Immune‐stimulating antibody conjugate (ISACs) incorporating STING agonists as payloads leverage both the targeting capability of the Fab region and the Fc region‐mediated tumor antigen‐dependent immune activation. Herein, a novel class of ISACs is reported, generated by engineering a quaternary ammonium‐cleavable linker to conjugate diABZI STING agonist 3 (dSA3) with the HER2‐targeting antibody Trastuzumab. The optimized ISAC (TZ‐dSA3‐12) demonstrated high potency, stability, enhanced solubility, and reduced off‐target toxicity. The data showed that TZ‐dSA3‐12 potently activates the STING pathway in the tumor microenvironment through the synergistic action of the Fab and Fc regions of antibodies (activity switch‐on). In contrast, TZ‐dSA3‐12 exhibited ≈75 fold lower activity than dSA3 in normal immune cells, where activation relies solely on the Fc region without Fab‐mediated tumor antigen binding (activity switch‐off). Furthermore, systemic administration of TZ‐dSA3‐12 at a dose (1 mg kg⁻¹) elicited robust and sustained antitumor effect in a manner dependent on the activation of innate immunity and adaptive immunity, including macrophages, dendritic cells (DCs) and CD8⁺ T cells, while minimizing systemic cytokine release. Notably, TZ‐dSA3‐12 also induced immunological memory to combat the growth of rechallenged tumors. This innovative quaternary ammonium‐linked STING agonist‐ISAC represents a promising avenue for the future development of STING‐targeted immunotherapy.

Despite remarkable achievements in antibody‒drug conjugates (ADCs), payloads remain limited. The identification of ADC payloads with novel mechanisms will increase therapeutic options and expand indications. Herein, we describe the use of dihydroorotate dehydrogenase inhibitors (DHODHi) as a novel payload class that provides highly potent ADCs for antitumor and antiviral therapies. Technical innovations include the development of stability-controllable linkers to meet the distinct requirements of acute viral infections and chronic tumor conditions. The antitumor ADC TH-C8H exhibited significant efficacy against gastric cancer in vivo as monotherapy and enhanced efficacy when combined with the ferroptosis inducer RSL3. The antiviral ADC HG-C3 showed broad-spectrum anti-SARS-CoV-2 activity in vitro and in vivo. Our study expands the types of ADC payloads and provides novel insights into the development of innovative broad-spectrum ADCs.

Immune checkpoint inhibitors (ICIs) targeting PD-L1 and CD47 are clinically limited by severe off-target toxicities. To address this issue, immunotherapeutic prodrug strategies have been developed, aimed at preventing antibodies from binding to targets in healthy tissues and thereby reducing systemic toxicity. Existing strategies include prodrug technologies that mask the active sites of antibodies via peptide or polyethylene glycol (PEG) modification—yet these approaches also cause antibodies to lose their targeting ability. Herein, we propose an antibody prodrug strategy (termed FA-PEG-S-Ab) with active targeting capability. By modifying antibodies with folate-PEG-disulfide and PEG-disulfide linkages, we developed two novel prodrugs: FA-PEG-S-Atz (PD-L1-blocking prodrug) and FA-PEG-S-Hu5 (CD47-blocking prodrug). This strategy functions through two key steps: first, folate binding to folate receptor α (FRα)-mediated tumor-specific targeting enables the prodrugs to accumulate specifically in tumor tissues; subsequently, the high concentration of glutathione (GSH) in the tumor microenvironment (TME) specifically cleaves the disulfide bonds, removing the PEG shield, releasing the antibody, and restoring the antibody’s antigen-binding activity. In vitro experiments confirmed that the modified antibody prodrug FA-PEG-S-Hu5 exhibits high affinity for FRα (K_D = 4.02 × 10⁻⁹ M) and effectively masks the antibody’s binding activity (K_D from 1.05 × 10⁻¹¹ M to 2.10 × 10⁻⁸ M). Following activation by GSH in the TME, this masking effect is reversed, and the antibody regains its binding affinity (K_D = 2.14 × 10⁻¹⁰ M). Crucially, FA-PEG-S-Hu5 completely eliminates hemolytic toxicity. This “folate targeting delivery + TME activation” prodrug strategy is expected to provide a new solution for addressing the off-target toxicities of conventional ICIs.

Long-acting drug–Fc conjugate (CD388) has emerged as a new class of antivirals against influenza A and B, yet the mechanisms governing how Fc conjugation reshapes small-molecule function remain undefined. Here we establish a programmable multivalent antiviral conjugates (AVCs) platform by click chemistry that enables flexible combination of valencies and conjugated drugs, allowing systematic interrogation of valency-dependent antiviral mechanisms. Using zanamivir as a model, mono-, di-, and trivalent zanamivir are conjugated with Fc, with trivalent zanamivir-Fc conjugates (AVC-Z3) exhibiting higher activity than lower-valent AVCs. AVC-Z3 induces inter-virion aggregation, thereby coupling blockade of viral entry with restriction of viral release. This cross-linking–mediated dual-stage inhibition represents a mechanistic shift from neuraminidase suppression to physical viral immobilization. Furthermore, trivalent peramivir-Fc conjugate (AVC-P3) is synthesised and exhibits stronger antiviral activity than AVC-Z3 in vitro, with much longer half-life in vivo than AVC-Z3, and CD388 with YTE mutation. The in vivo antivirus efficacy of AVC-P3 surpasses other AVCs, baloxavir, and CD388 in H1N1-, H3N2-, H5N1-, and influenza B-infection mice models. Together, our findings define ligand valency as a determinant of antiviral mechanism.

Antibody‐antibiotic conjugates (AACs) provide a transformative platform for eradicating intracellular methicillin‐resistant Staphylococcus aureus (MRSA). However, current AACs largely follow antibody‐drug conjugates (ADCs) design principles and rely on host‐lysosomal proteases for activation, rendering them ineffective against extracellular (planktonic) bacteria. Furthermore, we demonstrate that high bacterial burden induces host‐lysosomal dysfunction, fundamentally compromising the efficacy of host‐dependent AACs against intracellular reservoirs. To address these limitations, we developed a pathogen‐centric AAC (AZO‐AAC) featuring an azobenzene‐based linker that is triggered by bacterial‐secreted azoreductase. This strategy shifts the activation mechanism from host‐cell machinery to the pathogen itself, enabling antibiotic release independent of host‐cell integrity. Surprisingly, our designed AZO‐AAC achieves nanomolar‐scale eradication of both planktonic and intracellular MRSA, reaching levels below the limit of detection in scenarios where traditional AACs fail. In murine models of peritonitis and septicemia, a single dose (60 mg/kg) of AZO‐AAC resulted in a six‐log reduction in bacterial burden and the preservation of normal tissue architecture. By decoupling activation from host‐cell status, this pathogen‐responsive platform provides a robust strategy for the targeted elimination of complex, multi‐niche MRSA infections.