Assembly for a drug delivery device, piston rod and drug delivery device

An assembly for a drug delivery device, comprising:

• a housing having a distal end and a proximal end,
• a piston rod,
• a drive member, the drive member being moveable in a proximal direction with respect to the housing and with respect to the piston rod for setting a dose of a drug,
• at least one stop member which is provided on one of the piston rod and the drive member and which is radially moveable with respect to the one of the piston rod and the drive member on which it is provided,
• at least one interaction member which is provided on the other one of the piston rod and the drive member,
• wherein, in a non-locking mode of the assembly, the stop member is radially biased, and the stop member and the interaction member are arranged axially spaced apart from each other by an axial distance, the axial distance being reduced by a dosing distance when the drive member is moved in the proximal direction,
• and wherein, when the dosing distance corresponds to the axial distance, the assembly is configured to switch into a locking mode, wherein, for switching into the locking mode, the radial bias of the stop member is permitted to decrease at least partially to bring the stop member and the interaction member in mechanical cooperation, wherein, when the stop member and the interaction member mechanically cooperate, the stop member and the interaction member are configured to axially interlock the piston rod and the drive member such that, in the locking mode, movement of the drive member in the proximal direction with respect to the piston rod is prevented.