** Pharmacy benefit manager's shares up 25 pct at $60.31 premarket after health insurer UnitedHealth Group says unit will buy company in deal valued at about $12.8 bln
** Offer of $61.50/share is premium of 27 pct to Catamaran's Friday close on the Nasdaq
** UnitedHealth's shares up 3.4 pct at $122
** Up to Friday's close, Catamaran's U.S. shares had fallen about 7 pct this year while UnitedHealth shares had risen about 17 pct.
** Catamaran's Toronto-listed shares were little changed this year up to Friday's close