A compound of Formula (B) or a pharmaceutically acceptable salt thereof, wherein: the aliphatic moiety is selected from a polymer, R P , and a group selected from: polymer-L-(CH 2 ) m - and polymer-L-(CH 2 -CH 2 -O) p -(CH 2 ) m -; R P is selected from optionally substituted C 1-6 alkyl, optionally substituted C 1-3 alkyl-O-(CH 2 -CH 2 -O) p -(CH 2 ) m -, and optionally substituted C 3-7 cycloalkyl; L is a linking group; m and p are each independently an integer from 1 to 10; D is a residue of a biologically active drug; Z 1 is O; Z 3 is selected from O and N(R N ), or Z 3 is absent; A is O or N, wherein when A is O then R 3 is absent; R N is selected from H and optionally substituted C 1-6 alkyl; R 3 is selected from H and C 1-6 alkyl, or R 3 and R 1 , together with A and the carbon atom to which R 1 is attached, form an optionally substituted 4 to 7 membered aliphatic heterocyclic ring; or R 3 and R 2 , together with A, the carbon atom to which R 1 is attached, and the carbon atom to which R 2 is attached, form an optionally substituted 4 to 8 membered aliphatic heterocyclic ring; M A is a self-immolative group having any one of formulae (a)-(i): wherein x denotes a point of attachment to Z 1 and y denotes a point of attachment to Z 3 ; R 1 and R 2 are independently selected from the group consisting of hydrogen, optionally substituted C 1-6 alkyl, optionally substituted C 6-10 aryl and optionally substituted 5- to 14-membered heteroaryl; or R 1 and R 2 are joined together with the carbon atoms to which they are attached to form an optionally substituted C 3-7 cycloalkyl ring, an optionally substituted 4 to 7 membered aliphatic heterocyclic ring, an optionally substituted C 6-10 aryl or an optionally substituted 5- to 14-membered heteroaryl; or R 1 and R 2 are joined together to form a ribose ring system; R 7 and R 8 are independently selected from H and C 1-6 alkyl; and E is a cleavable moiety. the aliphatic moiety is selected from a polymer, R P , and a group selected from: polymer-L-(CH 2 ) m - and polymer-L-(CH 2 -CH 2 -O) p -(CH 2 ) m -; R P is selected from optionally substituted C 1-6 alkyl, optionally substituted C 1-3 alkyl-O-(CH 2 -CH 2 -O) p -(CH 2 ) m -, and optionally substituted C 3-7 cycloalkyl; L is a linking group; m and p are each independently an integer from 1 to 10; D is a residue of a biologically active drug; Z 1 is O; Z 3 is selected from O and N(R N ), or Z 3 is absent; A is O or N, wherein when A is O then R 3 is absent; R N is selected from H and optionally substituted C 1-6 alkyl; R 3 is selected from H and C 1-6 alkyl, or R 3 and R 1 , together with A and the carbon atom to which R 1 is attached, form an optionally substituted 4 to 7 membered aliphatic heterocyclic ring; or R 3 and R 2 , together with A, the carbon atom to which R 1 is attached, and the carbon atom to which R 2 is attached, form an optionally substituted 4 to 8 membered aliphatic heterocyclic ring; M A is a self-immolative group having any one of formulae (a)-(i): wherein x denotes a point of attachment to Z 1 and y denotes a point of attachment to Z 3 ; R 1 and R 2 are independently selected from the group consisting of hydrogen, optionally substituted C 1-6 alkyl, optionally substituted C 6-10 aryl and optionally substituted 5- to 14-membered heteroaryl; or R 1 and R 2 are joined together with the carbon atoms to which they are attached to form an optionally substituted C 3-7 cycloalkyl ring, an optionally substituted 4 to 7 membered aliphatic heterocyclic ring, an optionally substituted C 6-10 aryl or an optionally substituted 5- to 14-membered heteroaryl; or R 1 and R 2 are joined together to form a ribose ring system; R 7 and R 8 are independently selected from H and C 1-6 alkyl; and E is a cleavable moiety. <EOS>