Disrupting the Feed-Forward Cycle of RyR1 Ca ²⁺ Leak and Oxidative Stress Mitigates Doxorubicin-Induced Skeletal Myopathy

Doxorubicin (DOX) is a highly effective and widely used chemotherapeutic agent employed to treat various types of cancer. Unfortunately, DOX also has some undesirable and off-target effects, particularly debilitating muscle weakness and fatigue. The mechanism behind this DOX-induced skeletal myotoxicity (DISM) remains unclear. Here, we show that acute DOX exposure, at clinically relevant concentrations, impairs isometric force production and accelerates fatigue in ex vivo murine flexor digitorum brevis (FDB) muscles. Mechanistically, we found that DOX increases the open probability of single RyR1 and disrupts calcium (Ca 2+ )-dependent inactivation (CDI). This results in a persistent sarcoplasmic reticulum (SR) Ca 2+ leak, elevated basal cytosolic Ca 2+ , and abnormal Ca 2+ release during action potentials. This abnormal intracellular Ca 2+ handling ultimately leads to increased cellular reactive oxygen species (ROS) production, which, in turn, exacerbates the functional instability of RyR1. Interestingly, the cytosolic basal Ca 2+ elevation precedes ROS generation, suggesting that it initiates a destructive cross-talk between Ca 2+ dysregulation and oxidative stress. Further, our study suggests that DOX disrupts the interaction between RyR1 and FKBP12 and that pharmacological stabilization of this complex with the novel triazole compounds MP-001 and MP-034 normalizes RyR1 function, Ca 2+ and ROS homeostasis, as well as muscle force and fatigue resistance. Our findings indicate that DISM is initiated by DOX destabilization of the RyR1-FKBP12 complex (abnormal SR Ca 2+ leak) and then exacerbated by the Ca-ROS vicious cycle. Limiting RyR1-mediated Ca 2+ leak with MP compounds represents a promising therapeutic strategy for anti-DISM, aiming to normalize muscle function in patients undergoing DOX chemotherapy.