Intracellular Delivery of Antibody-Based Therapeutics for Targeted Oncogenic c-Myc Degradation in Mammalian Cells and Mice.

Targeting the intrinsically disordered oncoprotein c-Myc remains challenging due to its lack of druggable pockets, hindering small-molecule inhibitor development for decades. Antibody-based strategies utilizing TRIM21-mediated targeted protein degradation (TPD), such as TRIM-Away, offer a promising alternative for endogenous c-Myc degradation but are limited by ineffective intracellular antibody delivery and uneven endogenous TRIM21 levels in various cancer cells. To overcome these, herein we disclose an acid-responsive nanoplatform using amorphous carbonated calcium phosphate nanoparticles (ACCP NPs) for effective intracellular co-delivery of TRIM21 and antibodies. Under mild biomimetic conditions, TRIM21-antibody complexes were effectively mineralized and encapsulated in the formed nanoparticles. Following endocytic uptake, these acid-sensitive ACCP NPs disassembled in endo/lysosomes, facilitating subsequent endo/lysosomal escape and cytosolic cargo release, leading to eventual intracellular TPD. In vitro, mineralized TRIM21-antibody complexes from ACCP NPs effectively degraded various intracellular targets, particularly c-Myc, across diverse cell types, including those that are TRIM21-deficient. Specifically, m-T21-c-Myc Ab nanoparticle degraded c-Myc via the ubiquitin-proteasome system and concurrently reduced MAX levels, synergistically inhibiting c-Myc transcriptional activity. In vivo, m-T21-c-Myc Ab significantly suppressed tumor growth without major organ toxicity. This work establishes ACCP NP-based TPD as a versatile and efficient platform for TRIM21/antibody-mediated degradation and a promising strategy for targeting other "undruggable" proteins.