ESPE Abstracts

Introduction: The survival rates for childhood cancer have significantly increased over the past decades but the management of treatment-related complications remains challenging. Thus, the introduction of novel drugs requires careful investigation of potential off-target side effects. The developing skeleton is severely affected by the anticancer treatment which may result in growth retardation and short stature. Venetoclax is a selective Bcl-2 inhibitor which is under investigation in pediatric cancer clinical trials. However, there are no data regarding the potential side-effects of venetoclax on the bone tissue.

Objectives: The overall aim was to investigate the effects of venetoclax on bone growth. Mitochondrial activity and the Bcl-2 pathway are crucial during bone development, we hypothesized that venetoclax might suppress bone growth by inhibiting Bcl-2 in the growth plate.

Methods: (a) Culture of ATDC5 cells: The chondrogenic ATDC5 cells were cultured with venetoclax (0.625-2.5 μΜ) or DMSO and cell viability was assessed after 48 or 72 hrs with the CellTiter-Glo® Luminescent Cell Viability Assay. (b) Culture of fetal rat metatarsals: E20 rat metatarsals were cultured ex vivo with venetoclax (1 nM-10 μΜ) for 12 days (n=15). Photos and length measurements were taken at designated time-points to monitor growth and histomorphometric analysis was performed. (c) In vivo studies: Female NMRI nu/nu mice (7-week-old, n=4-5) were treated with venetoclax (100 or 200 mg/kg via oral gavage) or vehicle for 14 days. X-rays were taken to evaluate longitudinal bone growth and changes in the growth plate were assessed with histomorphometry.

Results: Venetoclax decreased cell viability in ATDC5 cells after 48 or 72 hrs culture. Venetoclax-treated metatarsals showed decreased growth from day 0 to day 12 compared to the control group (2.5 μΜ: 65.4 ± 3.4 % vs control 76.4 ± 1.5%, P<0.05; 5 μΜ: 20.2 ± 1.4 % and 10 μΜ: 9.5 ± 1.1% vs control 76.4 ± 1.5%, P<0.001). Furthermore, the height of resting + proliferative zone and the size of hypertrophic cells were significantly reduced in the venetoclax groups. Venetoclax in the highest dose significantly reduced tibia growth in vivo (P<0.05) and both doses reduced growth plate height compared to vehicle (100 mg/kg: 130.2 ± 3.3 μm, P< 0.01; 200 mg/kg: 130.6 ± 4.9 μm, P< 0.05 vs vehicle: 151.4 ± 4.9 μm).

Conclusion: Our findings show that venetoclax impairs chondrogenesis and induces growth retardation. Based on these, further investigations will enable the identification of the mechanisms involved in the observed side-effects on bone.