Effects of pharmacogenetics on pharmacokinetics and toxicity of doxorubicin in Egyptian breast cancer patients

Abstract This study investigates the impact of single nucleotide polymorphisms in genes (SLC22A16 and CBR1) involved in the pharmacokinetics and toxicity of doxorubicin (DOX) in Egyptian female patients with breast cancer. Patients administered DOX (60 mg/m2) for 4 cycles every 3 weeks. The peak DOX plasma concentration was measured using a validated chromatographic method. The genotyping for the selected SNPs, SLC22A16 T > C (rs714368), and CBR1 C > T (rs20572), was performed by RT-PCR. Patients were monitored for hematological and cardiac toxicities. The variant carriers of CBR1 C > T (rs20572) exhibited significantly higher DOX concentration, but no significant association to DOX-induced hematological toxicity. On the other hand, SLC22A16 T > C (rs714368) had no significant influence on DOX plasma concentration, but was significantly correlated with lower risk of neutropenia (OR 0.31, 95% CI 0.12–0.75, p = 0.01) and leukopoenia (OR 0.18, 95% CI 0.07–0.5, p = 0.001). DOX-related cardiotoxicity was correlated with the cumulative dose of DOX (R = 0.238, p = 0.017), but not with any of the two examined SNPs. Genetic polymorphisms in SLC22A16 and CBR1 may explain the inter-individual variations in DOX pharmacokinetics and toxicity. Using pharmacogenetic testing is important to customise drug therapy for cancer patients treated with anthracyclines.


Introduction
Breast cancer is the most prevailing cancer among women worldwide as it represents 1 in 4 cancers diagnosed (Siegel et al. 2024).According to the estimates of the American Cancer Society, about 300, 000 new cases will be diagnosed with invasive breast cancer in women in the Unites States, and will result in more than 40, 000 deaths (Siegel et al. 2024).In Egypt, nearly 32% of all female cancers are attributed to breast cancer (Ibrahim and Shash 2022).Unlike western countries, Egyptian women tend to be diagnosed at a younger age with breast cancer with more than 50% of the cases are <50 years of age (Ibrahim and Shash 2022).Additionally, Egypt has approximately double the mortality rate from breast cancer compared to other developing countries (41% versus 23%) (Azim et al. 2023).
Cytotoxic drugs have been widely used both alone and in combination for breast cancer treatment.Among the most commonly used and effective chemotherapy agents are the anthracycline antibiotics, such as doxorubicin (DOX).Anthracyclines belong to a class of antineoplastic antibiotics, which interfere with cancer cell replication (Venkatesh and Kasi 2023).The AC regimen, a combination of doxorubicin [Adriamycin®] and cyclophosphamide, is commonly administered to breast cancer patients in Egypt due to its better safety and efficacy profile compared to other regimens (Early Breast Cancer Trialists' Collaborative Group (EBCTCG)) 2005).Usually, the AC regimen can be used as adjuvant (after surgery), neoadjuvant (prior to surgery), or palliative (supportive) chemotherapy (Abdel-Razeq et al. 2018;Zaheed et al. 2019).Nausea, vomiting, myelosuppression, alopecia, and cardiotoxicity are the common adverse effects of anthracyclines (Venkatesh and Kasi 2023).These chemotherapy-induced toxicities usually result in disruption of therapy, dose reduction, reduced or failure of the anti-cancer effect (Liutkauskiene et al. 2018;Prieto-Callejero et al. 2020).Doxorubicin (DOX) is one of the most commonly used anthracyclines.Although its mechanism of action is not fully known, it is proposed that DOX exerts its cytotoxic effect through various mechanisms including DNA intercalation, generation of reactive oxygen species and interruption of DNA repair mediated by topoisomerase II (Thorn et al. 2011;Shi et al. 2023).Doxorubicin is mainly eliminated by metabolism and transported into cells by the organic cation transporter 6 (hOCT6) which is coded by SLC22A16 gene, then metabolised to the secondary alcohol doxorubicinol through carbonyl (CBR) and aldo-keto (AKR) reductases (Lal et al. 2007;Breysse et al. 2020).This interplay between various influx and efflux transporters and drug metabolising enzymes plays a key role in determining the intracellular concentration of DOX, and hence its efficacy and toxicity.The key players involved in the disposition, mechanism of action, and toxicity of DOX are illustrated in Figure 1.
The disposition and pharmacokinetics of DOX show wide inter-individual variations, which is at least partially related to the genetic variations, including single nucleotide polymorphisms, in the proteins involved in its uptake, efflux and metabolism (Huang 2007;Lee 2010).Alteration in the activity of these proteins could impact the cellular concentration of DOX, and hence its anti-cancer efficacy and toxicity (Bagdasaryan et al. 2022).For example, it has been reported that the alcohol metabolite doxorubcinol, while exhibiting less potent antitumor activity, it is at least 10-times more cardiotoxic than the parent drug DOX (Heibein et al. 2012).Therefore, genetic variations in the enzymes/transporters involved in the formation of this metabolite could affect the risk of DOX-induced cardiac toxicity.
Despite the importance of screening for SNPs in genes coding DOX metabolising enzymes and transporters, there is limited or no information among breast cancer patients in Egypt.Therefore, this study focused on investigating the impact of SNPs in genes involved in doxorubicin transport (e.g.SLC22A16) and metabolism (e.g.CBR1) on its pharmacokinetics and related toxicity in Egyptian breast cancer patients treated with DOX-based regimens.

