Background: Epstein-Barr Virus (EBV) is associated with a diverse collection of malignancies, including nasopharyngeal carcinoma (NPC). One viral-encoded protein, EBNA1, is consistently expressed in all known EBV-associated malignancies and is critical for the replication and maintenance of the EBV genome in latently infected cells. VK-2019 is a first-in-class, orally bioavailable EBNA1 inhibitor that blocks latent replication and proliferation in preclinical studies. Methods: We conducted a Phase I/IIa clinical trial of VK-2019 as a single therapeutic agent in patients with advanced (recurrent or metastatic) NPC. An accelerated titration design allowed dose escalation to proceed rapidly from 60 mg/day to 1800 mg/day. Blood samples for detailed pharmacokinetic analyses were collected after the first and multiple doses. In addition, tumor biopsies were collected from 2 patients at the 1800 mg/day dose level at baseline and during treatment and analyzed for EBV copy number and viral and cellular gene expression. Results: A total of 22 advanced NPC patients were enrolled. VK-2019 was well tolerated with mostly grade 1-3 adverse events being reported. VK-2019 is rapidly absorbed with a biphasic distribution and a terminal T_1/2 of ~12h after single or multiple doses. C_max and AUC increases with dose escalation through 920 mg with large variability at 1800mg. Accumulation has been observed with multiple doses up to 460mg. Based on preclinical efficacy models, target exposures are at or above expected activity levels for patients at least on the 920 mg daily dose and above. A partial response ( > 30% decrease in tumor size) was observed for one patient. Preliminary results from biopsies from 2 patients show decreases in EBV copy number and viral gene expression (LMP2 and gp150). We also observed decreases in EBER-positive cells in one patient. Conclusions: In this study, VK-2019 was well tolerated, exhibiting an excellent safety profile and exposure above in vitro efficacy. An MTD has not been yet established. Preliminary results indicate a decrease in EBV copy number and viral gene expression in latently infected tumor cells in some patients. Alternate dose schedules are justified to determine the clinical efficacy of an EBNA1 inhibitor in patients with advanced NPC. Funded by NCI, R01-CA235633 and Cullinan Oncology. Clinical trial information: NCT04925544.