Palbociclib

{{Short description|Medication for HR+ HER2– breast cancer}} {{Use dmy dates|date=March 2020}} {{cs1 config |name-list-style=vanc |display-authors=6}} {{Infobox drug | image = Palbociclib skeletal formula.svg | image_class = skin-invert-image | alt = | caption = | image2 = Palbociclib ball-and-stick model from xtal 2023.png | image_class2 = bg-transparent

| pronounce = {{IPAc-en|ˌ|p|æ|l|b|oʊ|ˈ|s|aɪ|k|l|ɪ|b}}<br />{{respell|PAL|boh|SYE|klib}} | tradename = Ibrance, others | Drugs.com = {{drugs.com|monograph|palbociclib}} | MedlinePlus = a615013 | DailyMedID = Palbociclib | pregnancy_AU = D | pregnancy_AU_comment = | pregnancy_category= | routes_of_administration = By mouth | ATC_prefix = L01 | ATC_suffix = EF01

| legal_AU = S4 | legal_AU_comment = <ref>{{cite web | title=Prescription medicines: registration of new chemical entities in Australia, 2017 | website=Therapeutic Goods Administration (TGA) | date=21 June 2022 | url=https://www.tga.gov.au/resources/publication/publications/prescription-medicines-registration-new-chemical-entities-australia-2017 | access-date=9 April 2023 | archive-date=10 April 2023 | archive-url=https://web.archive.org/web/20230410060848/https://www.tga.gov.au/resources/publication/publications/prescription-medicines-registration-new-chemical-entities-australia-2017 | url-status=live }}</ref><ref>{{cite web | title=Prescription medicines and biologicals: TGA annual summary 2017 | website=Therapeutic Goods Administration (TGA) | date=21 June 2022 | url=https://www.tga.gov.au/resources/publication/publications/prescription-medicines-and-biologicals-tga-annual-summary-2017 | access-date=31 March 2024}}</ref> | legal_CA = Rx-only | legal_CA_comment = <ref>{{cite web | title=Health Canada New Drug Authorizations: 2016 Highlights | website=Health Canada | date=14 March 2017 | url=https://www.canada.ca/en/health-canada/services/publications/drugs-health-products/health-canada-new-drug-authorizations-2016-highlights.html | access-date=7 April 2024}}</ref> | legal_UK = POM | legal_US = Rx-only | legal_EU = Rx-only | legal_EU_comment = <ref>{{cite web | title=Ibrance EPAR | website=European Medicines Agency | date=9 November 2016 | url=https://www.ema.europa.eu/en/medicines/human/EPAR/ibrance | access-date=16 May 2024}}</ref> | legal_status =

| CAS_number = 571190-30-2 | PubChem = 5330286 | DrugBank = DB09073 | ChemSpiderID = 4487437 | UNII = G9ZF61LE7G | KEGG_Ref = {{keggcite|correct|kegg}} | KEGG = D10372 | ChEBI_Ref = {{ebicite|correct|EBI}} | ChEBI = 85993 | ChEMBL = 189963 | PDB_ligand = LQQ | synonyms = PD-0332991

| IUPAC_name = 6-Acetyl-8-cyclopentyl-5-methyl-2-<nowiki/>{[5-(1-piperazinyl)-2-pyridinyl]amino}pyrido[2,3-d]pyrimidin-7(8''H'')-one | C=24 | H=29 | N=7 | O=2 | smiles = O=C2N(c1nc(ncc1/C(=C2/C(=O)C)C)Nc3ncc(cc3)N4CCNCC4)C5CCCC5 | StdInChI = 1S/C24H29N7O2/c1-15-19-14-27-24(28-20-8-7-18(13-26-20)30-11-9-25-10-12-30)29-22(19)31(17-5-3-4-6-17)23(33)21(15)16(2)32/h7-8,13-14,17,25H,3-6,9-12H2,1-2H3,(H,26,27,28,29) | StdInChIKey = AHJRHEGDXFFMBM-UHFFFAOYSA-N }}

'''Palbociclib''', sold under the brand name '''Ibrance''' among others, is a medication developed by Pfizer for the treatment of HR-positive and HER2-negative breast cancer. It is a selective inhibitor of the cyclin-dependent kinases CDK4 and CDK6.<ref name=Finn2009/><ref name="pmid24369047"/> Palbociclib was the first CDK4/6 inhibitor to be approved as a cancer therapy.<ref name="pfizer.com">{{cite press release|url=https://www.pfizer.com/news/press-release/press-release-detail/updated_data_from_phase_3_trial_of_ibrance_palbociclib_plus_letrozole_in_er_her2_metastatic_breast_cancer_confirm_improvement_in_progression_free_survival|title=Updated Data from Phase 3 Trial of Ibrance (palbociclib) Plus Letrozole in HR+, HER2- Metastatic Breast Cancer Confirm Improvement in Progression-Free Survival|publisher=Pfizer|access-date=2017-12-16|archive-date=22 June 2021|archive-url=https://web.archive.org/web/20210622095415/https://www.pfizer.com/news/press-release/press-release-detail/updated_data_from_phase_3_trial_of_ibrance_palbociclib_plus_letrozole_in_er_her2_metastatic_breast_cancer_confirm_improvement_in_progression_free_survival|url-status=live}}</ref>

