Eli Lilly/Amylin’s once-weekly Byetta could be slapped with a black-box warning on concerns the diabetes drug might put patients at an increased risk of developing thyroid cancer.

Alarm bells were raised after Curtis Rosebraugh, head of the US Food and Drug Administration’s Office of Drug Evaluation II, said in a memo posted to the agency’s website that there were similarities in data on intravenous dosing and the extended-release form of Byetta (exenetide) with that from rodent studies of Novo Nordisk’s long-acting Victoza (liraglutide), in which some of the animals developed thyroid cancers.

Victoza received the US green light at the beginning of the year as a once-daily human glucagon-like peptide-1 (GLP-1) analogue for type 2 diabetes after an extensive safety review, but the drug carries a black-box warning regarding the risk of thyroid c-cell tumours.

The move came on the back of preclinical tests showing that the drug caused such tumours in rodents and, while there have been no reported cases of medullary thyroid carcinoma in patients given Victoza, the FDA concluded that an associated risk cannot be ruled out.

It was also noted that this effect could well extend to the whole class of GLP-1 receptor agonists, and analysts fear that the comments in Rosenbraugh’s memo could be the first indication that once-weekly Byetta, which will be sold as Bydureon following its approval, may be required to carry the same boxed warning as Victoza.

“A relationship between Bydureon and human thyroid cancer has not been established, but cannot be ruled out,” a spokeswoman for Lilly told Bloomberg.

Last month, Bydureon’s path to market across the pond was delayed after the FDA issued a complete response letter for the drug, with requests raised primarily relating to the finalisation of the product labelling “with accompanying risk evaluation and mitigation strategy and clarification of existing manufacturing processes”.