Next-generation Inhibitors Research Articles

Outcomes of FLT3-Mutated Acute Myeloid Leukemia Patients: A Single Center Real-World Experience

Background:Internal tandem duplications (ITDs) within the FMS-like tyrosine kinase 3 (FLT3) gene (FLT3-ITD) is a common driver mutation and confers a poor prognosis in patients (pts) with acute myeloid leukemia (AML) due to proliferative presentation, short remission durations and early relapse. The addition of midostaurin to induction therapy has become standard therapy in newly diagnosedFLT3-mutated (FLT3-mu) AML. Next-generation FLT3 inhibitors (FLT3i) as gilteritinib and quizartinib are more specific, more potent, and may have fewer toxicities associated with off-target therapies but are only available in clinical trials or for salvage therapy. Real-world experience in smaller academic institutions is scanty. Methods:We reviewedFLT3-ITD mu AML pts at our institution from October 2016-December 2019. FLT3i were given as a single agent or in combination with cytotoxic chemotherapy (CCT) or low intensity therapy (LIT) (hypomethylating agents). AllFLT3-ITD DNA was subjected to PCR amplification using primers targeting the juxtamembrane domain of FLT3, and products were analyzed by capillary electrophoresis. We analyzed the characteristics of these pts, responses to therapy, and outcomes. Results:Sixteen pts with newly diagnosedFLT3-ITD mu AML were identified. The median age was 47 years [range, 17-76 years] and the median follow-up was 9.3 months [range, 1.2-29.5 months]. The median time from AML diagnoses to FLT3 detection was 4 days [range, 3-5 days]. Patient characteristics are shown in Table 1. The most common co-occurring mutations wereNPM1(44%) andNRAS(31%); 2 pts (12%) had both mutations. Five pts (31%) received CCT and 11 pts (69%) received CCT+FLT3i. The most frequently used FLT3i was midostaurin 25 mg twice daily on days 8 through 21 (14 days) (10 pts, 63% of all pts); 1 pt received sorafenib (6% of all pts). The complete remission rate (CR) and the overall response rate (ORR) (CR+CR with incomplete hematologic recovery [CRi]) were 31% and 88%, respectively. The two pts who did not achieve a remission were older than 65 years. Among the pts who received CCT+FLT3i, CR and OOR were 50% and 100%, respectively. The median number of cycles to remission was 1 [range, 1-2 cycles], and the median number of consolidation cycles was 0 [range, 0-3 cycles]. Overall, the median duration of response in frontlineFLT3-ITD pts who did not receive allogeneic hematopoietic stem cell transplantation (Allo-HSCT) was 5 months [range, 1-9 months] (all of them received CCT+FLT3i). There was no statistically significant difference in response in pts with NPM1 mutation compared with pts with NRAS mutation (P=0.7), or with no additional mutations. Seven pts (44%) received an Allo-HSCT (6 after CCT+FT3i, and 1 after CCT alone): 2 (28%) from matched unrelated donor, 1 (14%) from a haploidentical transplant, 1 (14%) from dual cord transplant, and the remaining (44%) from matched related donors. Two patients relapsed after Allo-HSCT (28%). Both of these patients had received FLT3i before Allo-HSCT; one of the two relapsed pts received ivosidenib and azacitidine (IDH1 mutation) after Allo-HSCT, achieved remission then received the 2nd Allo-HSCT with subsequent remission. Two pts (29%) received maintenance therapy after Allo-HSCT, one with midostaurin, and the other with azacitidine + gilteritinib; both pts are still on remission (24 and 6 months after CR, respectively). All 7FLT3-ITD pts who did not receive Allo-HSCT relapsed. Gilteritinib was used as a single agent (n=3) or in combination with LIT (n=1) in these pts after relapse. CR rate was 25% and CRI 50%; one pt relapsed (33%). All pts who responded are still receiving gilteritinib. Conclusion:Combination of FLT3i based therapy with CCT is effective therapy inFLT3-ITD frontline pts in real-world setting. Duration of response is short in the absence of Allo-HSCT. Disclosures Cortes: Arog:Research Funding;Pfizer:Consultancy, Research Funding;Amphivena Therapeutics:Research Funding;Daiichi Sankyo:Consultancy, Research Funding;Astellas:Research Funding;Telios:Research Funding;Novartis:Consultancy, Research Funding;BioPath Holdings:Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding;Merus:Research Funding;Bristol-Myers Squibb:Research Funding;Takeda:Consultancy, Research Funding;Sun Pharma:Research Funding;BiolineRx:Consultancy, Research Funding;Immunogen:Research Funding;Jazz Pharmaceuticals:Consultancy, Research Funding.

