Merck & Co has suffered a setback with the news that regulators in the USA have rejected its bone cancer drug ridaforolimus.

The US Food and Drug Administration has issued a complete response letter for ridaforolimus, an investigational oral mTOR inhibitor developed as maintenance therapy  for metastatic soft tissue or bone sarcoma. The drug, licensed from Ariad Pharmaceuticals, is specifically for patients who have stable disease or better after four or more cycles of chemotherapy.

However, the FDA says it cannot approve the application in its present form, and "that additional clinical trial(s) would need to be conducted to further assess safety and efficacy" of the drug, which is also known as Taltorvic. The rejection comes as no surprise given that the agency's Oncologic Drugs Advisory Committee voted 13 to one against approval in March.

That negative opinion came after Merck presented data which showed that patients on ridaforolimus had a median progression-free survival of 17.7 weeks compared with 14.6 weeks for those on placebo. However the FDA panel believed that no significant difference was observed between the groups in terms of overall survival and although patients did experience a longer disease-free period before their cancer returned when receiving the oral drug, the delay was not significant. Furthermore, they were also concerned about side effects.

Despite the CRL, Eric Rubin, head of clinical research oncology, said "Merck remains confident in the potential" of the drug "for an indication where patients have limited options". He added that the firm "remains committed to bringing forward this promising therapy…and look forward to further discussions with the FDA".

Merck is in ongoing discussions with European regulators for ridaforolimus, which is also being studied in combination with other mechanisms in several tumour types.