Abemaciclib

Introduction Overview

Generic Name

abemaciclib

Brand Name

Verzenio

Dosage Form

Oral tablets: 50 mg, 100 mg, 150 mg, and 200 mg.

Drug Class

Cyclin-dependent kinase 4 and 6 (CDK4/6) inhibitor; targeted anticancer therapy.

Pharmacokinetics

Absorption:
Abemaciclib is taken by mouth twice daily and may be taken with or without food. Peak plasma concentrations occur about 8 hours after an oral dose. If a dose is missed or vomited, the next dose should be taken at the usual scheduled time rather than replacing the missed dose.

Distribution:
Abemaciclib is widely distributed, with an apparent volume of distribution of several hundred liters. It is highly protein bound in plasma. Distribution into tumor tissue supports its use as a systemic therapy for hormone receptor-positive, HER2-negative breast cancer.

Metabolism:
Abemaciclib is extensively metabolized in the liver, primarily by CYP3A. Several metabolites are pharmacologically active. Because CYP3A is central to metabolism, strong or moderate CYP3A inhibitors can raise exposure and strong or moderate CYP3A inducers can lower exposure.

Elimination:
After a radiolabeled dose, most drug-related material is recovered in feces, mainly as metabolites, and only a small amount is recovered in urine. The mean plasma elimination half-life is about 18 hours. Abemaciclib can increase serum creatinine by inhibiting renal tubular secretion transporters without necessarily reducing glomerular filtration; alternative kidney function markers may be needed if renal function is unclear.

Dosage Forms

Available tablet strengths:
- 50 mg
- 100 mg
- 150 mg
- 200 mg

Usual adult dosing:
- In combination with endocrine therapy such as tamoxifen, an aromatase inhibitor, or fulvestrant: 150 mg by mouth twice daily.
- As monotherapy for indicated metastatic disease after endocrine therapy and prior chemotherapy: 200 mg by mouth twice daily.

Early breast cancer duration:
For adjuvant treatment of high-risk early breast cancer, continue abemaciclib until completion of 2 years of therapy, disease recurrence, or unacceptable toxicity.

Advanced or metastatic breast cancer duration:
Continue until disease progression or unacceptable toxicity.

Administration:
Take doses at approximately the same times each day. Swallow tablets whole. Do not chew, crush, split, or take tablets that are broken, cracked, or not intact.

Mechanism Of Action

Abemaciclib inhibits CDK4 and CDK6, kinases that help move cells from the G1 phase into the S phase of the cell cycle. In many hormone receptor-positive breast cancers, estrogen receptor signaling increases cyclin D activity, which activates CDK4/6. Activated CDK4/6 phosphorylates retinoblastoma protein (Rb), allowing cell-cycle progression and tumor cell proliferation.

By inhibiting CDK4/6, abemaciclib reduces Rb phosphorylation, slows cell-cycle progression, and can cause G1 arrest in susceptible cancer cells. It is commonly paired with endocrine therapy because endocrine therapy lowers estrogen-driven growth signaling while abemaciclib blocks a downstream cell-cycle pathway.

For students, the shortcut is: endocrine therapy lowers the growth signal, and abemaciclib blocks the cell-cycle engine that the growth signal turns on.

Pharmacological Actions

Inhibits CDK4 and CDK6 activity.
Reduces phosphorylation of retinoblastoma protein.
Promotes G1 cell-cycle arrest in susceptible tumor cells.
Enhances the effect of endocrine therapy in HR-positive, HER2-negative breast cancer.
Delays recurrence risk in selected high-risk early breast cancer when used as adjuvant therapy.
Delays disease progression in advanced or metastatic HR-positive, HER2-negative breast cancer.
Can be used continuously on a twice-daily schedule rather than the intermittent schedules used by some other CDK4/6 inhibitors.

Therapeutic Or Clinical Uses

With endocrine therapy for adjuvant treatment of adults with HR-positive, HER2-negative, node-positive early breast cancer at high risk of recurrence.
High-risk early breast cancer generally includes either at least 4 positive pathologic axillary lymph nodes, or 1 to 3 positive nodes plus tumor grade 3 or tumor size at least 50 mm. The FDA removed the former Ki-67 testing requirement in 2023.
With an aromatase inhibitor as initial endocrine-based therapy for adults with HR-positive, HER2-negative advanced or metastatic breast cancer.
With fulvestrant for adults with HR-positive, HER2-negative advanced or metastatic breast cancer whose disease progressed after endocrine therapy.
As monotherapy for adults with HR-positive, HER2-negative advanced or metastatic breast cancer after progression on endocrine therapy and prior chemotherapy in the metastatic setting.

