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A new type of osteoporosis drug being developed by Merck & Co. increased bone density over two years in postmenopausal women with mild osteoporosis, the company reported Tuesday.

The drug, known chemically as odanacatib, increased bone density in a midstage trial by about 5.5 percent in the lower spine, while a comparison group of women getting dummy pills on average had bone density decrease about 0.2 percent from its level at the start of the study.

Over the same two years, bone density in women getting the drug increased 3-4 percent at the hip and upper thigh bone, while those on dummy pills saw decreases of nearly 1 percent at both sites.

Art Santora, head of clinical research for musculoskeletal conditions at Merck, said existing treatments generally increase bone mass in the first year of treatment, then just maintain it, but odanacatib continued to increase bone mass in the second year.

The results were for the 50-milligram dose of odanacatib. Three lower doses also were tested, but women getting the lowest dose saw bone density decrease. About 400 women, aged 64 on average, got one of the four drug doses or a dummy pill weekly for two years in the study, plus calcium and vitamin D supplements.

Merck now is focusing on the 50-milligram dose and has begun an international, final-stage study of it that is to include about 17,000 women.

The new data were reported Tuesday at the American Society for Bone and Mineral Research conference in Montreal.

Slightly more than one-third of the women – in the group getting placebo as well as those getting odanacatib – had side effects including nausea, headache, muscle spasms and rash; about 8 percent getting the 50-milligram dose of the drug stopped taking it.

Santora said odanacatib works by inhibiting an enzyme called cathepsin K. The enzyme breaks down the collagen matrix that holds bone together, and the drug prevents the enzyme from breaking down calcium and protein deep in the bone, he said.

Osteoporosis drugs already on the market, which can reduce fracture risk by half, instead work by blocking the function of bone cells called osteoclasts that break down old bone, Santora said.

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Merck’s former blockbuster osteoporosis treatment, Fosamax, lost patent protection earlier this year, except for a version that includes high doses of vitamin D.