Shares of Puma Biotechnology Inc. dropped 18 percent on Monday after the Westwood drug developer reported clinical trial results for its experimental breast cancer drug that were less dramatic than investors expected.

The company, which has been a Wall Street darling since going public last year, presented mid-stage clinical data on its lead drug candidate, neratinib, at the American Association for Cancer Research’s annual meeting in San Diego. Puma licensed the drug from Pfizer Inc.

While the data suggested neratinib “has potent activity” when given with a long-used generic chemotherapy called paclitaxel, a key subset of women whose tumors are linked to specific protein called HER2 didn’t see as much benefit as anticipated.

Chief Executive Alan Auerbach said his company is designing late-stage studies of neratinib in both HER2-positive patients and patients with a high risk of breast cancer recurrence. Puma also is studying the drug as a treatment for other types of cancer.

Shares closed down $19.47 on the New York Stock Exchange.