BACKGROUND: Cyclin-dependent kinase 4/6 (CDK4/6) inhibitors marked a milestone in the breast cancer treatment. Due to the potential impact of adverse effects on treatment decisions and patient outcomes, careful consideration of the varying toxicities of CDK4/6 inhibitors is crucial, as three inhibitors-palbociclib, abemaciclib, and ribociclib-have been approved with differences in adverse event profiles. However, limitations in clinical trials call for urgent real-world safety studies to evaluate and compare the risk of adverse events (AEs) among these CDK4/6 inhibitors. Therefore, this study aimed to analyze AEs of CDK4/6 inhibitors and provide insights for clinical drug selection, using real world database.
METHODS: The AEs of CDK4/6 inhibitors in the FDA Adverse Event Reporting System (2015-2022) were analyzed. Four disproportionality methods were used to detect safety signals: reporting odds ratio (ROR), proportional reporting ratio, Bayesian Confidence Neural Network Propagation, and Multi-Item Gamma Poisson Shrinker. Venn analysis was used to compare and select common and specific AEs.
RESULTS: This study included 73,042 patients treated with palbociclib, 25,142 with ribociclib, and 7563 with abemaciclib. All three inhibitors had 27 common AEs. Palbociclib exhibited the highest ROR for hematologic toxicities, while ribociclib showed the highest ROR for macrocytosis, nail disorders, and hepatic lesions. Abemaciclib displayed the highest ROR for mucosal toxicity. Common signals for both palbociclib and ribociclib included hematologic toxicities, decreased immune responsiveness, and aphthous ulcers. Myelosuppression, oral pain, and pseudocirrhosis were common signals for palbociclib and abemaciclib. Anemia, hepatotoxicity, and pneumonitis were observed as common signals for ribociclib and abemaciclib. Furthermore, specific AEs associated with palbociclib included fatigue, alopecia, and stomatitis. For ribociclib, specific AEs included electrocardiogram QT prolongation, thrombocytopenia, and decreased hemoglobin. Abemaciclib was specifically linked to diarrhea, vomiting, and interstitial lung disease.
CONCLUSIONS: Our analysis revealed that palbociclib showed a higher risk of hematologic toxicity. Ribociclib showed higher risks of hepatotoxicity, nephrotoxicity, and QT prolongation. Abemaciclib showed higher risks of hepatotoxicity, gastrointestinal effects, interstitial lung disease, and thrombosis. These findings provide valuable insights for CDK4/6 inhibitor selection.

BACKGROUND: Current standard treatment for metastatic breast cancer (MBC) involves cyclin-dependent kinase 4/6 (CDK4/6) inhibitors with endocrine therapy, showing potential in enhancing anti-tumor immune responses.
METHODS: This report details a clinical case of MBC where palbociclib was co-administered with letrozole. The integration of allogeneic tumor vaccination to this treatment led to heightened interferon-γ production, expansion of CD8+ and NK cell populations, and positive delayed-type hypersensitivity reactions, indicating successful development of anti-tumor immunity. The induced production of interferon-γ by tumor vaccination was associated with manageable modulation of sensitivity to palbociclib-letrozole therapy. Administration of the BioNTech/Pfizer Covid-19 vaccine compromised the anti-tumor immune response by reducing cytotoxic cell populations and increasing immunosuppressive cytokine production. The patient undergoing combined treatment achieved a progressive-free survival of 42 months.
CONCLUSIONS: Incorporating active tumor vaccination with CDK4/6 inhibitor therapy presents a feasible approach for metastatic breast cancer. The precise regulation of the microenvironment emerges as a crucial factor and warrants careful consideration.

Spiradenocarcinomas are rare malignant skin adnexal tumors. We describe a novel case of a patient with an aggressive CDKN2A-mutated spiradenocarcinoma who responded to a CDK4/6 inhibitor. This case highlights the unique nature of spiradenocarcinomas as well as the potential benefit of targeted therapy.

