Int J App Pharm, Vol 16, Issue 3, 2024, 37-42Reviewl Article

THE APPLICATION OF BIOANALYTICAL METHOD OF TAMOXIFEN AND ITS ACTIVE METABOLITES FOR THERAPEUTIC DRUG MONITORING IN BREAST CANCER PATIENTS: A REVIEW

MUHAMMAD IKHSAN1, YAHDIANA HARAHAP1,2*

1Faculty of Military Pharmacy, Republic of Indonesia Defense University, Bogor, West Java, Indonesia. 2Faculty of Pharmacy, Universitas Indonesia, Depok, West Java, Indonesia
*Corresponding author: Yahdiana Harahap; *Email: yahdiana03@yahoo.com

Received: 23 Nov 2023, Revised and Accepted: 27 Mar 2024

ABSTRACT

Breast cancer is the most common cancer around the world and in Indonesia. The most widely used agent for breast cancer treatment is tamoxifen, with a fixed dose of 20 mg per day. Tamoxifen is metabolized by cytochrome P450 3A4 (CYP3A4) and 2D6 (CYP2D6) to endoxifen and 4-hydroxytamoxifen, which have 30-to 100-fold more potent antiestrogenic activity than tamoxifen. High variations of CYP3A4 and CYP2D6 genes can lead to interpatient variability in its metabolites concentration. The dose can be increased to 40 or 60 mg per day based on individual needs. Therapeutic drug monitoring (TDM) is required to measure the concentration of tamoxifen and its metabolites to decide the individualized dose. The measurement of drug levels should use a sensitive, selective, accurate, precise, and reliable bioanalytical method. Various bioanalytical methods have been developed in several matrices: urine, scalp hair, serum, plasma, dried blood spot (DBS), and volumetric absorptive microsampling (VAMS) samples, with different sample preparations, and frequently using liquid chromatography-tandem mass spectrometry (LC-MS/MS). The bioanalytical method of tamoxifen and its metabolites in the DBS sample was more suitable in the TDM application due to the low invasive sampling technique, more stable sample, and rapid sample preparation. Therefore, it is more time-and cost-efficient than the other methods.

Keywords: Bioanalytical method, Breast cancer, Endoxifen, Tamoxifen, Therapeutic drug monitoring

© 2024 The Authors. Published by Innovare Academic Sciences Pvt Ltd. This is an open access article under the CC BY license (https://creativecommons.org/licenses/by/4.0/)
DOI: https://dx.doi.org/10.22159/ijap.2024v16i3.49957 Journal homepage: https://innovareacademics.in/journals/index.php/ijap

INTRODUCTION

Breast cancer is the most prevalent cancer around the world and in Indonesia. According to the Global Cancer Observatory (Globocan) 2020 database, there were 2.26 million new breast cancer cases and 684.99 thousand breast cancer-related deaths around the world. In Indonesia, there were 65.85 thousand new cases of breast cancer and 22.43 thousand people died of breast cancer. During the last five years, the number of new breast cancer cases was 7.79 million cases around the world and 201.14 thousand cases in Indonesia [1, 2].

Breast cancer is caused by many factors, which are genetic and non-genetic factors such as obesity, physical inactivity, alcohol consumption, short breastfeeding periods, unhealthy diet, and pollution exposure [3]. Surgery, radiation, chemotherapy, immunotherapy, endocrine therapy, and targeted therapy are currently used to treat breast cancer [3-5]. The best kind of treatment for breast cancer is endocrine therapy since estrogen binding to the estrogen receptor (ER) on cancer cells accounts for about 70% of occurrences. Endocrine therapy directly blocks the actions of estrogen at ER and inhibits estrogen-dependent growth of cancer cells [6]. Tamoxifen has been the most widely used in the treatment of endocrine therapy for breast cancer in the last 40 y [6-8]. Cytochrome P450 3A4 (CYP3A4) and 2D6 (CYP2D6) convert tamoxifen to endoxifen and 4-hydroxytamoxifen. Both metabolites have been shown to have antiestrogenic activity that is 30-to 100-fold higher than tamoxifen itself. Highly variable CYP3A4 and CYP2D6 genes can lead to interpatient heterogeneity in metabolite concentrations [6].

