  We study the formation and evolution of a failed filament eruption observed
in NOAA active region 11121 near the southeast limb on November 6, 2010. We use
a time series of SDO/AIA 304, 171, 131, 193, 335, 94 {\AA} images, SDO/HMI
magnetograms, plus ROSA and ISOON H\alpha images, to study the erupting active
region. We identify coronal loop arcades associated with a quadrupolar magnetic
configuration, and show that the expansion and cancelation of the central loop
arcade system over the filament is followed by the eruption of the filament.
The erupting filament reveals a clear helical twist and develops a same sign of
writhe in the form of inverse \gamma-shape. The observations support the
"magnetic breakout" process with the eruption been triggered by quadrupolar
reconnection in the corona. We suggest that the formation mechanism of the
inverse \gamma-shape flux rope may be the MHD helical kink instability. The
eruption has failed due to the large-scale, closed, overlying magnetic loop
arcade that encloses the active region.
