Ibrance (Palbociclib)
palbociclib
Indications
| Indication | Approved Population | Therapy Type | Status |
|---|---|---|---|
| HR+/HER2− advanced or metastatic breast cancer — initial endocrine therapy | Adults (including men) | Combination (+ aromatase inhibitor) | FDA Approved |
| HR+/HER2− advanced or metastatic breast cancer — after endocrine progression | Adults (including men) | Combination (+ fulvestrant) | FDA Approved |
| PIK3CA-mutated, endocrine-resistant HR+/HER2− locally advanced or metastatic breast cancer | Adults, after adjuvant endocrine therapy recurrence | Combination (+ inavolisib + fulvestrant) | FDA Approved |
Palbociclib is a selective, reversible CDK4/6 inhibitor approved exclusively for HR-positive, HER2-negative advanced or metastatic breast cancer. It is always used in combination with endocrine therapy — never as monotherapy. The PALOMA-2 trial established palbociclib plus letrozole as a standard first-line regimen, demonstrating a median PFS of 24.8 months versus 14.5 months with letrozole alone (HR 0.58). PALOMA-3 established palbociclib plus fulvestrant for endocrine-pretreated patients, with median PFS of 11.2 versus 4.6 months (HR 0.50). OS in the overall PALOMA-3 population showed a numerical but non-significant improvement (34.9 vs 28.0 months; HR 0.81; P=0.09), with a statistically significant benefit observed in the subgroup with sensitivity to prior endocrine therapy. Pre/perimenopausal women receiving palbociclib with an aromatase inhibitor or fulvestrant should also receive an LHRH agonist. Men should be considered for concurrent LHRH agonist therapy.
Well-differentiated neuroendocrine tumors: Some evidence for CDK4/6 inhibitor activity in RB-proficient neuroendocrine tumors; investigational. Evidence quality: Low.
Liposarcoma: CDK4 amplification common in well-differentiated/dedifferentiated liposarcoma; palbociclib has shown activity in early-phase trials. Evidence quality: Moderate.
Dosing
Adult Dosing by Clinical Scenario
| Clinical Scenario | Starting Dose | Maintenance Dose | Maximum Dose | Notes |
|---|---|---|---|---|
| First-line HR+/HER2− advanced breast cancer with aromatase inhibitor | 125 mg PO once daily × 21 days, then 7 days off | 125 mg (21/7 schedule) | 125 mg/day | 28-day cycle; take with food; continue with continuous AI until progression Pre/perimenopausal: add LHRH agonist |
| HR+/HER2− after endocrine progression with fulvestrant | 125 mg PO once daily × 21 days, then 7 days off | 125 mg (21/7 schedule) | 125 mg/day | Fulvestrant 500 mg on Days 1, 15, 29, then monthly Pre/perimenopausal: add LHRH agonist |
| PIK3CA-mutated disease with inavolisib + fulvestrant | 125 mg PO once daily × 21 days, then 7 days off | 125 mg (21/7 schedule) | 125 mg/day | Triplet combination; PIK3CA mutation confirmed by FDA-approved test |
| Co-administration with strong CYP3A inhibitor | 75 mg PO once daily (21/7) | 75 mg (21/7) | 75 mg/day | Increase to prior dose after 3–5 half-lives of inhibitor upon discontinuation |
| Severe hepatic impairment (Child-Pugh C) | 75 mg PO once daily (21/7) | 75 mg (21/7) | 75 mg/day | No adjustment for mild-moderate (Child-Pugh A–B) Unbound AUC increased 77% in severe impairment |
Dose Reductions for Toxicity
| Dose Level | Dose |
|---|---|
| Starting dose | 125 mg/day |
| First reduction | 100 mg/day |
| Second reduction | 75 mg/day |
| If further reduction needed | Discontinue palbociclib |
Neutropenia-Specific Dose Modifications
| Neutropenia Grade (CTCAE) | Action |
|---|---|
| Grade 1–2 | No dose adjustment required |
| Grade 3 on Day 1 of cycle | Withhold; repeat CBC within 1 week; resume at same dose when ≤Grade 2 |
| Grade 3 on Day 15 of first 2 cycles | Continue current dose to complete cycle; recheck CBC Day 22. If Grade 4 on Day 22, follow Grade 4 guidelines below |
| Grade 3 with fever ≥38.5°C and/or infection | Withhold until ≤Grade 2; resume at next lower dose |
| Grade 4 | Withhold until ≤Grade 2; resume at next lower dose |
Monitor CBC prior to starting therapy, at the beginning of each cycle, on Day 15 of the first 2 cycles, and as clinically indicated. For patients who experience only Grade 1–2 neutropenia in the first 6 cycles, subsequent monitoring can be reduced to every 3 months (prior to the beginning of a cycle). The median time to first episode of any-grade neutropenia is 15 days and the median duration of Grade ≥3 episodes is 7 days — neutropenia with palbociclib is characteristically reversible, non-cumulative, and rarely associated with fever.
