Preliminary efficacy and safety of total neoadjuvant therapy combined with PD-L1 blockade in pMMR/MSS locally advanced rectal cancer (ESTIMATE): A prospective, single-arm, phase II trial.

e15602 Background: Patients with locally advanced rectal cancer (LARC) experience benefits from total neoadjuvant therapy (TNT). This study aims to explore the efficacy and safety of combining TNT with immunotherapy for pMMR/MSS LARC patients. Methods: ESTIMATE is a prospective, single-center, phase II study (ChiCTR2400079718) enrolling pMMR/MSS LARC patients with tumors located 10cm away from the anal margin. Treatment involved induction chemotherapy with immunotherapy ([mFOLFOX6, days 1-2 + Envafolimab 200mg, ih, day 1]; q2w; 2 cycles), long-course concurrent chemoradiotherapy (LCRT) with immunotherapy ([IMRT, 50Gy/25f, 2Gy + Cape 825mg/m2, bid]; 5w + Envafolimab 200mg, ih, day 1; q2w; 3 cycles), consolidation chemotherapy with immunotherapy ([mFOLFOX6 + Envafolimab; 2 cycles), and then total mesorectal excision (TME) 3 weeks later or watch and wait (W&W). The primary endpoint is the pathological complete response (pCR) rate. Results: Until Nov. 16th, 2023, 35 patients were enrolled, with 28 completing the TNT regimen. The efficacy analysis revealed that out of 28 patients, 12 (42.9%) achieved clinical complete response (cCR), with 7 individuals choosing the W&W strategy. 21 patients underwent TME, yielding 8 with a tumor regression grade (TRG) of 0, 7 with TRG of 1, and 6 with TRG of 2. The rates of pCR, major pathological response (MPR), and complete response (CR) were 38.1% (8/21), 71.4% (15/21), and 53.6% (15/28), respectively. 25 (89.3%) patients experienced treatment-related adverse events (AEs) of any grade. Non-hematological AEs included decreased appetite, fatigue, nausea, and vomiting, while hematological AEs mainly comprised thrombocytopenia, anemia, and leukopenia or neutrophilia. Grade III AEs were primarily leukopenia or neutrophilia (2/28, 7.1%). Additionally, circulating tumor DNA (ctDNA) levels were measured in 7 patients who chose the W&W strategy after achieving cCR. Of these, 6 tested negative for minimal residual disease (MRD), while 1 had a positive MRD result (ctDNA: 0.56 MTM/ml). As of Jan. 26th, 2024, with a median follow-up of 5 months (range: 3-6), no recurrence was observed in any of the patients. Conclusions: The TNT approach, integrating LCRT with mFOLFOX6 and PD-L1 blockade, demonstrated a favorable CR rate and acceptable tolerability, offering a potential avenue for organ preservation in low rectal cancer patients with pMMR/MSS. Long-term benefits need validation through further follow-up. Clinical trial information: ChiCTR2400079718. [Table: see text]