Differential expression of CCR8 in tumors versus normal tissue allows specific depletion of tumor-infiltrating T regulatory cells by GS-1811, a novel Fc-optimized anti-CCR8 antibody

The presence of T regulatory (Treg) cells in the tumor microenvironment is associated with poor prognosis and resistance to therapies aimed at reactivating anti-tumor immune responses. Therefore, depletion of tumor-infiltrating Tregs is a potential approach to overcome resistance to immunotherapy. However, identifying Treg-specific targets to drive such selective depletion is challenging. CCR8 has recently emerged as one of these potential targets. Here, we describe GS-1811, a novel therapeutic monoclonal antibody that specifically binds to human CCR8 and is designed to selectively deplete tumor-infiltrating Tregs. We validate previous findings showing restricted expression of CCR8 on tumor Tregs, and precisely quantify CCR8 receptor densities on tumor and normal tissue T cell subsets, demonstrating a window for selective depletion of Tregs in the tumor. Importantly, we show that GS-1811 depleting activity is limited to cells expressing CCR8 at levels comparable to tumor-infiltrating Tregs. Targeting CCR8 in mouse tumor models results in robust anti-tumor efficacy, which is dependent on Treg depleting activity, and synergizes with PD-1 inhibition to promote anti-tumor responses in PD-1 resistant models. Our data support clinical development of GS-1811 to target CCR8 in cancer and drive tumor Treg depletion in order to promote anti-tumor immunity.

Phase 1b study of GS-3583, a novel FLT3 agonist Fc fusion protein, in patients with advanced solid tumors

2566

Background: We have previously shown that systemic administration of GS-3583, a Fms-like tyrosine kinase 3 (FLT3) agonist Fc fusion protein leads to expansion of conventional dendritic cells (cDC), both subtype 1 (cDC1) and subtype 2 (cDC2), in the periphery of healthy volunteers (Rajakumaraswamy N, et al. J Clin Oncol. 2021;39[suppl_15]:2559.). This mechanism may increase cDC in the tumor microenvironment and promote T cell mediated antitumor activity in patients with solid tumors. Methods: This ongoing, Phase 1b, open label study is investigating the safety, tolerability, immunogenicity, pharmacokinetics, and pharmacodynamics of escalating multiple doses of GS-3583 monotherapy in adult patients with advanced solid tumors using a standard 3 + 3 design. GS-3583 was administered intravenously on Days 1 and 15 of Cycle 1 and on Day 1 of each subsequent 28-day cycle for up to 52 weeks or until progressive disease or unacceptable toxicity. Dose-limiting toxicity (DLT) was evaluated during the first 28 days of GS-3583 therapy at each dose level. Results: At the time of the Dec 3, 2021 data cut-off, 9 patients have enrolled in 3 dose escalation cohorts. Median (range) age was 71 (44-79); 4 (44%) patients were male. Tumor types were pancreatic (n=3), ovarian (n=4), and rectal (n=2). To date, no DLTs or discontinuation due to adverse events (AE) have been observed. Three patients had Grade ≥3 AEs which were also recorded as serious AEs, none of which were considered related to GS-3583. Dose dependent increase in GS-3583 exposure was observed in the evaluated dose range 2 to 12 mg with target-mediated drug disposition appearing to be saturated at doses above Dose Level 2. GS-3583 accumulation was observed at higher dose levels. GS-3583 treatment resulted in expansions of cDC1 and cDC2 at all 3 doses (Table); a dose-dependent trend in the magnitude and the durability of cDC expansion was observed. At the highest dose evaluated, GS-3583 produced ≥100-fold expansion of both cDC1 and cDC2 at multiple time points. Dose escalation on the study is still ongoing. Conclusions: GS-3583 was safe and well tolerated and induced dose-dependent expansion of cDCs in the periphery in patients with advanced solid tumors up to doses of 12 mg. These findings support further dose escalation and clinical development of GS-3583 in combination with agents that would stimulate the expanded cDCs to produce anti-tumor responses. Clinical trial information: NCT04747470. [Table: see text]

Phase 1b dose escalation study to evaluate the safety, tolerability, pharmacokinetics, and preliminary efficacy of GS-3583, a FLT3 agonist Fc fusion protein, in patients with advanced solid tumors

