Erzhi Pill Enhances Methotrexate Efficacy in Rheumatoid Arthritis Bone Homeostasis via Activation of the IDO1-KYN-AhR Pathway

EZP enhances bone protection of MTX in RA by bidirectionally regulating OC and OB. • EZP augments the osteoprotective effect of MTX by IDO1-KYN-AhR pathway. • Salidroside and ferulic acid from EZP are responsible for synergizing with MTX. • We provide a promising integrative strategy for managing bone disorders in RA. Progressive joint destruction and systemic bone loss are key factors contributing to disability and secondary osteoporosis in rheumatoid arthritis (RA). However, current clinical drugs, including first-line methotrexate (MTX), provide limited bone protection. Enhancing the regulatory effect of MTX on RA bone homeostasis through combination with traditional Chinese medicine (TCM) may offer a new perspective for RA treatment. This study investigated whether Erzhi Pill (EZP), a kidney-tonifying TCM formula, enhances MTX efficacy in regulating bone homeostasis in RA via the IDO1-KYN-AhR signaling pathway. Using primary osteoblast/osteoclast models and a kidney deficiency collagen-induced arthritis (KD-CIA) rat model, we demonstrated that the MTX+EZP combination was superior to MTX monotherapy in promoting osteoblast differentiation/function, suppressing osteoclastogenesis, and mitigating bone destruction and loss in KD-CIA rats. These effects were associated with the restoration of suppressed IDO1-KYN-AhR signaling activity. Furthermore, siRNA-mediated knockdown or pharmacological inhibition of IDO1 or AhR reversed the regulatory effects of MTX+EZP on osteoblast-osteoclast differentiation/function and its bone-protective actions in KD-CIA rats. Finally, salidroside and ferulic acid were identified as the potential active compounds in EZP responsible for synergizing with MTX. In summary, our findings demonstrate that EZP enhances the efficacy of MTX in regulating RA bone homeostasis by activating the IDO1-KYN-AhR signaling pathway, representing a promising integrative strategy for managing RA-induced bone metabolism disorders.