Phase 2 study of parsaclisib (INCB050465), a highly selective, next-generation PI3Kδ inhibitor, in relapsed or refractory diffuse large B-cell lymphoma (CITADEL-202)

Parsaclisib, a potent, highly selective, next-generation PI3Kδ inhibitor, was evaluated as monotherapy in CITADEL-202 (NCT02998476), an open-label, multicenter, phase 2 study in patients with relapsed or refractory diffuse large B-cell lymphoma. Patients enrolled into 2 groups (A, Bruton tyrosine kinase [BTK] inhibitor naïve, n = 55; B, BTK inhibitor experienced, n = 5) received oral parsaclisib 20 mg once daily for 8 weeks, then 20 mg once weekly while deriving benefit. The futility boundary was crossed at the interim analysis of Group A, resulting in a negative study. Parsaclisib monotherapy demonstrated an objective response rate (ORR) of 25.5% (8 complete metabolic responses/6 partial metabolic responses) and a median duration of response of 6.2 months. ORR in Group B was 20.0% (1 complete metabolic response). Parsaclisib monotherapy demonstrated manageable toxicities with no new safety signals reported. Further evaluation of parsaclisib in combination with standard therapies and active investigational agents is underway.

Abstract PO-59: Anti-membrane IgM monoclonal antibody, mAb4, is a novel, next-generation BTK inhibitor mediating B-cell leukemia and lymphoma cell death

Abstract Introduction: The concept that the B-cell receptor complex (BCRC) initiates a driver pathway in leukemia-lymphoma has been validated by clinical data. However, the BCRC has been overlooked as a therapeutic target due to homology between the BCRC's membrane IgM (mIgM) subunit and circulating serum IgM (sIgM). Previously, we reported the generation of a first-in-class antibody, mAb4, to a neoepitope specific to the BCRC, which inhibits cell growth (Welt et al. 2016; US Patent Nos. 9,926,381 and 10,227,419). Bruton's tyrosine kinase (BTK) inhibitors have emerged as valuable clinical therapies but can induce severe off-target effects. BTK is a downstream mediator of the BCRC's signaling pathway. We aim to characterize the impact of targeting the BCRC on BTK phosphorylation and its resulting biologic effects. We hypothesize that mAb4 binding to the BCRC initiates antitumor effects in part by inhibition of BTK phosphorylation. Methods: Antibody reagents detecting phosphorylation of BTK at Tyr551 (clones A16064A, 797837, and M4G3LN), Tyr223 (clones A16128B and 720101), and Ser180 (clones 3i5 and 3D3), were used to compare inhibition of phosphorylation by a BTK inhibitor drug, and by mAb4, in Burkitt lymphoma cells (CA46) over 0-96 hours, by ELISA. Flow cytometry will be used to further confirm these biologic effects and examine cell cycle phase inhibition. Results: We find that both mAb4 and a BTK inhibitor drug inhibit phosphorylation at Tyr551 and Tyr223, but not Ser180, across all Ab reagents used in ELISA. mAb4-treated leukemia and lymphoma cells undergo growth inhibition and apoptosis while the BTK inhibitor drug mediates only cell growth inhibition, as reported by others. Preliminary flow cytometry results suggest that mAb4 treatment induces a specific cell cycle arrest. Conclusions: Here we show that phospho-tyrosine BTK inhibition is mediated upon mAb4 binding to the BCRC's mIgM. Though BTK exhibits identical patterns of phosphorylation inhibition following treatment with either mAb4 or a BTK inhibitor drug, each treatment results in different cell survival outcomes. While studies show that BTK inhibitors induce a cell growth inhibitory effect, we find that mAb4 mediates BCRC internalization, and in low-density cultures, cell growth inhibition, anticlonogenic activity, and apoptosis. Further studies will examine spleen tyrosine kinase (SYK) phosphorylation and regulation of CXCR4 (a chemokine receptor), as mobilization of tumor cells into the blood may additionally be an important clinical parameter in BTK inhibitor drug efficacy. Due to the specific targeting of mAb4 to IgM-expressing B-cells, we expect that the toxicities associated with small-molecule BTK inhibitors upon interaction with nonlymphatic tissue, including bleeding, infection, cytopenia, arrhythmia, and secondary malignancies, will be avoided with mAb4 treatment. With increased antitumor activity and expected reduced toxicities, mAb4 represents a compelling candidate for clinical development. Citation Format: Rachel Welt, Jonathan Welt, Virginia Raymond, David Kostyal, Sydney Welt. Anti-membrane IgM monoclonal antibody, mAb4, is a novel, next-generation BTK inhibitor mediating B-cell leukemia and lymphoma cell death [abstract]. In: Proceedings of the AACR Virtual Meeting: Advances in Malignant Lymphoma; 2020 Aug 17-19. Philadelphia (PA): AACR; Blood Cancer Discov 2020;1(3_Suppl):Abstract nr PO-59.

