A coaxial stabilizing and sealing catheter provides increased support to a guide catheter that is seated in an ostium of a branch artery by extending through and out of a distal end of the guide catheter and anchoring in position within the vessel that is being treated. The stabilizing and sealing catheter includes, as part of a relatively flexible distal section, a balloon that locks the distal end of the stabilizing and sealing catheter in place to provide support to the therapeutic devices that are introduced and further seals the artery to prevent blood from entering the artery. A sealing cuff at a proximal end of the stabilizing and sealing catheter provides a seal between the exterior of the stabilizing and sealing catheter and the interior of the guide catheter. In addition, a fixing mechanism it utilized to lock the guide catheter in place to exchange therapeutic devices.