Hydrophilic linkers and their uses for conjugation of drugs to a cell binding molecules
Cell binding agent-drug conjugates comprising hydrophilic linkers, and methods of using such linkers and conjugates are provided.
1. A hydrophilic linker of formula (I)
wherein:
Y represents a functional group that enables reaction of the hydrophilic linker with a cell-binding agent;
Q and T are either —P(═O)(OM)-, or —S(O 2 )—, or —S(O)—;
m and n are integer from 0 to 5, but not 0 at the same time; provided that when m=1·n=0, Q is not —P(═O)(OM)-; when n=1, m=0, T is not —P(═O)(OM)-; and when Q or T is —S(O 2 )—, m and n are not 0:
Z represents a functional group that enables linkage of the hydrophilic linker to a cytotoxic drug via a disulfide, thioether, thioester, hydrazone, ether, ester, carbamate, carbonate, secondary, tertiary, or quaternary amine, imine, cycloheteroalkane, heteroaromatic, alkoxime or amide bond;
R 1 , R 2 , R 3 , and R 4 , are the same or different and are absent, linear alkyl having from 1-6 carbon atoms, branched or cyclic alkyl having from 3 to 6 carbon atoms, linear, branched or cyclic alkenyl or alkynyl, or ester, ether or amide having 2-6 carbon atoms, or polyethyleneoxy unit of formula (OCH 2 CH 2 ) p , wherein p is an integer from 1 to about 1000, or combination thereof,
or R 1 , R 2 , R 3 and R 4 are respectively a chain of two or more atoms selected from the group consisting of C, N, O, S, Si, and P that covalently connects a cell-surface binding ligand, a phosphinate or sulfonyl group, a conjugated drug and among themselves (R 1 , R 2 , R 3 and R 4 );
R 5 and R 6 , are the same or different and are absent, H, linear alkyl having from 1-6 carbon atoms, branched or cyclic alkyl having from 3 to 6 carbon atoms, linear, branched or cyclic alkenyl or alkynyl, or ester, ether or amide having 2-6 carbon atoms, or polyethyleneoxy unit of formula (OCH 2 CH 2 ) p , wherein p is an integer from 1 to about 1000, or combination thereof;
M is H, or Na, or K, or N + R 1 R 2 R 3 or a pharmaceutical salt.
2. The hydrophilic linker of claim 1 , wherein Z comprises a thiol, disulfide, amino, carboxy, aldehydes, maleimido, haloacetyl, hydrazine or hydroxyl group.
3. A cell-binding agent-drug conjugate of formula (II):
wherein:
Cb represents a cell-binding agent;
Drug represents a drug linked to the cell-binding agent via the hydrophilic linker by a disulfide, thioether, thioester, peptide, hydrazone, ether, ester, carbamate, carbonate, cycloheteroalkane, heteroaromatic, alkoxime or amide bond;
Q and T are either —P(═O)(OM)-, or —S(O 2 )—, or —S(O)—;
m and n are integer from 0 to 5, but not 0 at the same time; provided that when m=1, n=0, Q is not —P(═O)(OM)-; when n=1, m=0, T is not —P(═O)(OM)-; and when Q or T is —S(O)—, m and n are not 0:
R 1 , R 2 , R 3 , and R 4 , are the same or different and are absent, linear alkyl having from 1-6 carbon atoms, branched or cyclic alkyl having from 3 to 6 carbon atoms, linear, branched or cyclic alkenyl or alkynyl, or ester, ether or amide having 2-6 carbon atoms, or polyethyleneoxy unit of formula (OCH 2 CH 2 ) p , wherein p is an integer from 1 to about 1000, or combination thereof,
or R 1 , R 2 , R 3 and R 4 are respectively a chain of two or more atoms selected from the group consisting of C, N, O, S, Si, and P that covalently connects a cell-surface binding ligand, a phosphinate or sulfonyl group, a conjugated drug and among themselves (R 1 , R 2 , R 3 and R 4 );
R 5 and R 6 , are the same or different and are absent, H, linear alkyl having from 1-6 carbon atoms, branched or cyclic alkyl having from 3 to 6 carbon atoms, linear, branched or cyclic alkenyl or alkynyl, or ester, ether or amide having 2-6 carbon atoms, or polyethyleneoxy unit of formula (OCH 2 CH 2 ) p , wherein p is an integer from 1 to about 1000, or combination thereof;
M is H, or Na, or K, or N + R 1 R 2 R 3 or a pharmaceutical salt.
