Effect of the antineoplastic agent methotrexate on experimental heterotopic new bone formation in rats

Heterotopic new bone formation was induced by implanting pieces of demineralized bone matrix in the abdominal wall of 22 growing Sprague-Dawley rats. The animals were divided into three groups and were given, 24 hr after initiation of the bone induction process, a single i.v. injection of methotrexate, 100 or 250 mg/kg body weight, or placebo, followed after 2 hr by leucovorin rescue. A slight and transient arrest in weight gain was noted in the methotrexate-treated animals. New bone formation during 3 weeks after implantation was analyzed by the amount of ash in implants, and as a measure of bone formation at the end of the experiment, short-time incorporation of 45Ca and [3H]proline was used. The ash content of implants was reduced by 56 and 68% in the two methotrexate groups. Uptake of both nucleotides was also greatly reduced in heterotopic bone, whereas metaphysial and diaphysial tibia and teeth were not affected. The results indicate a pronounced inhibition of methotrexate on bone induction, persisting for at least 3 weeks. Methotrexate reduces the bone-forming potential, with possible consequences for the success of limb-saving surgery and fracture healing.