By Mary Welch

With the FDA approval of Mylotarg, American Home Product Corp.'s (AHP) targeted chemotherapy agent, Protein Design Labs Inc. had its fourth product commercialized under its antibody humanization patent licensing deals.

"We're certainly pleased since this is the fourth humanized antibody to reach market that was licensed under our patent licenses," said Cary Queen, Protein Design Labs' (PDL) senior vice president. "We expect and hope that there will be several other humanized antibodies that will reach the market soon."

Mylotarg (gemtuzumab ozogamicin for injection) is the first approved targeted chemotherapy agent using monoclonal antibody technology. It was approved under accelerated approval provisions for the treatment of patients 60 years and older in first relapse with CD33-positive acute myeloid leukemia (AML) who are not considered candidates for cytotoxic chemotherapy.

Mylotarg, which will be marketed by Wyeth-Ayerst Laboratories, the pharmaceutical division of AHP, is the first drug specifically approved to treat relapsed AML patients.

Mylotarg is a recombinant humanized antibody linked with a potent antitumor antibiotic called calicheamicin, which was isolated by Radnor, Pa.-based Wyeth-Ayerst from a bacterium in a caliche clay soil sample from Texas. The antibody portion of Mylotarg binds specifically to the CD33 antigen, a glycoprotein commonly expressed in myeloid leukemic cells.

The antibody was developed by Fred Hutchinson Cancer Research Center in Seattle, and then licensed to Wyeth-Ayerst. The anti-CD33 antibody was humanized by Celltech Group, of Slough, UK.

"Celltech and American Home Products were working on this, I think even before we got the patents," Queen said. "But in order for American Home Products to market the humanized antibody, they had to obtain a non-exclusive license from us under our antibody humanization patents."

AHP, of Madison, N.J., obtained the license in 1999 to market Mylotarg in the U.S. and Europe. With the FDA approval, it exercised its option to obtain a non-exclusive license to market it in Japan and other Asian countries.

With this signing, Fremont, Calif.-based PDL will receive an additional non-refundable, non-creditable signing and licensing from AHP and will be entitled to annual maintenance fees and royalties on any Mylotarg sales.

"We're not releasing any figures, but the deal is a standard one for us," Queen said.

Generally, that means the licensing fee is about $1 million with the annual maintenance fees and royalties in the 3 percent to 4 percent range.

Protein Design Labs' stock (NASDAQ:PDLI) closed Friday at $132.625, up $1.61.