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BOSTON ( TheStreet) -- The "armed antibody" drug TDM-1 under development by Roche and ImmunoGen ( IMGN) 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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Adam Feuerstein writes regularly for TheStreet.com. In keeping with TSC's editorial policy, he doesn't own or short individual stocks, although he owns stock in TheStreet.com. He also doesn't invest in hedge funds or other private investment partnerships. Feuerstein appreciates your feedback; click here to send him an email.

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