Forteo — is this bone drug too good to be true?
by Dr. Susan E. Brown, PhD
Forteo™ (teriparatide) is a new and different drug on the scene as a treatment for osteoporosis. Other bone drugs, like Fosamax, Actonel, Boniva, or even estrogen work to halt bone breakdown. Forteo, on the other hand, works to increase new bone formation. Short-term studies, in fact, report a common 9% increase in lumbar spine bone density and a 2–3% increase in hip density accompanied by significant reductions in fracture incidence. Sound too good to be true? Sure does. So let’s take a second look and try to understand this new drug from what the manufacturer, Eli Lilly, tells us in their 2008 Forteo Product Monograph.
What exactly is Forteo?
Forteo is a man-made (well, E. coli-made) recombinant parathyroid hormone that has an amino acid sequence identical to part of the human parathyroid hormone (PTH). Forteo was approved by the US FDA in 2001 as an osteoporosis drug.
How does Forteo work?
In the body, natural parathyroid hormone serves to regulate bone metabolism in many ways. Chronic elevation of PTH, as in hyperparathyroidism, results in increased bone breakdown, a loss of calcium, and osteoporosis. On the other hand, daily injections of the PTH drug Forteo have the paradoxical effect of increasing bone mass and reducing osteoporosis. As scientists report, it seems to be that the pattern of exposure to parathyroid hormone determines its effect on the skeleton.
Through activation of various bone metabolic pathways, the PTH drug (Forteo) increases the number of active osteoblasts (bone-building cells), decreases the naturally programmed death of osteoblast cells, and recruits bone-lining cells as osteoblasts. This drug appears to act largely upon the bone-building osteoblast cells stimulating them to overactivity. Safety studies on this drug in rats, in fact, have shown findings of excessive new bone formation and bone cancer. Interestingly enough, the cancer this drug was found to cause in rat studies (osteosarcoma — I’ll say a little more about this shortly) is a cancer often associated with high osteoblast activity and rapid bone growth.
By how much does Forteo reduce fractures?
Reduction of spinal fractures
Among 1637 postmenopausal women with severe osteoporosis and one or more vertebral fractures at baseline, the drug produced a 9.3% absolute fracture risk reduction over 19 months.
Reduction of hip, pelvis, wrist, rib, humerus, and other fractures
Over 19 months in these same high risk women, Forteo reduced nonvertebral fractures by an absolute 2.9% reduction in fractures.
So, overall, the short-term studies on Forteo reported above show this drug to reduce fracture risk substantially more than the bisphosphonate drugs like Fosamax. In addition, Forteo in these studies increased bone density more than the bisphosphonates.
Just how much does Forteo increase bone density?
In this same 19-month study of high risk osteoporotic women, Forteo increased bone density by an average 9.7% in the spine and 2.6% in total hip, while the wrist lost bone density and total body mineral content remained the same.
Why are there just short-term studies on Forteo, and is it safe?
As the manufacturer reports, the longest studies on the safety and efficacy of Forteo were of only a two-year duration. Why are there only short-term studies on this new drug? There are only short-term studies because in the animal safety studies, Forteo was shown to cause a high incidence of osteosarcoma (a rare malignant, often fatal, bone tumor), as well as osteoblastoma (abnormal mass of tissue in bone) and osteoma (small benign bone lesions). As with all drug safety tests, the drug doses tested were from 3-, 20-, and at times, 60-fold higher than those used in human medications. Notably, bone tumors were observed at all Forteo doses, with the incidence reaching 40–50% in the higher-dose groups.
Given this major safety concern, human trials with this drug were terminated early and guidelines were set to limit the duration of its use in humans. In fact, as the manufacturer specifically states, “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.” Also, given the cancer-causing effect of the drug, the FDA required a “black box” label warning, clearly stating the increased cancer risk shown in rat studies.
Expensive, experimental, possibly cancer-causing, and inconvenient
As near as I can calculate, since its development, studies on this drug have included only 1943 patients. According to the manufacturer, a study of one year is considered “long term,” while the maximum length of any study was only two years. Thus, not only is Forteo relatively new, but it is also experimental and, I would say, inadequately tested. In addition, it is very expensive — the drug costs upwards of $600–$700 per month! Further, it is inconvenient to use as it must be given as a daily self-administered injection. Most importantly, since this drug has shown to cause cancer in rat safety studies, its use by humans is limited to two years or less. While there may be significant bone density gains while using Forteo, the research is clear that these gains are lost once the drug is stopped. To maintain bone density gains, antiresorptive drugs such as Fosamax must be used after stopping Forteo.
Is there a better way?
This is the question I constantly ask myself — is there a better way to increase bone strength and reduce needless osteoporotic fractures? Is there a way that does not run the risk of causing cancer, or lead to excessive new bone formation — a way that is not only safe, but also good for the entire body? Yes, in most cases there is another way, and that way is through a comprehensive, life-supporting bone-building program that enlivens natural human healing and the body’s regenerative capacity.
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Original Publication Date: 06/10/2009
Last Modified: 10/24/2013
Principal Author: Dr. Susan E. Brown, PhD