Preclinical Antitumor Activity of a Novel Anti-c-KIT Antibody-Drug Conjugate against Mutant and Wild-type c-KIT-Positive Solid Tumors.

Purpose: c-KIT overexpression is well recognized in cancers such as gastrointestinal stromal tumors (GIST), small cell lung cancer (SCLC), melanoma, non-small cell lung cancer (NSCLC), and acute myelogenous leukemia (AML). Treatment with the small-molecule inhibitors imatinib, sunitinib, and regorafenib resulted in resistance (c-KIT mutant tumors) or limited activity (c-KIT wild-type tumors). We selected an anti-c-KIT ADC approach to evaluate the anticancer activity in multiple disease models.Experimental Design: A humanized anti-c-KIT antibody LMJ729 was conjugated to the microtubule destabilizing maytansinoid, DM1, via a noncleavable linker (SMCC). The activity of the resulting ADC, LOP628, was evaluated in vitro against GIST, SCLC, and AML models and in vivo against GIST and SCLC models.Results: LOP628 exhibited potent antiproliferative activity on c-KIT-positive cell lines, whereas LMJ729 displayed little to no effect. At exposures predicted to be clinically achievable, LOP628 demonstrated single administration regressions or stasis in GIST and SCLC xenograft models in mice. LOP628 also displayed superior efficacy in an imatinib-resistant GIST model. Further, LOP628 was well tolerated in monkeys with an adequate therapeutic index several fold above efficacious exposures. Safety findings were consistent with the pharmacodynamic effect of neutropenia due to c-KIT-directed targeting. Additional toxicities were considered off-target and were consistent with DM1, such as effects in the liver and hematopoietic/lymphatic system.Conclusions: The preclinical findings suggest that the c-KIT-directed ADC may be a promising therapeutic for the treatment of mutant and wild-type c-KIT-positive cancers and supported the clinical evaluation of LOP628 in GIST, AML, and SCLC patients. Clin Cancer Res; 24(17); 4297-308. ©2018 AACR.

A New Triglycyl Peptide Linker for Antibody-Drug Conjugates (ADCs) with Improved Targeted Killing of Cancer Cells.

A triglycyl peptide linker (CX) was designed for use in antibody -: drug conjugates (ADC), aiming to provide efficient release and lysosomal efflux of cytotoxic catabolites within targeted cancer cells. ADCs comprising anti-epithelial cell adhesion molecule (anti-EpCAM) and anti-EGFR antibodies with maytansinoid payloads were prepared using CX or a noncleavable SMCC linker (CX and SMCC ADCs). The in vitro cytotoxic activities of CX and SMCC ADCs were similar for several cancer cell lines; however, the CX ADC was more active (5-100-fold lower IC50) than the SMCC ADC in other cell lines, including a multidrug-resistant line. Both CX and SMCC ADCs showed comparable MTDs and pharmacokinetics in CD-1 mice. In Calu-3 tumor xenografts, antitumor efficacy was observed with the anti-EpCAM CX ADC at a 5-fold lower dose than the corresponding SMCC ADC in vivo Similarly, the anti-EGFR CX ADC showed improved antitumor activity over the respective SMCC conjugate in HSC-2 and H1975 tumor models; however, both exhibited similar activity against FaDu xenografts. Mechanistically, in contrast with the charged lysine-linked catabolite of SMCC ADC, a significant fraction of the carboxylic acid catabolite of CX ADC could be uncharged in the acidic lysosomes, and thus diffuse out readily into the cytosol. Upon release from tumor cells, CX catabolites are charged at extracellular pH and do not penetrate and kill neighboring cells, similar to the SMCC catabolite. Overall, these data suggest that CX represents a promising linker option for the development of ADCs with improved therapeutic properties. Mol Cancer Ther; 15(6); 1311-20. ©2016 AACR.

Development of Anilino-Maytansinoid ADCs that Efficiently Release Cytotoxic Metabolites in Cancer Cells and Induce High Levels of Bystander Killing.

Antibody anilino maytansinoid conjugates (AaMCs) have been prepared in which a maytansinoid bearing an aniline group was linked through the aniline amine to a dipeptide, which in turn was covalently attached to a desired monoclonal antibody. Several such conjugates were prepared utilizing different dipeptides in the linkage including Gly-Gly, l-Val-l-Cit, and all four stereoisomers of the Ala-Ala dipeptide. The properties of AaMCs could be altered by the choice of dipeptide in the linker. Each of the AaMCs, except the AaMC bearing a d-Ala-d-Ala peptide linker, displayed more bystander killing in vitro than maytansinoid ADCs that utilize disulfide linkers. In mouse models, the anti-CanAg AaMC bearing a d-Ala-l-Ala dipeptide in the linker was shown to be more efficacious against heterogeneous HT-29 xenografts than maytansinoid ADCs that utilize disulfide linkers, while both types of the conjugates displayed similar tolerabilities.

Design of Coltuximab Ravtansine, a CD19-Targeting Antibody-Drug Conjugate (ADC) for the Treatment of B-Cell Malignancies: Structure-Activity Relationships and Preclinical Evaluation.

Coltuximab ravtansine (SAR3419) is an antibody-drug conjugate (ADC) targeting CD19 created by conjugating a derivative of the potent microtubule-acting cytotoxic agent, maytansine, to a version of the anti-CD19 antibody, anti-B4, that was humanized as an IgG1 by variable domain resurfacing. Four different linker-maytansinoid constructs were synthesized (average ∼3.5 maytansinoids/antibody for each) to evaluate the impact of linker-payload design on the activity of the maytansinoid-ADCs targeting CD19. The ADC composed of DM4 (N(2')-deacetyl-N(2')-[4-mercapto-4-methyl-1-oxopentyl]maytansine) conjugated to antibody via the N-succinimidyl-4-(2-pyridyldithio)butyrate (SPDB) linker was selected for development as SAR3419. A molar ratio for DM4/antibody of between 3 and 5 was selected for the final design of SAR3419. Evaluation of SAR3419 in Ramos tumor xenograft models showed that the minimal effective single dose was about 50 µg/kg conjugated DM4 (∼2.5 mg/kg conjugated antibody), while twice this dose gave complete regressions in 100% of the mice. SAR3419 arrests cells in the G2/M phase of the cell cycle, ultimately leading to apoptosis after about 24 h. The results of in vitro and in vivo studies with SAR3419 made with DM4 that was [(3)H]-labeled at the C20 methoxy group of the maytansinoid suggest a mechanism of internalization and intracellular trafficking of SAR3419, ultimately to lysosomes, in which the antibody is fully degraded, releasing lysine-N(ε)-SPDB-DM4 as the initial metabolite. Subsequent intracellular reduction of the disulfide bond between linker and DM4 generates the free thiol species, which is then converted to S-methyl DM4 by cellular methyl transferase activity. We provide evidence to suggest that generation of S-methyl DM4 in tumor cells may contribute to in vivo tumor eradication via bystander killing of neighboring tumor cells. Furthermore, we show that S-methyl DM4 is converted to the sulfoxide and sulfone derivatives in the liver, suggesting that hepatic catabolism of the payload to less cytotoxic maytansinoid species contributes to the overall therapeutic window of SAR3419. This compound is currently in phase II clinical evaluation for the treatment of diffuse large B cell lymphoma.