In particular, the polypeptide constructs of the present invention comprise compounds of the formula (I): ##STR1## in which, B is a hydrocarbon backbone, P is a polypeptide capable of targeting particular cells, tissues or organs of the body, A may be the group --NR'--NR-- or the group --NR'--NR--L-- in which L may be an aliphatic or aromatic linker group, R, R', and R may be the same or