Characterization of INCB086550: A Potent and Novel Small-Molecule PD-L1 Inhibitor.

Blocking the activity of the programmed cell death protein 1 (PD-1) inhibitory receptor with therapeutic antibodies against either the ligand (PD-L1) or PD-1 itself has proven to be an effective treatment modality for multiple cancers. Contrasting with antibodies, small molecules could demonstrate increased tissue penetration, distinct pharmacology, and potentially enhanced antitumor activity. Here, we describe the identification and characterization of INCB086550, a novel, oral, small-molecule PD-L1 inhibitor. In vitro, INCB086550 selectively and potently blocked the PD-L1/PD-1 interaction, induced PD-L1 dimerization and internalization, and induced stimulation-dependent cytokine production in primary human immune cells. In vivo, INCB086550 reduced tumor growth in CD34+ humanized mice and induced T-cell activation gene signatures, consistent with PD-L1/PD-1 pathway blockade. Preliminary data from an ongoing phase I study confirmed PD-L1/PD-1 blockade in peripheral blood cells, with increased immune activation and tumor growth control. These data support continued clinical evaluation of INCB086550 as an alternative to antibody-based therapies. SIGNIFICANCE: We have identified a potent small-molecule inhibitor of PD-L1, INCB086550, which has biological properties similar to PD-L1/PD-1 monoclonal antibodies and may represent an alternative to antibody therapy. Preliminary clinical data in patients demonstrated increased immune activation and tumor growth control, which support continued clinical evaluation of this approach. See related commentary by Capparelli and Aplin, p. 1413. This article is highlighted in the In This Issue feature, p. 1397.

Characterization of human cancer xenografts in humanized mice.

BACKGROUND: Preclinical evaluation of drugs targeting the human immune system has posed challenges for oncology researchers. Since the commercial introduction of humanized mice, antitumor efficacy and pharmacodynamic studies can now be performed with human cancer cells within mice bearing components of a human immune system. However, development and characterization of these models is necessary to understand which model may be best suited for different agents. METHODS: We characterized A375, A549, Caki-1, H1299, H1975, HCC827, HCT116, KU-19-19, MDA-MB-231, and RKO human cancer cell xenografts in CD34+ humanized non-obese diabetic-scid gamma mice for tumor growth rate, immune cell profiling, programmed death ligand 1 (PD-L1) expression and response to anti-PD-L1 therapy. Immune cell profiling was performed using flow cytometry and immunohistochemistry. Antitumor response of humanized xenograft models to PD-L1 therapy was performed using atezolizumab. RESULTS: We found that CD4+ and CD8+ T-cell composition in both the spleen and tumor varied among models, with A375, Caki-1, MDA-MB-231, and HCC827 containing higher intratumoral frequencies of CD4+ and CD8+ T cells of CD45+ cells compared with other models. We demonstrate that levels of immune cell infiltrate within each model are strongly influenced by the tumor and not the stem cell donor. Many of the tumor models showed an abundance of myeloid cells, B cells and dendritic cells. RKO and MDA-MB-231 tumors contained the highest expression of PD-L1+ tumor cells. The antitumor response of the models to atezolizumab was positively associated with the level of CD4+ and CD8+ tumor-infiltrating lymphocytes (TILs). CONCLUSIONS: These data demonstrate that there are tumor-intrinsic factors that influence the immune cell repertoire within tumors and spleen, and that TIL frequencies are a key factor in determining response to anti-PD-L1 in tumor xenografts in humanized mice. These data may also aid in the selection of tumor models to test antitumor activity of novel immuno-oncology or tumor-directed agents.

A Potent and Selective Dual Inhibitor of AXL and MERTK Possesses Both Immunomodulatory and Tumor-Targeted Activity.

TYRO3, AXL, and MERTK constitute the TAM family of receptor tyrosine kinases, which play important roles in tumor growth, survival, cell adhesion, as well as innate immunity, phagocytosis, and immune-suppressive activity. Therefore, targeting both AXL and MERTK kinases may directly impact tumor growth and relieve immunosuppression. We describe here the discovery of INCB081776, a potent and selective dual inhibitor of AXL and MERTK that is currently in phase 1 clinical trials. In cellular assays, INCB081776 effectively blocked autophosphorylation of AXL or MERTK with low nanomolar half maximal inhibitory concentration values in tumor cells and Ba/F3 cells transfected with constitutively active AXL or MERTK. INCB081776 inhibited activation of MERTK in primary human macrophages and partially reversed M2 macrophage-mediated suppression of T-cell proliferation, which was associated with increased interferon-γ production. In vivo, the antitumor activity of INCB081776 was enhanced in combination with checkpoint blockade in syngeneic models, and resulted in increased proliferation of intratumoral CD4+ and CD8+ T cells. Finally, antitumor activity of INCB081776 was observed in a subset of sarcoma patient-derived xenograft models, which was linked with inhibition of phospho-AKT. These data support the potential therapeutic utility of INCB081776 as an immunotherapeutic agent capable of both enhancing tumor immune surveillance and blocking tumor cell survival mechanisms.