TPS3147

Background: Productive antitumor immune responses in nonclinical models depend on a type of dendritic cell (DC), conventional DC subtype 1 (cDC1), which in the context of cancer, primes tumor-reactive T cells through presentation of tumor-derived antigens. FMS-related tyrosine kinase 3 ligand (FLT3L) is a hematopoietic growth factor that binds to and activates FLT3 on terminally differentiated DCs. Activated FLT3 promotes proliferation, inhibits cell death, and is required for the differentiation, expansion, and maintenance of DCs in peripheral and lymphoid organs. GS-3583 is a fusion protein composed of the extracellular domain of recombinant human FLT3L fused to an engineered fragment crystallizable (Fc) region of human immunoglobulin G4. GS-3583 has PK properties that support sustained cDC in patients and potential combination with established immunotherapies. This phase 1b, open-label, multicenter, dose-finding study will evaluate safety, tolerability, PK, and preliminary efficacy of GS-3583 monotherapy in patients with advanced solid tumors (NCT04747470). Methods: Approximately 33 adults aged ≥18 years with a histologically or cytologically confirmed locally advanced or metastatic malignant solid tumor that is refractory to or intolerant of standard therapy or for which no standard therapy is available will be enrolled. The study employs a 3+3 dose escalation design in which GS-3583 is administered intravenously for up to 52 weeks or until progressive disease or unacceptable toxicity. Up to five dose escalation cohorts have been planned. The maximum tolerated dose is the highest dose with incidence of DLT in < 33% of 6 or more patients in the first 28 days of GS-3583 dosing; recommended phase 2 dose will be determined. Assessments include safety, PK, pharmacodynamics including cDCs, immunogenicity, and efficacy by RECIST 1.1 in CT/MRI imaging conducted every 8 weeks. Accrual at approximately 3-4 centers in the US is ongoing. Clinical trial information: NCT04747470.

GS-3583, a novel FLT3 agonist Fc fusion protein, to expand conventional dendritic cells in healthy volunteers

2559

Background: Conventional dendritic cells subtype 1 (cDC1) play a vital role in the priming and expansion of tumor specific CD8+ T cells and their recruitment to tumor microenvironment (TME). However, cDC1s are often underrepresented in the TME. Systemic administration of Fms-like tyrosine kinase 3 ligand (FLT3L), a hematopoietic growth factor that binds to FLT3 on myeloid and lymphoid progenitor cells, leads to expansion of cDC1s in the periphery which can then be recruited into the TME. We hypothesize that FLT3 pathway stimulation using GS-3583, a FLT3 agonist Fc fusion protein, has the potential to promote T cell mediated anti-tumor activity. Methods: This was a first-in-human placebo-controlled study of GS-3583 in healthy volunteers to evaluate the safety, PK, and PD of escalating single doses (ranging from 75μg to 2000μg) of GS-3583. The study was blinded to the subjects and the investigator. Each dose cohort enrolled 8-12 healthy subjects who received GS-3583 or placebo as single IV infusion at 3:1 ratio. Subjects were observed in the phase 1 unit for 15 days and then for 12 weeks as outpatients. As part of the PD evaluation, we investigated the changes in the number of cDC1s and cDC subtype 2 (cDC2) cells. Results: As of 8th Feb 2021, selected safety, PK and PD data from the first 3 cohorts were available. GS-3583 was well tolerated and all subjects had been discharged. To date, there have been no serious or grade 3 or higher adverse events. Preliminary PK analysis suggested dose-dependent increase in GS-3583 exposure (AUC and Cmax). Preliminary PD analysis shows that administration of GS-3583 resulted in dose-dependent increases in cDC1/cDC2 cells that peaked at day 5 or day 8 and returned to baseline within three weeks of drug administration. Conclusions: GS-3583 was safe and well tolerated and induced dose dependent expansion of dendritic cells in the periphery. In patients with cancer, this increase in dendritic cells can be utilized to enhance anti-tumor responses to immuno-oncology therapies.[Table: see text]

Ulocuplumab (BMS-936564 / MDX1338): a fully human anti-CXCR4 antibody induces cell death in chronic lymphocytic leukemia mediated through a reactive oxygen species-dependent pathway