The next tier of EGFR resistance mutations in lung cancer.

EGFR mutations account for the majority of druggable targets in lung adenocarcinoma. Over the past decades the optimization of EGFR inhibitors revolutionized the treatment options for patients suffering from this disease. The pace of this development was largely dictated by the inevitable emergence of resistance mutations during drug treatment. As a result, a rapid understanding of the structural and molecular biology of the individual mutations is the key for the development of next-generation inhibitors. Currently, the field faces an unprecedented number of combinations of activating mutations with distinct resistance mutations in parallel to the approval of osimertinib as a first-line drug for EGFR-mutant lung cancer. In this review, we present a survey of the diverse landscape of EGFR resistance mechanisms with a focus on new insights into on-target EGFR kinase mutations. We discuss array of mutations, their structural effects on the EGFR kinase domain as well as the most promising strategies to overcome the individual resistance profiles found in lung cancer patients.

Apalutamide for the treatment of metastatic castration-sensitive prostate cancer.

Prostate cancer is the fifth leading cause of cancer-related death among men with the majority of deaths linked to metastatic disease. Accumulating clinical data have confirmed the substantial survival benefit of the addition of docetaxel orandrogen signaling inhibitors to androgen deprivation therapy for the treatment of metastatic castration-sensitive prostate cancer (mCSPC). Apalutamide, a next-generation androgen receptor inhibitor, has recently been shown to provide an added survival benefit in the treatment of mCSPC and consequently approved for this indication. This review summarizes the body of evidence with regards to the preclinical activity and clinical efficacy of apalutamide with a specific focus on its efficacy in the treatment of mCSPC.

Targeting NAD+ Biosynthesis Overcomes Panobinostat and Bortezomib-Induced Malignant Glioma Resistance

To improve therapeutic responses in patients with glioma, new combination therapies that exploit a mechanistic understanding of the inevitable emergence of drug resistance are needed. Intratumoral heterogeneity enables a low barrier to resistance in individual patients with glioma. We reasoned that targeting two or more fundamental processes that gliomas are particularly dependent upon could result in pleiotropic effects that would reduce the diversity of resistant subpopulations allowing convergence to a more robust therapeutic strategy. In contrast to the cytostatic responses observed with each drug alone, the combination of the histone deacetylase inhibitor panobinostat and the proteasome inhibitor bortezomib synergistically induced apoptosis of adult and pediatric glioma cell lines at clinically achievable doses. Resistance that developed was examined using RNA-sequencing and pharmacologic screening of resistant versus drug-naïve cells. Quinolinic acid phosphoribosyltransferase (QPRT), the rate-determining enzyme for de novo synthesis of NAD+ from tryptophan, exhibited particularly high differential gene expression in resistant U87 cells and protein expression in all resistant lines tested. Reducing QPRT expression reversed resistance, suggesting that QPRT is a selective and targetable dependency for the panobinostat-bortezomib resistance phenotype. Pharmacologic inhibition of either NAD+ biosynthesis or processes such as DNA repair that consume NAD+ or their simultaneous inhibition with drug combinations, specifically enhanced apoptosis in treatment-resistant cells. Concomitantly, de novo vulnerabilities to known drugs were observed. IMPLICATIONS: These data provide new insights into mechanisms of treatment resistance in gliomas, hold promise for targeting recurrent disease, and provide a potential strategy for further exploration of next-generation inhibitors.

Isoform-Selective PI3K Inhibitors for Various Diseases.