Materials
Doxorubicin hydrochloride (DOX) and daunorubicin hydrochloride (purity > 99.0%) were purchased from Sigma-Aldrich (St. Louis, MO, USA).All solvents and chemicals were either HPLC grade (methanol and acetonitrile) or analytical grade (sodium acetate and acetic acid) and were purchased from Sigma-Aldrich (St. Louis, MO, USA).Pre-designed TaqMan® genotyping assay kits for the selected SNPs rs714368 and rs20572 were purchased from Applied Biosystems (Foster City, CA, USA).GeneJET Genomic DNA Purification Kit #K0721 was purchased from Thermo Fisher Scientific Inc., USA.

Study population
This was an observational single centre prospective study in a cohort of 100 female Egyptian patients with histologically confirmed breast cancer treated with adjuvant or neoadjuvant doxorubicin-based chemotherapy.Patients received treatment with AC chemotherapy regimen which combines doxorubicin (60 mg/m 2 IV infusion over 20 min) and cyclophosphamide (600 mg/m 2 IV over 30 min) for 4 cycles every 3 weeks.The study was conducted at the Clinical Oncology and Nuclear Medicine Department, Menoufia University, Egypt from October 2021 to December 2022.Patients were included in the study if they have performance status 0 or 1 according to Eastern Cooperative Oncology Group (ECOG) score (Azam et al. 2019), no contraindication to chemotherapy, adequate bone marrow, liver, and kidney functions as defined elsewhere (Lal et al. 2008, p. 1).Women of childbearing age were required to be on acceptable forms of contraception.Exclusion criteria include any of the following: pregnancy, lactation, uncontrolled comorbid conditions (such as ischaemic heart disease, diabetes mellitus, and hypertension), active infection, baseline ejection fraction < 50%, performance status ≥ 2 according to ECOG score, biological response modifiers, immunotherapy, or endocrine therapy during chemotherapy.The medication profiles of the recruited patients were extensively examined for any drugs that might interact with DOX pharmacokinetics or pharmacodynamics using the Lexicomp® application.Medications expected to interact with DOX were either replaced or the patients were excluded from the study.The study was performed according to the guidelines of Declaration of Helsinki (Rickham 1964) and approved by the Ethical Committee of Menoufia University, Egypt.All patients wrote their informed consent for participation in this study.

Study endpoints
For evaluation of hematological toxicities (leukopoenia, neutropenia, and anaemia), complete blood counts (CBC) with differential including platelet count, total leukocyte count, haemoglobin, and absolute neutrophil count were monitored before each cycle and as clinically indicated for potential dose modification.For evaluation of cardiac toxicity, cumulative doxorubicin dosage was monitored and ejection fraction was assessed before treatment initiation and after 6 months of treatment.
The evaluation of the acute and late adverse effects of DOX therapy was performed according to Common Terminology Criteria for Adverse Events (CTCAE) (Trotti et al. 2003).The reported adverse events were categorised according to the time of occurrence into: early; during the first two cycles, and overall; occurred during the whole first course of chemotherapy (4 cycles).The severity of symptoms, on the other hand, was categorised into: recurrent; if the symptom was of any grade and present during 4 cycles and recurrent severe; if it was grade 3 and/or 4 during at least two cycles.

