Tackling the tumor microenvironment - how can complex tumor models in vitro aid oncology drug development?

INTRODUCTION: Identifying effective cancer drugs remains an inefficient process. Drug efficacy in traditional preclinical cancer models translates poorly into therapy in the clinic. Implementation of preclinical models that incorporate the tumor microenvironment (TME) is needed to improve selection of active drugs prior to clinical trials. AREAS COVERED: Progression of cancer results from the behavior of cancer cells in concert with the host's histopathological background. Nonetheless, complex preclinical models with a relevant microenvironment have yet to become an integral part of drug development. This review discusses existing models and provides a synopsis of active areas of cancer drug development where implementation would be of value. Their contribution to finding therapeutics in immune oncology, angiogenesis, regulated cell death and targeting tumor fibroblasts as well as optimization of drug delivery, combination therapy, and biomarkers of efficacy is considered. EXPERT OPINION: Complex tumor models in vitro (CTMIVs) that mimic the organotypic architecture of neoplastic tumors have boosted research into TME influence on traditional cytoreductive chemotherapy as well as the detection of specific TME targets. Despite advances in technical prowess, CTMIVs can only address specific aspects of cancer pathophysiology.

Exploring Metabolic Signatures of Ex Vivo Tumor Tissue Cultures for Prediction of Chemosensitivity in Ovarian Cancer.

Predicting patient response to treatment and the onset of chemoresistance are still major challenges in oncology. Chemoresistance is deeply influenced by the complex cellular interactions occurring within the tumor microenvironment (TME), including metabolic crosstalk. We have previously shown that ex vivo tumor tissue cultures derived from ovarian carcinoma (OvC) resections retain the TME components for at least four weeks of culture and implemented assays for assessment of drug response. Here, we explored ex vivo patient-derived tumor tissue cultures to uncover metabolic signatures of chemosensitivity and/or resistance. Tissue cultures derived from nine OvC cases were challenged with carboplatin and paclitaxel, the standard-of-care chemotherapeutics, and the metabolic footprints were characterized by LC-MS. Partial least-squares discriminant analysis (PLS-DA) revealed metabolic signatures that discriminated high-responder from low-responder tissue cultures to ex vivo drug exposure. As a proof-of-concept, a set of potential metabolic biomarkers of drug response was identified based on the receiver operating characteristics (ROC) curve, comprising amino acids, fatty acids, pyrimidine, glutathione, and TCA cycle pathways. Overall, this work establishes an analytical and computational platform to explore metabolic features of the TME associated with response to treatment, which can leverage the discovery of biomarkers of drug response and resistance in OvC.

Pathophysiologic and Pharmacologic Considerations to Improve the Design and Application of Antibody-Drug Conjugates.

Antibody-drug conjugates (ADC) have emerged as one of the pillars of clinical disease management in oncology. The biggest hurdle to widespread development and application of ADCs has been a narrow therapeutic index. Advances in antibody technologies and formats as well as novel linker and payload chemistries have begun to facilitate structural improvements to ADCs. However, the interplay of structural characteristics with physiologic and pharmacologic factors determining therapeutic success has garnered less attention. This review elaborates on the pharmacology of ADCs, the pathophysiology of cancerous tissues, and the reciprocal consequences on ADC properties and functions. While most currently approved ADCs utilize either microtubule inhibition or DNA damage as primary mechanisms of action, we present arguments to expand this repertoire and highlight the need for payload mechanisms that exploit disease-specific vulnerabilities. We promote the idea that the choice of antibody format, targeting antigen, linker properties, and payload of an ADC should be deliberately fit for purpose by taking the pathophysiology of disease and the specific pharmacology of the drug entity into account, thus allowing a higher probability of clinical success.

Pevonedistat and azacitidine upregulate NOXA (PMAIP1) to increase sensitivity to venetoclax in preclinical models of acute myeloid leukemia.

