Most targets of ADC have unique requirements, including homogenous expression of ADC-accessible tumor surface antigens that internalize well and trafficked to lysosomes. Ideally the ADC targets should have minimal expression of targets in normal tissues of critical importance. These targets could be on tumor membranes or tumor associated cells and in the tumor microenvironment. In the past, the target identification technologies have relied on differential expression- immunohistochemistry (IHC) based target identification. But given the complexity, labor-intensive- time consuming aspects of IHC analysis, there is an urgent need to employ new technologies to rapidly discover new targets for ADCs. New technologies have included (not limited to) tumor genomics and LC-MS/MS proteomics of the membrane proteins, TCGA RNA Seq, and functional genomic mRNA profiling (FGmRNA-profiling). Specially interesting is the FGmRNA-profiling of tumors and normal tissues for new targets.
Multi-antigen antibodies for ADCs offer unique advantage in specificity for the differential targeting of tumor antigen versus normal tissue antigens. Bispecific ADCs can not only improve TI, they offer better targeting of tumor antigens. Medimmune’s Her2- bispecific- biparatopic ADC (MEDI-4276) allowed a better targeting of Her2- axis, including improved internalization, and lysosomal trafficking. Due-mab antibody technology of Medimmune that utilizes mono-valent interaction with the single target (non-tumor tissues) and bivalent interaction for paired targets (tumor tissues) is uniquely positioned to advance the bispecific ADC landscape. Additionally, there are more than 50 different formats for ADCs and many of these formats are adaptable to ADCs. Currently a large number of bispecific ADCs using these platform are being developed.
Stable linkers are critical for the target specific safe delivery of highly potent cytotoxic warheads to tumors. Additionally stable linkers allow the maintenance of ADCs’ extracellular stability (systemic and normal tissue compartments) and intracellular release (e.g., tumors) in the acid environment, the reducing environment and accessibility to deconjugation/release pathways, including lysosomal proteases. Currently, the linkers landscape include hydrazones (acid environment based release), dual peptide (lysosomal protease (s) dependent release) , disulfide (disulfide exchange using typically glutathione) and thioether (proteolytic degradation similar to proteins). Typically there are two types of linkers, cleavable an non-cleavable, though, most ADC construct use cleavable linkers. Acid labile hydrazone linkers take the advantage of highly acidic compartment of tumors. Protease cleavable dipeptide linkers such as Val-Ala and Val-Cit take advantage of the unique lysosomal proteases highly dominated in many tumors while disulfide linkers take the advantage of highly abundant reducing hypoxia rich environment of some tumors.
Non-cleavable stable linkers are designed to degrade ADCs naturally similar to all naked antibodies by the proteolytic degradation. The non-cleavable linkers require intracellular uptake and are not degraded by blood enzymes allowing very high degree of extracellular stability. The thioether technology developed by Genentech was used in generation of non-cleavable linker for ado-trastuzumab emtansine (Kadcyla™; Genentech/Roche). Although there are great stability and safety advantages for non-cleavable linkers for maintaining high systemic stability, they often modify cytotoxic warheads to largely polar entities during proteolytic degradation and reduce the bystander potency of warheads. In some cases, polar warheads generated during proteolytic degradation of non-cleavable linkers enabled ADCs get stuck inside the cell (e,g., lysosomes) and are unable to make their site of action (e.g., DNA in nucleus).
ADCs are different from chemotherapeutic cytotoxic drugs (small molecule based cellular uptake mainly through diffusion) because they need tumor target binding and internalization for the intracellular delivery of the cytotoxic warheads. Given very limited intracellular distribution of ADCs ( typically less 1% of the injected dose) even with high target expression on tumors, it is imperative that translational approaches include those patients whose tumors have high expression of targeted antigens, at least during the expansion stage of Phase I trials at or near the MTD dose. If the candidate ADC shows good activity or insufficient activity in tumors with high and homogeneous expression of antigens, this may serve as a major Go or No Go decision criteria. Given the limitations of invasive tumor biopsies, non-invasive PET bioimaging approaches are increasingly being to identify patients who have high expression of tumor antigens. Additionally, unique sensitivity of certain DNA repair deficient tumors to certain warheads (e.g., PBD) is being further assessed. Translational approaches may also include defining tumor resistance pathways and the pairing of the warheads with the targets. If a tumor has a very high expression of topoisomerase II (TOP-II) but resistance to tubulin inhibition or DNA damaging agents, it may be prudent to use TOP-II inhibitor based ADC. Biomarkers such as circulating tumor DNA, tumor derived exosomes are increasingly being used