Pharmacokinetic analysis
Peak doxorubicin plasma concentrations were measured in blood samples drawn in plain EDTA tubes and centrifuged at 1000 g for 10 min immediately after the end of DOX infusion of the first cycle of chemotherapy.Doxorubicin plasma concentrations were determined using a previously reported reversed phase HPLC-fluorescence method using daunorubicin as an internal standard (Barpe et al. 2010).Patients' plasma samples spiked with daunorubicin (1000 ng/mL) underwent a single step protein precipitation using 1 mL methanol followed by centrifugation at 1000 g for 10 min.Chromatographic quantification of the analytes in the supernatant was conducted using RP-C18 column (Hypersil BDS, 250 mm x 4.6 mm i.d., 5 μm particle size) and a mobile phase consisting of acetonitrile: 0.1 M sodium acetate (pH 4.7 adjusted with acetic acid) (31:69, v/v).Isocratic elution was performed at a flow rate of 1.2 mL/min.The analytes were detected and quantified with fluorescence detector at emission and excitation wavelengths of 560 and 480 nm, respectively.

Pharmacogenetic analysis
We selected candidate SNPs of genes participating in anthracycline pharmacokinetics related to doxorubicin transport and metabolism.The selected SNPs were SLC22A16 T > C (rs714368) and CBR1 C > T (rs20572).Genotyping for the selected SNPs rs714368 and rs20572 was performed by pre-designed TaqMan® genotyping assay kits.Genomic DNA was extracted from blood samples collected in EDTA tubes using the Thermo Scientific GeneJET Genomic DNA Purification Kit and stored at −80 °C.The extracted DNA was used to perform polymerase chain reaction (PCR) using AB 7500 Real-time PCR system (Applied Biosystems, Foster City, CA, USA), after assessment of its purity and concentration as mentioned previously (Abdelkawy et al. 2020).

Statistical analysis
Data were analysed using the IBM SPSS Statistics ver.23 (Chicago, Illinois, USA).Parametric data were presented as mean ± SD, while categorical data were presented as frequencies and percentages.Chi square (χ2) test was performed to test the difference between expected and observed genotype frequencies for the Hardy-Weinberg equilibrium (HWE).
Mann-Whitney and Kruskal-Wallis tests were performed to compare between different genotypes in plasma concentrations.Univariate and multivariate analyses were used to assess the predictive power of different variables.Statistical significance was considered at p < 0.05.

Patients' characteristics
At the beginning, 155 female breast cancer patients were recruited for screening and assessing their eligibility for the study.A total of 45 patients did not meet the eligibility criteria, and therefore were excluded as shown in Figure 2(a).Later on, 10 subjects did not complete the follow up, and therefore a total of 100 subjects successfully completed the pharmacokinetic and pharmacogenetic analysis studies. Figure 2(b) illustrates the overall study design from subject's recruitment till end of the study.
The baseline clinical characteristics and demographics of study patients, including age, complete blood count, body mass index (BMI), liver and kidney functions, progesterone receptor (PR), human epithelial growth factor receptor-2 (HER-2) and oestrogen receptor (ER) status were summarised in Table 1.The median age, height, and BSA of the study subjects at the time of study initiation were 45.8 years (range: 30-75), 162 cm (range: 150-178), and 1.89 m 2 (range: 1.47-2.28),respectively.

Pharmacokinetic analysis
The utilised HPLC-fluorescence method was fully validated and had acceptable accuracy (range: 100%-114%), precision (relative standard deviation less than 15%), and sensitivity (limit of quantification: 20 ng/mL).Therefore, the method was successful in determining the peak DOX plasma concentration from blood samples collected immediately following the first infusion.As shown in supplementary Figure 1, the retention times for DOX and daunorubicin were 5.1 and 8.5 min, respectively.

Frequencies of genotypes
The frequencies of the two SNP in the study subjects were following Hardy-Weinberg equilibrium.Statistical analysis did not show any significant difference (p > 0.05) in the frequencies of the two SNP genotypes among any of the baseline clinical characteristics and demographics.Table 2 illustrates the frequencies of alleles and genotypes of the SNPs under investigation.