Dysregulation of apoptotic machinery is one mechanism by which acute myeloid leukemia (AML) acquires a clonal survival advantage. B-cell lymphoma protein-2 (BCL2) overexpression is a common feature in hematologic malignancies. The selective BCL2 inhibitor, venetoclax (VEN) is used in combination with azacitidine (AZA), a DNAmethyltransferase inhibitor (DNMTi), to treat patients with AML. Despite promising response rates to VEN/AZA, resistance to the agent is common. One identified mechanism of resistance is the upregulation of myeloid cell leukemia-1 protein (MCL1). Pevonedistat (PEV), a novel agent that inhibits NEDD8-activating enzyme, and AZA both upregulate NOXA (PMAIP1), a BCL2 family protein that competes with effector molecules at the BH3 binding site of MCL1. We demonstrate that PEV/AZA combination induces NOXA to a greater degree than either PEV or AZA alone, which enhances VEN-mediated apoptosis. Herein, using AML cell lines and primary AML patient samples ex vivo, including in cells with genetic alterations linked to treatment resistance, we demonstrate robust activity of the PEV/VEN/AZA triplet. These findings were corroborated in preclinical systemic engrafted models of AML. Collectively, these results provide rational for combining PEV/VEN/AZA as a novel therapeutic approach in overcoming AML resistance in current therapies.

Patient-Derived Explants of Colorectal Cancer: Histopathological and Molecular Analysis of Long-Term Cultures.

Colorectal cancer (CRC) is one of the most common cancers worldwide. Although short-term cultures of tumour sections and xenotransplants have been used to determine drug efficacy, the results frequently fail to confer clinically useful information. Biomarker discovery has changed the paradigm for advanced CRC, though the presence of a biomarker does not necessarily translate into therapeutic success. To improve clinical outcomes, translational models predictive of drug response are needed. We describe a simple method for the fast establishment of CRC patient-derived explant (CRC-PDE) cultures from different carcinogenesis pathways, employing agitation-based platforms. A total of 26 CRC-PDE were established and a subset was evaluated for viability (n = 23), morphology and genetic key alterations (n = 21). CRC-PDE retained partial tumor glandular architecture and microenvironment features were partially lost over 4 weeks of culture. Key proteins (p53 and Mismatch repair) and oncogenic driver mutations of the original tumours were sustained throughout the culture. Drug challenge (n = 5) revealed differential drug response from distinct CRC-PDE cases. These findings suggest an adequate representation of the original tumour and highlight the importance of detailed model characterisation. The preservation of key aspects of the CRC microenvironment and genetics supports CRC-PDE potential applicability in pre- and co-clinical settings, as long as temporal dynamics are considered.

Application of LDH assay for therapeutic efficacy evaluation of ex vivo tumor models.

The current standard preclinical oncology models are not able to fully recapitulate therapeutic targets and clinically relevant disease biology, evidenced by the 90% attrition rate of new therapies in clinical trials. Three-dimensional (3D) culture systems have the potential to enhance the relevance of preclinical models. However, the limitations of currently available cellular assays to accurately evaluate therapeutic efficacy in these models are hindering their widespread adoption. We assessed the compatibility of the lactate dehydrogenase (LDH) assay in 3D spheroid cultures against other commercially available readout methods. We developed a standardized protocol to apply the LDH assay to ex vivo cultures, considering the impact of culture growth dynamics. We show that accounting for growth rates and background release levels of LDH are sufficient to make the LDH assay a suitable methodology for longitudinal monitoring and endpoint assessment of therapeutic efficacy in both cell line-derived xenografts (xenospheres) and patient-derived explant cultures. This method has the added value of being non-destructive and not dependent on reagent penetration or manipulation of the parent material. The establishment of reliable readout methods for complex 3D culture systems will further the utility of these tumor models in preclinical and co-clinical drug development studies.

Synergistic therapeutic benefit by combining the antibody drug conjugate, depatux-m with temozolomide in pre-clinical models of glioblastoma with overexpression of EGFR.

