5-year PFS Research Articles

EPEN-14. MOLECULAR MARKERS AND PREDICTORS OF OUTCOME FOR PAEDIATRIC INTRACRANIAL EPENDYMOMA IN THE PROSPECTIVE, MULTICENTRE E-HIT2000 TRIAL AND SUBSEQUENT HIT-REGISTRIES: A POOLED ANALYSIS OF 244 PATIENTS

Abstract BACKGROUND Throughout the past decade, molecular tools have revolutionised ependymoma classification, yet their application has been limited in clinical trial cohorts. METHODS We present the molecular analysis of paediatric ependymoma patients from the E-HIT2000 trial (n = 165), or subsequent HIT2000-interim and I-HIT-MED registries (n = 79) based on DNA-methylation profiling with multi-level classification using the Heidelberg Brain Tumor Classifier v12.5 and the analysis of chromosome 1q, 6q, and CDKN2A copy number status. Prior to further analysis, cases without ependymoma group prediction were excluded (n = 16/244). RESULTS 5-year PFS and OS rates varied by group: EPN-PFA (n=146/228) 45±4% and 76±4%; EPN-PFB (n=19/228) 90±7% and 100%; EPN-ZFTA (n=59/228) 63±7% and 87±5%; and EPN-YAP1 (n=4/228) 50±25% and 100%. EPN-PFA was subclassified into nine subtypes. Poor median PFS was observed for PFA1c (n=22, 5-yr-PFS 32±10%/5-yr-OS 77±9%), PFA1d (n=13, 35±14%/72±14%), PFA1e (n=15, 47±13%/59±13%), and PFA2a (n=16, 27±12%/50±14%). Presence of 1q gain was associated with inferior survival (n = 39/146, 29±8%/65±8%). 6q loss impacted PFS, but not OS (n=8, 25±14%/100%). Among 59 EPN-ZFTA cases, 46 clustered with EPN ZFTA-RELA fused, and 12 clustered within recently described subgroups of EPN-ZFTA (EPN-ZFTA alt., Cluster 1: 1/12, Cluster 2: 11/12). Strikingly, 6/11 EPN ZFTA-alt. (Cluster 2) cases showed infratentorial (4/6) or supratentorial midline location (2/6). Homozygous deletion of CDKN2A (CDKN2A-/-; n=8) was only observed in EPN ZFTA-RELA. Presence of EPN ZFTA-alt. (cluster 2) (5-yr-PFS: 36±15%; p<0.001), and CDKN2A-/- (19±16%; p<0.0001) was associated with inferior PFS compared to EPN ZFTA-RELA CDKN2A-wt (81±6%). OS was only impacted for patients with EPN ZFTA-RELA CDKN2A-/- (5-yr-OS: 38±20%, p>0.0001). CONCLUSIONS We present strong evidence supporting the inclusion of molecular criteria into the next generation of clinical trials for children with ependymoma.

Combined aqupla, paclitaxel liposome, and docetaxel treatment: survival and biomarker outcomes in recurrent ovarian cancer patients.

As one lethal malignancy in women's reproductive systems, ovarian cancer (OC) is frequently detected at an advanced phase during diagnosis. when the disease has spread widely. The absence of obvious symptoms and powerful screening tools in the early stages makes treatment difficult and the prognosis poor. Despite the clinical remission that can be achieved in some patients after initial treatment, the recurrence rate is conspicuous, posing a considerable challenge in treating recurrent OC (ROC). In the retrospective analysis, we compared the effects of two treatment regimens, aqupla combined with paclitaxel liposome (NP group) versus aqupla combined with docetaxel (ND group), on survival and biomarkers in patients with ROC. The study included 121 OC patients, and clinical data were collected through an electronic medical record system, outpatient review records, and a follow-up record system. The results revealed a notably higher overall remission rate in the ND group than the NP group, but revealed no notable inter-group discrepancy in toxicities, implying that the aqupla combined with docetaxel regimen may be more effective in platinum-sensitive ROC patients. Additionally, post-treatment CA125 levels were lower in patients in the ND group, suggesting that the regimen may be more effective in reducing tumour load. Survival analysis further revealed that treatment regimen, FIGO stage, number of recurrent lesions, and pretreatment CA125 level were independent prognostic factors affecting patients' 5-year OS and PFS. Overall for ROC patients, especially platinum-sensitive patients, the aqupla in combination with docetaxel regimen provided an improved survival benefit with a comparable safety profile, highlighting the importance of individualised treatment strategies.

Radiation therapy (RT)-free pembrolizumab plus chemotherapy (P+C) for PD-L1 TPS ≥50% locally advanced non-small cell lung cancer (LA-NSCLC): Primary analysis from multicenter single arm phase II study (Evolution trial; WJOG11819L).

LBA8050 Background: Standard of care for unresectable LA-NSCLC is chemoradiation therapy (CRT) followed by durvalumab (D). Survival curves of P monotherapy/P+C for PD-L1 TPS ≥50% stage IV NSCLC suggested possible comparable survival to CRT for stage III patients (pts). Moreover, some studies of neoadjuvant C+immunotherapy (I) for stage III pts have demonstrated high pathological complete response and major pathological response rates, implying potentially outstanding efficacy of C+I for earlier stage. We thus hypothesized P+C without RT in PD-L1 ≥50% LA-NSCLC pts provides a comparable efficacy to CRT followed by D while avoiding CRT-induced severe toxicities. Methods: This is a phase II study conducted by West Japan Oncology Group. P with platinum plus pemetrexed (PEM) (non-squamous) or P with carboplatin plus nab-paclitaxel (squamous) was administered every 3 weeks without RT. After four cycles of induction P+C, P with PEM (non-squamous) or P alone (squamous) was continued until progression or 2 years. The primary endpoint was 2 year-PFS rate (threshold/expected: 20%/45%). Results: Between May 2020 and February 2022, 21 pts were enrolled. Median age was 73 (range, 53-89). Stage IIIA/B/C included 11 (52%)/7 (33%)/3 (14%), respectively. Histologic subtypes were 14 (67%) adenocarcinoma, 5 (24%) squamous cell carcinoma, and 2 (10%) others. Median follow-up period was 29.9 (range, 0.3-44.2) months. Median number of P administrations was 30 (range, 1-35). The primary endpoint was met with 2-year PFS rate of 67% (90% CI: 46-83%). At the time of data cut-off, 13 (62%) of 21 pts were still progression-free. Median PFS and OS were not reached. Two-year OS rate was 85%. Centrally reviewed tumor response: 8 (38%) CR; 9 (43%) PR; 3 (14%) SD; and 1 (5%) NE were confirmed, resulting in overall response rate (ORR) of 81% and disease control rate of 95%. ORRs/2-year PFS rates of PD-L1 TPS 50-79% and 80-100% were 67%/56% and 92%/75%, respectively. Non-hematological AEs ≥grade 3 were observed in 11 (52%) pts, including: 2 (10%) pneumonitis; 2 (10%) pneumonia; 1 (5%) diarrhea; 1 (5%) ALT elevation; and 1 (5%) acute heart failure. There was one (5%) grade 5 AE (pneumonia). Conclusions: RT-free P+C provided long-lasting responses in approximately two-thirds of pts. Higher PD-L1 TPS cases achieved higher RR, including some CRs and higher 2-year PFS rate. To confirm our hypothesis that RT-free P+C can be a less toxic curative option in selected LA-NSCLC pts with PD-L1 TPS ≥50%, further data is warranted. Clinical trial information: NCT04153734 .

