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) --

NPS Pharmaceuticals


said Monday that 53% of patients with hypoparathyroidism responded to treatment with the company's experimental drug NPSP558 compared to a 2% placebo response rate, meeting the primary goal of a pivotal study.

Based on results from the positive study, NPS says it will seek U.S. approval of NPSP558 in 2012. NPS is expected to complete the U.S. approval filing for its first drug,

Gattex as a treatment for short bowel syndrome

, before the end of the year.

NPS shares closed Friday at $5.61.

Hypoparathyroidism is a rare endocrine disorder in which the body fails to produce enough parathyroid hormone to regulate levels of calcium and phosphorus. No replacement hormone therapy exists for hypoparathyroidism. Instead, patients are treated with large doses of oral calcium and vitamin D to reduce the severity of symptoms, although long-term use of these supplements has side effects, including kidney stones and kidney damage.

NPSP558 is a custom-engineered replica of human parathyroid hormone.

In the phase III study, 53% of patients responded to treatment with NPSP558 2% of placebo-treated patients. Response in the study was defined as a 50% or greater reduction in oral calcium and vitamin D therapy and a total serum calcium concentration that was normalized or maintained compared to baseline after 24 weeks of treatment.

After 24 weeks, 43% of patients treated with NPSP558 were able to achieve independence from vitamin D and calcium supplements compared to 5% for patients treated with placebo.

"Considering hypoparathyroidism is the only endocrine disorder for which we do not have an approved replacement hormone to treat the underlying condition, these data indicate that NPSP558 may offer a valuable option to achieve a physiological treatment and outcome by delivering the missing hormone," said Dr. Bart Clarke of the Mayo Clinic and an investigator in the NPSP558 phase III study, in a statement.

Thirteen of the 135 patients discontinued the study early, six in the NPSP558 arm and seven in the placebo arm. Overall, the incidence of serious adverse events was similar in both groups, NPS said, although detailed safety data were not disclosed.

Further analysis of the study continues.

NPS's partner,


, acquired by Japanese drug maker

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in September 2011, has marketed a similar version of NPSP558 outside the U.S. since 2006 for the treatment of osteoporosis in post-menopausal women at high risk of fractures.

If approved, NPSP558 could generate U.S. sales of $47 million in 2014 and $131 million in 2015, according to a forecast from Stifel Nicolaus analyst Maged Shenouda.

--Written by Adam Feuerstein in Boston.

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