* Thoratec shares up as much as 16.7 pct
* HeartWare shares down as much as 21 pct
* Share moves follow release of data on Friday
* Data showed high thrombosis risk with HeartWare device
By Toni Clarke
BOSTON, April 18 (Reuters) - Shares of Thoratec Corp (THOR.O) rose as much as 16.7 percent on Monday while those of rival HeartWare International Inc (HTWR.O) fell as much as 21.0 percent after data from a trial of HeartWare's heart device showed a higher-than-expected rate of blood clots.
Data released late on Friday showed patients implanted with HeartWare's mechanical heart-assist device, known as the HVAD, had an unexpectedly high 9.2 percent chance of developing a potentially life-threatening blood clot per year.
A rival device made by Thoratec, known as HeartMate II, carries an estimated risk of heart-pump-related thrombosis of about 3 percent. [ID:nN18123042]
HeartWare's device is approved in Europe and awaiting approval in the United States as a bridge-to-transplant (BTT), or to keep patients alive while they wait for heart transplant. The company is currently enrolling patients in a trial to test the device as a long-term treatment for patients who are not eligible for a heart transplant -- known as destination therapy (DT).
While analysts said they do not expect Friday's data to stop U.S. regulators approving the device as a bridge-to-transplant, some said it could slow enrollment in the ongoing destination therapy trial, and others said it could increase the risk that U.S. regulators would require more data before approving the device.
Data showed that the 250 patients in the trial contributed a total of 174 patient years, for an overall thrombosis rate of 9.2 percent per patient year. There were 16 cases of pump-related blood clots, of which 11 required the device to be replaced.
HeartWare said it is confident that rate will decline in future because its analysis showed that a subset of patients who took 325 milligrams of aspirin, rather than the previously recommended 81 milligrams, had a far lower risk of developing a blood clot. The company recently changed its clinical trial protocol to boost the recommended aspirin dose to 325 milligrams. Aspirin helps thin the blood and can potentially reduce clotting.
Analysts at UBS, who maintain their "buy" recommendation on HeartWare's stock, said that although a post-hoc analysis suggests a strong relationship between device replacement due to pump-related thrombosis and aspirin, the theory remains unproven, which could cause the U.S. Food and Drug Administration to require additional data.
"Our instinct is that these pump thrombus issues will be adequately addressed over time," they said. However, they said, "to the extent the FDA takes a conservative view of pump exchange, biologic plausibility may not be satisfactory and additional data may conceivably be required."
Sean Lavin, an analyst at Lazard Capital Markets, said he expects the rate of enrollment in the destination trial to slow, and market share in Europe and the U.S. in future to be lower than he previously thought.
"With this in mind, our HeartWare revenue forecasts fall while our Thoratec forecasts pick up extra pumps," he said.
One added cause for concern, he said, is that the thrombosis rate with HeartWare's device increased over time.
"If the rate keeps increasing, the pump would likely not be viable for the DT market, which is likely to be much larger than BTT," he said. "It will be imperative to see if patients on higher aspirin show a strongly increasing thrombi rate over time, or if their rates remain relatively low."
Analysts at J.P. Morgan said that while thrombus concerns are likely to weigh on HeartWare's shares, the company's competitive position remains strong.
"In the end, we expect this weekend's results to temper Street expectations for rapid adoption of the HVAD following its US approval (anticipated by year end) and HeartWare's shares are likely to come under some pressure Monday as a result," they said. "Our own view, however, is that the HVAD remains a compelling option for patients, with this latest issue just one piece of a multifaceted decision clinicians will face between two very good pumps."
HeartWare's shares fell 15.3 percent to $71.05 in late-morning trading on Nasdaq. Earlier in the day they fell as low as $66.15. Thoratec's shares rose 12.6 percent to $30.88. Earlier they rose as high as $32.00 a share.
(Reporting by Toni Clarke, editing by Dave Zimmerman)