Impact of gene polymorphisms on doxorubicin plasma concentration
To investigate the association of DOX pharmacokinetics and gene polymorphisms, we measured peak DOX plasma concentration immediately after the end of the infusion of cycle 1 of the chemotherapy regimen.Our results illustrated high inter-individual variability with regard to DOX pharmacokinetics as indicated by more than 10-fold variation in its peak plasma concentration.As illustrated in Figure 3, the association of SLC22A16 T > C (rs714368) SNP with DOX plasma concentrations was not statistically significant (p > The medians (IQR) of DOX plasma concentrations were 740.0 ng/ mL (IQR 279.0-1221.0ng/mL), 707.0 ng/mL (IQR 388.7-1394.0ng/mL) and 652.0 ng/mL (IQR 564.0-1475.0ng/mL) in patients with TT, TC and CC alleles, respectively, as shown in Figure 3(A).However, CBR1 C > T (rs20572) SNP was significantly associated with DOX plasma concentrations (p < 0.0001).

Toxicity of doxorubicin-based chemotherapy
Table 3 lists the hematological and non-hematological toxicities in the study subjects.According to the overall toxicity, forty-two patients (i.e.incidence of 42%) had grade 1-4 leukopoenia, of which 6 patients exhibited grade 3-4 severe leukopoenia, with an incidence of 6%, and 10 patients with recurrent leukopoenia (patients having leukopoenia of any grade in the 4 cycles of treatment), with an incidence of 10%.Forty-eight patients (i.e.incidence of 48%) exhibited grade 1-4 neutropenia, of which 14 patients with grade 3-4 severe neutropenia, with an incidence of 14%, and 4 patients with recurrent severe neutropenia (patients having grade 3 and/or 4 neutropenia during at least two cycles), with an incidence of 4%.Sixty-two patients (i.e.incidence of 62%) exhibited grade 1-4 anaemia, of which 26 patients with recurrent anaemia (patients having anaemia of any grade during the 4 cycles of treatment), with an incidence of 26%.
The most commonly reported non-hematological toxicities were alopecia, fatigue, nausea and anorexia, with an occurrence of more than 40% as shown in Table 3. Regarding cardiac toxicity, there was a significant difference between ejection fraction before (64.92 ± 5.56) and six months after (63.88 ± 4.73) the first cycle of doxorubicin treatment (p ˂ 0.001).Moreover, a statistically significant correlation was found between DOX cumulative dose and % decrease in ejection fraction six months after the first cycle of doxorubicin treatment (p = 0.017).However, no significant correlation was found between DOX plasma concentration and % decrease in ejection fraction six months after the first cycle of doxorubicin treatment (p = 0.394) as shown in Figure 4.
It should be noted that the average single DOX dose was 106 mg (range 80-120 mg), while the average cumulative dose after 4 cycles of therapy was 425 mg (range 320-480 mg).These doses are within the common clinical practice and similar to the dose range published in earlier studies.Additionally, using a single plasma concentration (C max ) as a pharmacokinetic endpoint for collaboration analysis may be inferior to using the area under the curve (AUC) which is a better indicator of body exposure.This is due to the fact that plasma concentration is affected by both clearance and distribution factors.Therefore, we analysed the data using adjusted C max to body surface area as a relevant parameter for drug distribution.As shown in supplementary figure 2, the correlation parameters did not vary significantly using adjusted peak plasma concentration.
However, the univariate analysis failed to illustrate any significant correlation between any of the two SNPs SLC22A16 T > C (rs714368) and CBR1 C > T (rs20572) and the DOX-related anaemia and cardiac toxicity (Table 4).

