CRANBURY, NJ ( TheStreet) -- An experimental therapy from Amicus Therapeutics ( FOLD) and GlaxoSmithKline ( GSK) failed to improve kidney response in Fabry disease patients compared to placebo in a late-stage study, the companies disclosed Wednesday night.

The negative study result is a significant setback for the drug, known as Amigal, and will raise more questions about the viability of Amicus' "pharmacological chaperone" technology. A similar Amicus drug for Gaucher disease failed in 2009.

Amicus shares closed Wednesday at $5.77 but fell 55% to $2.60 in after-hours trading.

Fabry is a rare, inherited disease in which a genetic mutation stops an enzyme from breaking down a fatty substance known as globotriaosylceramide, or GL3. The buildup of GL3 in blood vessels throughout the body causes severe damage to kidneys, heart, brain, and other organ systems.

The phase III study enrolled Fabry patients with severe disease, meaning they have high levels of GL3 built up in their kidneys. The patients were randomized in a blinded fashion to treatment with Amigal or a placebo for six months. The study's primary endpoint compared response to Amigal versus placebo at six months, with response defined as a 50% reduction in GL3 levels measured via a kidney biopsy.

After six months, 13 of 32, or 41%, of Amigal-treated patients demonstrated a kidney response compared to 9 of 32, or 28%, of the placebo-treated patients. The difference, while favoring Amigal, was not statistically significant, which is why the study failed.

Amicus and Glaxo provided little explanation in their joint announcement Wednesday night to explain the negative results. The 28% placebo response was unusually high while the 41% of Amigal patients who responded was lower than the results observed in previous studies.

Moreover, the median reduction of GL3 in the kidneys of placebo patients was just 6% from baseline through six months, which seems very low given 28% of placebo patients demonstrated a kidney response per the study's definition.

By comparison, the median reduction of GL3 in the kidneys of Amigal patients was 41%.

Amicus and Glaxo chose not to hold a conference call Wednesday night to discuss the study results.

Kidney response rates from the study after 12 months are expected in the first half of 2013. A separate phase III study investigating the combination of Amigal and Fabrazyme, an currently approved therapy for Fabry disease from the Genzyme unit of Sanofi ( SNY), will report results in 2014.

-- Reported by Adam Feuerstein in Boston.

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