Due to the complex and diverse structure of ADCs, as well as the low concentration of released small-molecule drugs in biological samples, pharmacokinetics (PK) evaluation presents several challenges. These challenges primarily lie in PK characteristics, PK-PD correlations, target analytes, bioanalytical methodologies, and data interpretation.
PK Study Design and Species Selection for ADCs
Given the specificity of ADCs, it is recommended to conduct single-dose and multiple-dose PK studies in relevant animal species. If rodents lack pharmacological relevance or only non-rodent species are relevant, PK studies may be limited to non-rodent species. Inter-species PK differences can significantly impact the dose-response predictions in toxicology studies, and each ADC's PK evaluation should follow a case-by-case approach based on its characteristics. The ADC formulation, concentration, and administration method should align as closely as possible with those used in toxicology or clinical trials, with confirmation of the administration formulation concentration during studies.
Key ADC PK Analytes and Parameters
Common analytes for ADC PK profiling include:
- Intact ADC and/or total antibody (both conjugated and unconjugated).
- Free small-molecule drug and/or naked antibody (if the ADC competes with antibody binding to the target, this detection also becomes significant).
Core PK studies for ADCs assess the stability of the ADC, plasma concentration-time curve, absorption, distribution, metabolism, and excretion (ADME). For novel small molecules, integrating in vivo and in vitro methodologies to quantify system exposure, plasma protein binding, and excretion characteristics is advisable. Distribution into tumors and normal tissues should also be studied, and comprehensive evaluations of small-molecule drug metabolites, including system exposure, metabolite profiling, and clearance routes, may be required.
ADC Metabolism and Excretion Mechanisms
The primary elimination mechanisms for ADCs include two pathways:
- Decoupling – linker cleavage releases the small-molecule drug while retaining the antibody framework.
- Decomposition – antibody degradation into peptides/amino acids and release of free or linked small-molecule toxins.
In vivo ADC concentration reduction primarily occurs via enzymatic degradation and detachment of small-molecule drugs. The differing clearance rates between the total antibody and ADC reflect the ADC's in vivo stability and indirectly provide insights into linker stability and payload release rates. Additionally, ADC PK studies benefit from immunogenicity assessments to monitor anti-ADC antibody (ATA) responses, which contribute to ADC clearance and may influence therapeutic efficacy and safety.
ADC Distribution and Clinical Relevance
ADC distribution in vivo typically resembles that of the unconjugated antibody, as antibodies largely determine the ADC's structure. Initially confined to the bloodstream, ADCs exhibit limited tissue penetration but tend to accumulate in high-blood-flow organs (e.g., liver, kidneys). ADCs' biodistribution depends on target antigen expression and cellular internalization rates. ADC drugs may accumulate in target and non-target tissues, to potential pharmacological or toxicological effects due to the subsequent release of cytotoxic payloads.
Understanding ADC biodistribution is essential for correlating its pharmacology and toxicology profile, supporting optimized ADC design, and guiding clinical safety assessments.
Combined Approaches in ADC PK Research
The complexity of ADC PK necessitates a combined approach using in vivo, in vitro, animal, and human studies. Integrated approaches-such as stability studies across species and target-expressing cell lines-clarify ADC metabolism, aid in metabolite identification, and establish preclinical relevance. For instance, in T-DM1 development, ADC and DM1 metabolism studies in rats provided foundational data for human trials and emphasized the importance of monitoring drug-drug interactions (DDI) when ADCs are co-administered with CYP3A4/5 inhibitors.