Tamoxifen is currently administered to all patients as a fixed dose of 20 mg tamoxifen per day for the treatment of breast cancer. The fixed-dose frequently results in a lower concentration of tamoxifen and its metabolites than the minimum effective concentration. The fixed dose also may potentially result in a higher concentration than the threshold of toxic level. The more rational therapy should use individualized doses to achieve a high degree of accuracy in the drug concentration. The most direct approach to individualizing tamoxifen dose is therapeutic drug monitoring (TDM) which quantifies the levels of tamoxifen and its metabolites. Depending on the measured drug concentrations, the dose can be adjusted for each patient [6, 9]. The recommended threshold concentration of endoxifen is 5.97 ng/ml in serum [10, 11] and 3.3 ng/ml in dried blood spot samples [12]. The dose can be increased to 40 or 60 mg per day when the measured level is lower than the threshold level [13].

TDM of tamoxifen requires a sensitive, selective, accurate, precise, and reliable bioanalytical method. Various bioanalytical methods of tamoxifen and its metabolites have been developed. Liquid chromatography-tandem mass spectrometry (LC-MS/MS) is the most often used analytical instrument due to being more sensitive and selective in determining very low analyte concentrations than other instruments such as gas chromatography-tandem mass spectrometry (GC-MS) and high-performance liquid chromatography with ultraviolet detector (HPLC-UV) [8, 10-12, 14-19, 20-24]. Many biological matrices have been used, such as urine [19], scalp hair [23], serum [20], plasma [8, 16, 17, 24], dried blood spot (DBS) [10-12, 18], and volumetric absorptive microsampling (VAMS) [25]s. The sample preparations were also different: protein precipitation, liquid-liquid extraction, and solid-phase extraction [8, 10-12, 14-19, 20-24].

This article reviews bioanalytical methods of tamoxifen and its metabolites and their application for TDM in breast cancer patients to reach individualized medicine. The literature was collected from online databases such as Google Scholar, PubMed, ScienceDirect, and Directory of Open Access Journals. The data used in the review were gathered from English-language articles published between 2008 and 2023.

Breast cancer

Breast cancer is a malignant tumor that develops as a result of DNA damage and gene mutation in breast cells such as ductal, lobular, and both. The development and occurrence of breast cancer are caused by a variety of factors, including genetic and non-genetic factors (table 1) [3, 26-30].

Table 1: The primary factors responsible for the occurrence and development of breast cancer

Factors Detail References
Genetic The woman born to mother with breast cancer [3, 29]
Reproductive The woman with premature menarche, late menopause, nulliparity, short breastfeeding period, high testosterone level [3, 26]
Lifestyle-related Obesity, smoking, alcohol consumption, physical inactivity, unhealthy diet [3, 27-29]
Environmental Exposure to pollution and exogenous estrogen [3, 29]

Breast cancer patients can be treated locally and systemically. The local treatments include surgery and radiotherapy. The systemic therapies are chemotherapy, immunotherapy, endocrine therapy, and targeted therapy [3-5, 31]. Cancer cell proliferation is associated with abnormal expression of estrogen receptor (ER) and human epidermal growth factor 2 (HER2) [32]. Approximately 70% of breast cancer cases were caused by abnormal expression of ER. Therefore, endocrine therapy is the most widely used treatment for breast cancer [33-35].

Tamoxifen

Tamoxifen is a triphenylethylene derivative that acts as a selective estrogen receptor modulator (SERM). It antagonizes the effect of estrogen on breast tissue and reproductive organs. During the last 40 y, tamoxifen has been the most frequently used endocrine agent in breast cancer treatment because it can reduce recurrence and mortality rates after years of treatment [33-35]. On the other hand, tamoxifen has many side effects, especially above toxic levels, such as diarrhea, anorexia, nausea, vomiting [36], stroke, uterine cancer, and cataract [33].

Tamoxifen pharmacokinetics

Tamoxifen is rapidly metabolized into a variety of metabolites in the liver (fig. 1). Almost 90% of tamoxifen is demethylated by CYP3A4 to N-desmethyltamoxifen during phase I metabolism, and then CYP2D6 oxidizes it to 4-hydroxy-N-desmethyltamoxifen (endoxifen). The minor route of tamoxifen metabolism is the hydroxylation reaction of tamoxifen to 4-hydroxytamoxifen. The active metabolites of tamoxifen are endoxifen and 4-hydroxytamoxifen, which have antiestrogenic action that is 30-100 times greater than tamoxifen itself [6-8, 33, 34, 37]. The structure of tamoxifen and its active metabolites can be seen in fig. 2.