There is an ongoing clinical dilemma of how best to treat patients who present themselves with visceral crisis. The time needed to undo the state of visceral crisis is the most relevant outcome for this patient group. We describe four patients treated with CDK4/6 inhibitor plus endocrine therapy for HR+/HER2- metastatic breast cancer who presented themselves with a visceral crisis. Two of them are male and three of them had synchronous metastatic breast cancer. Two patients had lymphangitis carcinomatosis of the lungs, one extensive disease of the eye and one of the liver. Time to first clinical response was observed within a few weeks in three patients. For one patient a switch to chemotherapy was needed. These cases show that treatment with CDK4/6 inhibitors can achieve a rapid response in patients experiencing visceral crisis. We conclude that chemotherapy is not the sole possibility in visceral crisis, and that CDK4/6 inhibitors can be considered as well.

Abemaciclib, a cyclin-dependent kinase 4/6 (CDK4/6) inhibitor, causes severe hepatotoxicity, a severe adverse event associated with the loss of treatment opportunities. We report a case of liver injury (grade 4) during treatment with abemaciclib, in which the patient was switched to palbociclib and successfully treated with this CDK4/6 inhibitor. A 73-year-old woman with bone metastatic breast cancer (hormone-positive, HER2-negative) was treated with abemaciclib, fulvestrant, denosumab, and precipitated calcium carbonate with cholecalciferol and magnesium carbonate (pCCCM). On day 17, the patient developed skin rashes on her trunk and arms. On day 22, abemaciclib and pCCCM were discontinued due to drug eruption. Grade 3 aspartate aminotransferase (AST) and grade 4 alanine aminotransferase (ALT) levels increased on day 29. Therefore, fulvestrant and denosumab were suspended as the causes of severe hepatotoxicity, in addition to the two drugs suspected of causing the skin eruption. On day 43, AST and ALT levels did not improve, and the patient was referred to a hepatologist. The hepatologist diagnosed hepatotoxicity as a drug-induced liver injury through additional tests and interviews. Fulvestrant treatment was resumed on day 78, and palbociclib on day 92, and denosumab and pCCCM on day 134. On day 287, treatment with the CDK4/6 inhibitor was continued without evidence of liver dysfunction. This case suggests that rechallenge with palbociclib after severe liver injury with abemaciclib may allow for continued treatment with CDK4/6 inhibitors.

In hormone receptor-positive metastatic breast cancer (HR+ MBC), endocrine resistance is commonly due to genetic alterations of ESR1, the gene encoding estrogen receptor alpha (ERα). While ESR1 point mutations (ESR1-MUT) cause acquired resistance to aromatase inhibition (AI) through constitutive activation, far less is known about the molecular functions and clinical consequences of ESR1 fusions (ESR1-FUS). This case series discusses 4 patients with HR+ MBC with ESR1-FUS in the context of the existing ESR1-FUS literature. We consider therapeutic strategies and raise the hypothesis that CDK4/6 inhibition (CDK4/6i) may be effective against ESR1-FUS with functional ligand-binding domain swaps. These cases highlight the importance of screening for ESR1-FUS in patients with HR+ MBC while continuing investigation of precision treatments for these genomic rearrangements.

