Oral Presentation ESA-SRB 2023 in conjunction with ENSA

RANKL inhibition creates a pro-osteoclastic environment, leading to an overshoot in serum TRAP and accelerated bone resorption following treatment withdrawal. (#133)

Albert Kim 1 , Ariel Castro-Martinez 1 , Vicoria Taylor 1 , Suraj Dhakal 1 , James Smith 1 , Jacqueline Center 1 2 , Christian Girgis 3 4 , Peter Croucher 1 2 , Michelle McDonald 1 2 4
  1. Skeletal Diseases Program, Garvan Institute of Medical Research, Darlinghurst, NSW, Australia
  2. St Vincent’s Clinical School, School of Medicine and Health, UNSW Sydney, Kensington, NSW, Australia
  3. Department of Diabetes and Endocrinology, Westmead Hospital, Westmead, NSW, Australia
  4. Faculty of Medicine & Health, University of Sydney, Sydney, NSW, Australia

Denosumab withdrawal triggers rapid bone mineral density (BMD) loss via accelerated bone resorption. Development of optimal sequential therapy is hindered by poor understanding of the cellular mechanisms and sub-optimal serum turnover marker assessment. We compared temporal changes in RANKL, serum TRAP5b and osteoclast precursors to CTX, P1NP and BMD, to define alternative tools to guide sequential treatment.

Seven week female C57BL/6 mice received 2-weeks of saline or thrice-weekly OPG:Fc (10mg/kg) to inhibit RANKL, then withdrawn from therapy (OPG-W). Following longitudinal BMD and serum measurement, mice were harvested at weeks 2, 8, 11 and 13 for RANKL, TRAP5b, P1NP and CTX. Week 8, marrow-flushed, long-bone samples were assessed for RANKL mRNA. Week 6 bone marrow samples were analysed for osteoclast precursors (NK1.1- Ter119- CD3- Ly6G-, B220-, CD11blo, CD117int, CD115+).

Following OPG:Fc withdrawal, BMD increased 24% at week 8 in OPG-W (p<0.01), declining at week 10 and normalised to vehicle at week 13. At week 8, serum TRAP, CTX and P1NP were all suppressed in OPG-W (p<0.001). At week 11, serum TRAP was elevated in OPG-W (p=0.01), P1NP and CTX remained equivalent to vehicle. At week 13, serum TRAP, P1NP and CTX were all greater in OPG-W (p<0.01).

Serum RANKL levels at week 2 were elevated with OPG:Fc (p<0.001), peaking 13-fold higher at week 8 (p<0.0001), returning to vehicle at week 11. Prior to the overshoot in serum TRAP levels in OPG-W (at week 11), bone RANKL mRNA was elevated at week 8 (p<0.01) and osteoclast precursors at week 6 (p<0.05).

Rebound BMD decline preceded the increase in clinical turnover markers (P1NP, CTX). Elevated serum TRAP occurs earlier, prior to bone loss, and may better guide sequential therapy. Increased serum and bone RANKL levels and an accumulation of osteoclast precursors were detected prior to the overshoot in TRAP.