A Ligand-Drug Conjugate having the formula: or or a salt thereof, wherein D is a Drug unit, and Ab is an antibody; and wherein: S is a sulfur atom of the antibody; M 1 is a succinimide ring or hydrolyzed succinimide or together with BU forms a dilactam resulting from the reaction of the base of BU with the succinimide ring; BU is a Basic Unit selected from the group consisting of -(CH 2) x NH 2, -(CH 2) x NHR a, and -(CH 2) x NR a 2, wherein x is an integer of from 0-4 and each R a is independently selected from the group consisting of C 1-6 alkyl and C 1-6 haloalkyl, or two R a groups are combined with the nitrogen to which they are attached to form an azetidinyl, pyrrolidinyl or piperidinyl group provided that there are no less than 2 intervening atoms between the base of the Basic unit and the nitrogen atom of the succinimide (hydrolyzed or non-hydrolyzed) or dilactam; the Drug unit is a cytotoxic agent; the subscript p ranges from 1 to 20; and L O is an optional secondary linker assembly having the formula: wherein -A- is an optional Stretcher unit, the subscript a' is 0 or 1; -W- is an optional Cleavable unit, the subscript w' is 0 or 1; and -Y- is an optional Spacer unit, and the subscript y' is 0 or 1; and the wavy lines indicate the points of attachment to the self-stabilizing linker assembly and the Drug unit, optionally wherein: (i) w' is 1 and W is conjugated directly to the Drug unit, (ii) w' is 1 and W is conjugated directly to the Spacer unit, (iii) w' is 1 and W is conjugated directly to the Drug unit or Spacer unit via a cleavable peptide, disulfide or hydrazine bond, (iv) W is absent, or (v) W is present and the Spacer unit and Stretcher unit are absent. S is a sulfur atom of the antibody; M 1 is a succinimide ring or hydrolyzed succinimide or together with BU forms a dilactam resulting from the reaction of the base of BU with the succinimide ring; BU is a Basic Unit selected from the group consisting of -(CH 2) x NH 2, -(CH 2) x NHR a, and -(CH 2) x NR a 2, wherein x is an integer of from 0-4 and each R a is independently selected from the group consisting of C 1-6 alkyl and C 1-6 haloalkyl, or two R a groups are combined with the nitrogen to which they are attached to form an azetidinyl, pyrrolidinyl or piperidinyl group provided that there are no less than 2 intervening atoms between the base of the Basic unit and the nitrogen atom of the succinimide (hydrolyzed or non-hydrolyzed) or dilactam; the Drug unit is a cytotoxic agent; the subscript p ranges from 1 to 20; and L O is an optional secondary linker assembly having the formula: wherein -A- is an optional Stretcher unit, the subscript a' is 0 or 1; -W- is an optional Cleavable unit, the subscript w' is 0 or 1; and -Y- is an optional Spacer unit, and the subscript y' is 0 or 1; and the wavy lines indicate the points of attachment to the self-stabilizing linker assembly and the Drug unit, optionally wherein: (i) w' is 1 and W is conjugated directly to the Drug unit, (ii) w' is 1 and W is conjugated directly to the Spacer unit, (iii) w' is 1 and W is conjugated directly to the Drug unit or Spacer unit via a cleavable peptide, disulfide or hydrazine bond, (iv) W is absent, or (v) W is present and the Spacer unit and Stretcher unit are absent. (i) w' is 1 and W is conjugated directly to the Drug unit, (ii) w' is 1 and W is conjugated directly to the Spacer unit, (iii) w' is 1 and W is conjugated directly to the Drug unit or Spacer unit via a cleavable peptide, disulfide or hydrazine bond, (iv) W is absent, or (v) W is present and the Spacer unit and Stretcher unit are absent.