Merck & Co has halted a late-stage trial of its closely-watched compound vintafolide in ovarian cancer, news which has seen partner Endocyte's stock collapse 62%.

The US major noted that an interim futility analysis by a data safety monitoring board has recommended that a Phase III study be stopped because vintafolide did not demonstrate efficacy regarding progression-free survival in patients with platinum-resistant ovarian cancer. The trial was evaluating vintafolide in combination with pegylated liposomal doxorubicin compared to the latter plus placebo.

The trial being pulled comes just over a month after the European Medicines Agency's Committee for Medicinal Products for Human Use issued a positive opinion for a conditional approval of the drug, to be marketed as Vynfinit for the treatment of women with a sub-type of platinum-resistant ovarian cancer for which there are limited approved treatments. The CHMP also recommended two companion diagnostic medicines, Folcepri (etarfolatide) and Neocepri (folic acid) , that would help identify patients who may benefit from the drug, though it seems fair to assume that conditional approval will now be revisited.

The failure is a blow for Merck but the damage has been severe to Endocyte. Chief executive Ron Ellis said the firm is reviewing and validating the data in collaboration with Merck "in order to gain a better understanding of the interim trial results and make our formal decision regarding the execution of the trial".

He added that the DSMB did not identify any safety concerns, noting that the companies are monitoring data from a Phase IIb trial of vintafolide in non-small cell lung cancer, particularly overall survival results, "which as previously announced, demonstrated initially positive trends in favour of the combination therapy arm".

Endocyte said that late-stage data on vintafolide in NSCLC should be ready by the end of the year, and chief financial officer Michael Sherman noted that Merck will determine and be financially responsible for the ongoing development of vintafolide. Some observers believe the axe could soon fall on the compound completely.