The present invention relates to a prefillable drug delivery device having a housing receiving a needle hub which is movable with respect to said needle hub between a retracted position and a protection position. The device also includes a tubular barrel intended to receive a drug that is pre-mountable on said housing so that said needle hub closes its distal end. The device further includes a sealing member actionable once said tubular barrel is pre-assembled on said housing, capable of transitioning from an inactive state where no sealing contact is present between said needle hub and said tubular barrel, to an active state, in which sealing contact is performed. In the active state of said sealing member, said tubular barrel is movable in translation with respect to said needle hub between a proximal position and a distal position, for realizing delivery of the drug.