Phosphoinositide 3-kinases (PI3Ks) are a family of ubiquitously distributed lipid kinases that control a wide variety of intracellular signaling pathways. Over the years, PI3K has emerged as an attractive target for the development of novel pharmaceuticals to treat cancer and various other diseases. In the last five years, four of the PI3K inhibitors viz. Idelalisib, Copanlisib, Duvelisib, and Alpelisib were approved by the FDA for the treatment of different types of cancer and several other PI3K inhibitors are currently under active clinical development. So far clinical candidates are non-selective kinase inhibitors with various off-target liabilities due to cross-reactivities. Hence, there is a need for the discovery of isoform-selective inhibitors with improved efficacy and fewer side-effects. The development of isoform-selective inhibitors is essential to reveal the unique functions of each isoform and its corresponding therapeutic potential. Although the clinical effect and relative benefit of pan and isoformselective inhibition will ultimately be determined, with the development of drug resistance and the demand for next-generation inhibitors, it will continue to be of great significance to understand the potential mechanism of isoform-selectivity. Because of the important role of type I PI3K family members in various pathophysiological processes, isoform-selective PI3K inhibitors may ultimately have considerable efficacy in a wide range of human diseases. This review summarizes the progress of isoformselective PI3K inhibitors in preclinical and early clinical studies for anticancer and other various diseases.

Molecular mechanisms of Guadecitabine induced FGFR4 down regulation in alveolar rhabdomyosarcomas

Fibroblast growth factor receptor 4 (FGFR4) aberrant expression and activity have been linked to the pathogenesis of a variety of cancers including rhabdomyosarcomas (RMS). We found that treatment of alveolar rhabdomyosarcoma (aRMS) cells with Guadecitabine (SGI-110), a next-generation DNA methyltransferase inhibitor (DNMTi), resulted in a significant reduction of FGFR4 protein levels, 5 days post treatment. Chromatin immunoprecipitation-sequencing (ChIP-seq) in aRMS cells revealed attenuation of the H3K4 mono-methylation across the FGFR4 super enhancer without changes in tri-methylation of either H3K4 or H3K27. These changes were associated with a significant reduction in FGFR4 transcript levels in treated cells. These decreases in H3K4me1 in the FGFR4 super enhancer were also associated with a 240-fold increase in KDM5B (JARID1B) mRNA levels. Immunoblot and immunofluorescent studies also revealed a significant increase in the KDM5B protein levels after treatment in these cells. KDM5B is the only member of KDM5 (JARID1) family of histone lysine demethylases that catalyzes demethylation of H3K4me1. These data together suggest a pleiotropic effect of DNMTi therapy in aRMS cells, converging to significantly lower FGFR4 protein levels in these cells.

Upregulation of CD38 expression on multiple myeloma cells by novel HDAC6 inhibitors is a class effect and augments the efficacy of daratumumab

Multiple myeloma (MM) is incurable, so there is a significant unmet need for effective therapy for patients with relapsed or refractory disease. This situation has not changed despite the recent approval of the anti-CD38 antibody daratumumab, one of the most potent agents in MM treatment. The efficiency of daratumumab might be improved by combining it with synergistic anti-MM agents. We therefore investigated the potential of the histone deacetylase (HDAC) inhibitor ricolinostat to up-regulate CD38 on MM cells, thereby enhancing the performance of CD38-specific therapies. Using quantitative reverse transcription polymerase chain reaction and flow cytometry, we observed that ricolinostat significantly increases CD38 RNA levels and CD38 surface expression on MM cells. Super-resolution microscopy imaging of MM cells by direct stochastic optical reconstruction microscopy confirmed this rise with molecular resolution and revealed homogeneous distribution of CD38 molecules on the cell membrane. Particularly important is that combining ricolinostat with daratumumab induced enhanced lysis of MM cells. We also evaluated next-generation HDAC6 inhibitors (ACY-241, WT-161) and observed similar increase of CD38 levels suggesting that the upregulation of CD38 expression on MM cells by HDAC6 inhibitors is a class effect. This proof-of-concept illustrates the potential benefit of combining HDAC6 inhibitors and CD38-directed immunotherapy for MM treatment.

Next-generation epigenetic inhibitors.

Selective bromodomain inhibitors provide new opportunities to modulate transcription

Advances in Targeting RET-Dependent Cancers.

RET alterations have been characterized as oncogenic drivers in multiple cancers. The clinical validation of highly selective RET inhibitors demonstrates the utility of specific targeting of aberrantly activated RET in patients with cancers such as medullary thyroid cancer or non-small cell lung cancer. The remarkable responses observed have opened the field of RET-targeted inhibitors. In this review, we seek to focus on the impact of therapeutic RET targeting in cancers. SIGNIFICANCE: Successful clinical translation of selective RET inhibitors is poised to alter the therapeutic landscape of altered cancers. Questions that clearly need to be addressed relate to the ability to maintain long-term inhibition of tumor cell growth, how to prepare for the potential mechanisms of acquired resistance, and the development of next-generation selective RET inhibitors.

