Infectious complications of cyclin-dependent kinases 4 and 6 inhibitors in patients with hormone-receptor-positive metastatic breast cancer: a systematic review and meta-analysis

The combination of cyclin-dependent kinase 4 and 6 (CDK 4/6) inhibitors plus endocrine therapy (ET) improved the survival outcomes and became the standard of care in the treatment of metastatic hormone-positive breast cancer. However, these combinations increased the risk of neutropenia compared with ET alone. While the infection-related mortalities did not seem to be increased, the exact risk of infections with CDK 4/6 inhibitor and ET combinations is relatively understudied. Therefore, we performed a meta-analysis of CDK 4/6 inhibitor clinical trials to assess the infection risk of adding CDK4/6 inhibitors to ET. We systemically searched the PubMed database for relevant clinical trials. For each study, all grade and grade 3 or higher infections, upper respiratory tract infections (URTI), urinary tract infections (UTI), pneumonia, and febrile neutropenia rates were recorded whenever available. The hazard ratios (HR) with a 95% confidence interval (CI) of infection risk were calculated via the generic inverse-variance method with a random-effects model. Nine eligible studies were included in the analyses (MONALEESA-2,3,7, MONARCH-2,3, MONARCH plus, PALOMA-1,2,3). In the meta-analysis of these studies, CDK 4/6 inhibitors plus ET arms were associated with increased all grade infections (HR 1.77, 95% CI 1.56–2.01, p < 0.00001), grade 3 or higher infections, (HR 1.77, 95% CI 1.28–2.43, p = 0.0005), UTIs (HR 1.59, 95% CI 1.19–2.12, p = 0.002), and febrile neutropenia (HR 4.28, 95% CI 1.73–10.62, p = 0.002). In this meta-analysis, we observed that adding CDK4/6 inhibitors to ET significantly increased the risk of all grade, grade 3 or higher infections, and urinary tract infections. We propose that closer follow-up for infections should be considered for metastatic breast cancer patients using CDK 4/6 inhibitors. This may help clinicians to recognize infections earlier which prevents early death from infection.


Introduction
Breast cancer is the most commonly diagnosed cancer in women and a leading cause of cancer mortality. Approximately 2.2 million people (11.7%) were diagnosed with breast cancer in 2020 [1]. Hormone-receptor (HR) positive breast cancer constitutes the largest subgroup, with approximately 2/3 of all breast cancer.
HR-positive breast cancer is an estrogen-dependent disease. Therefore, the treatment of HR-positive advanced breast cancer has been aimed at effectively blocking the estrogen-receptor signaling pathway or decreasing estrogen levels for many years. Accordingly, ET is the first-line treatment for HR-positive and human epidermal growth factor receptor 2 (HER2) negative advanced breast cancer other than for patients presenting with the visceral crisis. However, resistance to hormonal blockade is inevitable over time, and managing endocrine resistance is the most crucial aspect of ET. New treatment approaches are needed to combat this endocrine resistance.
Endocrine resistance in breast cancer is linked with alterations in the cyclin-D-CDK 4/6-Rb pathway causing the loss of regulation of this critical Rb checkpoint that results in the activation of growth factor pathways to bypass endocrine resistance. Considering the instrumental roles of CDK 4/6 in the cell cycle, the inhibition of these cell cycle control points came forward as a therapeutic option in the treatment of HR-positive advanced breast cancer. CDK4/6 inhibitors (ribociclib, palbociclib, and abemaciclib) are oral selective inhibitors of CDK4 and CDK6 that inhibits DNA synthesis by blocking the cell cycle in the G1 to S phase [2]. The addition of CDK4/6 inhibitors to standard ET has improved progression-free survival (PFS) and overall survival of (OS) by helping overcome endocrine resistance.
The most frequent adverse events of CDK 4/6 inhibitors are neutropenia, nausea, diarrhea, and fatigue. The neutropenia is especially frequent and reported in more than half of the patients in the pivotal (MONARCH-3, PALOMA-3, and MONALEESA-3) clinical trials [3][4][5]. However, febrile neutropenia rates are surprisingly low in these trials (one patient, three patients, and five patients, respectively), possibly due to reversibility of neutrophil maturation arrest with these agents rather than a true myelotoxicity [6].

