Common dihydropyrimidinase ( DPYS ) genetic variations do not predict fluoropyrimidine-related chemotherapy toxicity in a Canadian cohort.

Known genetic variations in dihydropyrimidine dehydrogenase (gene name DPYD ) do not fully predict patients at risk for severe fluoropyrimidine-associated chemotherapy toxicity. Dihydropyrimidinase (gene name DPYS ), the second catabolic enzyme in fluoropyrimidine metabolism, has been noted as a potential determinant of variation in fluoropyrimidine metabolism and response. In this study, we genotyped for DPYS c.-1T>C (rs2959023), c.265-58T>C (rs2669429) and c.541C>T (rs36027551) in a Canadian cohort of 248 patients who were wild type for Clinical Pharmacogenetics Implementation Consortium recommended DPYD variants and had received a standard dose of fluoropyrimidine chemotherapy. None of our patients were found to carry the DPYS c.541C>T variant, while the minor allele frequencies were 63% and 54% for c.-1T>C and c.265-58T>C, respectively. There was no association between DPYS c.-1T>C wild type and heterozygote [odds ratio (OR) (95% confidence interval, CI) = 1.10 (0.51-2.40)] or homozygote variant carriers [OR (95% CI) = 1.22 (0.55-2.70)], or between DPYS c.265-58T>C wild-type patients and heterozygote [OR (95% CI) = 0.93 (0.48-1.80)] or homozygote variant carriers [OR (95% CI) = 0.76 (0.37-1.55)] in terms of fluoropyrimidine-associated toxicity. Therefore, in our cohort of mostly Caucasian Canadians, genetic variations in DPYS do not appear to be a significant contributor to severe fluoropyrimidine-associated toxicity.

Impact of organic anion transporting polypeptide, P-glycoprotein, and breast cancer resistance protein transporters on observed tamoxifen and endoxifen concentration and adverse effects.

OBJECTIVE: Drug transporters are important determinants of drug disposition and response. Tamoxifen is an antiestrogen for breast cancer therapy known for adverse drug reactions (ADRs). In this study, the involvement of OATP transporters in tamoxifen and endoxifen transport was studied in vitro while the impact of single nucleotide variation (SNV) in OATP and efflux transporters P-glycoprotein ( ABCB1 ) and Breast Cancer Resistance Protein ( ABCG2 ) on ADRs during tamoxifen therapy were assessed. METHODS: Patients receiving tamoxifen for breast cancer, who were CYP2D6 normal metabolizers were enrolled ( n = 296). Patients completed a survey that captured ADRs and a blood sample was collected. Tamoxifen and endoxifen plasma concentration were measured, while DNA was genotyped for SNVs in ABCB1, ABCG2, SLCO1A2, SLCO1B1 , and SLCO2B1 . HEK293T cells were used to determine the extent of OATP-mediated transport of tamoxifen and endoxifen. RESULTS: Common SNVs of ABCB1, ABCG2, SLCO1A2 , and SLCO1B1 were not associated with tamoxifen or endoxifen concentration. However, tamoxifen concentration was significantly higher in carriers of SLCO2B1 c.935G>A (129.8 ng/mL) compared to wildtype (114.9 ng/mL; P = 0.036). Interestingly, subjects who carried SLCO1A2 c.38A>G reported significantly less dizziness ( P = 0.016). In-vitro analysis demonstrated increased cellular accumulation of tamoxifen in cells overexpressing OATP1A2 and 1B1, but endoxifen uptake was not effected in OATP overexpressing cells. CONCLUSIONS: We showed that OATP1A2 , a transporter known to be expressed at the blood-brain barrier, is capable of tamoxifen transport. Additionally, OATP1A2 c.38A>G was associated with reduced ADRs. Taken together, our findings suggest genetic variation in OATP transporters may be an important predictor of tamoxifen ADRs.

Genetic Variation in miR-27a Is Associated with Fluoropyrimidine-Associated Toxicity in Patients with Dihydropyrimidine Dehydrogenase Variants after Genotype-Guided Dose Reduction.

