Palbociclib is an oral targeted medicine used to treat hormone receptor-positive, HER2-negative breast cancer. It belongs to a class of medicines called CDK4/6 inhibitors, and was the first medicine in this class to receive FDA approval. It is a prescription-only medicine, typically dosed on a 3-weeks-on/1-week-off schedule.

• Therapeutic Class: Anti-cancer (oncology) targeted therapy for breast cancer.

• Pharmacological Class: Cyclin-dependent kinase 4 and 6 (CDK4/6) inhibitor.

• Drug Family: Small-molecule CDK4/6-targeted therapies.

• ATC Classification: L01EF01 (Palbociclib).

Approved: Hormone receptor-positive (HR+), HER2-negative advanced or metastatic breast cancer — combined with an aromatase inhibitor as initial endocrine-based therapy, or combined with fulvestrant in patients with disease progression following endocrine therapy; also approved in combination with trastuzumab (with or without pertuzumab) and endocrine therapy for maintenance treatment of HR-positive, HER2-positive advanced/metastatic breast cancer following induction treatment.

Commonly used for: Women (and eligible men) with HR+ advanced or metastatic breast cancer, as part of a broader hormone-based or HER2-targeted treatment strategy.

Off-label use (only under medical guidance): Being studied in other hormone-driven or CDK4/6-dependent cancers in clinical trials; not routine outside its approved indications.

● Oral capsules and tablets: 125 mg (also available in lower strengths such as 100 mg and 75 mg for dose reduction).

Palbociclib blocks two enzymes, CDK4 and CDK6, that cancer cells need to progress through the cell division cycle. By inhibiting these enzymes, palbociclib stops hormone receptor-positive breast cancer cells from dividing and growing, working alongside hormone-based therapies (or, in certain settings, HER2-targeted therapies) that address the cancer's other growth drivers.

Standard dose: 125 mg taken orally once daily for 21 consecutive days, followed by 7 days off treatment, to complete a 28-day cycle (Schedule 3/1).

Condition-wise dose: In combination with trastuzumab (with or without pertuzumab) and endocrine therapy for maintenance treatment, use the same 125 mg dosing schedule (21 days on, 7 days off), along with the recommended dosing of the combination partners.

Route: Oral, taken with food; capsules/tablets should be swallowed whole and not chewed, crushed, or opened.

Missed dose: If a dose is missed or vomiting occurs, do not take an additional dose the same day— take the next scheduled dose as usual.

Overdose: May worsen known side effects such as severe neutropenia; requires medical attention.

Common: Low white blood cell counts (neutropenia, the most common side effect), fatigue, nausea, diarrhoea, and hair thinning.

Serious: Severe neutropenia with an increased risk of infection (including rare cases of febrile neutropenia), interstitial lung disease/pneumonitis (rare but serious lung inflammation), and embryo-fetal toxicity.

Emergency symptoms: Fever with signs of infection, new or worsening shortness of breath or cough, or unusual bruising or bleeding.

Recovery advice: Neutropenia is very common but usually manageable through routine blood count monitoring and dose adjustment; it is a key reason blood tests are scheduled regularly throughout treatment.

Pregnancy: Can cause fetal harm; effective contraception is required during treatment and for a period after stopping.

Breastfeeding: Not recommended during treatment and for a period after the last dose.

Liver disease: No dose adjustment needed in mild to moderate impairment; used with caution and monitoring in severe impairment.

Kidney disease: No dose adjustment needed in mild to moderate impairment; used with caution in severe impairment.

Elderly: Commonly used in this age group; monitored particularly for blood count changes.

Children: Not approved for use in children.

● Strong CYP3A4 inhibitors (e.g., ketoconazole, clarithromycin): can increase palbociclib levels, requiring dose reduction.

● Strong CYP3A4 inducers (e.g., rifampin): can reduce palbociclib effectiveness; co-administration is generally avoided.

● Other myelosuppressive drugs: may increase the risk of low blood counts when combined.

● Grapefruit juice: may increase palbociclib levels and should generally be avoided.

● Complete blood counts before starting each cycle and on Day 14 of the first two cycles, then as clinically indicated.

● Monitoring for new or worsening respiratory symptoms suggestive of interstitial lung disease/pneumonitis.

● Liver function tests periodically.

● Pregnancy testing before starting in patients of reproductive potential, given the fetal toxicity risk.

Abemaciclib, Ribociclib (other CDK4/6 inhibitors, different dosing schedules and side-effect emphasis).

Used with an aromatase inhibitor or fulvestrant for HR+/HER2- advanced breast cancer; also used with trastuzumab (with or without pertuzumab) and endocrine therapy for maintenance treatment of HR+/HER2+ advanced breast cancer following induction treatment.

● Breast Cancer Medicines

● CDK4/6 Inhibitors

● Hormone Receptor-Positive Cancer Therapies

1. Fight Breast Cancer With Advanced Treatment

2. What is ER-positive Breast Cancer?

1. https://ibrance.pfizerpro.com/dosing/administration

2. https://www.fda.gov/drugs/resources-information-approved-drugs/fda-approves-palbociclib-trastuzumab-or-without-pertuzumab-and-endocrine-therapy-maintenance

The information on this page is for educational purposes only and is not a substitute for medical advice. Please consult your oncologist to know if this treatment is appropriate for your condition.