Pharmacology
Mechanism of Action
Palbociclib is a selective, reversible inhibitor of cyclin-dependent kinases 4 and 6 (CDK4/6). These kinases, when complexed with cyclin D, phosphorylate the retinoblastoma protein (Rb), which permits cell cycle progression from G1 to S phase. By inhibiting CDK4/6-mediated Rb phosphorylation, palbociclib arrests cell proliferation in the G1 phase. In HR-positive breast cancer, CDK4/6 is a key downstream effector of estrogen receptor signaling, and combined inhibition of CDK4/6 with endocrine therapy produces synergistic growth suppression. Preclinical models demonstrated that palbociclib preferentially inhibits ER-positive breast cancer cell lines and can reverse endocrine resistance when combined with anti-estrogen agents. Unlike cytotoxic chemotherapy, CDK4/6 inhibitor-induced neutropenia reflects reversible cell cycle arrest of bone marrow progenitors rather than cell death, explaining its non-cumulative, predictable pattern.
ADME Profile
| Parameter | Value | Clinical Implication |
|---|---|---|
| Absorption | Bioavailability 46%; Tmax 6–12 h; pH-dependent solubility (high below pH 4); food reduces variability in ~13% of patients | Must take with food; PPIs have no effect on palbociclib PK (unlike erlotinib); no acid-suppression interaction |
| Distribution | Vd = 2,583 L (apparent); protein binding ~85%; extensive tissue distribution | Very large Vd indicates extensive peripheral tissue binding; accumulation ratio 2.4 at steady state |
| Metabolism | CYP3A (primary) and SULT2A1; major pathways: oxidation, sulfonation; minor: acylation, glucuronidation; weak time-dependent CYP3A inhibitor in vivo | Strong CYP3A inhibitors increase AUC by 87%; strong inducers decrease AUC by 85%; palbociclib itself increases CYP3A substrate exposure (midazolam AUC +61%) |
| Elimination | t½ 29 ± 5 h; feces ~74%, urine ~18% (6.9% unchanged); steady state by day 8 | Once-daily dosing appropriate; 21/7 schedule allows hematologic recovery during the 7-day off period |
Side Effects
Adverse reaction data are from the FDA prescribing information. PALOMA-2 (N=444, palbociclib + letrozole) provides first-line data; PALOMA-3 (N=345, palbociclib + fulvestrant) provides post-endocrine progression data. Neutropenia is the hallmark toxicity, occurring in 80–83% of patients.
WBC decreased 97% (grade 3: 35%), neutrophils decreased 95% (grade 3: 56%, grade 4: 12%), blood creatinine increased 96% (grade 3: 2%), hemoglobin decreased 78% (grade 3: 6%), platelets decreased 63% (grade 3–4: 2%), AST increased 52% (grade 3: 3%), ALT increased 43% (grade 3: 2%). The creatinine elevation is typically mild and results from palbociclib inhibiting renal tubular secretion of creatinine rather than true nephrotoxicity — GFR measured by alternative markers (e.g., cystatin C) is generally unaffected.
