BioWorld International Correspondent

LONDON - PPL Therapeutics plc announced further progress in its xenotransplantation program with the birth of double gene knockout piglets, proving that animals without an alpha 1,3 galactosyl transferase (GT) gene are viable, and paving the way for the company to begin a conventional breeding program with cloned knockouts.

CEO Geoff Cook told BioWorld International, "This is a significant milestone in our xenotransplantation program, and we got there ahead of time; we expected this to be early next year."

The four female double GT knockouts, produced with the company's proprietary gene targeting and nuclear transfer cloning technology, were born at PPL Therapeutics Inc. in Blacksburg, Va., on July 25. They continue to thrive, but a fifth piglet died shortly after birth of unknown causes. "We don't think the one piglet died as a result of the double [gene] knockout, though we don't know conclusively at present. Given that four survive it seems unlikely to be the cause," Cook said.

The GT gene produces a sugar, alpha 1,3 galactose, on the surface of pig cells that is recognized by the human immune system as foreign. That would trigger the hyperacute rejection of transplanted organs or cells within minutes. The double knockout is therefore a vital step in producing pigs with organs that can be used for human implants.

PPL, the company behind Dolly, the cloned sheep, has a collaboration with the University of Pittsburgh's Thomas E. Starzl Transplantation Institute. Organs and cells taken from the double knockout pigs will be used in pivotal studies to test for the elimination of hyperacute rejection and long-term survival of the xenografts.

In addition to overcoming early hyperacute rejection, PPL, based in Edinburgh, Scotland, has shown proof of concept, and has patents on methods of avoiding other aspects of rejection, including delayed rejection, coagulopathy and chronic T-cell-mediated rejection. While the double GT knockout pigs provide a platform for adding up to three more genes, Cook said the company does not intend to take the research further because it is aiming to sell its regenerative medicine program.

"Although we announced earlier this year we were going to spin out the program, we didn't stop after the single knockout because the icing on the cake is a double knockout. This concludes the work."

He added that it is hard to say whether or not achieving the double knockout adds value to the program. "In the current market you can stick your finger in the air in terms of value." Negotiations with potential partners are ongoing and Cook said he intends to complete the spinout by the end of the year. He added that PPL hopes to retain an equity stake in any spinout.

The sale is prompted by the need to conserve funds and allow PPL to concentrate on its human protein products.

In January PPL scooped Immerge BioTherapeutics Inc., of Charlestown, Mass., when it announced in a press release the birth of single GT gene knockout piglets, born on Christmas Day. Immerge's piglets with the same gene knockout were born in September 2001, but the news of their arrival was announced in a paper in Science, a day after PPL's press release. PPL says it has generated more than 60 male and female single GT gene knockout pigs since the first litter of females was born in December 2001.