Large-scale vortices in protoplanetary disks are expected to form and survive for long periods of time in accordance with disk asymmetries observed in the sub-mm and mm wavelength ranges. Such vortices are known to trap in their core large amounts of dust grains that we assume to agglomerate into big boulders weakly coupled to the gas. Boulders are rapidly released by the core and their dynamics is explored when the vortex is located at $50AU$ from the star, a region of the disk where self-gravity plays a significant role. Global 2D simulations are used to follow the boulder dynamics in the star+vortex gravitational field during the very early stage of the evolution. Boulders are trapped in the co-orbital region associated to the vortex where they tend to accumulate during long time periods. Collisional evolution and growth of the boulders are discussed based on the standard coagulation models. We discuss collisional evolution and growth of the boulders based on the standard coagulation model. Finally, we propose a new scenario for the formation of Cold Classical Kuiper-Belt objects.