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A little over 10 years ago Forteo (terioparatide) emerged as a different kind of drug to treat osteoporosis. Instead of halting bone breakdown, it stimulates bone production. But safety statistics for the drug are very hard to follow.

Forteo, the brand name for the generic drug teriparatide, offers a different approach to treating osteoporosis. Unlike Actonel, Boniva, or Fosamax, estrogen replacement therapy or SERMs, Forteo is a synthetic version of the hormone the body produces to stimulate the production of new bone.

There are convincing results from clinical trials that suggest that this new recombinant hormone, produced by genetically modified bacteria in a factory but almost identical to the hormone in the human body, really works. Clinical trials found that it gave users a two to three percent increase in bone mineral density in the hip and a nine percent increase in bone mineral density in the spine, with a corresponding decrease in fracture rates. But is it really worth the average retail price of $3,520.54 you will have to plunk down every month at the pharmacy if your insurance does not cover it? And is Forteo really safe?

What is Forteo?

Forteo is a synthetic version of human parathyroid hormone. It has an amino acid sequence that is identical to the biologically active part of the human hormone. It is manufactured in the lab with the help of genetically modified E. coli bacteria that have been bioengineered to produce the hormone.

How does Forteo work?

Parathyroid hormone regulates the flow of calcium in and out of bones. When parathyroid hormone levels are high, calcium flows out of bones into the bloodstream. When parathyroid hormone levels are low, calcium flows out of the bloodstream into bones.

Since human parathyroid hormone is associated with removing calcium from bone, it seems odd that Forteo is prescribed to help demineralized bones absorb calcium to build new bone. But the paradoxical effect of daily injections of Forteo is that bones absorb more calcium and build new bone.

Forteo increases the number of the bone-building cells known as osteoblasts. It slows down the rate at which they undergo apoptosis, the process of cellular death that removes them to make way for other kinds of cells in the bone. 

Better than bisphosphonates like Fosamax​

Just how well does Forteo work?

  • In a clinical study sponsored by Eli Lilly, the maker of the medication, among 1637 women who had had at least one spine fracture and who had been diagnosed with severe osteoporosis when they started taking the drug, there was a 9.3 percent reduction in the risk of fractures over the next 19 months.
  • Also in the same study, using Forteo resulted in a 2.9 percent reduction in the rate of fractures in the same group of women.

In statistical terms, there is high certainty that Forteo improves bone health. But in practical terms, it may help to understand what a "9.3 percent reduction the risk of spine fractures" means. There are limited data to work with, but women with severe osteoporosis have about a 0.3 percent risk of death due to broken hip over a 19-month period. If they take Forteo, then they have about a 0.27 percent risk of death from hip fracture over the same period. That's a 0.03 percent reduction in risk of death for taking $66,890.26 worth of the medication.

OK, if Forteo reduces my chances of death from a broken hip by 3 in 10,000 over the next couple of years, and if I somehow have insurance that gives me a $4 month copay, is still a good idea to take it?

Forteo increases risk of certain kinds of cancer

We don't really know how much Forteo increases the risk of bone cancer if it is used long-term. That is because it has never been studied long-term. And that's because safety studies with animals found higher doses caused cancer so the maker of the drug explicitly warns that it is not for long-term use.

When drug companies use lab rats for safety testing, they deliberately give the animals much higher doses of the tested drug (adjusted for the fact that they have much smaller bodies than humans) to see what would happen if it were overdosed. The safety testing for Forteo involved giving animals three, 20, and even 60 times more of the drug than would be prescribed. At higher doses, Forteo was found to cause osteomas (small, benign bone lesions), osteoblastomas (abnormal accumulation of osteoblasts in bone), and osteosarcomas (malignant, potentially fatal, cancers of bone). At higher doses, 40 to 50 percent of test animals developed bone tumors.

That wasn't a good sign for the long-term safety of Forteo. The US FDA required a "black box" warning for the drug, and the product literature includes the statement “The safety and efficacy of Forteo have not been evaluated beyond two years (median 19 months in women and 10 months in men). Consequently, the maximum lifetime exposure to Forteo for an individual patient is 18 months.”

Should you use Forteo?

You are not a lab rat. You are not going to take three, 20, or 60 times the recommended dose of the drug. (You wouldn't want to inject $200,000 worth of the drug every month, would you?) The first eight years of reporting to the FDA have found that people who use Forteo for no more than 18 months have about a 400 percent greater risk of the potentially deadly form of bone cancer called osteosarcoma. But let's put that in perspective. Your absolute risk of becoming a cancer statistic due to taking Forteo is one in 185,000. That figure is based on reports from cancer registries for nine years from 2009 to 2018.

Forteo's worst side effect is scary, and deadly, but very, very unlikely. You are far more likely to be bothered by side effects such as  pain, headache, nausea, dizziness, and depression. Some people will have a problem with the fact that Forteo is taken by injection that you have to give yourself every day. But if your doctor recommends Forteo, and you can pay for it, take it. Just be sure to take it exactly as prescribed, only as long as it is prescribed (no more than 18 months in your lifetime), and to follow up with conventional osteoporosis treatments to preserve the bone you have gained.

  • Dempster DW, Zhou H, Recker RR, Brown JP, Recknor CP, Lewiecki EM, Miller PD, Rao SD, Kendler DL, Lindsay R, Krege JH, Alam J, Taylor KA, Melby TE, Ruff VA. Remodeling- and modeling-based bone formation with teriparatide versus denosumab: a longitudinal analysis from baseline to 3 months in the AVA study. J Bone Miner Res. 2018. 33:298–306. doi: 10.1002/jbmr.3309.
  • Gilsenan A, Harding A, Kellier-Steele N, Harris D, Midkiff K, Andrews E. The Forteo Patient Registry linkage to multiple state cancer registries: study design and results from the first 8 years. Osteoporos Int. 2018 Oct. 29(10):2335-2343. doi: 10.1007/s00198-018-4604-8. Epub 2018 Jul 5. PMID: 29978254.
  • Vahle JL, Sato M, Long GG, Young JK, Francis PC, Engelhardt JA, et al. Skeletal changes in rats given daily subcutaneous injections of recombinant human parathyroid hormone (1–34) for 2 years and relevance to human safety. Toxicol Pathol. 2002.30:312–321. doi: 10.1080/01926230252929882.
  • Photo courtesy of SteadyHealth

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