Fig. 1: Tamoxifen metabolic pathway
Orange: compounds which have antiestrogenic activity

Fig. 2: The structure of tamoxifen and its active metabolites (4-hydroxytamoxifen and endoxifen)

The phase II reactions of tamoxifen are the sulfation and glucuronidation reaction catalyzed by sulfotransferase (SULT) and uridine 5'-diphospho-glucuronosyltransferase (UGT). SULT and UGT play a vital role in tamoxifen and its metabolite elimination [37].

Tamoxifen pharmacogenetics

Many studies reported that there are more than 100 polymorphisms of CYP2D6. CYP3A4 has fewer polymorphisms compared to CYP2D6. SULT and UGT, the vital enzymes in tamoxifen elimination, also have polymorphisms. These polymorphisms are associated with variation of catalytic activity in the tamoxifen metabolism that significantly correlates with interpatient variability in the concentration of endoxifen and 4-hydroxy tamoxifen. The therapy using tamoxifen should be optimized by identifying genetic variation in CYP2D6, CYP3A4, SULT, and UGT [37-40].

Therapeutic Drug Monitoring (TDM)

In the last few years, many studies reported the relationship between pharmacokinetics and pharmacogenetics, introducing the concept of “individualized”, “personalized” or “precision” medicine. The high genetic variability of drug-metabolizing enzymes causes various therapeutic effects among patients. This concept suggested therapeutic drug monitoring (TDM) which measures the drug concentrations in biological matrices to ensure that the concentrations are in the therapeutic window. Measured concentration can be used to decide the individualized dose to reach the minimum effective concentration (MEC) and prevent achieving the minimum toxic concentration (MTC). Therefore, the treatments would be more accurate and precise for each patient [9, 41-44].

TDM of tamoxifen

TDM of tamoxifen can be carried out by measuring the level of its active metabolites. The recommended threshold concentration of endoxifen is 5.97 ng/ml in serum [10, 11] and 3.3 ng/ml in dried blood spot samples [12]. The initial dose of tamoxifen is 20 mg per day for 8 w. The dose can be increased to 40 or 60 mg per day when the measured level is lower than the threshold level [13]. Therapeutic drug monitoring is proposed to select the right dose of tamoxifen for each breast cancer patient [6, 37].

Biological matrices for TDM

There are many biological matrices used for TDM. Currently, the gold standard matrices are serum and plasma. The other matrices, such as urine, saliva, cerebrospinal fluid, hair, peripheral blood, tissue biopsies, sweat, tears, and breast milk, also can be alternative matrices for TDM. The considerations in the matrix selection are the physicochemical properties of the analyte. Moreover, minimizing the stress level of breast cancer patients is important to be considered. These two critical requirements can be fulfilled by the microsampling technique using peripheral blood as the matrices. The microsampling technique needs very low sample volumes (<50 µl) which is less invasive than the serum or plasma sampling technique [43, 45].

Bioanalytical method of tamoxifen and its metabolites

Urine

The bioanalytical method of tamoxifen in urine has been developed by Rathi et al., 2016 [19]. Tamoxifen is rapidly metabolized to 3-hydroxy-4-methoxy tamoxifen, which can be found in urine. The urine sample was collected in a container, then added azide sodium as a preservative. The sample was extracted with a solid-phase extraction procedure. 50 µl of internal standard (17a-methyltestosterone, chlorotestosterone acetate, d3-testosterone, and d3-epitestosterone and the sample were poured into the XAD-2 column. The first step was washing the XAD-2 column with distilled water, followed by elution with 0.5 ml methanol repeated 5 times. The remaining methanol in the screw test tube was appropriately squeezed out of the XAD-2 column. The hydrolysis procedure was conducted at 60 °C for one hour. The solution was cooled to room temperature and its pH was adjusted to range between 9 and 10. 5 ml of tertiary-butyl methyl ether was added into the mixture, shaken for 10 min, and centrifugated at 3000 rpm for 5 min. The organic layer was separated, added 1 mg of Na2SO4, then evaporated under nitrogen at 60 °C.

The second step was the derivatization of the dried sample by adding 50 µl of derivatizing agent (N-methyl-N-(trimethylsilyl) trifluoroacetamide (MSTFA), iodotrimethylsilane (iodo-TMS), and dithioerythritol (DTE) 1000:2:2). The sample was derivatized at 60 °C for 30 min. 20 µl of aliquot was injected into GC-MS.

