A compound having the formula A-L-B wherein A is represented by denotes the attachment point of A to L; L is a bond or a self-immolative spacer; B is represented by denotes the attachment point of B to L; A 1 is selected such that A'-OH is a therapeutic, diagnostic or theranostic agent which contains an -OH group that is attached to a 5- or 6-membered aromatic or heteroaromatic ring; A 2 and A 3 are independently selected such that A 2 -NH-A 3 is a therapeutic, diagnostic or theranostic agent which contains an -NH 2 or -NH- moiety; X is selected from -N(R a)-, -N(R b)- and -CR c 2 -; X 1 is -(CR d 2) m -; X 2 is -(CR e 2) n -; X 3 is -CR f 2 -; Y is -(CR g 2) p -; W is independently selected from -OH, -N(R h)(R i), -C -heterocyclic group, or -heterocyclic group, wherein the heterocyclic group is selected from azetidinyl, pyrrolidinyl, piperidinyl, piperazinyl, N' -(C 1-4 -alkyl)piperazinyl, or morpholino, and wherein if W is -heterocyclic group then the heterocyclic R a group is attached to the -C 1-4 -alkyl or -(C 2-4 -alkylene)- group via the N atom, or if W is -C -heterocyclic group then the heterocyclic group is attached to the C moiety via the N atom; is selected from -H, -C 1-4 -alkyl, -C -C 1-4 -alkyl, -C -O-C 1-4 -alkyl, -C -N(C 1-4 -alkyl) 2, -S 2 -C 1-4 -alkyl, and -(C 2-4 -alkylene)-O-(C 1-4 -alkyl), wherein -C 1-4 -alkyl or -(C 2-4 -alkylene)- can be optionally substituted by W; R b is an acyl group of a monopeptide selected from -proteinogenic amino acids attached via a carboxy group; R c groups are independently selected from -H and -C 1-4 -alkyl; R d groups are independently selected from -H and -C 1-4 -alkyl; R e groups are independently selected from -H and -C 1-4 -alkyl; R f groups are independently selected from -H and -C 1-4 -alkyl; R g groups are independently selected from -H and -C 1-4 -alkyl; R h is independently selected from -H, -C 1-4 -alkyl and --CH 2 CH 2 OH; R i is independently selected from -H, -C 1-4 -alkyl and -CH 2 CH 2 OH; m is 0, 1 or 2; n is 1 or 2, provided that m+n is 2 or 3; p is 0, 1, or 2, provided that when X = -N(R a)- or -N(R b)- then p = 1 or 2; or any pharmaceutically acceptable salt, solvate or ester thereof. A is represented by denotes the attachment point of A to L; L is a bond or a self-immolative spacer; B is represented by denotes the attachment point of B to L; A 1 is selected such that A'-OH is a therapeutic, diagnostic or theranostic agent which contains an -OH group that is attached to a 5- or 6-membered aromatic or heteroaromatic ring; A 2 and A 3 are independently selected such that A 2 -NH-A 3 is a therapeutic, diagnostic or theranostic agent which contains an -NH 2 or -NH- moiety; X is selected from -N(R a)-, -N(R b)- and -CR c 2 -; X 1 is -(CR d 2) m -; X 2 is -(CR e 2) n -; X 3 is -CR f 2 -; Y is -(CR g 2) p -; W is independently selected from -OH, -N(R h)(R i), -C -heterocyclic group, or -heterocyclic group, wherein the heterocyclic group is selected from azetidinyl, pyrrolidinyl, piperidinyl, piperazinyl, N' -(C 1-4 -alkyl)piperazinyl, or morpholino, and wherein if W is -heterocyclic group then the heterocyclic R a group is attached to the -C 1-4 -alkyl or -(C 2-4 -alkylene)- group via the N atom, or if W is -C -heterocyclic group then the heterocyclic group is attached to the C moiety via the N atom; is selected from -H, -C 1-4 -alkyl, -C -C 1-4 -alkyl, -C -O-C 1-4 -alkyl, -C -N(C 1-4 -alkyl) 2, -S 2 -C 1-4 -alkyl, and -(C 2-4 -alkylene)-O-(C 1-4 -alkyl), wherein -C 1-4 -alkyl or -(C 2-4 -alkylene)- can be optionally substituted by W; R b is an acyl group of a monopeptide selected from -proteinogenic amino acids attached via a carboxy group; R c groups are independently selected from -H and -C 1-4 -alkyl; R d groups are independently selected from -H and -C 1-4 -alkyl; R e groups are independently selected from -H and -C 1-4 -alkyl; R f groups are independently selected from -H and -C 1-4 -alkyl; R g groups are independently selected from -H and -C 1-4 -alkyl; R h is independently selected from -H, -C 1-4 -alkyl and --CH 2 CH 2 OH; R i is independently selected from -H, -C 1-4 -alkyl and -CH 2 CH 2 OH; m is 0, 1 or 2; n is 1 or 2, provided that m+n is 2 or 3; p is 0, 1, or 2, provided that when X = -N(R a)- or -N(R b)- then p = 1 or 2;