# Presentation: 376150

## CYP2B6 and Drug InteractionsFDA Advisory Committee for Pharmaceutical SciencesClinical Pharmacology SubcommitteeNovember 18th, 2003.

- Zeruesenay Desta PhD
- David A Flockhart MD, PhD
- Indiana University School of Medicine

## Outline

- CYP2B6 expression
- CYP2B6 substrates
- In vitro
- In vivo?
- CYP2B6 inhibitors
- CYP inducers
- Relevant in vivo data

## CYP2B6 expression: Late 1990s

**Low level of protein expression**
**Minor component of the total hepatic CYPs (<1% of total P450 in the liver)**
**Thought to play a relatively minor role in  human drug metabolism**

## CYP2B6 Expression: Initial Studies

| Detection (%)  | Pmol/mg protein (range) | References |
| --- | --- | --- |
| 13/13 (100%)**  |   | Yamano et al 1989 |
| 12/12 (100%)*  |   | Forrester et al., 1992 |
| 12/14 (86%), Thai  |   | Kirby et al 1993 |
| 12/50 (24%)  | 1.41.5 (0-7) | Mimura et al 1993 |
| 9/30 (30%) (Japanese)  | 0.25  (0-2) | Shimada et al 1994 |
| 26/30 (87%), Caucasian  | 1.0 (0-2.5) | Shimada et al 1994 |
| 2/10 (20%)  |   | Baker et al 1995 |
| ?/40, Japanese  | 0.140.62 | Guengerich et al 1995 |
| ?/32, Caucasian  | 1.41.8 | Guengerich et al 1995 |
| ?/16  | 3 (0-17) | Imaoka et al 1996 |
| 8/30 (27%)  |   | Boobis et al 1996 |
| High level in Thai |   | Kim  et al 1997 |
| 7/10 (70%)  | 19.3623.9) | Tateishi et al 1997 |
| 2/10 (20%)  | 2.655.94 | Tateishi et al 1997 |
| 6/30 (20%)  |   | Edwards et al 1998 |

## CYP2B6 Expression: Recent Studies

| Detection (%)  | Pmol/mg protein (range) | Variability | References |
| --- | --- | --- | --- |
| 17/17   (100) | 1521 (0.5-70) | 247-fold | Code et al. 1997 |
| 24/26   (92) | 10.99.5 (0-28) |   | Yang et al., 1998 |
| 19/19   (100) | 9.7717.6 (0.7-71.1) | 107-fold | Ekins et al 1998 |
| 28/28   (100) | 24.9 18.2 (2-82) | 41-fold | Stresser et al, 1999 |
| 43/48   (90) | 1.7 (0.4-8, n=43 livers) | 20 | Gervot et al 1999 |
| 15/16   (94) |   | 100-fold (n=15) | Faucette et al 2000 |
| 12/12    (100) | 44.6 (1.5-148.4) | 99-fold | Hesse et al 2000 |
| 136/136 (100) | 16.916.6 (0.5-95.5) | 191-fold | Lang et al. 2001 |
|  |  |  |  |

## CYP2B6 Expression: 2003

**New mono- and polyclonal antibodies of higher sensitivity and specificity for CYP2B6 protein immuno-quantification:**
**Greater frequency of detection (almost in all human livers tested)**
**Higher protein quantities:**
**Average: ~6% of the total liver CYP450 content**
**Absolute maximum amounts: up to 25 to 43.5%**

## The CYP2B6 gene

**CYP2B subfamily:**
**CYP2B6 gene (functional)**
- CYP2B7 gene (nonfunctional)
- CYP2B7-like pseudogene)
**Mapped to chromosome 19 [19q12 and 19q13.2]**
**Contains 9 exons - encodes a protein with 491 amino acids**

## CYP2B6: substrate drugs (in vitro)

**HIV medications: **
**Nevirapine **
**Efavirenz**
**DPC 963**
**Anticancer drugs:**
**Cyclophosphamide**
**Ifosfamide**
**Tamoxifen**
**ThioTEPA**
**CNS acting drugs: **
**S-mephobarbital**
**Propofol **
**Bupropion**
**Ketamine**
**Selegiline  **
**Methadone**** **
**S-mephenytoin**

