Receptor Panel - Case Study 3

The Problem	The Solution
Example Study	Availability

The problem

During phase II clinical trials, an oncology candidate demonstrated induction of a specific cytochrome P450. The induction event was not predicted in rodent studies or human hepatocyte studies. Unfortunately the induced enzyme was largely responsible for the metabolism of a co-administered drug, and the candidate drug was necessarily abandoned.

Therefore, in order to prevent such situations in the future, the customer wished to access a preclinical model that was more predictive of cytochrome P450 induction in man.

The solution

The Pregnane X Receptor (PXR) and the Constitutive Androstane Receptor (CAR) are closely related nuclear hormone receptors that play a critical role in cytochrome P450 induction. However, current preclinical systems for predicting PXR/CAR mediated induction have a number of drawbacks:

• Although human hepatocytes can be used to predict induction, inter-experimental variability is high due to the impossibility of accessing identical fresh tissue samples over time, and it is believed that some inductions (especially those mediated by CAR) may not be supported by the typically achieved 72-hour hepatocyte incubation.
• Rodent studies can also be used to predict induction. However, there are profound species differences in PXR/CAR ligand sensitivity.

A study in the Humanized PXR-CAR Mouse model can therefore be used to more accurately predict human drug induction in vivo. If desired, the single humanized and knockouts can be used to elucidate the contribution of each nuclear receptor.

When predicting human drug induction, the Humanized PXR-CAR Mouse model represents an alternative to human hepatocytes that is more reproducible and potentially more predictive, especially of CAR-mediated induction.

Study performed by CXR Biosciences:

Study Design

Groups	Humanized PXR-CAR Mouse - test compound Humanized PXR-CAR Mouse - control Pxr-Car Knockout Mouse - test compound Pxr-Car Knockout Mouse - control C57BL/6 wild type - test compound C57BL/6 wild type - control
Optional Groups	Humanized PXR-CAR Mouse - test compound & co-administered drug Pxr-Car Knockout Mouse - test compound & co-administered drug C57BL/6 wild type - test compound & co-administered drug
Group size	4 per group
Study duration	7 days
Dosing	Daily dosing, route as appropriate
Endpoints	PK levels on days 2, 5, & 7*

^* (using CXR's microsampling procedure)

Results

Compared to wild type mice, the Humanized PXR-CAR mice demonstrate increased sensitivity to potent human ligands of PXR and CAR (e.g. rifampicin and CITCO, respectively), and decreased sensitivity to potent murine ligands, such as 1,4-bis [2-(3,5-dichloropyridyloxy)] benzene (TCPOBOP) and dexamethasone (Scheer et al. 2008), thus demonstrating their utility as a more predictive model of human drug induction in vivo.

Induction was assessed both by cytochrome P450 enzyme activity and Western blotting (shown below):

These models can also be used to demonstrate the in vivo effects of test compounds on the PK of drugs that are likely to be co-administered in the clinic, therefore predicting undesirable drug-drug interactions.

For instance, in the example below the Test Compound is a known potent human inducer. After 5 days treatment with the Test Compound, due to higher levels of cytochrome P450 induction, PK levels of the co-administered drug midazolam are significantly reduced in the Humanized CAR mice:

Midazolam (0.9 mg/kg/IP) PK in wild type (A) and Humanized CAR (B) mice before (Day 1) and after (Day 5) treatment with Test Compound. Values are Mean ± SD (n=4).

These results demonstrate that the Humanized PXR-CAR Mouse model represents a novel tool for predicting human drug induction events and their in vivo consequence, before test compounds enter the clinic.

Availability

The first Receptor and Cytochrome P450 models are available now, with additional models to follow.

Off the shelf mice: Mice may be purchased directly from Taconic. To purchase transADMET mice, please visit the relevant model webpages:

• Pxr-Car Knockout Mouse (Taconic model 8222 )
• Humanized PXR-CAR Mouse (Taconic model 8223 )
• Humanized CYP3A4/3A7 Mouse (Taconic model 8842)
• Cyp3a (7-gene) Knockout Mouse (Taconic model 8841)
• Humanized Liver & Gut CYP3A4 Mouse (Taconic model 9049)
• Humanized Liver CYP3A4 Mouse (Taconic model 9048)
• Humanized Gut CYP3A4 Mouse (Taconic model 9047)
• Cyp3a (8-gene) Knockout Mouse (Taconic model 9011, available as a Taconic Transgenic Model™)

Contract services: CXR Biosciences is the co-exclusive supplier of contract research services using transADMET mice. CXR also offers consultancy and advice to our customers. For more information on contract research services at CXR using the transADMET mice, contact CXR at transADMET@cxrbiosciences.com.