Bendamustine esters and related compounds, and medical use thereof

A compound of formula I ** ** Formula or a pharmaceutically acceptable salt thereof, wherein R1 is C1-C3, R2 and R3 independently represent C1-C3 alkanediyl, Y1 and Y2 represent oxygen, and R4 and R5 They form a ring structure together with the interjacent nitrogen R4 and R5, in which the annular structure is selected from the group consisting of pyrrolidine, piperidine, piperazine and morpholine.