Week 3, September 2025: Antibody Conjugates: Expanding Beyond ADCs

Antibody conjugates extend the therapeutic potential of monoclonal antibodies by linking them to diverse payloads, broadening their mechanisms of action. While antibody–drug conjugates (ADCs) remain the most clinically validated, new modalities are emerging, including degrader–antibody conjugates (DACs), antibody–bottlebrush conjugates (ABCs), immune-stimulating antibody conjugates (ISACs), theranostic antibody conjugates (ARCs/AICs), and novel peptide- or nanoparticle-based hybrids. Together, these approaches reflect a growing effort to expand therapeutic design space and address challenges that traditional ADCs cannot fully solve.

1. Antibody–Drug Conjugates (ADCs)

ADCs link antibodies with cytotoxic small molecules to selectively deliver potent drugs to target cells. They are the most established antibody conjugate class, with more than 15 approvals across oncology. Advances in linker chemistry, site-specific conjugation, and payload diversity continue to refine efficacy and safety.

2. Antibody–Bottlebrush Conjugates (ABCs)

A paper published this month described ABCs, which are designed to overcome ADC limitations by attaching antibodies to polymer-based bottlebrush prodrugs. This architecture allows much higher and tunable drug-to-antibody ratios while maintaining target binding and uptake. ABCs offer flexibility to incorporate payloads with widely varying potencies and even non-traditional cargos such as imaging agents or photocatalysts. Early preclinical studies show improved efficacy compared with conventional ADCs, suggesting ABCs could broaden the utility of antibody conjugates beyond cytotoxics alone.

3. Immune-Stimulating Antibody Conjugates (ISACs)

Meanwhile, ISACs replace cytotoxic drugs with immune-stimulatory payloads such as TLR or STING agonists. The goal is to convert “cold” tumors into immune-responsive environments by activating local immunity at the tumor site. Clinical trials since 2021 have highlighted significant challenges, including systemic toxicity, cytokine release, and limited efficacy, but design refinements in payload selection and localized delivery approaches are ongoing. ISACs remain an early-stage but conceptually powerful class, especially as combination partners for immunotherapies.

4. Theranostic and Radioimmunoconjugates (ARCs/AICs)

Theranostic antibody conjugates combine therapeutic and diagnostic potential. A recent review describes how two main forms are advancing:

• ARCs (Antibody–Radionuclide Conjugates): Antibodies labeled with isotopes for targeted radiotherapy or imaging, already clinically validated in hematologic cancers.

• AICs (Antibody–Isotope Conjugates): Newer variants that expand isotope choices (e.g., Lutetium-177, Actinium-225) and linker strategies for more precise radiotherapeutic delivery. ASP Isotopes Inc. and IsoBio, Inc. announced series seed funding of IsoBio, Inc. to advance AICs for cancer treatment earlier this year.

Together, ARCs and AICs extend antibody conjugates into nuclear medicine, with promise for both solid tumors and theranostic applications.

5. Antibody–Toxin Conjugates (ATCs / Immunotoxins)

Antibody–toxin conjugates couple antibodies with potent protein toxins, such as diphtheria toxin, Pseudomonas exotoxin, or ricin A chain. Unlike ADCs that carry small-molecule drugs, ATCs deliver enzymatic toxins capable of irreversibly inactivating essential cellular processes, often by blocking protein synthesis. These constructs have been investigated for decades in oncology and infectious disease. ATCs highlight the principle that antibodies can serve as delivery vehicles for diverse classes of biologically active payloads, not only small molecules. A recent article published in Nature Portfolio describes a promising ATC targeting CD47 linked to the bacterial toxin listeriolysin O for cancer immunotherapy.