Five-year survival outcomes from the PIT-1 trial: Pemetrexed-cisplatin plus bevacizumab or concurrent thoracic radiation therapy followed by surgery in stage IIIA (N2) nonsquamous non-small cell lung cancer.

8064 Background: Personized Induction Therapy-1 (PIT-1) is a multicenter, randomized phase II selection design trial of pemetrexed-cisplatin plus bevacizumab (BEV arm) or concurrent thoracic radiation therapy (TRT arm) followed by surgery in patients with stage IIIA (N2) nonsquamous non-small cell lung cancer (NSCLC). The TRT arm was chosen as the investigational induction treatment strategy for a future phase III study based on the results of a numerically higher 2-year progression-free survival (PFS, primary endpoint) in the TRT arm and fatal postoperative complications observed only in the BEV arm. We report updated, exploratory analyses of survival, approximately 5 years after the last patient was randomly assigned. Methods: Patients with stage IIIA (N2) nonsquamous NSCLC were randomly assigned (1:1) induction therapy consisting of pemetrexed and cisplatin plus bevacizumab (n = 44) or concurrent TRT (n = 44) followed by surgery. Among them, 38 patients in the BEV arm and 37 patients in the TRT arm underwent surgery. Five-year overall survival (OS), 5-year PFS, and patterns of postoperative recurrence were compared between arms. Prognostic factors of OS were analyzed in surgically resected patients using Cox’s proportional hazard model. Results: The median follow-up was 66.4 months. In 82 treated patients, the 5-year OS and PFS rates were 63.5% (95% CI: 46.9-76.1) and 26.2% (95% CI: 14.1-40.0) in the BEV arm (n = 42), and 57.2% (95% CI: 40.5-70.8) and 27.5% (95% CI: 14.9-41.7) in the TRT arm (n = 40), respectively. There were no statistical differences in OS ( P = 0.572) and PFS ( P = 0.356). In 75 surgically resected patients, pathological nodal down stage was the only significant prognostic factor of OS ( P = 0.014). Age, sex, smoking status, clinical T stage, clinical N stage based on a computed tomography scan, preoperative serum CEA and CYFRA level, EGFR mutation status, pathological complete response, and major pathological response did not have a significant prognostic impact on OS. The patterns of postoperative recurrence were different between arms: locoregional only, in four (11%) and one (3%); distant metastasis only, in 16 (42%) and 21 (57%); and both, in five (13%) and four (11%) in the BEV and TRT arms, respectively. The recurrence rate of ipsilateral hilar or mediastinal lymph nodes (the irradiation field) was significantly lower in the TRT arm (3%) than in the BEV arm (21%, P= 0.01). Conclusions: Five-year survival outcomes were not different between the BEV and TRT arms. The TRT arm demonstrated sufficient local control, but insufficient control of distant metastasis so this is an important issue that requires improvement in the future. Clinical trial information: s031180402.

Real-world clinical outcomes and prognostic factors of olaparib maintenance in Korean patients with primary or recurrent high-grade ovarian cancer with exploratory analysis of location of BRCA1 and BRCA2 mutations.

e17561 Background: Olaparib is approved for first-line (1L) and second or subsequent-line (2L+) maintenance therapy in BRCA-mutated ovarian cancer (OC). However, the correlation between location of BRCA mutation and benefit of olaparib maintenance therapy has not been thoroughly investigated. We aimed to analyze real-world clinical outcomes and identify prognostic factors for olaparib maintenance therapy in Korean patients with BRCA-mutated OC, focusing on BRCA mutation sites. Methods: This retrospective cohort study included Korean patients with BRCA-mutated OC who received olaparib as 1L or 2L+ at the single center between January 2015 and April 2023. Survival outcomes and toxicity were correlated with BRCA mutation location summaries and patient characteristics by maintenance therapy line. Results: For the 1L group (n = 70), 2-year PFS and 3-year OS were 75.5% (95% CI, 56.4–87.1) and 98.5% (95% CI, 90.0–99.8). For the 2L+ group (n = 83), 2-year PFS and 3-year OS were 54.8% (95% CI, 56.4–87.1) and 87.3% (95% CI, 76.7–93.3). Primary stage (IV vs. ≤ III) was a prognostic factor for PFS in the 1L group (adjusted hazard ratio [aHR], 5.74; 95% CI, 1.23–26.73; P = 0.026). Notably, Korean-specific pathogenic variant p.Leu1780Pro BRCA1 mutation carriers (n= 8) showed significantly worse PFS than non-carriers (n = 32) in the 1L group (Log-rank P = 0.014). For the 2L+ group, platinum-free interval ≥ 12 months was a good prognostic factor for both PFS (aHR, 0.42; 95% CI, 0.23–0.75) and OS (aHR, 0.31; 95% CI, 0.11–0.91). Additionally, somatic BRCA1 mutation was a poor prognostic factor for PFS in the 2L+ group (hazard ratio [HR], 3.39; 95% CI, 1.22–9.38; P = 0.029). In the 1L group, the longer interval between the last chemotherapy and maintenance therapy was associated with the lower risk of hematologic toxicity induced by olaparib (odd ratio [OR], 0.75; 95% CI 0.57–0.99; P = 0.04). In addition, age ≥ 65 years (OR, 3.33; 95% CI, 1.05–10.63; P = 0.04) and the dose reduction of the last chemotherapy (OR, 3.80; 95% CI, 1.02–14.18; P = 0.047) were associated with a higher risk of hematologic toxicity in the 1L group. In the 2L+ group, germline BRCA1 (OR, 0.36; 95% CI, 0.14–0.92; P = 0.03) and BRCA2 (OR, 7.47; 95% CI, 1.96–28.39; P = 0.003) mutations showed contrariwise risks for hematologic toxicity. Drug discontinuation due to toxicity was 1.4% and 6.0% in the 1L and the 2L+ groups and no acute myeloid leukemia/myelodysplastic syndrome cases were observed. Conclusions: Olaparib maintenance therapy was effective and tolerable in Korean patients with BRCA-mutated OC. Prognostic factors for survival outcomes and risk factors of hematologic toxicity in olaparib maintenance therapy were different between 1L and 2L+. The prognostic significance of Korean-specific p.Leu1780Pro BRCA1 mutation on olaparib should be further researched.