4. The conjugate of claim 3 , wherein the Drug is selected from the group consisting of
1) therapeutic agents comprising
a). Alkylating agents comprising chlorambucil, chlornaphazine, cyclophosphamide, dacarbazine, estramustine, ifosfamide, mechlorethamine, mechlorethamine oxide hydrochloride, mannomustine, mitobronitol, melphalan, mitolactol, pipobroman, novembichin, phenesterine, prednimustine, thiotepa, trofosfamide, uracil mustard, CC-1065 adozelesin, carzelesin, bizelesin, duocarmycin, KW-2189, CBI-TMI, pyrrolobenzodiazepine dimers, tomaymycin dimers, indolinobenzodiazepine dimers, imidazobenzothiadiazepine dimers, oxazolidinobenzodiazepine dimers, carmustine, lomustine, chlorozotocin, fotemustine, nimustine, ranimustine, busulfan, treosulfan, improsulfan piposulfan, Triazenes, dacarbazine, carboplatin, cisplatin, oxaliplatin, benzodopa, carboquone, meturedopa, uredopa, altretamine, triethylenemelamine, triethylenephosphoramide, triethylenethiophosphaoramide or trimethylolomelamine1;
b). Plant Alkaloids comprising vincristine, vinblastine, vindesine, vinorelbine, navelbin, paclitaxel, docetaxol, Maytansinoids comprising DM1, DM2, DM3, DM4, DM5, DM6, or DM7, maytansine, ansamitocins, cryptophycin 1, cryptophycin 8, epothilones, eleutherobin, discodermolide, bryostatins, dolostatins, auristatins, tubulysins, cephalostatins, pancratistatin, sarcodictyin A or spongistatin;
c). DNA Topoisomerase Inhibitors comprising 9-aminocamptothecin, camptothecin, crisnatol, daunomycin, etoposide, etoposide phosphate, irinotecan, mitoxantrone, novantrone, retinoic acids, retinols, teniposide, topotecan, 9-nitrocamptothecin, or mitomycins;
d). Anti-metabolites comprising methotrexate, trimetrexate, denopterin, pteropterin, aminopterin, mycophenolic acid, tiazofurin, ribavirin, EICAR, hydroxyurea, deferoxamine, ancitabine, azacitidine, 6-azauridine, capecitabine, carmofur, cytarabine, dideoxyuridine, doxifluridine, enocitabine, 5-Fluorouracil, floxuridine, ratitrexed, cytarabine, cytosine arabinoside, fludarabine, azathioprine, fludarabine, mercaptopurine, thiamiprine, thioguanine, and a folic acid replenisher;
e). Hormonal therapies comprising Receptor antagonists comprising Anti-estrogen selected from the group consisting of megestrol, raloxifene, and tamoxifen; LHRH agonists comprising goserelin, and leuprolide acetate; Anti-androgens comprising bicalutamide, flutamide, calusterone, dromostanolone propionate, epitiostanol, goserelin, leuprolide, mepitiostane, nilutamide, testolactone, and trilostane;
f). Vitamin D3 compounds comprising CB 1093, EB 1089 KH 1060, cholecalciferol, or ergocalciferol;
g). Photodynamic therapies comprising verteporfin, phthalocyanine, photo sensitizer Pc4, or demethoxy-hypocrellin A;
h). Cytokines comprising Interferon-alpha, Interferon-gamma, tumor necrosis factor (TNFs), or human proteins containing a TNF domain;
i). Kinase inhibitors comprising, imatinib, gefitinib, pegaptanib, sorafenib, dasatinib, sunitinib, erlotinib, nilotinib, lapatinib, axitinib, pazopanib, vandetanib, mubritinib, ponatinib, bafetinib, bosutinib, cabozantinib, vismodegib, iniparib, ruxolitinib, CYT387, axitinib, tivozanib, sorafenib, or ispinesib;