Targeting ADAM17 inhibits human colorectal adenocarcinoma progression and tumor-initiating cell frequency.

ADAM17 (a disintegrin and metalloproteinase 17)/TACE (TNFα converting enzyme) has emerged as a potential therapeutic target in colorectal cancer (CRC) and other cancers, due in part to its role in regulating various tumor cell surface proteins and growth factors and cytokines in the tumor microenvironment. The emergence of MEDI3622, a highly potent and specific antibody-based ADAM17 inhibitor, has allowed testing of the concept that targeting ADAM17 may be an important new therapeutic approach for CRC patients. We demonstrate that MEDI3622 is highly efficacious on tumor growth in multiple human CRC PDX models, resulting in improved survival of animals bearing tumor xenografts. MEDI3622 was further found to impact Notch pathway activity and tumor-initiating cells. The promising preclinical activity seen here supports further clinical investigation of this treatment approach to improve therapeutic outcome for patients diagnosed with metastatic CRC, including patients with KRAS-mutant tumors for whom other therapeutic options are currently limited.

Antibody-Drug Conjugates Bearing Pyrrolobenzodiazepine or Tubulysin Payloads Are Immunomodulatory and Synergize with Multiple Immunotherapies.

Immunogenic cell death (ICD) is the process by which certain cytotoxic drugs induce apoptosis of tumor cells in a manner that stimulates the immune system. In this study, we investigated whether antibody-drug conjugates (ADCS) conjugated with pyrrolobenzodiazepine dimer (PBD) or tubulysin payloads induce ICD, modulate the immune microenvironment, and could combine with immuno-oncology drugs to enhance antitumor activity. We show that these payloads on their own induced an immune response that prevented the growth of tumors following subsequent tumor cell challenge. ADCs had greater antitumor activity in immunocompetent versus immunodeficient mice, demonstrating a contribution of the immune system to the antitumor activity of these ADCs. ADCs also induced immunologic memory. In the CT26 model, depletion of CD8+ T cells abrogated the activity of ADCs when used alone or in combination with a PD-L1 antibody, confirming a role for T cells in antitumor activity. Combinations of ADCs with immuno-oncology drugs, including PD-1 or PD-L1 antibodies, OX40 ligand, or GITR ligand fusion proteins, produced synergistic antitumor responses. Importantly, synergy was observed in some cases with suboptimal doses of ADCs, potentially providing an approach to achieve potent antitumor responses while minimizing ADC-induced toxicity. Immunophenotyping studies in different tumor models revealed broad immunomodulation of lymphoid and myeloid cells by ADC and ADC/immuno-oncology combinations. These results suggest that it may be possible to develop novel combinatorial therapies with PBD- and tubulysin-based ADC and immuno-oncology drugs that may increase clinical responses. Cancer Res; 77(10); 2686-98. ©2017 AACR.

Targeting CD73 in the tumor microenvironment with MEDI9447.

MEDI9447 is a human monoclonal antibody that is specific for the ectoenzyme CD73 and currently undergoing Phase I clinical trials. Here we show that MEDI9447 is a potent inhibitor of CD73 ectonucleotidase activity, with wide ranging immune regulatory consequences. MEDI9447 results in relief from adenosine monophosphate (AMP)-mediated lymphocyte suppression in vitro and inhibition of mouse syngeneic tumor growth in vivo. In contrast with other cancer immunotherapy agents such as checkpoint inhibitors or T-cell agonists, MEDI9447 drives changes in both myeloid and lymphoid infiltrating leukocyte populations within the tumor microenvironment of mouse models. Changes include significant alterations in a number of tumor micro-environmental subpopulations including increases in CD8(+) effector cells and activated macrophages. Furthermore, these changes correlate directly with responder and non-responder subpopulations within animal studies using syngeneic tumors. Combination data showing additive activity between MEDI9447 and anti-PD-1 antibodies using human cells in vitro and mouse tumor models further demonstrate the potential value of relieving adenosine-mediated immunosuppression. Based on these data, a Phase I study to test the safety, tolerability, and clinical activity of MEDI9447 in cancer patients was initiated (NCT02503774).

