A compound of the formula (Ia): wherein R 1 is selected from -C C 1-22 alkyl, -C C 2-22 alkenyl, -C C 5-12 aryl, -C C 1-22 alkylC 5-12 aryl, -C C 1-22 alkylC 3-12 cycloalkyl, -C C 5-10 arylC 2-22 alkenyl, -C C 4-12 cycloalkyl, -C C 3-12 cycloalkylC 1-22 alkyl, -C C 3-12 cycloalkylC 2-22 alkenyl, -C C 1-22 alkyl, -C C 2-22 alkenyl, -C C 5-10 aryl, -C C 1-22 alkylC 5-12 aryl, -C C 1-22 alkylC 3-12 cycloalkyl, -C C 4-12 cycloalkyl, -C C 5-10 arylC 1-22 alkyl, -C C 5-10 arylC 2-22 alkenyl, -C C 3-12 cycloalkylC 1-22 alkyl, -C C 3-12 cycloalkylC 2-22 alkenyl, -C C 1-22 alkyl, -C C 2-22 alkenyl, -C C 5-10 aryl, -C C 1-22 alkylC 5-12 aryl, -C C 1-22 alkylC 3-12 cycloalkyl, -C C 4-12 cycloalkyl, -C C 5-10 arylC 1-22 alkyl, -C C 5-10 arylC 2-22 alkenyl, -C C 3-12 cycloalkylC 1-22 alkyl, -C C 3-12 cycloalkylC 2-22 alkenyl, -S 2 C 1-22 alkyl, -S 2 C 2-22 alkenyl, -S 2 C 5-10 aryl, -S 2 C 4-12 cycloalkyl, S 2 C 5-10 arylC 1-22 alkyl, -S 2 C 5-10 arylC 2-22 alkenyl, -S 2 C 3-12 cycloalkylC 1-22 alkyl and -S 2 C 3-12 cycloalkylC 2-22 alkenyl, each optionally substituted with one or more C 1-2 alkyl, halo, or trihaloC 1-2 alkyl; R 2 represents a side chain of an amino acid selected from serine or threonine; R 3 represents a side chain of an amino acid selected from leucine, phenylalanine, norleucine, norvaline or t-butylglycine; R 4 represents a side chain of an amino acid selected from alanine, threonine, serine, valine, t-butylglycine, 2-aminobutyric acid or 2-aminoisobutyric acid; X is a residue of the side chain of an amino acid selected from diaminobutyric acid, diaminopropionic acid, lysine or ornithine; and k, m, n and p are individually selected from 1, 2, or 3; with the proviso that when R 3 is the side chain residue of leucine or phenylalanine, R 4 is the side chain residue of threonine and k, m, n and p are 2, R 1 is not S-6-methyloctanoyl or 6-methylheptanoyl; or pharmaceutically acceptable salts thereof. R 1 is selected from -C C 1-22 alkyl, -C C 2-22 alkenyl, -C C 5-12 aryl, -C C 1-22 alkylC 5-12 aryl, -C C 1-22 alkylC 3-12 cycloalkyl, -C C 5-10 arylC 2-22 alkenyl, -C C 4-12 cycloalkyl, -C C 3-12 cycloalkylC 1-22 alkyl, -C C 3-12 cycloalkylC 2-22 alkenyl, -C C 1-22 alkyl, -C C 2-22 alkenyl, -C C 5-10 aryl, -C C 1-22 alkylC 5-12 aryl, -C C 1-22 alkylC 3-12 cycloalkyl, -C C 4-12 cycloalkyl, -C C 5-10 arylC 1-22 alkyl, -C C 5-10 arylC 2-22 alkenyl, -C C 3-12 cycloalkylC 1-22 alkyl, -C C 3-12 cycloalkylC 2-22 alkenyl, -C C 1-22 alkyl, -C C 2-22 alkenyl, -C C 5-10 aryl, -C C 1-22 alkylC 5-12 aryl, -C C 1-22 alkylC 3-12 cycloalkyl, -C C 4-12 cycloalkyl, -C C 5-10 arylC 1-22 alkyl, -C C 5-10 arylC 2-22 alkenyl, -C C 3-12 cycloalkylC 1-22 alkyl, -C C 3-12 cycloalkylC 2-22 alkenyl, -S 2 C 1-22 alkyl, -S 2 C 2-22 alkenyl, -S 2 C 5-10 aryl, -S 2 C 4-12 cycloalkyl, S 2 C 5-10 arylC 1-22 alkyl, -S 2 C 5-10 arylC 2-22 alkenyl, -S 2 C 3-12 cycloalkylC 1-22 alkyl and -S 2 C 3-12 cycloalkylC 2-22 alkenyl, each optionally substituted with one or more C 1-2 alkyl, halo, or trihaloC 1-2 alkyl; R 2 represents a side chain of an amino acid selected from serine or threonine; R 3 represents a side chain of an amino acid selected from leucine, phenylalanine, norleucine, norvaline or t-butylglycine; R 4 represents a side chain of an amino acid selected from alanine, threonine, serine, valine, t-butylglycine, 2-aminobutyric acid or 2-aminoisobutyric acid; X is a residue of the side chain of an amino acid selected from diaminobutyric acid, diaminopropionic acid, lysine or ornithine; and k, m, n and p are individually selected from 1, 2, or 3; with the proviso that when R 3 is the side chain residue of leucine or phenylalanine, R 4 is the side chain residue of threonine and k, m, n and p are 2, R 1 is not S-6-methyloctanoyl or 6-methylheptanoyl; or pharmaceutically acceptable salts thereof.