The effect of rapamycin on bone growth in rabbits.

mTOR is a nutrient-sensing protein kinase that regulates numerous cellular processes. Our prior studies using the mTOR inhibitor, rapamycin, indicate an important role for mTOR in chondrogenesis. We extended our observations to a physiological, in vivo model of bone growth, direct infusion of rapamycin into the proximal tibial growth plates of rabbits. Rapamycin or DMSO vehicle was infused directly into growth plates by an osmotic minipump for 8 weeks. Tibial growth was followed radiographically. At the end of the experiment, growth plates were recovered for histological analysis. Six animals were studied. No untoward effects of rapamycin infusion were found. Bone growth of limbs exposed to rapamycin was slower than control limbs, particularly during the period of most rapid growth. Histological analysis revealed that growth plate height in the rapamycin-infused limbs was reduced. Both the hypertrophic and proliferative zones were significantly smaller in the rapamycin-infused limbs. Direct infusion of rapamycin into proximal tibial growth plates decreased the size of the growth plate and inhibited overall long bone growth. Rapamycin appears to affect both the proliferative and hypertrophic zones of the tibial growth plate. Our results indicate that nutrients may exert a direct effect on long bone growth via mTOR-mediated modulation of chondrogenesis at the growth plate. and suggest that the possible inhibitory effects of rapamycin on skeletal growth warrant further attention before its use in children. (c) 2009 Orthopaedic Research Society.