==Mechanism of action== {{further|CDK inhibitor}} It is a selective inhibitor of the cyclin-dependent kinases CDK4 and CDK6.<ref name=Finn2009>{{cite journal | vauthors = Finn RS, Dering J, Conklin D, Kalous O, Cohen DJ, Desai AJ, Ginther C, Atefi M, Chen I, Fowst C, Los G, Slamon DJ | title = PD 0332991, a selective cyclin D kinase 4/6 inhibitor, preferentially inhibits proliferation of luminal estrogen receptor-positive human breast cancer cell lines in vitro | journal = Breast Cancer Research | volume = 11 | issue = 5 | article-number = R77 | year = 2009 | pmid = 19874578 | pmc = 2790859 | doi = 10.1186/bcr2419 | doi-access = free }}</ref><ref name="pmid24369047">{{cite journal | vauthors = Rocca A, Farolfi A, Bravaccini S, Schirone A, Amadori D | title = Palbociclib (PD 0332991) : targeting the cell cycle machinery in breast cancer | journal = Expert Opinion on Pharmacotherapy | volume = 15 | issue = 3 | pages = 407–420 | date = February 2014 | pmid = 24369047 | doi = 10.1517/14656566.2014.870555 | s2cid = 23229580 }}</ref>

In the G1 phase of the cell cycle, mammalian cells must pass a checkpoint, known as the restriction point "R", in order to complete the cell cycle and divide. CDK4 and CDK6 complex with Cyclin D drive the phosphorylation of the retinoblastoma protein, Rb, which allows the cell to pass R and commit to division.<ref name=":1">{{cite journal | vauthors = Xu H, Yu S, Liu Q, Yuan X, Mani S, Pestell RG, Wu K | title = Recent advances of highly selective CDK4/6 inhibitors in breast cancer | journal = Journal of Hematology & Oncology | volume = 10 | issue = 1 | article-number = 97 | date = April 2017 | pmid = 28438180 | pmc = 5404666 | doi = 10.1186/s13045-017-0467-2 | doi-access = free }}</ref> Regulation of one or more proteins involved in this checkpoint is lost in many cancers. However, by inhibiting CDK4/6, palbociclib ensures that the cyclin D-CDK4/6 complex cannot aid in phosphorylating Rb. This prevents the cell from passing R and exiting G1, and in turn from proceeding through the cell cycle.<ref name=":1" />

== Administration == Palbociclib is taken daily orally with food in a cycle of 21 days of active medication, followed by a week of abstinence. Currently, palbociclib is prescribed as a combination therapy with either letrozole or fulvestrant.<ref name=":0">{{Cite web|url=https://www.accessdata.fda.gov/drugsatfda_docs/label/2016/207103s002lbl.pdf|title=IBRANCE FDA Drug Label|access-date=2017-12-15|archive-date=1 April 2021|archive-url=https://web.archive.org/web/20210401191142/https://www.accessdata.fda.gov/drugsatfda_docs/label/2016/207103s002lbl.pdf|url-status=dead}}</ref> Patients should also not consume CYP3A inhibitors or inducers while taking palbociclib. FDA information also cautions against consuming grapefruit products while taking palbociclib, in order to avoid negative interactions.<ref name=":0" />

==Approvals and indications== ===HR+ breast cancer=== The drug was reviewed and approved under the Food and Drug Administration’s (FDA) accelerated Priority Review and Breakthrough Therapy designation programs on February 3, 2015, as a treatment (in combination with letrozole) for patients with estrogen receptor positive (ER+) advanced breast cancer.<ref>{{cite web | url = http://www.onclive.com/web-exclusives/FDA-Approves-Palbociclib-for-Metastatic-Breast-Cancer | title = FDA Approves Palbociclib for Metastatic Breast Cancer | publisher = OncLive | date = 3 Feb 2015 | access-date = 4 February 2015 | archive-date = 31 October 2018 | archive-url = https://web.archive.org/web/20181031210023/https://www.onclive.com/web-exclusives/fda-approves-palbociclib-for-metastatic-breast-cancer | url-status = live }}</ref> This was an accelerated approval.<ref name=FDA-AA>{{cite web | url = http://www.pfizer.com/news/press-release/press-release-detail/pfizer_receives_u_s_fda_accelerated_approval_of_ibrance_palbociclib | title = Pfizer Receives U.S. FDA Accelerated Approval of IBRANCE (palbociclib) | publisher = Pfizer | date = 3 Feb 2015 | access-date = 4 February 2015 | archive-date = 18 January 2021 | archive-url = https://web.archive.org/web/20210118044026/https://www.pfizer.com/news/press-release/press-release-detail/pfizer_receives_u_s_fda_accelerated_approval_of_ibrance_palbociclib | url-status = live }}</ref>

