Background: TAK-676 is a novel synthetic STING agonist developed to induce an inflammatory state through release of interferon (IFN), production of proinflammatory chemokines and cytokines, and activation of innate and adaptive immune cells in the tumor microenvironment (TME). To gain early mechanistic insights on the effects of TAK-676 in the native TME alone and in combination with standard-of-care chemotherapies, we completed a Phase 0 intratumoral microdosing trial in head and neck squamous cell carcinoma patients using the Comparative In Vivo Oncology (CIVO) platform. CIVO enables direct evaluation of biomarker response in trackable, distinct regions of drug exposure upon resection. These studies were designed to identify rational combinations that could be evaluated in future clinical trials with therapeutic intent. Methods: This IRB-approved study was conducted at four centers within the United States. Fifteen subjects provided Informed Consent and enrolled in the trial. Microdoses (≤1/100^th systemic dose) of TAK-676, carboplatin, 5-FU, paclitaxel, and combinations thereof, were simultaneously injected via CIVO to eligible patients presenting with a surface-accessible primary or metastatic lesion ≥ 2 cm and planned surgical resection of the target lesion at 24 (n=8) or 72–96 (n=7) hours. Injection sites were identified via co-injection of fluorescent tracking markers (CIVO GLO). Multiplexed ISH, IHC, and spatial molecular profiling via GeoMx and CosMx were used to resolve molecular responses at each injection site. Patients were monitored for adverse events (AEs) for 28 days following injection. Results: TAK-676 exposure was associated with increased IFN signaling in all samples analyzed at the 24-hour cohort. This was accompanied by increased expression of pro-inflammatory cytokines and chemokines, “M1-like” macrophages, NK cells, and CD8+ T cells. Spatial profiling with GeoMx and CosMx revealed an IFN signature and elevated pro-inflammatory markers across multiple cell types throughout the TME. In addition, exposure to TAK-676 plus chemotherapy resulted in localized enhancement of apoptosis vs. chemotherapy alone across multiple tumors. The responses observed at 24 hours were transient and not detected in the 72–96-hour cohort. No AEs related to injection procedure or contents were reported. Conclusions: Intratumor microdosing via CIVO of TAK-676 alone and in combination with chemotherapy was shown to promote an early pro-inflammatory response, providing evidence of an on-target mechanism of action. The absence of effect at the late timepoints could be due to pharmacokinetics of drug exposure or compensatory signaling within the TME. In this Phase 0 clinical trial we safely demonstrated precise, mechanistic effects of TAK-676 on the native TME of intact human tumors. Clinical trial information: NCT04541108.