CytomX Therapeutics (CTMX) said Wednesday it is expanding a licensing agreement with Regeneron Pharmaceuticals (REGN) to develop conditionally activated bispecific cancer treatments.

CytomX said it will receive a $37 million target nomination payment for two newly chosen development programs.

Regeneron holds the right to select up to six more targets, bringing the overall potential milestone value of the broadened agreement to roughly $4 billion, CytomX said.

Shares CytomX were more than 5% higher premarket trading, while Regeneron was down 0.5%.