Anti-C5a Antibody STSA-1002 Shows Favorable Safety, Reduced Mortality in ARDS

Targeting Complement in Viral Pneumonia-Induced ARDS

Acute respiratory distress syndrome (ARDS) remains a significant driver of mortality in intensive care, particularly when precipitated by viral pneumonia, and specific treatments targeting its pathophysiology are lacking [1]. A growing body of evidence points to the overactivation of the complement system as a central mechanism of lung injury in ARDS. This inflammatory cascade, driven by mediators such as the anaphylatoxin C5a, can lead to diffuse alveolar damage and profound hypoxemia [2]. Consequently, modulating this response by directly inhibiting C5a has become a key therapeutic strategy under investigation, aiming to mitigate the excessive inflammation that characterizes the condition [2, 3].

Trial Design and Patient Cohort

To evaluate a potential complement-modulating therapy, researchers conducted a phase 1b/2, multi-center, double-blind, placebo-controlled trial of STSA-1002. The study enrolled adults aged 18 to 85 years with ARDS secondary to viral pneumonia, defined by moderate to severe hypoxemia with a partial pressure of oxygen to fraction of inspired oxygen ratio (PaO₂/FiO₂) of ≤ 200mmHg. A total of 49 patients were enrolled and randomized between December 9, 2023, and March 20, 2025. The 47 patients who received the study drug were assigned in a 1:1:1 ratio to receive either STSA-1002 1350mg, STSA-1002 750mg, or a matched placebo. The dosing schedule consisted of administrations on days 1, 3, and 7, with an optional dose on day 14±1 at the physician's discretion. The final treatment groups consisted of 17 patients in the 1350mg arm, 15 in the 750mg arm, and 15 in the placebo arm. This trial is registered with ClinicalTrials.gov, NCT06038916.

Primary Endpoint: Time to Clinical Improvement

The trial's primary endpoint was the time to clinical improvement (TTCI) within 28 days. This was defined as the first instance of achieving either a peripheral oxygen saturation to fraction of inspired oxygen ratio (SpO₂/FiO₂) > 235 or a PaO₂/FiO₂ > 200 mmHg, sustained for at least 48 hours. The median TTCI was 6.0 days in the high-dose 1350mg group, compared to 8.4 days in the 750mg group and 7.4 days in the placebo group. To account for events like death which can preclude clinical improvement, the investigators used a competing risk model to calculate subdistribution hazard ratios (sHR), a statistical method appropriate for settings with such competing events. The unadjusted sHR for the 1350mg group versus placebo was 1.55 (95% CI, 0.68-3.55), and for the 750mg group it was 1.04 (95% CI, 0.46-2.38). After adjusting for baseline PaO₂/FiO₂, the sHRs were 1.22 (95% CI, 0.52-2.86) and 1.13 (95% CI, 0.47-2.75) for the high-dose and low-dose groups, respectively.

Safety Profile and Mortality Outcomes

The study reported that STSA-1002 was well tolerated at both dose levels, demonstrating a favorable safety profile in this critically ill population. While the primary endpoint did not show a statistically significant difference, the analysis of 28-day all-cause mortality revealed a notable divergence between the groups. The mortality rate in the high-dose STSA-1002 1350mg group was 5.88% (1 of 17 patients). This was substantially lower than the mortality rates observed in the 750mg group, which was 26.67% (4 of 15 patients), and the placebo group, which was 40% (6 of 15 patients). These data suggest a potential dose-dependent survival benefit associated with STSA-1002.

Clinical Implications and Future Directions

This phase 1b/2 trial of the anti-C5a antibody STSA-1002 provides important data for the management of viral pneumonia-induced ARDS. Although the study did not meet its primary endpoint for time to clinical improvement with statistical significance, the findings on mortality are clinically relevant. The observed reduction in 28-day all-cause mortality from 40% in the placebo group to 5.88% in the high-dose STSA-1002 group represents a strong signal of potential efficacy. Given that ARDS carries a high mortality rate and lacks specific pharmacotherapies, a treatment that could substantially reduce death, even without accelerating recovery time, would be a significant clinical advance. The favorable safety profile combined with the mortality signal provides a clear rationale for the investigators' conclusion that these findings warrant confirmation in a larger, adequately powered phase 3 trial to definitively assess the clinical benefit of STSA-1002.

References

1. Wang Y, Huang X, Cheng Z, et al. Anti-C5a antibody STSA-1002 for patients with acute respiratory distress syndrome due to viral pneumonia: a phase 1b/2, multicenter, randomized, double-blind, placebo-controlled trial.. Chest. 2026. doi:10.1016/j.chest.2026.04.052

2. Fang Y, Wang X, Wang X, et al. Preclinical evaluation of STSA-1002, a novel human and rhesus monkeys cross-reactive monoclonal antibody targeting C5a, in acute respiratory distress syndrome models. International Immunopharmacology. 2025. doi:10.1016/j.intimp.2025.115338

3. Gawande MS, Zade N, Kumar P, Gundewar S, Weerarathna IN, Verma P. The role of artificial intelligence in pandemic responses: from epidemiological modeling to vaccine development. Molecular Biomedicine. 2025. doi:10.1186/s43556-024-00238-3