Under the terms, Seagen bought 3.7 million Pieris shares at $3.51 each, the companies said in a statement.
At last check Pieris shares were 2% higher at $2.55. Seagen shares were trading 2.5% lower at $136.73.
Pieris shares have wavered on Thursday. The shares have traded as high as $2.78 and as low as $2.43.
In a collaboration the companies will jointly evaluate the safety and efficacy of Pieris's cinrebafusp alfa (PRS-343) and Seagen's Tuksya (tucatinib) as a combined treatment for gastric cancer.
The effort to combine cinrebafusp alfa with Tukysa in gastric cancer patients follows "the generation of compelling preclinical data," Pieris President and Chief Executive Stephen S. Yoder said in a statement.
"Preclinical data exploring the combination of cinrebafusp alfa and Tukysa are encouraging and support evaluating the combination" in a planned Phase 2 clinical trial, said Marjorie Green, a physician who is senior vice president for late-stage development at Seagen.
The companies have also amended their current immuno-oncology collaboration agreement to jointly develop and commercialize the second of as many as three products, they added.
Pieris, Boston, will have “an option to participate in co-promotion of that program in the U.S.” The company will also be “entitled to increased royalties from that program” if it chooses that option, the companies said.
Seagen, Bothell, Wash., would be solely responsible for developing and commercializing that program.