The GC-MS system was performed using an ultra-1-dimethylpolymer polysiloxane fused silica capillary (17 m x 0.22 mm, 0.11 µm), operated in SIM and scan modes. Helium was flowing at a rate of 0.2 ml/min. The oven temperature was set to rise from 180 °C for one minute, hold, to 229 °C at a rate of 3 °C per minute, then from 229 °C to 300 °C at a rate of 40 °C per minute. The total time for running was 23.1 min.

The bioanalytical method was applied to analyze the urine samples of one healthy subject taking a single dose of tamoxifen (20 mg). The range of the tamoxifen metabolite concentrations in the urine samples was 3–90 ng/ml.

Scalp hair

The bioanalytical method of tamoxifen in scalp hair has been developed by Drooger et al., 2015 [23]. The scalp hair sample was extracted with a liquid-liquid extraction procedure. The samples were collected from around 150-300 hair strands, and the root ends were then cut into pieces measuring around 1 mm long. Tamoxifen-d5, N-desmethyl-tamoxifen-d5, 4-hydroxy-tamoxifen-d5, endoxifen-d5 as the internal standard (50 µl), methanol (950 µl), and hair samples (10-30 mg) were combined, then agitated for 24 h at room temperature while being shielded from light. The organic phase was separated and then evaporated under nitrogen at 60 °C. The dried sample was reconstituted with sodium carbonate buffer pH 8.8 (200 µl) and a combination of hexane/2-propanol (95:5) (1 ml). The mixture was vortexed and centrifugated at 18,000 x g for ten minutes. The organic layer (800 µl) was dried up under nitrogen at 60 °C. The dried sample was reconstituted in the mixture of water/acetonitrile/formic acid, 60:40:0.1 v/v/v (100 µl), and then centrifugated at 4,000 x g for five minutes. 5 µl of aliquot was injected into the ultra-performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS).

The UPLC-MS/MS system was performed using an Acquity UPLC® BEH C18 column (100 mm x 2.1 mm, 1.7 µm). The mixture of ammonium format 0.2 mmol and acetonitrile, both containing 0.1% formic acid, was used as the mobile phase. The flow rate was 0.3 ml/min with a linear gradient. The run time was 10 min.

The bioanalytical method was used in the determination of tamoxifen and its active metabolites from 8 breast cancer women taking 20 mg of tamoxifen every day for a period of 6 mo to 5 y. Tamoxifen, endoxifen, 4-hydroxytamoxifen, and N-desmethyltamoxifen had average concentrations of 1.15 pmol/mg, 0.0931 pmol/mg, 0.0471 pmol/mg, and 1.46 pmol/mg, respectively. The different hair pigmentation was related to the measured concentrations of the analyte. In comparison to samples of blond and brown hair, the concentrations in black hair were higher. In addition, UV irradiation also influenced the analytical result.

Serum

The bioanalytical method of tamoxifen in serum has been developed by Teunissen et al., 2011 [20] and applied by Pistilli et al., 2020 [21]. The serum sample was extracted with a protein precipitation procedure. 50 µl of the serum sample was added to 150 µl of internal standard (tamoxifen-d5, N-desmethyltamoxifen-d5, N-desmethyl-4-hydroxytamoxifen-d5, and 4-hydroxytamoxifen-d5), vortexed for 10 s and then centrifugated at 11.300 x g for 10 min. 120 µl of the supernatant was separated and dried up under nitrogen at 30 °C. The dried sample was reconstituted in 60 µl of the mobile phase. 15 µl of aliquot was injected into the high-performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS).

The HPLC-MS/MS system was performed using a C18 column (150 mm x 2.1 mm, 2.6 µm). The mixture of acetonitrile–5 mmol ammonium formate buffer pH 3.5 (3:7) was used as the mobile phase. The flow rate was 0.4 ml/min with a gradient. The run time was 10 min.

The bioanalytical method was applied to determine the level of tamoxifen and its active metabolites in serum to assess the rate of nonadherence in 188 breast cancer patients. The tamoxifen serum level<60 ng/ml was defined as the adherence threshold. 16.0% of patients had a serum level below the adherence threshold. However, there were only 12.3% of patients reported nonadherence through interviews. Therapeutic drug monitoring may be required to identify patients who did not adhere and were at risk for poorer outcomes.

Plasma

Several bioanalytical methods of tamoxifen in plasma have been developed, including by Antunes et al., 2013 using high-performance liquid chromatography with photodiode array detector (HPLC-PDA) with a run time of 16 min [17], Arellano et al., 2014 using UPLC-MS/MS with a run time of 6 min [16], Antunes et al., 2015 using LC-MS/MS with a run time of 8 min [18], and Bobin-Dubigeon et al., 2019 using UPLC-MS/MS with a run time of 4.5 min [8]. The method by Bobin-Dubigeon et al., 2019 was the fastest one, suitable for time-and cost-efficiency.