j). antibiotics comprising calicheamicin γ1, δ1, α1 or β1, dynemicin A, deoxydynemicin, esperamicin, kedarcidin, C-1027, maduropeptin, neocarzinostatin chromophore, chromoprotein enediyne antiobiotic chromomophores, aclacinomysins, actinomycin, authramycin, azaserine, bleomycins, cactinomycin, carabicin, carminomycin, carzinophilin; chromomycins, dactinomycin, daunorubicin, detorubicin, 6-diazo-5-oxo-L-norleucine, doxorubicin, morpholino-doxorubicin, cyanomorpholino-doxorubicin, 2-pyrrolino-doxorubicin, deoxydoxorubicin, epirubicin, esorubicin, idarubicin, marcellomycin, nitomycins, mycophenolic acid, nogalamycin, olivomycins, peplomycin, potfiromycin, puromycin, quelamycin, rodorubicin, streptonigrin, streptozocin, tubercidin, ubenimex, zinostatin, or zorubicin;
k). Polyketides comprising bullatacin or bullatacinone; gemcitabine, epoxomicins, bortezomib, thalidomide, lenalidomide, pomalidomide, tosedostat, fosbretabulin, vemurafib, gentespib, tecemotide, Isoprenylation inhibitors, Dopaminergic neurotoxins, Cell cycle inhibitors, Actinomycin D, bleomycin A2, bleomycin B2, peplomycin, daunorubicin, adriamycin, idarubicin, epirubicin, pirarubicin, zorubicin, mtoxantrone, MDR inhibitors, Ca +2 ATPase inhibitors, Vorinostat, Romidepsin, Panobinostat, Valproic acid, Mocetinostat, Belinostat, PCI-24781, Entinostat, SB939, Resminostat, Givinostat, AR-42, CUDC-101, sulforaphane, Trichostatin A, Celecoxib, glitazones, epigallocatechin gallate, Disulfiram, Salinosporamide A, aminoglutethimide, mitotane, trilostane; aceglatone, aldophosphamide glycoside; aminolevulinic acid, amsacrine, arabinoside, bestrabucil, bisantrene, edatraxate, defofamine, demecolcine, diaziquone, eflornithine, elformithine, elliptinium acetate, etoglucid, gallium nitrate, gacytosine, hydroxyurea, ibandronate, lentinan, lonidamine, mitoguazone, mitoxantrone, mopidamol, nitracrine, pentostatin, phenamet, pirarubicin, podophyllinic acid, 2-ethylhydrazide, procarbazine, razoxane, rhizoxin, sizofiran, spirogermanium, tenuazonic acid, triaziquone; 2,2′,2″-trichlorotriethylamine, T-2 toxin, verrucarin A, roridin A, anguidine, urethane, siRNA, or antisense drugs;
2). Anti-autoimmune disease agents comprising cyclosporine, cyclosporine A, aminocaproic acid, azathioprine, bromocriptine, chlorambucil, chloroquine, cyclophosphamide, amcinonide, betamethasone, budesonide, hydrocortisone, flunisolide, fluticasone propionate, fluocortolone danazol, dexamethasone, Triamcinolone acetonide, beclometasone dipropionate, DHEA, enanercept, hydroxychloroquine, meloxicam, methotrexate, mofetil, mycophenylate, prednisone, sirolimus, or tacrolimus;
3). Anti-infectious disease agents comprising amikacin, astromicin, netilmicin, sisomicin, isepamicin, hygromycin B, arbekacin, bekanamycin, dibekacin, tobramycin, framycetin, paromomycin, ribostamycin, spectinomycin, streptomycin, verdamicin, azidamfenicol, chloramphenicol, florfenicol, thiamphenicol, geldanamycin, herbimycin, biapenem, doripenem, ertapenem, imipenem, meropenem, panipenem, carbacephem, cefacetrile, cefaclor, cefradine, cefadroxil, cefalonium, cefaloridine, cefalotin, cefalothin, cefalexin, cefaloglycin, cefamandole, cefapirin, cefatrizine, cefazaflur, cefazedone, cefazolin, cefbuperazone, cefcapene, cefdaloxime, cefepime, cefminox, cefoxitin, cefprozil, cefroxadine, ceftezole, cefuroxime, cefixime, cefdinir, cefditoren, cefepime, cefetamet, cefmenoxime, cefodizime, cefonicid, cefoperazone, ceforanide, cefotaxime, cefotiam, cefozopran, cephalexin, cefpimizole, cefpiramide, cefpirome, cefpodoxime, cefprozil, cefquinome, cefsulodin, ceftazidime, cefteram, ceftibuten, ceftiolene, ceftizoxime, ceftobiprole, ceftriaxone, cefuroxime, cefuzonam, cefotetan, cefmetazole, flomoxef, and latamoxef, bleomycin, vancomycin, teicoplanin, ramoplanin, Glycylcyclines, penam clavam, clindamycin, and