The DNA methyltransferase inhibitor, guadecitabine, targets tumor-induced myelopoiesis and recovers T cell activity to slow tumor growth in combination with adoptive immunotherapy in a mouse model of breast cancer

BackgroundMyeloid derived suppressor cells (MDSCs) present a significant obstacle to cancer immunotherapy because they dampen anti-tumor cytotoxic T cell responses. Previous groups, including our own, have reported on the myelo-depletive effects of certain chemotherapy agents. We have shown previously that decitabine increased tumor cell Class I and tumor antigen expression, increased ability of tumor cells to stimulate T lymphocytes, depleted tumor-induced MDSC in vivo and augmented immunotherapy of a murine mammary carcinoma.ResultsIn this study, we expand upon this observation by testing a next-generation DNA methyltransferase inhibitor (DNMTi), guadecitabine, which has increased stability in the circulation. Using the 4 T1 murine mammary carcinoma model, in BALB/cJ female mice, we found that guadecitabine significantly reduces tumor burden in a T cell-dependent manner by preventing excessive myeloid proliferation and systemic accumulation of MDSC. The remaining MDSC were shifted to an antigen-presenting phenotype. Building upon our previous publication, we show that guadecitabine enhances the therapeutic effect of adoptively transferred antigen-experienced lymphocytes to diminish tumor growth and improve overall survival. We also show guadecitabine’s versatility with similar tumor reduction and augmentation of immunotherapy in the C57BL/6 J E0771 murine breast cancer model.ConclusionsGuadecitabine depleted and altered MDSC, inhibited growth of two different murine mammary carcinomas in vivo, and augmented immunotherapeutic efficacy. Based on these findings, we believe the immune-modulatory effects of guadecitabine can help rescue anti-tumor immune response and contribute to the overall effectiveness of current cancer immunotherapies.

Successful Use of Brigatinib Followed By Allogeneic Stem Cell Transplantation in Primary Refractory Anaplastic Lymphoma-Kinase Positive Large B Cell Lymphoma

Anaplastic lymphoma-kinase (ALK) positive large B cell lymphoma (LBCL) is a rare, aggressive lymphoma with high relapse rates and poor response to standard cytotoxic therapy. Brigatinib, a next-generation multi-kinase inhibitor that blocks signal transduction associated with the ALK mutational re-arrangement, has shown substantial efficacy in patients with ALK-positive non-small cell lung cancer. We present a unique and successful case of a young patient with primary refractory ALK-positive LBCL who received salvage chemotherapy plus brigatinib followed by allogeneic stem cell transplantation. A 26 year-old previously healthy male presented with weight loss, night sweats, and abdominal pain. Exam was notable for a large abdominal mass. PET/CT scan revealed a large, FDG-avid heterogeneous peripherally enhancing pelvic mass, ascites, and diffuse abdominal lymphadenopathy. Testicular ultrasound was unremarkable. Large atypical plasmablastic cells with amphophilic cytoplasm and prominent nucleoli were noted within areas of necrosis. The abnormal cells were negative for CD19, CD20, PAX5, CD30, and CD117. They were positive for CD138 and granular, cytoplasmic ALK-1. Genomic testing revealed RANBP2-ALK fusion. The patient was diagnosed with ALK-positive LBCL. CSF cytology revealed no malignant cells and bone marrow biopsy revealed normal hematopoiesis. Therapy with cyclophosphamide, doxorubicin, vincristine, etoposide, and methylprednisolone (CHOEP) was initiated. Interval PET/CT revealed primary refractory disease with an enlarging abdominal mass and resultant hydronephrosis. He had worsening leukocytosis and liver dysfunction. Salvage therapy with dexamethasone, cytarabine, cisplatin (DHAP) plus brigatinib was initiated after a risk and benefit discussion regarding the off-label use of brigatinib. He achieved a complete metabolic response and subsequently underwent an umbilical cord blood stem cell transplant with a cyclophosphamide, fludarabine, and total body irradiation-based conditioning regimen followed by tacrolimus and mycophenolate mofetil for graft-versus-host-disease (GVHD) prophylaxis. Maintenance brigatinib was initiated post-transplant. The patient remains in complete remission over a year post-transplant with no unexpected toxicities and no GVHD. ALK-positive LBCL carries a poor prognosis with no standard therapeutic options. This case demonstrates safety and efficacy with the novel use of brigatinib. ALK inhibitors may represent a successful therapeutic strategy and further investigation in a clinical trial setting is warranted.

Next-generation androgen receptor inhibitors in non-metastatic castration-resistant prostate cancer.

