Why This Study Matters
Small cell lung cancer (SCLC) remains one of the most aggressive and lethal malignancies, with limited treatment options beyond frontline chemo-immunotherapy. Per the authors, the vast majority of patients relapse within months, with median overall survival rarely exceeding 12 months. Antibody-drug conjugates (ADCs) in SCLC have mostly centered on lineage-specific targets such as DLL3 or CD276. Izalontamab brengitecan (iza-bren; BL-B01D1) instead co-targets EGFR and HER3 — targets that are not classical in SCLC — using a bispecific IgG1 backbone conjugated to a topoisomerase I inhibitor payload with a drug-to-antibody ratio of 8.
Study Design
BL-B01D1-101 is an open-label, multicenter, dose-escalation and dose-expansion phase I study. The SCLC cohort was expanded in the phase Ib stage. All patients received iza-bren 2.5 mg/kg on days 1 and 8 of each 3-week cycle (single arm; no randomization). Primary end points were ORR (RECIST v1.1) and safety/tolerability; secondary end points were disease control rate, duration of response (DOR), PFS and OS. Biomarker associations with EGFR/HER3 expression were exploratory. No formal hypothesis testing was planned. The number of sites is not specified in the source. Data cutoff: December 5, 2024. Sponsor: Sichuan Baili Pharmaceutical (supplied the study drug).
Patient Population
Eligible patients had SCLC with progression after at least one platinum-based chemotherapy and a PD-(L)1 inhibitor, ECOG performance status 0 or 1, and at least one RECIST-measurable lesion. Patients with active brain metastases were excluded. Of 52 patients, 80.8% were male, median age was 60.5 years, 98.1% had ECOG 1, and 32.6% had brain metastasis at screening. Prior lines of therapy: one line in 22 (42.3%), two lines in 11 (21.2%), three or more in 19 (36.5%). Previous anti–PD-1/L1 immunotherapy was reported in 84.6% and prior irinotecan in 36.5%.
Primary Endpoint Results
Twenty-five patients achieved a partial response and 17 had stable disease; six patients had no postbaseline assessment and were counted as nonresponders. Among responders, median DOR was 4.9 months (95% CI, 4.2 to 6.7). At data cutoff, 45 PFS events (86.5%) and 31 OS events (59.6%) had occurred.
| Outcome | Total (N = 52) | 1 prior line / 2L (n = 22) | 2+ prior lines (n = 30) |
|---|---|---|---|
| ORR, % (95% CI) | 48.1 (34.0 to 62.4) | 72.7 (49.8 to 89.3) | 30.0 (14.7 to 49.4) |
| DCR, % (95% CI) | 80.8 (67.5 to 90.4) | 90.9 (70.8 to 98.9) | 73.3 (54.1 to 87.7) |
| Median DOR, mo (95% CI) | 4.9 (4.2 to 6.7) | 4.9 (3.1 to 6.7) | 4.4 (3.0 to 7.0) |
| Median PFS, mo (95% CI) | 4.1 (3.0 to 5.5) | 6.2 (3.7 to 8.2) | 3.0 (2.6 to 4.4) |
| Median OS, mo (95% CI) | 12.2 (9.1 to 13.2) | 15.0 (8.7 to NR) | 10.3 (7.9 to 12.4) |
Subgroup Analyses
These are descriptive subgroups from a small single-arm cohort; the authors caution that sample size limits definitive conclusions about subgroups.
| Subgroup | n | ORR (95% CI) | DCR (95% CI) |
|---|---|---|---|
| Second-line (one prior line) | 22 | 72.7% (49.8 to 89.3) | 90.9% (70.8 to 98.9) |
| Platinum-sensitive (treatment-free interval >6 months) | 13 | 69.2% (38.6 to 90.9) | 84.6% (54.6 to 98.1) |
| Previously treated with irinotecan | 19 | 31.6% (12.6 to 56.6) | 68.4% (43.4 to 87.4) |
| Two or more prior lines | 30 | 30.0% (14.7 to 49.4) | 73.3% (54.1 to 87.7) |
Exploratory biomarkers. Tissue was available from 26 patients; 23 were response-evaluable. ORR was 68.8% (11/16) in HER3-positive versus 14.3% (1/7) in HER3-negative patients, while EGFR-positive (50.0%, 5/10) and EGFR-negative (53.8%, 7/13) patients had comparable ORR. The authors describe this as a small, hypothesis-generating subset; no confidence intervals or p-values were reported for these comparisons.
Safety Profile
All 52 patients had a treatment-related adverse event. Grade 3 or higher TRAEs occurred in 75% of patients, leading to dose reductions in 46% and treatment discontinuation in 13%. Treatment-related serious adverse events occurred in 26 patients (50.0%). No treatment-related interstitial lung disease was observed. The text reports two grade 5 TRAEs (3.8%; one respiratory failure and one gastrointestinal infection), whereas Table 3 lists Grade 5 events in 4 patients (7.7%) in the “any TRAE” row; two sudden deaths of unknown cause were judged not related to study drug by investigators.
| TRAE (n = 52) | Any grade | Grade 3 | Grade 4 |
|---|---|---|---|
| Anemia | 44 (84.6%) | 17 (32.7%) | 1 (1.9%) |
| Thrombocytopenia | 39 (75.0%) | 11 (21.2%) | 10 (19.2%) |
| Leukopenia | 38 (73.1%) | 12 (23.1%) | 3 (5.8%) |
| Neutropenia | 37 (71.2%) | 15 (28.8%) | 8 (15.4%) |
| Stomatitis | 19 (36.5%) | 5 (9.6%) | 0 |
| Asthenia | 22 (42.3%) | 1 (1.9%) | 0 |
Interpretation and Broader Context
The authors conclude:
In summary, iza-bren demonstrated encouraging antitumor activity and a manageable safety profile in relapsed ES-SCLC.
— Zhao et al., Journal of Clinical Oncology, 2026
A phase III randomized trial of iza-bren versus topotecan (NCT06500026) is ongoing and is expected to provide more definitive evidence.
Limitations
This is a single-arm phase Ib cohort of 52 patients with no comparator and no formal hypothesis testing, so comparisons with other second-line treatments are indirect; the authors state only that second-line results were “numerically superior to the current second-line treatments.” Subgroup results come from small samples, and the authors caution that sample size limits definitive conclusions about them. Six patients had no postbaseline assessment and were counted as nonresponders. Grade 3 or higher toxicity was frequent, driven by hematologic events. The biomarker and in vitro findings are hypothesis-generating, based on 23 response-evaluable patients, and the authors note that the mechanistic role of EGFR remains uncertain. The source also reports differing grade 5 counts between its text and Table 3, and the number of sites is not specified.