Outcome of chemo-immunotherapy for extensive-stage small-cell lung cancer according to potential clinical trial eligibility: 3-year outcomes from prospective cohort study.

8087 Background: Chemo-immunotherapy is the standard 1st-line therapy for patients with extensive-stage small-cell lung cancer (ES-SCLC). Our large previous real-world prospective analysis showed outcomes of chemo-immunotherapy for these patients according to potential clinical trial eligibility with a minimum follow-up period of 1 year. However, long-term outcomes have not been studied in the real-world setting. Methods: We conducted a 32-hospital prospective cohort study of consecutive patients with ES-SCLC who received carboplatin and etoposide with atezolizumab as 1st-line therapy between September 2019 and September 2020. Patients who met eligibility criteria for pivotal phase 3 clinical trials were considered “trial-eligible.” We present 3-year outcomes from this study. Results: In total, 207 patients with ES-SCLC were analyzed. The median (range) time from the start of treatment to data cutoff (September 30, 2023) was 42.2 (35.8.-48.2) months. The median age was 72 years, and 64 patients (31%) were elderly (≥75 years). Most patients (89%) had a performance status (PS) of 0 or 1. As a result, 132 (64%) were categorized as trial-eligible patients. The 3-year PFS and OS probability of all patients was 6.1 % (95%CI:3.5-10.4%) and 20.9 % (95%CI:15.6-27.3%), respectively. Patients achieving 3-year OS included significantly higher proportions of trial-eligible patients (30 out of 132 versus 5 out of 75, respectively; p = 0.002). Kaplan-Meier estimates of the 3-year OS rate were 26.7% for the trial-eligible group and 9.5 % for the trial-ineligible group. Conclusions: This is the first real-world study to show the long-term efficacy of chemo-immunotherapy for ES-SCLC by using the largest prospective cohort of its kind. Additionally, this study demonstrated that trial eligibility was associated with long-term efficacy. Our study suggests that long-term clinical outcomes among trial-eligible patients may not translate to ineligible patients.

The Reduction of CD4+ T Lymphocytes after the Treatment of Follicular Lymphoma with the Bendamustine Containing Regimen May Predict the Occurrence of Infection and Efficacy

To investigate the effectiveness, safety, and related prognostic factors of the treatment of follicular lymphoma (FL) with a regimen containing Bendamustine. The clinical data of 129 FL patients who were treated with Bendamustine containing regimen were collected from January 1,2020 to October 30,2022 in the Hematology Department of Lianyungang Second People's Hospital and Jiangsu Provincial People's Hospital. The patients were divided into three groups: Bendamustine plus Rituximab (BR), Bendamustine plus Obinutuzumab (GB), Rituximab + Cyclophosphamide + Epirubicin / Doxorubicin + Vindesine + Prednisone (R-CHOP). The efficacy, safety and related prognostic factors of the treatment of FL with a regimen based on Bendamustine were retrospectively analyzed. The ORR was 98% for the BR group, 94% for the GB group, and 72.3% for the R-CHOP group, while the CR rate was 61.2%,70% and 40.4%, respectively. The ORR and CR rates of the R-CHOP group were statistically different from those of the BR group and GB group (P < 0.05). The 3-year PFS rate of the BR group, GB group, and R-CHOP group was 89.6%, 90.9%, 48.9%, respectively. There was a statistically significant difference in 3-year PFS between the R-CHOP group, BR group, and GB group (P < 0.05), while there was no statistically significant difference in 3-year OS（P ＞0.05）. Hematological adverse reactions were mainly bone marrow suppression. Lymphocytes and CD4+T lymphocytes decreased to the lowest level about 6 months after treatment, and the incidence of lymphopenia in BR group and GB group was higher than that in R-CHOP group, with a statistical difference (P < 0.05). The higher incidence of non-Hematological adverse reactions were pulmonary infection, EB virus infection, hepatitis B virus reactivation, and gastrointestinal reactions without statistical difference in 3 groups (P >0.05), and were all controllable. The Receiver operating characteristic of CD4+T lymphocyte count showed that AUC of BR group was 0.802, and the critical value was 258/uL; AUC of GB group was 0.754 with a critical value of 322/uL. The treatment of FL with the Bendamustine containing regimen has good efficacy and controllable adverse reactions, but lymphocytopenia was significant after treatment, and the curative efficacy in combination with various CD20 monoclonal antibodies was different. The lowest CD4+T lymphocyte count can be used as a predictive factor for the occurrence of infection and efficacy of the Bendamustine containing regimen for FL.

A phase II trial of reirradiation combined with pembrolizumab in patients with locoregional inoperable recurrence or second primary squamous cell carcinoma of the head and neck (HNSCC).

6049 Background: Better outcomes are needed for patients (pts) with locoregional inoperable recurrence or second primary HNSCC after prior radiation therapy. Methods: We conducted a phase II single arm trial of hyperfractionated reirradiation (ReRT, 1.2 Gy BID for total 60Gy to gross disease + margin without elective nodal radiation) plus pembrolizumab (P) (200mg q3 weeks starting day 1 of ReRT). P was continued until confirmed CR or if no CR, until disease progression or up to 24 months. All pts underwent a 3 month post ReRT PET/CT and subsequently CT neck/chest after every 3rd cycle of P. Pts included had: Locoregional recurrence or second primary HNSCC (excluding skin or salivary), were unresectable or not willing to undergo resection, one RT course completed at least 6 months prior, with &gt;50% tumor volume treated at doses &gt;45Gy, ECOG PS 0-1, a target lesion by RECIST, no distant metastasis, and no prior anti-PD-1/PD-L1. Primary endpoint was PFS, and the hypothesis was that the median PFS would increase from the historical control of 8 months (mo) to 12 mo, α=0.05, power 78%, n=48. Secondary endpoints: ORR by RECIST, OS, toxicity, QOL (EORTC). The Kaplan-Meier method was used to estimate PFS and OS. Results: In the 48 evaluable pts, median age was 65.5 (range 44-79), 62.5% male. Primary site: hypopharynx (8.3%), larynx (6.3%), nasopharynx (10.4%), oral cavity (43.7%), oropharynx (31.3%, one HPV+ pt). Median doses of P was 8, median RT dose was 60 Gy and 20.8% received proton RT, remainder IMRT. Median follow up was 49.1 months. The median PFS was 8.3 mo (95%CI 5.5-10.3), and median OS was 13.8 mo (95% CI 9.5-21.8). 1 and 3-year PFS were 28.9% and 22.3% respectively, and for OS 54.2% and 25.7%, respectively. 42 pts were evaluable for response and the ORR was 54.8% (33.3% CR, 21.4%PR), 21.4% SD, 23.8% PD. In an exploratory analysis, the median PFS was 13.8 mo (95% CI 9.1-NR) and the median OS was 57.8 mo (19.6-NR) in patients that had a response (CR/PR, n=23), with a 1 and 3-year PFS of 51.8% and 42.4%, respectively, and for OS 78.3% and 51.7%, respectively. All patients had at least one treatment related AE (TRAE), defined as possibly, probable, or definitely related to treatment. 39% of pts had a G3 TRAE and the most common ones were: aspiration, dehydration, anemia, dysphagia, dyspnea. 4 patients had a G5 TRAE (found down deceased, epistaxis, aspiration pneumonia, iRAE pneumonitis). Conclusions: Our prospective trial uniquely evaluated ReRT plus pembrolizumab in patients with locoregional recurrence/second primary HNSCC that did not undergo salvage surgery. The primary endpoint of PFS was not improved compared to historical control. PD-L1 biomarker analysis is ongoing and will be reported subsequently. Clinical trial information: NCT02289209 .

