Background: Stratum D of the INSIGHT study investigates the feasibility and safety of s.c. application of IMP321 (eftilagimod alpha) combined with the PD-L1 inhibitor avelumab in advanced stage solid tumors. The MHC class II agonist IMP321 activates antigen-presenting cells followed by CD8 T-cell activation. The addition of avelumab aims at enhancing activity by combining IMP321’s activating effects on immune cells with the release of immune inhibitory effects caused by interruption of the PD-1/PD-L1 axis. Methods: This investigator-initiated phase I trial consists of four strata: intratumoral (A) or intraperitoneal IMP321 (B); s.c. IMP321 with SOC (C) or with PD-L1 inhibition (D). This abstract focuses on Stratum D. Patients (pts) receive 800mg avelumab i.v. q2w along with s.c. IMP321 injections (6mg IMP321 in cohort 1 and 30mg IMP321 in cohort 2). 12 pts are planned in stratum D: 6 pts in cohort 1 and 6 pts in cohort 2. Primary endpoint is safety. Results: So far, 8 pts have been enrolled (6 in cohort 1 and 2 in cohort 2). In 6 pts (cohort 1) treated for different tumor indications (gastric, gallbladder, colon cancer, pleural mesothelioma), no dose limiting toxicities occurred. 3 serious adverse events (SAEs) (1 acute kidney injury grade 5 in 1 pt, 2 preileus grade 3 in 1 pt) were reported, none of them was related to any of the study drugs. In total, 34 adverse events (AEs; grade 1-2, 21; grade 3, 12; no grade 4; grade 5, 1) have been documented in 5 pts. Most common grade 1-2 AEs were pain, nausea, and injection site reaction in 50%, 33%, and 17% of the pts. Most common grade 3 AEs were nausea/vomiting, preileus/ileus, and ascites in 33%, 33%, and 17% of the pts. One AE grade 5 (acute kidney injury) was reported. 4 AEs grade 1-2 were possibly or definitely related to IMP321 (injection site reaction 2x; fever; lipohypertrophy), 6 AEs grade 1-2 were possibly or definitely related to avelumab (nausea 2x; chills; fever; dyspnea; lipohypertrophy). All AEs grade 3-5 were unrelated to any of the study drugs. Of the 8 pts enrolled so far, 4 had disease progression (acc. to RECIST 1.1), 1 partial response, 1 stable disease with some extent of tumor shrinkage, and 2 have not had tumor assessment yet. Conclusions: Combination treatment with avelumab 800mg and IMP321 6mg is safe and well tolerated. Cohort 2 will be presented at the meeting. Clinical trial information: NCT03252938.