9067 Background: To date, few studies have associated elevated Absolute Eosinophil Count (AEC) with improved outcome to a single agent immune checkpoint inhibitor (ICI) as first or subsequent line of therapy in patients with NSCLC. Here, we investigated prognostic utility of AEC and the sex difference in patients undergoing standard-of-care first-line ICI with or without chemotherapy for metastatic NSCLC. Methods: This was a retrospective cohort study of 310 patients with Stage IV NSCLC treated with first-line ICI-based therapies (alone or in combination with chemotherapy) under IRB approved protocol 2021C0069. Demographic data are summarized in the table. Peripheral AEC measured by cells/microL was collected within 1 week before the start of therapy. Eosinophil count was evaluated in males and females by both descriptive statistics and Chi-squared test as a continuous and categorical variable, respectively. High level of baseline AEC was defined as 75th percentile (270 cells/microL). Overall Survival (OS) was plotted using the Kaplan-Meier method and p-values from log-rank test were reported. PD‐L1 was assessed by tumor proportion score (TPS) and classified into < 1%, 1 to 49% and ≥ 50%. Results: 14% male vs 6% female patients were found to have peripheral eosinophilia (AEC ≥ 500) at the start of immunotherapy, p = 0.02. Patients with lower AEC had a median OS of 18.6 months vs 29.3 months in those with higher AEC ( p = 0.02, HR: 1.465, 95% Cl: 1.073-2.001). Patients with higher baseline AEC showed a sex difference, with median OS of 23.9 months in males vs NR median OS in females with over 60% survival at the time of last follow up ( p = 0.04, HR: 1.934, 95% Cl: 1.040-3.597). This difference was seen with ICI monotherapy ( p = 0.02, HR: 2.630, 95% Cl: 1.192-5.805), but not ICI with chemotherapy ( p = 0.83, HR: 1.116, 95% Cl: 0.4054-3.070). Higher AEC was associated with TPS ≥ 50% in both males and females ( p = 0.004 and 0.001, respectively), though no significant sex difference was observed. Conclusions: Our data highlight critical sex differences in interpretation of baseline AEC in patients on ICI therapy for metastatic NSCLC. While males have a higher frequency of peripheral eosinophilia, higher AEC correlates to improved OS predominantly in females. Further, this female-biased outcome is seen only with ICI monotherapy, even though higher AEC is associated with high PD-L1 expression in both sexes. Future studies are indicated to assess the longitudinal trend in eosinophil count as well as their active versus passive role in anti-tumor immunity and toxicity in the presence or absence of chemotherapy. [Table: see text]

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e21094 Background: Cancer-associated cachexia and weight change from treatment-related side effects profoundly influence treatment outcomes for patients with NSCLC. Limited data are available to compare the association between weight change and OS for patients with NSCLC treated with pembrolizumab alone or combined with chemotherapy. Therefore, we evaluated the change in weight after the first cycle of pembrolizumab-based treatments and its association with OS in patients with metastatic NSCLC. Methods: This retrospective study included 306 patients with NSCLC treated with a first line pembrolizumab-based regimen at Ohio State University from 2018-2021. Baseline characteristics were collected within seven days before cycle one treatment. Percent of weight change (PWC) was calculated as (cycle two weight – baseline weight)/baseline weight x 100%. Severe weight loss was defined as ≥75th percentile (PWC ≤-3.5%). Kruskal Wallis test was used to compare the significance of median PWC between treatment groups. Cox proportional-hazards models were used to test associations with survival. Results: The median age was 64.0 (IQR 56.7-72.6) years. The median baseline BMI was 26.1 (IQR 22.4-30.7). There was no difference between the median PWCs of patients treated with pembrolizumab monotherapy (-1.2%, IQR -3.7-0.8%) vs. pembrolizumab with chemotherapy (-1.0%, IQR -3.4-0.6%), p = 0.587. The patients whose weight remained stable (PWC -1% to 1%, n = 87 (28.4%)) had the highest median OS at 45.6 months (95% CI 27.6 – not reached); while those with severe weight loss (PWC ≤-3.5%, n = 78 (25.5%)) and weight gain (PWC ≥1%, n = 65 (21.2%)) had significantly decreased survival with median OS 15.6 (95% CI 9.3-20.9) and 16.3 (95% CI 10.1-25.6) months respectively, overall model p-value < 0.001. The PWC remained significantly associated with OS after adjusting for performance status, PD-L1, and treatment with p < 0.001 (Table 1). Conclusions: For patients with metastatic NSCLC treated with first line pembrolizumab-based regimen, similar PWCs were seen between patients treated with pembrolizumab alone or combined with chemotherapy. Regardless of treatment, weight gain and severe weight loss after the first cycle of treatment were associated with reduced OS. Further in-depth analysis is warranted to investigate the potential causes of weight changes, especially for patients with positive PWC. [Table: see text]