LETROZOLE;VRIBOCICLIB SUCCINATE

Clinical safety rating: avoid

Tamoxifen may reduce letrozole levels Estrogen-containing medications should not be used Decreases bone mineral density increasing fracture risk.

How it works

Letrozole is a nonsteroidal aromatase inhibitor that inhibits the conversion of androgens to estrogens, reducing estrogen levels in postmenopausal women. Vribociclib succinate is a CDK4/6 inhibitor that blocks cell cycle progression by inhibiting retinoblastoma protein phosphorylation, leading to cell cycle arrest in G1 phase.

What the body does with it

Metabolism	Letrozole is metabolized by CYP3A4 and CYP2A6 into an inactive metabolite. Vribociclib succinate is primarily metabolized by CYP3A4 and to a lesser extent by CYP3A5.
Excretion	Letrozole: ~90% renal (as glucuronide conjugates, minor unchanged), ~10% fecal. Vribociclib: primarily hepatic metabolism; ~70% fecal, ~30% renal (mostly metabolites).
Half-life	Letrozole: ~2 days (42 hours), terminal half-life supports once-daily dosing. Vribociclib: ~32-52 hours, terminal half-life allows once-daily dosing with steady-state reached in ~1 week.
Protein binding	Letrozole: ~60% bound, primarily to albumin. Vribociclib: ~70% bound, primarily to albumin.
Volume of Distribution	Letrozole: ~1.9 L/kg (large, indicating extensive tissue distribution). Vribociclib: ~22 L/kg (very large, extensive tissue binding and distribution).
Bioavailability	Both oral: Letrozole ~99.9% (well absorbed), Vribociclib ~50% (high first-pass metabolism; food increases AUC by 23-31%).
Onset of Action	Oral: Letrozole suppresses serum estradiol within 2-3 days; Vribociclib anti-tumor effect measurable after weeks of therapy.
Duration of Action	Letrozole: estradiol suppression persists 6-10 days after last dose. Vribociclib: CDK4/6 inhibition persists for several days; dosing schedule (e.g., 21 days on, 7 off) manages toxicity.

Classification & Brands

Dosing & administration

Letrozole 2.5 mg orally once daily; Vribociclib succinate 600 mg (equivalent to 564 mg vribociclib base) orally once daily for 21 days followed by 7 days off, in combination with letrozole.

Dosage form	TABLET
Renal impairment	No dose adjustment required for mild to moderate renal impairment (CrCl ≥30 mL/min). For severe renal impairment (CrCl <30 mL/min), not recommended.
Liver impairment	Child-Pugh A: No dose adjustment. Child-Pugh B: Reduce vribociclib dose to 400 mg once daily. Child-Pugh C: Not recommended.
Pediatric use	Safety and efficacy not established in pediatric patients; no recommended dosing.
Geriatric use	No specific dose adjustment required based on age; monitor renal function and potential for QT prolongation.

Use during pregnancy

1st trimester	Consult provider
2nd trimester	Consult provider
3rd trimester	Consult provider

Clinical note

Tamoxifen may reduce letrozole levels Estrogen-containing medications should not be used Decreases bone mineral density increasing fracture risk.

FDA category	Contraindicated
Breastfeeding	No data on letrozole or vribociclib succinate in human milk. Letrozole has low molecular weight, likely excreted. Vribociclib is highly protein-bound, with unknown M/P ratio. Due to potential adverse effects in the infant, breastfeeding is contraindicated during therapy and for a period after the last dose.
Teratogenic Risk

Warnings & precautions

■ FDA Black Box Warning

None

Side Effect Profile

Common Effects	Fatigue Hypercholesterolemia high cholesterol Increased sweating Hot flashes Bone pain Dizziness Joint pain Edema swelling Flushing sense of warmth in the face ears neck and trunk
Serious Effects

Absolute Contraindications

["Hypersensitivity to letrozole or vribociclib succinate","Pregnancy","Pre-existing severe neutropenia (absolute neutrophil count < 1000/mm3)","Severe hepatic impairment"]

Clinical Precautions

Precautions

["Neutropenia and bone marrow suppression","Hepatotoxicity","Embryo-fetal toxicity","Severe cutaneous adverse reactions (e.g., Stevens-Johnson syndrome)","Interstitial lung disease/pneumonitis","QT interval prolongation"]

Clinical Tips & Counseling

Drug interactions

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LETROZOLE;VRIBOCICLIB SUCCINATE

Clinical safety rating: avoid

Tamoxifen may reduce letrozole levels Estrogen-containing medications should not be used Decreases bone mineral density increasing fracture risk.

