An early review of a recent study showed Genentech Inc.‘s cancer drug Herceptin can significantly increase the risk of heart problems, the company said in a letter released by U.S. regulators on Wednesday.

The study “demonstrated a significant increase in cardiotoxicity in patients who were randomized to the Herceptin-containing (group) as compared to patients who received chemotherapy alone,” the letter said.

The drug’s label already includes information about possible heart failure and heart ventricle problems. The study aimed to “characterize” the risk and determine the value of continual heart monitoring as a way to detect problems earlier, the letter said.

The randomized, late-stage study of 2,043 breast cancer patients found a higher incidence of damage to the heart muscle, known as cardiac toxicity, in the 1,019 patients taking Herceptin. Such damage can lead to heart failure or even death.

“A statistically significant increase in the 3-year cumulative incidence of…congestive heart failure and cardiac death was observed in patients who received the Herceptin-containing regimen (4.1 percent) compared with control (0.8 percent),” the letter said.

Final analysis of the results, which were first presented at a conference in May, is ongoing, it said.

A Genentech spokeswoman could not be immediately reached for comment.

Genentech sent the undated letter to doctors this month. The U.S. Food and Drug Administration released it on its Web site on Wednesday at http://www.fda.gov/medwatch/safety/2005/HerceptinDDL_0805.FINAL.pdf.