Xiuxia Sun

Antibody-Maytansinoid Conjugates Designed to Bypass Multidrug Resistance

Conjugation of cytotoxic compounds to antibodies that bind to cancer-specific antigens makes these drugs selective in killing cancer cells. However, many of the compounds used in such antibody-drug conjugates (ADC) are substrates for the multidrug transporter MDR1. To evade the MDR1-mediated resistance, we conjugated the highly cytotoxic maytansinoid DM1 to antibodies via the maleimidyl-based hydrophilic linker PEG(4)Mal. Following uptake into target cells, conjugates made with the PEG(4)Mal linker were processed to a cytotoxic metabolite that was retained by MDR1-expressing cells better than a metabolite of similar conjugates prepared with the nonpolar linker N-succinimidyl-4-(maleimidomethyl)cyclohexane-1-carboxylate (SMCC). In accord, PEG(4)Mal-linked conjugates were more potent in killing MDR1-expressing cells in culture. In addition, PEG(4)Mal-linked conjugates were markedly more effective in eradicating MDR1-expressing human xenograft tumors than SMCC-linked conjugates while being tolerated similarly, thus showing an improved therapeutic index. This study points the way to the development of ADCs that bypass multidrug resistance.

Design of Antibody−Maytansinoid Conjugates Allows for Efficient Detoxification via Liver Metabolism

Antibody-maytansinoid conjugates (AMCs) are targeted chemotherapeutic agents consisting of a potent microtubule-depolymerizing maytansinoid (DM1 or DM4) attached to lysine residues of a monoclonal antibody (mAb) using an uncleavable thioether linker or a stable disulfide linker. Most of the administered dose of an antibody-based therapeutic is slowly catabolized by the liver and other tissues of the reticuloendothelial system. Maytansinoids released from an AMC during this catabolic process could potentially be a source of toxicity. To investigate this, we isolated and identified liver metabolites in mice for three different [(3)H]AMCs with structures similar to those currently undergoing evaluation in the clinic. We then synthesized each metabolite to confirm the identification and assessed their cytotoxic potencies when added extracellularly. We found that the uncleavable mAb-SMCC-[(3)H]DM1 conjugate was degraded to a single major maytansinoid metabolite, lysine-SMCC-[(3)H]DM1, that was nearly 50-fold less cytotoxic than maytansine. The two disulfide-linked conjugates, mAb-SPP-[(3)H]DM1 and mAb-SPDB-[(3)H]DM4, were also found to be catabolized to the analogous lysine-linked maytansinoid metabolites. However, subsequent reduction, S-methylation, and NADPH-dependent oxidation steps in the liver yielded the corresponding S-methyl sulfoxide and S-methyl sulfone derivatives. The cytotoxic potencies of the oxidized maytansinoids toward several human carcinoma cell lines were found to be 5- to 50-fold less potent than maytansine. Our results suggest that liver plays an important role in the detoxification of both cleavable and uncleavable AMCs.

IMGN853, a Folate Receptor-α (FRα)–Targeting Antibody–Drug Conjugate, Exhibits Potent Targeted Antitumor Activity against FRα-Expressing Tumors

A majority of ovarian and non–small cell lung adenocarcinoma cancers overexpress folate receptor α (FRα). Here, we report the development of an anti-FRα antibody–drug conjugate (ADC), consisting of a FRα-binding antibody attached to a highly potent maytansinoid that induces cell-cycle arrest and cell death by targeting microtubules. From screening a large panel of anti-FRα monoclonal antibodies, we selected the humanized antibody M9346A as the best antibody for targeted delivery of a maytansinoid payload into FRα-positive cells. We compared M9346A conjugates with various linker/maytansinoid combinations, and found that a conjugate, now denoted as IMGN853, with the N-succinimidyl 4-(2-pyridyldithio)-2-sulfobutanoate (sulfo-SPDB) linker and N2′-deacetyl-N2′-(4-mercapto-4-methyl-1-oxopentyl)-maytansine (DM4) exhibited the most potent antitumor activity in several FRα-expressing xenograft tumor models. The level of expression of FRα on the surface of cells was a major determinant in the sensitivity of tumor cells to the cytotoxic effect of the conjugate. Efficacy studies of IMGN853 in xenografts of ovarian cancer and non–small cell lung cancer cell lines and of a patient tumor-derived xenograft model demonstrated that the ADC was highly active against tumors that expressed FRα at levels similar to those found on a large fraction of ovarian and non-small cell lung cancer patient tumors, as assessed by immunohistochemistry. IMGN853 displayed cytotoxic activity against FRα-negative cells situated near FRα-positive cells (bystander cytotoxic activity), indicating its ability to eradicate tumors with heterogeneous expression of FRα. Together, these findings support the clinical development of IMGN853 as a novel targeted therapy for patients with FRα-expressing tumors. Mol Cancer Ther; 14(7); 1605–13. ©2015 AACR.