Dihydropyrimidine dehydrogenase (DPYD) is the rate-limiting enzyme involved in the metabolism of fluoropyrimidine-based chemotherapy. However, single-nucleotide variants (SNVs) in DPYD only partially explain fluoropyrimidine-induced toxicity. The expression of DPYD has previously been shown to be regulated by microRNA-27a (miR-27a) and a common miR-27a SNV (rs895819) has been associated with an increased risk of toxicity in patients harboring a DPYD variant who received standard fluoropyrimidine dosing. We investigated if the miR-27a rs895819 SNV was associated with toxicity in DPYD wildtype patients and carriers of DPYD variants who received a reduced dose. The regulation of DPYD using miR-27a was investigated in HepG2 cells utilizing a miR-27a mimic. miR-27a overexpression decreased DPYD mRNA expression compared to control cells (p < 0.0001). In a cohort of patients that received pre-emptive DPYD genotyping, 45 patients had a DPYD variant and 180 were wildtype. Patients heterozygous for rs895819 had an increased risk of toxicity, which was seen in both patients who were wildtype for DPYD variants (OR (95%CI) = 1.99 (1.00-3.99)) and DPYD variant carriers (OR (95%CI) = 8.10 (1.16-86.21)). Therefore, miR-27a rs895819 may be a clinically relevant predictor of fluoropyrimidine-associated toxicities. Furthermore, toxicity was more profound in DPYD variant carriers, even after DPYD genotype-guided dose reduction. This suggests that patients may benefit from miR-27a genotyping to guide fluoropyrimidine dosing.

In-vitro characterization of coding variants with predicted functional implications in the efflux transporter multidrug resistance protein 4 (MRP4, ABCC4).

MRP4 (gene ABCC4) is a polymorphic efflux transporter that has been implicated in drug-induced toxicity. We selected ten commonly observed MRP4 coding variants among Europeans for experimental characterization including nine variants predicted to be deleterious or functional (combined annotation-dependent depletion score >15). We assessed protein localization and activity by quantifying intracellular accumulation of two prototypic substrates, taurocholic acid (TCA) and estradiol 17-ß-glucuronide (E217ßG), in HEK293T over-expressing MRP4 wildtype or variant where cellular substrate loading was optimized through co-transfection with an uptake transporter. V458M, a novel variant not previously studied, and T1142M, showed reduced activity compared to MRP4 wildtype for E217ßG and TCA (P < 0.01), while L18I, G187W, K293E, and R531Q moderately increased activity in a substrate-dependent manner. Protein expression analysis indicated reduced cell surface expression for V458M (P < 0.01) but not T1142M compared to wildtype. Reduced activity may result from altered surface expression (V458M) or intrinsic activity as both variants map within the nucleotide-binding domains of MRP4. G187W showed a trend for reduced surface expression (P = 0.054) despite transport comparable or increased to wildtype suggesting enhanced intrinsic activity. Our findings suggest moderately altered MRP4 activity in six out of nine predicted functional variants with likely different mechanisms and substrate-specific effects. Cell-based studies using multiple known substrates are warranted to more accurately predict functional variants in this clinically important transporter.

Patient-specific genetic factors predict treatment failure in sofosbuvir-treated patients with chronic hepatitis C.

BACKGROUND & AIMS: According to pivotal clinical trials, cure rates for sofosbuvir-based antiviral therapy exceed 96%. Treatment failure is usually assumed to be because of virological resistance-associated substitutions or clinical risk factors, yet the role of patient-specific genetic factors has not been well explored. We determined if patient-specific genetic factors help predict patients likely to fail sofosbuvir treatment in real-world treatment situations. METHODS: We recruited sofosbuvir-treated patients with chronic hepatitis C from five Canadian treatment sites, and performed a case-control pharmacogenomics study assessing both previously published and novel genetic polymorphisms. Specifically studied were variants predicted to impair CES1-dependent production of sofosbuvir's active metabolite, interferon-λ signalling variants expected to impact a patient's immune response to the virus and an HLA variant associated with increased spontaneous and treatment-induced viral clearance. RESULTS: Three hundred and fifty-nine sofosbuvir-treated patients were available for analyses after exclusions, with 34 (9.5%) failing treatment. We identified CES1 variants as novel predictors for treatment failure in European patients (rs115629050 or rs4513095; odds ratio (OR): 5.43; 95% confidence interval (CI): 1.64-18.01; P = .0057), replicated associations with IFNL4 variants predicted to increase interferon-λ signalling (eg rs12979860; OR: 2.25; 95% CI: 1.25-4.06; P = .0071) and discovered a novel association with a coding variant predicted to enhance the activity of IFNL4's receptor (rs2834167 in IL10RB; OR: 1.81; 95% CI: 1.01-3.24; P = .047). CONCLUSIONS: Ultimately, this work demonstrates that patient-specific genetic factors could be used as a tool to identify patients at higher risk of treatment failure and allow for these patients to receive effective therapy sooner.

