MEMANTINE HYDROCHLORIDE AND DONEPEZIL HYDROCHLORIDE
Clinical safety rating: safe
Drugs that make urine alkaline (eg carbonic anhydrase inhibitors) may decrease excretion Generally well-tolerated with dizziness being the most common side effect.
Memantine is an NMDA receptor antagonist, noncompetitively blocking glutamate-mediated excitotoxicity. Donepezil is a reversible acetylcholinesterase inhibitor, increasing acetylcholine levels in the brain.
| Metabolism | Memantine undergoes minimal hepatic metabolism via CYP450 enzymes (not significantly metabolized). Donepezil is metabolized by CYP3A4 and CYP2D6, with glucuronidation. |
| Excretion | Memantine: ~75-90% excreted unchanged in urine via renal tubular secretion and glomerular filtration, with ~10% as metabolites. Donepezil: ~57% excreted in urine (17% unchanged) and ~15% in feces. Biliary excretion is minimal for both. |
| Half-life | Memantine: terminal half-life 60-80 hours, requiring dose titration to steady state in ~3-4 weeks. Donepezil: terminal half-life ~70 hours, achieving steady state in 14-21 days. Renal impairment prolongs memantine half-life; hepatic impairment affects donepezil. |
| Protein binding | Memantine: ~45% bound to plasma proteins (primarily albumin). Donepezil: ~96% bound (primarily to albumin and α1-acid glycoprotein). |
| Volume of Distribution | Memantine: Vd ~10-12 L/kg, indicating extensive tissue distribution beyond extracellular fluid. Donepezil: Vd ~16-18 L/kg, consistent with high lipophilicity and deep tissue penetration. |
| Bioavailability | Both are oral: memantine ~100% (rapid and complete absorption, no first-pass effect); donepezil ~100% (peak plasma 3-4 hours post-dose, minimal first-pass metabolism). |
| Onset of Action | Clinical effects (cognitive improvement) observed after 2-4 weeks of daily oral dosing. Single-dose does not produce immediate symptomatic benefit. |
| Duration of Action | Sustained 24-hour coverage with once-daily dosing due to long half-lives. Clinical benefit persists over months of continuous therapy; effects wane over 6-12 weeks after discontinuation. |
Memantine ER/donepezil: 28 mg/10 mg orally once daily in the evening. Titrate from 7 mg/5 mg (or 14 mg/5 mg) over 4 weeks.
| Dosage form | CAPSULE, EXTENDED RELEASE |
| Renal impairment | CrCl 30-49 mL/min: maximum memantine ER/donepezil dose 14 mg/10 mg daily. CrCl 15-29 mL/min: 7 mg/10 mg daily. CrCl <15 mL/min or dialysis: not recommended. |
| Liver impairment | Child-Pugh A or B: no adjustment. Child-Pugh C: not studied; use caution. |
| Pediatric use | Not approved for pediatric use; safety and efficacy not established. |
| Geriatric use | Start at lowest available strength (7 mg/5 mg) and titrate slowly; monitor for adverse effects. No specific dose adjustment beyond renal function. |
| 1st trimester | Consult provider |
| 2nd trimester | Consult provider |
| 3rd trimester | Consult provider |
Clinical note
Drugs that make urine alkaline (eg carbonic anhydrase inhibitors) may decrease excretion Generally well-tolerated with dizziness being the most common side effect.
| FDA category | Animal |
| Breastfeeding | No data on excretion in human milk. Memantine is excreted in rat milk. Donepezil is likely excreted due to low molecular weight. M/P ratio unknown. Due to potential for adverse reactions in nursing infants, discontinue breastfeeding or discontinue drug, considering importance to mother. |
| Teratogenic Risk |
■ FDA Black Box Warning
None.
| Common Effects | Dizziness |
| Serious Effects |
["Hypersensitivity to memantine, donepezil, or piperidine derivatives","Concurrent use with other NMDA antagonists (e.g., amantadine, ketamine) is not recommended"]
| Precautions | ["Risk of bradycardia and heart block due to vagotonic effects of donepezil","Urinary retention","Seizures","Peptic ulcer disease risk (donepezil increases gastric acid secretion)","Concomitant use of other NMDA antagonists (e.g., amantadine, ketamine) with memantine may increase adverse effects","Use caution in renal impairment (memantine) and hepatic impairment (donepezil)"] |
| Food/Dietary | No known significant food interactions. However, take with a full glass of water to prevent esophageal irritation. Avoid grapefruit juice if taking with other CYP3A4-metabolized drugs, though donepezil is minimally affected. |
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| Memantine: No adequate studies in pregnant women; animal studies show fetal harm at high doses. Donepezil: Limited data; cholinesterase inhibitors may increase uterine tone potentially causing preterm labor. FDA Pregnancy Category C for both. Risk cannot be ruled out; use only if benefit outweighs risk. First trimester: potential for neural tube defects based on animal data. Second and third trimesters: risk of preterm labor and low birth weight. |
| Fetal Monitoring | Monitor maternal blood pressure, liver function, and gastrointestinal status. Assess fetal growth via ultrasound if prolonged use. Neonatal monitoring for respiratory depression, feeding difficulties, and cholinergic crisis if used near term. |
| Fertility Effects | In animal studies, memantine caused decreased fertility indices at high doses. Donepezil has not shown effects on fertility. Human data inadequate; potential for reversible impairment of spermatogenesis and menstrual irregularities. |
| Clinical Pearls | Titrate memantine to 20 mg daily before adding donepezil to minimize cholinergic side effects. Monitor for bradycardia, especially with concurrent beta-blockers or digoxin. Use with caution in severe renal impairment (CrCl < 30 mL/min); reduce memantine dose. |
| Patient Advice | Take the combination tablet at bedtime to reduce side effects like nausea or dizziness. · Do not crush or chew the extended-release tablet; swallow whole. · Report slow heart rate, fainting, or muscle cramps to your doctor immediately. · Avoid alcohol, as it may worsen dizziness and confusion. · If you have severe kidney problems, your doctor may need to adjust the dose. |