BACKGROUND: Cyclin-dependent kinase 4/6 (CDK4/6) inhibitors are the standard treatment for advanced hormone receptor-positive breast cancer. Although interstitial lung disease is a rare (1-3.3%) but serious adverse event associated with CDK4/6 inhibitors, the incidence of interstitial lung disease in Japanese patients in the real world and the risk factors of interstitial lung disease are not clear.
METHODS: We retrospectively investigated the incidence of interstitial lung disease in 224 patients with advanced breast cancer who received CDK4/6 inhibitors at our hospital between 31 January 2017 and 31 January 2021. The correlation of age (>50 vs ≤50 years), presence or absence of previous history of interstitial lung disease, lung metastasis, smoking history and chest radiation with the development of interstitial lung disease was evaluated.
RESULTS: In total, 177 cases received palbociclib, 39 cases received abemaciclib and 8 cases received both palbociclib and abemaciclib, constituting a palbociclib group (n = 185) and an abemaciclib group (n = 47). At a median observation period of 607 days, 8.0% (18/224) cases (13 definite and 5 probable cases) had interstitial lung disease; 6.5% (12/185) of palbociclib-treated and 13% (6/47) of abemaciclib-treated cases. The median time to interstitial lung disease onset was 178 (range, 14-750) days. There was no significant correlation between the background factors studied and the development of interstitial lung disease.
CONCLUSIONS: The frequency of CDK4/6 inhibitor-induced interstitial lung disease was higher than that reported in clinical trials. We did not identify any risk factors for the development of interstitial lung disease in this study, and thus, larger studies that include patient predisposition are required.

A 72-year-old woman with metastatic ER/PR-positive breast cancer who had been on ribociclib and letrozole for 1 year developed severe life-threatening colitis. She presented to emergency department with features of acute abdomen and diarrhoea. The diagnosis of colitis was confirmed radiologically as well as by histopathological examination of the biopsy specimen and the patient clinically improved after withholding ribociclib and receiving corticosteroids compatible with ribociclib-induced colitis. The mechanism of injury in CDK 4/6 inhibitor-induced colitis is unknown but may be related to recruitment of inflammatory cells. Whether the development of colitis is associated with tumour response is an interesting and unanswered question.

A 58‒year‒post‒menoposal woman was presented with left chest pain and shortness of breath because her breast cancer metastasized to the skin, lung, and pleural dissemination. In late‒line treatment for hormone receptor‒positive HER2‒negative advanced/recurrent breast cancer, we experienced a patient with tumor shrinkage leading to pain relief who was treated with a second combination of a CDK4/6 inhibitor and fulvestrant. Due to her poor performance status, she was treated with combined therapy to avoid severe adverse events. The CDK4/6 inhibitor was reintroduced after 1.5 years withdrawal period of endocrine therapy during anticancer drugs and radiation treatment. It has also been reported that withdrawal of CDK4/6 inhibitors might restore susceptibility related to the inhibitory signal. Rather than sequentially administering combined endocrine therapy with a CDK4/6 inhibitor, the withdrawal strategy of endocrine therapy continuing to administer anticancer drugs should be considered in case of reintroduction of CDK4/6 inhibitor.

Rheumatoid arthritis is a common autoimmune disease that requires new therapeutic agents. Cyclin-dependent kinase 4/6 inhibitors have recently been approved for metastatic breast cancer patients and have also been reported to improve the arthritis score in collagen-induced arthritis mouse models. We report a 56-year-old woman who had previously been diagnosed with rheumatoid arthritis and treated with methotrexate. At age 40, she underwent surgery with curative intent for breast cancer but subsequently developed lung metastases. Palbociclib, a cyclin-dependent kinase 4/6 inhibitor, was administered in combination with fulvestrant (anti-oestrogen drug) for metastatic breast cancer. One month later, serum matrix metalloproteinase-3 and C-reactive protein levels were markedly decreased, and her rheumatoid arthritis symptoms, which had worsened just prior to the detection of metastatic lung disease, showed amelioration. Methotrexate, which had been used to treat her rheumatoid arthritis, could subsequently be administered in a reduced dose. The cyclin-dependent kinase 4/6 inhibitor was also effective for the metastatic breast cancer, and, to date, the patient\'s disease has remained stable for more than one year. Based on the results of basic research, cyclin-dependent kinase 4/6 inhibitors are promising new therapeutic agents for rheumatoid arthritis patients, although these drugs have not, as yet, been used in a clinical setting. To the best of our knowledge, this is the first report to describe a patient whose rheumatoid arthritis responded to a cyclin-dependent kinase 4/6 inhibitor administered for metastatic breast cancer.