Research-based drugmakers in the US currently have a record 460 medicines in late-stage development for the prevention or treatment of rare diseases, says a new industry report.

The biggest number of orphan drugs now in clinical trials or awaiting Food and Drug Administration approval is the 107 targeted at rarer cancers, according to the report, which is published by the Pharmaceutical Research and Manufacturers of America (PhRMA).

In addition, 79 orphan drugs are now in development for cancers of the blood, while 67 are for genetic disorders such as cystic fibrosis, 37 for neurological disorders including multiple sclerosis and muscular dystrophy and 31 each are being developed for skin cancers and infectious diseases, the latter group including conditions such as anthrax and West Nile virus. 

20 products on the list are for use in transplants, while 18 are being developed to treat autoimmune disorders, 14 are for respiratory conditions, 12 for blood disorders and 11 for eye diseases. Cancer-related conditions and gastrointestinal disorders account for 10 products each, while six are in development for cardiovascular diseases and five for growth disorders. PhRMA lists another 37 products as being in development for "other" rare conditions.

Solid tumours of the liver and thyroid, cancer of the blood and melanoma account for more than one-third of all rare-disease drugs currently in development, it says.

The US National Institutes of Health (NIH) lists 7,000 diseases as "rare," and notes that about half of these affect children. The government currently defines a rare or orphan condition is as one that affects fewer than 200,000 Americans, with about 80% of such conditions having fewer than 6,000 patients. Nevertheless, taken together, rare diseases are common, affecting 25-50 million Americans, says PhRMA.

A comparison of the industry association's three previous reports on medicines in development for rare diseases reveals a steady increase in research bringing new medicines to the clinical trial stage and beyond; PhRMA's first report on the subject, in 1989, found 133 products in development, rising to 176 in 1991, 303 in 2007 and 460 today, it says.

Researchers attribute the increases partly to progress made in basic science. Adds PhRMA chief executive John Castellani: "biopharmaceutical research is entering an exciting new era, with a growing understanding of the genome. Scientific advances have given researchers new tools to explore more rare diseases, which are often more complex than common diseases." 

Also, the US Orphan Drug Act, which was passed in January 1983, provides some incentives for research into rare diseases such as longer periods of market exclusivity. Since the Act was passed, more than 350 orphan drugs have been approved in the US, compared to fewer than 10 during the 1970s and, as of January 24, 2011, the FDA had designated a total of 2,313 medicines as orphan drugs, says PhRMA. 

However, the group also notes that the development of orphan products is particularly challenging; for instance, it can be difficult to find sufficient patients for clinical trials and enough medical experts to conduct clinical trials and sit on review boards.

Moreover, balancing access to a medicine against safety issues can be particularly difficult when a rare disease, left untreated, has devastating or fatal consequences, it adds. For example, a medicine that can help control infantile spasms, in which young patients can suffer hundreds of seizures a day, also carries the risk of damaging peripheral vision.

• February 28 is the fourth annual worldwide Rare Disease Day, an awareness-raising event coordinated by EURORDIS - the European alliance of organizations representing patients with rare diseases - at the international level and by alliances of patient organizations at national levels.