CD74-Targeted Cathepsin-Inhibitor Antibody–Drug Conjugate Triggers Apoptosis in DLBCL

Transcriptomic analyses of public datasets (TCGA and GTEx) revealed that both <i>CD74</i> and <i>Cathepsin L</i> (CTSL) are significantly overexpressed in diffuse large B-cell lymphoma (DLBCL) compared to normal tissues, and that their expression levels are highly correlated to each other (Spearman R = 0.64, <i>p</i> = 3 × 10^-46). Kaplan-Meier analysis showed that elevated expression of both genes is associated with reduced overall survival (OS), defining a high-risk <i>CD74</i>+/<i>CTSL</i>+ DLBCL subgroup. This is the first study demonstrating coordinated overexpression of CD74 and CTSL and proposing their dual targeting via antibody-drug conjugates (ADCs) to improve outcomes in relapsed or refractory DLBCL. Cysteine cathepsins, a family of proteases, are upregulated in many cancers, facilitating tumor invasion and metastasis. Cathepsins are overexpressed and play key roles in DLBCL progression. GB111-NH₂, a potent broad-spectrum cathepsin inhibitor, significantly reduced cathepsin activity in lymphoma cell lines and patient samples. GB111-NH₂ treatment increased apoptosis and caspase-3 activation in DLBCL patient cells and chronic lymphocytic leukemia (CLL) mononuclear cells. Here, we developed a modified cathepsin inhibitor, M-GB, containing a maleimide linker for site-specific antibody conjugation. While M-GB alone has poor cell permeability, when conjugated to an antibody, it forms an ADC (M-GB-ADC) that selectively induces lymphoma cell death. One M-GB-ADC demonstrated high specificity for CD74-expressing lymphoma cells while exhibiting minimal toxicity to non-target cells in vitro. Our findings highlight the potential of another M-GB-ADC as a targeted therapy for overcoming rituximab resistance and treatment failure in DLBCL. This strategy enhances therapeutic efficacy and represents a preclinical proof-of-concept treatment option by directing a cathepsin-inhibitor payload specifically to malignant B cells.