Cytotoxic boronic acid peptide analogs

A method of inhibiting growth of a cancer cell comprising contacting said cell with an effective growth-inhibiting amount of a compound of the formula (II):

• ##STR10## or a physiologically acceptable salt thereof, wherein A and A.sub.2 are individually L-amino acid residues selected from the group consisting of Ala, Pro, Gly, Glu, Leu, Lys, Phe, Ser, Val, Ile, Arg, Tyr, Thr, Asp, Asn and Gly;
• R.sup.1 is C.sub.1 -C.sub.6 (alkyl) which is unsubstituted or is substituted with an aromatic substituent or one or more in-chain bivalent groups selected from the group consisting of --O--, --CO--, --S--, --NH--, --CONH--, CH.dbd.CH-- AND --SO.sub.2 --;
• Y.sup.1 and Y.sup.2 are each H, or taken together form a moiety derived from a dihydroxy compound having at least two hydroxy groups separated by at least two connecting atoms in a chain or ring, said chain or ring comprising carbon atoms, and optionally, a heteratom or heteroatoms which can be N, S or O;
• with the proviso that where the heteroatom is O, R.sup.2 cannot be H;
• and R.sup.2 is H or an N-terminal protecting group.