Patrick Henry Beusker

Design, Synthesis, and Evaluation of Linker-Duocarmycin Payloads: Toward Selection of HER2-Targeting Antibody-Drug Conjugate SYD985.

Antibody-drug conjugates (ADCs) that are currently on the market or in clinical trials are predominantly based on two drug classes: auristatins and maytansinoids. Both are tubulin binders and block the cell in its progression through mitosis. We set out to develop a new class of linker-drugs based on duocarmycins, potent DNA-alkylating agents that are composed of a DNA-alkylating and a DNA-binding moiety and that bind into the minor groove of DNA. Linker-drugs were evaluated as ADCs by conjugation to the anti-HER2 antibody trastuzumab via reduced interchain disulfides. Duocarmycin 3b, bearing an imidazo[1,2-a]pyridine-based DNA-binding unit, was selected as the drug moiety, notably because of its rapid degradation in plasma. The drug was incorporated into the linker-drugs in its inactive prodrug form, seco-duocarmycin 3a. Linker attachment to the hydroxyl group in the DNA-alkylating moiety was favored over linking to the DNA-binding moiety, as the first approach gave more consistent results for in vitro cytotoxicity and generated ADCs with excellent human plasma stability. Linker-drug 2 was eventually selected based on the properties of the corresponding trastuzumab conjugate, SYD983, which had an average drug-to-antibody ratio (DAR) of about 2. SYD983 showed subnanomolar potencies against multiple human cancer cell lines, was highly efficacious in a BT-474 xenograft model, and had a long half-life in cynomolgus monkeys, in line with high stability in monkey and human plasma. Studies comparing ADCs with a different average DAR showed that a higher average DAR leads to increased efficacy but also to somewhat less favorable physicochemical and toxicological properties. Fractionation of SYD983 with hydrophobic interaction chromatography resulted in SYD985, consisting of about 95% DAR2 and DAR4 species in an approximate 2:1 ratio and having an average DAR of about 2.8. SYD985 combines several favorable properties from the unfractionated ADCs with an improved homogeneity. It was selected for further development and recently entered clinical Phase I evaluation.

The Preclinical Profile of the Duocarmycin-Based HER2-Targeting ADC SYD985 Predicts for Clinical Benefit in Low HER2-Expressing Breast Cancers

SYD985 is a HER2-targeting antibody–drug conjugate (ADC) based on trastuzumab and vc-seco-DUBA, a cleavable linker-duocarmycin payload. To evaluate the therapeutic potential of this new ADC, mechanistic in vitro studies and in vivo patient-derived xenograft (PDX) studies were conducted to compare SYD985 head-to-head with T-DM1 (Kadcyla), another trastuzumab-based ADC. SYD985 and T-DM1 had similar binding affinities to HER2 and showed similar internalization. In vitro cytotoxicity assays showed similar potencies and efficacies in HER2 3+ cell lines, but in cell lines with low HER2 expression, SYD985 was 3- to 50-fold more potent than T-DM1. In contrast with T-DM1, SYD985 efficiently induced bystander killing in vitro in HER2-negative (HER2 0) cells mixed with HER2 3+, 2+, or 1+ cell lines. At pH conditions relevant for tumors, cathepsin-B cleavage studies showed efficient release of the active toxin by SYD985 but not by T-DM1. These in vitro data suggest that SYD985 might be a more potent ADC in HER2-expressing tumors in vivo, especially in low HER2-expressing and/or in heterogeneous tumors. In line with this, in vivo antitumor studies in breast cancer PDX models showed that SYD985 is very active in HER2 3+, 2+, and 1+ models, whereas T-DM1 only showed significant antitumor activity in HER2 3+ breast cancer PDX models. These properties of SYD985 may enable expansion of the target population to patients who have low HER2-expressing breast cancer, a patient population with still unmet high medical need. Mol Cancer Ther; 14(3); 692–703. ©2015 AACR.