The plasma sample (100 µl) was added to formic acid 1% (100 µl), and then vortexed for 30 s. N-desmethyltamoxifen-d5, 4-hydroxytamoxifen-d5, and endoxifen-d5 as the internal standard (400 µl), and methanol (100 µl) were added to the mixture, followed by shaking and centrifugation at 18,000 x g for ten minutes at 4 °C. The separated supernatant (300 µl) was combined with 300 µl of ammonium format 2 mmol containing formic acid 0.2%. 7 µl of aliquot was injected into UPLC-MS/MS.

The UPLC-MS/MS system was carried out using a C18 (50 x 2.1 mm, 1.7 µm) column. The mobile phase was ammonium format 2 mmol–acetonitrile, both containing formic acid 0.5%. The flow rate was 0.6 ml/min with a gradient.

The application of the bioanalytical method was in the analysis of tamoxifen and its active metabolites in 10 breast cancer patients. The measured concentrations of tamoxifen, N-desmethyltamoxifen, endoxifen, and 4-hydroxytamoxifen were 108–330 ng/ml, 152–329 ng/ml, 3.55–15.21 ng/ml, and 0.91–2.63 ng/ml, respectively. Interpatient variability of the concentration was associated with CYP2D6 polymorphisms. Therapeutic drug monitoring was needed to individualize the dose of tamoxifen according to plasma concentration.

Dried blood spot (DBS)

Several bioanalytical methods of tamoxifen in DBS samples have been developed by Antunes et al., 2015 [18], Tré-Hardy et al., 2016 [11], Harahap et al., 2019 [12], and Maggadani et al., 2021 [25]. All of them used LC-MS/MS, with the run time of 8 min, 6.5 min, 4 min, and 5 min, respectively. The method by Harahap et al., 2019 was the fastest one, suitable for time-and cost-efficiency.

Whole blood (20 µl) was spotted on the DBS card and allowed to dry for 2 h. The blood spot was cut, added methanol (1 ml), and vortexed for one minute, followed by sonication for 25 min. The sample (850 µl) was evaporated under nitrogen at 55 °C. The dried sample was reconstituted in 0.1% formic acid in water–0.1% formic acid in acetonitrile (35:65), vortexed, and centrifugated at 3000 rpm for ten minutes. 10 µl of aliquot was injected into LC-MS/MS.

The LC-MS/MS system was performed using a C18 (2.1 x 100 mm, 1.7 µm) column. The mobile phase was 0.1% formic acid in water–0.1% formic acid in acetonitrile (35:65), flowing at a rate of 0.25 ml/min with an isocratic system.

The application of the bioanalytical method was in the determination of tamoxifen and its active metabolites in 29 breast cancer patients. The measured concentrations of tamoxifen, endoxifen, and 4-hydroxytamoxifen were 30.29–188.63 ng/ml, 1.45–28.77 ng/ml, 0.21–11.28 ng/ml, respectively. Interpatient variability was associated with polymorphisms of CYP2D6 and CYP3A4 and drug interactions. There were 2 patients with endoxifen concentrations below the clinical threshold (<3.30 ng/ml).

Volumetric absorptive microsampling (VAMS)

The bioanalytical method of tamoxifen in the VAMS sample has been developed by Maggadani et al., 2021 [25]. The tip of the VAMS sampler was dipped into the whole blood for 2 s and dried for 1-3 h. The dried tip was removed from the plastic handle and then extracted with 1 ml of methanol (containing 100 ng/ml of propranolol), vortexed for 1 min, followed by sonication for 25 min. The sample (850 µl) was evaporated under nitrogen at 50 °C. The dried sample was reconstituted in 100 µl of 0.1% formic acid in water–0.1% formic acid in acetonitrile (5:95), vortexed, and centrifugated at 805 x g for 10 min. 10 µl of aliquot was injected into LC-MS/MS.

The LC-MS/MS system was carried out using a C18 (2.1 x 100 mm, 1.7 µm) column. 0.1% formic acid in water and 0.1% formic acid in acetonitrile was used as the mobile phase, eluted with a gradient system. The composition of 5:95 was held for 3 min, then altered to the composition of 70:30 and held for 2 min. The flow rate was 0.20 ml/min.

