Genzyme (NASDAQ:GENZ) reported first-quarter earnings yesterday, and several of its larger products seem to be doing very well. Cerezyme, Fabrazyme, and Renagel all showed solid growth over the same period last year. However, Genzyme's newest drug, Aldurazyme, is on the verge of becoming a disappointment.

Aldurazyme was developed by BioMarin Pharmaceutical (NASDAQ:BMRN). In 1998, Genzyme and BioMarin formed a joint venture in which Genzyme would be responsible for worldwide sales and marketing. Aldurazyme was approved for treating the genetic, debilitating illness mucopolysaccharidosis I (MPS I) in both the U.S. and Europe in the middle of last year. With three full quarters of sales complete, we can start to assess the product's launch.

First-quarter sales were $7.3 million, compared with $6.7 million in the fourth quarter of last year. This was not received well by BioMarin's investors, and its stock fell 4% following Genzyme's release. The revenue results are causing some concern that the 2004 sales guidance of $40 million to $45 million will not be met. If the sequential growth in the second quarter is as anemic as it was in the past quarter, there will be real cause for concern.

While Aldurazyme is clearly not a major drug for Genzyme at this time, presumably, the firm became a partner in the product due to its sales potential. Several thousand patients worldwide have MPS I, and at an annual cost of treatment at just under $200,000 per patient, the overall market for this drug is worth close to $500 million. Ideally, Aldurazyme would grow to become an important product for both Genzyme and BioMarin. The first-quarter sales figure, however, has raised some doubts that this will happen. Unless sales growth starts to pick up in the coming quarters, Aldurazyme may remain a minor product. While it is not yet time to panic, this is a situation that bears watching.

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Fool contributor Charly Travers owns shares of BioMarin Pharmaceutical.