Updated from 2:46 p.m. EST

Shares of

Myogen

( MYOG) were among the worst-performing health-related stocks Thursday, falling 21.3% after the company said patient enrollment for part of a clinical study to evaluate a pulmonary hypertension treatment would be delayed until the fourth quarter of 2005.

The biopharmaceutical company had hoped to complete enrollment of the phase III ambrisentan trial by the first half of the year. The company said that enrollment for its first group of patients, called Aries-1, has been affected by a number of factors, "including a decline in the availability of treatment naive PAH patients due to increasing market penetration of bosentan as well as the expanded use of sildenafil after the release of clinical data in October 2004." Other clinical studies are competing for this group as well, the company continued. The second group, called Aries-2, is expected to complete enrollment in June 2005, as originally scheduled. Shares traded down $1.92 to $7.08.

TriPath Imaging

( TPTH) fell 2.8% after the maker of diagnostic tests posted fourth-quarter earnings and sales that fell below expectations. The company posted a profit of $308,000, or 1 cent a share, on sales of $18.2 million. Analysts polled by Thomson First Call were expecting earnings of 2 cents a share on sales of $18.7 million. Earnings for the full year came in at $605,000, or 2 cents a share, on sales of $68.5 million. Sales matched the company's reduced sales guidance that was issued in October. Shares traded down 25 cents to $8.54.

Shares of

Digene

( DIGE) fell 3.4% after the maker of DNA and RNA testing systems posted a second-quarter profit and sales that fell short of expectations. The company earned $304,000, or 1 cent a share, on sales of $27 million. Analysts were expecting earnings of 5 cents a share on sales of $27.2 million. Looking ahead, Digene said that third-quarter earnings would be flat to 7 cents a share on sales of $28 million to $31 million. Analysts had been expecting earnings of 5 cents a share on sales of $30.1 million. Shares traded down 90 cents to $25.55.

Icagen

( ICGN) fell 8.8% on its first day of trading as a public company. The biopharmaceutical company sold 5 million shares at $8 apiece. The pricing came in below the original range of $10 to $12 a share. J.P. Morgan, UBS Investment Bank and CIBC World Markets led the underwriting syndicate. Shares traded down 70 cents to $7.30.

Shares of

AmSurg

(AMSG)

fell 10.6% after the operator of surgery centers trimmed its 2005 earnings outlook and warned that results would fall below expectations. The company now expects earnings of $1.33 to $1.36 a share, down from previous guidance of $1.42 to $1.45 a share. Sales are expected to be between $400 million and $420 million. Analysts had been expecting earnings of $1.42 a share on sales of $400.2 million. AmSurg cited several factors for the reduced outlook. Three of its Florida centers under development will open later because of hurricane-related delays, while five other centers will open later than originally scheduled because of various construction and regulatory issues. The company also said that four centers that were supposed to be acquired during the fourth quarter would now be acquired during the first quarter of 2005. Shares traded down $2.84 to $23.95.

Other health care movers included

Pfizer

(PFE) - Get Report

, down 16 cents to $23.91;

Aastrom Biosciences

(ASTM)

, up 3 cents to $3.68;

Merck

(MRK) - Get Report

, down 5 cents to $28.41;

StemCells

(STEM)

, down 38 cents to $4.82;

Amgen

(AMGN) - Get Report

, down 51 cents to $62.14;

Johnson & Johnson

(JNJ) - Get Report

, down 35 cents to $65.65;

Schering-Plough

( SGP), up 3 cents to $19; and

ViaCell

( VIAC), down 54 cents to $9.94.