The health care sector dominated after-hours news Monday as the most active stocks mostly traded flat to higher.

Ventana Medical Systems

(VMSI)

catapulted more than 50% postclose after Switzerland-based

Roche

disclosed a

$75 per-share buyout bid

for the medical-device maker. The offer, Roche said, represents a 55% premium to the Ventana stock's prior three-month average. Roche said it has previously expressed interest, to no avail, in taking lesser Ventana equity positions. Ventana shares were trading $27.16 higher at $78.90.

Houston's

Encysive

( ENCY) jumped more than 11% amid word of a

structural overhaul

after shares were hit hard last week on

disappointing regulatory developments

regarding Thelin, the biopharma's hypertension drug. CEO Bruce Given is stepping down, to be replaced by Chief Operating Officer George Cole. In addition, Encysive is halting a key Thelin trial and will slash about 70% of its U.S. workforce. Shares were up a dime, or 5.3%, at $1.99.

Theravance

(THRX)

climbed 6.2% to $33.25 on positive results from a midphase study on TD-5108, its proposed treatment for chronic constipation. The California-based company said all three tested doses achieved the primary endpoint.

But semiconductor firm

IXYS

(IXYS)

plunged 7.8% on a convertible senior bond offering -- $60 million worth, with a $15 million initial purchasers' option to cover overallotments -- as well as lowered guidance. The company now expects revenue at $70 million to $72 million for the quarter ending June 30 -- a decline from last quarter -- compared with its prior outlook, which called for sequentially flat revenue of about $74 million. Two analysts polled by Thomson Financial are seeking $75 million. Shares were down 76 cents to $9 in recent late trading.