Background: There is a significant unmet need in esophagogastric cancers (EGC) due to limited effective treatments (Tx) and poor prognosis. Dendrimers are highly branched nanoparticles that enable sustained delivery of cytotoxic drugs and achieve selective tumor targeting via an enhanced permeability and retention effect. DEP cabazitaxel is a highly optimised dendrimer formulation of cabazitaxel. Unlike standard cabazitaxel, DEP cabazitaxel is highly water soluble, does not contain toxic excipients such as polysorbate-80 associated with anaphylaxis, and avoids the need for steroid/antihistamine pre-medication. The objective of this Phase 1/2 trial was to assess preliminary efficacy and safety of DEP cabazitaxel in patients (pts) with advanced solid tumors including EGC, for which standard cabazitaxel is not approved. Methods: In the P2 part of the trial, advanced EGC pts with measurable disease were treated with DEP cabazitaxel 20 mg/m² cabazitaxel, the recommended Phase 2 dose, IV 3-weekly. Anti-tumour activity was assessed by RECIST v1.1, safety by CTCAE v4.03. (EudraCT 2017-003424-76). Results: Fifteen pts with advanced EGC were enrolled; 9 adenocarcinoma (ADENOCA) (3 gastric, 2 esophageal and 4 EG junction) and 6 esophageal squamous cell carcinoma (ESCC); median age 62 yrs (26–73 yrs), with all pts having ≤3 prior anticancer Tx, and the majority (9) having progressed on or immediately after 1^st line Tx (1L). All had ≥1 prior platinum-based regimen and 4 pts had received prior taxanes. Pts received a median of 4.5 DEP cabazitaxel cycles without the need for routine steroid/antihistamine or primary G-CSF prophylaxis. In all evaluable pts, disease control rate (DCR) was 80%. Disease control was observed in both histological subtypes and was durable, including stable disease (SD) for up to 27 weeks and partial responses (PR) for up to 17 weeks, with objective response rate (ORR) of 30%. In EG ADENOCA pts, DCR was 100%, ORR was 33%, and median progression free survival (mPFS) was 4.0 months. In ESCC pts, DCR was 50% and ORR was 25%, with mPFS of 1.9 months. DEP cabazitaxel was well-tolerated with mostly mild (62.9%, Grade (G) 1)/moderate (20.2%, G2) treatment-related adverse events (TRAEs), including nausea, vomiting, fatigue, neuropathy, neutropenia (only 1 pt had G-CSF Tx, 1 pt had secondary G-CSF prophylaxis) and anemia, that were like those observed with standard cabazitaxel Tx. Of severe TRAEs (G3/4; 16.9%) most (80%) were observed in 2 pts, including neutropenia, anemia, thrombocytopenia, fistula, elevated liver enzymes. Conclusions: DEP cabazitaxel exhibited highly encouraging anti-tumor activity in >1L, advanced EGC in multiple anatomic locations and of different histological sub-types. The preliminary efficacy and safety results compare favorably to available treatment options and highlight the promising clinical potential of DEP cabazitaxel in advanced EGC. Clinical trial information: EudraCT 2017-003424-76.