First-Line Maintenance Therapy with Lurbinectedin Plus Atezolizumab in ES-SCLC: Clinical Impact of the IMforte Phase 3 Trial

Highlights

• The IMforte phase 3 trial is the first to show statistically significant improvements in both progression-free and overall survival with lurbinectedin plus atezolizumab as first-line maintenance in ES-SCLC.
• Lurbinectedin plus atezolizumab nearly halved the risk of disease progression and reduced the risk of death by 27% compared to atezolizumab alone.
• Hematologic adverse events were more frequent with the combination, with careful monitoring and supportive care necessary.

Study Background and Disease Burden

Extensive-stage small-cell lung cancer (ES-SCLC) remains a major clinical challenge, accounting for approximately 13-15% of all lung cancers and characterized by rapid progression, early dissemination, and poor prognosis. Despite recent advances incorporating immune checkpoint inhibitors (ICIs) such as atezolizumab or durvalumab with platinum-etoposide chemotherapy, median overall survival rarely exceeds 13-15 months, and most patients eventually experience disease relapse. Maintenance strategies—treatments administered after initial induction to prolong disease control—have been underexplored in ES-SCLC, creating an unmet need for approaches that extend survival without unacceptable toxicity.

Lurbinectedin, a selective inhibitor of oncogenic transcription, received accelerated approval in relapsed SCLC based on promising activity, while atezolizumab (a PD-L1 inhibitor) is standard in first-line ES-SCLC. The IMforte trial was designed to test whether combining these agents as maintenance therapy could improve outcomes for patients with ES-SCLC who do not progress after standard induction therapy.

Study Design

IMforte was a randomized, open-label, phase 3 trial conducted at 96 centers across 13 countries.

– **Population:** Adults (≥18 years) with treatment-naïve, extensive-stage SCLC who achieved at least stable disease after four cycles of induction therapy (atezolizumab, carboplatin, and etoposide).
– **Induction Phase:** 4 cycles of atezolizumab, carboplatin, and etoposide administered every 21 days.
– **Randomization:** After induction, patients without progression were randomized 1:1 to:
– Lurbinectedin (3.2 mg/m2 IV, every 3 weeks, with G-CSF prophylaxis) plus atezolizumab (1200 mg IV every 3 weeks), or
– Atezolizumab alone (1200 mg IV every 3 weeks).
– **Endpoints:**
– Primary: Progression-free survival (PFS), as assessed by an independent review facility (IRF), and overall survival (OS), both measured from randomization at start of maintenance.
– Secondary: Safety and tolerability, as well as characterization of adverse events by treatment group.
– **Analysis Sets:**
– Efficacy: Full analysis set (all randomized to maintenance, regardless of treatment received).
– Safety: All patients who received at least one dose of study drug.

Key Findings

**Patient Disposition and Baseline Characteristics**
– Of 895 patients screened, 660 (74%) entered induction, and 483 (73% of induction-completers) were randomized to the maintenance phase: 242 to lurbinectedin plus atezolizumab, 241 to atezolizumab alone. Baseline demographics and disease characteristics were balanced between arms.

**Efficacy**
– **Progression-Free Survival (PFS):** Median IRF-assessed PFS was significantly longer in the lurbinectedin plus atezolizumab group (HR 0.54, 95% CI 0.43–0.67; p<0.0001), indicating a 46% reduction in risk of disease progression or death compared to atezolizumab monotherapy.
– **Overall Survival (OS):** The combination also led to longer median OS (HR 0.73, 95% CI 0.57–0.95; p=0.017), corresponding to a 27% reduction in risk of death.
– Absolute survival curves demonstrated persistent benefit over time, supporting the clinical significance of these findings.
– The benefit was consistent across most pre-specified subgroups, including age, sex, and performance status.

**Safety**
– **Grade 3-4 Adverse Events:** Observed in 38% of patients receiving combination therapy versus 22% with atezolizumab alone.
– Most common (combo group): anemia (8%), neutropenia (7%), thrombocytopenia (7%).
– Most common (atezolizumab): hyponatremia (2%), dyspnea (2%), pneumonia (2%).
– **Grade 5 (Fatal) Adverse Events:** 5% in the combination arm vs 3% in atezolizumab alone.
– **Myelosuppression:** Neutropenia and leukopenia were notably higher with lurbinectedin, consistent with its known toxicity profile; proactive use of granulocyte colony-stimulating factor (G-CSF) was mandated.
– **Non-hematologic Toxicities:** No unexpected safety signals; immune-related events were similar between groups.

Expert Commentary

The IMforte trial marks a significant advance in the management of ES-SCLC, a setting where therapeutic innovation has lagged. By demonstrating meaningful gains in both PFS and OS, the combination of lurbinectedin and atezolizumab establishes a new reference for maintenance therapy. However, the increased risk of hematologic toxicity necessitates vigilant monitoring and supportive care.

While the absolute survival benefit is modest, it is clinically meaningful given the aggressive course of ES-SCLC and the lack of effective alternatives post-induction. Importantly, the trial’s international, multicenter nature and inclusion of a broad patient population enhance its generalizability to routine practice. Limitations include the open-label design (though endpoints were IRF-assessed) and the relatively short follow-up for late adverse events or long-term survival outliers.

Mechanistically, lurbinectedin’s cytotoxic effects may complement the immunomodulatory activity of atezolizumab, providing a rationale for this combination. Further research may clarify predictive biomarkers for optimal patient selection.

Conclusion

The IMforte phase 3 trial provides robust evidence that lurbinectedin plus atezolizumab as first-line maintenance therapy significantly extends both progression-free and overall survival for patients with ES-SCLC, compared with maintenance atezolizumab alone. This combination should be considered a new therapeutic option following induction chemoimmunotherapy, with careful attention to hematologic monitoring and supportive care. Ongoing research should focus on optimizing patient selection and management of toxicity to maximize the benefit-risk profile.

References

1. Paz-Ares L, Borghaei H, Liu SV, Peters S, Herbst RS, Stencel K, et al.; IMforte investigators. Efficacy and safety of first-line maintenance therapy with lurbinectedin plus atezolizumab in extensive-stage small-cell lung cancer (IMforte): a randomised, multicentre, open-label, phase 3 trial. Lancet. 2025 Jun 14;405(10495):2129-2143. doi: 10.1016/S0140-6736(25)01011-6.
2. Horn L, Mansfield AS, Szczęsna A, et al. First-Line Atezolizumab plus Chemotherapy in Extensive-Stage Small-Cell Lung Cancer. N Engl J Med. 2018;379(23):2220-2229.
3. National Comprehensive Cancer Network (NCCN) Guidelines: Small Cell Lung Cancer. Version 3.2024.