Arcus Medica, a 25-person Philadelphia science and medical communications agency founded six years ago by three medical professionals who sought to change careers, has combined with San Francisco-based W20 Group.

Under terms of the deal, whose cash value wasn’t disclosed, Arcus’ founders — Mary Seideman, a pediatric oncologist; her husband, medical researcher Jonathan Seideman; and Stan Eapen, a pharmacist — become minority owners of W20, an 800-person health-care marketing firm headed by Jim Weiss, a former executive with Sanofi and Genentech.

W20′s controlling investor is NewMountain Capital, the New York-based buyout giant whose other locally based properties include the Radnor lab-equipment giant Advantor (formerly VWR). Arcus group will work closely with W20′s Scientific Strategy group, headed by Ujwal Pyati, who as a Harvard student once earned a Young Investigator Award from the Bala Cynwyd-based Alex’s Lemonade Foundation.

The W20 group has offices in Philadelphia in the Graham Building, at 30 S. 15th St., eight floors above Arcus.

“Our client mix is quite similar" — indeed, “we share quite a few clients,” said W20 spokesperson Pete Duckler. W20 employs 42 at its Philadelphia office and 17 in New Hope. Even as the two combine operations, the firm expects to add staff for the group, rather than eliminating positions as a result of the merger, he added.

Arcus Medica says it helps drugmakers and other clients “translate complex clinical and scientific data” to “help brands influence key audiences." W20 calls itself “the leading independent provider of analytics-driven, digital-first marketing communications to the health-care sector.”

The firm says it convinces doctors and patients to use clients’ products with “clear, compelling storytelling” based on proprietary customer and product data and “peer to peer medical engagement.”

Before joining Arcus, Mary Seideman was a medical director at Genentech, Chester County-based Cephalon, and GlaxoSmithKline, where she worked on Rituxan and Treanda. Jonathan Seideman worked on autoimmune and inflammatory diseases at Centocor. Eapen worked, like Mary Seideman, at Genentech and at Glaxo, where he marketed Rituxan for lymphoma.

Arcus Medica’s financial adviser for the deal was Delancey Street Partners, the West Conshohocken investment bank headed by B. Andrew Schmucker, former investment banking chief (corrected) at the Philadelphia brokerage Janney Montgomery Scott.