Implementation of pharmacogenomics: Where are we now?

Pharmacogenomics (PGx), examining the effect of genetic variation on interpatient variation in drug disposition and response, has been widely studied for several decades. However, as cost, as well as turnaround time associated with PGx testing, has significantly improved, the use of PGx in the clinical setting has been gaining momentum. Nevertheless, challenges have emerged in the broader clinical implementation of PGx. In this review, we will outline current models of PGx delivery and methodologies of evaluation, and discuss clinically relevant PGx tests and associated medications. Additionally, we will describe our approach for the broad implementation of pre-emptive DPYD genotyping in patients taking fluoropyrimidines in Ontario, Canada, as an example of clinically actionable PGx testing with sufficient clinical evidence of patient benefit that can become a new standard of patient care. We will highlight challenges associated with PGx testing, including a lack of diversity in PGx studies as well as general limitations that impact the broad adoption of PGx testing. Lastly, we examine the future of PGx, discussing new clinical targets, methodologies and analysis approaches.

A Comparative Analysis of Drug Therapy, Disease Phenotype, and Health Care Outcomes for Men and Women with Inflammatory Bowel Disease.

BACKGROUND: Sex and gender refer to biological and social differences between men and women. While well-evaluated in other disciplines, their roles in inflammatory bowel disease (IBD) are not well-defined. This study aimed to characterize differences in healthcare outcomes in men and women with IBD. METHODS: A retrospective single-centre cohort study was conducted to evaluate differences between men and women receiving care for Crohn's disease (CD) and ulcerative colitis (UC) at the Western University Personalized Medicine Clinic from March 2012 to September 2019. The primary endpoint was the proportion of IBD drugs used for all drug classes. Additional outcomes in healthcare utilization and disease phenotype were assessed. Student's t test and Fisher's exact test were used to assess differences RESULTS: A total of 1015 participants were included (CD = 656; UC = 359). In UC and CD, 47.9% and 59.0% were women, respectively. Overall, women were more likely prescribed budesonide than men (23.6% vs. 13.4%; p < 0.0001), while more men were exposed to prednisone for IBD management (73.5% vs. 67.4%; p = 0.04). Immunomodulator use was higher in men with CD versus women (86.6% vs. 78.3%; p = 0.008) and of those exposed, women more commonly experienced ADRs (29.5% vs. 21.2%; p = 0.01). Though no sex-related difference was identified, age was a predictor of biologic exposure in women with CD and men with UC, with those > 55 being less likely to receive biologics. CONCLUSIONS: These findings highlight differences in disease course and treatment approaches between men and women with IBD and support the consideration of sex and gender when researching disease outcomes.

High infliximab trough concentrations are associated with sustained histologic remission in inflammatory bowel disease: a prospective cohort study.

BACKGROUND: The threshold concentration of infliximab during maintenance therapy has not been well-defined in relation to histologic remission. The aim of the study is to dentify the maintenance-phase infliximab concentration associated with histologic remission in inflammatory bowel disease patients (IBD). METHODS: A prospective cohort study was carried out in 104 IBD patients seen at a tertiary care centre in London, Canada. Infliximab trough concentrations were collected during the maintenance phase of treatment and compared between participants with and without evidence of histologic remission. Participants were additionally evaluated for sustained histologic remission, and relapse to active disease. RESULTS: Participants in histologic remission attained higher mean concentrations of infliximab during the maintenance phase (10.34 ± 0.69 µg/ml) compared to those with persistent disease activity (6.23 ± 0.67 µg/ml, p-value < 0.0001). Additionally, during the maintenance phase, sustained histologic remission was also associated with a higher mean concentration of infliximab (10.81 ± 5.46 µg/ml) compared to those who relapsed to active disease (5.68 ± 3.70, p < 0.001). Overall, participants with a mean infliximab trough concentration greater than 8ug/ml were more likely to have histologic remission (area under the receiver operating characteristic curve, AUROC = 0.72, 95%CI = 0.65-0.84, p < 0.0001) and sustained histologic remission (AUC = 0.77, 95%CI = 0.63-0.91, p = 0.002). CONCLUSION: Maintenance-phase infliximab trough concentrations greater than 8 µg/ml, which is higher than the currently recommended target concentration, are highly associated with histologic remission and sustained histologic remission.

