Receptor Panel - Case Study 2

The Problem	The Solution
Example Study	Availability

The problem

A test compound was known to cause significant induction of a number of cytochrome P450 enzymes in both rodents and man. The client wished to dissect the pathways of induction in vivo, in order to interpret clinical induction data, and assess the likelihood of drug-drug interactions and other related toxicities in specific patient populations.

The challenge was to dissect the key causes of drug induction in an in vivo system.

The solution

The Pregnane X Receptor (PXR) and the Constitutive Androstane Receptor (CAR) are closely related nuclear hormone receptors that play a critical and interlinked role in Cytochrome P450 induction.

A study performed by CXR Biosciences in the Pxr-Car Knockout Mouse model was used to elucidate the contribution of the PXR/CAR nuclear receptor system to the observed enzyme induction. PK and enzyme induction were compared in the knockout and wild type control mice after repeat dosing with test compound. If required, single knockouts could be used to elucidate the individual contribution of each nuclear receptor, and organ samples could be taken to assess any PXR- or CAR-mediated toxicity.

The Pxr-Car Knockout Mouse can be used to mechanistically dissect causes of induction and define the contribution of PXR and CAR to downstream effects on PK and toxicity.

Study performed by CXR Biosciences:

Study Design

Groups	Pxr-Car Knockout Mouse - test compound Pxr-Car Knockout Mouse - control C57BL/6 wild type - test compound C57BL/6 wild type - control
Optional Groups	Pxr Knockout Mouse - test compound Pxr Knockout Mouse - control Car Knockout Mouse - test compound Car Knockout Mouse - control
Group size	2-4 males & 2-4 females per group
Study duration	7 days
Dosing	Oral daily dose
Endpoints	PK samples taken at 2 and 8 hours post-dose on days 1 and 7, terminal PK samples RNA extracted from liver for determination of mRNA levels Microsomes prepared from liver for enzyme activity analysis Optional organ samples taken for toxicological analysis

• High levels of Cytochrome P450 induction (as measured by microsomal activity) were seen in wild type mice by day 7, and this resulted in rapid metabolism of the test compound and therefore minimal plasma levels at day 7.
• In contrast, significantly lower levels of enzyme induction were seen in the Pxr-Car Knockout mice. Due to this, at day 7, accumulation of test compound was observed in these mice:
  
  

  	Liver Microsomal Activity (pmol test item/30 min/mg)		[Test item Plasma] (ng/mL)
  Test Compound treated male groups	Pxr-Car Knockout Mouse	Wild Type	Pxr-Car Knockout Mouse	Wild Type
  2 hours	NA	NA	18,374 ± 1,273	18,594 ± 1,962
  Terminal (7 days)	192 ± 42	1,543 ± 58	29,179 ± 12,777	0 ± 0

  Note: These results are based on actual data obtained in a confidential study carried out for a customer.

Upon further examination, this accumulation was not seen in Pxr Knockout animals, suggesting CAR plays a key role in driving enzyme induction in vivo, and it is CAR-mediated induction that will likely be the key underlying cause of any induction-related drug-drug interactions in man.

These results demonstrate that the Pxr-Car Knockout Mouse model can be used to mechanistically dissect the causes of induction, and define the contribution of PXR and CAR to downstream effects on PK and toxicity in an in vivo system.

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.