A new joint development program testing Eisai Co. Ltd.'s Lenvima (lenvatinib mesylate), an oral tyrosine kinase inhibitor, both alone and together with Merck & Co. Inc.'s anti-PD-1 therapy, Keytruda (pembrolizumab), across a dozen cancers could yield a multibillion-dollar payout for the Tokyo-based company while opening up new markets for both partners. The far-reaching deal completes a triplet of immuno-oncology-focused bets by Merck following earlier tie-ups with Astrazeneca plc last year and Ablynx NV in 2015.