Unlike the development of most small or large molecules, which typically measures only one part or the metabolite for pharmacokinetic analysis, multiple parts of ADCs need to be measured to characterize their PK properties. Therefore, a thorough understanding of the clinical pharmacology of ADCs is critical for selecting a safe and effective dose for patients. Biological Analysis of ADC In order to characterize the PK characteristics of different ADC components, several analytical methods are required: (1) ELISA determines the kinetic curves of the conjugate and total antibody. (2) TFC-MS/MS can quantify the free drugs or metabolites. (3) High-resolution mass spectrometry for in vivo drug antibody ratio (DAR) analysis. In addition, two types of ELISA immunoassays are used to quantitatively measure the ADC analytes: The first type measures total antibodies, namely ADCs with DAR greater than or equal to zero. The second assay measures drug-binding antibodies, defined as ADCs with DAR greater than or equal to 1. Other analytical methods include size exclusion chromatography (SEC) and hydrophobic interaction chromatography (HIC). Cytotoxic Payload ADC cytotoxic payloads should have the following characteristics: (1) Proper lipid solubility. (2) Stable in blood. (3) The target of payloads locates inside the cells. (4) The payload molecules are small in size, lack immunogenicity, and are soluble in water buffers so that they can be easily coupled. Currently, the commonly used cytotoxic effector molecules are microtubule inhibitors (e.g. auristatins, maytansinoids), DNA damaging agents (e.g. calicheamicin, duocarmycins, anthracyclines, pyrrolobenzodiazepine dimers) and DNA transcription inhibitors (amatoxin and quinoline alkaloid (SN-38)). Drug Antibody Ratio (DAR) Studies have shown that ADCs with a high DAR (7 to 14) are cleared faster and have reduced in vivo efficacy compared to those with a DAR lower than 6. DAR values and their effect on the stability and PK also depend on coupling locations and linker sizes. Lysine or cysteine is often modified to produce ADC. Lysine is one of the most commonly used amino acid residues to link a substrate to an antibody. Lysine is usually present on the surface of an antibody and, therefore, easily conjugated. Mylotargs, Kadcylas, and Besponsas all use lysine bio-binding technologies. And other amino acids such as cysteine and tyrosine can also be modified. For instance, ADCs such as Adcetriss, Polivys, Padcevs, Enhertus, Trodelvys, and Blenreps have been synthesized by modifying cysteine with maleimide. Linker The latest generation of linkers is more stable in the systemic circulation, such as peptide and glucuronic acid linkers. The two most common linkers are introduced below: (1) Cleavable linkers are sensitive to the intracellular environment and release free effector molecules and antibodies through catabolism and dissociation. They are usually stable in the blood but are rapidly cleaved to release effector molecules in a low pH and protease-rich lysosome environment. In addition, if the payloads are transmembrane, they can eliminate tumors through the potential bystander effect. (2) Non-cleavable linker is a new generation linker with better plasma stability compared to cleavable linkers. Since non-cleavable linkers can provide greater stability and tolerance, they can reduce off-target toxicity and also provide a larger therapeutic promise. Immunogenicity In 11 clinical trials of eight ADCs, the baseline incidence of anti-drug antibody (ADAs) ranged from 1.4% to 8.1%, and the post-baseline incidence of ADAs ranged from 0-35.8%, values within the range of therapeutic monoclonal antibodies. In general, the incidence of ADA of ADCs is less in patients targeting hematologic cancer than in patients targeting solid cancer; Most ADA is specific to the monoclonal antibody domain of the ADC. Furthermore, in most patients, the hapten-like structure of these ADCs does not generate any more risk of an immune response than therapeutic monoclonal antibodies. ADC Pharmacokinetic Model The application of models can integrate PK, efficacy, and safety data to meet the needs of ADC drug development at different stages, such as target selection, antibody affinity, linker stability, animal-to-human extrapolation, dose selection and adjustment, exposure-response relationships, DDI studies, etc. ADC's kinetic model is complicated due to its multiple clearance pathways (dissociation and catabolism) and complex PK characteristics of various analytes. Future Vision Clinical pharmacology plays a very important role in ADC drug research. An in-depth and comprehensive elucidation of the PK/PD characteristics is crucial to promote the development of ADC drugs with lower toxicity and higher efficiency. Backed by advanced biological analysis techniques, researchers and clinicians will witness wider applications of ADC drugs in the field of cancer treatment sooner or later.

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