Effects of Drug–Antibody Ratio on Pharmacokinetics, Biodistribution, Efficacy, and Tolerability of Antibody–Maytansinoid Conjugates

Antibody-drug conjugates (ADCs) are being actively pursued as a treatment option for cancer following the regulatory approval of brentuximab vedotin (Adcetris) and ado-trastuzumab emtansine (Kadcyla). ADCs consist of a cytotoxic agent conjugated to a targeting antibody through a linker. The two approved ADCs (and most ADCs now in the clinic that use a microtubule disrupting agent as the payload) are heterogeneous conjugates with an average drug-to-antibody ratio (DAR) of 3-4 (potentially ranging from 0 to 8 for individual species). Ado-trastuzumab emtansine employs DM1, a semisynthetic cytotoxic payload of the maytansinoid class, which is conjugated via lysine residues of the antibody to an average DAR of 3.5. To understand the effect of DAR on the preclinical properties of ADCs using maytansinoid cytotoxic agents, we prepared a series of conjugates with a cleavable linker (M9346A-sulfo-SPDB-DM4 targeting folate receptor α (FRα)) or an uncleavable linker (J2898A-SMCC-DM1 targeting the epidermal growth factor receptor (EGFR)) with varying DAR and evaluated their biochemical characteristics, in vivo stability, efficacy, and tolerability. For both formats, a series of ADCs with DARs ranging from low (average of ∼2 and range of 0-4) to very high (average of 10 and range of 7-14) were prepared in good yield with high monomer content and low levels of free cytotoxic agent. The in vitro potency consistently increased with increasing DAR at a constant antibody concentration. We then characterized the in vivo disposition of these ADCs. Pharmacokinetic analysis showed that conjugates with an average DAR below ∼6 had comparable clearance rates, but for those with an average DAR of ∼9-10, rapid clearance was observed. Biodistribution studies in mice showed that these 9-10 DAR ADCs rapidly accumulate in the liver, with maximum localization for this organ at 24-28% percentage injected dose per gram (%ID/g) compared with 7-10% for lower-DAR conjugates (all at 2-6 h post-injection). Our preclinical findings on tolerability and efficacy suggest that maytansinoid conjugates with DAR ranging from 2 to 6 have a better therapeutic index than conjugates with very high DAR (∼9-10). These very high DAR ADCs suffer from decreased efficacy, likely due to faster clearance. These results support the use of DAR 3-4 for maytansinoid ADCs but suggest that the exploration of lower or higher DAR may be warranted depending on the biology of the target antigen.

Understanding How the Stability of the Thiol-Maleimide Linkage Impacts the Pharmacokinetics of Lysine-Linked Antibody–Maytansinoid Conjugates

