Doctors are putting pressure on the government to seal a legal loophole that allows pharmaceutical companies to license existing therapies as orphan drugs and then charge exorbitant prices for them.

In a letter to prime minister David Cameron and health secretary Andrew Lansley, published in the British Medical Journal, 20 consultants and a patient group claim that European legislation designed to encourage new research into rare diseases is being manipulated, with many drug companies merely modifying and licensing existing medicines as orphan drugs to obtain sole marketing rights and pricing freedoms.

For example, doctors have been using the drug 3,4-diaminopyridine for more than 20 years to treat two rare muscle diseases, on an unlicensed basis and at an annual cost of around £800 to £1,000 per patient. However, since BioMarin was issued with a licence to market the drug (Firdapse) throughout Europe, the price has rocketed to £40,000 to £70,000 per patient per year.

Although this is a more recent example, “massive price rises” have occurred for other orphan diseases for years, including N-carbamylglutamate for N-acetylglutamate synthetase deficiency and ibuprofen and indometacin for patent ductus arteriosus, the signatories point out, and stress that the unnecessary cost to the National Health Service is likely to be above £10 million a year.

In the current difficult economic climate and the growing need for austerity, it seems “vital to ensure that systems are in place to prevent excessive commercial profits being made at the expense of patients and public spending”, the letter notes. It calls for an urgent review by the Health Select Committee of the current situation, and notes that if evidence of artificially high pricing is uncovered, then the Office of Fair Trading should consider pursuing the issue.

In a separate BMJ investigation, Sam Richmond, a consultant neonatologist at Sunderland Royal Infirmary and a signatory of letter, commented: “If drug companies are undertaking research where nobody else was interested – and some are – then a monopoly may be justified. But if it’s a product already in use, they should clear off, or sell at a price comparable with the existing price.”