| Adverse Effect | Incidence | Clinical Note |
|---|---|---|
| Neutropenia | 80% | Grade 3: 56%, Grade 4: 10%; median onset Day 15; median Grade ≥3 duration 7 days; reversible and non-cumulative |
| Infections | 60% | Grade 3: 6%, Grade 4: 1%; most common: nasopharyngitis, URTI, UTI, oral herpes, sinusitis |
| Leukopenia | 39% | Grade 3: 24%, Grade 4: 1%; correlates with neutropenia pattern |
| Fatigue | 37% | Grade 3: 2%; one of the most commonly reported non-hematologic effects |
| Nausea | 35% | Grade 3: <1%; usually mild; not a common cause of dose modification |
| Alopecia | 33% | Grade 1 (thinning): 30%; Grade 2 (noticeable): 3%; typically non-total; reversible |
| Stomatitis | 30% | Grade 3: 1%; includes aphthous ulcers, mouth ulceration, mucosal inflammation |
| Diarrhea | 26% | Grade 3: 1%; generally mild and self-limiting |
| Anemia | 24% | Grade 3: 5%, Grade 4: <1%; monitor hemoglobin with CBC |
| Rash | 18% | Grade 3: 1%; includes maculopapular, pruritic, erythematous, acneiform forms |
| Asthenia | 17% | Grade 3: 2%; distinct from fatigue in severity grading |
| Thrombocytopenia | 16% | Grade 3: 1%, Grade 4: <1%; typically mild; monitor with CBC |
| Vomiting | 16% | Grade 3: 1%; manage supportively |
| Decreased appetite | 15% | Grade 3: 1%; nutritional support if weight loss significant |
| Dry skin | 12% | Emollients helpful; no grade 3–4 events |
| Pyrexia | 12% | Rule out febrile neutropenia and infection; no grade 3–4 events |
| Dysgeusia | 10% | Altered taste; no grade 3–4 events |
| Adverse Effect | Incidence | Clinical Note |
|---|---|---|
| Epistaxis | 9% | Usually mild; assess platelet count |
| Lacrimation increased | 6% | EGFR-pathway-independent mechanism |
| Dry eye | 4.1% | Lubricating drops; refer if persistent |
| Vision blurred | 3.6% | Ophthalmologic evaluation if worsening |
| Febrile neutropenia | 2.5% (PALOMA-2); 0.9% (PALOMA-3) | Across both trials: 1.8%; one death from neutropenic sepsis in PALOMA-3 |
| ILD/Pneumonitis | 1% | Grade 3–4 in 0.1% (trials); fatal cases reported postmarketing |
| Adverse Effect | Estimated Frequency | Typical Onset | Required Action |
|---|---|---|---|
| Grade ≥3 Neutropenia | 66% | Median Day 15; duration median 7 days | CBC monitoring per protocol; withhold for Grade 3 Day 1 or Grade 4; dose reduce per algorithm. Reversible and non-cumulative. |
| Febrile neutropenia | 1.8% | Variable | Withhold palbociclib; initiate antibiotics; resume at next lower dose. One death from neutropenic sepsis reported (PALOMA-3). |
| ILD / Pneumonitis | 1% (trials); fatal cases postmarketing | Variable | Interrupt palbociclib immediately for suspected ILD; permanently discontinue for severe ILD/pneumonitis. Fatal cases reported in postmarketing surveillance. |
| Venous thromboembolism | Reported (clinical trials + postmarketing) | Variable | Standard VTE management; CDK4/6 inhibitor class signal. PE reported in 0.9% of PALOMA-2 palbociclib arm. |
| Palmar-plantar erythrodysesthesia | Rare (postmarketing) | Variable | Reported in postmarketing surveillance; manage symptomatically. |
Neutropenia is the defining toxicity of palbociclib, affecting approximately 80% of patients with Grade ≥3 in 66%. However, it differs fundamentally from chemotherapy-induced myelosuppression. CDK4/6 inhibition causes reversible cell cycle arrest (quiescence) of bone marrow progenitors rather than cytotoxic cell death. This results in neutropenia that is predictably cyclic (nadir around Day 15), self-resolving during the 7-day off period, non-cumulative across cycles, and rarely complicated by febrile neutropenia (1.8%). The risk of serious infection is disproportionately low relative to the ANC nadir, reflecting functional preservation of mature neutrophils. Dose reductions for neutropenia maintain efficacy while improving tolerability.