Antibody-drug conjugates (ADCs) have become a widely investigated modality for cancer therapy, in part due to the clinical findings with ado-trastuzumab emtansine (Kadcyla). Ado-trastuzumab emtansine utilizes the Ab-SMCC-DM1 format, in which the thiol-functionalized maytansinoid cytotoxic agent, DM1, is linked to the antibody (Ab) via the maleimide moiety of the heterobifunctional SMCC linker. The pharmacokinetic (PK) data for ado-trastuzumab emtansine point to a faster clearance for the ADC than for total antibody. Cytotoxic agent release in plasma has been reported with nonmaytansinoid, cysteine-linked ADCs via thiol-maleimide exchange, for example, brentuximab vedotin. For Ab-SMCC-DM1 ADCs, however, the main catabolite reported is lysine-SMCC-DM1, the expected product of intracellular antibody proteolysis. To understand these observations better, we conducted a series of studies to examine the stability of the thiol-maleimide linkage, utilizing the EGFR-targeting conjugate, J2898A-SMCC-DM1, and comparing it with a control ADC made with a noncleavable linker that lacked a thiol-maleimide adduct (J2898A-(CH2)3-DM). We employed radiolabeled ADCs to directly measure both the antibody and the ADC components in plasma. The PK properties of the conjugated antibody moiety of the two conjugates, J2898A-SMCC-DM1 and J2898A-(CH2)3-DM (each with an average of 3.0 to 3.4 maytansinoid molecules per antibody), appear to be similar to that of the unconjugated antibody. Clearance values of the intact conjugates were slightly faster than those of the Ab components. Furthermore, J2898A-SMCC-DM1 clears slightly faster than J2898A-(CH2)3-DM, suggesting that there is a fraction of maytansinoid loss from the SMCC-DM1 ADC, possibly through a thiol-maleimide dependent mechanism. Experiments on ex vivo stability confirm that some loss of maytansinoid from Ab-SMCC-DM1 conjugates can occur via thiol elimination, but at a slower rate than the corresponding rate of loss reported for thiol-maleimide links formed at thiols derived by reduction of endogenous cysteine residues in antibodies, consistent with expected differences in thiol-maleimide stability related to thiol pKa. These findings inform the design strategy for future ADCs.

Discovery and Optimization of HKT288, a Cadherin-6 Targeting ADC for the Treatment of Ovarian and Renal Cancer.

Despite an improving therapeutic landscape, significant challenges remain in treating the majority of patients with advanced ovarian or renal cancer. We identified the cell-cell adhesion molecule cadherin-6 (CDH6) as a lineage gene having significant differential expression in ovarian and kidney cancers. HKT288 is an optimized CDH6-targeting DM4-based antibody-drug conjugate (ADC) developed for the treatment of these diseases. Our study provides mechanistic evidence supporting the importance of linker choice for optimal antitumor activity and highlights CDH6 as an antigen for biotherapeutic development. To more robustly predict patient benefit of targeting CDH6, we incorporate a population-based patient-derived xenograft (PDX) clinical trial (PCT) to capture the heterogeneity of response across an unselected cohort of 30 models-a novel preclinical approach in ADC development. HKT288 induces durable tumor regressions of ovarian and renal cancer models in vivo, including 40% of models on the PCT, and features a preclinical safety profile supportive of progression toward clinical evaluation.Significance: We identify CDH6 as a target for biotherapeutics development and demonstrate how an integrated pharmacology strategy that incorporates mechanistic pharmacodynamics and toxicology studies provides a rich dataset for optimizing the therapeutic format. We highlight how a population-based PDX clinical trial and retrospective biomarker analysis can provide correlates of activity and response to guide initial patient selection for first-in-human trials of HKT288. Cancer Discov; 7(9); 1030-45. ©2017 AACR.This article is highlighted in the In This Issue feature, p. 920.

Abstract 4576: IMGN853, an anti-Folate Receptor I antibody-maytansinoid conjugate for targeted cancer therapy

