CAMBRIDGE, Mass. (AP) _ Drug developer Transkaryotic Therapies Inc. announced on Monday that it is abandoning efforts to win federal approval for the only drug that it markets, sending shares plummeting in after-hours trading.

The company has been seeking U.S. Food and Drug Administration approval for Replagal, a treatment for the rare genetic disorder Fabry disease. But the company said it is unwilling to conduct a head-to-head trial comparing Replagal to another treatment for Fabry disease that already has FDA approval.

Genzyme Corp.'s drug, known as Fabrazyme, was approved by the FDA in April and given seven years market exclusivity.

``We always characterized this initiative as one that faced significant challenges and one that was not a necessary part of our business plan,'' TKT president and chief executive Michael J. Astrue said.

Shares of TKT fell nearly 10 percent, or $1.65, in after-hours trading on the Nasdaq Stock Market after closing at $17.05 in the regular session, up 55 cents.

Cambridge, Mass.-based TKT will continue to market and sell Replagal in the 27 countries where it has already been approved, mostly in the European Union. The company will also continue to supply Replagal to U.S. patients for clinical trials.

Fabry disease is a rare inherited genetic fat-storage disorder caused by deficiency of an enzyme involved in the digestion of lipids. The disease, which primarily affects men, can cause pain in the extremities and lead to potentially fatal complications. Only about 5,000 people worldwide have the disease.

TKT also announced on Monday that it expects fourth quarter sales of Replagal to roughly equal the $15.2 million in third-quarter sales.

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On the Web: http://www.tktx.com.