The CXCR4 receptor (Chemokine C-X-C motif receptor 4) is highly expressed in different hematological malignancies including chronic lymphocytic leukemia (CLL). The CXCR4 ligand (CXCL12) stimulates CXCR4 promoting cell survival and proliferation, and may contribute to the tropism of leukemia cells towards lymphoid tissues. Therefore, strategies targeting CXCR4 may constitute an effective therapeutic approach for CLL. To address that question, we studied the effect of Ulocuplumab (BMS-936564), a fully human IgG4 anti-CXCR4 antibody, using a stroma--CLL cells co-culture model. We found that Ulocuplumab (BMS-936564) inhibited CXCL12 mediated CXCR4 activation-migration of CLL cells at nanomolar concentrations. This effect was comparable to AMD3100 (Plerixafor--Mozobil), a small molecule CXCR4 inhibitor. However, Ulocuplumab (BMS-936564) but not AMD3100 induced apoptosis in CLL at nanomolar concentrations in the presence or absence of stromal cell support. This pro-apoptotic effect was independent of CLL high-risk prognostic markers, was associated with production of reactive oxygen species and did not require caspase activation. Overall, these findings are evidence that Ulocuplumab (BMS-936564) has biological activity in CLL, highlight the relevance of the CXCR4-CXCL12 pathway as a therapeutic target in CLL, and provide biological rationale for ongoing clinical trials in CLL and other hematological malignancies.

BMS-936564/MDX-1338: a fully human anti-CXCR4 antibody induces apoptosis in vitro and shows antitumor activity in vivo in hematologic malignancies

PURPOSE

CXCR4 has been identified as a prognostic marker for acute myeloid leukemia (AML) and other malignancies. We describe the development and characterization of a fully human antibody to CXCR4 and its application for therapy of AML, non-Hodgkin lymphoma (NHL), chronic lymphoid leukemia (CLL), and multiple myeloma.

EXPERIMENTAL DESIGN

Human transgenic mice were immunized with CXCR4-expressing cells, and antibodies reactive with CXCR4 were analyzed for apoptosis induction and ability to interfere with CXCL12-induced migration and calcium flux. In vivo efficacy was determined in multiple AML, NHL, and multiple myeloma xenograft tumors in severe combined immunodeficient mice.

RESULTS

BMS-936564/MDX-1338 is a fully human IgG(4) monoclonal antibody that specifically recognizes human CXCR4. In vitro studies show that MDX-1338 binds to CXCR4-expressing cells with low nanomolar affinity, blocks CXCL12 binding to CXCR4-expressing cells, and inhibits CXCL12-induced migration and calcium flux with low nanomolar EC(50) values. When given as monotherapy, MDX-1338 exhibits antitumor activity in established tumors including AML, NHL, and multiple myeloma xenograft models. In addition, we show that MDX-1338 induced apoptosis on a panel of cell lines and propose that antibody-induced apoptosis is one of the mechanisms of tumor growth inhibition.

CONCLUSIONS

BMS-936564/MDX-1338 is a potent CXCR4 antagonist which is efficacious as monotherapy in tumor-bearing mice and is currently in phase I for the treatment of relapsed/refractory AML, NHL, CLL, and multiple myeloma.

A nonfucosylated human antibody to CD19 with potent B-cell depletive activity for therapy of B-cell malignancies

A human anti-CD19 antibody was expressed in fucosyltransferase-deficient CHO cells to generate nonfucosylated MDX-1342. Binding of MDX-1342 to human CD19-expressing cells was similar to its fucosylated parental antibody. However, MDX-1342 exhibited increased affinity for FcγRIIIa-Phe158 and FcγRIIIa-Val158 receptors as well as enhanced effector cell function, as demonstrated by increased potency and efficacy in antibody-dependent cellular cytotoxicity (ADCC) and phagocytosis assays. MDX-1342 showed dose-dependent improvement in survival using a murine B-cell lymphoma model in which Ramos cells were administered systemically. In addition, low nanomolar binding to cynomolgus monkey CD19 and increased affinity for cynomolgus monkey FcγRIIIa was observed. In vivo administration of MDX-1342 in cynomolgus monkeys revealed potent B-cell depletion, suggesting its potential utility as a B-lymphocyte depletive therapy for malignancies and autoimmune indications.

