A conjugate having formula (VIIu): and its pharmaceutically acceptable salts, wherein T is a targeting or binding ligand; L is a releasable linker that connects to the nitrogen atom inside the bracket; n is 1-20, k is 0-4 and m is 1-10; the portion inside the round bracket is a potent antimitotic agent wherein R 1, R 2, R 3, and R 4 are independently C 1 -C 8 alkyl, heteroalkyl; C 2 -C 8 heterocyclic, carbocyclic, alkylcycloalkyl, heterocycloalkyl; C 3 -C 8 aryl, Ar-alkyl, heteroalkylcycloalkyl, alkylcarbonyl; or two R's can be 3-7 members of a carbocyclic, cycloalkyl, or heterocyclic, heterocycloalkyl ring system; Y is N or CH; in addition, R 1, R 3, and R 4 can be H, and R 2 can be absent; wherein Ar refers to an aromatic or hetero aromatic group, composed of one or several rings, comprising three to fourteen carbon atoms; R 5, R 6, R 8 and R 10 are independently selected from H and C 1 -C 4 alkyl or heteroalkyl; R 7 is independently selected from H, R 14 or -R 14 C(=O)X 1 R 15 or -R 14 X 1 R 15, wherein R 14 and R 15 are independently selected from C 1 -C 8 alkyl, or heteroalkyl; C 2 -C 8 alkenyl, alkynyl; heterocyclic, carbocyclic, cycloalkyl; C 3 -C 8 aryl, heterocycloalkyl, heteroaralkyl, heteroalkylcycloalkyl, alkylcarbonyl; X 1 is selected from O, S, S-S, NH, or NR 14; R 9 is independently H, -O-, -OR 14a, -OC(=O)R 14a -, -OC(=O)NHR 14a -, -OC(=O)R 14a SSR 15a, OP(=O)(OR 14a), or OR 14a OP(=O)(OR 15a), wherein R 14a and R 15a are independently C 1 -C 8 alkyl, heteroalkyl; C 2 -C 8 alkenyl, alkynyl, heterocyclic, carbocyclic; C 3 -C 8 aryl, cycloalkyl, alkylcycloalkyl, heterocycloalkyl, heteroalkylcycloalkyl, heteroaralkyl, alkylcarbonyl; R 11 is independently H, R 14b, -R 14b C(=O)R 16, -R 14b X 2 R 16, -R 14b C(=O)X 2, wherein X 2 is -O-, -S-, -NH-, -N(R 14b)-, -O-R 14b -, -S-R 14b -, -S(=O)-R 14b -, or -NHR 14b -; wherein R 14b is C 1 -C 8 alkyl, heteroalkyl; C 2 -C 8 alkenyl, alkynyl, heterocyclic, carbocyclic; C 3 -C 8 aryl, cycloalkyl, alkylcycloalkyl, heterocycloalkyl, heteroalkylcycloalkyl, heteroaralkyl, alkylcarbonyl; R 16 is H, OH, R 14b, or one to four amino acid units; or R 11 is: wherein X 2 is O, S, N-R 8; R 8 is H, C 1 -C 6 alkyl or heteroalkyl; Z 1 is H, CH 2 OP (OR 18) 2, PO(OR 18) 2, C OP (OR 18) 2, P (OR 18)OP (OR 18) 2, C R 18, C NHR 18, SO 2 (OR 18), C 4 -C 12 glycosides, or C 1 -C 8 alkyl, carboxyalkyl, heterocyclic; R 18 is H, C 1 -C 8 alkyl, carboxyalkyl, heteroalkyl; C 2 -C 8 alkenyl, alkynyl, heterocyclic; C 3 -C 8 aryl, alkylcarbonyl; and Y 1 and Y 2 are N or CH independently. wherein T is a targeting or binding ligand; L is a releasable linker that connects to the nitrogen atom inside the bracket; n is 1-20, k is 0-4 and m is 1-10; the portion inside the round bracket is a potent antimitotic agent wherein R 1, R 2, R 3, and R 4 are independently C 1 -C 8 alkyl, heteroalkyl; C 2 -C 8 heterocyclic, carbocyclic, alkylcycloalkyl, heterocycloalkyl; C 3 -C 8 aryl, Ar-alkyl, heteroalkylcycloalkyl, alkylcarbonyl; or two R's can be 3-7 members of a carbocyclic, cycloalkyl, or heterocyclic, heterocycloalkyl ring system; Y is N or CH; in addition, R 1, R 3, and R 4 can be H, and R 2 can be absent; wherein Ar refers to an aromatic or hetero aromatic group, composed of one or several rings, comprising three to fourteen carbon atoms; R 5, R 6, R 8 and R 10 are independently selected from H and C 1 -C 4 alkyl or heteroalkyl; R 7 is independently selected from H, R 14 or -R 14 C(=O)X 1 R 15 or -R 14 X 1 R 15, wherein R 14 and R 15 are independently selected from C 1 -C 8 alkyl, or heteroalkyl; C 2 -C 8 alkenyl, alkynyl; heterocyclic, carbocyclic, cycloalkyl; C 3 -C 8 aryl, heterocycloalkyl, heteroaralkyl, heteroalkylcycloalkyl, alkylcarbonyl; X 1 is selected from O, S, S-S, NH, or NR 14; R 9 is independently H, -O-, -OR 14a, -OC(=O)R 14a -, -OC(=O)NHR 14a -, -OC(=O)R 14a SSR 15a, OP(=O)(OR 14a), or OR 14a OP(=O)(OR 15a), wherein R 14a and R 15a are independently C 1 -C 8 alkyl, heteroalkyl; C 2 -C 8 alkenyl, alkynyl, heterocyclic, carbocyclic; C 3 -C 8 aryl, cycloalkyl, alkylcycloalkyl, heterocycloalkyl, heteroalkylcycloalkyl, heteroaralkyl, alkylcarbonyl; R 11 is independently H, R 14b, -R 14b C(=O)R 16, -R 14b X 2 R 16, -R 14b C(=O)X 2, wherein X 2 is -O-, -S-, -NH-, -N(R 14b)-, -O-R 14b -, -S-R 14b -, -S(=O)-R 14b -, or -NHR 14b -; wherein R 14b is C 1 -C 8 alkyl, heteroalkyl; C 2 -C 8 alkenyl, alkynyl, heterocyclic, carbocyclic; C 3 -C 8 aryl, cycloalkyl, alkylcycloalkyl, heterocycloalkyl, heteroalkylcycloalkyl, heteroaralkyl, alkylcarbonyl; R 16 is H, OH, R 14b, or one to four amino acid units; or R 11 is: wherein X 2 is O, S, N-R 8; R 8 is H, C 1 -C 6 alkyl or heteroalkyl; Z 1 is H, CH 2 OP (OR 18) 2, PO(OR 18) 2, C OP (OR 18) 2, P (OR 18)OP (OR 18) 2, C R 18, C NHR 18, SO 2 (OR 18), C 4 -C 12 glycosides, or C 1 -C 8 alkyl, carboxyalkyl, heterocyclic; R 18 is H, C 1 -C 8 alkyl, carboxyalkyl, heteroalkyl; C 2 -C 8 alkenyl, alkynyl, heterocyclic; C 3 -C 8 aryl, alkylcarbonyl; and Y 1 and Y 2 are N or CH independently.