Teriparatide

What is Teriparatide?

Teriparatide (brand names FORTEO and BONSITY) is the recombinant 1-34 N-terminal fragment of human parathyroid hormone (PTH). It contains the biologically active region of full-length PTH (1-84) and binds the same PTH1 receptor with equivalent affinity. Approved by the FDA on November 26, 2002, it was the first anabolic osteoporosis therapy — meaning it builds new bone, in contrast to bisphosphonates and denosumab, which slow bone loss.

Teriparatide is produced in E. coli and administered as a once-daily subcutaneous injection from a multi-dose pen device.

Mechanism of Action

The key pharmacological insight is that the duration of PTH receptor activation determines the biological response:

• Continuous high PTH (as in hyperparathyroidism) → net bone resorption
• Intermittent brief PTH spikes (as with daily teriparatide injection) → net bone formation

A daily teriparatide injection produces a sharp serum PTH peak that lasts ~4 hours and then falls back to baseline. This intermittent stimulation:

• Activates osteoblast differentiation from lining cells and progenitors
• Reduces osteoblast apoptosis, extending the bone-forming life of each osteoblast
• Increases trabecular and cortical bone formation with a net positive bone balance

The result is meaningful gains in bone mineral density and reductions in fracture risk in patients with severe osteoporosis.

Clinical Evidence

Fracture Prevention Trial (NEJM 2001):

• 1,637 postmenopausal women with prior vertebral fracture randomized to teriparatide 20 µg, teriparatide 40 µg, or placebo for median 19 months
• Vertebral fracture risk reduced by 65–69%
• Non-vertebral fracture risk reduced by 35–53%
• Lumbar spine BMD increased 9% over placebo

Glucocorticoid-Induced Osteoporosis (NEJM 2007):

• 428 patients on chronic glucocorticoids randomized to teriparatide vs risedronate for 36 months
• Teriparatide produced significantly greater BMD gains and fewer new vertebral fractures

Approval History and Indications

• November 2002 — FDA approval for postmenopausal osteoporosis at high fracture risk, men with primary or hypogonadal osteoporosis at high risk, and glucocorticoid-induced osteoporosis
• 2017 — Bonsity (generic teriparatide) approved
• 2019 — First biosimilar (terivatide) approved

Place in Therapy

Teriparatide is reserved for patients with severe osteoporosis at high fracture risk because of the daily-injection regimen, cost, and a lifetime treatment-duration limit of 24 months (originally based on rat carcinogenicity findings; this limit was removed from the US label in 2020 but the typical clinical course remains 18-24 months).

It is typically followed by an antiresorptive agent (bisphosphonate, denosumab) to consolidate the BMD gains.

Distinction from Palopegteriparatide

Teriparatide and palopegteriparatide (Yorvipath) contain the same active 34-amino-acid peptide but produce opposite clinical effects:

Same peptide, different time-concentration profile, different therapeutic outcome.

Safety Profile

The most common adverse events are nausea, leg cramps, dizziness, and arthralgia. Hypercalcemia and orthostatic hypotension after the first few doses are typically transient.

A boxed warning for osteosarcoma was included in the original label based on rat carcinogenicity studies; this warning was removed in November 2020 after long-term observational data in humans showed no increased osteosarcoma risk over 15+ years of post-marketing use. The 24-month lifetime treatment limit was simultaneously removed.

Relationship to Abaloparatide

Abaloparatide (Tymlos) is a synthetic 34-amino-acid analogue of PTHrP (parathyroid hormone-related peptide), not native PTH. It binds the same PTH1 receptor but with selectivity for the receptor's transient (RG) conformation, which may produce stronger anabolic-to-resorptive effects. Both drugs are anabolic osteoporosis options; they are typically substituted, not combined.