WASHINGTON (Reuters) - The U.S. Food and Drug Administration proposed on Thursday identifying cheaper versions of biologic drugs with a suffix

to distinguish them from their more expensive, branded counterparts.

The FDA said its draft guidance is designed to prevent the inadvertent substitution of non-interchangeable products and to make it easier to monitor and track usage once the products are on the market.

Biologic drugs are made from living organisms and, unlike most traditional drugs, cannot be easily replicated. Copies of biologic products are known as biosimilars as they are similar, not identical, to the original.

The FDA is proposing that original biologic products and their biosimilars share a core drug substance name. That name would be followed by a unique suffix composed of four lowercase letters with no meaning.

The agency is proposing that originator drugs also contain a suffix, albeit a different one from the biosimilar. As an example, the agency offered the hypothetical drug replicamab. The original biologic might be named replicamab-cznm while the biosimilar could be named replicamab-hixf.

The agency is seeking public comment as to whether a distinct suffix should be required for products considered

interchangeable with the reference version. A product approved as interchangeable may be substituted for the reference product at the pharmacy counter.

The generic drug industry disagrees with the proposal and argues both products should have the same name.

"Adding a random collection of letters to the product's nonproprietary name confers no additional safety benefit, and in fact would require the healthcare professional to be armed at all times with a code-breaking reference," Dr. Bertrand Liang, chairman of the Biosimilars Council, a division of the Generic Pharmaceutical Association, said in a statement.

Brand name drugmakers and some patient groups want differences between the products to be defined. A coalition of patient advocacy groups known as Patients for Biologics Safety and Access said distinguishable names are needed "to ensure accurate tracking of medication utilizaton and adverse events."

The FDA has only approved one biosimilar - Novartis AG's Zarxio, or filgrastim-sndz. The drug is similar to Amgen Inc's Neupogen, or filgrastim, which is used to reduce the incidence of infection in patients taking certain cancer drugs.

In a blog post, FDA officials said the agency must consider what to do to address previously approved biologic products that do not have a suffix and is seeking public comment on the matter. In the meantime it is proposing designating names that contain a suffix for six previously licensed biologic products.

(Reporting by Toni Clarke; Editing by Bill Rigby and Alan Crosby)