Eli Lilly and Zymeworks said Wednesday they are expanding a cancer drug development partnership.

The companies announced a team-up in January but did not disclose terms. Without explaining the initial terms of the partnership, the companies said Zymeworks could get an additional $375 million in milestones and other payments. It will also receive royalties on sales of any approved drugs, and Lilly will make an investment in Zymeworks.

The companies say they will study “bi-specific antibodies,” or drugs that use the immune system to treat diseases by targeting two different proteins.

Shares in Indianapolis-based Eli Lilly and Co. were unchanged at $64.68 on Wednesday. Zymeworks is a privately held Canadian company.