Search

Immunotherapy approaches focusing on T cells have provided breakthroughs in treating solid tumors. However, there remains an opportunity to drive anticancer immune responses via other cell types, particularly myeloid cells. ATRC-101 was identified via a target-agnostic process evaluating antibodies produced by the plasmablast population of B cells in a patient with non-small cell lung cancer experiencing an antitumor immune response during treatment with checkpoint inhibitor therapy. Here, we describe the target, antitumor activity in preclinical models, and data supporting a mechanism of action of ATRC-101. Immunohistochemistry studies demonstrated tumor-selective binding of ATRC-101 to multiple nonautologous tumor tissues. In biochemical analyses, ATRC-101 appears to target an extracellular, tumor-specific ribonucleoprotein (RNP) complex. In syngeneic murine models, ATRC-101 demonstrated robust antitumor activity and evidence of immune memory following rechallenge of cured mice with fresh tumor cells. ATRC-101 increased the relative abundance of conventional dendritic cell (cDC) type 1 cells in the blood within 24 h of dosing, increased CD8+ T cells and natural killer cells in blood and tumor over time, decreased cDC type 2 cells in the blood, and decreased monocytic myeloid-derived suppressor cells in the tumor. Cellular stress, including that induced by chemotherapy, increased the amount of ATRC-101 target in tumor cells, and ATRC-101 combined with doxorubicin enhanced efficacy compared with either agent alone. Taken together, these data demonstrate that ATRC-101 drives tumor destruction in preclinical models by targeting a tumor-specific RNP complex leading to activation of innate and adaptive immune responses.

Background We have previously demonstrated adaptive antibody responses targeting public tumor antigens in cancer patients. ATRC-101, a clinical stage, engineered version of an antibody identified in such a patient, displays robust single-agent activity in syngeneic tumor models requiring Fc receptors (FcRs) expressed by innate immune cells and the presence of CD8+ T cells. The novel target of ATRC-101 was found to be a tumor-restricted ribonucleoprotein (RNP) complex, and because RNP complexes drive T cell responses in infectious and autoimmune disease via innate immune cells, we further characterized the mechanism-of-action of ATRC-101. Here we describe changes in immune cell populations in a tumor model proximal to treatment initiation with ATRC-101. Methods Mice bearing EMT6 tumors received ATRC-101 beginning on day 7 post-tumor inoculation. Tissues were harvested between days 7 and 14 and analyzed by flow cytometry and immunohistochemistry. Transcriptome analysis was performed using RNA sequencing on whole tumors taken on days 7, 9, and 12. Results The earliest significant changes induced by ATRC-101, relative to vehicle, were noted just 24 hours after dosing: increased numbers of cDC1 cells in blood, and decreased numbers of cDC2 cells in blood and M-MDSCs in tumor. A significant increase of CD8+ T cells was observed in blood 48 hours after dosing and in tumor 96 hours after dosing. Increased numbers of NK cells were also observed in blood and tumor at this later time. Multiplex analysis of circulating cytokines demonstrated a very early increase in myeloid chemo-attractants, such as MCP1 and MIP1a.Whole exome sequencing of tumor samples showed that ATRC-101 dosing drives a significant increase, relative to vehicle, in the expression of interferon-stimulated genes. Co-culturing experiments demonstrated that induced, bone marrow-derived dendritic cells are activated by ATRC-101 and its target in a dose-dependent fashion. Conclusions Dosing with ATRC-101 in the EMT6 syngeneic tumor model, in which ATRC-101 displays notable single-agent activity, leads to changes in immune cell composition in the blood and tumor, with the earliest changes observed in myeloid or myeloid-derived cell populations, and to the early appearance of myeloid chemo-attractants. We believe these data indicate that ATRC-101 acts proximally on the myeloid cell populations in the tumor, leading to a remodeling of the tumor environment and an adaptive immune response that includes CD8+ T cells driving tumor regression. Our data demonstrate that ATRC-101, bound to its target which is an RNP complex, can activate myeloid cells and are consistent with this activation occurring via FcR and Toll-like receptor (TLR) pathways.

BackgroundWe previously described ATRC-101, a fully human, engineered IgG1 antibody which is currently under evaluation in the clinic as a monotherapy for solid tumors. A chimeric version of this antibody expressed on a mouse IgG2a (mATRC-101) has shown robust anti-tumor activity as a monotherapy in the EMT6 syngeneic tumor model. In order to assess the potential utility of ATRC-101 in combination with chemotherapeutic agents, non-clinical studies were performed to assess the efficacy of mATRC-101 in combination with chemotherapeutic agents, including Doxorubicin and pegylated liposomal Doxorubicin (PLD), and the impact of these anti-tumor small molecules on mATRC-101 immunoreactivity in mouse tumor and normal tissues.MethodsFemale BALB/c mice with established EMT6 tumors were dosed with mATRC-101 (1 or 3 mg/kg) or vehicle intraperitoneally (IP) twice weekly plus Doxorubicin (2 or 5 mg/Kg) or vehicle (saline) IV once weekly following randomization on Day 6. Statistical analyses of tumor volumes were performed using the normalized area above the curve and the normalized growth rate metrics developed at Atreca. One-sided log-rank (Mantel-Cox) test was used to assess survival advantage relative to the indicated reference group. P-values ≤0.05 were considered significant. In monotherapy studies with Doxorubicin and PLD, EMT6 tumor and non-tumor bearing mice were dosed with vehicle or Doxorubicin (2, 10 mg/kg) or PLD (1, 2, 5,10 mg/kg). Normal mouse tissues were collected at 24 hours and 2 weeks after the last dose. Reactivity for mATRC-101 in EMT6 tumor and normal mouse tissues was evaluated by immunohistochemistryResultsThe combination of 3 mg/kg mATRC-101 and 5 mg/kg Doxorubicin demonstrated significant tumor growth inhibition compared to either monotherapy, or vehicle (pConclusionsThis study provides non-clinical evidence that administration of mATRC-101 in combination with Doxorubicin increases anti-tumor activity in the EMT6 model. Exposure to Doxorubicin or PLD in EMT6 tumor-bearing mice increased mATRC-101 immunoreactivity in the tumor, in a dose-dependent manner Moreover, mATRC-101 immunoreactivity in normal tissues was not influenced by Doxorubicin or PLD. Taken together, these findings support clinical evaluation of the combination of ATRC-101 and doxorubicin.