Plasmin-activated doxorubicin prodrugs containing a spacer reduce tumor growth and angiogenesis without systemic toxicity

To generate doxorubicin (Dox) specifically at the tumor site, the chemotherapeutic agent was incorporated into a prodrug by linkage to a peptide specifically recognized by plasmin, which is overproduced in many cancers. ST‐9905, which contains an elongated self‐elimination spacer, is activated more rapidly in vitro by plasmin than is ST‐9802. Prodrug activation in vitro depended on the level of urokinase produced by tumor cells and was inhibited by aprotinin, a plasmin inhibitor. Comparison of equimolar concentrations of ST‐9905, ST‐9802, and Dox in EF43.fgf‐4 and MCF7 models revealed that both prodrugs, in sharp contrast to Dox, displayed antiproliferative and antiangiogenic activities without discernible toxicity. Although MCF7 cells are poor urokinase producers in vitro, prodrug efficacy in this model may be explained by production of plasmin by tumor‐infiltrating host cells. Mice treated with equitoxic concentrations (maximum tolerated doses) of prodrugs showed 100% survival and negligible body weight loss, in contrast to results after Dox treatment. ST‐9905 was substantially more effective than ST‐9802 and induced similar tumor growth inhibition as Dox but without apparent toxicity. This finding may be explained by the elongated spacer, which facilitates enzymatic prodrug activation. These data validate both the use of elongated spacers in vivo and the concept of targeting anticancer prodrugs to tumor‐associated plasmin.

EXPLORATION OF HIGH-PRESSURE CYCLOADDUCTS OF FURANS AND CITRACONIC ANHYDRIDE AS PRECURSORS FOR CD-RING FRAGMENTS OF PACLITAXEL AND ITS ANALOGUES

The high-pressure promoted Diels–Alder reactions between several furans and citraconic anhydride have been studied and the cycloadducts obtained have been explored in new straightforward routes to the CD-ring fragment of paclitaxel. The reaction between furan and citraconic anhydride afforded the exo cycloadduct diastereoselectively, whereas a variety of 2-substituted furans afforded approximate 1:1 mixtures of exo regioisomers. Separation of both regioisomers was accomplished after either diastereoselective esterification or regioselective reduction of the anhydride function. Ether cleavage of the bicyclic compounds by either high-pressure promoted ether cleavage or Boord elimination afforded several potential CD-ring precursors which can be used in the total synthesis of paclitaxel analogues.

Abstract 5360: The preclinical profile of the duocarmycin-based HER2-targeting ADC SYD985 predicts for clinical benefit in low HER2-expressing breast cancers

SYD985 is a HER2-targeting ADC based on trastuzumab and vc-seco-DUBA, Synthon9s proprietary cleavable linker-duocarmycin payload. Vc-seco-DUBA was coupled to cysteines after partial reduction on the interchain disulfides of trastuzumab. SYD985 was obtained after purification by Hydrophobic Interaction Chromatography to yield a well-defined ADC consisting of predominantly DAR 2 and DAR 4 species. To evaluate the therapeutic potential of this new ADC, and to prepare clinical development, mechanistic in vitro studies and in vivo PDX studies were conducted to compare SYD985 head-to-head to T-DM1 (Kadcyla®), another trastuzumab-based ADC with a toxin of the maytansinoid class in combination with a non-cleavable linker. SYD985 and T-DM1 had similar binding-affinities to HER2 and showed similar internalization. In vitro cytotoxicity assays showed similar potencies and efficacies in HER2 3+ cell lines, but in cell lines with low HER2 expression, SYD985 was 3 to 50-fold more potent than T-DM1. In contrast to T-DM1, SYD985 efficiently induced bystander killing in vitro of HER2 negative (HER2 0) cells when mixed with HER2 3+, 2+, or 1+ cells. SYD985 efficiently killed HER2 0 cells, even in the presence of only 20% of HER2 3+ cells. At pH conditions relevant for tumors, cathepsin B cleavage studies showed efficient release of the active toxin from SYD985 but not from T-DM1. These in vitro data suggest that SYD985 might be a more potent ADC in HER2 expressing tumors in vivo, allowing the treatment of tumor tissues with low or heterogeneous membrane expression of HER2. In line with this, in vivo anti-tumor studies in breast cancer PDX models showed that SYD985 is very active in HER2 3+, 2+, and 1+ models whereas T-DM1 only showed significant anti-tumor activity in HER2 3+ breast cancer PDX models. We conclude that the properties of SYD985 may enable expansion of the target population to patients who have low HER2 expressing breast cancer, a patient population with unmet high medical need. Early phase clinical evaluation with SYD985 is ongoing. Citation Format: Willem Dokter, Miranda van der Lee, Patrick Groothuis, Ruud Ubink, Monique van der Vleuten, Tanja van Achterberg, Eline Loosveld, Desiree Damming, Myrthe Rouwette, David Egging, Diels van den Dobbelsteen, Patrick Beusker, peter goedings, Gijs Verheijden, Jacques Lemmens, Marco Timmers. The preclinical profile of the duocarmycin-based HER2-targeting ADC SYD985 predicts for clinical benefit in low HER2-expressing breast cancers. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 5360. doi:10.1158/1538-7445.AM2015-5360

