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CHICAGO -- An investigational agent that's structurally similar to the breast cancer drug trastuzumab (Herceptin) also had similar efficacy and safety in a clinical trial, a researcher said here.

The findings are the first report that an agent "biosimilar" to an approved biologic drug has the same effects on cancer as the original, according to , of the University of California San Francisco.

In principle, Rugo told reporters at the annual meeting of the , the new drug, if approved, could broaden access to trastuzumab, a "lifesaving" medication for women with metastatic breast cancer that is positive for the HER2 receptor.

Lack of access is not a major issue in the U.S., Rugo told 名媛直播, because most patients have insurance in one form or another that covers trastuzumab. "I've not ever seen a situation where it wasn't covered for a standard indication," she said.

But around the world, the cost of the branded product is a barrier to access, she said.

"It is expected that having biosimilars will reduce the costs -- by some amount -- for these agents around the world," she said, but trying to pin down that amount is "speculation."

An ASCO spokesman, , of the Dana-Farber Cancer Institute in Boston, agreed with Rugo that it's hard to predict what effect approval of the drug would have on access to treatment.

"In general, one solution to high prices is to have more competition," he told 名媛直播. "This would fit that bill."

Herceptin and other biologic agents are complicated proteins with a high molecular weight, Rugo said, and they are highly targeted as well as costly.

Rugo and colleagues conducted a phase III, double-blind randomized trial to compare the two agents in combination with either docetaxel (Taxotere) or paclitaxel (Taxol), with the goal of comparing safety and efficacy in treating women with previously untreated metastatic HER2-positive breast cancer.

The primary efficacy endpoint was the overall response rate at the end of eight 3-week cycles of the medications, she said, with equivalence defined in terms of the ratio of the rates, as required by the FDA.

The investigators also looked at the difference between response rates, which is the way the European regulators want to define equivalence.

The so-called HERITAGE study enrolled 500 patients at 95 sites around the world, of whom 458 could be evaluated for efficacy, Rugo said.

Some 69.6% of patients treated with Myl-1401O had a response, according to the Response Evaluation Criteria In Solid Tumors (RECIST 1.1), Rugo reported. In comparison, the rate among the patients getting trastuzumab was 64%.

The ratio of the two rates was 1.09 (95% CI 0.954-1.237), which put it within a pre-defined equivalence margin, Rugo and colleagues found.

The difference between the rates was 5.5 percentage points, she said, which met the European standard for equivalence.

The number of patients with at least one serious adverse event was similar between the arms, she said -- 94, or 38.1% in the Myl-1401O arm and 89, or 36.2% in the trastuzumab arm. The most common events were neutropenia, febrile neutropenia, leukopenia, and pneumonia, she said.

The new agent is not yet approved in the U.S., although a spokesman for manufacturer Mylan said it might be sent to the FDA later this year.

The maker of trastuzumab, South San Franciscio-based Genentech, said in a statement that it welcomes the "FDA's efforts to implement a science-based pathway for the approval of biosimilars" but added that its drug will inevitably have more data to support it than any new agent.

"We believe doctors and patients should have a choice in the medicines patients receive, and the depth and breadth of clinical data and physician experience with Herceptin is an important consideration, the company statement said.

"It will be important to closely monitor safety and effectiveness to ensure there are no meaningful differences that would impact patients," the statement continued. "It's also important to consider that biosimilars are approved under an abbreviated FDA pathway. At the time of approval, biosimilars will have a more limited set of clinical data than the original medicines."

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