Data sources
We conducted a systematic meta-analysis following the Preferred Reporting Items for Systematic Reviews and Metaanalysis guidance (PRISMA). We have searched PubMed (articles published between January 1, 2015, and March 31, 2021) for relevant clinical trials evaluating the addition of CDK4/6 inhibitors to endocrine therapy in HR-positive HER-2 negative metastatic breast cancers. Search terms included "Abemaciclib'' OR "Ribociclib" OR "Palbociclib" AND "Adverse events'' OR "Infections'' as well as combinations of these terms.

Study selection and data extraction
Randomized controlled trials testing the addition of CDK 4/6 inhibitors to ET in HR-positive HER-2 negative metastatic breast cancers are included. When more than one report of the same trial was available, the most recent information was considered in the analysis. Irrelevant trials (n = 863), trials including the chemotherapy arm (MonarcHER trial [18]), trials conducted in the neoadjuvant settings (PALLET trial [19]), and trials without a placebo arm (TREnd trial [20]) were excluded. Duplicated articles, chapters of books, case reports, editorial letters, review articles, opinion papers, animal studies, and studies including patients without cancer and studies without data for HRs and confidence intervals (CIs) were also excluded. The flow diagram of the study selection process is shown in Fig. 1. Only studies written in English were included and analyzed. Two reviewers (O.B. and E.E) independently extracted data from the studies. Any disagreement was resolved by discussion with the senior author. The number of patients, all grade and grade 3 or higher infection, URTIs, UTIs, pneumonia, and febrile neutropenia rates were extracted from each study.

Statistics
The individual study quality and risk of bias were evaluated with the Cochrane risk-of-bias tool [21] for randomized trials version 2 by OB and EE. The meta-analysis was performed using the generic inverse-variance method with a random-effects model. The principal summary measure used was the hazard ratios with a 95% two-sided confidence interval. All analyses were done using the Review Manager software, version 5.3 (The Nordic Cochrane Center, The Cochrane Collaboration, Copenhagen, Denmark). The heterogeneity within each subgroup is reported using the I-square statistics. The p values below 0.05 were considered statistically significant.
A total of 6 studies reported separate data for the URTIs. Three thousand nine hundred and seventy-five patients were enrolled in the studies, with 1544 (38.8%) being in the placebo plus ET arm and 2431 (61.2%) in the CDK 4/6 inhibitor plus ET arm. Although there was a trend toward increased URTI rate in CDK4/6 inhibitors plus ET arm, the magnitude of risk increase was lower and did not reach statistical significance (HR 1.22, 95% CI 0.99-1.49, p = 0.06) (Fig. 3a). A moderate degree of heterogeneity was noted in the studies (I 2 : 40%).
A total of 4 studies reported separate data for UTIs. Two thousand six hundred and sixty-eight patients were enrolled in the studies, with 1004 (37.6%) being in the placebo plus ET arm and 1664 (62.4%) in the CDK 4/6 inhibitors plus ET arm. UTIs are increased in CDK4/6 inhibitors plus ET arm (HR 1.59, 95% CI 1.19-2.12, p = 0.002) (Fig. 3b).
A total of 7 studies reported separate data for febrile neutropenia; a total of 4395 patients were enrolled in the studies, with 1686 (38.4%) being in the placebo plus ET arm and 2709 (61.6%) in the CDK 4/6 inhibitors plus ET arm. Febrile neutropenia is increased in CDK4/6 inhibitors plus ET arm (HR 4.28, 95% CI 1.73-10.62, p = 0.002) (Fig. 4). Although a total of three studies reported separate data for pneumonia, the definition of the respiratory event had a broad term that may encompass a variegate spectrum of lung diseases, with different clinical phenotypes except for underlying infective complications (bronchiolitis, pneumonia). Therefore, pneumonia was not noted in this study due to scant data on pneumonia.
Despite the increased rate of all grade and grade 3 infections, the risk of infection-related deaths was not significantly increased in the pooled analysis of the studies and event rates were very low (seven vs. three deaths in the CDK 4/6 + ET and ET arms, respectively; HR 1.00, 95% CI 0.30-3.32, p > 0.99).

