Hepatocyte stability assay

Hepatocyte stability assay protocol

Data from Cyprotex's Hepatocyte Stability assay

Figure 1
In vitro in vivo clearance correlation in Cyprotex’s human hepatocyte stability assay. In vitro CL_int data, for 29 literature compounds including acid, base and neutral compounds, was scaled (mL/min/kg Scaled CL_int,ub) and compared to derived values of in vivo intrinsic clearance (Derived in vivo CL_int,ub) back-calculated from observed in vivo clearance using the well-stirred model. Dashed line shows line of regression. Dotted lines show 2-fold of unity line (solid).
A range of literature compounds were assessed in the Cyprotex hepatocyte stability assay (1µM; 0.5 million cells/mL; 60 min incubation; n=3 assays) and intrinsic clearance (CL_int µL/min/10⁶ cells) determined. Predicted in vivo CL_int (mL/min/kg) values were determined using a hepatocellularity of 120 x 10⁶ cells/g liver² and a human liver weight of 25.7 g/liver/kg³, and taking into account fu_inc (fraction unbound in vitro incubation). Observed in vivo CL_int,ub were back-calculated from observed hepatic clearance using the well-stirred model, human liver blood flow of 20.7 ml/min/kg⁴ and fu_b (fraction unbound in blood)⁴.

Figure 2
Comparison of CL_int values generated in 3 separate assays, based on n=1 per assay. Incubations performed using human hepatocytes, 0.5x10⁶ cells/mL, Williams E media, 1 µM substrate concentration.

Questions and answers on hepatocyte stability

Please provide an overview of Cyprotex's hepatocyte stability assay.

The hepatocytes are incubated with the test compound at 37°C. Samples are removed at the appropriate time points into acetonitrile to terminate the reaction. Following centrifugation, internal standard is added and the supernatant is analyzed by LC-MS/MS. The disappearance of test compound is monitored over a 60 minute time period. An example of a typical depletion profile is shown in Figure 3.

The ln peak area ratio (compound peak area/internal standard peak area) is plotted against time and the gradient of the line determined.

What are the benefits of using hepatocytes for drug metabolism studies?

The liver is the main organ of drug metabolism in the body. Hepatocytes contain both phase I and phase II drug metabolizing enzymes, which are present in the intact cell, and provide a valuable in vitro model for predicting in vivo hepatic clearance.

How do you overcome the problems with inter individual variability in humans?

Cyprotex's hepatocyte stability assay uses cells pooled from a minimum of three different individual donors both male and female. This reduces the problems associated with inter individual variation in drug metabolism.

What is the difference between Qualified and Calculated CL_int value?

Calculated CL_int is the actual CL_int value based on the half life obtained in the hepatocyte stability assay. The qualified CL_int value takes into account the lower limit of quantification for the assay when the half life for a test compound exceeds 3* the assay duration. This half life corresponds to approximately 20% compound disappearance at the last assay timepoint. The CL_int values associated with this half life will differ depending on the cell density used in the incubation (see equation above). Where compound disappearance is <20% over the time course then this may be due to analytical variability and further work may be required to ensure the result is reproducible. Where the calculated CL_int is close to the LoQ for the assay then repeating the assay could help to confirm the result. Alternatively depending on the assay conditions used it may be possible to extend the incubation time (max 2hr for suspension hepatocytes) or use the low clearance assay (HµRELhumanPool^TM co-culture 72hr incubation) to achieve greater sensitivity.

How do I interpret the data from the hepatocyte stability assay?

Scaling and in vitro - in vivo extrapolation of clearance

In vivo hepatic clearance can be predicted from in vitro CL_int using the well stirred model⁵. In vitro CL_int values are scaled to an in vivo CL_int value using species-specific hepatocellularity values that relate the number of hepatocytes in a whole liver, to those in the assay and correcting for nonspecific binding in the assay (fu_inc)⁴. This in vivo CL_int can then be used to predict in vivo hepatic clearance by taking into account the free fraction in blood and liver blood flow.

Where:

CL_int = intrinsic clearance (mL/min/kg)

CL_h = hepatic clearance (mL/min/kg)

Q_h = liver blood flow (mL/min/kg)

SF = Species dependent Hepatocyte Scaling factor

fu_b = Fraction unbound in blood (if unknown can assume = 1)

fu_inc = Fraction unbound in the incubation (if unknown can assume = 1)

Application of this approach for a set of marketed compounds is shown in Figure 1. CL_int data was generated in the human hepatocyte stability assay and scaled to a predicted in vivo intrinsic clearance using literature values for fu_b, fu_inc, Q_h and scaling factors. Demonstrating that predicted hepatic clearance in preclinical species correlates well with measured in vivo clearance for the test compound builds confidence that clearance is predominantly hepatic and that human CL_int is predictive of in vivo CL_h.

Rank order and clearance categories

One of the main uses of hepatocytes stability data is that compounds can be ranked in terms of their intrinsic clearance values.

Classification bands can be used to categorise compounds into low, medium or high clearance. These classification bands are calculated from a rearrangement of the well stirred model⁵ detailed in the following equation assuming extraction ratios of 0.3 and 0.7 (the fraction of drug which is eliminated from the blood by an organ) for the low and high boundaries respectively. This can be scaled to intrinsic clearance (µL/min/10⁶ cells) using the relevant liver weights³ and hepatocellularity^2,6. Due to lack of literature information the monkey hepatocellularity was assumed to be 120x10⁶ cell/g liver. For simplicity fu has been assumed to be 1, this represents the worse case scenario and where compounds are more highly plasma protein bound less stringent criteria would apply.

Where:

CL_h = hepatic Clearance (mL/min/kg)

Q_h = liver blood flow (mL/min/kg)

E = Extraction ratio

fu = fraction unbound in plasma (assumed at 1)

CL_int =

Where:

CL_H = E x Q_H

Q_H = liver blood flow (mL/min/kg)⁷

E = Extraction Ratio
CL_H = Hepatic Clearance (mL/min/kg)

fu = fraction unbound in plasma (assumed at 1)

Clearance Category	Intrinsic Clearance (µL/min/106 cells)
	Human	Monkey	Dog	Rat	Mouse
Low	<3.5*	<5.2	<1.9*	<5.1	<3.3*
High	>19.0	>28.3	>10.5	>27.5	>17.8

*Note these cutoffs (assuming fu=1) are at or below the lower limit of quantification for the hepatocyte stability assay

Please contact us to discuss further if you require more information on this topic.

What stage in the drug discovery process does the hepatocyte stability assay tend to be used?

Clients tend to use the microsomal stability assay as a primary screen early in the drug discovery process. The hepatocyte stability assay is then used as a secondary screen for the more favourable compounds from the primary screening.

What may cause differences in the rates of clearance obtained from hepatocyte and microsomal assays?

Compounds that do not readily permeate cell membranes or are subject to efflux may appear to be more stable in hepatocyte incubations than in microsomal incubations. Compounds that undergo Phase II metabolism may appear less stable in hepatocyte compared to microsomal incubations.