Side Or Adverse Effects

Diarrhea, which is very common and may be severe, especially during the first month.
Neutropenia, leukopenia, anemia, lymphopenia, and thrombocytopenia.
Infections, including upper respiratory tract infection, urinary tract infection, and nasopharyngitis.
Nausea, vomiting, abdominal pain, decreased appetite, dyspepsia, and stomatitis.
Fatigue, headache, dizziness, rash, pruritus, nail changes, and alopecia.
Increased ALT or AST and other signs of hepatotoxicity.
Interstitial lung disease or pneumonitis, which can be severe or life-threatening.
Venous thromboembolic events, including deep vein thrombosis and pulmonary embolism.
Increased serum creatinine from transporter inhibition, which may not represent true loss of glomerular filtration.

Precautions

Start antidiarrheal therapy, increase oral fluids, and contact the oncology team at the first sign of loose stools.
Monitor complete blood counts before treatment, every 2 weeks for the first 2 months, monthly for the next 2 months, and as clinically indicated.
Monitor liver function tests before treatment, every 2 weeks for the first 2 months, monthly for the next 2 months, and as clinically indicated.
Evaluate new or worsening cough, shortness of breath, chest pain, or hypoxia for possible ILD or pneumonitis.
Evaluate leg swelling, chest pain, sudden shortness of breath, tachycardia, or neurologic symptoms for possible thromboembolism.
Review pregnancy status before treatment in females of reproductive potential.
Use effective contraception during treatment and for 3 weeks after the last dose.
Do not breastfeed during treatment and for at least 3 weeks after the final dose.
Review all prescription, over-the-counter, vitamin, and herbal products for CYP3A interactions.
Use dose interruptions, dose reductions, or discontinuation according to toxicity severity.

Warnings

Severe diarrhea can cause dehydration and infection; early management is essential.
Neutropenia can lead to serious or fatal infection.
ILD or pneumonitis can be severe, life-threatening, or fatal.
Hepatotoxicity can occur; liver enzyme monitoring is required.
Venous thromboembolism and pulmonary embolism have occurred, including fatal cases.
Abemaciclib can cause fetal harm when administered during pregnancy.
Avoid ketoconazole; dose reduction may be needed with other strong CYP3A inhibitors or moderate CYP3A inhibitors.
Avoid strong and moderate CYP3A inducers because they may reduce abemaciclib exposure and effectiveness.
Avoid grapefruit products because they can increase abemaciclib exposure.

Contraindication

Current U.S. prescribing information lists no formal contraindications for Verzenio. Pregnancy risk, breastfeeding avoidance, CYP3A interactions, severe toxicities, and pediatric non-establishment are important warnings or precautions, but they are not label-listed absolute contraindications.

Drug Interactions

Ketoconazole: avoid because it is predicted to markedly increase abemaciclib exposure.
Other strong CYP3A inhibitors, such as clarithromycin, itraconazole, posaconazole, voriconazole, or ritonavir-containing regimens, can increase exposure and may require dose reduction.
Moderate CYP3A inhibitors, such as diltiazem or verapamil, can increase exposure and may require monitoring or dose adjustment.
Strong or moderate CYP3A inducers, such as rifampin, carbamazepine, phenytoin, or St. John's wort, can lower abemaciclib exposure and should generally be avoided.
Grapefruit and grapefruit juice can inhibit CYP3A and should be avoided.
Other myelosuppressive or hepatotoxic drugs may increase overlapping toxicity and require closer monitoring.
When abemaciclib is used with fulvestrant, tamoxifen, or an aromatase inhibitor, follow the safety information and dosing instructions for the endocrine therapy as well.

Preparations Of The Drug

Verzenio 50 mg tablets.
Verzenio 100 mg tablets.
Verzenio 150 mg tablets.
Verzenio 200 mg tablets.
Store at controlled room temperature according to the product labeling.
Dispense and store tablets so they remain intact and protected from damage.
Do not use broken, cracked, or otherwise damaged tablets.
Prescription-only oncology medicine requiring regular laboratory and toxicity monitoring.

Student Key Points

Abemaciclib is a CDK4/6 inhibitor used in HR-positive, HER2-negative breast cancer.
The usual combination dose is 150 mg twice daily; monotherapy dose is 200 mg twice daily.
Diarrhea is the signature toxicity and needs action at the first loose stool.
CBC and liver enzymes are monitored frequently early in treatment.
Formal contraindications are none, but serious warnings still matter.
The 2023 FDA expansion removed the Ki-67 requirement for high-risk early breast cancer selection.