P-GEMOX with sequential or sandwiched radiotherapy for early-stage extranodal natural killer/T-cell lymphoma.

7075 Background: Combined modality therapy (CMT), namely chemotherapy combined with radiotherapy, has been recommended for patients with localized extranodal natural killer/T-cell lymphoma (ENKTL). However, the optimal CMT has not been fully clarified. As chemotherapy is more easily accessible than immediate radiotherapy in the routine clinical practice, the “chemotherapy-first” CMT including sequential or sandwiched radiotherapy deserves further exploration. In addition, the optimal non-anthracycline-based regimen needs to be confirmed. Methods: The P-GEMOX regimen was administrated as follows: pegaspargase 2000 IU/m2 intramuscular on day 1, gemcitabine 1000 mg/m2 intravenous on day 1 and 8, oxaliplatin 130 mg/m2 intravenous on day 1 and was repeated every 21 days. Patients who achieved at least stable disease (SD) subsequently underwent involved field radiotherapy (IFRT). The dose of IFRT ranged from 50 to 55 Gy. Performing sequential radiotherapy (4 cycles of CT+RT) or sandwich radiotherapy (2 cycles of CT+RT+2 cycles of CT) is determined by the clinician based on the patients' conditions. The primary endpoint was the overall response rate (ORR) after 2 cycles of chemotherapy, and secondary study endpoints were CR, PFS, and OS. Safety was also evaluated. Results: From August 2020 to April 2023, thirty-seven patients (23 men, 14 women; median age: 55, 26-76 years) with stage I/II ENKTL who had received P-GEMOX were collected. 14 patients were stage IE and 23 were IIE. ALL patients completed at least 2 cycles of P-GEMOX, which resulted in 94.6% (35/37) of ORR including 30 patients with CR and 5 patients with PR. Among the patients in remission, 15 patients received sequential radiotherapy, 15 received sandwich radiotherapy, 4 were waiting for or in the process of radiotherapy, and 1 patient progressed before radiotherapy. With a median follow-up of 14.8 months (range: 2.6-38.1 months), the 3-year PFS and OS were 87.2% (95%CI 69.3%-95%) and 90.6% (95%CI 72%-97.1%), respectively. Sandwich radiotherapy had an obvious trend for longer PFS than sequential radiotherapy (18moths-PFS 100% vs. 78%; P=0.073). During the treatment, 21.6% (8/37) of the patients experienced grade 3–4 treatment-emergent adverse events (TEAEs). The most grade 3–4 hematological TEAEs were neutropenia (7/37, 18.9%) and thrombocytopenia (4/37, 10.8%). As for grade 3–4 non-hematological TEAEs, elevated aminotransferase, hyperbilirubinemia and hypofibrinogenemia were 16.2%, 10.8% and 8.1%, respectively. The adverse events of this CMT were well-tolerated and manageable. No treatment-related death occurred. Conclusions: Our study demonstrated that CMT consisted of P-GEMOX showed favorable efficacy with acceptable toxicity, and sandwich radiotherapy (CT+RT+CT) seems give more favorable PFS than sequential radiotherapy. The CMT protocol could be an effective treatment option for early-stage ENKTL patients.

Prospective validation of ctHPVDNA for detection of minimal residual disease and prediction of recurrence in patients with HPV-associated head and neck cancer treated with surgery.

6010 Background: Prediction of minimal residual disease (MRD) following surgery, and thus the need for adjuvant therapy, is currently based on clinicopathologic risk factors which have poor individual prognostic capacity. We previously reported that MRD detection by ctHPVDNA droplet digital (dd)PCR as early as post-operative day (POD) 1 is predictive of recurrence in stage I-II HPV+HNSCC patients. Here, we applied a significantly more sensitive custom whole genome hybrid-capture-based next generation sequencing assay, termed HPV-DeepSeek, to validate the prognostic capacity of ctHPVDNA detection and explore the optimal timing of MRD testing in HPV+HNSCC patients. We tested the primary hypothesis that patients with MRD detection within 6 weeks of surgery would have inferior PFS at 2 years and the secondary hypothesis that patients with ctHPVDNA positivity detected at any point following treatment completion would have inferior PFS at 2 years. Methods: 98 patients with HPV+HNSCC were prospectively enrolled with a mean follow-up of 712 days. All patients underwent surgery as primary treatment and clinicopathologic adjusted adjuvant treatment. 10ml blood samples were collected before surgery, in the post-operative period (POD 1-42), and serially in follow-up. MRD was defined as a lack of ctHPVDNA clearance during the 6 weeks following surgery. Cell free DNA was extracted from plasma and run on HPV-DeepSeek, and on existing ddPCR assays for head to head comparisons. Results: 96/98 (98%) of patients had detectable ctHPVDNA pre-treatment. 30/98 patients (31%) were MRD positive. Patients who were MRD positive had significantly worse 2 years PFS compared to MRD negative patients (78% vs 98%, P=0.0009). Predictive performance improved by limiting time points to POD 7-42 (2 year PFS 60% vs 97%, P&lt;0.0001) as significantly fewer patients were MRD positive &gt;1 week after surgery, suggesting the use of an ultra-sensitive assay such as HPV-DeepSeek requires adjustment of MRD time points. Patients with detectable ctHPVDNA following completion of all treatment had significantly worse 2-year PFS compared to patients without detectable ctHPVDNA (0% vs 97%, P&lt;0.0001). 7/98 patients had cancer recurrence during follow-up that was detected by ctHPVDNA with a mean lead time of 207 days (35-518) to clinical diagnosis. Conclusions: ctHPVDNA detection by HPV-DeepSeek is a highly specific biomarker of MRD in HPV+HNSCC, accurately predicts disease progression and detects recurrence earlier than standard care clinical.