PURPOSE: Depatux-m is an antibody drug conjugate (ADC) that targets and inhibits growth of cancer cells overexpressing the epidermal growth factor receptor (EGFR) or the 2-7 deletion mutant (EGFRvIII) in tumor models in vitro and in vivo. Treatment of patients suffering from relapsed/refractory glioblastoma (GBM) with a combination of depatux-m and temozolomide (TMZ) tended to increase overall survival. As a first step to understand the nature of the interaction between the two drugs, we investigated whether the interaction was synergistic, additive or antagonistic. METHODS: The efficacy of ADCs, antibodies, TMZ and radiation was tested in xenograft models of GBM, U-87MG and U-87MG EGFRvIII. Both models express EGFR. U-87MG EGFRvIII was transduced to express EGFRvIII. Changes in tumor volume, biomarkers of cell death and apoptosis after treatment were used to measure efficacy of the various treatments. Synergism of depatux-m and TMZ was verified in three-dimensional cultures of U-87MG and U-87MG EGFRvIII by the method of Chou and Talalay. RESULTS: Combined with TMZ and radiotherapy (RT), depatux-m inhibited xenograft growth of U-87MG and U-87MG EGFRvIII more than either treatment with depatux-m or TMZ + RT. Durability of the response to depatux-m + TMZ + RT or depatux-m + TMZ was more pronounced in U-87MG EGFRvIII than in U-87MG. Efficacy of depatux-m + TMZ was synergistic in U-87MG EGFRvIII and additive in U-87MG. CONCLUSION: Adding depatux-m enhances the efficacy of standard of care therapy in preclinical models of GBM. Durability of response to depatux-m + TMZ in vivo and synergy of the drug-drug interaction correlates with the amount of antigen expressed by the tumor cells.

Patient-derived ovarian cancer explants: preserved viability and histopathological features in long-term agitation-based cultures.

Ovarian carcinoma (OvC) remains a major therapeutic challenge due to its propensity to develop resistance after an initial response to chemotherapy. Interactions of tumour cells with the surrounding microenvironment play a role in tumour survival, invasion capacity and drug resistance. Cancer models that retain tissue architecture and tumour microenvironment components are therefore essential to understand drug response and resistance mechanisms. Herein, our goal was to develop a long-term OvC patient-derived explant (OvC-PDE) culture strategy in which architecture and cell type heterogeneity of the original tumour would be retained. Samples from 25 patients with distinct OvC types and one with a benign tumour, were cultured for 30 days in agitation-based culture systems with 100% success rate. OvC-PDE cultures retained the original tumour architecture and main cellular components: epithelial cells, fibroblasts and immune cells. Epithelial cells kept their original levels of proliferation and apoptosis. Moreover, the major extracellular components, such as collagen-I and -IV, were retained in explants. OvC-PDE cultures were exposed to standard-of-care chemotherapeutics agents for 2 weeks, attesting the ability of the platform for drug assays employing cyclic drug exposure regimens. We established an OvC-PDE dynamic culture in which tumour architecture and cell type heterogeneity were preserved for the different OvC types, replicating features of the original tumour and compatible with long-term drug exposure for drug efficacy and resistance studies.

Discovery of A-1331852, a First-in-Class, Potent, and Orally-Bioavailable BCL-XL Inhibitor.

Herein we describe the discovery of A-1331852, a first-in-class orally active BCL-XL inhibitor that selectively and potently induces apoptosis in BCL-XL-dependent tumor cells. This molecule was generated by re-engineering our previously reported BCL-XL inhibitor A-1155463 using structure-based drug design. Key design elements included rigidification of the A-1155463 pharmacophore and introduction of sp3-rich moieties capable of generating highly productive interactions within the key P4 pocket of BCL-XL. A-1331852 has since been used as a critical tool molecule for further exploring BCL-2 family protein biology, while also representing an attractive entry into a drug discovery program.

Targeting Multiple EGFR-expressing Tumors with a Highly Potent Tumor-selective Antibody-Drug Conjugate.

