Bone healing response to systemic bisphosphonate-prostaglandin E2 receptor 4 agonist treatment in female rats with a critical-size femoral segmental defect

Despite the wide body of research into prevention and treatment of nonunion, current bone fracture therapies remain suboptimal in their efficacy. Previous animal studies show that MES-1022, a bone-targeted prodrug that activates the prostaglandin E2 receptor EP4, stimulates bone healing when applied locally in uneventful defects. Here we investigated the healing capacity of systemically administered MES-1022 in a rat femoral critical size segmental defect. Ten-week-old female Sprague-Dawley rats (n = 8/group) underwent a 5 mm osteotomy of the left femoral midshaft, stabilized by a unilateral external fixator. Rats received weekly subcutaneous injections of MES-1022 at 5 mg/kg (MES1022-Hi), 1.7 mg/kg (MES1022-Lo), or Vehicle without a defect site scaffold. Serum bone markers and open field activity were measured pre-osteotomy and throughout the study. Rats were sacrificed after 12 weeks and osteotomized femora were imaged via microcomputed tomography (microCT) followed by histology and immunohistochemistry to assess healing. Complete bridging of the defect occurred in one rat from the MES1022-Hi group and zero from MES1022-Lo and Vehicle groups. However, healing outcomes in both MES-1022 groups for bone volume fraction, bone volume, bridging score, callus tissue composition, callus blood vessel density, P1NP levels, TRAcP-5b levels, and physical activity did not differ from Vehicle. Fracture callus osteoclast density and spleen weight were increased in MES1022-Hi rats relative to Vehicle. Overall, systemic administration of MES-1022 alone may not suffice for treatment of large segmental bone defects. Additional studies are needed to determine whether systemic MES-1022 is a useful therapeutic in conjunction with local scaffolds like bone graft substitutes.