Abstract: Peptide analogs of neurotensin are disclosed which are resistant to enzymatic degradation and which retain high binding affinity for neurotensin receptors. Pharmaceutical compositions of these compounds are useful for diagnostic and therapeutic purposes.

Abstract: Stable analogs of cyclic peptides containing disulfide linkages are disclosed. The disulfide linkage is modified by one of four methods: (a) sulfide contraction, (b) isosteric substitution, (c) thioketal expansion, or (d) alkylation expansion. In sulfide contraction the disulfide bond (—S—S—) is replaced with a monosulfide bond (—S—) in which a bifunctional effector molecule, such as a ligand or chemotoxic agent, is bound to the new peptide linkage. In isosteric substitution, one sulfur atom is replaced with a carbon atom and at least one of the carbon atoms at the modified site is a bifunctional effector molecule. In thioketal expansion, an alkylidene unit (—CR1C2—) is inserted between the two sulfur atoms. In alkylation expansion, an alkyl moiety of from C2 to C3, is inserted between the two sulfur atoms.