lincomycin, daptomycin, A54145, calcium-dependent antibiotics, azithromycin, cethromycin, clarithromycin, dirithromycin, erythromycin, flurithromycin, josamycin, ketolide, midecamycin, miocamycin, oleandomycin, rifamycins, rokitamycin, roxithromycin, spectinomycin, spiramycin, troleandomycin, telithromycin, aztreonam, tigemonam, Oxazolidinones, amoxicillin, ampicillin, azidocillin, azlocillin, benzylpenicillin, benzathine benzylpenicillin, benzathine phenoxymethylpenicillin, clometocillin, procaine benzylpenicillin, carbenicillin, cloxacillin, dicloxacillin, epicillin, flucloxacillin, mecillinam, mezlocillin, meticillin, nafcillin, oxacillin, penamecillin, penicillin, pheneticillin, phenoxymethylpenicillin, piperacillin, propicillin, sulbenicillin, temocillin, ticarcillin, bacitracin, colistin, polymyxin B, alatrofloxacin, balofloxacin, ciprofloxacin, clinafloxacin, danofloxacin, difloxacin, enoxacin, enrofloxacin, floxin, garenoxacin, gatifloxacin, gemifloxacin, grepafloxacin, kano trovafloxacin, levofloxacin, lomefloxacin, marbofloxacin, moxifloxacin, nadifloxacin, norfloxacin, orbifloxacin, ofloxacin, pefloxacin, trovafloxacin, grepafloxacin, sitafloxacin, sparfloxacin, temafloxacin, tosufloxacin, trovafloxacin, pristinamycin, quinupristin/dalfopristin, mafenide, prontosil, sulfacetamide, sulfamethizole, sulfanilimide, sulfasalazine, sulfisoxazole, trimethoprim, trimethoprim-sulfamethoxazole, Steroid antibacterials, doxycycline, chlortetracycline, clomocycline, demeclocycline, lymecycline, meclocycline, metacycline, minocycline, oxytetracycline, penimepicycline, rolitetracycline, tetracycline, annonacin, arsphenamine, bactoprenol inhibitors, DADAL/AR inhibitors, dictyostatin, discodermolide, eleutherobin, epothilone, ethambutol, etoposide, faropenem, fusidic acid, furazolidone, isoniazid, laulimalide, metronidazole, mupirocin, mycolactone, NAM synthesis inhibitors, nitrofurantoin, paclitaxel, platensimycin, pyrazinamide, rifampin, tazobactam tinidazole, or uvaricin;
4). Anti-viral drugs comprising aplaviroc, maraviroc, vicriviroc, enfuvirtide, PRO 140, raltegravir, elvitegravir, globoidnan A, bevirimat, vivecon, oseltamivir, zanamivir, peramivir, abacavir, aciclovir, adefovir, amdoxovir, apricitabine, brivudine, cidofovir, clevudine, dexelvucitabine, didanosine, elvucitabine, emtricitabine, entecavir, famciclovir, fluorouracil, 3′-fluoro-2′,3′-dideoxythymidine and 3′-fluoro-2′,3′-dideoxyguanosine, fomivirsen, ganciclovir, idoxuridine, lamivudine, β-1-thymidine, β-1-2′-deoxycytidine, penciclovir, racivir, stampidine, stavudine, taribavirin, telbivudine, tenofovir, trifluridine valaciclovir, valganciclovir, zalcitabine, zidovudine amantadine, ateviridine, capravirine, diarylpyrimidines, delavirdine, docosanol, emivirine, efavirenz, foscarnet, imiquimod, interferon alfa, loviride, lodenosine, methisazone, nevirapine, NOV-205, peginterferon alfa, podophyllotoxin, rifampicin, rimantadine, resiquimod, tromantadine; amprenavir, atazanavir, boceprevir, darunavir, fosamprenavir, indinavir, lopinavir, nelfinavir, pleconaril, ritonavir, saquinavir, telaprevir, tiprananvir, umifenovir, calanolide-a, ceragenin, cyanovirin-n, epigallocatechin gallate, foscarnet, griffithsin, hydroxyurea, KP-1461, miltefosine, pleconaril, ribavirin, or seliciclib;
5). a radioisotopes of 3 H, 11 C, 14 C, 18 F, 32 P, 35 S, 64 Cu, 68 Ga, 86 Y, 99 Tc, 111 In, 123 I, 124 I, 125 I, 131 I, 133 Xe, 177 Lu, 211 At, or 213 Bi; and
6) the pharmaceutically acceptable salts or acids of any of the above drugs.
