* Breast cancer drug seen as successor to Herceptin
* T-DM1 has higher response, fewer side-effects in study
* T-DM1 developed with ImmunoGen
* ImmunoGen shares up nearly 5 percent (Adds further details, background on drug's potential)
Ben Hirschler and Katie Reid
MILAN/ZURICH, Oct 8 (Reuters) - Roche Holding AG's (ROG.VX) experimental breast cancer drug T-DM1 delivers a higher response rate with fewer side effects than the standard treatment of Herceptin plus chemotherapy, researchers said on Friday.
The strong results from a clinical trial boost prospects for the medicine, which the Swiss company is developing with ImmunoGen Inc (IMGN.O) as a successor to its blockbuster Herceptin, which had 2009 sales of some $5 billion.
T-DM1 is the first of a new kind of "armed antibody" that can carry a cell-killing payload into cancer cells.
Shares of ImmunoGen, whose fortunes are closely tied to the success of T-DM1, rose nearly 5 percent in afternoon trading in New York.
A mid-stage Phase II study measured how good the different drugs were at shrinking tumours in 137 women with HER-2 positive breast cancer, which had spread, and who had not previously received chemotherapy.
After six months, those on T-DM1 had an overall response rate of 48 percent, against 41 percent for patients on Herceptin, or trastuzumab, plus Sanofi-Aventis SA's (SASY.PA) Taxotere, or docetaxel, researchers at the European Society for Medical Oncology (ESMO) congress in Milan reported.
Importantly, the rates of clinically relevant adverse events were much lower in the T-DM1 group at 37 percent, compared to 75 percent for women on Herceptin plus Taxotere, endorsing a key rationale behind the drug's development.
Roche, the world's largest maker of cancer drugs, has had a tough year so far as its drug development pipeline has been hit by setbacks. The T-DM1 data may go some way to restoring investors' faith in the group's research capabilities.
The drug is already in final-stage Phase III testing.
The Swiss group's chief executive, Severin Schwan, said earlier on Friday at a press conference in Tokyo that the TDM-1 data was "stunning," although he did not reveal any details of the results at the time. [ID:nTOE69705N]
A full presentation of the T-DM1 data will be made to oncologists at the ESMO meeting on Monday. ESMO officials said they announced key findings early because a bar on release of the data was broken.
"We are encouraged by the results. The study demonstrated that T-DM1 has very good anti-tumour activity as well as much lower toxicity when evaluated side-by-side to the older 'standard,'" Edith Perez, principal investigator of the study, said in a statement.
T-DM1's image was tarnished in August when U.S. regulators rejected Roche's bid to gain marketing approval based only on Phase II trials, delaying its path to market by two years. But Roche remains confident it has a potential winner on its hands.
The fact that the drug delivers its toxic payload directly into cells is thought to be key to why it causes fewer cases of common chemotherapy side effects like hair loss and low white blood cell counts. However, the incidence of nausea in the study was slightly higher on T-DM1.
In addition to fewer adverse side effects, Roche believes its new drug also offers greater convenience, since it eliminates the need for administering chemotherapy.
Commercially, it should help protect Roche's breast cancer franchise, since Herceptin could be exposed to so-called "biosimilar" generic competition in Europe from around 2015.
It also keeps the company in the breast cancer innovation race in the face of newer medicines that aim to rival Herceptin, like GlaxoSmithKline Plc's (GSK.L) Tykerb. (Editing by David Cowell; editing by John Wallace)