This study shows that cells with MAPC characteristics can be isolated not only from BM, but also from brain and muscle tissue. Whether MAPC originally derived from BM are circulating or all organs contain stem cells with MAPC characteristics currently is being studied. Presence of MAPC in multiple tissues may help explain the "plasticity" found in multiple adult tissues.

The conjugation of toxins, dyes, peptides, or proteins to monoclonal antibodies is often performed via free thiol groups generated by either partial reduction methods or engineering free cysteine residues into the antibody sequence. Antibodies from the rabbit Oryctolagus cuniculus have an additional intrachain disulfide bond, whereby the light chain variable kappa domain is bridged to the constant kappa region between cysteine residues at positions 80 and 171, respectively. Chimerization of rabbit antibodies with human constant domains allows for the generation of a free thiol group at the light chain position 80 (C80) that can be used for site-specific conjugation. An efficient process for the purification and simultaneous removal of cysteinylation at the C80 site was developed. The unpaired C80 was shown to be efficiently conjugated using several different maleimido-based ligands. REsidue SPEcific Conjugation Technology (RESPECT) antibody-drug conjugates prepared using rabbit-human chimeric anti-human mesothelin rabbit antibodies and maleimido-PEG-auristatin conjugated to C80 were shown to be highly potent and specific in vitro and effective in vivo in reduction of tumor growth in a highly aggressive mesothelin-expressing xenograft tumor model.

No abstract

The random conjugation of toxins, dyes, peptides, or other payloads to monoclonal antibodies often targets free thiol groups generated by partial reduction methods or lysine residues using succinimide- or isothiocyanate-based chemistry. There remains a need for conjugation technologies targeting specific amino acid residues as a way to produce a homogeneous antibody-drug conjugate (ADC) product with a defined drug-to-antibody ratio (DAR). Our REsidue-SPEcific Conjugation Technology (RESPECT) utilizes two methods by which payloads can be conjugated to specific residues in an antibody. Our cysteine-specific conjugation method exploits a unique intrachain disulfide bond in the light chain of rabbit antibodies between residues 80 and 171 of the variable and constant domains, respectively. Our humanization strategy allows retention of the cysteine at position 80 with a free thiol group that is both amenable for residue-specific conjugation and compatible with optimal antibody biophysical properties including antigen binding and structural stability. This platform has been optimized via antibody engineering strategy stems from in silico modeling, extensive mutagenesis, and crystallographic studies, which have allowed defining the contribution of neighboring residues to the retention of a reactive thiol group as well as the desired humanized antibody’s properties. Our C-terminal lysine-specific linkage method employs the transglutaminase enzyme that catalyzes the formation of a stable isopeptide bond between the γ-carboxyamide group (acyl donor) of a glutamine and the ϵ-amino group (acyl acceptor) of a lysine. While we found no acyl acceptor sites in recombinant wild-type IgG, all antibodies investigated lacked the C-terminal Lys447 due to cleavage by carboxypeptidase B in the antibody production cell line. Blocking the cleavage of Lys447 by addition of a C-terminal amino acid resulted in transamidation of Lys447 by a variety of acyl donor substrates in the presence of any non-acidic, non-proline amino acid residue at position 448. Antibody-drug conjugates (ADCs) prepared using our RESPECT technology targeting the tumor associated-mesothelin protein produced uniform drug-to-antibody ratios (DAR) and were shown to be highly potent and specific in vitro and effective in vivo in reduction of tumor growth in a highly aggressive mesothelin-expressing xenograft tumor model. Citation Format: Earl Albone, Jared Spidel, Xin Cheng, Young Chul Park, Sara Jacob, Arielle Verdi, Andrew Milinichik, Ben Vaessen, J. Bradford Kline, Luigi Grasso. RESPECT (REsidue-SPEcific Conjugation Technology): A platform technology utilizing native cysteine and lysine residues for the generation of homogeneous antibody-drug conjugates [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 65. doi:10.1158/1538-7445.AM2017-65