ABBV-321 (serclutamab talirine), a next-generation EGFR-targeted antibody-drug conjugate (ADC) incorporates a potent pyrrolobenzodiazepine (PBD) dimer toxin conjugated to the EGFR-targeting ABT-806 affinity-matured AM1 antibody. ABBV-321 follows the development of related EGFR-targeted ADCs including depatuxizumab mafodotin (depatux-m, ABT-414), ABT-806 conjugated to monomethyl auristatin F (MMAF), and ABBV-221 (losatuxizumab vedotin), AM1 antibody conjugated to monomethyl auristatin E (MMAE). The distinct tumor selectivity of ABBV-321 differentiates it from many previous highly active antibody PBD conjugates that lack a therapeutic window. Potency of the PBD dimer, combined with increased binding of AM1 to EGFR-positive tumor cells, opens the possibility to target a wide array of tumors beyond those with high levels of EGFR overexpression or amplification, including those insensitive to auristatin-based ADCs. ABBV-321 exhibits potent antitumor activity in cellular and in vivo studies including xenograft cell line and patient-derived xenograft glioblastoma, colorectal, lung, head and neck, and malignant mesothelioma tumor models that are less sensitive to depatux-m or ABBV-221. Combination studies with ABBV-321 and depatux-m suggest a promising treatment option permitting suboptimal, and potentially better tolerated, doses of both ADCs while providing improved potency. Collectively, these data suggest that ABBV-321 may offer an extended breadth of efficacy relative to other EGFR ADCs while extending utility to multiple EGFR-expressing tumor indications. Despite its highly potent PBD dimer payload, the tumor selectivity of ABBV-321, coupled with its pharmacology, toxicology, and pharmacokinetic profiles, support continuation of ongoing phase I clinical trials in patients with advanced EGFR-expressing malignancies.

5-Azacitidine Induces NOXA to Prime AML Cells for Venetoclax-Mediated Apoptosis.

PURPOSE: Patients with acute myeloid leukemia (AML) frequently do not respond to conventional therapies. Leukemic cell survival and treatment resistance have been attributed to the overexpression of B-cell lymphoma 2 (BCL-2) and aberrant DNA hypermethylation. In a phase Ib study in elderly patients with AML, combining the BCL-2 selective inhibitor venetoclax with hypomethylating agents 5-azacitidine (5-Aza) or decitabine resulted in 67% overall response rate; however, the underlying mechanism for this activity is unknown. EXPERIMENTAL DESIGN: We studied the consequences of combining two therapeutic agents, venetoclax and 5-Aza, in AML preclinical models and primary patient samples. We measured expression changes in the integrated stress response (ISR) and the BCL-2 family by Western blot and qPCR. Subsequently, we engineered PMAIP1 (NOXA)- and BBC3 (PUMA)-deficient AML cell lines using CRISPR-Cas9 methods to understand their respective roles in driving the venetoclax/5-Aza combinatorial activity. RESULTS: In this study, we demonstrate that venetoclax and 5-Aza act synergistically to kill AML cells in vitro and display combinatorial antitumor activity in vivo. We uncover a novel nonepigenetic mechanism for 5-Aza-induced apoptosis in AML cells through transcriptional induction of the proapoptotic BH3-only protein NOXA. This induction occurred within hours of treatment and was mediated by the ISR pathway. NOXA was detected in complex with antiapoptotic proteins, suggesting that 5-Aza may be "priming" the AML cells for venetoclax-induced apoptosis. PMAIP1 knockout confirmed its major role in driving venetoclax and 5-Aza synergy. CONCLUSIONS: These data provide a novel nonepigenetic mechanism of action for 5-Aza and its combinatorial activity with venetoclax through the ISR-mediated induction of PMAIP1.

Characterization of ABBV-221, a Tumor-Selective EGFR-Targeting Antibody Drug Conjugate.

Depatuxizumab mafodotin (depatux-m, ABT-414) is a tumor-selective antibody drug conjugate (ADC) comprised of the anti-EGFR antibody ABT-806 and the monomethyl auristatin F (MMAF) warhead. Depatux-m has demonstrated promising clinical activity in glioblastoma multiforme (GBM) patients and is currently being evaluated in clinical trials in first-line and recurrent GBM disease settings. Depatux-m responses have been restricted to patients with amplified EGFR, highlighting the need for therapies with activity against tumors with nonamplified EGFR overexpression. In addition, depatux-m dosing has been limited by corneal side effects common to MMAF conjugates. We hypothesized that a monomethyl auristatin E (MMAE) ADC utilizing an EGFR-targeting antibody with increased affinity may have broader utility against tumors with more modest EGFR overexpression while mitigating the risk of corneal side effects. We describe here preclinical characterization of ABBV-221, an EGFR-targeting ADC comprised of an affinity-matured ABT-806 conjugated to MMAE. ABBV-221 binds to a similar EGFR epitope as depatux-m and retains tumor selectivity with increased binding to EGFR-positive tumor cells and greater in vitro potency. ABBV-221 displays increased tumor uptake and antitumor activity against wild-type EGFR-positive xenografts with a greatly reduced incidence of corneal side effects relative to depatux-m. ABBV-221 has similar activity as depatux-m against an EGFR-amplified GBM patient derived xenograft (PDX) model and is highly effective alone and in combination with standard-of-care temozolomide in an EGFRvIII-positive GBM xenograft model. Based on these results, ABBV-221 has advanced to a phase I clinical trial in patients with advanced solid tumors associated with elevated levels of EGFR. Mol Cancer Ther; 17(4); 795-805. ©2018 AACR.

