An antibody-drug conjugate compound having the formula: Ab-(L-D) p wherein: Ab is an antibody; L is a linker having the formula: -Str-(Pep) m -(Sp) n - where Str is a stretcher unit covalently attached to the antibody; Pep is an optional peptide unit of two to twelve amino acid residues, Sp is an optional spacer unit covalently attached to a dimer drug moiety, and m and n are independently selected from 0 and 1; p is an integer from 1 to 8; D is the dimer drug moiety having the formula: where R 1 is selected from H, P 3 H 2, C NR a R b, or a bond to L; R 2 is selected from H, P 3 H 2, C NR a R b, or a bond to L; R a and R b are independently selected from H and C 1 -C 6 alkyl optionally substituted with one or more F, or R a and R b form a five or six membered heterocyclyl group; T is a tether group selected from C 3 -C 12 alkylene, Y, (C 1 -C 6 alkylene)-Y-(C 1 -C 6 alkylene), (C 1 -C 6 alkylene)-Y-(C 1 -C 6 alkylene)-Y-(C 1 -C 6 alkylene), (C 2 -C 6 alkenylene)-Y-(C 2 -C 6 alkenylene), and (C 2 -C 6 alkynylene)-Y-(C 2 -C 6 alkynylene); where Y is independently selected from O, S, NR 1, aryl, and heteroaryl; where alkylene, alkenylene, aryl, and heteroaryl are independently and optionally substituted with F, OH, O(C 1 -C 6 alkyl), NH 2, NHCH 3, N(CH 3) 2, OP 3 H 2, and C 1 -C 6 alkyl, where alkyl is optionally substituted with one or more F; or alkylene, alkenylene, aryl, and heteroaryl are independently and optionally substituted with a bond to L; D' is a drug moiety selected from: and where the wavy line indicates the site of attachment to T; X 1 and X 2 are independently selected from O and NR 3, where R 3 is selected from H and C 1 -C 6 alkyl optionally substituted with one or more F; R 4 is H, CO 2 R, or a bond to L, where R is C 1 -C 6 alkyl or benzyl; and R 5 is H or C 1 -C 6 alkyl; wherein the antibody binds to Napi2b. Ab is an antibody; L is a linker having the formula: -Str-(Pep) m -(Sp) n - where Str is a stretcher unit covalently attached to the antibody; Pep is an optional peptide unit of two to twelve amino acid residues, Sp is an optional spacer unit covalently attached to a dimer drug moiety, and m and n are independently selected from 0 and 1; p is an integer from 1 to 8; D is the dimer drug moiety having the formula: where R 1 is selected from H, P 3 H 2, C NR a R b, or a bond to L; R 2 is selected from H, P 3 H 2, C NR a R b, or a bond to L; R a and R b are independently selected from H and C 1 -C 6 alkyl optionally substituted with one or more F, or R a and R b form a five or six membered heterocyclyl group; T is a tether group selected from C 3 -C 12 alkylene, Y, (C 1 -C 6 alkylene)-Y-(C 1 -C 6 alkylene), (C 1 -C 6 alkylene)-Y-(C 1 -C 6 alkylene)-Y-(C 1 -C 6 alkylene), (C 2 -C 6 alkenylene)-Y-(C 2 -C 6 alkenylene), and (C 2 -C 6 alkynylene)-Y-(C 2 -C 6 alkynylene); where Y is independently selected from O, S, NR 1, aryl, and heteroaryl; where alkylene, alkenylene, aryl, and heteroaryl are independently and optionally substituted with F, OH, O(C 1 -C 6 alkyl), NH 2, NHCH 3, N(CH 3) 2, OP 3 H 2, and C 1 -C 6 alkyl, where alkyl is optionally substituted with one or more F; or alkylene, alkenylene, aryl, and heteroaryl are independently and optionally substituted with a bond to L; D' is a drug moiety selected from: and where the wavy line indicates the site of attachment to T; X 1 and X 2 are independently selected from O and NR 3, where R 3 is selected from H and C 1 -C 6 alkyl optionally substituted with one or more F; R 4 is H, CO 2 R, or a bond to L, where R is C 1 -C 6 alkyl or benzyl; and R 5 is H or C 1 -C 6 alkyl; wherein the antibody binds to Napi2b.