Lipid- and polyion complex-based micelles as agonist platforms for TNFR superfamily receptors.

Receptor clustering is important for signaling among the therapeutically relevant TNFR superfamily of receptors. In nature, this clustering is driven by trimeric ligands often presented in large numbers as cell surface proteins. Molecules capable of driving similar levels of clustering could make good agonists and hold therapeutic value. However, recapitulating such extensive clustering using typical biotherapeutic formats, such as antibodies, is difficult. Consequently, generating effective agonists of TNFR superfamily receptors is challenging. Toward addressing this challenge we have used lipid- and polyion complex-based micelles as platforms for presenting receptor-binding biologics in a multivalent format that facilitates receptor clustering and imparts strong agonist activity. We show that receptor-binding scFvs or small antibody mimetics that have no agonist activity on their own can be transformed into potent agonists through multivalent presentation on a micelle surface and that the activity of already active multivalent agonists can be enhanced. Using this strategy, we generated potent agonists against two different TNFR superfamily receptors and mouse tumor model studies demonstrate that these micellar agonists have therapeutic efficacy in vivo. Due to its ease of implementation and applicability independent of agonist molecular format, we anticipate that this strategy could be useful for developing agonists to a variety of receptors that rely on clustering to signal.

Doxil synergizes with cancer immunotherapies to enhance antitumor responses in syngeneic mouse models.

Based on the previously described roles of doxorubicin in immunogenic cell death, both doxorubicin and liposomal doxorubicin (Doxil) were evaluated for their ability to boost the antitumor response of different cancer immunotherapies including checkpoint blockers (anti-PD-L1, PD-1, and CTLA-4 mAbs) and TNF receptor agonists (OX40 and GITR ligand fusion proteins) in syngeneic mouse models. In a preventative CT26 mouse tumor model, both doxorubicin and Doxil synergized with anti-PD-1 and CTLA-4 mAbs. Doxil was active when CT26 tumors were grown in immunocompetent mice but not immunocompromised mice, demonstrating that Doxil activity is increased in the presence of a functional immune system. Using established tumors and maximally efficacious doses of Doxil and cancer immunotherapies in either CT26 or MCA205 tumor models, combination groups produced strong synergistic antitumor effects, a larger percentage of complete responders, and increased survival. In vivo pharmacodynamic studies showed that Doxil treatment decreased the percentage of tumor-infiltrating regulatory T cells and, in combination with anti-PD-L1, increased the percentage of tumor-infiltrating CD8(+) T cells. In the tumor, Doxil administration increased CD80 expression on mature dendritic cells. CD80 expression was also increased on both monocytic and granulocytic myeloid cells, suggesting that Doxil may induce these tumor-infiltrating cells to elicit a costimulatory phenotype capable of activating an antitumor T-cell response. These results uncover a novel role for Doxil in immunomodulation and support the use of Doxil in combination with checkpoint blockade or TNFR agonists to increase response rates and antitumor activity.

A Monoclonal Antibody to ADAM17 Inhibits Tumor Growth by Inhibiting EGFR and Non-EGFR-Mediated Pathways.

ADAM17 is the primary sheddase for HER pathway ligands. We report the discovery of a potent and specific ADAM17 inhibitory antibody, MEDI3622, which induces tumor regression or stasis in many EGFR-dependent tumor models. The inhibitory activity of MEDI3622 correlated with EGFR activity both in a series of tumor models across several indications as well in as a focused set of head and neck patient-derived xenograft models. The antitumor activity of MEDI3622 was superior to that of EGFR/HER pathway inhibitors in the OE21 esophageal model and the COLO205 colorectal model suggesting additional activity outside of the EGFR pathway. Combination of MEDI3622 and cetuximab in the OE21 model was additive and eradicated tumors. Proteomics analysis revealed novel ADAM17 substrates that function outside of the HER pathways and may contribute toward the antitumor activity of the monoclonal antibody.

A versatile polymer micelle drug delivery system for encapsulation and in vivo stabilization of hydrophobic anticancer drugs.

Chemotherapeutic drugs are widely used for the treatment of cancer; however, use of these drugs is often associated with patient toxicity and poor tumor delivery. Micellar drug carriers offer a promising approach for formulating and achieving improved delivery of hydrophobic chemotherapeutic drugs; however, conventional micelles do not have long-term stability in complex biological environments such as plasma. To address this problem, a novel triblock copolymer has been developed to encapsulate several different hydrophobic drugs into stable polymer micelles. These micelles have been engineered to be stable at low concentrations even in complex biological fluids, and to release cargo in response to low pH environments, such as in the tumor microenvironment or in tumor cell endosomes. The particle sizes of drugs encapsulated ranged between 30-80 nm, with no relationship to the hydrophobicity of the drug. Stabilization of the micelles below the critical micelle concentration was demonstrated using a pH-reversible crosslinking mechanism, with proof-of-concept demonstrated in both in vitro and in vivo models. Described herein is polymer micelle drug delivery system that enables encapsulation and stabilization of a wide variety of chemotherapeutic drugs in a single platform.