In March 2017, the FDA granted regular approval to palbociclib for hormone receptor (HR) positive, human epidermal growth factor receptor 2 (HER2) negative advanced or metastatic breast cancer, in combination with an aromatase inhibitor.<ref>{{cite web|url=https://www.fda.gov/Drugs/InformationOnDrugs/ApprovedDrugs/ucm549978.htm|title=Palbociclib (Ibrance)|publisher=FDA|date=2017-03-31|access-date=7 July 2017|archive-date=23 April 2019|archive-url=https://web.archive.org/web/20190423133437/https://www.fda.gov/Drugs/InformationOnDrugs/ApprovedDrugs/ucm549978.htm|url-status=dead}}</ref>

A phase III trial, PALOMA-2, was fully enrolled by February 2015 and reported positive results in April 2016.<ref>{{Cite web |url=http://seekingalpha.com/news/3173893-late-stage-study-expanded-use-pfizers-ibrance-successful-global-regulatory-applications |title=Late-stage study of expanded use of Pfizer's Ibrance successful; global regulatory applications to follow. April 2016 |work=Seeking Alpha |date=19 April 2016 |access-date=20 April 2016 |archive-date=25 April 2020 |archive-url=https://web.archive.org/web/20200425005101/https://seekingalpha.com/news/3173893-late-stage-study-expanded-use-pfizers-ibrance-successful-global-regulatory-applications |url-status=live }}</ref> The results of PALOMA-2 trial, published in November 2016, showed significantly longer progression-free survival in patients on palbociclib in combination with letrozole, compared to patients on letrozole and placebo. Progression-free survival was assessed by radiologically-confirmed disease progression by RECIST criteria or death during the study. At the time of publication, there was insufficient data on overall survival, and a final analysis is planned after a total of 390 deaths occur per protocol and in agreement with regulatory agencies. Of note, it was noted that the addition of palbociclib caused higher rates of myelotoxic events in the study.<ref name=Finn2016>{{cite journal | vauthors = Finn RS, Martin M, Rugo HS, Jones S, Im SA, Gelmon K, Harbeck N, Lipatov ON, Walshe JM, Moulder S, Gauthier E, Lu DR, Randolph S, Diéras V, Slamon DJ | title = Palbociclib and Letrozole in Advanced Breast Cancer | journal = The New England Journal of Medicine | volume = 375 | issue = 20 | pages = 1925–1936 | date = November 2016 | pmid = 27959613 | doi = 10.1056/NEJMoa1607303 | doi-access =free }}</ref>

The drug was approved for use in the European Union in November 2016 as a treatment for hormone receptor (HR) positive, human epidermal growth factor receptor 2 (HER2) negative locally advanced or metastatic breast cancer, either in combination with an aromatase inhibitor or, for women who have received prior endocrine therapy, in combination with fulvestrant. In pre- or perimenopausal women, a luteinizing hormone releasing hormone agonist should also be given.<ref>{{Cite web |url=http://www.ema.europa.eu/ema/index.jsp?curl=pages/medicines/human/medicines/003853/human_med_002034.jsp&mid=WC0b01ac058001d124 |title=Ibrance (palbociclib) European public assessment report |access-date=6 May 2017 |archive-date=29 May 2017 |archive-url=https://web.archive.org/web/20170529145304/http://www.ema.europa.eu/ema/index.jsp?curl=pages/medicines/human/medicines/003853/human_med_002034.jsp&mid=WC0b01ac058001d124 |url-status=live }}</ref>

In December 2017, palbociclib, was accepted for use by the NHS after going through the Scottish Medicines Consortium's process for medicines used to treat very rare and end-of-life breast cancer.<ref>{{Cite news|url=https://www.bbc.com/news/uk-scotland-42314944|title=Breast cancer drug approved for NHS use|date=2017-12-11|work=BBC News|access-date=2017-12-11|archive-date=25 April 2020|archive-url=https://web.archive.org/web/20200425004037/https://www.bbc.com/news/uk-scotland-42314944|url-status=live}}</ref>