Efficacy and safety of the combination of tisleizumab with chemoradiotherapy in oligorecurrence esophageal squamous cell carcinoma: A prospective phase II study.

4057 Background: The postoperative recurrence and metastasis rate of esophageal squamous cell carcinoma (ESCC) remains high and prognosis is poor. PD-1 immune checkpoint inhibitors have shown efficacy in targeting advanced esophageal cancer. This phase II study aims to explore the efficacy and safety of concurrent chemoradiotherapy and immunotherapy for ESCC patients with oligometastatic recurrence after radical surgery. Methods: This phase II study enrolled ESCC patients from 3 hospitals in China, diagnosed oligometastatic recurrence after radical surgery. The 1st stage, with 10 patients enrolled, was designed to explore the safety level and the 2nd stage was to explore efficacy. The oligorecurrence is defined as progression occurring three months after the operation with no more than three metastatic regions. The local recurrence was defined as tumor bed or anastomotic recurrence. The regional lymph node recurrences were regarded as mediastinal region and celiac lymph, respectively. The distal recurrences were considered as supraclavicular, retroperitoneal region and other distal organs. All of the positive regional lymph nodes were collectively counted as one lesion if regional lymph node recurrences existed. Tisleizumab was delivered on D1,22. Nab-paclitaxel 150mg/m2 (starting on D2,23) and cisplatin, 75mg/m2(starting on D2-4,22-24) were administered for two cycles. The radiotherapy was delivered with 50.4-60Gy, 1.8-2Gy per fraction. After CRT, patients received Tisleizumab every 3 weeks for 3-6 months and 2-4 cycle consolidation chemotherapy. The primary endpoint was 1-year OS, the second endpoints were 1-year PFS, ORR and toxicities. Results: Between Jan 2021 and May 2023,50 patients were screened for enrolment and 44 patients met the eligibility criteria. The metastatic regions were made up by the local (15.9%),regional (47.7%), distal (13.6%) and mixed recurrence (22.7%). In the 1st stage, one patient had a grade-3 Immune-related erythra out of ten patients. All patients were delivered chemoradiotherapy, 36(81.8%) patients had completed two cycles of immunotherapy. The ORR rate was 84.1%. During concurrent chemoradiotherapy, the incidence of grade≥3 treatment-related adverse events was nausea (9.1%), vomit (4.5%), anorexia (4.5%), leukopenia (13.6%), hepatitis (2.3%), radiation esophagitis (1.4%), erythra (4.5%). There were 5 cases of Immune-related adverse, including 2 cases of erythra, 1 case of hypothyroidism, hepatitis, gastroenteritis, respectively. With the median follow-up time of 19 months (range:4-30 months), 1-year OS and PFS was 83.2% and 79.2%. The median OS and the median PFS were not reached. Conclusions: Concurrent chemoradiotherapy and immunotherapy yielded satisfactory 1-year OS rate and manageable toxicities in oligometastatic recurrence patients with ESCC after radical surgery. Clinical trial information: NCT04821765 .

Combination of pirtobrutinib and lentiviral transduced bispecific anti-CD20/CD19 (LV20.19) CAR T-cell therapy to improve outcomes in patients with relapsed/refractory lymphoma.

7043 Background: Combination trials of BTK inhibitors (BTKi) with CAR-T have suggested potential synergistic benefit by improving CAR-T immunophenotype and clinical outcomes but are limited by increased toxicity. We report immunophenotype, cytokine profile, and patient outcomes with the new non-covalent BTKi, pirtobrutinib (pirto) when given prior to LV20.19 CAR-T and compare the final CAR product to pts who did not receive pirtobrutinib prior to CAR. Methods: Pts received LV20.19 CAR-T as part of a phase 1/2 trial (NCT04186520). Only pts who received pirto £4 weeks prior to apheresis were included. To determine the impact of pirto on LV20.19 CAR-T, we calculated both polyfunctionality (PFA) and polyfunctional strength index (PSI) for each product using the Isoplexis. Descriptive statistics, t-tests and Kaplan-Meier method were used as appropriate. Results: 11 pts received pirto prior to LV20.19 CAR-T (Table). Median age was 65 (50-80) yrs and median prior lines of therapy were 4 (2-8). There were 4 MCL pts, 5 Richter’s (RT)/CLL, 1 MZL and 1 DLBCL. Median duration of pirto was 4 (1-20) mo. Pts were on pirto for a median of 12 (1-22) days prior to apheresis. All except 1 pts received target CAR-T dose. The day 28 ORR was 82% (CR=7, PR=2). After a median follow up of 13 mo, 3 pts died (PD, Covid, Guillain Barre). The median PFS and OS were both 30.8 mo while the 1-year PFS and OS rates were 9 and 15 mo respectively. 9 pts had CRS and 2 had ICANS, all grades 1-2, while 4 had IEC-HS. One pt had afib recurrence and 1 pt had CMV viremia within 30 days of CAR-T. Among the pts with immunophenotypic data (n=10), there were no differences in naïve or more differentiated T-cell percentages in apheresis and final CAR-T products when compared to pts who did not receive pirto before LV20.19 CAR-T (n=57). Although not statistically significant, both PFA (CD4=61.8 vs 59, CD8= 49.8 vs 46.9) and PSI (CD4= 1752 vs 1608, CD8=1299 vs 1079) were higher with pirto pre-treated pts, suggesting a potential trend towards improved CAR-T functionality. Conclusions: These data represent the largest experience of pirto prior to CAR-T apheresis and demonstrate that pirto can be safely used as a bridge to LV20.19 CAR-T without negatively impacting their immunophenotype and potentially improving functionality. These data support our planned phase 1 clinical trial to assess the safety of pirto as bridging and maintenance therapy with LV20.19 CAR-T (NCT05990465). [Table: see text]

Long term results of a new treatment strategy for follicular low grade lymphoma (FLGL).

