Teriparatide

Teriparatide is a recombinant peptide comprising the N-terminal 34 amino acids of human parathyroid hormone (PTH). It was FDA-approved in 2002 (brand name Forteo, Eli Lilly; biosimilars now available) for postmenopausal women with osteoporosis at high fracture risk, for men with primary or hypogonadal osteoporosis, and for men and women with glucocorticoid-induced osteoporosis. It is the first approved anabolic (bone-building) treatment for osteoporosis and carries the strongest fracture reduction evidence of any single osteoporosis agent.

Teriparatide carries a **black box warning for osteosarcoma** risk based on findings in Fischer 344 rats given supraphysiological doses over their lifetimes. This risk has not been demonstrated in humans at therapeutic doses after more than two decades of clinical use, but a 2-year cumulative lifetime dosing limit remains in place and use is contraindicated in patients with elevated baseline osteosarcoma risk (Paget's disease, prior skeletal radiation, metabolic bone disease other than osteoporosis, and pre-existing hypercalcemia).

Teriparatide binds PTH1R (PTH/PTHrP receptor), a class B GPCR expressed on osteoblasts, renal tubular cells, and other tissues. The pharmacological key is administration timing: intermittent daily subcutaneous injection produces a transient PTH1R pulse, which has opposite effects to continuous PTH elevation seen in primary hyperparathyroidism.

**Intermittent exposure (teriparatide dosing):** - Activates Wnt/β-catenin signaling → osteoblast differentiation and proliferation - Suppresses sclerostin and DKK1 → removes endogenous Wnt pathway inhibition - Inhibits osteoblast apoptosis (Bcl-2 upregulation) - Net result: increased bone formation markers (P1NP, ALP), increased trabecular bone volume, improved microarchitecture

**Continuous PTH elevation (hyperparathyroidism):** - Activates RANKL → osteoclast activation - Net result: bone resorption predominates

This time-dependent pharmacological duality is a canonical example of how receptor exposure pattern determines therapeutic outcome. Osteoclast activation does occur with teriparatide (bone resorption markers also rise), but the anabolic response predominates with the 20 mcg daily pulse.