The agency’s founders CEO Scott Clark, MD Oliver Parsons and director Moira Gitsham, along with London MD Stéphanie Bunten, are understood to have given notice of their intention to leave having failed to reach a resolution with agency owner Huntsworth Health about their future involvement.
Tonic was sold to Huntsworth Health in 2009 and the earn-out period ended in July 2013.
The current plan will see the four staff remain with the agency until January 2015 and Huntsworth said they will continue to actively work in the business.
They are not understood to have made plans for their next moves.
Tonic Life's London deputy MD Tamsin Tierney will continue to work with president of Tonic North America Maryellen Royle, and Huntsworth Health VP strategic development Annabelle Sandeman will support the London operation.
Huntsworth said a search has already been initiated for a new Tonic Group CEO and other senior positions.
Former Tonic Life MD and then Huntsworth Health MD for European integration Neil Flash left to take up the role of UK MD at global healthcare agency dna Medical Communications at the start of this year.