A compound having the formula: R 1 -P 1 -P 2 -R 2 wherein R 1 is H, C 1-4 alkyl, acetyl, formyl, benzoyl or trifluoroacetyl; R 2 is OH or NH 2; P 1 is a peptide having the sequence: His-X2-X3-GTFTSDYSKYL-X15-X16-X17-X18-A-X20-DFI-X24-WLE-X28-A wherein: X2 is selected from Aib, Ac3c, Ac4c and Ac5c; X3 is selected from Gln and His; X15 is selected from Asp and Glu; X16 is selected from Glu and Ψ; X17 is selected from Arg and Ψ; X18 is selected from Ala and Arg; X20 is selected from Lys and His; X24 is selected from Glu and Ψ; X28 is selected from Ser and Ψ; and P 2 is absent or is a sequence of 1-20 amino acid units independently selected from the group consisting of Ala, Leu, Ser, Thr, Tyr, Cys, Glu, Lys, Arg, Dbu, Dpr and Orn; wherein the compound contains one and only one Ψ and wherein said Ψ is a residue of Lys, Arg, Orn or Cys in which the side chain is conjugated to a substituent having the formula -Z 2 Z 1; -Z 1 is a fatty chain having a polar group at one end of the chain and a connection to Z 2, -X-at the end of the chain distal from the polar group, wherein the polar group comprises a carboxylic acid or a carboxylic acid bioisostere, a phosphonic acid, or a sulfonic acid group; and -X- is a bond, -CO-, -SO-, or -SO 2 -; -Z 2 - is a spacer of formula: wherein: each Y is independently -NH, -NR, -S or -O, where R is alkyl, a protecting group or forms a linkage to another part of the spacer Z 2; each X is independently a bond, CO-, SO-, or SO 2 -; with the proviso that when Y is -S, X is a bond; each V is independently a bivalent organic moiety linking Y and X; and n is 1-10; or a pharmaceutically acceptable salt or solvate thereof. R 1 is H, C 1-4 alkyl, acetyl, formyl, benzoyl or trifluoroacetyl; R 2 is OH or NH 2; P 1 is a peptide having the sequence: His-X2-X3-GTFTSDYSKYL-X15-X16-X17-X18-A-X20-DFI-X24-WLE-X28-A wherein: X2 is selected from Aib, Ac3c, Ac4c and Ac5c; X3 is selected from Gln and His; X15 is selected from Asp and Glu; X16 is selected from Glu and Ψ; X17 is selected from Arg and Ψ; X18 is selected from Ala and Arg; X20 is selected from Lys and His; X24 is selected from Glu and Ψ; X28 is selected from Ser and Ψ; X2 is selected from Aib, Ac3c, Ac4c and Ac5c; X3 is selected from Gln and His; X15 is selected from Asp and Glu; X16 is selected from Glu and Ψ; X17 is selected from Arg and Ψ; X18 is selected from Ala and Arg; X20 is selected from Lys and His; X24 is selected from Glu and Ψ; X28 is selected from Ser and Ψ; and P 2 is absent or is a sequence of 1-20 amino acid units independently selected from the group consisting of Ala, Leu, Ser, Thr, Tyr, Cys, Glu, Lys, Arg, Dbu, Dpr and Orn; wherein the compound contains one and only one Ψ and wherein said Ψ is a residue of Lys, Arg, Orn or Cys in which the side chain is conjugated to a substituent having the formula -Z 2 Z 1; -Z 1 is a fatty chain having a polar group at one end of the chain and a connection to Z 2, -X-at the end of the chain distal from the polar group, wherein the polar group comprises a carboxylic acid or a carboxylic acid bioisostere, a phosphonic acid, or a sulfonic acid group; and -X- is a bond, -CO-, -SO-, or -SO 2 -; -Z 2 - is a spacer of formula: wherein: each Y is independently -NH, -NR, -S or -O, where R is alkyl, a protecting group or forms a linkage to another part of the spacer Z 2; each X is independently a bond, CO-, SO-, or SO 2 -; with the proviso that when Y is -S, X is a bond; each V is independently a bivalent organic moiety linking Y and X; and n is 1-10; or a pharmaceutically acceptable salt or solvate thereof. each Y is independently -NH, -NR, -S or -O, where R is alkyl, a protecting group or forms a linkage to another part of the spacer Z 2; each X is independently a bond, CO-, SO-, or SO 2 -; with the proviso that when Y is -S, X is a bond; each V is independently a bivalent organic moiety linking Y and X; and n is 1-10; or a pharmaceutically acceptable salt or solvate thereof.