The application of the bioanalytical method was in the analysis of tamoxifen and its metabolite concentration from 30 breast cancer patients. The measured concentrations were 36.14–233.31 ng/ml for tamoxifen, 3.78–30.03 ng/ml for endoxifen, 1.55–6.56 ng/ml for 4-hydroxytamoxifen, and 48.80–385.01 ng/ml for N-desmethyltamoxifen. There was interpatient variability related to polymorphisms of CYP2D6 and CYP3A4 and drug interactions.

Bioanalytical method of tamoxifen for TDM in breast cancer patients

Various bioanalytical methods of tamoxifen and its active metabolites in several matrices are summarized in table 2. Endoxifen and 4-hydroxytamoxifen are biomarkers in therapeutic drug monitoring of tamoxifen; however, the method by Rathi et al., 2016 [19] was unable to determine them. Moreover, there was derivatization, which required more reagents for derivatizing agents and a longer time for sample preparation so it would be more expensive.

Table 2: Summary of several bioanalytical method of tamoxifen and its metabolites

Matrices Sample preparation Analytical instruments Run time (min) References
Urine Solid-phase extraction GC-MS 23.1 Rathi et al., 2016 [19]
Scalp hair Liquid-liquid extraction UPLC-MS/MS 10 Drooger et al., 2015 [23]
Serum Protein precipitation HPLC-MS/MS 10 Teunissen et al., 2011 [20] and Pistilli et al., 2020 [21]
Plasma Liquid-liquid extraction HPLC-PDA 16 Antunes et al., 2013 [17]
Plasma Liquid-liquid extraction UPLC-MS/MS 6 Arellano et al., 2014 [16]
Plasma Liquid-liquid extraction LC-MS/MS 8 Antunes et al., 2015 [18]
Plasma Protein precipitation UPLC-MS/MS 4.5 Bobin-Dubigeon et al., 2019 [8]
DBS Liquid-liquid extraction LC-MS/MS 8 Antunes et al., 2015 [18]
DBS Protein precipitation LC-MS/MS 6.5 Tré-Hardy et al., 2016 [11]
DBS Protein precipitation LC-MS/MS 4 Harahap et al., 2019 [12]
DBS Protein precipitation LC-MS/MS 5 Maggadani et al., 2021 [25]
VAMS Protein precipitation LC-MS/MS 5 Maggadani et al., 2021 [25]

The simplicity of the sample preparation procedure is important to be considered. The method by Drooger et al., 2015 [23] was relatively difficult to be applied due to the cutting process into very small samples (1 mm). In addition, liquid-liquid extraction is needed a longer time and is more expensive than protein precipitation. The methods in serum, plasma, and DBS use protein precipitation as sample preparation with methanol.

Therapeutic drug monitoring will be done repeatedly so that the method should be comfortable for breast cancer patients. Serum and plasma sampling techniques are more invasive. In addition, the methods in serum and plasma require the phlebotomist to take samples and the refrigerator to freeze samples so they would be more expensive.

DBS and VAMS are the newer biosampling techniques that collect the microvolume sample from finger capillary blood, which is less invasive than venipuncture. The sample is easy to be taken by the patient themselves without a phlebotomist. The collected sample is dried thus, it is more stable and can be stored at room temperature without the refrigerator requirement [10, 11, 45]. According to table 2, the shortest run time was the method by Harahap et al., 2020 which used DBS as the biosampling technique. The method can be used to quantify the concentration of tamoxifen and its active metabolites, which are 4-hydroxytamoxifen and endoxifen [12]. The comparative study conducted by Maggadani et al., 2021 showed that there is no statistically significant difference between using VAMS and DBS in the analysis of tamoxifen and its metabolites [25]. In addition, the cost of the WhatmanTM 903 DBS card is 7 times lower than the MitraTM VAMS sampler. The card also takes less space in storage [46].

CONCLUSION

Several bioanalytical methods of tamoxifen and its metabolites have been developed in various biological matrices: urine, scalp hair, serum, plasma, DBS, and VAMS samples, with different sample preparations. The most frequently used analytical instrument was LC-MS/MS. The bioanalytical method of tamoxifen and its metabolites in the DBS sample was more suitable in the TDM application due to the low invasive sampling technique, more stable sample, and rapid sample preparation. Therefore, it is more time-and cost-efficient than the other methods. This method can be applied for therapeutic drug monitoring in breast cancer patients.

FUNDING

Nil

AUTHORS CONTRIBUTIONS

All authors have contributed equally

CONFLICT OF INTERESTS

Declared none

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