== Adverse effects == A majority of patients taking palbociclib experience neutropenia, a condition where a patient has an abnormally low number of neutrophils. This side effect impacts the immune system, and is thus likely responsible for the second most common side effect, infection.<ref name="Research">{{Cite web|url=https://www.fda.gov/Drugs/InformationOnDrugs/ApprovedDrugs/ucm549978.htm|title=Approved Drugs - Palbociclib (IBRANCE) | work =Center for Drug Evaluation and Research| publisher = U.S. Food and Drug Administration |access-date=2017-12-16|archive-date=23 April 2019|archive-url=https://web.archive.org/web/20190423133437/https://www.fda.gov/Drugs/InformationOnDrugs/ApprovedDrugs/ucm549978.htm|url-status=dead}}</ref> Leukopenia and anemia are also frequent among patients taking palbociclib.<ref name="Research" /> More than 10% of patients also experience side effects such as fatigue, nausea, diarrhea, respiratory infection, headache, thrombocytopenia, vomiting, and decreased appetite.<ref>{{cite journal | vauthors = Turner NC, Ro J, André F, Loi S, Verma S, Iwata H, Harbeck N, Loibl S, Huang Bartlett C, Zhang K, Giorgetti C, Randolph S, Koehler M, Cristofanilli M | title = Palbociclib in Hormone-Receptor-Positive Advanced Breast Cancer | journal = The New England Journal of Medicine | volume = 373 | issue = 3 | pages = 209–219 | date = July 2015 | pmid = 26030518 | doi = 10.1056/NEJMoa1505270 | url = https://jdc.jefferson.edu/cgi/viewcontent.cgi?article=1177&context=medfp | access-date = 10 December 2019 | url-status = live | archive-url = https://web.archive.org/web/20210923064631/https://jdc.jefferson.edu/cgi/viewcontent.cgi?article=1177&context=medfp | archive-date = 23 September 2021 | doi-access = free }}</ref><ref name="Research" /> The FDA also indicates that patients should be vigilant to monitor themselves for any sign of pulmonary embolism. The FDA further cautions that women should be aware that the medication can have a harmful effect on a fetus, and thus should not be taken while pregnant.<ref name=":0" />

== Mechanisms of resistance ==

=== Resistance to palbociclib === FDA- and EMA-approval for palbociclib hinges upon the clinical trials’ results regarding progression-free survival (PFS). However, though data has proven promising (one study<ref>{{cite journal | vauthors = Turner NC, Slamon DJ, Ro J, Bondarenko I, Im SA, Masuda N, Colleoni M, DeMichele A, Loi S, Verma S, Iwata H, Harbeck N, Loibl S, André F, Puyana Theall K, Huang X, Giorgetti C, Huang Bartlett C, Cristofanilli M | title = Overall Survival with Palbociclib and Fulvestrant in Advanced Breast Cancer | journal = The New England Journal of Medicine | volume = 379 | issue = 20 | pages = 1926–1936 | date = November 2018 | pmid = 30345905 | doi = 10.1056/nejmoa1810527 | doi-access = free }}</ref> had p-values very close to the significance threshold which, with further clinical trials, may be able to demonstrate statistical significance), there is {{as of|2019|lc=yes}} no significant improvement evident in overall survival (OS) when taking palbociclib. For example, in the PALOMA-2 trial, more than 70% of patients who were treated with palbociclib and letrozole progressed by 40 months.<ref name="Overcoming CDK4">{{cite journal | vauthors = Portman N, Alexandrou S, Carson E, Wang S, Lim E, Caldon CE | title = Overcoming CDK4/6 inhibitor resistance in ER-positive breast cancer | journal = Endocrine-Related Cancer | volume = 26 | issue = 1 | pages = R15–R30 | date = January 2019 | pmid = 30389903 | doi = 10.1530/erc-18-0317 | doi-access = free }}</ref> This has impeded the drug's worldwide acceptance as economic analyses have not found palbociclib to be cost-effective.

===''De novo'' resistance === De novo resistance to palbociclib implicates a variety of targets both upstream and downstream of the CDK4/6 to Rb pathway. Overexpression of the transcription factor E2F2 is capable of promoting resistance to CDK4/6 inhibition more than just loss of Rb alone (the direct downstream target of CDK4/6).<ref name="Mechanisms of Resistance to CDK4">{{cite journal | vauthors = Guarducci C, Bonechi M, Boccalini G, Benelli M, Risi E, Di Leo A, Malorni L, Migliaccio I | title = Mechanisms of Resistance to CDK4/6 Inhibitors in Breast Cancer and Potential Biomarkers of Response | journal = Breast Care | volume = 12 | issue = 5 | pages = 304–308 | date = October 2017 | pmid = 29234249 | pmc = 5704709 | doi = 10.1159/000484167 }}</ref> Palbociclib is less effective in patients whose breast cancer cells overexpress cyclin E1 or E2, and Brk (breast tumor-related kinase). Brk is an intracellular kinase that is overexpressed in 60% of breast cancers and whose amplification leads to increased phosphorylation of Y88 of the p27 protein, as well as increases the cyclin D and CDK4 activity, thereby rendering the cancer cells more resistant to palbociclib.<ref name="Mechanisms of Resistance to CDK4"/>

A 2018 study found that mutations in a pathway completely unrelated to the CDK4/6-Rb axis are also implicated in resistance to palbociclib. Loss of the FAT1 tumor suppressor promoted resistance to CDK4/6 inhibitors through the Hippo pathway, a signaling pathway known to function as a tumor suppressor.<ref name="Loss of the FAT1 Tumor Suppressor P">{{cite journal | vauthors = Li Z, Razavi P, Li Q, Toy W, Liu B, Ping C, Hsieh W, Sanchez-Vega F, Brown DN, Da Cruz Paula AF, Morris L, Selenica P, Eichenberger E, Shen R, Schultz N, Rosen N, Scaltriti M, Brogi E, Baselga J, Reis-Filho JS, Chandarlapaty S | title = Loss of the FAT1 Tumor Suppressor Promotes Resistance to CDK4/6 Inhibitors via the Hippo Pathway | journal = Cancer Cell | volume = 34 | issue = 6 | pages = 893–905.e8 | date = December 2018 | pmid = 30537512 | pmc = 6294301 | doi = 10.1016/j.ccell.2018.11.006 }}</ref> Previously, FAT1 had been suggested to be a tumor suppressor and oncogene, though never before explicitly linked to resistance. The team found that FAT1 suppressed cells required significantly higher doses of CDK4/6 inhibitors in order to block proliferation of breast cancer cells ''in vivo''.<ref name="Loss of the FAT1 Tumor Suppressor P"/>