Discussion
Anthracyclines, including DOX, is a key component of the chemotherapy regimens approved for breast cancer treatment.Despite their anticancer efficacy, the toxicities, especially hematological and cardiac toxicities, often complicate drug therapy and result in poor clinical outcomes.Several reports in the last decade have linked DOX toxicity to polymorphisms in the genes that alter its pharmacokinetics (Lal et al. 2007;2008;Faraji et al. 2016;Cui et al. 2021;Bagdasaryan et al. 2022).Due to the large inter-ethnic and inter-individual variabilities and the lack of such studies among breast cancer patients in Egypt, this study was conducted to explore the role of SNPs in SLC22A16 and CBR1 genes in the development of hematological and cardiac toxicities and its association to DOX pharmacokinetics in Egyptian breast cancer patients treated with DOX-based chemotherapy.
The current study reports a significant association between the expression of SLC22A16 T > C (rs714368) heterozygote and reduced risk of leukopoenia and neutropenia.Moreover, significantly higher peak DOX plasma concentrations were associated with the variant carriers of CBR1 C > T (rs20572) when compared to the reference genotypes, suggesting the possible decrease in intracellular conversion of doxorubicin to doxorubicinol.The clinical relevance of this study is to highlight the importance of personalised treatment of breast cancer based on genotype profile.Prior to starting breast cancer treatment, genotype analysis for these SNPs SLC22A16 T > C (rs714368) and CBR1 C > T (rs20572) is useful in prediction of the therapeutic decision and picking out the patients with expected high tolerability to AC regimen and thus high probability of completing chemotherapy without decrease in quality of life, treatment delays and dose reduction.
Regarding SLC22A16 T > C (rs714368), Okabe et al. (Okabe et al. 2005) showed in their in vitro studies that SLC22A16 mediated the uptake of DOX in Xenopus oocytes.Moreover, studies in Jurkat cells overexpressing SLC22A16 showed increased susceptibility to the cytotoxic effects of doxorubicin, which was attributed to the enhanced drug influx (Okabe et al. 2005).These studies suggested that alterations in SLC22A16 activity and expression might have great influences on pharmacodynamics and pharmacokinetics of doxorubicin in cancer patients.
In the current study, we found that patients with breast cancer carrying the rare CC genotypes of SLC22A16 T > C (rs714368) had lower peak plasma levels of doxorubicin in comparison to the reference genotypes or heterozygotes.However, no significant association was found between genotypes of SLC22A16 T > C (rs714368) and peak plasma levels of doxorubicin.This result was in contrast to a prior report by Lal et al. (Lal et al. 2007), who observed that the rare allele homozygotes of SLC22A16 T > C (rs714368) had a tendency towards higher exposure levels of doxorubicin.This disagreement could be due to the ethnic variations between Egyptian and Asian populations and the presence of other genetic confounders.Also, we found that carriers of variant allele (TC/CC) of SLC22A16 T > C (rs714368), compared with homozygous wild type (TT), exhibited a statistically significant lower incidence of leukopoenia and neutropenia.This finding is consistent with the observations of Bray et al. (Bray et al. 2010), who found a statistically significant lower incidence of hematological toxicities and dose delay of DOX-based chemotherapy in variant carriers of SLC22A16 T > C (rs714368) when compared with wild homozygotes.Although the proportion of homozygous variant alleles is less than 10%, the total proportion of variant carriers is about 40%.Thus, the effect seems to be related to the heterozygotes.The key role that this gene plays in DOX-induced hematological toxicity stems from the fact that it is expressed in bone marrow haematopoietic cells in normal adult tissues, and therefore it could have a great role in haematopoiesis (Tecza et al. 2018).Additionally, bone marrow becomes more vulnerable to the toxic effects of SLC22A16 substrates, such as the anthracyclines To emphasise the importance of this gene in predicting the incidence of hematological toxicities with DOX, Bray et al. (Bray et al. 2010) also investigated another two SNPs in SLC22A16 gene (SLC22A16 A > G (rs6907567) and SLC22A16 A > G (rs723685) which were in strong linkage disequilibrium with SLC22A16 T > C (rs714368).They observed that carriers of mutant allele had a statistically significant lower incidence of hematological toxicities and dose delay of DOX-based chemotherapy as compared to wild homozygotes.Concerning CBR1 C > T (rs20572), we found that peak plasma levels of doxorubicin were significantly higher in variant carriers (CT/TT) of CBR1 C > T (rs20572) when compared to the reference genotypes.Our findings are consistent with a study by Lal et al. (Lal et al. 2008: 1) who investigated the impact of polymorphism in carbonyl reductase gene on doxorubicin disposition in Asian patients with breast cancer.They showed that patients carrying the reference CC genotype of CBR1 C > T (rs20572) had a significantly lower exposure levels and higher clearance of doxorubicin when compared to heterozygous patients or homozygotes of the variant allele.Also, they found that exposure levels of doxorubicin were significantly higher in variant carriers of CBR1 C > T (rs20572).
To highlight the contribution of this gene to the variations in DOX pharmacokinetics, Lal et al. (Lal et al. 2008) investigated another SNP in CBR1 gene (CBR1 G > A rs1096) that affected the exposure levels of DOX.They found that exposure levels of doxorubicin were significantly lower in patients harbouring the reference genotypes GG of CBR1 G > A (rs1096) when compared to heterozygous patients or homozygotes of the variant allele.The two SNPs in CBR1 gene (rs20572 and rs1096), located very closely on chromosome 21, have high degree of linkage disequilibrium, illustrating the similar results of the two variants (Varatharajan et al. 2012).The exposure levels of doxorubicin were significantly higher in the variant carriers of the two SNPs in CBR1 gene (rs20572 and rs1096) when compared to the reference genotypes, suggesting the possibility of decreased catalytic activity of CBR1 and decreased intracellular conversion of doxorubicin to doxorubicinol in these patients.This is indicative of lower incidence of cardiotoxicity as doxorubicin biotransformation to doxorubicinol, 10-times more cardiotoxic than the parent drug DOX, by CBRs was correlated with doxorubicin cardiotoxicity (Forrest et al. 2000).
Additionally, Bains et al. found that allele A of CBR1 G > A rs1143663 (CBR1 V88I) was associated with decreased catalytic activity of CBR1 as compared to allele G when treated with daunorubicin and doxorubicin (Bains et al. 2009).Consequently, individuals with AA genotypes (CBR1 I88) may show a slower synthesis of cardiotoxic metabolites (daunorubicinol and doxorubicinol) as compared to individuals with GG genotypes (CBR1 V88).Similarly, Olson et al. found that mice treated with doxorubicin and having a null allele of Cbr1 (Cbr1+/−) had lower plasma levels of doxorubicinol as compared to mice with wild type allele Cbr1 (Cbr1+/+) and thus significantly lower incidence of anthracycline-related cardiotoxicity (Olson et al. 2003).Consequently, polymorphisms in CBR1 could contribute to alterations in doxorubicin pharmacokinetics and predict risk of anthracycline-related cardiotoxicity.
Also, we found that the variant carriers of CBR1 C > T (rs20572) had a trend towards reducing risk of leukopoenia (p = 0.059, Table 4).These findings are consistent with Cui et al. (Cui et al. 2021), who observed that variant carriers of CBR1 C > T (rs20572) also had a tendency towards reducing risk of leukopoenia (p = 0.052) in breast cancer patients.
Herein we report novel findings related to the influences of SLC22A16 T > C (rs714368) and CBR1 C > T (rs20572) polymorphisms on pharmacokinetics and toxicity of doxorubicin in Egyptian patients with breast cancer.Despite its novel clinical relevance, our study is limited by its modest size.Our sample size depended on previous studies (Okabe et al. 2005;Lal et al. 2008;Bray et al. 2010;Cui et al. 2021) and was relatively enough to detect statistically significant findings.However, our findings should be considered preliminary in nature until definitive conclusions can be derived from future studies with larger size.Another limitation is that we did not examine all genes that alter the pharmacokinetics and toxicity of DOX.Additionally, we did not measure the ratio of DOX to its major metabolite (doxorubicinol).This ratio might be a more appropriate marker of DOX metabolism, especially the metabolite is more cardiotoxic than the parent drug.Finally, all study subjects administered DOX in combination with cyclophosphamide, and therefore the observed toxic effects cannot be attributed 100% to DOX alone.

