The US biotech confirmed it sent the response to the Food and Drug Administration (FDA) last night but did not share any more information.
Details of the clinical hold have not been made public.
In the Phase III study Multikine (Leukocyte Interleukin, Injection) is being assessed as a treatment for advanced primary head and neck cancer.
According to a Cel-Sci filing "about 860 patients were enrolled in that study at the end of July 2016."
A testing trial
Cel-Sci has encountered a number of problems with the study.
Originally, the program was being run by PharmaNet, a US contract research organisation (CRO) that was bought by inVentiv in 2011.
Cel-Sci dismissed inVentiv in 2013 subsequently accusing the CRO breach of contract and seeking $50m (€47m) in damages.
At the time, Cel-Sci said the move “was necessary since the patient enrolment in the study dropped off substantially following a takeover of Pharmanet by inVentiv which caused many of the members of InVentiv’s study team to leave inVentiv.”
UK contractor Ergomed was hired – along with Icon’s Aptiv unit – to replace inVentiv in April 2013.
Last year Ergomed said it would add additional trial sties in a bid to accelerate patient recruitment.
A Cel-Sci spokesman told us: "Ergomed continues to manage the clinical trial on behalf of CelSci and all patients are being followed up as planned."