5. The conjugate of claim 3 , wherein Drug is selected from the group consisting of tubulysins, calicheamicins, auristatins, maytansinoids, CC-1065 compounds, morpholinos doxorubicins, taxanes, cryptophycins, epothilones, and benzodiazepine dimers, siRNA or a combination thereof, and pharmaceutically acceptable salts or acids of any of the above.
6. The conjugate of claim 3 , wherein the cell binding agent is selected from the group consisting of antibodies, proteins, vitamins, peptides, polymeric micelles, liposomes, lipoprotein-based drug carriers, nano-particle drug carriers, dendrimers, and combination thereof.
7. The conjugate of claim 3 , wherein the cell-binding agent is an antibody, a single chain antibody, an antibody fragment that binds to a target cell, a monoclonal antibody, a single chain monoclonal antibody, or a monoclonal antibody fragment that binds the target cell, a chimeric antibody, a chimeric antibody fragment that binds to the target cell, a domain antibody, a domain antibody fragment that binds to the target cell, a resurfaced antibody, a resurfaced single chain antibody, or a resurfaced antibody fragment that binds to the target cell, a humanized antibody or a resurfaced antibody, a humanized single chain antibody, or a humanized antibody fragment that binds to the target cell, a lymphokine, a hormone, a vitamin, a growth factor, a colony stimulating factor, or a nutrient-transport molecule.
8. The conjugate of claim 3 , wherein the drug is selected from the group consisting of a toxin, a chemotherapeutic agent, a drug moiety, an antibiotic, a radioactive isotope, and a nucleolytic enzyme.
9. The conjugate of claim 3 , wherein the conjugate comprises one or more linker components of 6-maleimidocaproyl, maleimidopropanoyl, valine-citrulline, alanine-phenylalanine, p-aminobenzyloxycarbonyl, 4-thio-pentanoate, 4-(N-maleimidomethyl)cyclohexane-1-carboxylate, 4-thio-butyrate, maleimidoethyl, 4-thio-2-hydroxysulfonyl-butyrate and (4-acetyl)aminobenzoate.
10. The conjugate of claim 3 , wherein Drug is selected from the group consisting of tubulysins, maytansinoids, taxanoids, CC-1065 compounds, daunorubicin and doxorubicin compounds, benzodiazepine dimers, calicheamicins and the enediyne antibiotics, actinomycin, azaserines, bleomycins, epirubicin, tamoxifen, idarubicin, dolastatins/auristatins, duocarmycins, thiotepa, vincristine, hemiasterlins, and esperamicins.
11. The conjugate of claim 3 , having in vitro, in vivo or ex vivo cell killing activity.
12. The conjugate of claim 3 , wherein the conjugate comprises either a peptide of 1-20 units of natural or unnatural amino acids, or a p-aminobenzyl unit, or a 6-maleimidocaproyl unit, or a disulfide unit, or a thioether unit, or a hydrozone unit, or an alkoxime unit.
13. The conjugate of claim 3 , wherein the conjugate is cleavable by a protease.
14. The conjugate of claim 3 , wherein the drug is derived from a drug containing a —SH, —CONHNH 2 , -≡, —COR″ or —NHR″, wherein R″ is H, CH 3 , Et, Pr, Bu, or Ph; and/or the cell-binding agent is an antibody or protein.
15. A pharmaceutical composition comprising a therapeutically effective amount of the conjugate of claim 3 , and a pharmaceutically acceptable salt, carrier, diluent, or excipient therefor.
16. A pharmaceutical composition comprising the conjugate of claim 3 , and a pharmaceutically acceptable carrier.
17. A pharmaceutical composition comprising a therapeutically effective amount of the conjugate of claim 3 , and a chemotherapeutic agent, radiation therapy agent, immunotherapy agent, autoimmune disorder agent, or anti-infectious agent.
