Merck & Co has called a halt to a late-stage trial of its vintafolide candidate, causing shares of development partner Endocyte to go into free-fall.

The data and safety monitoring board (DSMB) for the trial recommended that research be halted after a planned look at the data suggested that vintafolide was unable to improve progression-free survival (PFS) in patients with platinum-resistant ovarian cancer.

No safety issues were identified in the analysis of the PROCEED trial, but Endocyte and Merck have suspended screening and enrolment while they examine the implications of the finding.

PROCEED compared the combination of vintafolide and pegylated liposomal doxorubicin to a combination of doxorubicin and placebo.

"This is obviously not the outcome we expected," said Endocyte chief executive Ron Ellis on a conference call, adding that the company is "working with Merck to understand the data."

The results come shortly after vintafolide was recommended for approval in the EU by the European Medicines Agency's Committee for Medicinal Products for Human Use (CHMP) under the Vynfinit trade name, along with two companion diagnostics Folcepri (etarfolatide) and Neocepri (folic acid).

The CHMP's positive opinion was for a conditional approval, contingent on data from the phase III trials, so it seems likely that the drug will not now be given marketing authorisation.

Endocyte and Merck are still waiting for data from a phase IIb trial of vintafolide in non-small cell lung cancer (NSCLC), called TARGET, which met its primary PFS target and is due to generate overall survival data in the coming months.

The two companies were also planning to start a trial of vintafolide in triple-negative breast cancer later this year, although analysts have pointed out that Merck has full responsibility for the programme so may decide to walk away from the project.

If vintafolide is dropped, Endocyte will have to refocus its efforts on EC1456, a cancer drug for folate receptor-positive cancers in phase I testing. Merck licensed rights to vintafolide in 2012 in a deal valued at up to $1bn, including $120m upfront.

Shares in Endocyte are currently hovering above $6.50, having traded as high as $33.70 in the last 12 months.

