-life
Dosing Intervals Half-Life
Half-Life (PDR) Dosage Interval
Aspirin 6 hours 4 Hours
Aleve 12-17 hours 12 Hours
Normal Dose Interval is based on one Half-Life
PDR: Physicians Desk Reference√
Draw Half-Life curves for Aspirin and AleveEstimate the Amount and Percent of Drug remaining after
two Half-Lives
Half-Life (PDR) Time to Peak (PDR)
Aspirin 6 hours 1-2 hours
Aleve 12-17 hours 1-2 hours
Half-Life Plot for Aleve
0
5
10
15
20
0 1 2 3 4 5 6 7 8 9 10 111213141516 17181920212223242526
Hours
50%
25%
Half-Life Plot for Aspirin
0
5
10
15
20
0 1 2 3 4 5 6 7 8 9 10 111213 14 1516 1718 1920212223242526
Hours
50%
25%
Pharmacokinetics•Absorption•Distribution•Metabolism•Excretion
Molecular StructureBond Polarity
Why do Drugs Have Different Half-Lives?
SodiumO- Na+
√
Half-Life andLongevity of a Single Dose
Rule of thumb: For a single dose, a drug iseliminated from the body after five half-lives.
Half-Life (PDR)
Elimination time
Aspirin 6 hours 30 hours
Aleve 12 hours 60 hours
Rule of thumb: It takes five dose intervals for adrug to reach a therapeutic steady state level.
Half-Life and Therapeutic Steady State Level
About How Many Half-Lives? 5
√
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?
Digoxin: Treat arrhythmia t1/2 = 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?
t1/2 = 18-26 hours About 5 days
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.
