PD-L1/4-1BB–Directed Bispecific Antibody ATG-101 Shows Preclinical Efficacy Without Hepatotoxicity

ATG-101, a novel PD-L1/4-1BB bispecific antibody, demonstrated good in vivo efficacy, safety without hepatotoxicity, and pharmacokinetic/pharmacodynamic properties in cynomolgus monkeys, according to preclinical findings from a study presented in a poster during the 2021 SITC Annual Meeting.¹

Although PD-1/PD-L1 inhibitors have transformed the treatment paradigms of several tumor types, the therapies yield deep and durable responses in a minority of patients.

4-1BB is a promising costimulatory immunologic target in oncology; however, therapeutic attempts to target 4-1BB have been fraught with significant safety concerns, notably hepatotoxicity, or suboptimal agonistic potency.

ATG-101 is a novel bispecific antibody that dually targets PD-L1 and 4-1BB. The agent activates 4-1BB–positive T cells and exhausted T cells in a PD-L1–crosslinking dependent manner by which “cold” tumors are rendered “hot”. With this mechanism of action, the therapy is hypothesized to treat tumors without eliciting on-target, off-tumor liver toxicity.

“ATG-101 was developed by introducing lower affinity 4-1BB scFv into a human IgG1 PD-L1 monoclonal antibody. The mutation on CH2 abolishes the binding capacity to more FcγRs but retains the binding to FcγRn,” said senior author Bing Hou, PhD, executive director of Drug Discovery at Antengene Corporation Co, Ltd, in Shanghai, China, during a virtual presentation of the data.

ATG-101 has demonstrated potent in vivo efficacy in several syngeneic tumor models, including those resistant to PD-1/PD-L1 inhibitors. Moreover, the therapy was found to be well tolerated among cynomolgus monkeys in Good Laboratory Practice (GLP) toxicology studies.

Regarding binding affinity, the results of the preclinical study showed that ATG-101 had 0.177 nM hPD-L1 affinity, 11.4 nM h4-1BB affinity, and 3.25 nM PD-1/PD-L1 blockade (IC50).

“Higher affinity of PD-L1 binding vs 4-1BB ensures tumor microenvironment distribution of ATG-101,” Hou said.

Additionally, ATG-101 was not shown to have hFcγR I-III binding but was shown to have hFcγRn binding.

“The binding capacity of ATG-101 to FcγRn prevents the degradation of IgG in vivo, while the deficiency of other FcγRs binding ensures that ATG-101 showed 4-1BB agonist activity only when crosslinked by PD-L1–positive cells,” Hou added.

Additional results showed that ATG-101 mediates effective and PD-L1–dependent activation of CD8+ T cells. PD-L1 overexpressed HEK293 cells were mixed with parental HEK293 cells at different proportions. At 1 µg/mL of ATG-101, IFNγ release was induced with a PD-L1 proportion of as low as 6.25%. Moreover, when HEK293­–PD-L1 cells were 0%, meaning PD-L1 was not crosslinked, ATG-101 did not mediate 4-1BB signaling.

However, upon PD-L1 crosslinking, ATG-101 was found to activate PD-1–positive, TIM3-positive exhausted T cells in vitro. This suggests ATG-101 has the potential ability to reverse T-cell dysfunction and exhaustion. These results were observed by inducing exhausted T cells by human CD3+ T cells culturing with anti-CD3/CD28 beads for 6 days. On day 6, the percentage of terminally exhausted PD-1–positive/TIM3-positive and progenitor exhausted PD-1–positive/TIM3-negative T cells was increased. Additionally, ATG-101 induced IL-2 and INF-γ secretion by exhausted T cells in the presence of PD-L1–positive cells.

ATG-101 also demonstrated antitumor efficacy in PD-1/PD-L1–resistant tumor models. ATG-101, given at 3.25 mg/kg or 13 mg/kg, was found to potentially inhibit tumor growth in h4-1BB transgenic mice bearing PD-1/PD-L1 inhibitor resistant B16F10 (melanoma) and EL4 (lymphoma) tumor cells. The tumor growth inhibition rates were 55.74% and 57.42% with the 3.25 mg/kg and 13 mg/kg doses of ATG-101, respectively, in the B16F10 model. The rates were 35% and 50%, respectively, in the EL4 model.

Additionally, the findings showed an increase in the infiltration, proliferation, and activation of CD8+ T cells with ATG-101, as well as the infiltration of natural killer T cells and the CD8+/Treg ratio in tumor infiltrating lymphocytes.

Regarding safety, the results of the study demonstrated a low risk of cytokine release syndrome (CRS) with ATG-101. The human umbilical vein endothelial cell and peripheral blood mononuclear cell coculturing system, which is the most sensitive system in evaluating the risk of drug-induced CRS, showed that ATG-101 did not induce non-specific inflammatory cytokine releases.

Notably, ATG-101, dosed up to 100 mg/kg, did not demonstrate liver toxicity in cynomolgus monkeys over a 4-week GLP repeated-dose toxicity study that was followed by a 4-week recovery period. No alteration of aspartate aminotransferase or alanine aminotransferase was observed with ATG-101 during the study.

Additionally, ATG-101 was found to maintain long-term full receptor occupancy, which suggests the agent could provide prolonged clinical efficacy.

Finally, ATG-101 was found to be effective in achieving tumor growth inhibition in mice bearing MC38 tumors that were treated with atezolizumab (Tecentriq). Upon disease progression, half of the mice were switched to ATG-101 and the others continued with PD-L1 inhibitor treatment. In the half that received ATG-101, potent tumor growth inhibition and tumor regression, as well as prolonged survival, was observed.

The first-in-human phase 1 PROBE study (NCT04986865) is ongoing to evaluate ATG-101 in patients with metastatic or advanced solid tumors and mature B-cell non-Hodgkin lymphomas.²

References

1. Yuwen H, Li T, Ren Y, et al. ATG-101, a novel PD-L1/4-1BB bispecific antibody, augments anti-tumor immunity through immune checkpoint inhibition and PDL1-directed 4-1BB activation. Presented at: 2021 SITC Annual Meeting; November 10-14; 2021; Washington DC. Abstract 893.
2. A study of evaluating the safety and efficacy of ATG-101 in patients with metastatic/advanced solid tumors and mature B-cell non-Hodgkin lymphomas (PROBE). ClinicalTrials.gov. Updated September 14, 2021. Accessed November 12, 2021. https://clinicaltrials.gov/ct2/show/NCT04986865