Background: CEA CD3 TCB (RO6958688) targets CEA on tumor cells and is agonistic for CD3e on T cells. In mouse models, CEA CD3 TCB displays potent anti-tumor activity, leads to increased intra-tumoral T cell infiltration and activation and up-regulates the PD-1/PD-L1 pathway. Methods: Biodistribution was assessed in mice using SPECT/CT. Patient (pts) samples correspond to 2 dose-escalation studies in CEA+ solid tumors. Study 1 (S1): single agent weekly (qW) (0.052 to 600 mg, iv, n = 80), and Study 2 (S2): combination of RO6958688 qW (5 to 160 mg, iv) with 1200mg atezolizumab q3W (n = 38). Analytical methods: PK - population modeling approach; anti-drug antibodies (ADA) - ELISA; immunophenotyping in peripheral blood (PB) by flow cytometry (FCM), in pre- (BSL) and on-treatment (OT) biopsies by immunohistochemistry (IHC) and FCM; plasma cytokines - multiplex assays; PD-L1 - SP142 assay. Results: In mice, RO6958688 preferentially accumulated in CEA+ tumors. In pts with no ADAs tested thus far in both studies (S1 29; S2 21), RO6958688 showed near linear PK and exposure. In S1, OT biopsies demonstrated a statistically significant increase in density and activation profile of T cells (CD3: 2.6-fold, n = 21; CD3/CD8: 3.7 fold, n = 17; CD3/Ki67: 4-fold, n = 20; CD8/PD1: 1.7-fold, n = 15) without dose-dependence. In S2, preliminary data of T cell density (5-80mg) were similar to S1 (2-fold). In S1, a significant correlation was observed between treatment-induced tumor lesion reduction and increases of OT CD8/CD25 fluorescence intensity from BSL (p = 0.028). PD-L1 expression increased in OT biopsies in both studies. In S1, from week 4, a moderate expansion of activated CD8 T cells (HLA-DR/Ki67) but not of CD4, was detected in PB at doses > 60mg ( > 3.3 fold). Transient increases of several cytokines were seen in both studies with levels peaking within 24hrs. Conclusions: PK and PD results consistent with tumor inflammation and mechanism of action support that RO6958688 is the first tumor-targeted T cell bispecific to show intra-tumoral biological activity in pts with CEA+ solid tumors. Updated data will be presented. Clinical data are reported separately.