r both of L1 and L2 is an amino acid or polypeptide residue and any carboxyl group that is linked by less than about 5 atoms to the amidate N is esterified or amidated, the dotted lines represent facultative bonds and wherein, (i) P and Z are linked to form a compound of the formula Ib or (ii) L1 and Z are linked to form a compound of the formula Ic wherein