Genzyme cut its forecast after saying U.S. regulators decided that the Pompe disease treatment Myozyme produced at the 160-liter bioreactor scale and the same drug made at the 2,000-liter scale should be classified as two different products.

The company has domestic approval for Myozyme at the 160-liter scale, and it was hoping to secure clearance for the larger scale, which is already approved in more than 40 countries. However, the company said that because there are differences in the carbohydrate structures of the molecules, the Food and Drug Administration considers them different products and will require a separate biologics license application for 2,000-liter manufacturing.

Myozyme is the only treatment for Pompe disease, a severe and progressively debilitating inherited neuromuscular disorder that affects a very small number of people throughout the world.

Genzyme said the FDA suggested it begin its new application process by submitting results from a late-onset treatment study, which is complete. The company now believes the 2,000-liter Myozyme will be available in the U.S. in the first quarter of 2009.

Because of the delay, Genzyme expects Myozyme sales of approximately $275 million to $285 million in 2008, down from its previous estimate of $320 million to $330 million. The company is now predicting an adjusted profit of $3.90 a share for this year, compared with $4 a share previously expected, and GAAP earnings of $2.65 a share, also down 10 cents from its prior guidance.

Wall Street had anticipated $320 million in sales of Myozyme for the year, and analysts surveyed by Thomson Financial had expected a profit of $4.01 a share in 2008.

Genzyme will report its quarterly results Wednesday before the market opens.

Shares of the Cambridge, Mass., company fell 65 cents, or 0.9%, to $74.09 in regular trading. After the close, Genzyme dropped 3% to $71.90.

Genzyme's decline came on a day when other big biotechs saw mixed trading. Amgen ( AMGN - Get Report) and Genentech fell 1%, while Amylin was lower by 10.3%.

On the winning side, Vertex ( VRTX - Get Report) rose 2.2% ahead of its earnings, and ImClone ( IMCL) was up 5.2%. Human Genome Sciences advanced 2.5%, and Gilead ( GILD - Get Report) tacked on 0.7%.

The Amex Biotechnology Index eased 0.6%.