BioMarin Pharmaceutical ( BMRN) and its partner Serono ( SRA) said an experimental drug for treating a rare genetic disorder met the goals of a late-stage study. The drug, Phenoptin, is designed for patients with phenylketonuria, or PKU, a genetic condition that can cause brain damage, severe mental retardation, mental illness, seizures and tremors. Shares of BioMarin soared $2.55, or 20.5%, to $15 in after-hours trading. The stock had lost 49 cents to $12.45 in the regular session. Serono added 40 cents, or 2.2%, to $18.47. About 50,000 cases of PKU have been diagnosed in developed nations. The disease is caused by the deficiency of an enzyme that's required for metabolizing phenylalanine, an amino acid found in most protein-containing foods. If not enough of the enzyme is present, phenylalanine accumulates to abnormally high levels in the blood and brain, resulting in a variety of complications. BioMarin said data from a phase III study found that participants taking Phenoptin had a statistically significant reduction in their blood phenylalanine levels after six weeks of treatment compared with those receiving the placebo. Phenoptin received orphan drug designation to treat PKU from both the Food and Drug Administration and regulators in Europe. If Phenoptin becomes the first drug approved for treating the disease, it would have seven years of market exclusivity in the U.S. and 10 years in the European Union. The FDA has granted Phenoptin fast-track status, which is meant to hasten the review of drugs intended to treat serious or life-threatening conditions and that might address an unmet medical need. BioMarin and Serono expect to apply for the drug's approval in the U.S. and European Union in 2007. BioMarin has licensed the exclusive rights for Phenoptin outside of the U.S. and Japan to Serono. Separately, BioMarin said it's selling certain rights to its pediatric asthma drug Orapred. Privately held Alliant Pharmaceuticals will acquire the North American rights to the drug, and BioMarin will receive various payments and royalties. BioMarin will retain commercial rights to the drug outside of North America. "Orapred no longer fits well with our core business of developing and commercializing novel products for serious genetic and metabolic diseases," said BioMarin CEO Jean-Jacques Bienaime.