Overview of antibody-drug conjugates nonclinical and clinical toxicities and related contributing factors

Abstract

An antibody-drug conjugate (ADC), consisting of an antibody, a chemical linker, and a payload, can selectively deliver a highly cytotoxic payload to cancer cells and minimize the systemic toxicity of the payload by harnessing the selectivity of the antibody. However, many ADCs have unacceptable toxic effects due to the expression characteristics of target antigens, structural characteristics of antibodies, linker stability and payload properties, which have slowed their development progress. In this review, we describe the effects of the structure of each component of an ADC molecule on its toxicity, explore and discuss the toxicity profiles of various ADCs in the main body tissues/organs by using nonclinical and clinical data obtained from marketed ADCs, aiming to provide a reference for the development of novel ADC molecules and to facilitate related nonclinical and clinical studies.

Keywords: ADC; clinical study; non-clinical study; structural characteristics; toxicity.

Figure 1

Molecular structural characteristics likely contributing to ADC toxicities. A classic ADC consists of three components: antibody, chemical linker and potent cytotoxic payload. The antibody of an ADC recognizes the target antigen on the cell surface, then the ADC is internalized into the cells through endocytosis. Within the lysosome the payload is released and exerts its cytotoxic effect. The properties of each component may contribute to the toxicity of the ADC, and the payload is one of the most important contributors. The combination of individual components determines the toxicity profile of a certain ADC molecule.

Figure 2

IC₅₀ ranges of commonly used payloads. Doxorubicin has a relatively high IC₅₀ and results in poor ADC efficacy. It has been used in the development of early ADCs. Microtubule inhibitors with lower IC₅₀ (MMAE, MMAF, DM1, DM4) have been successfully used in ADCs. The DNA damaging agent PBD has a high cytotoxicity (the lowest IC₅₀) and the ADCs with PBD as a payload exhibit very obvious toxicity. At present, topoisomerase 1 inhibitors (SN38, DXd) are a popular payload for ADCs, and their IC₅₀ values are slightly higher than those of microtubule inhibitors [13].

Figure 3

Characteristics of developed site-specific conjugation techniques for ADCs. Site-specific conjugation techniques have been developed for ADCs, including the introduction of engineered reactive cysteine residues, unnatural amino acids, glycosyl, disulfide bond rebridging, enzymatic conjugation, and pClick technology, etc. The engineered cysteine residue conjugation employs genetic engineering technology to insert cysteine residues at specific positions and then couples sulfhydryl group on cysteine with the payload to synthesize site-specific ADCs [57]. The resultant product has high homogeneity, low toxicity and tunable stability. Disulfide bond rebridging refers to opening the inter-chain disulfide bond of the antibody to obtain a reactive cysteine. Disulfide bond rebridging usually leads to a narrower DAR distribution, while misbridging is a potential disadvantage [57, 62]. Unnatural amino acids can be constructed into recombinant proteins to obtain residual side chains that react chemically with small molecules of drugs to achieve site-specific antibody-drug conjugation [58]. However, the antibodies with unnatural amino acids may induce immunogenicity and the hydrophobicity of unnatural amino acids increases the risk of antibody aggregation [57]. Enzymatic coupling is frequently represented by glutamine transferase, generating a bond between an amine group on linker or a drug and an engineered glutamine residue on an antibody to form an ADC molecule. The extraneous amino acids may induce immunogenicity [57]. Glycogroups are convenient targets for site-specific conjugation. A pClick technology is recently developed for site-specific conjugation of ADC in a more convenient and efficient way [57].