1. A prodrug or a pharmaceutically acceptable salt thereof, comprising a conjugate of a drug-linker DL, where! - D represents a nitrogen containing biologically active moiety and -! L is a linker moiety biologically inactive - L1, represented by formula (I)! ! wherein the dashed line indicates the attachment to the nitrogen of the biologically active moiety by forming an amide bond ! X represents C (R4R4a), N (R4), O, C (R4R4a) -C (R5R5a), C (R5R5a) -C (R4R4a), C (R4R4a) -N (R6), N (R6) - C (R4R4a), C (R4R4a) -O or OC (R4R4a) ! X1 is C or S (O) ! X2 is C (R7, R7a) or C (R7, R7a) -C (R8, R8a) ! X3 represents O, S or N-CN ! R1, R1a, R2, R2a, R3, R3a, R4, R4a, R5, R5a, R6, R7, R7a, R8, R8a are independently selected from the group consisting of H and C1-4 alkyl ! if necessary, one or more pairs of R1a / R4a, R1a / R5a, R4a / R5a, R7a / R8a form a chemical bond ! if necessary, one or more pairs of R1 / R1a, R2 / R2a, R4 / R4a, R5 / R5a, R7 / R7a, R8 / R8a together with the atom to which they are attached, combine to form a C3-7 cycloalkyl or 4-7 membered heterocyclyl ! if necessary, one or more pairs of R1 / R4, R1 / R5, R1 / R6, R4 / R5, R4 / R6, R7 / R8, R2 / R3 together with the atoms to which they are attached are connected to form ring A ! if necessary, R3 / R3a together with the nitrogen atom to which they are attached, combine to form a 4-7 membered heterocycle ! A is selected from the group consisting of phenyl, naphthyl, indenyl, indanyl, tetralinyl, C3-10 cycloalkyl, 4-7 membered heterocyclyl and 9-11-membered heterobicycle and! wherein L1 is substituted with 1-4 groups L2-Z and, if desired, further substituted, provided that the hydrogen marked with the symbol * in formula (I), can be substituted, where! L2 represents a single the chemical