How it works

What the body does with it

Metabolism	Letrozole is metabolized by CYP3A4 and CYP2A6 into an inactive metabolite. Vribociclib succinate is primarily metabolized by CYP3A4 and to a lesser extent by CYP3A5.
Excretion	Letrozole: ~90% renal (as glucuronide conjugates, minor unchanged), ~10% fecal. Vribociclib: primarily hepatic metabolism; ~70% fecal, ~30% renal (mostly metabolites).
Half-life	Letrozole: ~2 days (42 hours), terminal half-life supports once-daily dosing. Vribociclib: ~32-52 hours, terminal half-life allows once-daily dosing with steady-state reached in ~1 week.
Protein binding	Letrozole: ~60% bound, primarily to albumin. Vribociclib: ~70% bound, primarily to albumin.
Volume of Distribution	Letrozole: ~1.9 L/kg (large, indicating extensive tissue distribution). Vribociclib: ~22 L/kg (very large, extensive tissue binding and distribution).
Bioavailability	Both oral: Letrozole ~99.9% (well absorbed), Vribociclib ~50% (high first-pass metabolism; food increases AUC by 23-31%).
Onset of Action	Oral: Letrozole suppresses serum estradiol within 2-3 days; Vribociclib anti-tumor effect measurable after weeks of therapy.
Duration of Action	Letrozole: estradiol suppression persists 6-10 days after last dose. Vribociclib: CDK4/6 inhibition persists for several days; dosing schedule (e.g., 21 days on, 7 off) manages toxicity.

Classification & Brands

Dosing & administration

Letrozole 2.5 mg orally once daily; Vribociclib succinate 600 mg (equivalent to 564 mg vribociclib base) orally once daily for 21 days followed by 7 days off, in combination with letrozole.

Dosage form	TABLET
Renal impairment	No dose adjustment required for mild to moderate renal impairment (CrCl ≥30 mL/min). For severe renal impairment (CrCl <30 mL/min), not recommended.
Liver impairment	Child-Pugh A: No dose adjustment. Child-Pugh B: Reduce vribociclib dose to 400 mg once daily. Child-Pugh C: Not recommended.
Pediatric use	Safety and efficacy not established in pediatric patients; no recommended dosing.
Geriatric use	No specific dose adjustment required based on age; monitor renal function and potential for QT prolongation.

Use during pregnancy

1st trimester	Consult provider
2nd trimester	Consult provider
3rd trimester	Consult provider

Clinical note

Tamoxifen may reduce letrozole levels Estrogen-containing medications should not be used Decreases bone mineral density increasing fracture risk.

FDA category	Contraindicated
Breastfeeding	No data on letrozole or vribociclib succinate in human milk. Letrozole has low molecular weight, likely excreted. Vribociclib is highly protein-bound, with unknown M/P ratio. Due to potential adverse effects in the infant, breastfeeding is contraindicated during therapy and for a period after the last dose.
Teratogenic Risk

Warnings & precautions

■ FDA Black Box Warning

None

Side Effect Profile

Common Effects	Fatigue Hypercholesterolemia high cholesterol Increased sweating Hot flashes Bone pain Dizziness Joint pain Edema swelling Flushing sense of warmth in the face ears neck and trunk
Serious Effects

Absolute Contraindications

["Hypersensitivity to letrozole or vribociclib succinate","Pregnancy","Pre-existing severe neutropenia (absolute neutrophil count < 1000/mm3)","Severe hepatic impairment"]