Drug Interactions
Palbociclib is metabolized primarily by CYP3A and SULT2A1. In vivo, palbociclib is a weak time-dependent inhibitor of CYP3A, which can increase plasma levels of co-administered CYP3A substrates. Unlike erlotinib, palbociclib PK is not affected by gastric pH — PPIs, H2-blockers, and antacids can be used freely.
Monitoring
- CBC with DifferentialBaseline, Day 1 & Day 15 of cycles 1–2, then Day 1 of each cycle
RoutineCritical monitoring parameter. After 6 cycles with max Grade 1–2 neutropenia, reduce to every 3 months. Grade ≥3 neutropenia drives dose modifications. - Pregnancy TestBefore initiation
RoutineVerify pregnancy status in females of reproductive potential before starting palbociclib. - Liver Function TestsBaseline, then as indicated
RoutineAST increased in 52%, ALT increased in 43% (PALOMA-2, all grades). Monitor for hepatotoxicity, especially with Child-Pugh C impairment. - Respiratory SymptomsEvery visit
Trigger-basedILD/pneumonitis reported in 1% of trial patients with fatal postmarketing cases. New dyspnea, cough, or hypoxia should prompt immediate evaluation. - Signs of InfectionEvery visit
Trigger-basedInfections occurred in 47–60% of patients. Report fever (≥38.5°C) promptly, especially if concurrent with neutropenia nadir. - VTE SymptomsEvery visit
Trigger-basedVTE is a recognized CDK4/6 inhibitor class effect. Evaluate for DVT/PE if leg swelling, pain, or new dyspnea develop.
Contraindications & Cautions
Absolute Contraindications
- The FDA prescribing information lists no formal absolute contraindications.
- Severe ILD/pneumonitis during prior palbociclib treatment — permanent discontinuation is mandated.
Relative Contraindications (Specialist Input Recommended)
- Active severe infection — resolve infection before initiating or resuming CDK4/6 inhibitor therapy; neutropenia may worsen infectious course.
- Severe hepatic impairment (Child-Pugh C) — dose reduction to 75 mg required; unbound AUC increased by 77%.
- Concurrent use of strong CYP3A inducers — palbociclib exposure reduced by 85%; avoidance is required (no established dose escalation).
Use with Caution
- Concurrent strong CYP3A inhibitors — reduce palbociclib to 75 mg; AUC increased 87%.
- Patients on narrow-TI CYP3A substrates — palbociclib may increase their exposure; dose adjustment of substrate may be needed.
- History of VTE — VTE is a CDK4/6 inhibitor class effect; standard thromboprophylaxis considerations apply.
- Elderly patients (≥65 years) — no overall differences in safety observed but represent 41% of PALOMA-2 and 25% of PALOMA-3.
Palbociclib can cause fetal harm based on animal data and mechanism of action. Embryo-fetal toxicity observed in rats and rabbits at exposures ≥4 times human clinical exposure. Females of reproductive potential must use effective contraception during treatment and for at least 3 weeks after the last dose. Males must use effective contraception during treatment and for 3 months after the last dose due to genotoxicity potential. Verify pregnancy status before initiation.
Patient Counselling
Purpose of Therapy
Palbociclib is a targeted therapy that slows cancer growth by blocking proteins called CDK4 and CDK6, which cancer cells need to divide. It is always used alongside hormone therapy, not on its own. The treatment follows a 3-weeks-on, 1-week-off cycle, allowing blood counts to recover during the off week.
How to Take
Take palbociclib with food at approximately the same time each day. Swallow capsules whole — do not open, crush, or chew them. Do not take a capsule that is broken or cracked. If a dose is missed or vomiting occurs after taking a dose, do not take an extra dose; take the next scheduled dose at the usual time.
