7033 Background: The IGF-1 receptor (IGF-IR) is expressed in thymoma (T) and thymic carcinoma (TC). Prolonged SD has been seen with anti-IGF-IR therapy in thymic tumors in phase I trials. Cixutumumab (C) is a fully human IgG1 monoclonal antibody (ab) that targets IGF-IR. Methods: Patients (pts) with recurrent T or TC, PS 0-2, measurable disease, and normal organ function received C at 20 mg/kg IV q3wks until progression or intolerable toxicity. Primary endpoint was response rate. Correlative studies included immune cell subset (central memory, effector memory, naïve and regulatory T cells), circulating endothelial progenitor (CEP) cell, serum IGF-1 and IGF-IR in PBMCs, anticytokine ab, and tumor mutational analysis. Results: Forty-five pts enrolled from two centers (20 female, T=33, med age 52 yrs, range, 26-86). Median number of prior systemic treatments: 3 (range, 1-11). Median number of cycles of C administered: 6 (range, 1-41). In 30 evaluable T pts there were 4 (13%) PR, 23 (77%) SD, and 3 (10%) PD. Corresponding numbers in 12 TC pts were 0, 5 (42%) and 7 (58%). Median PFS for T and TC was 40 and 9 wks respectively. C-related grade 3/4 AEs were hyperglycemia (8%), hyperuricemia, weight loss, lipase elevation, chest wall pain (5% each) and AST elevation (3%). Two pts died while on study: one due to respiratory insufficiency related to T after 5 cycles and one due to worsening myasthenia and an acute coronary event after 13 cycles. In 8 of 33 T pts autoimmune (AI) symptoms developed (4 new-onset) due to immune thrombocytopenic purpura, myositis, myocarditis, colitis, PRCA and food allergy. In the first 13 pts a trend towards increased numbers of central and effector memory T cells and a decrease in naïve T cells and CEPs was seen. High-titer anticytokine abs, including anti-IFNa, anti-IL-12, and anti-IL-17A, were found in 13/27 pts tested. No EGFR, KRAS or BRAF mutation, HER2 amplification or ALK translocations were detected in 11 tumors analyzed. Conclusions: C monotherapy is well tolerated and active in T. Accrual will continue until 37 T pts are enrolled. Activity in TC is unworthy of further investigation. CEP and immune cell subset analysis suggests therapy may impact angiogenesis and immune response. Development of AI symptoms during treatment needs further evaluation.