18. A compound of formula (III):
wherein:
Cb represents a cell-binding agent;
Q and T are either —P(═O)(OM)-, or —S(O 2 )—, or —S(O)—;
m and n are integer from 0 to 5, but not 0 at the same time; provided that when m=1, n=0, Q is not —P(═O)(OM)-; and when n=1, m=0, T is not —P(═O)(OM)-;
q is an integer from 1 to 20;
Z represents a functional group that enables linkage of the compound to a cytotoxic drug via a disulfide, thioether, thioester, hydrazone, ether, ester, carbamate, carbonate, secondary, tertiary, or quaternary amine, imine, cycloheteroalkane, heteroaromatic, alkoxime or amide bond;
R 1 , R 2 , R 3 , and R 4 , are the same or different and are absent, linear alkyl having from 1-6 carbon atoms, branched or cyclic alkyl having from 3 to 6 carbon atoms, linear, branched or cyclic alkenyl or alkynyl, or ester, ether or amide having 2-6 carbon atoms, or polyethyleneoxy unit of formula (OCH 2 CH 2 ) p , wherein p is an integer from 1 to about 1000, or combination thereof,
or R 1 , R 2 , R 3 and R 4 are respectively a chain of two or more atoms selected from the group consisting of C, N, O, S, Si, and P that covalently connects a cell-surface binding ligand, a phosphinate or sulfonyl group, a conjugated drug and among themselves (R 1 , R 2 , R 3 and R 4 );
R 5 and R 6 , are the same or different and are absent, H, linear alkyl having from 1-6 carbon atoms, branched or cyclic alkyl having from 3 to 6 carbon atoms, linear, branched or cyclic alkenyl or alkynyl, or ester, ether or amide having 2-6 carbon atoms, or polyethyleneoxy unit of formula (OCH 2 CH 2 ) p , wherein p is an integer from 1 to about 1000, or combination thereof;
M is H, or Na, or K, or N + R 1 R 2 R 3 or a pharmaceutical salt.
19. The conjugate of claim 18 , wherein the cytotoxic drug is a drug containing a —SH, —CONHNH 2 , -≡, —COR″ or —NHR″, wherein R″ is H, CH 3 , Et, Pr, Bu, or Ph; and/or the cell-binding agent is an antibody or protein.
20. A compound of formula (IV):
wherein:
Y represents a functional group that enables reaction of the compound with a cell-binding agent;
Q and T are either —P(═O)(OM)-, or —S(O 2 )—, or —S(O)—;
m and n are integer from 0 to 5, but not 0 at the same time; provided that when m=1, n=0, Q is not —P(═O)(OM)-; and when n=1, m=0, T is not —P(═O)(OM)-;
“Drug” represents a drug linked to the cell-binding agent via the hydrophilic linker by a disulfide, thioether, thioester, peptide, hydrazone, ether, ester, carbamate, carbonate, cycloheteroalkane, heteroaromatic, alkoxime or amide bond;
R 1 , R 2 , R 3 , and R 4 , are the same or different and are absent, linear alkyl having from 1-6 carbon atoms, branched or cyclic alkyl having from 3 to 6 carbon atoms, linear, branched or cyclic alkenyl or alkynyl, or ester, ether or amide having 2-6 carbon atoms, or polyethyleneoxy unit of formula (OCH 2 CH 2 ) p , wherein p is an integer from 1 to about 1000, or combination thereof,
or R 1 , R 2 , R 3 and R 4 are respectively a chain of two or more atoms selected from the group consisting of C, N, O, S, Si, and P that covalently connects a cell-surface binding ligand, a phosphinate or sulfonyl group, a conjugated drug and among themselves (R 1 , R 2 , R 3 and R 4 );
R 5 and R 6 , are the same or different and are absent, H, linear alkyl having from 1-6 carbon atoms, branched or cyclic alkyl having from 3 to 6 carbon atoms, linear, branched or cyclic alkenyl or alkynyl, or ester, ether or amide having 2-6 carbon atoms, or polyethyleneoxy unit of formula (OCH 2 CH 2 ) p , wherein p is an integer from 1 to about 1000, or combination thereof;
M is H or Na, or K or N + R 1 R 2 R 3 or a pharmaceutical salt.
21. The conjugate of claim 20 , wherein the drug is derived from a drug containing a —SH, —CONHNH 2 , -≡, —COR″ or —NHR″, wherein R″ is H, CH 3 , Et, Pr, Bu, or Ph; and/or the cell-binding agent is an antibody or protein.