Until recently, continuing androgen deprivation therapy (ADT) and closely monitoring patients until evolution towards metastatic castration-resistant prostate cancer (CRPC) were recommended in men with non-metastatic CRPC (nmCRPC). Because delaying the development of metastases and symptoms in these patients is a major issue, several trials have investigated next-generation androgen receptor (AR) axis inhibitors such as apalutamide, darolutamide, and enzalutamide in this setting. This review summarizes the recent advances in the management of nmCRPC, highlighting the favourable impact of next-generation AR inhibitors on metastases-free survival, overall survival and other clinically meaningful endpoints.

Matching-Adjusted Indirect Comparison of the Efficacy of Apalutamide and Enzalutamide with ADT in the Treatment of Non-Metastatic Castration-Resistant Prostate Cancer.

IntroductionApalutamide and enzalutamide are next-generation androgen receptor inhibitors that demonstrated efficacy in placebo-controlled studies (SPARTAN for apalutamide; PROSPER for enzalutamide) when used in combination with androgen deprivation therapy (ADT) for treatment of non-metastatic castration-resistant prostate cancer (nmCRPC). In the absence of comparative studies between these agents, the present study sought to indirectly compare metastasis-free survival (MFS) and overall survival (OS) in patients with nmCRPC who received these therapies.MethodsIndividual patient-level data from SPARTAN (apalutamide plus ADT) and published data from PROSPER (enzalutamide plus ADT) were utilized. An anchored matching-adjusted indirect comparison (MAIC) was conducted by weighting the patients from the SPARTAN study to match baseline characteristics reported for PROSPER. Hazard ratios (HRs) for MFS and OS were re-estimated for SPARTAN using weighted Cox proportional hazards models and indirectly compared with those of PROSPER using a Bayesian network meta-analysis.ResultsFrom the SPARTAN population (N = 1207), a total of 1171 patients were matched to the PROSPER population (N = 1401). The recalculated HRs (95% confidence interval) for apalutamide versus ADT based on the reweighted SPARTAN data to mimic the PROSPER patient population were 0.26 (0.21; 0.33) for MFS and 0.62 (0.41; 0.94) for OS. MAIC-based HRs (95% credible interval) for apalutamide versus enzalutamide were 0.91 (0.68; 1.22) for MFS and 0.77 (0.46; 1.30) for OS. The Bayesian probabilities of apalutamide being more effective than enzalutamide were 73.6% for MFS and 83.5% for OS.ConclusionsMAIC results suggest that nmCRPC patients treated with apalutamide have a higher probability of a more favorable MFS and OS compared with those treated with enzalutamide.Electronic Supplementary MaterialThe online version of this article (10.1007/s12325-019-01156-5) contains supplementary material, which is available to authorized users.

Abstract C084: Crizotinib resistance mechanisms and potential therapeutic strategies to overcome the resistance in ROS1-rearranged non-small cell lung cancer

Abstract ROS1 rearrangement was observed in about 1 % of NSCLC patients and in several other cancers such as cholangiocarcinoma, glioblastoma, or colorectal cancer. To date, crizotinib, an ALK/ROS1 inhibitor was approved and widely used for the treatment of ROS1 rearranged NSCLC. However, the tumor relapses within a few years due to the emergence of crizotinib resistance by the secondary mutations in ROS1 kinase domain (e.g. G2032R) or other unidentified mechanisms. In this study, we explored crizotinib resistance mechanisms using ENU mutagenesis screening and the analysis of crizotinib resistant patient’s pleural effusion. We found several secondary mutations in ROS1 kinase domain including previously reported mutations. From a crizotinib resistant patient’s specimen, no secondary mutation in ROS1 was observed. By establishing a cell line from the specimen, we performed kinase inhibitor screening combined with crizotinib, and found that a combination of crizotinib with dasatinib or a few other tyrosine kinase inhibitors can suppress the growth. These results suggested that a bypass pathway mediated resistance through Src family kinase conferred resistance in this crizotinib resistant case. For overcoming the secondary ROS1 mutations mediated crizotinib resistance, we characterized several ROS1 inhibitors including a new selective ROS1/NTRK inhibitor DS-6051b in preclinical models of ROS1-rarranged cancers by newly establishing cell lines from TKI naïve ROS1 fusion positive NSCLC patients, or developing resistant mutation harboring HCC78 (SCL34A2-ROS1) or Ba/F3-CD74-ROS1 cell line models. Using these preclinical models, we demonstrated that DS-6051b induced dramatic growth inhibition of both wild type and G2032R mutant ROS1–rearranged cancers in vitro and in vivo, suggesting that DS-6051b might be effective in treating wildtype and crizotinib resistant mutant ROS1-rearranged lung cancer. This study revealed that potential resistance mechanisms in ROS1-rearranged NSCLC are similar to those identified in ALK rearranged NSCLC, and next-generation ROS1 inhibitor provides future promising therapeutic strategies to overcome the secondary mutation mediated resistance. Citation Format: Ryohei Katayama, Bo Gong, Makoto Nishio, Naoya Fujita. Crizotinib resistance mechanisms and potential therapeutic strategies to overcome the resistance in ROS1-rearranged non-small cell lung cancer [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference on Molecular Targets and Cancer Therapeutics; 2019 Oct 26-30; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2019;18(12 Suppl):Abstract nr C084. doi:10.1158/1535-7163.TARG-19-C084

