Under an outline released Monday, the measure would give brand-name biologics 12 years of market exclusivity before a generic challenger could enter the market. The bill also seeks to address the thorny issue of full-scale clinical trials by leaving much of the authority to order the trials up to the Food and Drug Administration.

The bill is designed to provide a regulatory pathway for generic copies of biologic drugs and vaccines. Without such a pathway now, the FDA has approved fewer generic alternatives to branded products.

The measure would affect a range of products that use bacteria, cell cultures, or other biologic techniques in manufacturing. Examples include many hormones, cancer treatments and anemia drugs.

Brand-name manufacturers get 12 years without the threat of competition when putting new biologic products on the market. But without a grandfather clause, some biologics approved in the 1990s could face competition almost immediately.

"There are definitely products as of the date of enactment that will be immediately open" to generic challenges, a Democratic Senate aide who briefed reporters said Monday.

One example is Epogen, Amgen's anemia drug first approved in 1989.

A bipartisan group of lawmakers is backing the bill, including Sens. Orrin Hatch, R-Utah, and Hillary Rodham Clinton, D-N.Y.; and Sens. Edward Kennedy, D-Mass. and Mike Enzi, R-Wyo., the chair and ranking member of the Health, Education, Labor and Pensions Committee.

Their bill, headed for a markup before the panel Wednesday, envisions the FDA deciding on a case-by-case basis whether a generic manufacturer's application for a new biologic will require human clinical trials before it can be approved.

Current law allows most chemical drugs to be approved as generics without human trials, as long as sponsors can show they have an identical chemical structure and biologic mechanism as a branded predecessor.

But the process is more complicated with biologics, where manufacturing techniques are far more variable and produce drugs with very large and highly complex molecules.

That could mean that some generic firms may be forced to resort to human testing if they can't show their biologic version is safe, potent and pure, aides said.

But widespread human testing adds millions in development costs, a threat generic firms say undermines the advantage of bringing cheaper copies to the market.

Generic makers would also be asked to shell out $900,000 in user fees with each FDA application in order to help defray costs, aides said. The bill envisions an official new biologics user fee program beginning in 2012.

The bill's bipartisan backers could provide a smooth road in the Senate, but its ultimate fate remains uncertain. Rep. Henry Waxman, D-Calif., has not signed onto the bill and is promoting his own version of biologics legislation.

Senate aides said Democratic and Republican supporters would like to add the biologics provision into legislation reauthorizing the Prescription Drug User Fee Act. That bill overwhelmingly passed the Senate but is awaiting action in the House.

Senate lawmakers are angling to tack the biologics bill onto the user-fee legislation when it reaches a House-Senate conference later this summer.

"That's our hope," one Democratic aide said.