e19035 Background: FLGL usually presents with widespread but asymptomatic lymphadenopathy, low SUVs on PET and normal LDH. It's considered incurable and standard of care has been 'watch and wait' (W&amp;W) followed later by treatment if necessary. We have described a subset of patients that we termed clinically discordant indolent histology (“CDIH”) whose behavior is more aggressive. CDIH is defined as presence of any of these: B symptoms, high LDH, SUV &gt;14 on PET, Ki-67 index &gt;30% or involvement of areas such as lung, pleura, soft-tissue, CNS and bones. Our study aims to assess real-world long-term outcome of managing FLGL with and without CDIH. Methods: We analyzed 186 previously untreated FLGL cases treated at our center from June 2002-July 2023, without opting for W&amp;W. CDIH cases typically received R-CHOP x6, then 4 FND. Non-CDIH mainly received FND-R. All received 2 years maintenance rituximab. Main objective was to assess 10-year PFS and OS of stage IV cases, comparing it in a real-world setting against our previously published MD Anderson (MDACC) data. Secondary objectives were to compare our 10-year PFS and OS in stage IV cases against historical controls based on the FLIPI-2 score. We also sought to determine prognostic value of CDIH as compared to FLIPI-2, evaluate early relapse rate (2 yrs), and assess rate of transformation to aggressive lymphoma. Results: Median age: 58 (26-93). Median follow up: 96 mos (6-261). Stage III-IV: 76%. CDIH was present in 56%. Our PFS 10 yrs for non-CDIH=96% vs CDIH=77% (p=0.002). For stage IV, PFS 10 yrs = 82% vs MDACC's 50% using FND-R. OS 10-yr= 90% vs MDACC's 85%. 67 pts were followed &gt;10 years; a relapse occurred in only 5 (7%). Our PFS 10 yr for favorable FLIPI-2 (score 0)= 97%, for intermediate (1-2) =88%, unfavorable (3-5)= 72% (p=.15). Our PFS results were consistently superior to literature data across all FLIPI-2 scores. Transformation to DLBCL= 6/186 (3%) compared to 20-30% historical controls; 5 of 6 transformations were in cases with CDIH and 4 occurred within 3 yrs. Early (&lt;2 yr) relapse rate= 2% in contrast with 20% literature data; all 4 early relapses were CDIH. Conclusions: Our findings emphasize several advantages of initiating treatment early and basing it on presence or absence of CDIH. Our data highlight the need to 1- identify CDIH for potential use of doxorubicin treatment. 2- CDIH as prognostic factor seems more effective than FLIPI-2. 3- PFS 10 yr results are remarkably superior to both literature controls and to our previous experience at MDACC with FND-R. 4- Only 7% relapses beyond 10 yrs suggest potential for cure, according to our results and those from MDACC. [Table: see text]

Long-term follow-up of patients with hairy cell leukemia treated with different modalities at a single institution: Real-life experience over 25 years.

e19023 Background: Hairy cell leukemia (HCL) is an uncommon and indolent leukemia of mature B cells, accounting for 2% of all lymphoid neoplasms. We analyzed characteristics, treatment modalities, response rates, and survival of HCL patients followed at our institution. Methods: We retrospectively reviewed data from the medical records of 56 consecutive HCL patients evaluated in our Hematology/Oncology clinic between 1998 and 2023. Results: The median age at diagnosis was 59 years (range, 37-94). Forty-nine patients (87%) were male. First-line therapy was cladribine in 30 patients (54%), cladribine + rituximab in 6 patients (11%), observation in 5 patients (9%), and another modality (fludarabine, pentostatin, single agent-rituximab, and other) in the rest of patients. The median follow-up was 122 months (1-291). The overall response rate was 96%. CR and PR rates were 68% and 28%, respectively. Relapses were observed in 44% of patients. The median OS was not reached, and the median PFS was 181 months (95% CI, 128-252). The 5-year and 10-year PFS were 78% and 29%, respectively. Only 3 patients (5%) died as a result of HCL, either indirectly from immune suppression (pneumonia, 2 patients) or directly (cachexia in 1 patient, untreated because of advanced age and comorbidities). Conclusions: Treatment with purine analogues such as cladribine, plus or minus rituximab, achieves deep and durable responses in most HCL patients. Although relapses were commonly observed, the vast majority of HCL patients did not die as a result of HLC.

Phase II study of tislelizumab combined with concurrent chemoradiotherapy for locally advanced cervical cancer.

e17523 Background: Concurrent chemoradiotherapy (CRT) is the standard treatment for locally advanced cervical cancer (LACC). However, local recurrence and distant metastasis are the main modes of treatment failure in LACC, especially for patients with stage IIIA ~IVA as well as patients with large masses (&gt;4cm) and regional lymph node metastasis. This study aimed to evaluate the efficacy and safety of tislelizumab (anti-PD-1 antibody) combined with concurrent chemoradiotherapy for locally advanced cervical cancer. Methods: Eligible patients were age 18-75 years, ECOG performance status of 0-1 and untreated cervical cancer (2018 FIGO stage IIIA, IIIB, IVA, or cervical tumor diameter &gt; 4cm with regional lymph node metastasis, or paracervical invasion with regional lymph node metastasis). All patients received CRT plus tislelizumab 200mg Q3W for 1 year or disease progression or intolerable toxicity. The CRT includes at least 4 cycles of cisplatin 40mg/m2 QW + EBRT 45~50Gy/25f then BT 28~30Gy/4~5f. The primary endpoint was tumor regression ratio before brachytherapy. Secondary endpoints were 3-month and 6-month objective response rate (ORR), 1-year and 3-year OS and PFS, safety. Results: From August 24, 2022 to February 4, 2024, 30 patients were enrolled. 25 patients completed CRT and were available for evaluation. The median age was 59 years (range 40-75). The tumor regression ratio before brachytherapy was 90.6%. The ORR at 3 and 6 months were 100%, The OS and PFS rate at 1 year were 100%. The main adverse effect was neutropenia (36% for grade 3-4 and 20% for grade 1-2). Radiation enteritis incidence was 64% and were grade 1-2. Other adverse effect such as nausea, vomiting, and dizziness occurred during CRT and could be alleviated after symptomatic treatment. No immune-related adverse events were observed. Conclusions: Our results suggest that Tislelizumab combined with concurrent chemoradiotherapy showed valuable antitumor activity and controllable safety in LACC. We will keep strictly implementing our study protocol for further PFS and OS. Clinical trial information: NCT05588219 .

The efficacy and safety of ulinastatin in the prevention and treatment of radiotherapy-induced oral mucositis in locoregionally advanced nasopharyngeal carcinoma (LA-NPC): A multicenter, open-label, randomized controlled clinical trial.

