Antibody conjugates of cytotoxic drugs are being developed as target-specific therapeutic agents. Antibodies against various cancer cell-surface antigens have been conjugated with different cytotoxic agents that inhibit various essential cellular targets such as microtubules (maytansinoids, auristatins, taxanes: U.S. Pat. Nos. 5,208,020; 5,416,064; 6,333,410; 6,441,163; 6,340,701; 6,372,738; 6,436,931; 6,596,757; 7,276,497), DNA (calicheamicin, doxorubicin, CC-1065 analogs; U.S. Pat. Nos. 5,475,092; 5,585,499; 5,846,545; 6,534,660; 6,756,397; 6,630,579). Antibody conjugates with some of these cytotoxic drugs are actively being investigated in the clinic for cancer therapy (Richart, A. D., and Tolcher, A. W., 2007, Nature Clinical Practice, 4, 245-255).
The antibody-cytotoxic agent conjugates typically are prepared by the initial modification of reactive moieties on antibodies, such as lysine amino groups, or cysteine groups (generated by reduction of native disulfide bonds or by engineering of additional non-native cysteine residues on to antibodies using molecular biology methods). Thus antibodies are first modified with a heterobifunctional linker reagent, such as those previously described, exemplified by SPDB, SMCC and SIAB (U.S. Pat. No. 6,913,758 and U.S. Patent Publication No. 20050169933) to incorporate a linker with a reactive group such as mixed pyridyldisulfide, maleimide or haloacetamide. The incorporated reactive linker group in the antibody is subsequently conjugated with a cytotoxic agent containing a reactive moiety such as a thiol group. Another conjugation route is by reaction of a cytotoxic agent derivative containing a thiol-reactive group (such as haloacetamide, or maleimide) with thiol groups on the cell-binding agent. Thiol groups are incorporated on cell-binding agents such as an antibody by reduction of native disulfide residues (R. Singh et al., Anal. Biochem., 2002, 304, 147-156), or reduction of incorporated disulfide moieties (via SPDP, succinimidyl 3-(2-pyridyldithio)propionate, followed by reduction with dithiothreitol, D. G. Gilliland et al., Proc. Natl. Acad. Sci. USA., 1980, 77, 4539-4543), or by incorporation of additional non-native cysteine residues (J. B. Stimmel et al., J. Biol. Chem., 2000, 275, 30445-30450), or incorporation of thiol groups by reaction with 2-iminothiolane (R. Jue et al., Biochemistry, 1978, 17, 5399-5406), or methyl 3-mercaptopropionimidate ester (T. P. King et al., Biochemistry, 1978, 17, 1499-1506).
The antibody-cytotoxic agent conjugates with disulfide or thioether linkages are cleaved intracellularly, presumably in lysosomes, to deliver the active cytotoxic agent inside the cancer cell (H. K. Erickson et al., 2006, Cancer Research, 66, 4626-4433). In addition to the killing of target cells, antibody-cytotoxic agent conjugates with reducible disulfide linkage also kill proximate antigen-negative cells in mixed populations of antigen-negative and antigen-positive cells in vitro and in vivo in xenograft models, suggesting the role of target-cell released cytotoxic agent in improving potency against neighboring non-antigen-expressing cells in tumors with heterogeneous antigen expression (Y. V. Kovtun et al., Cancer Research, 2006, 66, 3214-3221).
Although, antibody-cytotoxic drug conjugates show cell killing activity in vitro and anti-tumor activity in vivo, their potency is diminished in many cases, especially when the antigen expression on the target cancer cell is low, or when the target cells are resistant to the treatment. This is often the case in the clinical setting, resulting in low to modest anti-tumor activity in patients. A potential approach to try to circumvent resistance is to synthesize new drugs that bear hydrophilic or lipophobic functionalities (see G. Szokacs et al., Nature Reviews, 5; 219-235, 2006). However, this process is cumbersome and several analogs have to be synthesized, and often modification in the structure of the drug results in loss of biological activity. Thus, there is a need for a different approach.
The method described in the art for preparing a cytotoxic conjugate of a cell binding agent and a drug via non-cleavable linker requires two reaction steps (U.S. Pat. No. 5,208,020 & US Publication No. 2005/0169933). First, the cell binding agent, such as an antibody, is modified with a bifunctional crosslinker that undergoes reaction with the reactive groups of the cell binding agent, such as the amine group on lysine residues or the sulfhydryl group on cysteine residues, to form covalent chemical bonds. Following modification of the cell binding agent, the product is purified to separate the desired modified cell binding agent from unreacted crosslinker. In a second step, known as a conjugation step, a reactive drug derivative, such as a thiol-containing maytansinoid, is added to the modified cell-binding agent for reaction with the modified cell-binding agent. Following this reaction, an additional purification is required to remove any unreacted drug species and other byproducts from the final conjugate. These multiple reaction and purification steps result in low yield of the final conjugate and can be expensive and cumbersome when one considers implementing these steps on a large scale. An additional drawback to these methods is the conjugate heterogeneity that is introduced when unreacted crosslinker remains linked to the cell-binding agent without attached drug. The unreacted crosslinker can then undergo additional side reactions such as hydrolysis and intramolecular or intermolecular reactions. There is therefore a need for functionalized, reactive drug derivatives, such as maytansinoids, that can be covalently linked via a non-cleavable bond to a cell binding agent, such as an antibody in essentially one reaction step.