Abstract

Background Cancer-induced bone pain (CIBP) remains a debilitating clinical challenge due to its complex pathogenesis and limited therapeutic options. Tetrahydropalmatine (THP), an active alkaloid from  Corydalis tuber , has shown analgesic potential, but its specific mechanisms in mitigating CIBP- especially its interactions with neural factors and immune cells-remain incompletely understood. This study aimed to investigate the efficacy of THP in alleviating CIBP and clarify its underlying mechanisms, with a focus on the roles of transient receptor potential vanilloid 1 (TRPV1), substance P (SP), and macrophage dynamics in a mouse model of CIBP. Methods Using a multidisciplinary approach, we established a CIBP model in male C57BL/6 mice (and TRPV1-knockout mice) via intramedullary injection of lung cancer cells. Behavioral assessments were performed to evaluate mechanical, thermal, and cold allodynia, as well as spontaneous pain, following daily oral administration of THP (80 mg/kg) from day 7 post-modeling. Molecular and cellular analyses included immunofluorescence staining, real-time PCR, ELISA (to quantify SP, cytokines, and Tac1 expression), calcium imaging (to measure TRPV1-mediated calcium influx in DRG neurons), scratch assays (to assess macrophage migration), and flow cytometry (to analyze macrophage polarization in RAW 264.7 cells). Results THP significantly alleviated CIBP-related allodynia and spontaneous pain in mice. Mechanistically, THP directly inhibited TRPV1 function in the primary phase (≤ 14 days post-modeling) (with an IC 50 of 77.9 µM), reducing SP release from DRG neurons and suppressing macrophage recruitment to DRG and sciatic nerve via the TRPV1-SP pathway. Importantly, THP directly promoted the polarization of recruited macrophages toward the anti-inflammatory M2 phenotype, as evidenced by downregulated iNOS, TNF-α, IL-1β and upregulated CD206, IL-4, IL-10 in vitro (RAW264.7 cells) and in vivo. These effects were abrogated in TRPV1-kn