Background: Immune checkpoint inhibitors (ICIs) relieve immunosuppression of tumor-reactive T cells and enhance antitumor immune response; however, not all patients benefit and some become refractory. EIK1001 is a Toll-like receptor (TLR)7/8 agonist that stimulates myeloid and plasmacytoid dendritic cells, activating immune and inflammatory responses. This dual activity provides another pathway, distinct from effects on checkpoint proteins, to enhance antitumor T-cell activity alone or in combination with ICIs. Methods: Study BDB001-102 was a Phase 1, open-label, dose-escalation/expansion study of EIK1001 combined with atezolizumab (comb Rx). Enrollment criteria included participants (pts) ≥ age 18 with confirmed, RECIST-measurable advanced solid tumors. Primary study objectives included safety and tolerability, and secondary objectives included evaluation of dose-limiting toxicities, pharmacokinetics (PK), pharmacodynamics, and preliminary efficacy by RECIST 1.1. During dose escalation, pts received a range of doses of EIK1001 (QW IV) in combination with atezolizumab (1200 mg Q3W). Results: Forty-one pts (median age 65 years [range = 32 to 79]) with multiple, distinct histological tumor types and a median of 3 prior Rx regimens were enrolled. Overall, a total of 28/41 (68.3%) receiving EIK1001 + atezolizumab experienced a treatment-related adverse event (TRAE). Of these, 4/41 (9.8%) experienced a ≥ Grade 3 TRAE, including fatigue, nausea, hyponatremia, and lymphedema. Only 1/41 (2.4%) experienced manageable cytokine release syndrome. There were no deaths due to TRAEs. Of the efficacy-evaluable pts (n = 37), complete response (CR) or partial response (PR) was observed for 3/37 (8.1%). Disease control (including CR, PR, or stable disease) was observed in 19/37 (51.4%). The median duration of response (DOR) was 13 months (range = 10 to 27). One responder was PD-L1 negative yet had a > 12-month DOR; another had a history of prior anti-PD-1 Rx yet experienced a 10-month DOR on comb Rx. EIK1001 PK was linear and dose-proportional, and was not affected by combination with atezolizumab. Conclusions: Overall, EIK1001 was well-tolerated with a manageable safety profile and showed encouraging preliminary efficacy across several tumor types in combination with atezolizumab. Responses were observed even in pts not anticipated to respond to atezolizumab monotherapy. Further development of EIK1001 is underway. Clinical trial information: NCT04196530.