** Shares of CytomX Therapeutics CTMX.O rise 3.5% to $3.27 premarket

** Co says it has expanded its licensing agreement with Regeneron Pharmaceuticals REGN.O to develop next-generation bispecific cancer therapies in a collaboration that could be worth up to about $4 billion

** The partnership combines CytomX’s Probody platform with Regeneron's Veloci-Bi technology to create conditionally activated bispecific antibodies designed to target tumors while limiting damage to healthy tissue

** As part of the expansion, CytomX will receive a $37 million payment after Regeneron selects two additional drug targets

** Regeneron also secures option to choose up to six more targets under the collaboration

** Regeneron will lead and fund preclinical and clinical development as well as commercialization, while CytomX is eligible to receive milestone payments tied to development, regulatory approvals and sales, as well as tiered royalties on future products

** YTD, CTMX down 25.8% and REGN down 21.9%, as of last close