Teriparatide

What it is

Teriparatide is the first 34 amino acids of human parathyroid hormone, the biologically active fragment. PTH's job is raising blood calcium, and one of the ways it does that is by pulling calcium out of bone.

Which makes it an odd choice for treating osteoporosis, and the resolution of that oddity is the most interesting thing about the drug.

The schedule inverts the effect

Chronic PTH elevation, as in hyperparathyroidism, causes bone loss. Intermittent PTH exposure causes bone formation.

The mechanism is a race between two cell populations. A brief daily pulse preferentially stimulates osteoblasts, the cells that build bone, and the signal clears before osteoclast activity catches up. Sustained exposure gives the osteoclasts time, and resorption dominates.

Teriparatide's one hour half-life is therefore not a limitation to be engineered away. It is the therapeutic design. A long-acting PTH analogue would cause the disease this one treats, and that is a rare and clean example of pharmacokinetics determining whether a drug helps or harms.

It also makes teriparatide genuinely anabolic. Bisphosphonates work by suppressing bone resorption, which preserves what exists. Teriparatide builds new bone, which is a different and better thing when there is not much left.

What the evidence shows

The Fracture Prevention Trial, published in the New England Journal of Medicine in 2001, randomised postmenopausal women with prior vertebral fractures to teriparatide or placebo. It reduced new vertebral fractures by roughly 65% and non-vertebral fragility fractures by about half, alongside substantial increases in bone mineral density.

The trial was stopped early, and the reason became the drug's defining regulatory story: osteosarcoma appeared in rats given high doses for most of their lifespan. That produced a boxed warning and a two year lifetime treatment limit.

The warning was removed in 2020. Two decades of post-marketing surveillance and a long-term observational study found no increase in osteosarcoma in humans, and the rat finding is now understood as species-specific, involving continuous exposure over a rodent's entire skeletal growth period.

Practical notes

• Treatment is a finite course. Roughly two years, after which an antiresorptive is usually started to hold the gains. Stopping without follow-on therapy loses much of the benefit.
• Sequence matters. Giving teriparatide after a bisphosphonate blunts its anabolic response. Anabolic first, then antiresorptive, is the better-supported order.
• Take the first dose sitting or lying down. Orthostatic hypotension is most likely in the first few administrations and then generally settles.

Side effects and warning signs

Commonly reported

• Nausea
• Leg cramps
• Dizziness and orthostatic hypotension, most likely after the first few doses
• Transient rises in serum calcium
• Injection-site reactions

Stop and get medical help

• Pre-existing hypercalcaemia, hyperparathyroidism or Paget's disease of bone
• Prior radiation therapy involving the skeleton
• Persistent hypercalcaemia symptoms: confusion, excessive thirst, constipation, abdominal pain
• Unexplained bone pain, given the osteosarcoma signal that shaped this drug's history

Sources

1. Neer et al., Effect of parathyroid hormone (1-34) on fractures and bone mineral density in postmenopausal women with osteoporosis, NEJM 2001trial
2. Effects of parathyroid hormone, alendronate, or both in men with osteoporosis, NEJM 2003trial

From the community

Discussion for this compound concentrates in r/osteoporosis.

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