6105 Background: Severe oral mucositis is a common radiation-induced toxicity in locoregionally advanced nasopharyngeal carcinoma (LA-NPC) patients treated with concurrent chemo-radiotherapy (CCRT). Ulinastatin (UTI) can reduce the inflammatory response by inhibiting the release of inflammatory factors, but its role in radiotherapy-induced oral mucositis (RTOM) is unclear. Therefore, we conducted a multicenter, open-label, randomized controlled clinical trial to investigate the efficacy and safety of UTI in the prevention and treatment of RTOM in LA-NPC patients. Methods: Patients with histologically confirmed LA-NPC who met the eligibility criteria were randomly assigned to UTI group and control group. All patients received radical intensity modulated radiation therapy (IMRT) and concurrent chemotherapy (cisplatin 100 mg/m2/3 weeks for 2 or 3 cycles). UTI of 100,000 units three times daily (5 days/week) was intravenously administrated from day 1 to the end of radiotherapy for UTI Group. The primary endpoint was the incidence of grade ≥ 3 acute RTOM during CCRT (Radiation Therapy Oncology Group, RTOG grading). The secondary endpoints included the cumulative incidence of RTOM, recovery rate (proportion of patients with grade ≥ 3 RTOM who recovered to grade ≤ 2 during CCRT), the onset time and duration of grade ≥ 3 RTOM, oral pain (Numerical rating scale, NRS), safety and survival outcomes. Results: From January, 2018, to December, 2021, 182 patients from 5 hospitals were enrolled. 179 patients were included for efficacy, safety and survival analysis (89 in the UTI group and 90 in the control group). All UTI group patients completed UTI treatment as planned, and both groups completed scheduled CCRT. The incidence of grade ≥ 3 RTOM was significantly lower in UTI group compared with control group (25.8% vs. 41.1%; P = 0.030). The recovery rate in UTI group was higher than that in control group (39.1% vs. 10.8%; P = 0.023). However, the onset time and the duration of grade ≥ 3 RTOM were similar between the two groups (Median [IQR] 26.00 [19.00, 33.00] days vs. 32.00 [20.50, 36.00] days; P= 0.621 and 12.00 [7.00, 18.00] days vs. 15.00 [8.00, 25.50] days; P= 0.209). The incidence of severe oral pain ( NRS score≥ 7 ) was significantly reduced in UTI group compared with the control group (22.5% vs. 36.7; P = 0.038). No UTI related adverse events were observed during treatment. With a median follow-up time of 41.6 months (IQR, 38.2 - 45.0 months), The 3-year OS, LRRFS, DMFS and PFS in UTI group and Control group were 96.5% vs. 94.3%, 91.2% vs. 87.2%, 95.2% vs. 92.1% and 89.9% vs. 85.1%, respectively (all P &gt; 0.05). Conclusions: Our study revealed that UTI can effectively reduce the incidence of graded ≥ 3 RTOM and severe oral pain without increasing adverse events and compromising survivals. Clinical trial information: NCT03387774 .

Neoadjuvant treatment with camrelizumab in combination with doxorubicin and ifosfamide: A phase II clinical study of high-risk soft tissue sarcoma.

e23548 Background: Surgery remains the cornerstone in the management of non-metastatic soft tissue sarcomas. However, high rate of recurrence and metastasis are not uncommon in patients(pts) with high-risk soft tissue sarcoma. Although doxorubicin plus ifosfamide resulted in better tumor shrinkage which facilitate surgical resection, approaches of reducing recurrence and metastasis are still needed. Chemotherapy combined with PD-1 antibodies showed promising activity in several tumors. To date, there is no study to evaluate the efficacy and safety of camrelizumab combined with chemotherapy for high-risk Soft Tissue Sarcoma. Methods: This is a phase II, open-label single-arm study (NCT04606108), aimed to include pts aged 14-65 with histopathologically confirmed high risk soft tissue sarcoma and ECOG 0-1, at least one measurable lesion. One course of treatment every three weeks (camrelizumab 200 mg on day 1, doxorubicin 37.5 mg/m2 or liposomal doxorubicin 25 mg/m2 on days 1-2, and ifosfamide 3 g/m2 on days 1-3). After completing the 2nd course of chemotherapy, the drug will be stopped for 14 days, and surgery or the next stage of treatment will be carried out according to the efficacy of the treatment. Six courses of preoperative chemotherapy will be administered 2 weeks ± 5 days after surgery based on the effectiveness, followed by radiotherapy (50 Gy in 25 fractions or 60 Gy in 30 fractions.) within 3 months after surgery. The primary objective is the Objective response rate (ORR). Using the Simon minimax 2 stage design, planned sample size is 62 pts with 80% power and alpha of 5% to detect an increase in ORR from 27% to 42%. Interim analysis requiring at least 9 confirmed responses in the first 30 pts to proceed to full accrual. Results: 35 pts completed neoadjuvant therapy and surgery, 30 pts received postoperative adjuvant therapy and 1 patient achieved partial response (PR). Median age was 36.4 years (14-61), 62.8% were females, and 57.1% were ECOG-PS 0. Surgery was performed in 35 pts including 16/35 Synovial Sarcoma (SS), 5/35 Liposarcoma (LPS), 2/35 Myxofibrosarcoma (MFS), 2/35 Undifferentiated pleomorphic sarcoma (UPS), 1/35 Leiomyosarcoma (LMS), 5/35 Malignant mesenchymal tumors and 4/35 other pathologic subtype. The 2-year PFS rate was 84.9%, the 2-year OS rate was 96.77%, and the 2-year RFS rate was 72.6%. The most common grade 3-4 adverse events included leukopenia (37.7%), Neutrophil count decreased (37.7%), Platelet count decreased (20.0%), Blennisthmia (3.1%), and Aspartate aminotransferase increased (0.8%). Conclusions: Camrelizumab combined with doxorubicin, liposomal doxorubicin, and ifosfamide in the perioperative treatment of soft tissue sarcoma pts were safe and tolerable. Survival follow-up is still ongoing to analyze the effect of perioperative immunotherapy combined with chemotherapy on long-term outcomes of pts with soft tissue sarcoma. Clinical trial information: NCT04606108 .

Pyrotinib in HER2-altered advanced salivary gland carcinomas: Analysis of two cohorts from an exploratory study.

6108 Background: The HER2 altered in a subset of salivary gland cancers (SGCs). Pyrotinib is an oral irreversible pan-HER receptor tyrosine kinase inhibitor targeting HER1, HER2, and HER4. In this study, we assessed the efficacy and safety of pyrotinib in patients with HER2-amplified/overexpressed/mutated advanced SGCs, including recurrent/metastatic(R/M) and locally advanced (LA) patients with high-risk of recurrence. Methods: R/M and high-risk LA SGCs with HER2-alteration were enrolled. Pyrotinib (400 mg) was given orally once daily. In LA cohort, pyrotinib were given as adjuvant therapy after standard surgery and postoperative radiation. Results: Between August 2019 and August 2023, 18 patients were enrolled (10 in R/M cohort and 8 in LA cohort). In R/M cohort, the median age was 58.5 years, and all patients were grade III. The ORR was 90%, including 2 CRs, 7 PRs and 1PD; median DOR was 5.7 months (range: 3.1-13.7) and median PFS was 7.6 months (range: 2.7-16.8). In LA cohort, the median age was 59.0 years, and all patients but one were grade III. All patients but one had T4 disease; 5 patients had N3 and 2 had N2b disease; 5 patients had ENE and 5 patients had positive margin. With a median follow up 21.3 months, only 3 patients experienced recurrence, and one patient died due to non-tumor reasons. The estimated 2-years PFS and 2-years OS were 75% and 100%. The most common adverse event was diarrhea, which occurred in 11/18 (61.1%) patients and 3/18 (16.7%) were grade III. Conclusions: Pyrotinib showed promising antitumor activity in recurrent/metastatic HER2-altered SGCs, and had a potential to prolong survival outcome as adjuvant treatment in LA patients setting, which needs further randomized studies. Clinical trial information: NCT05087706 .