Sources
- IBRANCE (palbociclib) [prescribing information]. New York, NY: Pfizer Inc; revised September 2025. Pfizer LabelingPrimary source for all dosing, adverse reaction incidence rates, pharmacokinetics, warnings, and dose modification guidance.
- FDA grants regular approval to palbociclib (IBRANCE) for HR+/HER2- advanced breast cancer. FDA News Release, March 31, 2017. FDA.govOfficial FDA announcement of regular approval based on PALOMA-2 results, converting the 2015 accelerated approval.
- Finn RS, Martin M, Rugo HS, et al. Palbociclib and letrozole in advanced breast cancer. N Engl J Med. 2016;375(20):1925-1936. doi:10.1056/NEJMoa1607303PALOMA-2 primary analysis: palbociclib + letrozole median PFS 24.8 vs 14.5 months (HR 0.58; P<0.001).
- Finn RS, Rugo HS, Dieras V, et al. Overall survival with first-line palbociclib plus letrozole versus placebo plus letrozole in patients with ER+/HER2- metastatic breast cancer (PALOMA-2). J Clin Oncol. 2024;42(9):994-1000. doi:10.1200/JCO.23.00137PALOMA-2 final OS: median 53.8 vs 49.8 months (HR 0.92); OS not significantly improved despite PFS benefit.
- Turner NC, Ro J, André F, et al. Palbociclib in hormone-receptor–positive advanced breast cancer. N Engl J Med. 2015;373(3):209-219. doi:10.1056/NEJMoa1505270PALOMA-3 primary analysis: palbociclib + fulvestrant PFS 11.2 vs 4.6 months (HR 0.50) in endocrine-pretreated patients.
- Turner NC, Slamon DJ, Ro J, et al. Overall survival with palbociclib and fulvestrant in advanced breast cancer. N Engl J Med. 2018;379(20):1926-1936. doi:10.1056/NEJMoa1810527PALOMA-3 OS analysis: median OS 34.9 vs 28.0 months (HR 0.81; P=0.09 ITT); benefit significant in endocrine-sensitive subgroup.
- Cristofanilli M, Turner NC, Bondarenko I, et al. Fulvestrant plus palbociclib versus fulvestrant plus placebo for treatment of HR+, HER2- metastatic breast cancer that progressed on previous endocrine therapy (PALOMA-3): final analysis. Lancet Oncol. 2016;17(4):425-439. doi:10.1016/S1470-2045(15)00613-0PALOMA-3 final PFS analysis confirming benefit across all prespecified subgroups.
- Finn RS, Crown JP, Lang I, et al. The cyclin-dependent kinase 4/6 inhibitor palbociclib in combination with letrozole versus letrozole alone as first-line treatment of ER+, HER2- advanced breast cancer (PALOMA-1/TRIO-18). Lancet Oncol. 2015;16(1):25-35. doi:10.1016/S1470-2045(14)71159-3PALOMA-1 phase 2 study: supported accelerated FDA approval with PFS HR 0.49.
- National Comprehensive Cancer Network (NCCN). Clinical Practice Guidelines in Oncology: Breast Cancer. Version 1.2025. NCCN.orgPositions CDK4/6 inhibitors (including palbociclib) + endocrine therapy as preferred first-line for HR+/HER2- metastatic breast cancer.
- Sun W, O'Dwyer PJ, Finn RS, et al. Characterization of palbociclib pharmacokinetics in patients with impaired renal function. Cancer Chemother Pharmacol. 2020;86(6):743-752. doi:10.1007/s00280-020-04164-7Renal impairment PK study: AUC increased 31–42% across impairment levels; no dose adjustment recommended.
- Royer B, Kaderbhaï C, Fumet JD, et al. Population pharmacokinetics of palbociclib in a real-world situation. Pharmaceuticals. 2021;14(3):181. doi:10.3390/ph14030181Real-world PopPK analysis confirming palbociclib PK parameters (CL/F, Vd) consistent with pivotal trial data.