Impact of Metabolizing Enzymes on Drug Response of Endocrine Therapy in Breast Cancer

Pilar H Saladores; Jana C Precht; Werner Schroth; Hiltrud Brauch; Matthias Schwab

Disclosures

Expert Rev Mol Diagn. 2013;13(4):349-65. 

In This Article

CYP2D6 Pharmacogenetics

Tamoxifen provides a significant clinical benefit in endocrine therapy; however, approximately 30–50% of the patients relapse or eventually die from the disease.[1,21] The causes for breast cancer relapse are attributed to a variety of factors, one of which may be traced to interindividual variability in DME, particularly CYP2D6 and its role in the formation of endoxifen. CYP2D6 is one of the most studied CYP450 enzymes and has been extensively reviewed.[22–25] The CYP2D6 gene is located on chromosome 22q13.1 and its sequence is characterized by numerous variants. Currently, 105 distinct alleles are known, listed on the CYP allele nomenclature website.[203] Many of these alleles lead to absent, decreased or increased CYP2D6 expression. Comprehensive genotype–phenotype correlation studies in Caucasians using debrisoquine or sparteine as probe-drug clearly indicate the impact of CYP2D6 polymorphisms on CYP2D6 function in vivo[26–28] and result in the discrimination of the poor (PM), intermediate (IM), extensive (EM) and ultrarapid (UM) metabolizer phenotypes (Figure 2). The EM status is determined by the presence of two normal functioning alleles (e.g., *1, *2, *35). The PM status is defined by a lack of CYP2D6 enzyme function and is determined by the presence of two nonfunctional alleles (so-called null alleles), of which more than 20 have been identified.[203] Approximately 99.9% of CYP2D6 PM metabolizers can be predicted by CYP2D6 null alleles either in a homozygous variant or a compound heterozygous genotype.[25,29] Subjects carrying one partially defective CYP2D6 allele (e.g., *41, *9, *10) together with another partially defective or null allele are classified as IM (e.g., *9/*41 or *4/*41), which is a distinct phenotypic CYP2D6 subgroup (Figure 2). Moreover, heterozygous carriers of one defective and one normal CYP2D6 allele (e.g., *1/*4) tend to also have a lower median CYP2D6 enzyme activity compared with wild-type carriers (e.g., *1/*1).[19,30] As this group is not as clearly defined as EMs and PMs but represents a rather heterogenous group, many of the current studies combine heterozygous carriers together with classically defined IMs into one decreased function phenotype. The UM phenotype can be inferred from CYP2D6 gene duplications/amplifications of fully functional alleles (e.g., *1xN/*2) resulting in excessive enzymatic function.[31,32] Since not all CYP2D6 duplications comprise functional genes, the prediction of the correct phenotype may be difficult.[33,34] Only up to 30% of the UM phenotype can be attributed to gene amplification.[26,27,35,36]

Figure 2.

Sparteine oxidation phenotypes and distribution in a German population. MRS: Urinary metabolic ratio for sparteine.Adapted with permission from [25].

In the European population, approximately 5–10% are PMs, 10–15% are IMs and approximately 5–10% are UMs (Figure 2). Furthermore, ethnic variations in reduced or null function alleles must also be considered as several studies have reported on the rarity of CYP2D6 PM alleles in individuals of African and Asian descent.[37] Table 1 highlights the major CYP2D6 variants with reported allele frequencies categorized by the main population groups. CYP2D6*17 is the most common reduced function allele in African populations while the IM *10 allele is more prevalent in Asian populations ranging from a frequency of 38 to 70%.[37–39]

Several CYP2D6 genotyping strategies are employed to determine pharmacogenetic DME variations from various DNA sources. Table 2 summarizes the currently available methods. There are two FDA-approved methods used in the molecular diagnostic setting: the AmpliChip® CYP450 test system (Roche, CA, USA) and the xTAG® assay CYP2D6 kit (Luminex). An advantage of the AmpliChip system is the high-density allele coverage of 33 variant CYP2D6 alleles including allele-specific gene duplication. The DNA source is critical for accurate genotyping. Because formalin-fixed paraffin-embedded (FFPE) material contains highly fragmented DNA, this source is not suitable for the long-range PCR-based AmpliChip CYP450 test.[40] Other multiplexing technologies include the MassARRAY® system (Sequenom) that incorporates MALDI-TOF MS, which allows for the use of archived materials, such as FFPE, as a source of DNA.[41] As an extension of validated CYP2D6 TaqMan® genotyping assays (Applied Biosystems), the Fluidigm platform may process several individual TaqMan assays simultaneously in a medium-throughput setting.[42]

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