A compound of the following formula or a pharmaceutically acceptable salt thereof, wherein: R 1 is alkyl, preferably C1-C6 alkyl, more preferably methyl; R 2 is H or alkyl, which alkyl may be unsubstituted or substituted with at least one substituent selected from -NH 2, -NH-C(=NH)-NH 2, -C NH 2, -C OH, -OH, -SH, -SCH 3 and a 5 to 10-membered carbocycle or 5 to 10-membered heterocycle containing oxygen, nitrogen or sulfur as heteroatom(s); R 3 is H or alkyl, which alkyl may be unsubstituted or substituted with at least one substituent selected from -NH 2, -NH-C(=NH)-NH 2, -C NH 2, -C OH, -OH, -SH, -SCH 3 and a 5 to 10-membered carbocycle or 5 to 10-membered heterocycle containing oxygen, nitrogen or sulfur as heteroatom(s); R 4 is alkyl, which alkyl is substituted with at least one -S-R A, R 5 is H or I; R 6 is alkyl substituted with at least one substituent selected from -NH 2, -NH-C(=NH)-NH 2, -NH-R B, -NH-C(=NH)-NH-R B, -NH-C(=NH)-NH-R 14, and -NH-C(=NH)-NH-C(=O)-R 15, R 7 to R 10 are each independently H or alkyl; R 14 and R 15 are independently C1-C10 alkyl, which alkyl may be substituted by at least one substituent selected from -NH-C(=O)-CH 3 and -C=CH; R A is a group which comprises at least one selected from: (i) a chelating moiety, (ii) a chelate formed by a chelating moiety (i) with a chelated radioactive or nonradioactive cation, (iii) a moiety carrying a covalently bound radioisotope, (iv) a phosphonate moiety, and (v) a fluorescent label; and R B is a group which comprises at least one selected from: (i) a chelating moiety, (ii) a chelate formed by a chelating moiety with a chelated radioactive or nonradioactive cation, (iii) a group carrying a covalently bound radioisotope, (iv) a phosphonate moiety, and (v) a fluorescent label, and R B may further comprise a group of formula which is bound to the remainder of the compound with the bond marked with, wherein R 1 to R 5 and R 7 to R 10 are as defined above, and wherein R 16 is an alkanediyl group, preferably a C1-C6 alkanediyl group, and R 17 is selected from -NH- and -NH-C(=NH)-NH-. R 1 is alkyl, preferably C1-C6 alkyl, more preferably methyl; R 2 is H or alkyl, which alkyl may be unsubstituted or substituted with at least one substituent selected from -NH 2, -NH-C(=NH)-NH 2, -C NH 2, -C OH, -OH, -SH, -SCH 3 and a 5 to 10-membered carbocycle or 5 to 10-membered heterocycle containing oxygen, nitrogen or sulfur as heteroatom(s); R 3 is H or alkyl, which alkyl may be unsubstituted or substituted with at least one substituent selected from -NH 2, -NH-C(=NH)-NH 2, -C NH 2, -C OH, -OH, -SH, -SCH 3 and a 5 to 10-membered carbocycle or 5 to 10-membered heterocycle containing oxygen, nitrogen or sulfur as heteroatom(s); R 4 is alkyl, which alkyl is substituted with at least one -S-R A, R 5 is H or I; R 6 is alkyl substituted with at least one substituent selected from -NH 2, -NH-C(=NH)-NH 2, -NH-R B, -NH-C(=NH)-NH-R B, -NH-C(=NH)-NH-R 14, and -NH-C(=NH)-NH-C(=O)-R 15, R 7 to R 10 are each independently H or alkyl; R 14 and R 15 are independently C1-C10 alkyl, which alkyl may be substituted by at least one substituent selected from -NH-C(=O)-CH 3 and -C=CH; R A is a group which comprises at least one selected from: (i) a chelating moiety, (ii) a chelate formed by a chelating moiety (i) with a chelated radioactive or nonradioactive cation, (iii) a moiety carrying a covalently bound radioisotope, (iv) a phosphonate moiety, and (v) a fluorescent label; and (i) a chelating moiety, (ii) a chelate formed by a chelating moiety (i) with a chelated radioactive or nonradioactive cation, (iii) a moiety carrying a covalently bound radioisotope, (iv) a phosphonate moiety, and (v) a fluorescent label; R B is a group which comprises at least one selected from: (i) a chelating moiety, (ii) a chelate formed by a chelating moiety with a chelated radioactive or nonradioactive cation, (iii) a group carrying a covalently bound radioisotope, (iv) a phosphonate moiety, and (v) a fluorescent label, (i) a chelating moiety, (ii) a chelate formed by a chelating moiety with a chelated radioactive or nonradioactive cation, (iii) a group carrying a covalently bound radioisotope, (iv) a phosphonate moiety, and (v) a fluorescent label, and R B may further comprise a group of formula which is bound to the remainder of the compound with the bond marked with, wherein R 1 to R 5 and R 7 to R 10 are as defined above, and wherein R 16 is an alkanediyl group, preferably a C1-C6 alkanediyl group, and R 17 is selected from -NH- and -NH-C(=NH)-NH-.