Mechanisms limiting the long-term anabolic effects of teriparatide (PTH 1-34) on bone.

Teriparatide, (recombinant human PTH 1-34) is an Food and Drug Administration (FDA)-approved anabolic therapy for osteoporosis. In patients with severe bone loss, daily teriparatide injections increase osteoblastic activity and bone turnover, leading to net gain in bone mass, increased BMD, and significantly reduced fracture risk. However, after 6-12 mo of treatment, its anabolic effects, reflected by increased serum bone turnover markers and LS-BMD gain, tend to diminish. Despite various efforts to address this challenge, the underlying mechanisms have remained poorly understood. This review discusses the main mechanisms proposed to limit teriparatide's bone anabolic potential, including osteoprogenitor depletion, changes in bone remodeling dynamics, counter-regulatory molecules (eg, Wnt inhibitors), the influence of mechanical loading, and downstream signaling adaptations. Understanding these mechanisms is important for optimizing osteoporosis therapy and developing strategies to extend teriparatide's "anabolic window."