Background/Purpose: Aromatase Inhibitors (AIs) block physiological estrogen (E2) production in peripheral tissues and are used clinically to reduce disease recurrences and improve overall survival rates in post-menopausal, hormone receptor-positive breast cancer patients. However, half of patients taking these drugs develop aromatase inhibitor induced arthralgia (AIIA), which is characterized by severe pain and inflammation in various joints and the surrounding musculoskeletal tissue. It has been proposed that AIIA is associated with E2 deficiency resulting from AI treatment. In this study, we designed a novel animal model to establish a better understanding of disease pathology and to create a platform that can be used to explore future interventional strategies.

Methods: Female BALB/C-Tg(NFκB-RE-luc)-Xen mice, were oophorectomized (OVX) and treated with AI (letrozole) by daily SC injections for 5 weeks. Control groups included OVX mice receiving vehicle control injections and non-OVX mice treated with AI. Knee joints were imaged on the BioSpec 94/30 micro-MRI. The primary weight-bearing joint (hind limb) was examined histopathologically and NFκB activity was measured by bioluminescent imaging. Serum was collected for cytokine analysis. Healthy human PBMCs were treated with AI, E2, or both and intracellular RNA sequencing was performed at 36 hours.

Results: Bioluminescent imaging showed significantly enhanced NFκB activation with AI treatment in the hind limbs compared to controls receiving vehicle treatment. Moreover, analysis of knee joints by MRI showed enhanced signal detection in the joint space and surrounding tissue following AI treatment. Surprisingly, the enhanced MRI detection and NFκB activation was observed with AI treatment independent of the OVX. This indicates that the induction of musculoskeletal-directed inflammation is not mediated by changes in physiological E2 levels, which is contrary to proposed mechanisms of disease pathogenesis. Histopathological analysis further demonstrated the same trend, as tenosynovitis and musculoskeletal infiltrates were detected in all mice receiving AI. Additionally, serum cytokine levels of IL-2, IL-4, IL-6, and CXCL1 were significantly elevated with AI treatment. IHC analysis of the infiltrates demonstrated a predominantly macrophage-mediated inflammatory response with scattered CD4+ T cells. RNA sequencing of human PBMCs after in vitro AI stimulation did not demonstrate an AI-specific gene expression pattern associated with immune system activation directly, indicating that the pathogenesis of AIIA may be mediated through other cell types in vivo.

Conclusion: Collectively, these data establish a novel mouse model of AIIA and identify an E2-independent stimulation of disease pathology via AI-mediated induction. This suggests that the pathogenesis of AIIA may not be mediated by E2 deficiency, as previously hypothesized, and indicates that AI-induced inflammation is not regulated directly through immune cell activation. Future studies will characterize this inflammatory mechanism in vivo with a focus on synovial cells and chondrocytes to provide insight into putative therapeutic strategies directed at mitigating disease pathology.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/aromatase-inhibitor-induced-musculoskeletal-inflammation-is-observed-independent-of-oophorectomy-in-a-novel-mouse-model/