A compound, or a pharmaceutically acceptable salt or ester thereof, comprises the structure: [(P1-S1j)p-L-(S2q-P2)r]t wherein: P1 and P2, which may be the same or different, are cyclic oligopeptide moieties, at least one of P1 and P2 having the motif B-Arg or B-(Me) Arg within the cyclic moiety, wherein B is a basic amino acid, a derivative thereof, or phenylalanine or a derivative thereof; S1 and S2 are spacer groups, which may be the same or different; L is a linker moiety containing at least two functional groups for attachment of the cyclic oligopeptides or spacer groups; n and q are independently 0 or 1; p and r are independently integers of 1 or more; and t is an integer of 1 or more, provided that, when t, p or r is greater than 1, the cyclic oligopeptide moiety, spacer group and/or the value of j or q may be the same or different between the multiple (P1-S1j) moieties or multiple (S2q-P2) moieties.