C. Hwang1, S. Ucer1, C. Pagani1, N. Patel1, A. Vaishampayan1, M. Sorkin1, M. T. Chung1, J. Li1, C. Breuler1, C. Priest1, A. N. Economides3, S. Agarwal1, Y. Mishina2, B. Levi1 1University Of Michigan,Section Of Plastic Surgery,Ann Arbor, MI, USA 2University Of Michigan,Department Of Biologic And Material Sciences, School Of Dentistry,Ann Arbor, MI, USA 3Regeneron Pharmaceuticals And Genetics Center,Tarrytown, NY, USA
Introduction: Heterotopic ossification (HO) is a debilitating formation of ectopic bone restricting joint mobility and causing chronic pain. Fibrodysplasia Ossificans Progressiva (FOP) is a congenital variant of HO caused by a genetic mutation in a bone morphogenetic receptor that causes severe, progressive lesions resulting in immobility and fatal mechanical respiratory failure at a premature age. With increased proclivity for osteogenesis at baseline, surgical extirpation is contraindicated in FOP patients due to universal recurrence. This disease presents a unique clinical challenge with no current treatments available. Previous research has shown mTOR inhibition by rapamycin to be a viable therapy for reducing trauma-induced HO. Mice of an FOP mouse model with a hyperactivating mutation in the ACVR1 receptor (ACVR1R206H/+) were examined to determine the effects of rapamycin on both primary and post-surgical HO.
Methods: Mice were designated for 2 distinct study arms. In the primary HO cohort, ACVR1R206H/+ P21 mice received bilateral hindlimb cardiotoxin (CTX) with Ad.cre injection and stratification to daily i.p. rapamycin (n=10 hindlimbs) v. PBS control (n=12). 3 weeks later, mice were scanned with in vivo µCT. Mice within the separate recurrence cohort received initial Ad.cre/CTX followed by baseline CT scans, Ad.cre reinjection bilaterally, and surgical removal of HO at the 3 week timepoint. After surgery, mice were randomized to daily rapamycin (5 mg/kg, n=12) or PBS (n=8) for 3 weeks with endpoint µCT. Contours were drawn manually around HO to compute total volumes at 800HU. Primary and recurrence cohorts were analyzed by Student’s t test and log-transform/ANOVA/Hochberg post-hoc respectively.
Results: Mice treated with rapamycin for 21 days showed 261-fold less ectopic bone when compared to PBS vehicle injection (p=0.002). In post-surgical mice, PBS injection showed statistically similar HO volumes to baseline volumes of pre-excision mice (p=0.054). However, rapamycin treatment reduced the recurrence of HO volume 11-fold (p=.044, Figure). Geometric means from de-transformed data for pre-excision v. PBS, pre-excision v. rapamycin, and PBS v rapamycin were 0.061, 1.398, and 22.961 respectively.
Conclusions: These studies demonstrate that rapamycin prevents primary development of HO and is also effective in preventing recurrence following surgical excision in a FOP mouse model. This study further corroborates rapamycin as a promising candidate for primary and post-surgical HO prophylaxis in children with FOP. Existing literature implicates similar molecular mechanisms among various etiologies of HO, suggesting a putative role for rapamycin even beyond FOP in post-traumatic and post-surgical HO patients.