=== Resistance due to endocrine therapy === Palbociclib is indicated in HR+, HER2- metastatic breast cancer cells. Because CDK4/6 inhibition acts directly downstream of the endocrine therapy targets, cross-therapy resistance could possibly develop as a result of patient progression on hormone-therapy. In general, however, endocrine-resistant tumors are able to maintain sensitivity to CDK4/6 inhibition, with particular success witnessed with patients given combination therapy of palbociclib with endocrine therapy.<ref name="Overcoming CDK4"/> For example, the PALOMA-3 trial which studied the benefit of palbociclib and fulvestrant vs. fulvestrant alone found that the combined treatment regimen improved PFS in both patients with an ''ESR1'' mutation and those with ''ESR1'' wildtype ctDNA. Such results indicate that CDK4/6 inhibitors are effective irrespective of ''ESR1'' mutation status, a marker that otherwise would reveal a patient to be endocrine-resistant.<ref>{{cite journal | vauthors = Turner NC, Ro J, André F, Loi S, Verma S, Iwata H, Harbeck N, Loibl S, Huang Bartlett C, Zhang K, Giorgetti C, Randolph S, Koehler M, Cristofanilli M | title = Palbociclib in Hormone-Receptor-Positive Advanced Breast Cancer | journal = The New England Journal of Medicine | volume = 373 | issue = 3 | pages = 209–219 | date = July 2015 | pmid = 26030518 | doi = 10.1056/nejmoa1505270 | url = https://jdc.jefferson.edu/cgi/viewcontent.cgi?article=1177&context=medfp | access-date = 10 December 2019 | url-status = live | archive-date = 23 September 2021 | archive-url = https://web.archive.org/web/20210923064631/https://jdc.jefferson.edu/cgi/viewcontent.cgi?article=1177&context=medfp | author-link4 = Sherene Loi | doi-access = free }}</ref>

Certain manifestations of endocrine therapy resistance may further sensitize the breast cancer to CDK4/6 inhibitors. For example, deficiencies in mismatch repair caused by the MutL mutation in ER+ breast cancer evades CHK2-mediated inhibition of CDK4, thereby leading to endocrine resistance.<ref name="Overcoming CDK4"/> CDK4/6 inhibitors, however, have been demonstrated to be highly effective in MutL-defective ER+ breast cancer cells, and MutL bears potential as a biomarker for identifying patients highly suitable for treatment with CDK4/6 inhibitors.

=== Other mechanisms of resistance === Researchers at the Dana Farber Institute found that breast cancer cells that developed resistance to palbociclib were resensitized to the drug after a seven-day "treatment holiday".<ref>{{cite journal | vauthors = Cornell L, Wander SA, Visal T, Wagle N, Shapiro GI | title = MicroRNA-Mediated Suppression of the TGF-β Pathway Confers Transmissible and Reversible CDK4/6 Inhibitor Resistance | journal = Cell Reports | volume = 26 | issue = 10 | pages = 2667–2680.e7 | date = March 2019 | pmid = 30840889 | pmc = 6449498 | doi = 10.1016/j.celrep.2019.02.023 }}</ref> Scientists linked the initial development of resistance to an increase in expression of CDK6 (but not CDK4, the other target of palbociclib), with the specific mechanism of CDK6 upregulation originating from suppression of the TGF-β pathway via the miR-432-5p microRNA. Scientists made this discovery after noticing that, in the lab, all breast cancer cells in the dish were found to acquire resistance at a similar time, a phenomenon contrary to classical models of acquiring resistance in which one or two cells become resistant and then expand to encompass more of the tumor as they divide. These palbociclib-resistance cells also did not contain a particular mutation, but rather became resistant and continued to spread this resistance to neighboring cells via exosomes. While the mechanism of this resistance has not yet been elucidated, it opens an avenue for further research into a completely novel method through which cancer cells acquire resistance. On the other hand, this suggests that patients which seemingly progress on palbociclib could benefit from longer-term treatment following a treatment holiday in which their tumors are "reset".

