Interpublic's Lowe Group has signed a letter of intent to acquire William Douglas McAdams Inc., New York, the sixth largest medical agency in the U.S., with billings of about $170 million. Its clients include Glaxo-Wellcome, Schering, Roche and Pharmacia & Upjohn. The agency will be renamed Lowe McAdams HealthCare and work closely with Lowe & Partners/SMS in New York.