# Presentation: 808460

## Tamoxifen, Endoxifen, and 

- CYP2D6

- Sally Usdin Yasuda, MS, PharmD
- Senior Reviewer, Division of Clinical Pharmacology 1
- Office of Clinical Pharmacology
- Center for Drug Evaluation and Research
- Food and Drug Administration

- Clinical Pharmacology Subcommittee October 18, 2006
- Rockville, Maryland

## Outline

- Exposure to tamoxifen and metabolites after administration of tamoxifen
- Pharmacology of tamoxifen, endoxifen, and other metabolites
- CYP2D6-mediated metabolism of tamoxifen and formation of endoxifen *in vitro*
- Role of CYP2D6 in formation of endoxifen *in vivo*
- CYP2D6 genotype
- Strong Inhibitors of CYP2D6

## Case Report

- 45 year-old woman presented with intense, intolerable hot flashes after being prescribed 20 mg of tamoxifen per day for a week.
- Placed on 10 mg per day of paroxetine for depression
- Resolution of hot flashes within a week
- Hot flashes resumed when taken off paroxetine

- Personal Communication from David Flockhart

##  Classic Understanding of Tamoxifen Pharmacology

**Estradiol (E2)**

**E2-ER**

**ERE**

**Notes:**
The antiestrogenic effects are thought to be related to the ability of tamoxifen or its metabolites to compete with estrogen for binding sites in target tissues such as breast. In cytosols derived from human breast adenocarcinomas, tamoxifen competes with estradiol for estrogen receptor protein.

## Hypothesis: CYP2D6 inhibition interferes with formation of 4-OH-tamoxifen

- 12 women with hx of breast cancer receiving tamoxifen (20 mg/day) as adjuvant treatment for at least 4 weeks before starting the study
- History of troublesome hot flashes for which treatment with a non-hormonal agent was considered to be appropriate
- Blood samples collected before and after 4 weeks of co-administration of tamoxifen with 10 mg/day paroxetine

## Paroxetine Administration Decreased the Concentration of One Metabolite

- Before

- After

**4OHTam**

- X

**    ****X**

- As modified from Stearns et al.
- JNCI 2003: 95:1758-1764.

- X

**    ****X**

- Separated, purified, identified and synthesized metabolite X:

- 4-hydroxy-N-desmethyl tamoxifen
- (Endoxifen)

**Concentration  of **
**endoxifen is ~ **
**10x > 4-OH-tamoxifen**

## Paroxetine and CYP2D6 genotype change the plasma concentrations of endoxifen 

- (but not of tamoxifen, N-desmethyl, or 4-hydroxy-tamoxifen )

- from Stearns et al.  JNCI 2003: 95:1758-1764, as communicated by D. Flockhart

**Notes:**
Endoxifen decreased by 64% in WT and by 24%  in women with variant allele.  Baslien eendoxifen concentrations were lower in women with variant allele than in WT.  Paroxetine had no statistically singiicant effect on enedoxifen levels in women with variant allele (*4, 6, 8). 

## What is the relative pharmacological activity of tamoxifen and its metabolites?

- Tamoxifen and N-desmethyl-tamoxifen have similar pharmacologic activity1
- 4-OH-tamoxifen is 30-100 times more potent as antiestrogen than tamoxifen2
- Endoxifen is equipotent to 4-OH-tamoxifen (and has 5-10x higher concentration)

- 1Nolvadex (Tamoxifen Citrate) label.  9-27-2005. Wilmington, Delaware,
- AstraZeneca Pharmaceuticals LP.
- 2Coezy E, Borgna JL, and Rochefort H. Cancer Res. 1982;4:317-23.

## 4-OH-Tamoxifen and 4-OH-N-Des-Tamoxifen have equal affinities for  Estrogen Receptor α

- 4-OH-Tamoxifen and 4-OH-N-Des-Tamoxifen have equal affinities for  Estrogen Receptor α

- Johnson MD, et al. Breast Cancer Research and Treatment, 2004; 85:151-159.

