The Potential of STING Agonists Is Explored in Cancer

Jason Luke, MD, FACP

Stimulator of interferon genes (STING) is an endoplasmic protein that induces the production of proinflammatory cytokines and has become a target of interest to obtain or boost antitumor immune responses.¹ First-generation STING agonists are structurally unstable, which has driven development of compounds with greater stability and more potency.¹

Numerous classes of STING agonists are being evaluated for use, including novel cyclic dinucleotides (CDNs), next-generation noncyclic dinucle- otides (non-CDNs), bacterial vectors, and ENPP1 (ectonucleotide pyrophosphatase/phosphodies- terase 1) inhibitors. Systemic delivery of STING agonists is also being studied using antibody-drug conjugates, nanoparticle vaccines, antigen-presenting cell (APC)-targeted tumor vaccines, and exosome-based therapies (FIGURE1).

“Most immunotherapies rely on a preexistent antibody antitumor response, and the vast major- ity of [patients’ physiologies] have ‘immunologi- cal ignorance’ toward cancer,” Jason Luke, MD, FACP, associate professor of medicine, Division of Hematology/Oncology, University of Pittsburgh School of Medicine in Pennsylvania, said during an interview with Targeted Therapies in Oncol- ogyTM. Luke, the director of the Cancer Immu- notherapeutics Center, UPMC Hillman Cancer Center, explained that “STING is essentially an on switch that should activate the immune system to recognize the cancer.”

First-Generation STING Agonists

Michael A. Curran, PhD

DMXAA: Surprising Results, Wrong Species

DMXAA (Vadimezan), a 5,6-dimethylxanthe- none-4-acetic acid and a vascular disrupting agent, was the first STING agonist to be extensively evaluated.1 “DMXAA showed potential, but no one fully understood the pathway or why it was working at the time,” Michael A. Curran, PhD, asso- ciate professor of immunology and coscientific director of the Oncology Research for Biologics and Immunotherapy Translation program at The University of Texas MD Anderson Cancer Center in Houston, said when interviewed by Targeted Therapies in Oncology^TM.

Preclinical data were encouraging, but didn't last for long.^2,3 A randomized phase 2 study (NCT00832494) incorporating DMXAA produced an adequate safety profile, but a follow-up phase 3 placebo-controlled trial (NCT00662597) showed no benefit to overall survival or progression-free survival with the addition of DMXAA to carboplatin and paclitaxel in both trials.4,5 In both trials, patients had non–small cell lung cancer. For NCT00832494, efficacy—including improved tumor response rate, time to tumor progression, and survival—was observed when DMXAA was added to the chemotherapy duo, as well as manageable safety. Further research revealed that DMXAA binds the ligand-binding site of murine STING with high affinity but is ineffective in humans because of interspecies differences in STING.⁶ Therefore, the focus turned toward devel- oping new agents with improved stability and the ability to bind to all known alleles of human STING.

ADU-S100/MIW815: First-In-Human Clinical Trial

ADU-S100 (MIW815)—a synthetic CDN that mimics an endogenous cyclic dinucleotide called cGAMP is the native substrate of human STING and activates all known variants of human STING—was the first STING agonist to be evaluated as a cancer immunotherapy in clinical trials.¹ In preclinical studies using a variety of murine tumor models, intratumoral injection of ADU-S100 induced tumor-specific CD8-positive T cells, leading to tumor clearance. Investigators found that combining ADU-S100 with immune checkpoint modulation produced a synergistic effect; the CD8 T-cell expansion driven by ADU-S100 enhanced the efficacy of checkpoint inhibition, generating stronger, more durable eradication of tumors, including at distal noninjected sites.^7-10

Two phase 1 dose-escalation trials have examined treatment of advanced/metastatic solid tumors and lymphomas with intratumoral ADU-S100. The first-in-human trial of ADU-S100 monotherapy (NCT02675439) enrolled 47 patients.

There was only 1 dose-limiting tox- icity (DLT), but clinical efficacy was limited, with a single confirmed partial response (PR). In NCT03172936, 66 patients received ADU-S100 in combination with the anti–PD-1 monoclonal antibody spartalizumab. There were no DLTs, and several patients experienced PRs.^11,12

The NCT02675439 trial also evaluated ADU-S100 in combination with ipilimumab (Yervoy), and a phase 2 trial (NCT03937141) evaluated ADU-S100 in combination with pembrolizumab (Keytruda) in head and neck squamous cell carcinoma (HNSCC). Both trials were discontinued due to lack of substantial antitumor activity.1 Additionally, Novartis removed ADU-S100 from its port- folio in late 2019 based on a review of the clinical data generated to date.¹³

Luke commented, “The ADU-S100 trials generated more questions than answers because no linear association between dose and likelihood of tumor size reduction was observed.”

