e14565 Background: The development of anticancer therapy tends to focusing on pan-cancer analysis, targeted therapy and immunotherapy with the advance of molecular and tumor microenvironment. Radiotherapy produces an immunomodulatory effect that may act synergistically with PD-1/PD-L1 inhibitor in multiple malignancies. Previous phase II trial showed HFRT/SBRT combined with PD-1 inhibitor and GM-CSF (PRaG regimen) has obtained promising results(Yuehong Kong et al. ASTRO 2021). RC48-ADC is a novel HER2-targeting antibody-drug conjugate (ADC). Preclinical studies indicated that ADC elicit immunogenic cell death and selectively radiosensitize to tumours. RC48-ADC combined with HFRT, PD-1/PD-L1 inhibitor sequential GM-CSF and IL-2 (PRaG3.0 regimen) could further improve the synergistic antitumor effects, this precise combination therapy paradigm provides an exploratory path for the treatment of pan-cancer. An exploratory phase II, open-label, multi-centre, single-arm study was conducted to evaluate the initial clinical efficacy and safety of PRaG3.0 regimen for treatment of HER2-expressing advanced solid tumors. Methods: Participants with advanced, confirmed HER2-expressing(IHC3+, 2+ or 1+) solid tumors that had progressed after the standard of care or intolerance were enrolled. In the cycle, those received RC48-ADC(2.0 mg/kg d1, every 3 weeks), then HFRT (2-3 doses of 5-8Gy) was delivered for one metastatic lesion every other day, followed by GM-CSF(200 μg d3-7), sequential IL-2(2million IU d8-12), and PD-1/PD-L1 inhibitor was dosing within one week after completion of HFRT. After RC48-ADC combined with PD-1/PD-L1 inhibitor sequential GM-CSF and IL-2 for at least 6 cycles, then maintenance with PD-1/PD-L1 inhibitor was administered until disease progression or unacceptable toxicity. The primary endpoint was objective response rate (ORR). This trial is registered with ClinicalTrials.gov, number NCT05115500. Results: With the cutoff date of 11 February 2022, a total of 8 patients were enrolled. Among 6 patients who had at least one tumor assessment, involved gastric cancer(3/6), breast cancer(1/6), cervical cancer(1/6), pancreatic cancer(1/6). The ORR was 66.7% (4/6) and disease control rate (DCR) was 83.3%(5/6). One cervical cancer and one pancreatic cancer achieved complete remission. The most common treatment-related adverse events (TRAEs) were alopecia (37.5%), fatigue (25%) and hepatic damage(12.5%). No Grade 3 and higher adverse events occurred. Conclusions: The PRaG3.0 regimen manifested manageable safety profile and encouraging efficacy. So PRaG3.0 regimen is considered as a promising salvage treatment for patients with HER2-expressing advanced solid tumors. And the conclusion should be validated in more patients subsequently. Clinical trial information: NCT05115500.

e14614 Background: Cancer immunotherapy has emerged as one of the most promising approaches in many kinds of advanced tumors. The PRaG therapy is an innovative cancer immunotherapy, when combined with radiotherapy, PD-1/L1 inhibitor and GM-CSF to generate the desired in situ vaccine effect to activate the immune response and modulate the tumor microenvironment. Previous studies have showed encouraging efficacy of the PRaG regimen in the treatment of advanced refractory tumors. RC48-ADC, an anti-HER2 antibody-drug conjugate with MMAE as cytotoxic payload has been recently demonstrated that ADCs are also able to induce immunogenic cell death and widespread release of cancer cell antigens, synergize with immunotherapy by promoting effector T-cell activation. The aim of this study is to explore efficacy and safety of PRaG3.0 regimen for therapy of HER2-expressing advanced solid tumors. Methods: Participants with advanced, confirmed HER2-expressing (IHC3+, 2+ or 1+) solid tumors that had progressed after standard of care, or intolerance were enrolled. In the cycle, those received RC48-ADC (2.0 mg/kg d1, every 3 weeks), then HFRT (2-3 doses of 5-8Gy) was delivered for one metastatic lesion every other day, followed by GM-CSF (200 μg d3-7), sequential IL-2 (2million IU d8-12), and PD-1/L1 inhibitor was dosing within one week after completion of HFRT. After RC48-ADC combined with PD-1/L1 inhibitor sequential cytokines for at least 6 cycles, then maintenance with PD-1/PD-L1 inhibitor was administered until disease progression or unacceptable toxicity. The primary endpoint was objective response rate (ORR). Results: As of Jan 2023, a total of 32 patients (n=6 for gynecological cancer, n=5 for pancreatic cancer, n=21 for other cancers) were enrolled, 26 of them completed at least 1 tumor assessment. The ORR was 38.5%, and the disease control rate was 69.2%. The ORR was 66.7% in gynecological cancer, 25.0% in pancreatic cancer, and 31.3% in other cancers. Notably, patients who were HER2 IHC1+ responded similarly to those who were IHC2+~3+, with ORR of 43.8% and 30.0%, respectively. Median progression-free survival was 7.2 months (95%CI: 4.9, 9.5). The most common treatment-related adverse events (TRAEs) included fatigue (28.1%), fever (28.1%), alopecia (28.1%) and anorexia (18.8%). Grade ≥3 TRAEs occurred in two patients (6.3%). In addition, activated plasmacytoid dendritic cell was significantly higher at baseline in responders (CR+PR) vs. nonresponders (SD+PD). Conclusions: The schedule of RC48-ADC was changed from once every 2 weeks to once every 3 weeks, and was still effective with significantly reduced side effects. These preliminary results suggest that PRaG3.0 regimen has a manageable safety profile and enhancing potential sensitivity in pretreated patients with HER2-expressing cancers. Clinical trial information: NCT05115500 .