Disclosed is a compound of formula (1) e.g. [(1R)-l1-({ [(2,3-difluorobenzoyl)amino] acetyl} amino)-3-methylbutyl]boronic acid or a pharmaceutically acceptable salt or boronic acid anhydride thereof, wherein Z1 and Z2 are each independently hydroxy, alkoxy, aryloxy, or aralkoxy or Z1 and Z2 together form a moiety derived from a boronic acid complexing agent and Ring A is selected from the group defined in the specification for treating cancer.