There are a variety of means through which cancer cells become resistant to palbociclib, with mechanisms of resistance involving multiple targets and processes throughout the pathway in which palbociclib operates. Previous research estimates that breast cancer cells show adaptation against palbociclib as early as 72 hours post-treatment.<ref name="Mechanisms of Resistance to CDK4"/> Evidence for this was "shown to be mediated by non-canonical activation of cyclin D1/CDK2 complexes, which in turn induced a recovery of cyclin E2 expression and [subsequent] S-phase entry despite CDK4/6 inhibition" upstream of these two targets.<ref name="Mechanisms of Resistance to CDK4"/> Furthermore, it is estimated that approximately 10% of patients have primary resistance to CDK4/6 inhibitors (resistance prior to drug therapy).<ref name="Inhibitors 2019">{{cite journal | vauthors = McCartney A, Migliaccio I, Bonechi M, Biagioni C, Romagnoli D, De Luca F, Galardi F, Risi E, De Santo I, Benelli M, Malorni L, Di Leo A | title = Mechanisms of Resistance to CDK4/6 Inhibitors: Potential Implications and Biomarkers for Clinical Practice | journal = Frontiers in Oncology | volume = 9 | article-number = 666 | date = 23 July 2019 | pmid = 31396487 | pmc = 6664013 | doi = 10.3389/fonc.2019.00666 | doi-access = free }}</ref> For example, patients who display evidence of functional Rb loss at baseline are not likely to benefit from CDK4/6 inhibition, nor are patients who display baseline evidence of increased cyclin E1 expression, or a high CCNE1/RB ratio.<ref name="Inhibitors 2019"/>

==Clinical trials== ===HR+ breast cancer=== The PALOMA-3 trial announced in April 2015 that the addition of palbociclib was superior to fulvestrant alone for progression-free survival.<ref>{{Cite news|url = http://www.pfizer.com/news/press-release/press-release-detail/pfizer_announces_paloma_3_trial_for_ibrance_palbociclib_stopped_early_due_to_efficacy_seen_in_patients_with_hr_her2_metastatic_breast_cancer_whose_disease_has_progressed_following_endocrine_therapy|title = Pfizer Announces PALOMA-3 Trial For IBRANCE (Palbociclib) Stopped Early Due To Efficacy Seen In Patients With HR+, HER2- Metastatic Breast Cancer Whose Disease Has Progressed Following Endocrine Therapy|date = April 15, 2015|access-date = 3 May 2015|archive-date = 25 April 2020|archive-url = https://web.archive.org/web/20200425004034/https://www.pfizer.com/news/press-release/press-release-detail/pfizer_announces_paloma_3_trial_for_ibrance_palbociclib_stopped_early_due_to_efficacy_seen_in_patients_with_hr_her2_metastatic_breast_cancer_whose_disease_has_progressed_following_endocrine_therapy|url-status = live}}</ref>

In the phase II PALOMA-1 trial reported at the April 2014 annual meeting of the American Association for Cancer Research, the addition of palbociclib to letrozole was shown to significantly slow the progression of advanced cancer (median progression-free survival increased from 10.2 months to 20.2 months), but was not shown to have a statistically significant effect on increasing patients' overall survival times.<ref>{{cite news | url = https://www.nytimes.com/2014/04/07/business/breast-cancer-drug-shows-groundbreaking-results.html | title = Breast Cancer Drug Shows 'Groundbreaking' Results | archive-url = https://web.archive.org/web/20210609112355/https://www.nytimes.com/2014/04/07/business/breast-cancer-drug-shows-groundbreaking-results.html | archive-date=9 June 2021 | vauthors = Pollack A | work = The New York Times | date = 6 April 2014 }}</ref><ref>{{cite news| vauthors = Beasley D |date=6 April 2014|title=Pfizer drug doubles time to breast cancer tumor growth in trial|url=https://news.yahoo.com/pfizer-drug-doubles-time-breast-cancer-tumor-growth-143050725--finance.html|publisher=Yahoo! News|agency=Reuters|access-date=7 April 2014|archive-date=6 April 2014|archive-url=https://web.archive.org/web/20140406205804/http://news.yahoo.com/pfizer-drug-doubles-time-breast-cancer-tumor-growth-143050725--finance.html|url-status=live}}</ref><ref>{{cite web | url = http://www.aacr.org/home/public--media/aacr-in-the-news.aspx?d=3335 | title = Palbociclib Shows Promising Results in Patients With Hormone Receptor-positive Metastatic Breast Cancer | archive-url = https://web.archive.org/web/20140408215525/http://www.aacr.org/home/public--media/aacr-in-the-news.aspx?d=3335 | archive-date=2014-04-08 | work = AACR in the News | date = 6 April 2014 }}</ref>

Pfizer announce on May 31, 2020, the results of a preplanned efficacy and futility analysis. The results indicate that, when used in combination with post-surgery endocrine therapy, it "is unlikely to show a statistically significant improvement in the primary endpoint of invasive disease-free survival (iDFS)."<ref>{{Cite press release|url=https://www.businesswire.com/news/home/20200529005610/en/Pfizer-Update-Phase-3-PALLAS-Trial-IBRANCE%C2%AE|title=Pfizer Provides Update on Phase 3 PALLAS Trial of IBRANCE® (Palbociclib) Plus Endocrine Therapy in HR+, HER2- Early Breast Cancer|date=29 May 2020|access-date=2 June 2020|archive-date=8 August 2020|archive-url=https://web.archive.org/web/20200808155237/https://www.businesswire.com/news/home/20200529005610/en/Pfizer-Update-Phase-3-PALLAS-Trial-IBRANCE%C2%AE|url-status=live}}</ref> The trial was conducted on male and female patients with HR-positive, HER2-negative early stage breast cancer.

