A federal judge in the United States on Friday issued a tentative ruling dismissing several patent-related claims that biotech giant Amgen brought against rival Transkaryotic Therapies over Amgen's best-selling product, an anemia treatment that generates billions of dollars in sales a year.

The claims in question relate to one of five patents that Amgen says Transkaryotic Therapies Inc (TKT) violated in the production of erythropoietin (EPO), which is used to combat anaemia.

US District Judge William Young has already ruled that TKT infringed Amgen's patent on erythropoietin, but left issues relating to the four other patents to be decided at the trial.

Young said he would think further about the ruling over the weekend and comment on Monday. The trial reconvenes on Monday morning.

TKT's stock shot up on Friday, rising above an already strong biotech sector. It closed at 42-7/8, up 11-3/16. Amgen's shares fell 3-1/16 to close at 62-5/8.

TKT declined to comment on the ruling. An Amgen spokesman said Young's ruling was a disappointment but did not significantly alter its position.

Analysts said they regarded the judge's preliminary ruling as favorable for TKT but added it was too early to say the fight over Amgen's billion-dollar EPO drug Epogen was decided.

``Keep in mind he's already ruled that they infringed one of them,'' said Amgen's David Kaye. ``We're disappointed but we continue to be confident in our patent position.''

TKT makes EPO by activating a gene in human cells, whereas Amgen isolates human DNA and puts it into hamster cells to grow.

The stakes in the suit are huge. Epogen generated about $1.8 billion in sales for Amgen in 1999 and worldwide revenues were some $4 billion, including sales through Amgen licensing partner Johnson & Johnson.

TKT has contended that Amgen's patents are too broad and the company has improperly tried to claim ownership of the process of making EPO by any method. TKT has noted that EPO exists in nature and its therapeutic qualities were not invented by Amgen.

Amgen, based in Thousand Oaks, Calif., wants the judge to rule that TKT violated its patents simply by using cells from a vertebrate life form -- in this case humans -- to make EPO.

``It's somewhat marginally helpful to TKT,'' said SG Cowen Securities analyst Eric Schmidt. ``I've always thought that TKT is likely to prevail but frankly it's a tossup what the judge is going to do.''

Dennis Harp, a Deutsche Banc Alex. Brown analyst, said TKT ''got a point on the scoreboard'' in the case, but Amgen still appears to be leading in the overall court battle.

``The judge ruled in a tentative 'finding of fact' that one of five Amgen patents in the lawsuit he was not convinced was infringed by TKT and Aventis,'' Harp said.

Harp said the judge was referring to the so-called ``698'' patent that covers the process of making erythropoietin.

Harp said the other four Amgen patents relevant in the trial, which Amgen says TKT has infringed, all relate to the composition of the drug.