Discussion
CDK4/6 inhibitors are standard of care options with significantly better progression-free survival, objective response rate, and overall survival in patients with advanced breast cancer [22][23][24][25][26] (Table 1). However, the association between CDK4/6 inhibitors and infection rates have not been specifically assessed before. This meta-analysis showed that CDK 4/6 inhibitors plus ET significantly increased infection rates in patients with HR + /HER2-advanced breast cancer. The increased risk of all grade infections, grade 3 or higher infections, UTIs, pneumonia, and febrile neutropenia was observed. To the best of our knowledge, this is the first metaanalysis assessing infection rates in patients receiving CDK 4/6 inhibitors.
Neutropenia was the most common toxicity reported with CDK 4/6 inhibitors, particularly in patients treated with palbociclib and ribociclib. Questions are still present about whether CDK 4/6 inhibitors increase infection risks related to myelosuppression. There was a higher incidence of neutropenia in the CDK 4/6 inhibitor arms, while infectionrelated deaths and febrile neutropenia were very low. Only 35 (1.3%) patients developed febrile neutropenia in the arm of CDK 4/6 inhibitors ( Table 2). The biological rationale for this results in contrast to chemotherapy which causes DNA damage and induces apoptosis of proliferating neutrophil precursors, the CDK 4/6 inhibitors prevent progression through the G1 to S checkpoint, which causes cell cycle arrest. Therefore, CDK 4/6 inhibitors, an important mechanism different from chemotherapy, reflect a cytostatic effect on the bone marrow by cell cycle arrest in hematopoietic precursor cells and cause quiescence without apoptosis [27]. The white blood cells may continue to function after withdrawal of CDK 4/6 inhibitors and also rapid recovery of the marrow may occur without long-term detrimental effects.  Considering infections remain the major cause of morbidity and mortality among patients with cancer still in the modern area, our results are of great significance for clinical implications. Early recognition and proper treatment may improve prognosis and patients' quality of life [28]. Many infections can easily be evaluated by complete blood count and infection parameters. Patients with high infection risk (patients with urinary catheters, central catheters) might be considered for closer follow-up to prevent further neutropenic fever.
When neutropenia or febrile neutropenia develops, due to CDK4/6 inhibitor-induced neutropenia is rapidly reversible; causitive agent should be discontinued. Granulocyte colony-stimulating factor (G-CSF, e.g., lenograstim) usage is not necessary in most cases [29]. Palbociclib and ribociclib are dosed intermittently (e.g., 21 days on followed by 7 days off) to allow for recovery of neutrophils. On the other hand, abemaciclib shows greater selectivity for CDK4 when compared to palbociclib and ribociclib, causing in lower rates of neutropenia, and thus can be dosed continuously. Dose modification might be necessary for hematologic toxicities for each agent [30].
There are three main limitations of our meta-analysis. First, we used the data from the published articles instead of individual patient data. Second, the data on the specific infection types was unavailable in all studies. Therefore, the interpretation of the results on the several infection types needs to be taken cautiously. Third, there was three different molecules were included. Also, trials included both first-and secondline treatments (aromatase inhibitors and estrogen-receptor antagonist, respectively). So, the heterogeneity between the studies limited the generality. Additionally, we could not be able to conduct additional analyses on the impact of infections on the quality of life due to a lack of data. However, despite these limitations, we could take a snapshot of the infection risk in patients treated with CDK 4/6 inhibitors in a large body of data collected from well-constructed clinical trials.

Conclusion
In this meta-analysis, we observed that adding CDK4/6 inhibitors to ET significantly increased infection rates. We propose that close vigilance for infections is required for metastatic breast cancer patients using CDK 4/6 inhibitors. Further research is needed to delineate the effects of infections on quality of life in patients treated with CDK 4/6 inhibitors.
Author contribution All authors contributed to the study conception and design. All authors, namely DCG, OB, EE, and SA, have made significant and substantive contributions to the reporting of the work. All authors have participated in reviewing relevant literature, drafting the manuscript, and reviewing and revising the final draft. DCG, OB, and EE performed material preparation, data collection, and analysis. OB wrote the first draft of the manuscript and all authors commented on previous versions. All authors read and approved the final manuscript.

Data availability
The data supporting the conclusions of this article are included within the paper and its additional file.

Declarations
Ethics approval This article does not contain any studies with human participants or animals performed by any authors.

Competing interests
The authors declare no competing interests.