=== Active clinical trials === According to the NIH National Cancer Institute there are currently 39 active clinical trials testing palbociclib on its own or in combination with other medications. While a majority of these are exploring the further uses of palbociclib to treat breast cancer, other trials are investigating the potential applications of palbociclib to head and neck cancers, non-small cell lung cancer, recurring brain metastasis, squamous cell carcinoma, central nervous system tumors, and other solid tumor types.<ref>{{Cite web|url=https://www.cancer.gov/about-cancer/treatment/clinical-trials/intervention/C49176|title=Intervention Dynamic Trial Listing Page|website=National Cancer Institute|access-date=2017-12-16|archive-date=23 January 2018|archive-url=https://web.archive.org/web/20180123195230/https://www.cancer.gov/about-cancer/treatment/clinical-trials/intervention/C49176|url-status=live}}</ref>

In December 2017, Pfizer announced that the PALOMA-2 trial, an ongoing phase III trial combining palbociclib with letrozole, showed a 44% reduction in risk of disease progression among trial subjects. The trial has also demonstrated greater than a year's improved median progression free survival for patients on the combined therapy (as compared to letrozole on its own). PALOMA-2 median patient follow-up time now exceeds three years, making it the longest traceable data for phase III study of a CDK4/6 inhibitor.<ref name="pfizer.com"/>

Palbociclib is currently in Phase 2 clinical trials as a combination treatment with oral SERD’s for ER+/HER2- breast cancer. Studies include palbociclib plus G1 Therapeutic’s rintodestrant<ref>{{ClinicalTrialsGov|NCT03455270|A Phase 1, Open-Label, Multicenter Study to Assess the Safety, Tolerability, Pharmacokinetics, and Preliminary Antitumor Activity of Ascending Doses of G1T48 Alone and in Combination With Palbociclib in Women With Estrogen Receptor-Positive, HER2-Negative Advanced Breast Cancer}}</ref> and Zentalis’s Zn-c5.<ref>{{ClinicalTrialsGov|NCT03560531|A Phase 1/2 Open Label, Multicenter Study to Assess the Safety, Tolerability, Pharmacokinetics, and Anti-Tumor Activity of ZN-c5 Alone and in Combination With Palbociclib in Subjects With Estrogen-Receptor Positive, Human Epidermal Growth Factor Receptor-2 Negative Advanced Breast Cancer}}</ref>

== Drugs with a similar mechanism of action == Palbociclib has several direct competitors currently on the market or in clinical trials. In September 2017, abemaciclib, another selective CDK4/6 inhibitor owned and manufactured by Eli Lilly, was approved for HR-positive, HER2-negative advanced metastatic breast cancer both in combination with fulvestrant and as a monotherapy. In March 2017, the FDA also approved ribociclib, owned by Novartis, as a combination therapy with aromatase inhibitors for indications similar to those of palbociclib and ribociclib. Notably, ribociclib seems to also have an inhibitory effect on Cyclin D3/CDK6 activity.<ref>{{Cite web|url=https://www.cancer.gov/publications/dictionaries/cancer-drug|title=NCI Drug Dictionary|website=National Cancer Institute|access-date=2017-12-16|date=2011-02-02|archive-date=10 August 2019|archive-url=https://web.archive.org/web/20190810012312/https://www.cancer.gov/publications/dictionaries/cancer-drug|url-status=live}}</ref> G1 Therapeutics<ref>{{Cite web|url=http://www.g1therapeutics.com/|title=G1 Therapeutics, Inc. | Optimizing Chemotherapy, Advancing Survival|access-date=16 December 2017|archive-date=4 August 2021|archive-url=https://web.archive.org/web/20210804210230/https://www.g1therapeutics.com/|url-status=live}}</ref> also has a CDK4/6 inhibitor, trilaciclib, which is up for FDA approval (target decision date of February 15, 2021) for use in small cell lung cancer.<ref>{{Cite web |title=FDA Grants Priority Review of Trilaciclib for Treating Small Cell Lung Cancer|url=https://www.uspharmacist.com/article/fda-grants-priority-review-of-trilaciclib-for-treating-small-cell-lung-cancer|access-date=2021-01-02|website=www.uspharmacist.com|language=en|archive-date=20 January 2021|archive-url=https://web.archive.org/web/20210120222659/https://uspharmacist.com/article/fda-grants-priority-review-of-trilaciclib-for-treating-small-cell-lung-cancer|url-status=live}}</ref>

