T-cell retargeting to eliminate CEACAM5-expressing cancer cells via CEACAM5xCD3 bispecific antibodies (BsAbs) showed limited clin. activity so far, mostly due to insufficient T-cell activation, dose-limiting toxicities, and formation of anti-drug antibodies (ADA).We present here the generation and preclin. development of NILK-2301, a BsAb composed of a common heavy chain and two different light chains, one kappa and one lambda, determining specificity (so-called κλ body format).NILK-2301 binds CD3ε on T-cells with its lambda light chain arm with an affinity of ≈100 nM, and the CEACAM5 A2 domain on tumor cells by its kappa light chain arm with an affinity of ≈5 nM.FcγR-binding is abrogated by the "LALAPA" mutation (Leu234Ala, Leu235Ala, Pro329Ala).NILK-2301 induced T-cell activation, proliferation, cytokine release, and T-cell dependent cellular cytotoxicity of CEACAM5-pos. tumor cell lines (5/5 colorectal, 2/2 gastric, 2/2 lung), e.g., SK-CO-1 (Emax = 89%), MKN-45 (Emax = 84%), and H2122 (Emax = 97%), with EC50 ranging from 0.02 to 0.14 nM.NILK-2301 binds neither to CEACAM5-neg. or primary colon epithelial cells nor to other CEACAM family members.NILK-2301 alone or in combination with checkpoint inhibition showed activity in organotypic tumor tissue slices and colorectal cancer organoid models.In vivo, NILK-2301 at 10 mg/kg significantly delayed tumor progression in colon- and a pancreatic adenocarcinoma model.Single-dose pharmacokinetics (PK) and tolerability in cynomolgus monkeys at 0.5 or 10 mg/kg i.v. or 20 mg s.c. showed dose-proportional PK, bioavailability ≈100%, and a projected half-life in humans of 13.1 days.NILK-2301 was well-tolerated.Data were confirmed in human FcRn TG32 mice.In summary, NILK-2301 combines promising preclin. activity and safety with lower probability of ADA-generation due to its format compared to other mols. and is scheduled to enter clin. testing at the end of 2023.