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. Plain Language Summary: Current limitations and our approach Cancer immunotherapy drugs like anti-PD-L1 antibodies help the immune system fight cancer, but they often do not stay in tumors long enough to work effectively. To improve this, we developed nanoscale lipid-based carriers, like tiny fat bubbles (called liposomes) densely coated with anti-PD-L1 antibodies. These engineered liposomes bind to tumors more effectively, stay longer, and deliver treatment more precisely than regular antibodies. Key discoveries We discovered that packing more drugs onto each tiny carrier makes them stick to tumors better and last longer. Also, how we deliver these drugs matters too—the carrier-delivered versions bind to tumors more effectively than regular antibody drugs. Importantly, in controlled lab tests with cells, these engineered antibodies work just as powerfully as regular immunotherapy—proving we have created a delivery system without sacrificing cancer-fighting strength. Conclusion and potential benefits This research introduces an innovative approach to cancer immunotherapy delivery using antibody-coated liposomes. By enhancing tumor targeting and retention while minimizing accumulation in healthy tissues, this method could improve treatment effectiveness while reducing side effects. The platform offers three key advantages: patients may require fewer treatment sessions with more durable results, clinicians gain enhanced options for treating melanoma and solid tumors, and researchers can extend this adaptable platform to other cancer therapies. Future directions Our immediate focus will be on validating these findings in animal models and expanding the technology to other immunotherapies. The use of clinically approved components positions this approach for potential rapid translation to human trials, pending successful preclinical results. Keywords: cancer immunotherapy, melanoma, immune checkpoint inhibitors, liposomes, anti-PD-L1 mAb, targeted drug delivery