IT-141, a Polymer Micelle Encapsulating SN-38, Induces Tumor Regression in Multiple Colorectal Cancer Models.

Polymer micelles are promising drug delivery vehicles for the delivery of anticancer agents to tumors. Often, anticancer drugs display potent cytotoxic effects towards cancer cells but are too hydrophobic to be administered in the clinic as a free drug. To address this problem, a polymer micelle was designed using a triblock copolymer (ITP-101) that enables hydrophobic drugs to be encapsulated. An SN-38 encapsulated micelle, IT-141, was prepared that exhibited potent in vitro cytotoxicity against a wide array of cancer cell lines. In a mouse model, pharmacokinetic analysis revealed that IT-141 had a much longer circulation time, plasma exposure, and tumor exposure compared to irinotecan. IT-141 was also superior to irinotecan in terms of antitumor activity, exhibiting greater tumor inhibition in HT-29 and HCT116 colorectal cancer xenograft models at half the dose of irinotecan. The antitumor effect of IT-141 was dose-dependent and caused complete growth inhibition and tumor regression at well-tolerated doses. Varying the specific concentration of SN-38 within the IT-141 micelle had no detectible effect on this antitumor activity, indicating no differences in activity between different IT-141 formulations. In summary, IT-141 is a potent micelle-based chemotherapy that holds promise for the treatment of colorectal cancer.

Ectopic expression of histone H2AX mutants reveals a role for its post-translational modifications.

Recent evidence from a wide variety of biological systems has indicated important regulatory roles for post-translation histone modifications in cellular processes such as regulation of gene expression, DNA damage response and recombination. Phosphorylation of histone H2AX at serine 139 is a critical event in the response to DNA damage, but the functional implications of this modification are not yet clear. To investigate the role of H2AX phosphorylation we ectopically expressed epitope-tagged H2AX or mutants at the phosphorylation site. GFP-tagged wild type H2AX, H2AX Ser139Ala or H2AX Ser139Glu proteins were efficiently expressed, localizing exclusively to the interphase nucleus and to condensed chromosomes during mitosis. Biochemical fractionation indicated that epitope-tagged H2AX proteins are incorporated into nucleosomes. Expression of H2AX Ser139Ala, which disrupts the phosphorylation site partially suppressed early G(2)/M arrest following ionizing radiation, and cells expressing this mutant were more sensitive to DNA damage. Conversely, expression of H2AX Ser139Glu, designed as phosphorylation mimic, induced a decrease in the number of cells in mitosis in the absence of DNA damage. Interestingly, this decrease induced by H2AX Ser139Glu was independent of the formation of 53BP1-containing foci and was partially suppressed in CHK2-deficient cells, suggesting a role for CHK2 in this process. Further analyses revealed that expression of either mutant lead to apoptosis and induced higher caspase-3/7 activity compared to expression of wild type H2AX. In addition, we also identified Lys119 as a site for ubiquitination that controls H2AX half-life. Phosphorylation of Ser139 and ubiquitination of K119 are not interdependent. Taken together these results demonstrate a role for H2AX Serine 139 phosphorylation in cell cycle regulation and apoptosis, and for Lysine 119 in the control of H2AX turnover.

DNA damage response: determining the fate of phosphorylated histone H2AX.

Phosphorylation of histone H2AX at Serine 139 is one of the earliest events after DNA damage and is required for the retention of factors involved in repair at the site of the break. Intriguingly, H2AX phosphorylation spreads from the vicinity of the break to both directions spanning large chromosomal regions. Phosphorylated H2AX (also known as gamma-H2AX) then progressively disappears with kinetics that correlates with the completion of DNA repair. Despite intense investigation on the kinases and stimuli involved in gamma-H2AX formation, the mechanism of gamma-H2AX disappearance has remained obscure. Three recent papers shed light on this process and suggest that H2AX may serve as a signaling platform that integrates repair and cell cycle checkpoints.

Truncated E-cadherin potentiates cell death in prostate epithelial cells.

