The FDA staff report questioned the unusually poor performance of the control group used in the study.
AstraZeneca Plc's experimental ovarian cancer drug, olaparib, showed an 83 percent reduction in the risk of disease progression, but a U.S. Food and Drug Administration staff review has questioned whether the result could be reproduced.
The company's shares fell 1.8 percent to 43.89 pounds on the London Stock Exchange.
Olaparib, one of several cancer drugs AstraZeneca has flagged as having strong potential in its defense of a $118 billion take-over bid by , is designed as a maintenance therapy for relapsed ovarian cancer in which tumors have responded completely or partially to platinum-based chemotherapy.
"AstraZeneca has put up some pretty lofty expectations," said Damien Conover, an analyst at Morningstar.
The FDA staff report, published on the agency's website on Monday, comes two days ahead of a meeting of outside experts who will discuss whether olaparib's benefits outweigh its risks and whether further data is needed before approval.
The FDA is not obligated to follow the recommendations of its advisory panels but typically does so.