A guanine nucleotide exchange factor-independent function of Vav1 in transcriptional activation

T cell antigen receptor (TCR) stimulation induces the tyrosine phosphorylation of several intracellular proteins including the protooncogene Vav1. Vav1 expression is necessary for normal T cell development and activation. We previously showed that overexpression of Vav1 in Jurkat T cells potentiates the activity of the transcription factor nuclear factor of activated T cells (NF-AT). The mechanism by which Vav1 participates in TCR signaling events is not clear. Vav1 contains a guanine nucleotide exchange factor (GEF) domain that has specificity for Rac and other Rho GTPases that have been recently implicated in T cell activation events. Significantly, in vitro tyrosine phosphoryation of Vav1 by Lck activates its exchange activity. This Lck-mediated phosphorylation of Vav1 has been reported to depend upon Tyr-174 in Vav1, a site implicated in Vav1 function by other studies as well. In this report, we demonstrated that Tyr-174 is not required for the TCR-induced phosphorylation of Vav1 in vivo. Moreover, mutation of Tyr-174 augmented the ability of Vav1 to up-regulate NF-AT activation as well as the Vav1 GEF function leading to Rac activation. However, we also showed that the GEF activity of Vav1 was neither sufficient nor necessary for potentiation of NF-AT, and thereby we identify a GEF-independent role of Vav1 in potentiating NF-AT-driven transcription. Oncogenic Vav1 in which the amino-terminal 67 amino acids were deleted had elevated GEF activity but did not potentiate NF-AT when overexpressed in Jurkat cells. We also showed that a GEF mutant form of Vav1 that had impaired GEF function could still potentiate NF-AT. These studies reveal a previously unrecognized negative regulatory function of Tyr-174 in Vav1 and suggest that domains other than the Vav1 GEF domain contribute to TCR signals leading to NF-AT activation.

Uncoupling of nonreceptor tyrosine kinases from PLC-gamma1 in an SLP-76-deficient T cell

Activation of nonreceptor protein tyrosine kinases (PTKs) is essential for T cell receptor (TCR) responsiveness; however, the function of individual PTK substrates is often uncertain. A mutant T cell line was isolated that lacked expression of SLP-76 (SH2 domain-containing leukocyte protein of 76 kilodaltons), a hematopoietically expressed adaptor protein and PTK substrate. SLP-76 was not required for TCR-induced tyrosine phosphorylation of most proteins, but was required for optimal tyrosine phosphorylation and activation of phospholipase C-gamma1 (PLC-gamma1), as well as Ras pathway activation. TCR-inducible gene expression was dependent on SLP-76. Thus, coupling of TCR-regulated PTKs to downstream signaling pathways requires SLP-76.

Novel regulation of an MHC class I gene response to interferon-gamma

The ability of IFN-gamma to increase the expression of MHC class I gene products is likely to enhance cytolytic T lymphocyte recognition of viral pathogens and tumor cells. The murine lymphoma AKR SL3-cl.F AZR (SL3-cl.F) responds aberrantly to treatment with interferon-gamma such that H-2Dk surface expression is augmented, but H-2Kk expression remains at constitutive levels. Somatic cell fusions have been used to demonstrate that the lesion responsible for this phenotype is cis-dominant, implicating a primary lesion within the SL3-cl.F H-2Kk gene. In this communication, we have used PCR to analyze the nucleotide sequence in regions of the SL3-cl.F H-2Kk promoter known to contain interferon-responsive enhancer elements. Comparison of the SL3-cl.F H-2Kk sequences to known consensus elements revealed complete identity. In order to identify the lesion responsible for the SL3-cl.F phenotype, two H-2Kk genomic clones were independently isolated from SL3-cl.F. Each clone exists as a 10.5-kbp EcoRI fragment containing the entire structural gene. The site of transcription initiation is at the center of this fragment; therefore, all regulatory elements within 5 kbp of the transcript start site which could alter steady-state message levels are included. Interestingly, IFN-gamma-augmented expression of the H-2Kk gene was restored following DNA-mediated transfection of either of these clones into fibroblast cell lines and the parental cell line SL3-cl.F. Because isolation of these clones required passage of the DNA through a prokaryotic host, which alters the pattern of DNA methylation, there was the possibility that demethylation was responsible for the newly acquired IFN-gamma-responsive phenotype. Treatment of SL3-cl.F with 5-azacytidine, which inhibits de novo methylation, did not restore IFN-gamma-augmented expression, however, thus excluding H-2Kk specific methylation as a potential mechanism. Collectively, these data demonstrate that the alteration responsible for the phenotype observed in SL3-cl.F does not involve known transcriptional regulatory elements. Potential mechanisms which might account for the mutant phenotype are discussed.