The Volume of Three-Dimensional Cultures of Cancer Cells InVitro Influences Transcriptional Profile Differences and Similarities with Monolayer Cultures and Xenografted Tumors.

Improving the congruity of preclinical models with cancer as it is manifested in humans is a potential way to mitigate the high attrition rate of new cancer therapies in the clinic. In this regard, three-dimensional (3D) tumor cultures in vitro have recently regained interest as they have been acclaimed to have higher similarity to tumors in vivo than to cells grown in monolayers (2D). To identify cancer functions that are active in 3D rather than in 2D cultures, we compared the transcriptional profiles (TPs) of two non-small cell lung carcinoma cell lines, NCI-H1650 and EBC-1 grown in both conditions to the TP of xenografted tumors. Because confluence, diameter or volume can hypothetically alter TPs, we made intra- and inter-culture comparisons using samples with defined dimensions. As projected by Ingenuity Pathway Analysis (IPA), a limited number of signal transduction pathways operational in vivo were better represented by 3D than by 2D cultures in vitro. Growth of 2D and 3D cultures as well as xenografts induced major changes in the TPs of these 3 modes of culturing. Alterations of transcriptional network activation that were predicted to evolve similarly during progression of 3D cultures and xenografts involved the following functions: hypoxia, proliferation, cell cycle progression, angiogenesis, cell adhesion, and interleukin activation. Direct comparison of TPs of 3D cultures and xenografts to monolayer cultures yielded up-regulation of networks involved in hypoxia, TGF and Wnt signaling as well as regulation of epithelial mesenchymal transition. Differences in TP of 2D and 3D cancer cell cultures are subject to progression of the cultures. The emulation of the predicted cell functions in vivo is therefore not only determined by the type of culture in vitro but also by the confluence or diameter of the 2D or 3D cultures, respectively. Consequently, the successful implementation of 3D models will require phenotypic characterization to verify the relevance of applying these models for drug development.

A "Prozone-Like" Effect Influences the Efficacy of the Monoclonal Antibody ABT-700 against the Hepatocyte Growth Factor Receptor.

ABT-700 is a therapeutic antibody against the hepatocyte growth factor receptor (MET). At doses or regimens that lead to exposures exceeding optimum in vivo, the efficacy of ABT-700 is unexpectedly reduced. We hypothesized that this reduction in efficacy was due to a "prozone-like" effect in vivo. A prozone-like effect, which is a reduction in efficacy beyond optimum exposure, is caused due a mechanism similar to the generation of false negative flocculation tests by excessive antibody titres. In vitro, we demonstrate that at higher ABT-700 concentrations, this "prozone-like" effect is mediated by a progressive conversion from bivalent to ineffective monovalent binding of the antibody. In vivo, the efficacy of ABT-700 is dependent on an optimum range of exposure as well. Our data suggest that the "prozone-like" effect is operative and independent of target expression. ABT-700 dose, regimen, exposure, and tumor burden are interdependent variables influencing the "prozone-like" effect and mediating and in vivo efficacy. By optimization of dosage and regimen we demonstrate that the "prozone-like" effect can be alleviated and ABT-700 efficacy at varying tumor loads can be further extended in combination with cisplatin. Our results suggest that optimization of exposure taking tumor burden into account may alleviate "prozone-like" effects without compromising efficacy.

ABBV-399, a c-Met Antibody-Drug Conjugate that Targets Both MET-Amplified and c-Met-Overexpressing Tumors, Irrespective of MET Pathway Dependence.

