Abstract Body: Introduction: Radiotherapy is a cornerstone treatment for thoracic malignancies, yet late-onset cardiotoxicity is a major concern. Direct radiation responses in human cardiomyocytes remain poorly characterized, as mechanistic studies have largely been done in murine models. We hypothesized that radiation-induced mitochondrial injury disrupts cardiac homeostasis, and that restoring this balance via an actionable countermeasure would mitigate cardiac dysfunction.

Aims: To elucidate mechanisms of radiation-induced cardiotoxicity in a human model system and evaluate honokiol (HKL) as a cardioprotective countermeasure.

Methods: iPSC-derived cardiomyocytes (iPSC-CMs, N=3 donors) were exposed to X-rays (0–20 Gy) acutely. Contractility, calcium handling, and mitochondrial respiration were assessed. Integrative analysis of irradiated iPSC-CM transcriptomics and proteomics with public irradiated mouse heart data identified SIRT3-related mitochondrial dysregulation, which identify honokiol as lead candidate. To overcome HKL's poor pharmacokinetic profile, HKL prodrug (HKLp) was tested in whole heart (20 Gy) and partial heart–lung (PHL, 40 Gy) mouse models.

Results: Acute irradiation above 4 Gy caused dose-dependent contractile impairment, arrhythmogenic calcium dysregulation including early and delayed after depolarizations (EAD/DAD), and mitochondrial superoxide overload with suppressed spare respiratory capacity. Honokiol treatment (3 μM) suppressed arrhythmogenic calcium waves at 3 hours and enhanced calcium transient amplitude at day 7 post-irradiation (p<0.05). In vivo, HKLp (intraperitoneal injection at 50 mg/kg, daily, for 5 days) after irradiation preserved ejection fraction at 4 weeks post-20 Gy (p=0.0047 vs vehicle). PHL (40 Gy) resulted in diastolic dysfunction at 6 months (E/A p<0.01 and E/E' p<0.05 vs sham), and HKLp attenuated both (not significant vs sham). Plasma proteomics in mouse samples showed HKLp intervention attenuated inflammatory and innate immune pathway enrichment post-irradiation.

Conclusions: HKLp confers durable cardioprotection against radiation-induced cardiac dysfunction in both mouse and human-based models. Given honokiol's anticancer activity, HKLp may widen the therapeutic window of radiotherapy for thoracic cancer patients.