Celgene Corp has halted a late-stage study evaluating Revlimid for a rare type of leukaemia after an imbalance was observed in the number of deaths in elderly patients treated with the drug versus patients on chemotherapy.

Specifically, the US firm will discontinue treatment with Revlimid (lenalidomide) in the Phase III ORIGIN trial, which enrolled 450 patients in over 100 sites in 26 countries. The study was evaluating Revlimid versus chlorambucil as single agent in patients aged over 65 with B-cell chronic lymphocytic leukaemia (CLL) with comorbidities that precluded treatment with more aggressive standard chemo-immunotherapies. The majority of them were suffering from diseases such as diabetes, congestive heart failure, renal impairment and elevated bilirubin count.

Data revealed 34 deaths out of 210 patients in the lenalidomide arm compared to 18 deaths out of 211 patients in the chlorambucil arm. The US Food and Drug Administration placed the study on clinical hold last week and Celgene notes that "no specific causality for this imbalance has been identified to date".

Celgene shares fell 2.6% to $133.05 on the news, a rare blow for the stock which has soared almost 70% this year. Revlimid is comfortably the biggest earner for the company and is best-known as a treatment, in combination with dexamethasone, for multiple myeloma. It is also approved in the USA for mantle cell lymphoma and transfusion-dependent anaemia due to low- or intermediate-1–risk myelodysplastic syndromes (MDS).

The company added that all other Celgene-sponsored CLL trials with lenalidomide are continuing in accordance with their respective protocols.