Purpose: Despite the importance of the MET oncogene in many malignancies, clinical strategies targeting c-Met have benefitted only small subsets of patients with tumors driven by signaling through the c-Met pathway, thereby necessitating selection of patients with MET amplification and/or c-Met activation most likely to respond. An ADC targeting c-Met could overcome these limitations with potential as a broad-acting therapeutic.Experimental Design: ADC ABBV-399 was generated with the c-Met-targeting antibody, ABT-700. Antitumor activity was evaluated in cancer cells with overexpressed c-Met or amplified MET and in xenografts including patient-derived xenograft (PDX) models and those refractory to other c-Met inhibitors. The correlation between c-Met expression and sensitivity to ABBV-399 in tumor and normal cell lines was assessed to evaluate the risk of on-target toxicity.Results: A threshold level of c-Met expressed by sensitive tumor but not normal cells is required for significant ABBV-399-mediated killing of tumor cells. Activity extends to c-Met or amplified MET cell line and PDX models where significant tumor growth inhibition and regressions are observed. ABBV-399 inhibits growth of xenograft tumors refractory to other c-Met inhibitors and provides significant therapeutic benefit in combination with standard-of-care chemotherapy.Conclusions: ABBV-399 represents a novel therapeutic strategy to deliver a potent cytotoxin to c-Met-overexpressing tumor cells enabling cell killing regardless of reliance on MET signaling. ABBV-399 has progressed to a phase I study where it has been well tolerated and has produced objective responses in c-Met-expressing non-small cell lung cancer (NSCLC) patients. Clin Cancer Res; 23(4); 992-1000. ©2016 AACR.

Expression Profile of BCL-2, BCL-XL, and MCL-1 Predicts Pharmacological Response to the BCL-2 Selective Antagonist Venetoclax in Multiple Myeloma Models.

BCL-2 family proteins dictate survival of human multiple myeloma cells, making them attractive drug targets. Indeed, multiple myeloma cells are sensitive to antagonists that selectively target prosurvival proteins such as BCL-2/BCL-XL (ABT-737 and ABT-263/navitoclax) or BCL-2 only (ABT-199/GDC-0199/venetoclax). Resistance to these three drugs is mediated by expression of MCL-1. However, given the selectivity profile of venetoclax it is unclear whether coexpression of BCL-XL also affects antitumor responses to venetoclax in multiple myeloma. In multiple myeloma cell lines (n = 21), BCL-2 is expressed but sensitivity to venetoclax correlated with high BCL-2 and low BCL-XL or MCL-1 expression. Multiple myeloma cells that coexpress BCL-2 and BCL-XL were resistant to venetoclax but sensitive to a BCL-XL-selective inhibitor (A-1155463). Multiple myeloma xenograft models that coexpressed BCL-XL or MCL-1 with BCL-2 were also resistant to venetoclax. Resistance to venetoclax was mitigated by cotreatment with bortezomib in xenografts that coexpressed BCL-2 and MCL-1 due to upregulation of NOXA, a proapoptotic factor that neutralizes MCL-1. In contrast, xenografts that expressed BCL-XL, MCL-1, and BCL-2 were more sensitive to the combination of bortezomib with a BCL-XL selective inhibitor (A-1331852) but not with venetoclax cotreatment when compared with monotherapies. IHC of multiple myeloma patient bone marrow biopsies and aspirates (n = 95) revealed high levels of BCL-2 and BCL-XL in 62% and 43% of evaluable samples, respectively, while 34% were characterized as BCL-2(High)/BCL-XL (Low) In addition to MCL-1, our data suggest that BCL-XL may also be a potential resistance factor to venetoclax monotherapy and in combination with bortezomib. Mol Cancer Ther; 15(5); 1132-44. ©2016 AACR.

ABT-414, an Antibody-Drug Conjugate Targeting a Tumor-Selective EGFR Epitope.

