An adapter assembly for establishing bidirectional fluid connection between a cartridge and a vial includes a housing having an open first end adapted to engage the cartridge and a second open end adapted to engage the vial. The adapter includes a needle assembly having a first tip and a second tip, with the needle assembly disposed within the housing and at least partially supported by a needle holder. The needle assembly is movable relative to the housing from an initial position in which the first and second tip are isolated from the cartridge and vial, to an end of use position, in which first tip is engaged with the vial and the second tip is engaged with the cartridge, establishing fluid communication therebetween. The needle assembly is maintained in the initial position by a locking structure which is released by rotationally advancing the needle holder relative to the housing.
