Certain embodiments of the invention provide a cyclic compound of formula I: wherein: Pro is a residue of L-proline; X1 is a residue of Arg or DArg; X2 is a residue of Phe or DPhe; X3 is a residue of Phe, DPhe or hPhe; X4 is a residue of a natural or unnatural amino acid; X5 is a residue of Ala, Asp, Glu, Lys, His, Phe, Ser, Leu or Gly; X6 is a residue of Phe, Ala, Gly, Ser, Lys, Asp, Leu, Nle, Trp, Tyr, Cha or hPhe; and DPro is a residue of D-proline; or a salt thereof. Certain embodiments also provide compositions comprising such compounds, as well as methods of using such compounds and compositions.