LAUSR

8576 Background: Thymic epithelial tumors (TETs) are rare but the most common tumor of the anterior mediastinum. Platinum-based combination chemotherapy is standard of care which is associated with a 50%-90% overall response rate (ORR) in metastatic disease. However, there is no standard chemotherapeutic option after failure of platinum-based combination chemotherapy. Genetic alterations associated with cell cycle including pRB, p16INK4A, and cyclin D1 are commonly observed in TETs. Based on these results, we conducted a phase II trial to evaluate the efficacy and safety of palbociclib in patients with recurrent or refractory advanced TETs. Methods: This is a phase II multicenter, open-label, single arm study of palbociclib monotherapy in patients with recurrent or metastatic advanced TETs who failed one or more cytotoxic chemotherapy. Patients receive oral palbociclib 125mg daily for 21 days followed by a 7-day break. The primary endpoint was the progression-free survival (PFS) rate at 6 months (H0 = 30% vs H1 = 48%). Results: Between August 2017 and October 2019, 48 patients were enrolled. The median number of previous chemotherapy was 1 (range: 1-4) and 21 (43.7%) of 48 patients received thymectomy. By WHO classification, Type A (n = 1), Type B1 (n = 2), Type B2 (n = 8), Type B3 (n = 13), Type C (n = 23), and unknown (n = 1). With medial follow-up of 14.5 months (range 0.8-38.2), the median cycle of palbociclib was 10 (range 1-40). The PFS at 6 months was 60% and the median PFS was 11.0 months (95% CI: 4.6-17.4). Six of 48 patients (12.5%) achieved partial response. The median overall survival was 26.4 months (95% CI: 17.4-35.4). The most common adverse events of any grade included neutropenia (62.5%), anemia (37.5%) and thrombocytopenia (29.1%). Conclusions: Palbociblib monotherapy is well tolerable and encouraging efficacy in patients with TETs who failed platinum-based combination chemotherapy. Updated results will be presented. Clinical trial information: NCT03219554.