Efficacy and safety of total neoadjuvant therapy combined with PD-L1 blockade in PMMR/MSS locally advanced rectal cancer (ESTIMATE): an open-label, prospective, phase II trial

Purpose: Although total neoadjuvant therapy (TNT) improves outcomes in locally advanced rectal cancer (LARC), patients with proficient mismatch repair (pMMR)/microsatellite stability (MSS) tumors respond poorly to conventional regimens. Patients and Methods: This phase II trial (ESTIMATE) evaluated a novel TNT protocol integrating PD-L1 blockade (envafolimab) in 33 pMMR/MSS LARC patients with tumors located ≤10 cm from the anal verge. The regimen comprised: induction with mFOLFOX6 plus subcutaneous envafolimab (200 mg q2w × 2 cycles), concurrent chemoradiation (50 Gy/25 fractions with capecitabine 825 mg/m 2 plus envafolimab q2w × 3 cycles), and consolidation with mFOLFOX6/Envafolimab ×2 cycles. Patients achieving a clinical complete response (cCR) with undetectable circulating tumor DNA (ctDNA) can opt for watch-and-wait (W&W). Results: The complete response (CR) rate, which integrates pathological complete response (pCR) and cCR, reached 51.5% (17/33), with 21.2% (7/33) achieving organ preservation. Pathological assessment revealed 38.5% pCR (10/26) and 73.1% major response (19/26) after total mesorectal excision (TME). Transcriptomic profiling identified UBD overexpression and RPL21 downregulation as predictive biomarkers. Adverse events occurred in 87.9% (29/33) of the patients, predominantly grade 1–2. Grade 3 neutropenia occurred in 9.1% (3/33) of the patients, and injection-site reaction occurred in 1 patient (3%). With a median follow-up of 27 months (IQR 25-29), the 2-year DFS and OS rates were 90.9% (95% CI: 77.0%–97.6%) and 100% (95% CI: 90.1%–100%), respectively. Distant metastases developed in one of seven patients (14.3%) managed with W&W and two of 26 patients (7.7%) undergoing TME. Conclusions: PD-L1 blockade-enhanced TNT achieves encouraging efficacy, organ preservation, and acceptable tolerability in pMMR/MSS LARC, supported by mechanistically relevant biomarkers.