RELEASABLE CONJUGATES

1 . A compound of Formula (B)

or a pharmaceutically acceptable salt thereof, wherein:

the aliphatic moiety is selected from a polymer, R, and a group selected from:

polymer-L-(CH 2 ) m — and polymer-L-(CH 2 —CH 2 —O) p —(CH 2 ) m —;

R 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 selected from O, S, and N(R N );

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.