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.