Antibody drug conjugates (ADCs) employ the exquisite specificity of tumour-specific monoclonal antibodies (mAb) for the targeted delivery of highly potent cytotoxic drugs to the tumour site. The chemistry of the linker, which connects the drug to the mAb, determines how and when the drug is released from the mAb. This, as well as the chemistry of the drug, can dictate whether the drug can diffuse into surrounding cells, resulting in ‘bystander killing’. Initially, any bystander killing mechanism of action of an ADC was understood to involve an essential sequence of steps beginning with surface antigen targeting, internalisation, intracellular linker cleavage, drug release, and diffusion of drug away from the targeted cell. However, recent studies indicate that, depending on the linker and drug combination, this mechanism may not be essential and ADCs can be cleaved extracellularly or via other mechanisms. In this minireview, we will examine the role of bystander killing by ADCs and explore the emerging evidence of how this can occur independently of internalisation.

Aluminium (Al) compounds are used as adjuvants in human and veterinary prophylactic vaccines due to their improved tolerability compared to other adjuvants. These Al-based adjuvants form microparticles (MPs) of heterogeneous sizes ranging from ~0.5 to 10 µm and generally induce type 2 (Th2)-biased immune responses. However, recent literature indicates that moving from micron dimension particles toward the nanoscale can modify the adjuvanticity of Al towards type 1 (Th1) responses, which can potentially be exploited for the development of vaccines for which Th1 immunity is crucial. Specifically, in the context of cancer treatments, Al nanoparticles (Al-NPs) can induce a more balanced (Th1/Th2), robust, and durable immune response associated with an increased number of cytotoxic T cells compared to Al-MPs, which are more favourable for stimulating an oncolytic response. In this review, we compare the adjuvant properties of Al-NPs to those of Al-MPs in the context of infectious disease vaccines and cancer immunotherapy and provide perspectives for future research.

Solid tumors are inherently difficult to treat because of large regions of hypoxia and are often chemotherapy- or radiotherapy-resistant. It seems that cancer stem cells reside in hypoxic and adjacent necrotic tumor areas. Therefore, new treatments that are highly selective for tumors and can eradicate cells in both hypoxic and necrotic tumor regions are desirable. Antibody α-radioconjugates couple an α-emitting radionuclide with the specificity of a tumor-targeting monoclonal antibody. The large mass and energy of α-particles result in radiation dose delivery within a smaller area independent of oxygen concentration, thus matching key criteria for killing hypoxic tumor cells. With advances in radionuclide production and chelation chemistry, α-radioconjugate therapy is regaining interest as a cancer therapy. Here, we will review current literature examining radioconjugate therapy specifically targeting necrotic and hypoxic tumor cells and outline how α-radioconjugate therapy could be used to treat tumor regions harboring more resistant cancer cell types. Statement of Significance Tumor-targeting antibodies are excellent vehicles for the delivery of toxic payloads directly to the tumor site. Tumor hypoxia and necrosis promote treatment recurrence, resistance, and metastasis. Targeting these areas with antibody α-radioconjugates would aid in overcoming treatment resistance.

Antibody–drug conjugates (ADC) have revolutionized the field of cancer therapy. ADCs combine the high specificity of tumor-targeting monoclonal antibodies with potent cytotoxic drugs, which cannot be used alone because of their high toxicity. Till date, all ADCs have either targeted cell membrane proteins on tumors or the tumor vasculature and microenvironment. Here, we investigate ADCs of APOMAB (DAB4, or its chimeric derivative, chDAB4), which is a mAb targeting the La/SSB protein, which is only accessible for binding in dying or dead cancer cells. We show that DAB4-labeled dead cells are phagocytosed by macrophages, and that the apoptotic/necrotic areas within lung tumor xenografts are bound by DAB4 and are infiltrated with macrophages. We show that only DAB4–ADCs with a cleavable linker and diffusible drug are effective in two lung cancer models, particularly when given after chemotherapy. These results are consistent with other recent studies showing that direct internalization of ADCs by target cells is not essential for ADC activity because the linker can be cleaved extracellularly or through other mechanisms. Rather than targeting a tumor cell type specific antigen, DAB4–ADCs have the advantage of targeting a common trait in most solid tumors: an excess of post-apoptotic, necrotic cells either adjacent to hypoxic tumor regions or distributed more generally after cytotoxic therapy. Consequently, any antitumor effects are solely the result of bystander killing, either through internalization of the dead, ADC-bound tumor cells by macrophages, or extracellular cleavage of the ADC in the tumor microenvironment.

Purpose: Early detection of tumor treatment responses represents an unmet clinical need with no approved noninvasive methods. DAB4, or its chimeric derivative, chDAB4 (APOMAB®) is an antibody that targets the Lupus associated antigen (La/SSB). La/SSB is over-expressed in malignancy and selectively targeted by chDAB4 in cancer cells dying from DNA-damaging treatment. Therefore, chDAB4 is a unique diagnostic tool that detects dead cancer cells and thus could distinguish between treatment responsive and nonresponsive patients. Procedures: In clinically relevant tumor models, mice bearing subcutaneous xenografts of human ovarian or lung cancer cell lines or intraperitoneal ovarian cancer xenografts were untreated or given chemotherapy followed 24h later by chDAB4 radiolabeled with [⁸⁹Zr]Zr^IV. Tumor responses were monitored using bioluminescence imaging and caliper measurements. [⁸⁹Zr]Zr-chDAB4 uptake in tumor and normal tissues was measured using an Albira SI Positron-Emission Tomography (PET) imager and its biodistribution was measured using a Hidex gamma-counter. Results: Tumor uptake of [⁸⁹Zr]Zr-chDAB4 was detected in untreated mice, and uptake significantly increased in both human lung and ovarian tumors after chemotherapy, but not in normal tissues. Conclusion: Given that tumors, rather than normal tissues, were targeted after chemotherapy, these results support the clinical development of chDAB4 as a radiodiagnostic imaging agent and as a potential predictive marker of treatment response.