Anastrozole Half-Life and Elimination
The elimination half-life of anastrozole is approximately 50 hours (about 2 days), and this does not change in elderly patients. 11
Pharmacokinetic Details
Time to Complete Elimination:
- With a half-life of 50 hours, anastrozole takes approximately 10-14 days (5-7 half-lives) to be essentially cleared from the body after stopping therapy 1
- Steady-state plasma levels are reached after about 7 days of once-daily dosing 11
- Plasma concentrations at steady state are approximately 3-4 fold higher than after a single dose 1
Specific Pharmacological Suppression:
- Suppression of serum estradiol is maintained for up to 6 days after cessation of daily dosing with anastrozole 1 mg 11
- This indicates that the pharmacodynamic effect (estrogen suppression) persists slightly beyond the drug's presence in plasma
Elderly Population Considerations
No age-related pharmacokinetic changes occur with anastrozole 111:
- Pharmacokinetics were investigated in postmenopausal women ranging from <50 to >80 years of age
- No age-related effects were observed across this entire age spectrum
- No dosage adjustment is necessary in elderly patients
The FDA label explicitly states: "Anastrozole pharmacokinetics have been investigated in postmenopausal female volunteers and patients with breast cancer. No age-related effects were seen over the range <50 to >80 years" 11
Metabolism and Excretion
- 85% hepatic metabolism (primarily via N-dealkylation, hydroxylation, and glucuronidation) 11
- 10% renal elimination (unchanged drug) 11
- 85% of radiolabeled drug recovered in feces and urine combined 11
Clinical Implications
The 50-hour half-life supports once-daily dosing and means that:
- Missing a single dose has minimal immediate impact on estrogen suppression
- When discontinuing therapy, residual drug effects persist for approximately 2 weeks
- Steady therapeutic levels are achieved within one week of starting treatment
This pharmacokinetic profile is consistent across research studies showing half-lives of 41-48 hours 23 and 30-60 hours 4, with the FDA-approved label settling on 50 hours as the mean elimination half-life 11.