Clinical features and outcomes of patients with follicular lymphoma grade 3A: A real-world study in single center.

e19058 Background: Follicular lymphoma (FL) was classified into three grades based on the number of centroblasts (FL1-2, FL3A, and FL3B). Guidelines suggest that FL1-2 should be treated according to the treatment recommendations for classical FL. FL3B is usually treated as diffuse large B-cell lymphoma (DLBCL).There is no consensus yet on the management of FL3A. In order to explore the relationship between clinical features and outcomes of FL3A, we conducted this study. Methods: We conducted a retrospective study of newly diagnosed FL patients who were admitted to our hospital from January 2013 to December 2022. The data were collected from the hospital electronic medical record system, including but not limited to: demographic characteristics, pathological information, clinical features at diagnosis, treatment, and efficacy evaluation. The survival curve was calculated through Kaplan-Meier method and compared with Log Rank test.The prognostic factors of PFS was compared by COX regression analysis. Results: There were 253 FL3A patients admitted, accounting for 27.55% of all FL patients. After screening, a total of 223 FL3A patients were included in this study. Among 223 patients, the male to female ratio was close to 1:1; The median age is 57 years old. At diagnosed, 33 patients in early stage and 190 in advanced stage. Among advanced patients, 144 had no treatment indications according to the GELF criteria. 86.1% (192/223) of patients received systemic treatment, with the majority of rituximab contained regimens. The overall response rate (ORR) was 96%, and 62.9% of patients who responded to chemotherapy received maintenance therapy with CD20 monoclonal antibody. A median follow-up of 41 months, with an estimated 5-year OS of 91.3% and 5-year PFS of 73%. The PFS of advanced FL patients treated with RCHOP regimen (rituximab, cyclophosphamide, doxorubicin, vincristine, prednisone) was significantly longer than that of non-RCHOP regimen patients (P&lt;0.05). The PFS of patients receiving maintenance treatment was better than that of those not (P&lt;0.05). Conduct prognostic analysis on advanced FL3A patients. Univariate analysis showed that treatment plan selection, maintenance of treatment, PET scan of FDG metabolic maximum uptake value (SUVmax), Ki-67, lactate dehydrogenase (LDH), white blood cell count (WBC), IgA, IgM, all significantly affected PFS; Multivariate analysis shows SUVmax ≥ 15 and platelet count&lt;100 × 109/L and LDH&gt;210U/L are all correlated with poor PFS. Conclusions: This study found that FL3A accounts for 27.55% of all FLs, with a median age of 57 years old and mostly in the advanced stage. FL3A has a long survival period, with a 5-year PFS of 73%. COX regression analysis showed that SUVmax, platelet count, and LDH were all associated with PFS in advanced FL3A patients.Those may explore new clinical prognosis prediction models.

Plasma mutational burden in PIK3CA and TP53 independently predicts early progression in patients with HR+/HER2- metastatic breast cancer (MBC) enrolled in the GEICAM/2014-12 FLIPPER trial.

1029 Background: There is an unmet need for molecular diagnostics to predict therapy outcomes using blood-based biomarker monitoring to guide treatments in HR+/HER2- MBC patients. Here, we report initial results of serial ctDNA analyses in a trial of Fulvestrant (F) + CDK4/6 inhibitor (CDK4/6i) Palbociclib (Pa) vs. F + Placebo (Pl). Endocrine therapy + CDK4/6i are currently the cornerstones of treatment for HR+/HER2- MBC. However, between 10–30% of patients suffer primary resistance. Thus, early identification of HR+/HER2- MBC patients that progress during CDK4/6i treatment is required. Methods: The GEICAM FLIPPER trial (NCT02690480) is a multicenter, double-blind, randomized, phase II study comparing F+Pa vs F+Pl in postmenopausal women with HR+/HER2- MBC. Sequential plasma was collected from 187 patients at baseline, 12, 24, 36, and 48 weeks on treatment and at progression. In total, 903 samples were analyzed using the Plasma-SeqSensei BC IVD kit (Sysmex Inostics) that identifies and quantifies ctDNA mutations in six BC-related genes. The mutant allele frequency (MAF) of PIK3CA and TP53 variants was assessed in baseline samples (median value as cutoff). These variants, identified as the most prevalent, were used as surrogate markers for mutational burden. The Kaplan-Meier method was used to estimate median (m) PFS and overall survival (OS). Hazard ratio (HR) and odds ratio (OR) were calculated with 95% confidence interval (CI) using Cox proportional hazards regression model and logistic regression respectively, unadjusted or adjusted by age, site of disease, disease presentation at study entry and clinical subtype. Results: Previously we showed that F+Pa improved 1-year PFS (83.5% vs. 71.9%, p=0.064) and mPFS (31.8 vs. 22.0 months, p=0.001) vs F+Pl. Here, ctDNA analyses revealed that having ≥1 mutation in PIK3CA + TP53 was prognostic of early (≤12 months) progression (OR 2.37; 95% CI 1.1-5.12, p=0.0274) independent of treatment arm. Furthermore, having ≥2 mutations was associated with treatment resistance (not achieving objective response; OR 3.55; 95% CI 1.28-9.87, p=0.0151). Additionally, worse mPFS and mOS were seen in patients with ≥2 mutations in PIK3CA + TP53 (PFS: HR 3.6; 95% CI 2.25-5.76, p&lt;0.0001; OS: HR 2.92; 95% CI 1.66-5.12, p=0.002) and in patients with high MAF (PFS: HR 2.43; 95% CI 1.6-3.68, p&lt;0.0001; OS: HR 2.83; 95% CI 1.75-4.58, p&lt;0.0001). Finally, having ≥2 mutations in PIK3CA + TP53 was predictive of early progression (OR 22.32; 95% CI 4.12-120.9, p&lt;0.0001) and worse mOS (61.8 vs. 34.79 months) in the F+Pl arm (HR 5.91; 95% CI 2.65-13.16, p&lt;0.0001) but not in the F+Pa arm. Conclusions: Baseline plasma mutational burden in both PIK3CA and TP53 genes was significantly associated with Fulvestrant +/- Palbociclib treatment outcome in HR+/HER2- MBC and appeared to be predictive of Palbociclib benefit. Clinical trial information: NCT02690480 .