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BOSTON ( TheStreet) -- The "armed antibody" drug TDM-1 under development by Roche and ImmunoGen ( IMGN - Get Report) posted a higher response rate and lower toxicity than the blockbuster breast cancer drug Herceptin, according to results from the first head-to-head study of the two drugs to be presented at a European cancer meeting Monday.

The data from the TDM-1 phase II study in front-line breast cancer patients are early and very preliminary, but also encouraging because they suggest TDM-1 may be the next-generation successor to Herceptin that Roche has been looking for.

ImmunoGen, as Roche's partner on TDM-1, stands to profit as well if the drug is approved and successful.

In the phase II study, 137 women with HER2-positive metastatic breast cancer with no prior chemotherapy were randomly assigned to treatment with TDM-1 or Herceptin plus the chemotherapy drug Taxotere.

After six months of follow-up, the overall response rate for patients treated with TDM-1 was 48% compared to a 41% response rate for patients treated with Herceptin-Taxotere.

More striking was the difference in the rate of adverse events between the two drugs. The rate of "clinically relevant" adverse events (defined as Grade 3 or higher) was 37% in the TDM-1 arm compared to 75% in the Herceptin-Taxotere arm.

A full presentation of the T-DM1 study will be made Monday at the European Society of Medical Oncology (ESMO) annual meeting taking place in Milan.

Roche CEO Severin Schwan called the TDM-1 data "stunning" at a press event in Tokyo, according to a Reuters story Friday.

ImmunoGen shares were up 6% to $7.09 in late Friday trading.

T-DM1 is a so-called "armed antibody" that combines the targeted antibody drug Herceptin owned by Roche (already a potent cancer drug on its own) with a chemotherapy payload developed by ImmunoGen.

TDM-1 is designed to deliver a cancer-killing dose of chemotherapy directly inside tumors, sparring healthy cells from toxic side effects. This may explain the sharply lower toxicities observed in the front-line breast cancer study of TDM-1 compared to Herceptin-Taxotere.

For instance, the rates of hair loss and neutropenia (low white blood cell counts) in the Herceptin-Taxotere arm were 66% and 57%, respectively. By comparison, hair loss and neutropenia were reported in 1.5% and 7.5% of TDM-1 patients, respectively.

More patients treated with TDM-1 reported fever and nausea than those treated with Herceptin-Taxotere.

The TDM-1 data to be presented Monday are preliminary and may not hold up as the phase II study progresses. The study's primary endpoint is progression-free survival, which will determine whether TDM-1 can significantly delay the time before a patient's tumor starts to grow compared to Herceptin-Taxotere.

Roche, however, is not waiting for final results from this phase II study to push ahead with TDM-1 development in front-line metastatic breast cancer. A large, phase III study comparing TDM-1, Herceptin and another Roche drug, pertuzumab, has already been announced.

The TDM-1 data to be presented Monday may also help ease the sting felt in August when the U.S. Food and Drug Administration rejected Roche's effort to seek approval for T-DM1 based on results from a different phase II study involving patients with more advanced breast cancer.

--Written by Adam Feuerstein in Boston.

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