Aclasta, a drug for the bone disorder Paget’s disease developed by Swiss drugmaker Novartis, is facing another delay in the USA after the Food and Drug Administration asked for more clinical data on the product.

The FDA issued an ‘approvable’ letter for Aclasta (zoledronic acid), saying that it wanted to see additional data from an ongoing clinical trials programme before it would give Novartis a green light.

This is the second time the agency has asked for more information on Aclasta via an approvable letter, the first being in March 2005. Novartis said it now hopes to get approval for the bisphosphonate drug before the end of 2006.

Novartis has had more luck bringing Aclasta to the market outside the USA. Last April, it won approval for the product in 41 countries, including all 25 countries in the European Union. It is already on launched in some markets, including Germany.

Novartis is also developing the product for osteoporosis, and said it hopes to submit applications for this indication in the USA and Europe next year. The FDA is asking for clinical data from pivotal trials in this indication, said Novartis.

Last year, analysts at Goldman Sachs said that Novartis' zoledronic acid franchise - which also includes Zometa for bone metastases - could achieve peak sales of nearly $3 billion, providing Aclasta wins approval in the US market and for the follow-up osteoporosis indication.

Paget's disease is a chronic skeletal disorder that can cause pain, fractures and deformities, seriously impeding patients' ability to perform routine activities of daily living. It is the second most common metabolic bone disease after osteoporosis.

Aclasta will be delivered to Paget's disease patients in a single intravenous dose; currently, other bisphosphonates used to treat the disease are dosed orally for two months, and have side effects and complex dosing instructions that make compliance with treatment difficult.