Abstract #5480: Antibody-maytansinoid conjugates with hydrophilic linkers: cytotoxic therapeutics with enhanced potency against cancer cells with low antigen number and multidrug resistance
Authors: Brenda Kellogg, Erin Maloney, Michael Okamoto, Nathan Fishkin, Hans Erickson, Xiuxia Sun, Sharon Wilhelm, Robert Zhao, Jan Pinkas, Michele Mayo, Yelena Kovtun, Ravi Chari, Rajeeva Singh
Journal: Cancer Research (2009)

Conjugates of antibodies with maytansinoid cytotoxic agents are currently in clinical trials for various cancers. These \#8220;standard\#8221; antibody-maytansinoid conjugates (AMCs) are prepared by the modification of lysine residues in the antibody to incorporate approximately 4 maytansinoid (DMx) molecules per antibody (Ab) using either a hindered disulfide or a non-reducible linkage. Upon binding to the target antigen, the conjugate is internalized into the cancer cell and its antibody component is processed, thereby releasing the maytansinoid. It, in turn, inhibits microtubule function and results in cell death. Here we report on a new class of linkers - hydrophilic linkers - that enable the formation of antibody-maytansinoid conjugates with increased DMx/Ab loads compared to standard 4 DMx/Ab AMCs. These new hydrophilic linkers can employ either disulfide or non-reducible linkages, and are able to achieve high DMx/Ab loads (6-8 DMx/Ab) without compromising conjugate biochemical characteristics. High DMx/Ab load conjugates with these hydrophilic linkers were found to have 8-to-100-fold greater cytotoxic potency against cancer cells with low expression of the target antigen than standard AMCs in  in vitro  studies.   One AMC high DMx/Ab-load design studied consisted of a hydrophilic linker containing four ethylene glycol units (PEG 4 -linker) incorporating a non-reducible bond. AMC conjugates (7 DMx/Ab) with this design showed desirable pharmacokinetic properties in CD-1 mice, similar to the findings with standard AMCs containing a non-reducible linker. However, plasma samples from mice dosed with these high DMx/Ab-load PEG 4 -linker conjugates were found to demonstrate 10-fold greater cytotoxic bioactivity  in vitro  against an antigen-expressing human cancer cell line than standard AMCs. Additionally, the tolerability of a high DMx/Ab-load (6 DMx/Ab) PEG 4 -linker conjugate in CD-1 mice was found to be comparable to that of a standard AMC when normalized for DMx dose. Of particular importance, AMCs made with the hydrophilic non-reducible PEG 4  linker showed greater cytotoxic activity than standard AMCs toward multi-drug resistant (MDR) cancer cells  in vitro  at both high DMx/Ab loads or the same load (4 DMx/Ab) as the standard AMC. Additional hydrophilic linkers have been developed with either a disulfide or non-reducible bond, using the design that incorporates the polar sulfonate group. These sulfonate linker AMCs also were found to be highly potent toward cancer cells with low antigen number, as well as against MDR cancer cells  in vitro  and in an MDR-xenograft model  in vivo . These new linkers further expand the potential utility of AMCs for treatment of MDR cancers and those with low expression of the target antigen.   Citation Information:  In: Proc Am Assoc Cancer Res; 2009 Apr 18-22; Denver, CO. Philadelphia (PA): AACR; 2009. Abstract nr 5480.