Background:

Congenital tibial dysplasia is a severe pediatric condition that classically results in a persistent pseudarthrosis. A majority of these cases are associated with neurofibromatosis type I (NF1), a genetic disorder in which inactivation of the NF1 gene leads to overactivity of the Ras-MEK-MAPK (mitogen-activated protein kinase) signaling pathway. We therefore hypothesized that pharmaceutical inhibition of MEK-MAPK may be a beneficial therapeutic strategy.

Methods:

In vitro methods were used to demonstrate a role for the MEK inhibitor PD0325901 in promoting osteogenic differentiation in Nf1−/− calvarial osteoblasts. Local applications of rhBMP-2 and/or PD0325901 were then tested in a mouse model of NF1 tibial pseudarthrosis featuring localized double inactivation of the Nf1 gene in a fracture. Mice received no treatment, PD0325901 (10 mg/kg/day from two days before fracture to ten days after fracture), rhBMP-2 (10 μg), or a combination of rhBMP-2 and PD0325901.

Results:

Animals treated with the delivery vehicle alone, PD0325901, rhBMP-2, or the PD0325901+ rhBMP-2 combination showed union rates of 0%, 8%, 69%(p< 0.01), or 80%(p< 0.01), respectively, at twenty-one days after fracture. Mice treated with the rhBMP-2+ PD0325901 combination displayed a callus volume sixfold greater than the vehicle controls and twofold greater than the group receiving rhBMP-2 alone. Although MEK inhibition combined with rhBMP-2 led to increases in bone formation and union, the proportion of fibrous tissue in the callus was not significantly reduced.

Conclusions:

The data suggest that MEK inhibition can promote bone formation in combination with rhBMP-2 in the context of an NF1 pseudarthrosis. However, PD0325901 did not promote substantive bone anabolism in the absence of an exogenous anabolic stimulus and did not suppress fibrosis.

Clinical Relevance:

This study examines a signaling pathway-based approach to treating poor bone healing in a model of NF1 pseudarthrosis.