A conjugate having Formula la': or a pharmaceutically acceptable salt or solvate thereof wherein: Ab is an antibody, A is a Stretcher unit, a is 0 or 1, each W is independently an Amino Acid unit, w is an integer ranging from 0 to 12, Y is a Spacer unit, y is 0, 1, or 2, p ranges from 1 to 20, and D F is a Drug unit having the formula: wherein: independently at each location: R 2 is selected from H and C 1 -C 8 alkyl; R 3 is selected from H, C 1 -C 8 alkyl, C 3 -C 8 carbocycle, aryl, C 1 -C 8 alkyl-aryl, C 1 -C 8 alkyl-(C 3 -C 8 carbocycle), C 3 -C 8 heterocycle, and C 1 -C 8 alkyl-(C 3 -C 8 heterocycle); R 4 is selected from H, C 1 -C 8 alkyl, C 3 -C 8 carbocycle, aryl, C 1 -C 8 alkyl-aryl, C 1 -C 8 alkyl-(C 3 -C 8 carbocycle), C 3 -C 8 heterocycle, and C 1 -C 8 alkyl-(C 3 -C 8 heterocycle); R 5 is selected from H and methyl; or: R 4 and R 5 jointly form a carbocyclic ring and have the formula -(CR a R b) n -, wherein R a and R b are independently selected from H, C 1 -C 8 alkyl, and C 3 -C 8 carbocycle, and n is selected from 2, 3, 4, 5 and 6; R 6 is selected from H and C 1 -C 8 alkyl; R 7 is selected from H, C 1 -C 8 alkyl, C 3 -C 8 carbocycle, aryl, C 1 -C 8 alkyl-aryl, C 1 -C 8 alkyl-(C 3 -C 8 carbocycle), C 3 -C 8 heterocycle, and C 1 -C 8 alkyl-(C 3 -C 8 heterocycle); each R 8 is independently selected from H, OH, C 1 -C 8 alkyl, C 3 -C 8 carbocycle, and O-(C 1 -C 8 alkyl); R 9 is selected from H and C 1 -C 8 alkyl; R 10 is selected from aryl and C 3 -C 8 heterocycle; Z is O, S, NH, or NR 12, wherein R 12 is C 1 -C 8 alkyl; R 11 is selected from H, C 1 -C 20 alkyl, -(R 13 O) m -R 14, and -(R 13 O) m -CH(R 15) 2; m is an integer ranging from 1 to 1000; R 13 is C 2 -C 8 alkyl; R 14 is H or C 1 -C 8 alkyl; each occurrence of R 15 is independently H, COOH, -(CH 2) n -N(R 16) 2, -(CH 2) n -SO 3 H, or -(CH 2) n -SO 3 -C 1 -C 8 alkyl; each occurrence of R 16 is independently H, C 1 -C 8 alkyl, or -(CH 2) n -COOH; and n is an integer ranging from 0 to 6. Ab is an antibody, A is a Stretcher unit, a is 0 or 1, each W is independently an Amino Acid unit, w is an integer ranging from 0 to 12, Y is a Spacer unit, y is 0, 1, or 2, p ranges from 1 to 20, and D F is a Drug unit having the formula: wherein: independently at each location: R 2 is selected from H and C 1 -C 8 alkyl; R 3 is selected from H, C 1 -C 8 alkyl, C 3 -C 8 carbocycle, aryl, C 1 -C 8 alkyl-aryl, C 1 -C 8 alkyl-(C 3 -C 8 carbocycle), C 3 -C 8 heterocycle, and C 1 -C 8 alkyl-(C 3 -C 8 heterocycle); R 4 is selected from H, C 1 -C 8 alkyl, C 3 -C 8 carbocycle, aryl, C 1 -C 8 alkyl-aryl, C 1 -C 8 alkyl-(C 3 -C 8 carbocycle), C 3 -C 8 heterocycle, and C 1 -C 8 alkyl-(C 3 -C 8 heterocycle); R 5 is selected from H and methyl; or: R 4 and R 