Bone Abstracts

Introduction: Prostaglandin E₂ has bone-anabolic effects through EP4 receptor but its clinical utility is hindered by gastrointestinal side effects. To avoid these side effects, EP4 agonists (EP4a) were covalently linked to the bisphosphonate alendronate (ALN) to create two ALN-EP4a conjugate drugs, C1 and C2. When administered systemically, C1 and C2 will be target delivered to bone through ALN, where local hydrolytic enzymes liberate EP4a from ALN to exert bone anabolic effects. Although C1 and C2 both have the same ALN and EP4a components, C1 has a short linker between the two components while C2 does not, making C1 more labile in vitro. Here we seek to characterize and compare effects of C1/C2 in a curative in vivo study. We hypothesize that C1 and C2 show differential levels of bone anabolic effects due to presence or absence of the linker.

Methods: Three-month-old female Sprague–Dawley rats were ovariectomized (OVX) or sham operated, and allowed to lose bone for 3 months. Animals were then treated via tail–vein injections for 3 months and sacrificed at 9 months. Seven treatment groups were: C1-L (5 mg/kg biweekly), C1-H (5 mg/kg weekly), C2-L (15 mg/kg monthly), C2-H (15 mg/kg biweekly), vehicle for OVX and sham control (PBS biweekly), and ALN/EP4a mixture without conjugation (0.75 mg/kg each, biweekly).

Results: MicroCT showed C1 significantly increased vertebral vBMD and trabecular bone volume compared to OVX controls but not in C2 treated animals. Mechanical testing of C1 vertebrates and femurs revealed significant improvement in load bearing abilities compared to OVX but not in C2 treated animals. Undecalcified histomorphometry of proximal tibial metaphysis showed C2 treatment had no effect while C1 treatment significantly increased bone formation compared to sham.

Conclusions: C1 form of ALN-EP4a conjugate drug showed significant bone anabolic effects while C2 conjugate did not, possibly due to slow cleavage of EP4a from ALN of the C2 conjugate in the local bone environment.