Selectively Targeting Leukemic MOLT-4 Cells by MTX-cIBR Conjugate: Mechanism of Action and Cellular Entry

The ICAM-1-derived cIBR peptide selectively binds to the I-domain of LFA-1, a receptor highly expressed on leukemia T cells; thus, the MTX-cIBR conjugate can be used to target methotrexate (MTX) to leukemic T cells and reduce its off-target toxicity. However, the uptake, biological mechanism, and selectivity of MTX-cIBR compared with unconjugated MTX remain unclear. Therefore, this study is aimed at evaluating the uptake, cytotoxicity, selectivity, apoptosis, cell cycle effects, and DHFR-related activity of MTX-cIBR in leukemia T cells compared with unconjugated MTX. MTX-cIBR exhibited cytotoxic activity comparable to MTX in LFA-1-expressing MOLT-4 cells but showed lower toxicity toward LFA-1-negative K562 cells, indicating improved selectivity. MTX uptake occurred through RFC and mFBP transport systems, whereas MTX-cIBR no longer depended on these pathways, suggesting altered cellular uptake after conjugation with cIBR by utilizing the LFA-1 receptor. Both compounds predominantly induced apoptosis with minimal necrotic cell populations. MTX induced S-phase arrest at lower concentrations and G2/M induced arrest at higher concentrations, whereas MTX-cIBR consistently promoted S-phase accumulation. In addition, MTX and MTX-cIBR downregulated the expression of DHFR, FPGS, and TYMS in MOLT-4 cells. Computational analyses further demonstrated that MTX exhibited lower binding free energy (ΔG) and greater binding stability toward DHFR than MTX-cIBR. These findings suggest that MTX-cIBR retains selective cytotoxic activity toward LFA-1-expressing leukemia T cells through altered cellular uptake and exhibits different interaction characteristics with DHFR compared with unconjugated MTX.