Immunex

(IMNX)

and

American Home Products

(AHP)

won't proceed with large-scale advanced clinical trials of Enbrel as a treatment for heart failure, after early studies indicated that the drug wouldn't be effective in fighting the condition.

Additionally, Immunex, a biopharmaceutical company based in Seattle, said its experimental asthma drug Nuvance didn't benefit patients in two studies. A third study is continuing.

The disclosures sent shares of Immunex plummeting in after-hours action. The stock closed at $18.88 in regular-session Nasdaq trading, up 81 cents on the day, but tumbled 48% to $9.81 in after-hours

Instinet

action.