Concerns regarding patient safety prompted EPIX Pharmaceuticals Inc. to amend the protocol for its ongoing Phase II study of EP-2104R, an imaging agent to detect blood clots, and the trial's completion is expected to be delayed.

Following an FDA review from a 14-day, repeat-dose preclinical toxicology study, the Cambridge, Mass.-based company said it intends to include additional patient safety monitoring. Specific safety issues were not disclosed; however, CEO Michael Webb said in a press release that "the observations in the preclinical study, which occurred in both drug-treated and non-drug-treated animals, do not establish a clear relationship to EP-2104R."

The amended protocol is expected to extend the trial's timeline and increase costs, though EPIX said it had not yet determined the full impact.

The company could not be reached for comment. Its shares (NASDAQ:EPIX) lost 57 cents Wednesday to close at $9.81.

As the company's second investigational imaging agent, EP-2104R is in development for the detection of blood clots, a frequent precursor to stroke, heart attacks, deep vein thrombosis and atrial fibrillation. By binding to fibrin, a protein found in all blood clots, an injection of EP-2104R could allow doctors to visualize the clots directly using magnetic resonance imaging.

EPIX began the proof-of-concept study in April to evaluate EP-2104R in about 40 patients who presented with strong suspicion of clots from thrombo-embolic disease, with the aim of developing an imaging technique using the product in conjunction with MRI. The trial also is designed to determine the feasibility of imaging clots in several areas of the body during a single scanning session.

In addition to EP-2014R, the company also is in discussions with the FDA regarding its lead product, Vasovist, an imaging agent for use in the detection of vascular disease. Formerly known as MS-325, Vasovist received an approvable letter in January. EPIX's response to the FDA was accepted last month, and the agency set a target action date for late November. Though the company's response did not include data from new trials, which had been requested by the FDA, it provided a new analysis of existing data.

If approved, Vasovist likely would be the first imaging pharmaceutical approved in the U.S. for magnetic resonance angiography. The product is a blood-pool agent designed to bind reversibly to albumin, brightening the blood and making it possible for doctors to see images of arteries and veins for up to about one hour after injection.

Vasovist is partnered with Berlin-based Schering AG, which secured recommended approval from the European Committee for Medicinal Products for Human Use last month and expects final marketing approval in Europe later this year. EPIX also is collaborating with Schering on EP-2104R.

Both products were developed using EPIX's Target Visualization Technology, which creates imaging agents targeted at the molecular level, to detect conditions ranging from vascular disease to deep venous thrombosis to breast cancer.

EPIX has not yet released its second-quarter earnings. The company reported a net loss of $6.3 million, or 27 cents per share, for the first quarter. As of March 31, it had cash, cash equivalents and marketable securities totaling $158.6 million.