Targeting tumor-overexpressed EGFR with an antibody-drug conjugate (ADC) is an attractive therapeutic strategy; however, normal tissue expression represents a significant toxicity risk. The anti-EGFR antibody ABT-806 targets a unique tumor-specific epitope and exhibits minimal reactivity to EGFR in normal tissue, suggesting its suitability for the development of an ADC. We describe the binding properties and preclinical activity of ABT-414, an ABT-806 monomethyl auristatin F conjugate. In vitro, ABT-414 selectively kills tumor cells overexpressing wild-type or mutant forms of EGFR. ABT-414 inhibits the growth of xenograft tumors with high EGFR expression and causes complete regressions and cures in the most sensitive models. Tumor growth inhibition is also observed in tumor models with EGFR mutations, including activating mutations and those with the exon 2-7 deletion [EGFR variant III (EGFRvIII)], commonly found in glioblastoma multiforme. ABT-414 exhibits potent cytotoxicity against glioblastoma multiforme patient-derived xenograft models expressing either wild-type EGFR or EGFRvIII, with sustained regressions and cures observed at clinically relevant doses. ABT-414 also combines with standard-of-care treatment of radiation and temozolomide, providing significant therapeutic benefit in a glioblastoma multiforme xenograft model. On the basis of these results, ABT-414 has advanced to phase I/II clinical trials, and objective responses have been observed in patients with both amplified wild-type and EGFRvIII-expressing tumors. Mol Cancer Ther; 15(4); 661-9. ©2016 AACR.

Clearance of systemic hematologic tumors by venetoclax (Abt-199) and navitoclax.

The Bcl-2 family inhibitors venetoclax and navitoclax demonstrated potent antitumor activity in chronic lymphocytic leukemia patients, notably in reducing marrow load and adenopathy. Subsequent trials with venetoclax have been initiated in non-Hodgkin's lymphoma and multiple myeloma patients. Traditional preclinical models fall short either in faithfully recapitulating disease progression within such compartments or in allowing the direct longitudinal analysis of systemic disease. We show that intravenous inoculation of engineered RS4;11 (acute lymphoblastic leukemia) and Granta 519 (mantle cell lymphoma) bioluminescent reporter cell lines result in tumor engraftment of bone marrow, with additional invasion of the central nervous system in the case of Granta 519. Importantly, apoptosis induction and response of these systemically engrafted tumors to Bcl-2 family inhibitors alone or in combination with standard-of-care agents could be monitored longitudinally with optical imaging, and was more accurately reflective of the observed clinical response.

Exploiting selective BCL-2 family inhibitors to dissect cell survival dependencies and define improved strategies for cancer therapy.

The BCL-2/BCL-XL/BCL-W inhibitor ABT-263 (navitoclax) has shown promising clinical activity in lymphoid malignancies such as chronic lymphocytic leukemia. However, its efficacy in these settings is limited by thrombocytopenia caused by BCL-XL inhibition. This prompted the generation of the BCL-2-selective inhibitor venetoclax (ABT-199/GDC-0199), which demonstrates robust activity in these cancers but spares platelets. Navitoclax has also been shown to enhance the efficacy of docetaxel in preclinical models of solid tumors, but clinical use of this combination has been limited by neutropenia. We used venetoclax and the BCL-XL-selective inhibitors A-1155463 and A-1331852 to assess the relative contributions of inhibiting BCL-2 or BCL-XL to the efficacy and toxicity of the navitoclax-docetaxel combination. Selective BCL-2 inhibition suppressed granulopoiesis in vitro and in vivo, potentially accounting for the exacerbated neutropenia observed when navitoclax was combined with docetaxel clinically. By contrast, selectively inhibiting BCL-XL did not suppress granulopoiesis but was highly efficacious in combination with docetaxel when tested against a range of solid tumors. Therefore, BCL-XL-selective inhibitors have the potential to enhance the efficacy of docetaxel in solid tumors and avoid the exacerbation of neutropenia observed with navitoclax. These studies demonstrate the translational utility of this toolkit of selective BCL-2 family inhibitors and highlight their potential as improved cancer therapeutics.

Discovery of a Potent and Selective BCL-XL Inhibitor with in Vivo Activity.

A-1155463, a highly potent and selective BCL-XL inhibitor, was discovered through nuclear magnetic resonance (NMR) fragment screening and structure-based design. This compound is substantially more potent against BCL-XL-dependent cell lines relative to our recently reported inhibitor, WEHI-539, while possessing none of its inherent pharmaceutical liabilities. A-1155463 caused a mechanism-based and reversible thrombocytopenia in mice and inhibited H146 small cell lung cancer xenograft tumor growth in vivo following multiple doses. A-1155463 thus represents an excellent tool molecule for studying BCL-XL biology as well as a productive lead structure for further optimization.