Effector-to-target ratio as a translational bottleneck for CD33 T-cell engagers in acute myeloid leukemia: a...

A common, often unstated paradigm in T-cell engager (TCE) translation assumes that target-antigen density and drug exposure are the main efficacy drivers, with effector context secondary. This paradigm is visible in dose-escalation strategies and in the use of high-E:T screens as principal preclinical assays. We tested whether an alternative, receptor-saturation framework better organizes the published AMG 330 record in acute myeloid leukemia (AML). Receptor-saturation arguments motivate two qualitative expectations: at sufficiently high CD33 densities, EC $_{50}^{}$ should show little systematic dependence on additional antigen density, and response should become increasingly constrained by effector availability and functional state. We tested these expectations using two statistical reanalyses of published AMG 330 data-log-linear regression of EC $_{50}^{}$ on CD33 density across 11 AML cell lines, and a comparative effect-size analysis of effector-to-target (E:T) ratio versus CD33 expression in 38 primary AML samples-integrated with clinical exposure-response findings and effector-augmenting rescue studies. Both expectations were supported. CD33 density did not significantly predict EC $_{50}^{}$ across the 3.9-fold range tested ( $p=0.13$ ; bootstrap 95% CI on slope includes zero). In primary samples, E:T ratio dominated sustained lysis by approximately an order of magnitude over CD33 expression (92.4 vs. 8.5 percentage points; CD33 effect $p=0.7$ , not significant). Clinical exposure-response analyses identified baseline E:T ratio and T-cell PD-1 expression as response correlates. A similar target-effector dissociation has been reported in several other TCE programs. The AMG 330 record is more coherently organized by an effector-context framework than by the conventional target-density-and-exposure paradigm. These findings indicate that endogenous-effector assays, paired target-effector biomarkers, and rational effector-support strategies are likely to be clinically informative complements to conventional cell-line potency and dose-escalation approaches in future CD33 TCE development.