== Society and culture == === Economics === Palbociclib (Ibrance) "can be ordered through select" specialty pharmacies and "sells for $9,850 for 30 days or $118,200 for a year's supply before discounts."<ref name=standard>{{cite news | url=https://www.business-standard.com/article/pti-stories/pfizer-breast-cancer-drug-gets-early-fda-approval-115020400062_1.html | title=Pfizer breast cancer drug gets early FDA approval | work=Business Standard | agency=Associated Press | date=3 February 2015 | access-date=10 July 2020 | archive-date=25 October 2015 | archive-url=https://web.archive.org/web/20151025235019/http://www.business-standard.com/article/pti-stories/pfizer-breast-cancer-drug-gets-early-fda-approval-115020400062_1.html | url-status=live }}</ref> According to a statement by the New York–based Pfizer the price "is not the cost that most patients or payors pay" since most prescriptions are dispensed through health plans, which negotiate discounts for medicines or get government-mandated price concessions.<ref name=standard /> In the United States specialty pharmacies fill prescriptions for drugs that are usually high cost.<ref name="Forbes_2010">{{citation |title=The World's Most Expensive Drugs |url=https://www.forbes.com/2010/02/19/expensive-drugs-cost-business-healthcare-rare-diseases.html |date=19 February 2010 |access-date=25 June 2015 | vauthors = Herper M |work=Forbes |archive-date=28 April 2021 |archive-url=https://web.archive.org/web/20210428095442/https://www.forbes.com/2010/02/19/expensive-drugs-cost-business-healthcare-rare-diseases.html |url-status=live }}</ref><ref name="nytimes.com_2015_07_16">{{cite web | url=https://www.nytimes.com/2015/07/16/business/specialty-pharmacies-proliferate-along-with-questions.html | title=Specialty Pharmacies Proliferate, Along With Questions | work=New York Times | date=15 July 2015 | access-date=5 October 2015 | vauthors = Thomas K, Pollack A | location=Sinking Spring, Pa. | archive-date=25 February 2019 | archive-url=https://web.archive.org/web/20190225083506/https://www.nytimes.com/2015/07/16/business/specialty-pharmacies-proliferate-along-with-questions.html | url-status=live }}</ref>

Pfizer reported revenue of {{US$|4.753 billion}} for Ibrance in 2023.<ref>{{cite web | title=Pfizer's year in review | website=Pfizer 2023 Annual Report | date=31 December 2023 | url=https://www.pfizer.com/sites/default/files/investors/financial_reports/annual_reports/2023/ | access-date=30 December 2024}}</ref>

==== Resistance to cost ==== In February 2017, the National Institute for Health and Care Excellence (NICE) in the United Kingdom published a statement stating that the cost of Ibrance (approximately US$3700/28 days) did not make the added health benefits worth the cost.<ref>{{Cite news|url=https://www.reuters.com/article/us-pfizer-britain-cancer/uk-agency-says-pfizer-breast-cancer-drug-too-expensive-idUSKBN15I00B|title=UK agency says Pfizer breast cancer drug too expensive|date=3 February 2017|work=Reuters|access-date=2017-12-16|archive-date=12 April 2019|archive-url=https://web.archive.org/web/20190412004526/https://www.reuters.com/article/us-pfizer-britain-cancer/uk-agency-says-pfizer-breast-cancer-drug-too-expensive-idUSKBN15I00B|url-status=live}}</ref> It was stated that with the side effects caused by Ibrance, it was not a cost-effective medication for NICE to endorse.<ref name=":2">{{Cite news|url=https://www.nice.org.uk/news/article/breast-cancer-drug-costs-too-high-in-relation-to-benefits-for-routine-nhs-funding|title=Breast cancer drug costs too high in relation to benefits for routine NHS funding|work=NICE|access-date=2017-12-16|archive-date=25 April 2020|archive-url=https://web.archive.org/web/20200425005102/https://www.nice.org.uk/news/article/breast-cancer-drug-costs-too-high-in-relation-to-benefits-for-routine-nhs-funding|url-status=dead}}</ref> At the time, a year's treatment with palbociclib and a drug such as fulvestrant was priced at US$106,105 (£79,650).<ref name=":2" /> In November 2017 NICE announced that, after negotiation with Pfizer, the price would be discounted, and the drug would be recommended for use.<ref>{{Cite news|url=https://www.nice.org.uk/news/article/breast-cancer-patients-to-have-routine-access-to-two-life-extending-drugs-after-new-deal-say-nice-in-draft-guidance|title=Breast cancer patients to have routine access to two life extending drugs after new deal, say NICE in draft guidance|work=NICE|access-date=2017-12-16|archive-date=25 April 2020|archive-url=https://web.archive.org/web/20200425004033/https://www.nice.org.uk/news/article/breast-cancer-patients-to-have-routine-access-to-two-life-extending-drugs-after-new-deal-say-nice-in-draft-guidance|url-status=dead}}</ref>

== References == {{reflist}}

Category:1-Piperazinyl compounds Category:Disubstituted pyridines Category:Pyridopyrimidines Category:Protein kinase inhibitors Category:Specialty drugs Category:Drugs developed by Pfizer Category:Cyclopentyl compounds Category:Lactams Category:CDK inhibitors