The true upside of the many advances in cancer treatment is clear. Approximately 18.6 million people in the United States have a history of cancer, and the number of cancer survivors is expected to exceed 22 million by the year 2035.¹However, one downside affecting many of these individuals is the cardiotoxicity caused by certain systemic medications or radiation therapy.

At the 2025 European Society of Cardiology (ESC) Cardio-Oncology Annual Conference in Florence, Italy, several presenters explored the mechanisms of cancer treatment–related cardiovascular toxicity. The focus of three sessions was on the CDK4/6 inhibitor ribociclib; a potential therapeutic target (guanylate-binding protein 5 [GBP5]), to reduce cardiac damage after radiation therapy; and atrial fibrillation associated with the Bruton’s tyrosine kinase (BTK) inhibitor ibrutinib.^2-4

Ribociclib and Cardiotoxicity

According to Eva Pet, PhD Candidate in Cardio-Oncology, University Medical Centre Groningen, the Netherlands, and colleagues, ribociclib may induce cardiotoxicity through impaired E2F1-regulated spliceosome assembly.² (E2F1 is the transcription factor that is encoded by the E2F1 gene.) Ms. Pet briefly reviewed the increased risk of cardiovascular adverse events, especially heart failure, associated with CDK4/6 inhibitors such as ribociclib. In a recent safety analysis,⁵acute coronary syndrome, arrhythmia, angiopathy, and intracardiac thrombus were found to be “strongly correlated” with CDK4/6 inhibitors.

“Overexpression of E2F1 in these cardiomyocytes was performed, resulting in a threefold increase in expression of the E2F1 protein, which mitigated these ribociclib-induced cardiac effects.”

— EVA PET, PhD CANDIDATE

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The preclinical research of these investigators centered on how these targeted agents function and their cardiotoxic effects in dynamic human engineered heart tissues, focusing on the CDK4/6–Rb-E2F1 pathway. To mimic clinical drug administration, dynamic engineered heart tissues (generated by using human-induced pluripotent stem cell–derived cardiomyocytes) were treated with repeated doses of ribociclib (7 μM) over 2 weeks.

According to the study authors, once treated with ribociclib, “the dynamic engineered heart tissues exhibited a 17.0% ± 4.3% (P < .001) increase in tissue dilatation, an 8.9% ± 4% (P < .001) decrease in systolic force generation, and a 22.1% ± 5.7% (P < .001) increase in systolic stress, indicating significant cardiac dysfunction.” To better understand why, overexpression of E2F1 in these cardiomyocytes was performed, resulting in a threefold increase in expression of the E2F1 protein, which mitigated these ribociclib-induced cardiac effects. “We actually saw a rescue of that dilation as well as a rescue of the systolic force within these tissues,” stated Ms. Pet.

The researchers concluded that the significant cardiotoxic effects produced by ribociclib occurred through tissue dilation and reduced contractile force in dynamic engineered heart tissues. “RNA sequencing confirmed an overall effect of ribociclib on E2F1-mediated pathways, confirming its role in ribociclib-induced cardiotoxicity,” Ms. Pet said. E2F1 overexpression mitigates the reduced expression of genes related to spliceosome assembly and splice site recognition, pointing at a potential mechanism of cardiac dysfunction,” she added. As a result of these study findings, the investigators believe that careful cardiac monitoring is indicated for patients treated with CDK4/6 inhibitors.

Inactivation of GBP5 and Radiotherapy

Many patients who undergo thoracic radiotherapy are at increased risk for cardiac events and cardiac fibrosis, but the specific molecular mechanisms leading to cardiac fibrosis remain unclear. To learn more, Markus Benjamin Heckmann, MD, of the Clinic for Angiology, Pneumology, and Cardiology, University Hospital, Heidelberg, Germany, and colleagues expanded on their previous work with guanylate-binding protein 5 (GBP5), a gene that has been found to be upregulated after radiation therapy. In fact, GBP5, they noted, plays a role in the “assembly of the NLRP3 inflammasome, a complex responsible for triggering the release of proinflammatory cytokines.” In their preclinical study of cardiomyocyte-specific knock-in mice that received photo irradiation (16 Gy),³ they focused on the expression of a dysfunctional GBP5 molecule, which lacks the carboxyl terminus necessary for NLRP3 assembly, to assess acute and chronic cardiac inflammation after irradiation.

“GBP5 is a potential therapeutic target to reduce cardiac damage after radiation.”

— MARKUS BENJAMIN HECKMANN, MD

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The investigators found that mice carrying this dysfunctional GBP5 molecule were protected against radiation-induced mortality, with markedly prolonged survival compared with wild-type controls (P = .0276). Cardiac function was largely preserved after 5 months. In fact, reduced release of high-sensitivity cardiac troponin T (a protein released into the bloodstream when the heart muscle is injured) was also observed. In addition, the investigators shared that quantitative polymerase chain reaction results indicated disruption of NLRP3 activity, “leading to reduced cardiac fibrosis.” Similar findings were observed on protein levels.