Abstract 2461: SYD985, a novel HER2-targeting antibody-drug conjugate, shows strong antitumor activity in primary USC cell lines with low (1+) and moderate (2+) HER2/Neu expression

Introduction: Uterine serous carcinoma (USC) is an aggressive form of endometrial cancer which carries an extremely poor prognosis. Up to 35% of USC may overexpress the HER2/neu oncogene at strong (i.e., 3+) levels and and additional 45% of USC express HER2/Neu at moderate (2+) or low (1+) levels. For tumors with moderate/low HER2/neu expression, there is no sufficient targeted therapy. SYD985 (Synthon Biopharmaceuticals BV, Nijmegen, The Netherlands) is a novel HER2-targeting antibody-drug conjugate (ADC) composed of the mAb trastuzumab linked to a highly potent DNA-alkylating agent (i.e., duocarmycin). The objective of this study was to explore the anti-tumor activity of SYD985 and to compare it to trastuzumab emtansine (T-DM1) in USC in in vitro and in vivo studies. Methods: Nine primary USC cell lines were evaluated for HER2/neu surface expression by IHC and flow cytometry and gene amplification using FISH assays. The cytotoxicity of SYD985 and T-DM1 was evaluated using cell lines with differential HER2/Neu expression (i.e., 1+, 2+, and 3+). Proliferation and viability experiments were performed using propidium iodide-based, flow cytometry assays. In vivo activity of SYD985 in mouse xenograft models is ongoing. Results: SYD985 was 55 to 115 times more potent when compared to T-DM1 in primary USC cell lines with 1+ and 2+ HER2/neu expression. Specifically, in the HER2/neu 1+ the IC509s for SYD985 and T-DM1 were 0.065 μg/mL and 3.58 μg/mL, respectively (p = 0.004). In the HER2/neu 2+ cell lines the IC509s of SYD985 and T-DM1 were 0.016 μg/mL and 1.82 μg/mL, respectively (p = 0.005). In the HER2/neu 3+ cell lines a statistical trend was noted when comparing the cytotoxicity of SYD985 with T-DM1; the IC509s were 0.011 μg/mL and 0.035 μg/mL, respectively (p = 0.06). Conclusions: SYD985 is a novel ADC endowed with remarkable activity against not only USC with strong (3+) HER2/neu overexpression but also (as previously shown for SYD985 in breast cancer) versus USC with low to moderate (i.e., 1+/2+) HER2/neu expression. SYD985 is significantly more potent than T-DM1 in comparative in vitro experiments. Currently, in vivo experiments in USC xenografts not responsive to T-DM1 are ongoing. This is a significant discovery as we do not currently have an adequate targeted therapy to HER2/neu 1+ and 2+ expressing tumors. Thus, clinical studies with SYD985 in patients with biologically aggressive endometrial cancer (e.g., USC) resistant to standard salvage chemotherapy are warranted. Citation Format: Jonathan D. Black, Salvatore Lopez, Emiliano Cocco, Stefania Bellone, Elena Bonazzoli, Carlton Schwab, Diana English, Peter Goedings, Patrick Beusker, Miranda van der Lee, Marco Timmers, Wim Dokter, Thomas Rutherfor, Peter Schwartz, Alessandro Santin. SYD985, a novel HER2-targeting antibody-drug conjugate, shows strong antitumor activity in primary USC cell lines with low (1+) and moderate (2+) HER2/Neu expression. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 2461. doi:10.1158/1538-7445.AM2015-2461