Bone growth during rapamycin therapy in young rats

Area of Science:

• Pediatric Endocrinology
• Pharmacology
• Developmental Biology

Background:

• Rapamycin is a crucial immunosuppressant for pediatric renal transplant patients.
• Its anti-proliferative and anti-angiogenic properties raise concerns about potential impacts on linear growth in children.

Purpose of the Study:

• To investigate the effects of rapamycin on endochondral bone growth in a young rat model.
• To assess the impact of rapamycin on growth plate cartilage, chondrocyte proliferation, differentiation, and resorption.

Main Methods:

• Weanling rats received daily rapamycin (2.5 mg/kg) or saline for 2 or 4 weeks.
• Evaluated body/tibia length, serum markers (calcium, phosphate, IGF-I), and performed histomorphometric analysis of growth plates.
• Assessed gene and protein expression related to chondrocyte function and bone resorption.

Main Results:

• Two weeks of rapamycin reduced body and tibia length, enlarged the hypertrophic zone, and decreased chondrocyte proliferation (histone-4, mTOR).
• Changes in PTHrP, Ihh, RANKL, and VEGF indicated altered chondrocyte differentiation and reduced osteoclast activity.
• After four weeks, body/tibia length remained short, but markers for proliferation and differentiation showed improvement, with no catch-up growth observed.

Conclusions:

• Short-term rapamycin (2 weeks) significantly impaired endochondral bone growth in young rats.
• Despite some marker improvements at 4 weeks, sustained catch-up growth was not evident.
• Clinical studies are warranted to assess these growth effects in pediatric patients.