BACKGROUND: E-cadherin, a fundamental component of the adherens junction, is known to mediate aggregation-dependent cell survival. We have previously identified a novel, calpain-dependent proteolytic cleavage of E-cadherin that resulted in the generation of a stable 100-kDa E-cadherin fragment (E-cad(100)) in prostate epithelial cells in response to cell death stimuli. We postulated that the E-cad(100) fragment may play a role in abrogating survival of LNCaP cells following induction of apoptosis. METHODS: Wild-type E-cadherin and E-cad(100) were engineered, tagged with GFP, and stably expressed in LNCaP cells. These cell lines were characterized for E-cadherin-GFP/beta-catenin interactions, endogenous E-cadherin and beta-catenin expression, and sensitivity to apoptosis induced by PKC activation. RESULTS: E-cad(100)-GFP demonstrated a punctuate expression pattern, in contrast to E-cad(120)-GFP, which was membrane-localized. E-cad(100)-GFP, unlike E-cad(120)-GFP, failed to bind to and co-localize with beta-catenin. Transient or stable overexpression of E-cad(100) resulted in the downregulation of endogenous E-cadherin expression at the cell membrane. Activation of PKC in LNCaP cells which overexpressed E-cad(100) potentiated cell death. CONCLUSIONS: Truncated E-cadherin may play a role in the regulation of endogenous E-cadherin expression and epithelial cell survival.

Cleavage of beta-catenin by calpain in prostate and mammary tumor cells.

Mutations in the NH(2)-terminal regulatory domain of the beta-catenin gene lead to aberrant stabilization and accumulation of the protein and increased TCF/LEF-dependent transcription. Although these mutations are common in some cancers, they are infrequent in prostate and breast cancer. We have found that metastatic prostate cancer specimens, obtained through a rapid autopsy tissue procurement program, expressed a novel M(r) 75,000 proteolytic fragment of beta-catenin (beta-cat(75)). beta-Cat(75) was also expressed in multiple prostate and breast cancer cell lines and was closely associated with the activity of the calcium-dependent protease, calpain. In a prostate cancer cDNA microarray, m-calpain RNA levels were found to be significantly increased in metastatic disease compared with normal prostate. We showed calpain-dependent generation of beta-cat(75) in cell culture and in vitro. Molecular mapping revealed that calpain cleavage removed the NH(2)-terminal regulatory domain of the beta-catenin protein. Treatment of MCF-7 cells with ionomycin led to increased accumulation of beta-cat(75) in the nucleus and TCF-dependent transcriptional activity. Overexpression of a similar beta-catenin fragment that lacks the NH(2)-terminal 132 amino acids and has transforming potential activated TCF-dependent transcription. Given the low frequency of mutation-induced activation of beta-catenin in prostate and breast cancers, proteolytic cleavage of beta-catenin by calpain may represent a novel mechanism by which the protein is activated during tumorigenesis.

The role of calpain in the proteolytic cleavage of E-cadherin in prostate and mammary epithelial cells.

The E-cadherin protein mediates Ca(2+)-dependent interepithelial adhesion. Association of E-cadherin with the catenin family of proteins is critical for the maintenance of a functional adhesive complex. We have identified a novel truncated E-cadherin species of 100-kDa (E-cad(100)) in prostate and mammary epithelial cells. E-cad(100) was generated by treatment of cells with ionomycin or TPA. Cell-permeable calpain inhibitors prevented E-cad(100) induction by ionomycin. Immunoblotting for spectrin and mu-calpain confirmed calpain activation in response to ionomycin treatment. Both the mu- and m-isoforms of calpain efficiently generated E-cad(100) in vitro. The E-cad(100) fragment was unable to bind to beta-catenin, gamma-catenin, and p120, suggesting that this cleavage event would disrupt the E-cadherin adhesion complex. Mutational analysis localized the calpain cleavage site to the cytosolic domain upstream of the beta- and gamma-catenin binding motifs of E-cadherin. Because E-cadherin is inactivated in many adenocarcinomas we hypothesized that calpain may play a role in prostate tumorigenesis. A prostate cDNA microarray data base was analyzed for calpain expression in which it was found that m-calpain was up-regulated in localized prostate cancer, and to an even higher degree in metastatic prostate cancer compared with normal prostate tissue. Furthermore, we examined the cleavage of E-cadherin in prostate cancer specimens and found that E-cad(100) accumulated in both localized and metastatic prostate tumors, supporting the cDNA microarray data. These findings demonstrate a novel mechanism by which E-cadherin is functionally inactivated through calpain-mediated proteolysis and suggests that E-cadherin is targeted by calpain during the tumorigenic progression of prostate cancer.