Osteocyte mechanosensing following short-term and long-term treatment with sclerostin antibody

Sclerostin antibody romosozumab (EVENITY™, romosozumab-aqqg) has a dual mechanism of action on bone, increasing bone formation and decreasing bone resorption, leading to increases in bone mass and strength, and a decreased risk of fracture, and has been approved for osteoporosis treatment in patients with high risk of fragility fractures. The bone formation aspect of the response to sclerostin antibody treatment has thus far been best described as having two phases: an immediate and robust phase of anabolic bone formation, followed by a long-term response characterized by attenuated bone accrual. We herein test the hypothesis that following the immediate pharmacologic anabolic response, the changes in bone morphology result in altered (lesser) mechanical stimulation of the resident osteocytes, initiating a negative feedback signal quantifiable by a reduced osteocyte signaling response to load. This potential desensitization of the osteocytic network is probed via a novel ex vivo assessment of intracellular calcium (Ca²⁺) oscillations in osteocytes below the anteromedial surface of murine tibiae subjected to load after short-term (2 weeks) or long-term (8 weeks) treatment with sclerostin antibody or vehicle control. We found that for both equivalent load levels and equivalent strain levels, osteocyte Ca²⁺ dynamics are maintained between tibiae from the control mice and the mice that received long-term sclerostin antibody treatment. Furthermore, under matched strain environments, we found that short-term sclerostin antibody treatment results in a reduction of both the number of responsive cells and the speed of their responses, which we attribute largely to the probability that the observed cells in the short-term group are relatively immature osteocytes embedded during initial pharmacologic anabolism. Within this study, we demonstrate that osteocytes embedded following long-term sclerostin antibody treatment exhibit localized Ca²⁺ signaling akin to those of mature osteocytes from the vehicle group, and thus, systemic attenuation of responses such as circulating P1NP and bone formation rates likely occur as a result of processes downstream of osteocyte Ca²⁺ signaling.