Conclusions
In this study, the heterozygote expression of SLC22A16 T > C (rs714368) was associated with a lower incidence of neutropenia and leukopoenia.Thus, the SLC22A16 T > C (rs714368) might represent a potential protection for reducing incidence of hematological toxicity during doxorubicin-based chemotherapy.Additionally, the exposure levels of doxorubicin were significantly higher in variant carriers of CBR1 C > T (rs20572) when compared to the reference genotypes, suggesting the possible decrease in intracellular conversion of doxorubicin to doxorubicinol in these patients.Thus, CBR1 C > T (rs20572) could contribute to alterations in doxorubicin pharmacokinetics in Egyptian patients with breast cancer.
patients in this study or from their relatives if they are not capable of giving consent.

Figure 2 .
Figure 2. Study flow diagram (a) and overall study design (b).

Figure 4 .
Figure 4. correlation between decrease in ejection fraction (eF) and doxorubicin (DoX) plasma concentration (in ng/mL) and cumulative dose (in mg) in the study subjects.

Table 1 .
Demographics and baseline laboratory data of participants.

Table 3 .
Frequency of doxorubicin-based chemotherapy toxicities in the study patients (n = 100).

Table 4 .
Univariate analyses of the associations between SnPs and doxorubicin-based chemotherapy toxicities.

Table 5 .
Multivariate analysis of the associations between SnPs and patients' clinical characteristics and incidence of hematological toxicities.