NOVATO -- Raptor Pharmaceutical Corp. (Nasdaq: RPTP) today said the U.S. Food and Drug Administration approved its drug designed for the treatment of nephropathic cystinosis, a rare and possibly life-threatening metabolic disorder.

It's the first FDA-approved product for Raptor, started in 2005 by former executives and researchers of Novato-based BioMarin Pharmaceutical, itself a developer of treatments for rare diseases.

The treatment, called Procysbi, comes in the form of delayed-release capsules for adults and children 6 years or older, according to Raptor (415-382-8111, The kidney disorder, which leads to irreversible tissue damage and multiorgan failure, blindness, muscle wasting and premature death, is usually diagnosed in infancy and requires lifelong therapy.

Raptor anticipates on a U.S. commercial launch by the end of the second quarter this year. The FDA approval triggers the second of two $25 million payments to Raptor under a loan agreement with HealthCare Royalty Partners, according to Raptor.

Raptor signed the loan agreement with HC Royalty in December and received the first $25 million payment upon closing. The loan is interest-only for the first two years.

In a phase 3 study, Raptor said the drug Procysbi led to a depletion of cystine, an amino acid needed to make natural antioxidants that fight off damaging free radicals, over a 12-hour dosing period. Studies have shown that sustained cystine depletion in patients may significantly delay disease progression, according to Raptor.

The drug, according to Raptor, is the culmination of early research funded by the patient advocacy group Cystinosis Research Foundation. The FDA approval of Procysbi was based on a new drug application with data from six clinical trials, including a multicenter randomized, active-controlled Phase 3 trial of 43 patients with nephropathic cystinosis and extension data from that trial.

Commonly reported adverse reactions to the drug were vomiting, abdominal pain, discomfort, headaches, nausea, diarrhea, anorexia/decreased appetite, breath odor, fatigue, dizziness, skin odor and rash.

Clinical studies are ongoing in children less than 6 years of age to evaluate the safety and efficacy of Procysbi. More than 90 percent of the cystinosis population is 6 years or older.

Raptor said it is committed "to the principle that no eligible U.S. patient with nephropathic cystinosis will be denied access to Procysbi based on inability to pay." Raptor intends to begin shipping Procysbi within six to eight weeks.

Raptor shares at the end of Nasdaq regular trading Tuesday were $6.90 a share, up more than 5 percent. The price was $6.75 in after-hours trading.