Background: L is a novel anticancer agent that inhibits trans-activated transcription and modulates the tumor microenvironment. L is approved by the FDA for metastatic SCLC patients (pts) with progressive disease (PD) on or after platinum-based chemotherapy (CT). The LUPER study is assessing the safety, tolerability, and preliminary efficacy of L+P as second-line regimen for SCLC pts after failure of platinum-based CT. Phase 1 data are presented here. Methods: In this phase 1/2 trial (NCT04358237), adult pts with histologically confirmed SCLC, PD to a previous CT-containing regimen (≥4 weeks before study initiation), no prior exposure to immunotherapy, ECOG PS of 0-1, and measurable disease as per RECIST 1.1 are eligible. Pts with treated, stable, and asymptomatic brain metastases (BMs) are allowed. A 3+3 dose-escalation was done to determine the recommended phase 2 dose (RP2D) of L+P. L was dosed at 2.4 mg/m² and 3.2 mg/m² IV Q3W in the dose level (DL)1 and 2, respectively, in combination with fixed dose of P (200 mg IV Q3W). The RP2D was the highest DL at which 0/3 pts or ≤1/6 pts experienced dose-limiting toxicities (DLTs) during the first cycle. Treatment was administered until PD, unacceptable toxicity, or consent withdrawal. Secondary endpoints include safety as per CTCAE 5.0, preliminary efficacy, and pharmacokinetics. Results: Thirteen pts were enrolled across 3 hospitals in Spain (DL1, n = 7; DL2, n = 6). Median age was 66 (range 43–78) years, 46.2% were female, 61.5% had ECOG PS of 1, 38.5% had platinum-free interval < 90 days, 30.8% had LDH > upper normal limit, and 15.4% had BMs. One DLT (G3 asthenia) and one G4 neutropenia lasting > 3 days (controlled with G-CSF prophylaxis upon C2, without requiring dose delay or modification) occurred in the DL1. No DLT were reported in the DL2. The RP2D was identified as 3.2 mg/m² Land 200 mg P IV Q3W. At data cutoff (Jan 21, 2022), 5 (38.4%) pts remained on treatment (1 pt in DL1 discontinued due to COVID-19 in cycle 1). Median duration of treatment was 2.1 (0–11.8) months, 5 (38.5%) pts had ≥8 cycles, and median relative dose intensity of L and P were 91.1% and 95.7%, respectively. Immune-related AEs (G2 pneumonitis; G3 ALT increased) led to P discontinuation in 2 (15.4%) pts. Responses were shown in both DLs, with ORR of 30.8% (1 confirmed complete response and 3 partial responses); 3 pts had stable disease (SD; including 1 patient with SD > 12 weeks) and 5 (38.5%) pts experienced PD. Conclusions: This is the first report to demonstrate a manageable safety profile and preliminary efficacy of second-line L+P for relapsed SCLC pts. This combination warrants further confirmation in the ongoing expansion phase 2. Clinical trial information: NCT04358237.