Proceedings: AACR 102nd Annual Meeting 2011‐‐ Apr 2‐6, 2011; Orlando, FL Previously we reported (O. Ab; EORTC, 2010) that an antibody-maytansinoid conjugate (AMC) composed of an anti-FOLR1 antibody conjugated to the cytotoxic maytansinoid, DM4, via the disulfide-containing linker, SPDB, was potent in killing FOLR1-expressing cancer cells in vitro and in vivo. In light of the favorable results noted, we assessed the optimal antibody, linker, and maytansinoid agent for an AMC targeting FOLR1, as reported here. Antibody selection. Anti-FOLR1 antibodies were generated by immunizing mice with human FOLR1-expressing cells, and a panel of FOLR1-specific antibodies was identified by flow cytometry binding assay. Several FOLR1-antibodies with high binding affinity to both human and monkey FOLR1 were chosen for further evaluation and were humanized using ImmunoGens resurfacing technology. Antibodies were conjugated to DM1 via the non-cleavable SMCC linker and the conjugates tested for activity against FOLR1-positive KB cells in vitro and in vivo. All conjugates had comparable cytotoxic potencies in vitro. However, the in vivo anti-tumor activity of one conjugate, M9346A-SMCC-DM1, was significantly better than that of SMCC-DM1 conjugates of other FOLR1 antibodies. Based on this finding, the M9346A antibody was chosen for further development. Linker/maytansinoid selection. The M9346A antibody was linked to DM1 or DM4 via the disulfide-containing cleavable linkers SPP, SPDB or sulfo-SPDB, or via the non-cleavable SMCC linker. We compared the in vitro cytotoxic activities of these conjugates on KB, Igrov-1 and Jeg-3 cell lines. The conjugates with cleavable linkers displayed markedly greater in vitro activities than the SMCC conjugate. We then examined the in vivo activities of the conjugates in FOLR1-positive KB- and Ovcar 3-tumor models. Again, we found that the conjugates with cleavable linkers were more active in vivo than the noncleavable conjugate. Among the conjugates with cleavable linkers, the sulfo-SPDB-DM4 conjugate was the most active conjugate against the Ovcar-3 model, it had activity comparable to that of the SPDB-DM4 conjugate against KB tumors, and both were more active than the SPP-DM1 conjugate in the two xenograft models. Taking into consideration that sulfo-SPDB-DM4 was the most efficacious design in vivo and the potential of the hydrophilic sulfo-SPDB-linker to enable better activity against PgP-expressing cells (previously reported data), M9346A-sulfo-SPDB-DM4 was selected to be the candidate for development and designated IMGN853. IMGN853 is a promising candidate for the treatment of FOLR1-expressing tumors. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 4576. doi:10.1158/1538-7445.AM2011-4576

Studies on the Metabolism of Antibody–Drug Conjugates

Antibody–drug conjugates (ADCs) are targeted anticancer agents that utilize the specificity of monoclonal antibodies (Ab) to deliver potent cell-killing agents specifically to cancer cells that express the target antigen [1, 2]. The two most advanced ADCs brentuximab vedotin (SGN-35, Adcetris®) and trastuzumab emtansine (trastuzumab-DM1, trastuzumab-SMCC-DM1, trastuzumab-MCC-DM1, T-DM1) have shown favorable efficacy and safety in the clinic. SGN-35 recently received accelerated approval from the FDA for the treatment of patients with Hodgkin lymphoma and anaplastic large-cell lymphoma (ALCL) (see Chap. YY) and T-DM1 is in several advanced clinical trials for the treatment of HER2-positive metastatic breast cancer (see Chap. XX).

Abstract 5677: The anti-EGFR ADC, IMGN289 displays favorable pharmacokinetic properties.

IMGN289 is an antibody-drug conjugate (ADC) in preclinical development for the treatment of EGFR-positive cancers. IMGN289 utilizes the same SMCC thioether linker and DM1 cytotoxic agent used in trastuzumab emtansine (T-DM1), and both conjugates contain approximately 3.5 DM1 molecules per antibody. The stability of the SMCC linker is exemplified by several reports that have shown maytansinoid release only after hydrolysis of the antibody backbone within cells and tissue to yield lysine-SMCC-DM1. Despite these findings, pharmacokinetic (PK) data for T-DM1 point to a faster clearance for conjugated antibody than for total antibody. Additionally, reports of payload release from non-maytansinoid, cysteine-linked ADCs via thiol-maleimide exchange in plasma have led to speculation of DM1 release from T-DM1, and of this occurring via a similar mechanism. Consequently, we conducted a series of studies in mice to examine the PK behavior of IMGN289, its unmodified antibody component (J2898A) and a version of IMGN289 (J2898A-(CH2)3-DM1) in which the SMCC linker is replaced with an all-carbon linker incapable of cleavage via thiol-maleimide exchange. PK was assessed in CD-1 mice administered a single bolus iv dose of the test article. Total antibody and ADC concentrations (species with at least one linked DM1) were measured at periodic intervals over 35 days using standard sandwich ELISA assays. We also employed radioactive assays to directly measure the antibody and ADC components in plasma. For total antibody detection, the antibody or ADC was labeled with the radioactive tracer, N-succinimidyl-2,3-[3H]propionate. For ADC detection, tritium was incorporated into the C20-methoxy group of its maytansinoid moiety. The PK profiles for the ADCs and their unmodified antibody were found to be similar using the ELISA and radioactive assays for total antibody, having only slight changes in the PK parameters for the ADC. Using the ELISA assays, we found the total antibody and ADC PK profiles were indistinguishable for IMGN289, in contrast to the reported PK for T-DM1 in mice. The profiles developed using the radioactive assays were in good agreement with these findings, supporting that the ELISA assays were reliable. To confirm our results were not unique to IMGN289, we repeated the experiments with two additional antibody-SMCC-DM1 conjugates and found similar results. Additionally, the PK profile for the all- carbon linked conjugate, J2898A-C3-[3H]DM1, was similar to that of J2898A-SMCC-[3H]DM1 conjugate, suggesting little if any thiol-maleimide exchange in the latter, consistent with the finding of similar total antibody and ADC PK profiles for Ab-SMCC-DM1 conjugates. In summary, IMGN289 appears to retain the PK properties of its antibody component and avoids the payload release via thiol-maleimide exchange described for cysteine-maleimide linked ADCs. Citation Format: Jose F. Ponte, Xiuxia Sun, Nicholas C. Yoder, Nathan Fishkin, Sharon Wilhelm, Susan J. Hawes, Wayne Widdison, Robert A. Mastico, Jan Pinkas, Ravi J. Chari, Robert J. Lutz, Hans K. Erickson. The anti-EGFR ADC, IMGN289 displays favorable pharmacokinetic properties. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 5677. doi:10.1158/1538-7445.AM2013-5677