**Other drugs: **
**RP 73401**
**A****rtemisinin**
**Benzphetamine**
**Cinnarizine**
**Diazepam**
**Clopidogrel**
**Lidocaine N-deethylation**
**Ticlopidine**
**Dextromethorphan **
**R-mianserin**
**Benzyloxyresorufin**
**Antipyrine**
**4-Chloromethyl-7-ethoxycoumarin**
**3-Cyano-7-ethoxycoumarin**
**7-ethoxycoumarin**
**7-Ethoxy-4-trifluoromethylcoumarin**
**Imipramine**
**methoxyflurane**** **
**Sertraline**

## CYP2B6: Xenobiotics

**Styrene**
**6-aminochrysene**
**Dibenzo[a,h]anthracene**
**Aflatoxin B**
**Nicotine**
**Methylenedioxymethamphetamine (ecstasy)**
**2-dibromoethane**
**benzo[*****a*****]pyrene**
**Phenanthrene**
**6-aminochrysene**
**Methoxychlor**
**1,3-butadiene**
**2-chloro-1,1-difluoroethene**** **

**azinphos-methyl (AZIN)**
**chlorpyrifos (CPF)**
**diazinon (DIA)**
**parathion (PAR), **

**CYP2B6: endogenous substances**
**Testosterone**
**Estrone**
**17beta-estadiol**

## Human metabolism of efavirenz (Ward et al., J Pharmacol Exp Ther 2003)

## CYP2B6 catalyses the metabolism of efavirienz (1μM) 

## Effect of rifampin on efavirenz in healthy volunteers

## CYP2B6 Catalyses metabolism of an efavirenz analog:

*Drug Metab Dispos. 2003 Jan;31(1):122-32. *

**CYP2B6 Catalyses metabolism of an efavirenz analog:**
**DPC 963**

## CYP2B6 is a low affinity catalyst of S-mephenytoin metabolism to Nirvanol

**Heyn et al Drug Metab Dispos 1996;24:948-954**

**Km = 564****μM**

## N-Demethylation of S-mephenytoin (200μM) by CYP2C9 and CYP2B6

**Ko, Desta and Flockhart. Drug Metab Dispos 1998;26(8):775-778**

## Human Metabolism of Mephenytoin

## R-mephenytoin as CYP2B6 substrate probe?

## Inhibitors of CYP2B6

**Inhibitors:**
- Antidepressants (e.g. paroxetine and sertraline)
- Antiretrovirals (e.g. nelfinavir and ritonavir)
- Ticlopidine and clopidogrel
- ThioTEPA

## ThioTEPA is a specific cytochrome P450  inhibitor in vitro 

**IC50 ********5****μM**

## Inhibition of CYP2B6 by thioTEPA

**Ki=4.8 ± 0.3 μM (HLMs****)**

**Ki=6.2 ± 0.7 μM (CYP2B6)**

**ThioTEPA therapeutic concentration: 1.1-18.6 μM**

## Cyclophosphamide activation

## Effect of thioTEPA on the PK of CPA

**(Huitema et al. Cancer Chemother Pharmacol 2000;46:119-127) **

## Inducers of CYP2B6

- Rifampin
- Hyperforin
- Phenobarbital
- Ritonavir
- Phenytoin
- Carbamazepine
- HMG-CoA reductase inhibitors
- Nevirapine
- Efavirenz
- Clotrimazole
- Artemisinin

## Conclusions

- CYP2B6 is a significant contributor to hepatic CYP expression
- The number of substrate drugs for CYP2B6 is growing rapidly
- Efavirenz and buproprion are specific in vitro probes for CYP2B6 activity
- ThioTEPA is a specific in vitro inhibitor of CYP2B6
- No valuable, specific inhibitors of CYP2B6 in vivo have been demonstrated to date
- Efavirenz is a potentially valuable in vivo probe for CYP2B6 activity

## Effect of rifampin on efavirenz in healthy volunteers