Abstract A118: Identification of resistance mechanisms to FGFR4 targeted therapy in hepatocellular carcinoma

Abstract When on-target resistance mutations occur in response to potent and selective therapeutics, the target is often considered a validated driver of disease. Hepatocellular carcinoma (HCC) is a leading cause of cancer mortality worldwide whose drivers remain unvalidated because approved therapies utilize multi-kinase inhibitors or immune modulation. In this study, we validate for the first time a driver of disease in the FGF19 expressing subset of HCC through the identification of Fisogatinib resistance mutations in FGFR4. Fisogatinib (formerly known as BLU-554), an investigational agent, was developed to specifically inhibit the tyrosine kinase activity of FGFR4. Approximately one third of HCC patients harbor aberrant expression of fibroblast growth factor 19 (FGF19), a ligand hypothesized to drive paracrine FGFR4 activation and enhance mitogenic signaling in HCC cells. The identification of FGF19 amplifications in a subset of HCC patients through genomic profiling is further suggestive of the involvement of the FGF19/FGFR4 signaling axis in oncogenesis. The first-in-human, phase I study of Fisogatinib in patients with advanced HCC (NCT02508467) suggests that expression of FGF19 as assessed by immunohistochemistry is responsible for oncogenesis and predictive of response to treatment. We identified FGFR4 gatekeeper (V550) and hinge-1 (C552) mutations upon disease progression in two patients treated with Fisogatinib. These resistance mutations had been predicted by our structure-based studies and were also identified through in vitro and in vivo Fisogatinib resistance screens. We confirm by liquid chromatography tandem mass-spectrometry that Fisogatinib binding to FGFR4 is prevented when either the gatekeeper or the hinge-1 mutation is present and demonstrate the capability of these mutations to mediate resistance to Fisogatinib through genetic engineering. Using a gatekeeper-agnostic pan-FGFR inhibitor we show, in preclinical models, continued FGF19/FGFR4 pathway dependence. These results validate FGF19/FGFR4 as an oncogenic driver and inform the profile of potential next-generation inhibitors designed to facilitate improved patient outcomes. Citation Format: Megan A Hatlen, Oleg Schmidt-Kittler, Cori-Ann Sherwin, Emily Rozsahegyi, Nooreen Rubin, Michael Sheets, Joseph L Kim, Chandra Miduturu, Neil Bifulco, Natasja Brooijmans, Hongliang Shi, Timothy Guzi, Andy Boral, Christoph Lengauer, Marion Dorsch, Richard D Kim, Yoon-Koo Kang, Beni B Wolf, Klaus P Hoeflich. Identification of resistance mechanisms to FGFR4 targeted therapy in hepatocellular carcinoma [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference on Molecular Targets and Cancer Therapeutics; 2019 Oct 26-30; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2019;18(12 Suppl):Abstract nr A118. doi:10.1158/1535-7163.TARG-19-A118

Acquired On-Target Clinical Resistance Validates FGFR4 as a Driver of Hepatocellular Carcinoma.

Hepatocellular carcinoma (HCC) is a leading cause of cancer mortality worldwide with no clinically confirmed oncogenic driver. Although preclinical studies implicate the FGF19 receptor FGFR4 in hepatocarcinogenesis, the dependence of human cancer on FGFR4 has not been demonstrated. Fisogatinib (BLU-554) is a potent and selective inhibitor of FGFR4 and demonstrates clinical benefit and tumor regression in patients with HCC with aberrant FGF19 expression. Mutations were identified in the gatekeeper and hinge-1 residues in the kinase domain of FGFR4 upon disease progression in 2 patients treated with fisogatinib, which were confirmed to mediate resistance in vitro and in vivo. A gatekeeper-agnostic, pan-FGFR inhibitor decreased HCC xenograft growth in the presence of these mutations, demonstrating continued FGF19-FGFR4 pathway dependence. These results validate FGFR4 as an oncogenic driver and warrant further therapeutic targeting of this kinase in the clinic. SIGNIFICANCE: Our study is the first to demonstrate on-target FGFR4 kinase domain mutations as a mechanism of acquired clinical resistance to targeted therapy. This further establishes FGF19-FGFR4 pathway activation as an oncogenic driver. These findings support further investigation of fisogatinib in HCC and inform the profile of potential next-generation inhibitors.See related commentary by Subbiah and Pal, p. 1646.This article is highlighted in the In This Issue feature, p. 1631.