In closing, Dr. Heckmann and colleagues shared these clinical implications: “These results suggest that the carboxyl terminal GBP5 modulates the inflammasome pathway and is for myocardial inflammation after radiation essential. GBP5 is a potential therapeutic target to reduce cardiac damage after radiation.”

Ibrutinib-Mediated Atrial Fibrillation

The benefits of using BTK inhibitors to treat hematologic malignancies such as B-cell lymphomas and chronic lymphocytic leukemia are well known. Also well recognized are the cardiovascular adverse events linked to first-generation BTK inhibitors such as ibrutinib; they include atrial fibrillation, hypertension, and ventricular arrhythmia. However, all BTK inhibitors are not the same, as newer-generation BTK inhibitors such as acalabrutinib have been linked to fewer cardiotoxicities, with one study citing the incidence of all-grade atrial fibrillation or atrial flutter as 1.0% with acalabrutinib vs 9.4% with ibrutinib (P = .02).⁶

To better understand the potential mechanism behind the proarrhythmic process associated with ibrutinib treatment, Matthew Fleming, MD, PhD, Instructor of Cardiology, Vanderbilt-Ingram Cancer Center, Nashville, and colleagues turned to the use of high-content imaging and phenotypic screening of human-induced pluripotent stem cell–derived atrial-specific cardiomyocytes.⁴ Their study built on the earlier results of Bray et al with cell painting, a high-content, image-based assay for morphologic profiling using multiplexed fluorescent dyes.⁷

“We believe this system is adequate to study BTK inhibitor–mediated atrial fibrillation.”

— MATTHEW FLEMING, MD, PhD

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Dr. Fleming explained the study details. Monolayers of these atrial-specific cardiomyocytes were plated onto 96 well plates and treated with increasing concentrations of ibrutinib or DMSO (dimethyl sulfoxide)-vehicle control. After 72 hours of ibrutinib treatment, the investigators performed in situ staining with the cell painting assay. Morphologic profiling was performed, he added, by simultaneously multiplexing several different fluorescent dyes to label different cellular components (including the mitochondrial structure). Then, they measured and analyzed mitochondrial intensity, texture, shape, size, and relation to neighboring cellular structures in relation to treatment with ibrutinib or the vehicle control.

When the atrial-specific cardiomyocytes were treated with ibrutinib, the investigators observed dose-dependent changes in 34 parameters of mitochondrial structure. For instance, a statistically dose-dependent response was seen in mean mitochondrial intensity (P < .001 and P < .0001 for ibrutinib at 1 and 10 μM, respectively). In addition, Dr. Fleming noted, three-dimensional principal component analysis of all the measured mitochondrial parameters revealed significant differences (P < .0001) in the mitochondrial structure between ibrutinib- and vehicle-treated atrial-specific cardiomyocytes.

“Ibrutinib exposure causes marked changes in multiple indices of mitochondrial structure, implicating altered mitochondrial function in the development of ibrutinib-related atrial fibrillation,” the investigators concluded. In addition, according to Dr. Fleming, “we believe this system is adequate to study BTK inhibitor–mediated atrial fibrillation. And, the exciting news is that we actually see a difference in the treatment of these cells with ibrutinib vs acalabrutinib.”

DISCLOSURE: Ms. Pet reported no conflicts of interest. Dr. Heckmann has received speaker honoraria from Bristol Myers Squibb and Daiichi Sankyo. Dr. Fleming has received research support from an American Heart Association Career Development Award and a Vanderbilt Clinical Oncology Research Center Career Development Program K12 Award.

REFERENCES

1. American Cancer Society: Cancer Treatment and Survivorship Facts & Figures. Available at https://www.cancer.org/research/cancer-facts-statistics/survivor-facts-figures.html. Accessed July 3, 2025.
2. Pet E: CDK4/6 inhibitor ribociclib induces cardiotoxicity through impaired E2F1-regulated spliceosome assembly. 2025 ESC Cardio-Oncology Annual Conference. Presented June 20, 2025.
3. Heckmann MB: Targeted cardiac inactivation of guanylate binding protein 5 (GBP5) protects from cardiotoxicity after radiation. 2025 ESC Cardio-Oncology Annual Conference. Presented June 20, 2025.
4. Fleming M: Cell painting based high-content phenotypic screening to elucidate kinase signaling pathways in ibrutinib-mediated atrial fibrillation. 2025 ESC Cardio-Oncology Annual Conference. Presented June 20, 2025.
5. Zhang C, Shen G, Li S, et al: Cardiovascular events associated with CDK4/6 inhibitors: A safety meta-analysis of randomized controlled trials and a pharmacovigilance study of the FAERS database. Am J Cardiovasc Drugs 25:373-388, 2025.
6. Byrd JC, Hillmen P, Ghia P, et al: Acalabrutinib versus ibrutinib in previously treated chronic lymphocytic leukemia: Results of the first randomized phase III trial. J Clin Oncol 39:3441-3452, 2021.
7. Bray MA, Singh S, Han H, et al: Cell painting, a high-content image-based assay for morphological profiling using multiplexed fluorescent dyes. Nat Protoc 11:1757-1774, 2016.