MK-1454: Possibilities in Combination Therapy

Ulevostinag (MK-1454) is a synthetic CDN that produced an acceptable safety profile in a phase 1, open-label dose-escalation study in patients with advanced solid tumors or lymphomas (NCT03010176).¹⁴ MK-1454 was evaluated as monotherapy (arm 1) and in combination with pembrolizumab (arm 2). No complete responses or PRs were observed in arm 1, but the response rate in arm 2 was 24% (6 of 25; all PRs), with an 83% median reduction in the size of injected and noninjected lesions.¹⁴

Adverse events occurred in 83% of arm 1 patients and 82% of arm 2, with grade 3 or higher events occurring in 9% and 14% of patients, respectively.¹⁴ Currently, MK-1454 is being evaluated in a phase 2 trial (NCT04220866) in patients with metastatic or unresectable recurrent HNSCC. Intratumor- ally injected MK-1454 in combination with IV [intravenous] pembrolizumab is being compared with IV pembrolizumab alone.¹⁵

Next-Generation STING Agonists

BMS-986301

BMS-986301 is a novel CDN with promising preclinical results. In the CT26 and MC38 murine tumor models, more than 90% regression was observed in injected and noninjected tumors, compared with 13% with ADU-S100. Further, complete regression of 80% of injected and noninjected tumors was achieved with a single dose of BMS-986301 in combination with an anti–PD-1 agent; in contrast, there were no responses to anti–PD-1 alone.¹⁶ A phase 1 trial evaluating BMS-986301 intratumoral or IV injection as monotherapy or in combination with nivolumab (Opdivo) and ipilimumab in patients with advanced solid cancers is currently recruiting (NCT03956680).¹

E7766

E7766 is a macrocycle-bridged STING agonist that binds to both human and murine STING proteins.1 Compared with reference CDNs, E7766 shows activity across a broader range of major human STING variants.¹⁷ Complete regression or significant tumor growth delay was observed with single intratumoral injections in mouse models.¹⁸ In CT26 model mice bearing subcutaneous and liver tumors, a single injection of E7766 caused 90% of tumors to resolve, with no recurrence for over 8 months.19 A dose-dependent antitumor response and strong interferon-β induction were also observed in a mouse model of BCG-unresponsive non–muscle-invasive blad- der cancer following intravesical adminis- tration of E7766.²⁰ A phase 1/1b clinical trial evaluating the efficacy of E7766 monotherapy in patients with advanced solid tumors and lymphomas is under way (NCT04144140).¹

GSK3745417

GSK3745417, a non-CDN small molecule with a dimeric amidobenzimidazole scaffold, is being studied in a phase 1 dose-escalation study (NCT03843359) as monotherapy or in combination with dostarlimab (Jemperli) in patientswithrelapsed/refractorysolidtumors. GSK3745417 is administered via IV injection, and study completion is expected in 2025.^1,21

SB11285

Another CDN being evaluated for IV use in patients with advanced solid tumors is the small molecule SB 11285. Preclinical studies have shown significantly higher inhibition of tumor growth in mice with intratumoral injection of SB11285 vs control, and a synergistic effect was observed when SB11285 was combined with cyclophosphamide.²² A phase 1a/1b, nonrandomized dose-escalation study is under way to evaluate IV SB11285 as monotherapy or in combination with atezoli- zumab (Tecentriq) in patients with advanced solid tumors (NCT04096638).¹

Of the systemic drugs, Curran says SB 11285 may have the best therapeutic window. “SB 11285 is more graded in its activity without being hyperactive at lower doses, and it has a natural predilection in the way it was made for uptake by lymphocytes spe- cifically, so it’s not going to have as great a potential for organ toxicity.”

Ryvu’s STING Agonists

The biopharmaceutical company Ryvu Therapeutics is developing new-generation non-CDN, nonmacrocyclic small-molecule STING agonists (Ryvu’s agonists) intended for systemic administration. Initial preclin- ical study results indicated high cellular potency, with cytokine production in human immune cells at low nanomolar range.^1,23 Additionally, high activity is maintained regardless of the human STING variant. In the CT26 murine model, IV administration led to significant tumor growth inhibition, complete tumor regression, and development of immunological memory.^1,23

The Future of STING Agonists

Although preclinical data are exciting, clini- cal results with STING agonists remain elu- sive. “Every single company has preclinical modeling to indicate their version of this is the best,” said Luke, “but beyond ADU-S100 and DMXAA, we have zero clinical data to indi- cate there are direct applications in the field.”

He noted that there is even a lupus-like autoimmune condition in humans called STING-associated vasculopathy with onset in infancy in which STING is hyperactive.²⁴

This raises another concern mentioned by both Luke and Curran. “STING is everywhere, almost all cells have some degree of STING pathway, and it’s active in a lot of them,” Curran said. “If you’re giving something that accumulates more efficiently in...the liver and not the target tumor, then you’re going to have concerns about the appropriate therapeutic window.” Curran noted one way to circumvent this problem: targeting of STING agonists to tumors using systemic delivery approaches such as antibody-drug conjugation and other tumor-targeted delivery vehicles.

One fundamental obstacle associated with STING agonists is knowing which cells to target. “We may need to be targeting other antigen-presenting populations, such as monocytes or myeloid cells,” said Luke. Expanding on this topic, Curran suggested that a “pan- myeloid” approach is required. “There’s a dif- ferent benefit depending on the myeloid subset. On the suppressive myeloid side, you need to depolarize them from how the tumor has programmed them, but on the dendritic cell side, you’re looking to create an active dendritic cell population and wake up the tumor response.” STING agonists have the potential to do both.

The combination of STING agonists and current immunotherapies is certainly appealing. “CTLA-4 and PD-L1 blockades are really good at rescuing failed T-cell responses, but they require T cells and access to the core of the tumor,” Curran said. He continued, “STING agonists function as an in situ vac- cine to prime [the] T-cell response, but they also lower the myelosuppressive threshold that creates immune exclusion. The 2 [types of drug] together are ideal.”

There are many open-ended questions regarding STING agonist therapy at this time, but the immense potential keeps Luke engaged with the research. “This should work; we just haven’t figured out why it doesn’t yet in humans. That’s partly why I remain so interested. If we can keep working on it in a logical, hypothesis-driven way, then we should get there.”

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