A compound with the formula I: and salts and solvates thereof, wherein R L is a linker for connection to an antibody or an antigen-binding fragment of an antibody, which is selected from: (ia): wherein Q is: where Q x is such that Q is an amino-acid residue, a dipeptide residue, a tripeptide residue or a tetrapeptide residue, and wherein the superscripted labels C(=O) and NH indicate the group to which the atoms are bound; X is:, where a = 0 to 5, b1 = 0 to 16, b2 = 0 to 16, c1 = 0 or 1, c2 = 0 or 1, d = 0 to 5, wherein at least b1 or b2 = 0 and at least c1 or c2 = 0; G L is a linker for connecting to an antibody or an antigen-binding fragment of an antibody, wherein G L is selected from (G L1-1) (G L6) (G L1-2) (G L7) (G L2) (G L8) (G L3-1) (G L9) where the NO 2 group is optional (G L3-2) (G L10) where the NO 2 group is optional (G L3-3) (G L11) where the NO 2 group is optional (G L3-4) (G L12) where the NO 2 group is optional (G L4) (G L13) Where Hal = I, Br, Cl (G L5) (G L14) where Ar represents a C 5-6 arylene group, and X' represents C 1-4 alkyl; (ib): where R L1 and R L2 are independently selected from H and methyl, or together with the carbon atom to which they are bound form a cyclopropylene or cyclobutylene group; and e is 0 or 1. (ia): wherein Q is: where Q x is such that Q is an amino-acid residue, a dipeptide residue, a tripeptide residue or a tetrapeptide residue, and wherein the superscripted labels C(=O) and NH indicate the group to which the atoms are bound; X is:, where a = 0 to 5, b1 = 0 to 16, b2 = 0 to 16, c1 = 0 or 1, c2 = 0 or 1, d = 0 to 5, wherein at least b1 or b2 = 0 and at least c1 or c2 = 0; G L is a linker for connecting to an antibody or an antigen-binding fragment of an antibody, wherein G L is selected from (G L1-1) (G L6) (G L1-2) (G L7) (G L2) (G L8) (G L3-1) (G L9) where the NO 2 group is optional (G L3-2) (G L10) where the NO 2 group is optional (G L3-3) (G L11) where the NO 2 group is optional (G L3-4) (G L12) where the NO 2 group is optional (G L4) (G L13) Where Hal = I, Br, Cl (G L5) (G L14) where Ar represents a C 5-6 arylene group, and X' represents C 1-4 alkyl; Q is: where Q x is such that Q is an amino-acid residue, a dipeptide residue, a tripeptide residue or a tetrapeptide residue, and wherein the superscripted labels C(=O) and NH indicate the group to which the atoms are bound; X is:, where a = 0 to 5, b1 = 0 to 16, b2 = 0 to 16, c1 = 0 or 1, c2 = 0 or 1, d = 0 to 5, wherein at least b1 or b2 = 0 and at least c1 or c2 = 0; G L is a linker for connecting to an antibody or an antigen-binding fragment of an antibody, wherein G L is selected from (G L1-1) (G L6) (G L1-2) (G L7) (G L2) (G L8) (G L3-1) (G L9) where the NO 2 group is optional (G L3-2) (G L10) where the NO 2 group is optional (G L3-3) (G L11) where the NO 2 group is optional (G L3-4) (G L12) where the NO 2 group is optional (G L4) (G L13) Where Hal = I, Br, Cl (G L5) (G L14) where Ar represents a C 5-6 arylene group, and X' represents C 1-4 alkyl; (ib): where R L1 and R L2 are independently selected from H and methyl, or together with the carbon atom to which they are bound form a cyclopropylene or cyclobutylene group; and e is 0 or 1. where R L1 and R L2 are independently selected from H and methyl, or together with the carbon atom to which they are bound form a cyclopropylene or cyclobutylene group; and e is 0 or 1.