Conference Coverage

SRA737 + anti–PD-L1 therapy and low-dose gemcitabine shows early promise for SCLC



– Combined treatment with a programmed death-ligand 1 (PD-L1) inhibitor, the oral CHK1 inhibitor SRA737, and low-dose gemcitabine for small cell lung cancer (SCLC) resulted in dramatic antitumor activity and established a strong antitumor microenvironment in a preclinical model.

The findings provide a “strong rationale” for combining these agents in patients with SCLC, Triparna Sen, PhD, reported in a late-breaking abstract presentation at the annual meeting of the American Association for Cancer Research.

Dr. Sen and her colleagues assessed this triple regimen based on encouraging prior findings, including their own recent finding that DNA damage response (DDR) inhibition “actually increases antitumor immunity in this cancer type” by activating the STING/TBK1/RF3 innate immune pathway and increasing levels of chemokines-CXCL10 and CCL5 that induced activation of cytotoxic T lymphocytes.

“Based on this background and studies published in other cancer types, we hypothesized that ... SRA737... a very highly selective potent checkpoint inhibitor ... will upregulate the innate immune signaling, resulting in improved antitumor immune response in combination with anti–PD-L1,” she said, noting that bladder and colorectal cancer models were also studied.

The results varied by cancer type, but encouraging results in SCLC led to in vivo study, said Dr. Sen, who was a postdoctoral fellow, instructor, and member of the Byers Laboratory at MD Anderson Cancer Center, Houston, at the time the research was conducted.

She and her colleagues injected immunocompetent mice with Trp53, Rb1, and p130 triple-knockout SCLC cell lines that are “very highly representative of what we see in patients,” and resulting tumors were treated with SRA737 alone or in combination with an anti–PD-L1 agent.

PD-L1 alone did not work, and SRA737 with 5- out of 7-day dosing was associated with a significant delay in tumor growth.

“However, the combination does much better than either of the single agents alone. ... we never had tumor growth beyond baseline and there was regression as early as 12 days,” she said, noting that the combination activates the STING pathway.

Similar findings were seen for bladder and colorectal cancer models, she noted.

The next question is how chemotherapy plus immune checkpoint blockade – the currently approved first-line therapy in SCLC – can be improved, and how chemotherapy modulates the immune microenvironment in SCLC, she said.

To explore this, she and her colleagues treated the mice with subtherapeutic doses of SRA737 on 2 out of 7 days with low-dose gemcitabine, followed by anti–PD-L1 therapy, or with single-agent therapy and various double-agent combinations.

Again, none of the agents worked on their own.

“Even with the double combinations you see very, very modest benefit,” she said. “With the triple combination we wipe out the tumors; as early as 14 days we have 8 out of 10 complete responses, and we have followed the tumors up to 2 months and they stay gone.”

“In a nutshell, this works,” she added.

Additional analyses showed that the CD3+ T cells increase with the gemcitabine/SRA737 combination, and even more so with the triple-combination therapy.

“So we not only increase the CD3+ total T cells, we do increase CD8+ cytotoxic T cells,” she said. “Interestingly, we also decrease exhausted T-cell populations, and also [regulatory T] cells.”

Additionally, the M1 macrophage population was significantly higher with the triple regimen, there was a trend toward a decrease in the antimacrophage population, and there was a higher population of dendritic cells and myeloid-derived suppressor cells.

“What I believe is we are still scratching the surface, and we need to go deeper into the tumor microenvironment and see how these combinations really work,” she said, concluding that SRA737 is cytotoxic and induces micro-nuclei formation in a subset of SCLC and other cancer models in vitro, that in combination with anti–PD-L1 it activates innate immune signaling and causes tumor regression in SCLC, and that with low-dose gemcitabine it results in durable tumor regression in combination with SRA737 and anti–PD-L1.

“What is the most interesting is that this triple combination enhances antitumor immunity by increasing cytotoxic T-cell infiltration, decreasing T-cell exhaustion, and a favorable modulation of antigen presenting cells,” she said. “Why do we care? The anti–PD-L1 drug ... atezolizumab ... is right now FDA approved as a first-line treatment in combination with chemotherapy, and we already have DDR inhibitors in the clinic, we have PARP inhibitors in the clinic, we have checkpoint inhibitors in the clinic, SRA737 is in the clinic.

“So our preclinical data provides a strong rationale for combining low-dose gemcitabine with checkpoint inhibition and with anti–PD-L1 to enhance the clinical efficacy of these drugs,” she concluded.

Dr. Sen reported having no disclosures.

SOURCE: Sen T et al. AACR 2019, Abstract LB-148.

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