JERUSALEM (Reuters) - Swiss drugmaker Novartis will invest up to an extra $15 million in Gamida Cell, an Israeli developer of stem cell therapies, Gamida said on Sunday.

Novartis last year invested $35 million in the company for a 15 percent stake, in a deal that could reach $600 million if Novartis exercises a buyout option that expires in 2016.

The $15 million investment will be used to advance Gamida Cell's clinical programs, including the development of NiCord, an experimental treatment for patients with high risk hematological malignancies, or blood cancers such as leukemia and lymphoma and sickle cell disease.

Gamida plans to initiate a Phase III clinical trial with NiCord in mid-2016.

As part of the latest deal, Novartis will immediately invest $5 million in Gamida Cell for an additional 2.5 percent stake. Also, subject to the close of an equity financing by the end of 2017 to fund the late stage development of NiCord, Novartis will invest up to another $10 million.

Novartis will not have rights or options to Gamida Cell products or technology under the terms of the agreement.

Other Gamida shareholders include Clal Biotechnology Industries, Elbit Imaging, Israel Healthcare Venture, Teva Pharmaceutical Industries, Denali Ventures and Auriga Ventures.

(Reporting by Steven Scheer; editing by Susan Thomas)