Multivalent Atezolizumab-Liposome Conjugates as Active Immunotherapeutic Platforms for Enhanced PD-L1 Blockade in Melanoma.

Purpose: Anti-programmed death ligand-1 monoclonal antibody (anti-PD-L1 mAb) therapy has demonstrated notable clinical success. However, efficacy is often limited by transient interactions at the tumor-immune interface, limiting sustained immune activation. To address this, we hypothesized that liposomal delivery of anti-PD-L1 mAbs could enhance therapeutic efficacy. Nanosystems may increase binding avidity and prolong cell-surface retention, and enable multivalent antibody presentation through conjugation of multiple antibodies onto a single liposome. Methods: In this study, we engineered multivalent anti-PD-L1 liposomes (anti-PD-L1 LPs) by conjugating FDA-approved anti-PD-L1 mAbs (atezolizumab) at varying densities onto a clinically relevant liposomal formulation. The biophysical properties, cellular interactions, and therapeutic potential of anti-PD-L1 LPs were evaluated using two melanoma cell line models (BLM and MZ2Mel43), employing multiple analytical techniques. Results: Our results demonstrated that high-density anti-PD-L1 LPs exhibited superior binding avidity, prolonged membrane retention, and reduced non-specific cellular interaction with PD-L1-expressing cells compared to both low-density and non-targeted LPs. Additionally, in 3D tumor spheroid models, high-density anti-PD-L1 LPs showed deeper penetration, suggesting improved tissue accessibility compared with low-density formulations. Moreover, compared to free antibodies, anti-PD-L1 LPs displayed a higher association rate (k_on) and a significantly lower dissociation rate (k_off), resulting in an overall improved (lower) dissociation constant (K_D). Functional assays confirmed that anti-PD-L1 LPs achieved superior PD-L1 blockade compared to free antibodies. Importantly, in co-cultures of human peripheral blood mononuclear cells and tumor cells, anti-PD-L1 LPs maintained immunomodulatory activity comparable to free anti-PD-L1 antibodies. Conclusion: This study highlights the critical role of ligand density in enhancing binding strength, tumor retention, and tissue penetration of the anti-PD-L1 LP system. Our nanosystem offers a promising improvement over conventional anti-PD-L1 mAbs, supporting the broader application of this modular liposomal platform to other therapeutic antibodies in melanoma and other solid tumors.