Review Khot, Sharma & Shah Key term
PK–PD modeling and simulation: Practice of characterizing PK and PD of drug molecules using mathematical models and employing these models as a tool to simulate the PK–PD of drugs under previously untested scenarios.
in the biological matrix of interest. Depending on the stage of ADC development and the questions that need to be answered, the investigation of PK can range from in vitro to the clinic.
In vitro stability testing of ADC Before investing significant amount of resources behind an ADC molecule, it is important to make sure that the ADC is stable for in vivo evaluation. This is inves- tigated in vitro by either incubating the ADC in buffer solutions (determined based on the linker chemistry) or plasma of animals and human [4,5]. Following the ADC incubation, buffer/plasma samples are collected at pre- determined time points to measure the concentrations of various analytes. Usually bioanalytical scientists mea- sure total antibody (TmAb) and conjugated antibody (CmAb) concentrations using ELISA, and unconju- gated drug concentrations using LC–MS. Changes in drug–antibody ratio (DAR) is also quantified sometimes using tandem MS techniques [4–6]. An illustrative profile of all the analytes from in vitro stability experiment is provided in Figure 1A. Owing to the different size of the analytes involved, molar concentration units are always preferred when dealing with ADCs. In order to assure complete mass balance of the system, all the profiles are simultaneously characterized using PK models similar to the one shown in Figure 1B. This model assumes that the released drug is stable in the in vitro system, and each molecule of ADC that degrades leads to the production of DAR equivalents of drug molecules. The model is fit- ted to the data to estimate parameters like the antibody (mAb) degradation rate (kdeg
of drug dissociation from the ADC (kdis The estimates of kdeg
in Figure 1B) and the rate in Figure 1B) [6].
provide an idea about the inher-
ent stability of the mAb in plasma (or buffer), and these estimates can be further used while characterizing the in vivo PK of ADC (as done for trastuzumab-emtan-
them based on their stability [4,5]. Of note, the in vitro estimates of kdis
can be compared across several ADCs to triage can also be compared with the in vivo
for the same ADC (discussed later) to establish in vitro–in vivo correlation for ADC stability. estimates of kdis
Cellular PK of ADC Before evaluating an ADC in vivo, often in vitro investigations are performed to evaluate how cells
1634 Bioanalysis (2015) 7(13)
sine [T-DM1] in [6]). Moreover, the estimates of kdeg and kdis
(e.g., antigen expressing cancer cells, antigen negative normal cells) process the ADC. For these experiments, cells are incubated with different concentrations of ADCs at 4°C to allow the binding, and then cells are washed and incubated at 37°C to evaluate cellular PK of ADC and the released drug. At predetermined times following 37°C incubation, media and cell pellet is collected for measuring the concentrations of ADC (Cmab) and the released drug [7]. An illustrative pro- file of all the analytes from a cellular PK experiment conducted on target expressing cancer cell is provided in Figure 1C. All these profiles are simultaneously char- acterized using PK models similar to the one shown in Figure 1D [8]. Since there is not enough data to estimate all the parameters of the model shown in Figure 1D, in order to enable estimation of the key model param- eters by fitting the model to the data, the bioanalyti- cal scientists are requested for quantitation of a few chemomeasures (drug-related bioanalytical measures) and biomeasures (biological system related bioana- lytical measures). These typically include: affinity of ADC toward the cell surface receptor (surface plasmon resonance or cell-based assay), internalization rate of the ADC or the antigen (flow cytometry), number of receptors per cell (flow cytometry) and affinity of the drug toward the intracellular target (surface plasmon resonance). The investment to quantify chemomea- sures and biomeasures early in ADC development is worthwhile since they can be later employed for the development of in vivo PK models (discussed later). Model fitting of cellular level PK data for ADCs is valuable for estimating key parameters like the efflux rate of the released drug out of the cells, which can be a main determinant for maintaining intracellular drug concentrations. These fittings can also help in esti- mating the intracellular degradation rate of the ADC, which can be slower than ADC internalization rate and hence the rate limiting step for the intracellular drug delivery. Intracellular PK models have also proven to be valuable for estimating the intracellular concentra- tion of drug target, and demonstrating that binding of drug to intracellular target can be a major determinant for retaining the drug inside the cell [8,9]. Thus, cellular level ADC PK models can be very useful for extract- ing valuable information from the bioanalytical data, which can help in evaluating the performance of mAb, linker and drug in biological system.
Plasma/serum PK of ADC In order to evaluate in vivo ADME characteristics of ADCs, and to establish a reliable concentration– response relationship, plasma PK of ADC is routinely evaluated in preclinical settings and in the clinic. Plasma is preferred because it is an easily accessible
future science group
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