Abstract 2965:In vitroandin vivoactivity of a site-specific SeriMab antibody-drug conjugate (ADC) using an indolino-benzodiazepine DNA-alkylating agent

Previously we have described the characterization of a proprietary class of indolino-benzodiazepine dimers, IGNs, with high potency against many cancer lines. Antibody-drug conjugates (ADCs) made with mono-imine containing IGNs were designed to only alkylate DNA and not cause DNA crosslinking. ADCs with the IGN linked via lysine residues of the antibody were shown to be highly potent and antigen specific. Here we apply our SeriMab site-specific technology platform, which employs N-terminal conjugatable aldehydes derived from oxidation of a serine residue, to link an IGN molecule to the antibody using a peptide linker. This ADC (SeriMab-IGN-P1) has exactly 2 IGN molecules per antibody conjugated to the N-termini of the heavy chains, as determined by MS analysis. Serimab-IGN-P1 was found to maintain binding to the target antigen with comparable affinity to the unconjugated antibody by FACS analysis (Kd = 300 pM). The conjugate was highly potent (IC50 = 4 pM) against a cancer cell line with high target expression (3 × 106 antibodies bound per cell) as well as a cell line with much lower target expression (2 × 104 antibodies bound per cell, IC50 = 22 pM). SeriMab-IGN-P1 also demonstrated strong bystander killing activity against proximal target-negative cancer cells, likely due to a cell permeable metabolite identified in cells expressing the target antigen. The unique oxime linkage in SeriMab-IGN-P1 was found to be stable in circulation in mice for 3 days, as determined by affinity capture LC-MS; the conjugate species with 2 IGNs per antibody remained predominant over this period with an estimated t1/2 of >10 days for payload release. In vivo, SeriMab-IGN-P1 was found to be effective against human lung cancer xenografts (NCI-H2110), with anti-tumor activity observed at single doses as low as 2 μg/kg (payload dose), and complete responses observed at 5 μg/kg. This conjugate was also well tolerated in CD-1 mice at doses significantly higher than the minimum effective dose that produced tumor regression. These data are highly encouraging for new ADC therapies using the SeriMab site-specific conjugation platform with potent DNA-acting payloads. Citation Format: Dilrukshi Vitharana, Alan Wilhelm, Luke Harris, Katie Archer, Manami Shizuka, Erin Maloney, Olga Ab, Rassol Laleau, Xiuxia Sun, Jan Pinkas, Michael Miller, Ravi Chari, Thomas Keating, Nathan Fishkin. In vitro and in vivo activity of a site-specific SeriMab antibody-drug conjugate (ADC) using an indolino-benzodiazepine DNA-alkylating agent. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 2965.