Development of a Novel DNA Mono-alkylator Platform for Antibody-Drug Conjugates.

Although microtubule inhibitors (MTI) remain a therapeutically valuable payload option for antibody-drug conjugates (ADC), some cancers do not respond to MTI-based ADCs. Efforts to fill this therapeutic gap have led to a recent expansion of the ADC payload "toolbox" to include payloads with novel mechanisms of action such as topoisomerase inhibition and DNA cross-linking. We present here the development of a novel DNA mono-alkylator ADC platform that exhibits sustained tumor growth suppression at single doses in MTI-resistant tumors and is well tolerated in the rat upon repeat dosing. A phosphoramidate prodrug of the payload enables low ADC aggregation even at drug-to-antibody ratios of 5:1 while still delivering a bystander-capable payload that is effective in multidrug resistant (MDR)-overexpressing cell lines. The platform was comparable in xenograft studies to the clinical benchmark DNA mono-alkylator ADC platform DGN459 but with a significantly better tolerability profile in rats. Thus, the activity and tolerability profile of this new platform make it a viable option for the development of ADCs.

Discovery and Optimization of a STING Agonist Platform for Application in Antibody Drug Conjugates.

While STING agonists have proven to be effective preclinically as anti-tumor agents, these promising results have yet to be translated in the clinic. A STING agonist antibody-drug conjugate (ADC) could overcome current limitations by improving tumor accessibility, allowing for systemic administration as well as tumor-localized activation of STING for greater anti-tumor activity and better tolerability. In line with this effort, a STING agonist ADC platform was identified through systematic optimization of the payload, linker, and scaffold based on multiple factors including potency and specificity in both in vitro and in vivo evaluations. The platform employs a potent non-cyclic dinucleotide STING agonist, a cleavable ester-based linker, and a hydrophilic PEG8-bisglucamine scaffold. A tumor-targeted ADC built with the resulting STING agonist platform induced robust and durable anti-tumor activity and demonstrated high stability and favorable pharmacokinetics in nonclinical species.

Discovery and Preclinical Characterization of XMT-1660, an Optimized B7-H4-Targeted Antibody-Drug Conjugate for the Treatment of Cancer.

Antibody-drug conjugates (ADC) achieve targeted drug delivery to a tumor and have demonstrated clinical success in many tumor types. The activity and safety profile of an ADC depends on its construction: antibody, payload, linker, and conjugation method, as well as the number of payload drugs per antibody [drug-to-antibody ratio (DAR)]. To allow for ADC optimization for a given target antigen, we developed Dolasynthen (DS), a novel ADC platform based on the payload auristatin hydroxypropylamide, that enables precise DAR-ranging and site-specific conjugation. We used the new platform to optimize an ADC that targets B7-H4 (VTCN1), an immune-suppressive protein that is overexpressed in breast, ovarian, and endometrial cancers. XMT-1660 is a site-specific DS DAR 6 ADC that induced complete tumor regressions in xenograft models of breast and ovarian cancer as well as in a syngeneic breast cancer model that is refractory to PD-1 immune checkpoint inhibition. In a panel of 28 breast cancer PDXs, XMT-1660 demonstrated activity that correlated with B7-H4 expression. XMT-1660 has recently entered clinical development in a phase I study (NCT05377996) in patients with cancer.

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

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.

High-throughput screening using patient-derived tumor xenografts to predict clinical trial drug response.

Profiling candidate therapeutics with limited cancer models during preclinical development hinders predictions of clinical efficacy and identifying factors that underlie heterogeneous patient responses for patient-selection strategies. We established ∼1,000 patient-derived tumor xenograft models (PDXs) with a diverse set of driver mutations. With these PDXs, we performed in vivo compound screens using a 1 × 1 × 1 experimental design (PDX clinical trial or PCT) to assess the population responses to 62 treatments across six indications. We demonstrate both the reproducibility and the clinical translatability of this approach by identifying associations between a genotype and drug response, and established mechanisms of resistance. In addition, our results suggest that PCTs may represent a more accurate approach than cell line models for assessing the clinical potential of some therapeutic modalities. We therefore propose that this experimental paradigm could potentially improve preclinical evaluation of treatment modalities and enhance our ability to predict clinical trial responses.

Combination of antibody that inhibits ligand-independent HER3 dimerization and a p110α inhibitor potently blocks PI3K signaling and growth of HER2+ breast cancers.

We examined the effects of LJM716, an HER3 (ERBB3) neutralizing antibody that inhibits ligand-induced and ligand-independent HER3 dimerization, as a single agent and in combination with BYL719, an ATP competitive p110α-specific inhibitor, against HER2-overexpressing breast and gastric cancers. Treatment with LJM716 reduced HER2-HER3 and HER3-p85 dimers, P-HER3 and P-AKT, both in vitro and in vivo. Treatment with LJM716 alone markedly reduced growth of BT474 xenografts. The combination of LJM716/lapatinib/trastuzumab significantly improved survival of mice with BT474 xenografts compared with lapatinib/trastuzumab (P = 0.0012). LJM716 and BYL719 synergistically inhibited growth in a panel of HER2+ and PIK3CA mutant cell lines. The combination also inhibited P-AKT in HER2-overexpressing breast cancer cells and growth of HER2+ NCI-N87 gastric cancer xenografts more potently than LJM716 or BYL719 alone. Trastuzumab-resistant HER2+/PIK3CA mutant MDA453 xenografts regressed completely after 3 weeks of therapy with LJM716 and BYL719, whereas either single agent inhibited growth only partially. Finally, mice with BT474 xenografts treated with trastuzumab/LJM716, trastuzumab/BYL719, LJM716/BYL719, or trastuzumab/LJM716/BYL719 exhibited similar rates of tumor regression after 3 weeks of treatment. Thirty weeks after treatment discontinuation, 14% of mice were treated with trastuzumab/LJM716/BYL719, whereas >80% in all other treatment groups were sacrificed due to a recurrent large tumor burden (P = 0.0066). These data suggest that dual blockade of the HER2 signaling network with an HER3 antibody that inhibits HER2-HER3 dimers in combination with a p110α-specific inhibitor in the absence of a direct HER2 antagonist is an effective treatment approach against HER2-overexpressing cancers.