Abstract C050: A novel, potent, and selective hypoxia-inducible factor (HIF)-2α antagonist

Abstract The microenvironment of solid tumors is known to be hypoxic and requires induction of genes associated with metabolism, growth, proliferation, and angiogenesis for tumor cells to survive and metastasize. The master transcriptional regulator of hypoxia-induced genes is the Hypoxia-Inducible Factor (HIF), consisting of an oxygen-regulated alpha monomer, of which there are three isoforms (HIF-1α, HIF-2α, and HIF-3α), that can heterodimerize with a constitutively-expressed beta monomer (HIF-1β/ARNT) using Per-ARNT-SIM (PAS) protein-protein interaction domains. In normoxia, proline residues present in the oxygen-dependent degradation domain of the HIF-α subunits are hydroxylated, allowing for recognition by the von Hippel-Lindau (pVHL) E3-ubiquitin ligase complex and subsequent proteasomal degradation. Upon exposure to low oxygen conditions or in the case of VHL mutation or silencing, HIF-α subunits accumulate in the cell and mediate transcription of various pro-tumorigenic gene sets. In patients, overexpression of HIF is associated with poor prognosis, and both preclinical and clinical evidence is mounting that suggests inhibiting HIF-2α is a valid approach to destroy tumor cells, particularly in clear cell renal carcinoma, warranting development of next-generation inhibitors. Using a suite of in vitro and in vivo assays designed to evaluate HIF-2α-specific effects, herein we describe pharmacological properties associated with novel, potent, and selective small-molecule antagonists of HIF-2α. These compounds inhibited HIF-dependent reporter gene transcription as well as VEGF protein secretion in a human renal cell adenocarcinoma line. Compounds that were confirmed to bind the HIF-2α PAS-B domain by Microscale thermophoresis (MST) and Thermal shift assay (TSA) also significantly inhibited HIF-2α, but not HIF-1α, target gene expression in a hepatocellular carcinoma cell line (P<0.05). Further, NanoString analyses revealed pathway signatures upregulated in hypoxic conditions, including angiogenesis, metabolism, and metastasis, that were significantly decreased with antagonist treatment. Selective antagonists were evaluated in vivo to determine efficacy and PK-PD relationships. Additionally, syngeneic tumor models were used to characterize the effects of HIF-2α inhibition (alone or in combination with anti-PD-1 and anti-adenosine agents) on tumor growth, immune cell infiltration, and expression of genes associated with adenosine production/signaling. Collectively, these data support the rationale to target HIF-2α for the treatment of cancers, particularly indications with a HIF-2α-driven hypoxia signature, and describe the properties of novel and selective HIF-2α antagonists. Citation Format: Kelsey E Sivick Gauthier, Kenneth V Lawson, Dana Piovesan, Matthew J Walters, Ada Chen, Xiaoning Zhao, Cesar Meleza, Nikki Kimura, Tim Park, Steve Young, Anh Tran, Samuel L Drew, Lixia Jin, Manmohan Leleti, Elaine Ginn, Jay P Powers. A novel, potent, and selective hypoxia-inducible factor (HIF)-2α antagonist [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference on Molecular Targets and Cancer Therapeutics; 2019 Oct 26-30; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2019;18(12 Suppl):Abstract nr C050. doi:10.1158/1535-7163.TARG-19-C050

An Update on Safety and Preliminary Efficacy of Highly Specific Bruton Tyrosine Kinase (BTK) Inhibitor Zanubrutinib in Combination with PD-1 Inhibitor Tislelizumab in Patients with Previously Treated B-Cell Lymphoid Malignancies

An Update on Safety and Preliminary Efficacy of Highly Specific Bruton Tyrosine Kinase (BTK) Inhibitor Zanubrutinib in Combination with PD-1 Inhibitor Tislelizumab in Patients with Previously Treated B-Cell Lymphoid Malignancies