5 jointly form a carbocyclic ring and have the formula -(CR a R b) n -, wherein R a and R b are independently selected from H, C 1 -C 8 alkyl, and C 3 -C 8 carbocycle, and n is selected from 2, 3, 4, 5 and 6; R 6 is selected from H and C 1 -C 8 alkyl; R 7 is selected from H, C 1 -C 8 alkyl, C 3 -C 8 carbocycle, aryl, C 1 -C 8 alkyl-aryl, C 1 -C 8 alkyl-(C 3 -C 8 carbocycle), C 3 -C 8 heterocycle, and C 1 -C 8 alkyl-(C 3 -C 8 heterocycle); each R 8 is independently selected from H, OH, C 1 -C 8 alkyl, C 3 -C 8 carbocycle, and O-(C 1 -C 8 alkyl); R 9 is selected from H and C 1 -C 8 alkyl; R 10 is selected from aryl and C 3 -C 8 heterocycle; Z is O, S, NH, or NR 12, wherein R 12 is C 1 -C 8 alkyl; R 11 is selected from H, C 1 -C 20 alkyl, -(R 13 O) m -R 14, and -(R 13 O) m -CH(R 15) 2; m is an integer ranging from 1 to 1000; R 13 is C 2 -C 8 alkyl; R 14 is H or C 1 -C 8 alkyl; each occurrence of R 15 is independently H, COOH, -(CH 2) n -N(R 16) 2, -(CH 2) n -SO 3 H, or -(CH 2) n -SO 3 -C 1 -C 8 alkyl; each occurrence of R 16 is independently H, C 1 -C 8 alkyl, or -(CH 2) n -COOH; and n is an integer ranging from 0 to 6. R 2 is selected from H and C 1 -C 8 alkyl; R 3 is selected from H, C 1 -C 8 alkyl, C 3 -C 8 carbocycle, aryl, C 1 -C 8 alkyl-aryl, C 1 -C 8 alkyl-(C 3 -C 8 carbocycle), C 3 -C 8 heterocycle, and C 1 -C 8 alkyl-(C 3 -C 8 heterocycle); R 4 is selected from H, C 1 -C 8 alkyl, C 3 -C 8 carbocycle, aryl, C 1 -C 8 alkyl-aryl, C 1 -C 8 alkyl-(C 3 -C 8 carbocycle), C 3 -C 8 heterocycle, and C 1 -C 8 alkyl-(C 3 -C 8 heterocycle); R 5 is selected from H and methyl; or: R 4 and R 5 jointly form a carbocyclic ring and have the formula -(CR a R b) n -, wherein R a and R b are independently selected from H, C 1 -C 8 alkyl, and C 3 -C 8 carbocycle, and n is selected from 2, 3, 4, 5 and 6; R 6 is selected from H and C 1 -C 8 alkyl; R 7 is selected from H, C 1 -C 8 alkyl, C 3 -C 8 carbocycle, aryl, C 1 -C 8 alkyl-aryl, C 1 -C 8 alkyl-(C 3 -C 8 carbocycle), C 3 -C 8 heterocycle, and C 1 -C 8 alkyl-(C 3 -C 8 heterocycle); each R 8 is independently selected from H, OH, C 1 -C 8 alkyl, C 3 -C 8 carbocycle, and O-(C 1 -C 8 alkyl); R 9 is selected from H and C 1 -C 8 alkyl; R 10 is selected from aryl and C 3 -C 8 heterocycle; Z is O, S, NH, or NR 12, wherein R 12 is C 1 -C 8 alkyl; R 11 is selected from H, C 1 -C 20 alkyl, -(R 13 O) m -R 14, and -(R 13 O) m -CH(R 15) 2; m is an integer ranging from 1 to 1000; R 13 is C 2 -C 8 alkyl; R 14 is H or C 1 -C 8 alkyl; each occurrence of R 15 is independently H, COOH, -(CH 2) n -N(R 16) 2, -(CH 2) n -SO 3 H, or -(CH 2) n -SO 3 -C 1 -C 8 alkyl; each occurrence of R 16 is independently H, C 1 -C 8 alkyl, or -(CH 2) n -COOH; and n is an integer ranging from 0 to 6.