Breast cancer is considered to be the most common cancer among women worldwide. The majority of breast cancer cases (almost 80%) are classified as hormone-dependent cancer, since estrogen, acting via estrogen receptor alpha (ER-α) or estrogen receptor beta (ER-β), is the major inducer of the development and growth of the tumor. These are also called ER-positive breast cancers. The remaining cases are not induced by estrogen and are classified as hormone-independent, or ER-negative, cancers. Since the growth of hormone-dependent cancer cells can be down-regulated by the oppositely active hormones, several endocrine therapies that limit the actions of estrogen (through blocking its production or its receptors) have been developed over the past years. These endocrine therapies have played an important part in treating and improving the outcomes of women with all stages of the disease . Selective estrogen receptor modulators (SERMs) have also been studied for their anti-cancer activity.
Selective estrogen receptor modulators (SERMs) are a class of drugs that act on the estrogen receptor (ER); a characteristic that distinguishes these substances from pure ER agonists and antagonists as their action is different in various tissues, thereby granting the possibility to selectively inhibit or stimulate estrogen-like action in various tissues .
Tamoxifen, which is a SERM, is important for the treatment and prevention of estrogen receptor (ER) positive breast cancer commonly in premenopausal women. It has been shown to decrease disease recurrence and mortality rates by as much as 50% and 30% respectively. It has been also used as a prophylactic treatment for patients who are at high risk of developing breast cancer. Post-menopausal breast cancer patients are commonly treated nowadays with aromatase inhibitors (AIs) for 5 years, alone or combined with tamoxifen for a 3-5 year period. Tamoxifen monotherapy in postmenopausal women with breast cancer may be used for 10 years if the side effects from AIs are too bothersome .
Besides acting as SERMs, it has recently been found that some of tamoxifen's metabolites (as norendoxifen) also act as aromatase inhibitors in vitro. Aromatase converts steroids (e.g., testosterone to estradiol), the inhibition of which severely decreases the amount of available estrogen in the body .
The most common side effects of Tamoxifen:
* Hot flashes, the most common side effect of tamoxifen, affect up to 80% of women.
* Increased risk of endometrial hyperplasia and polyps
* Increased risk for cataract.
* Increased risk for thromboembolic events as well as clinical depression .
Pharmacokinetics of tamoxifen and its clinical implications:
Tamoxifen is a prodrug that is metabolized by several cytochrome P450 enzymes. It is a relatively weak antiestrogen in comparison to its active metabolites, particularly endoxifen. Two parallel pathways bioactivate tamoxifen to endoxifen (4-OH-N-desmethyltamoxifen) through several overlapping cytochrome P450 (CYP) enzymes; primarily CYP3A4/5 and CYP2D6.
For many years, 4-hydroxy-tamoxifen has believed to be primarily responsible for the clinical activity; however, the CYP2D6 metabolites 4-hydroxy-tamoxifen and endoxifen (4-OH-N-desmethyl-tamoxifen) have equal affinity for the estrogen receptor. Because serum concentrations of endoxifen are 6 to 12 times higher than 4-hydroxy-tamoxifen in patients receiving long-term tamoxifen therapy, many think endoxifen is the most significant tamoxifen metabolite.
The rate limiting step in tamoxifen metabolism is catalyzed by the highly polymorphic CYP2D6 enzyme so that CYP2D6 genotype can be translated into predicted metabolic activity phenotypes: poor metabolizer (PM), intermediate metabolizer (IM), extensive metabolizer (EM), ultrarapid metabolizer (UM), which are strongly predictive of endoxifen concentration during tamoxifen treatment.
There is substantial variation in CYP2D6 genotypes among different populations. CYP2D6\*1 is the wild-type allele and is associated with normal enzyme activity and the normal "extensive metabolizer" phenotype. The CYP2D6 alleles \*2, \*33, and \*35 are also considered to have near-normal activity. Other alleles include variants that produce a non-functioning enzyme (e.g., \*3, \*4, \*5, and \*6) or an enzyme with reduced activity (e.g., \*10, \*17, and \*41). There are large inter-ethnic differences in the frequency of these alleles, with \*3, \*4, \*5, \*6, and \*41 being more common in the Caucasian population, \*17 more common in Africans, and \*10 more common in Asians. Also it was found that \*1 and \*4 alleles are more common in Egyptians.
Patients with low-activity CYP2D6 phenotypes have substantially lower endoxifen steady-state concentrations . It is unclear whether patients with genotypes that confer low CYP2D6 activity have inferior efficacy from tamoxifen treatment, but if so, preemptive genotyping to guide tamoxifen dose selection could be a viable strategy to improve treatment effectiveness.
Despite the proven efficacy of tamoxifen, some women experience cancer recurrence during or after treatment. Therapeutic failure may be caused by tumor resistance to antiestrogen therapy or inadequate bioactivation of tamoxifen to its active metabolite, endoxifen.
Some studies have reported that patients with low endoxifen concentrations (below 5.9 ng/ml) have increased the risk of inferior tamoxifen efficacy with consequent cancer recurrence compared with its concentration in intermediate and rapid or ultra-rapid metabolizers taking the same dose. This can open the way for application of therapeutic drug monitoring of tamoxifen and its metabolite endoxifen for possible dose escalation especially a fixed dose of the drug is usually used which is 20 mg/day.
The issue of relying on genotyping profile of CYP2D6 and its variants or use of TDM of tamoxifen and its metabolite endoxifen or the combination of both parameters to correlate with clinical response, adverse effect and dose modification of tamoxifen is still under investigation.
