Phase 1 first-in-human dose-escalation study of IMSA101, a novel cyclic di-nucleotide STING agonist, for patients with advanced solid tumor malignancies.

Background: Despite progress in cancer therapeutics, there remains an unmet need for treatment of advanced solid tumors. The cGAS-cGAMP-STING pathway plays a pivotal role in innate antitumor immunity processes. IMSA101 is a small molecule analog of cGAMP and a potent STING agonist. Preclinical studies demonstrate antitumor activity of IMSA101 alone and in combination with immune-checkpoint inhibitors (ICIs).

Methods: IMSA101-101 was an open-label, multicenter, phase 1 first-in-human dose-escalation study to establish a recommended phase 2 dose (RP2D) of IMSA101 both as monotherapy and in combination with a programmed death ligand 1 (PD-(L)1)-ICI. Secondary objectives were to evaluate safety, tolerability and antitumor activity, and to characterize pharmacokinetics. Adult patients with advanced solid tumors with ≥2 Response Evaluation Criteria in Solid Tumors evaluable lesions, at least one of these suitable for injection, were enrolled. IMSA101 was administered by intratumoral injection with weekly injections for the first 3 weeks, followed by biweekly injections. The dose escalation explored doses of 100-1,200 µg (monotherapy) and 800-2,400 µg (combination therapy with ICI) in a 3+3 design. No formal statistical analysis was planned for this study.

Results: 40 patients (22 monotherapy, 18 combination therapy) received at least one dose of IMSA101. IMSA101 1,200 µg (monotherapy) and 2,400 µg (combination therapy) doses, well-tolerated and associated with signs of antitumor activity, were selected as provisional RP2Ds. The most common IMSA101-related treatment-emergent adverse events (TEAEs) were injection site pain (8 (36.4%)) and fatigue (4 (18.2%)) for monotherapy and chills (3 (16.7%)), injection site pain (2 (11.1%)), and fever (2 (11.1%)) for combination therapy. No clear dose-response relationship between IMSA101 and occurrence of TEAEs was observed. The elimination half-life of plasma IMSA101 was approximately 1.5-2 hours, with no reported plasma accumulation. With monotherapy, no patients achieved complete response (CR) or partial response (PR), so overall response rate (ORR) was not determined; 17 (77.3%) patients had progressive disease (PD) and one patient (4.5%, 400 µg cohort) had stable disease (SD) as best response. With combination therapy, ORR was 5.6%; remaining patients had PD (10 (55.6%)) and SD (2 (11.1%)) as their best response.

Conclusions: IMSA101 doses of 1,200 µg (monotherapy arm) and 2,400 µg (combination therapy arm) were well tolerated but demonstrated minimal signals of antitumor activity in patients with advanced solid tumors.

Trial registration number: NCT04020185.