## Endoxifen and 4-OH-Tamoxifen are Equipotent as Inhibitors of Estrogen Stimulated Cell Proliferation in MCF7 Cells

- Stearns V et al.  JNCI 2003; 95:1758-64

## In vitro studies suggest N-desmethyl-TAM accounts for majority of primary TAM oxidation

*In vitro* studies suggest N-desmethyl-TAM accounts for majority of primary TAM oxidation

- Desta et al. JPET 2004; 310:1062-1075

**Notes:**
Pathways for N-desmethyl and 4-hydroxy determined by correlation experiments in characterized HLMs and  inhibition with isoform specific chemical inhibitors and in a panel of expressed p450s.  These findings allowed for the path forward for studies of secondary metabolism.  

## CYP2D6 is the principal route of metabolism to Endoxifen

- Desta et al. JPET 2004; 310:1062-1075 and personal communication from D. Flockhart

- CYP2D6 is the principal route of metabolism to Endoxifen

**Notes:**
Next, these investigators evaluated the metabolism to endoxifen and other secondary metabolites uing N-desmethyll and 4-hydroxy as substrates.  They have shown that endoxifen is formed from both N-desmthyl and 4-hydroxy.  A series of experiments including correlation studies in a panel of HLMs showed significant correlation between endoxifen formation from N-desmethyl and CYP2D6 activity (upper right).  In chemical inhibition experiments in HLMs, Q almost completely inhibited formation of endoxifen from N-desmethyl, and other inhibitors showed much less effect, supporting CYP2D6.  Finally, in expressed P450s

## modified from Stearns et al.  JNCI 2003: 95:1758-1764

- CYP3A4

## Pharmacogenetics of CYP2D6 (debriosquine metabolic ratio)

- Alvan G, Bertilsson L, Dahl M.-L., Ingleman-Sundber M, and Sjöqvist F.
- Drug Metab Sip 2001; 29:580-585

## CYP2D6 Genotype, CYP2D6 Inhibitors, and Tamoxifen Exposure

- 80 pre- and postmenopausal women with newly diagnosed breast cancer starting tamoxifen (20 mg/day) as adjuvant therapy
- Blood samples for determination of tamoxifen and metabolites in plasma
- Genotype functional and variant alleles of:
- CYP3A5 (*1, *3)
- CYP2D6 (*1, *3, *4, *5, *6)
- CYP2C9 (*1, *2, *3)
- SULT1A1 (*1,*2)
- No statistically significant associations of candidate genotypes with tamoxifen or metabolite exposure except for CYP2D6

- Jin Y et al.  *JNCI* 2005; 97:30-39

## CYP2D6 Vt/Vt genotype has decreased endoxifen exposure

## CYP2D6 inhibitors decrease endoxifen exposure

*Strong*: paroxetine, fluoxetine
*Weak*: amiodarone, sertraline
*Other*: metoclopramide, citalopram

- Jin Y et al.  *JNCI* 2005; 97:30-39

## Commonly used antidepressants and endoxifen concentrations

- Jin Y et al.  *JNCI* 2005; 97:30-39

- CYP2D6 inhibitor:
- 	*Strong*: Paroxetine
- 	*Weak*: Sertraline

## Effect of CYP2D6 genotype on endoxifen/NDM ratio and endoxifen plasma concentration (n=158)

- *, P < 0.001

- *, P < 0.01

- Borges S et al.  Clin Pharmacol Ther 2006; 80:61-74.

## Conclusions

- Endoxifen is an active metabolite of tamoxifen, present in patients at 5-10 x greater concentration than 4-OH-tamoxifen
*In vitro *studies demonstrate the primary role of CYP2D6 in the formation of endoxifen.*  *
- Potent inhibitors of CYP2D6 reduce endoxifen concentrations in patients taking tamoxifen
- CYP2D6 genotype correlates with endoxifen concentrations in patients taking tamoxifen

## Acknowledgements

- Larry Lesko, PhD
- Shiew Mei Huang, PhD
- NAM Atiqur Rahman, PhD
- Felix Frueh, PhD
- Myong-Jin Kim, PharmD
- Todd Skaar, PhD
- David Flockhart, MD, PhD