Baclofen has a risk of encephalopathy in older adults receiving dialysis.

At least 23 case reports link the muscle relaxant baclofen to encephalopathy in patients receiving dialysis. To explore this issue, we conducted a study to quantify the risk of encephalopathy from baclofen in patients receiving dialysis. Linked healthcare databases were used to conduct a population-based cohort study of older adults receiving maintenance dialysis in Ontario, Canada (1997-2018) to compare new users of baclofen to non-users. The primary outcome was the 30-day risk of hospitalization with encephalopathy, defined as a main diagnosis of delirium, disorientation, transient alteration of awareness, or transient cerebral ischemic attack. Inverse probability of treatment weighting on the propensity score was used to balance comparison groups on indicators of baseline health. Weighted risk ratios (RR) were obtained using modified Poisson regression and weighted risk differences (RD) using binomial regression. We studied 360 new baclofen users and 6109 non-users (2638 [41%] women; median age 75). The median baclofen dose was 20 mg/day. Hospitalization with encephalopathy occurred in 26 of 360 baclofen users (7.2%) and in under six of 6109 non-users (under 0.1%); weighted risk ratios, 78.3 (95% confidence interval 27.9 to 219.2); weighted risk differences, 7.1% (4.5% to 9.8%). The median time from baclofen dispensing to hospitalization with encephalopathy was three days. Among patients receiving dialysis, approximately one in 14 were hospitalized with encephalopathy shortly after starting baclofen. Thus, baclofen should be avoided in older adults receiving dialysis, and other muscle relaxants considered in its place. Hence, if baclofen must be used, a low dose should be prescribed, and older adults should be carefully monitored for signs of encephalopathy.

Genetic and clinical predictors of arthralgia during letrozole or anastrozole therapy in breast cancer patients.

PURPOSE: Female patients with breast cancer frequently develop arthralgia when treated with aromatase inhibitors (AI). Although the mechanism of AI-induced arthralgia is unknown, potential biomarkers have been identified. The purpose of this study was to investigate the clinical and genetic predictors of AI-induced arthralgia in a prospective cohort of patients with estrogen receptor-positive breast cancer. METHODS: One hundred and ninety-six patients were enrolled at initiation of AI therapy with either letrozole or anastrozole. Patients completed two validated self-report questionnaires assessing pain, stiffness, and physical function at baseline, and repeated the questionnaires at two and at six months after the initiation of treatment with an AI. Germline DNA of all patients was genotyped for seven single-nucleotide polymorphisms (SNPs) previously identified by genetic screens and genome-wide association studies as associated with AI-induced arthralgia. RESULTS: More than 50% of the study group experienced arthralgia symptoms. Genetic analysis revealed that four SNPs, in CYP19A1 (rs4775936) and ESR1 (rs9322336, rs2234693, rs9340799), were associated with the development of arthralgia (adjusted P = 0.016, 0.018, 0.017, 0.047). High body mass index (BMI) was also associated with the development of arthralgia symptoms (adjusted P = 0.001). Patients prescribed letrozole were significantly more likely to develop arthralgia than patients on anastrozole (P = 0.018), and also more likely to discontinue AI therapy due to arthralgia. The CYP19A1 (rs4775936) SNP was significantly associated with discontinuation of therapy due to intolerable arthralgia. CONCLUSIONS: Our results suggested that BMI and AI drug (letrozole versus anastrozole) were clinical predictors of arthralgia, while genetic variants rs4775936, rs9322336, rs2234693, and rs9340799 were genetic predictors of AI-induced arthralgia. Significantly, rs4775936 was also a predictor of discontinuation of therapy.

In Vitro Functional Characterization and in Silico Prediction of Rare Genetic Variation in the Bile Acid and Drug Transporter, Na+-Taurocholate Cotransporting Polypeptide (NTCP, SLC10A1).

Na+-taurocholate cotransporting polypeptide (NTCP, SLC10A1) is a key hepatic uptake transporter for bile acids and drugs and is the main functional receptor for hepatitis B and D viruses. Next-generation sequencing has revealed that a large number of rare SLC10A1 variants exist in the population. Little data exist regarding head-to-head comparison of in silico algorithms to predict functional effects of pharmacogenetic variants when compared to direct in vitro functional assessment. This study aimed at characterizing rare SLC10A1 variants in vitro and to assess the performance of seven in silico algorithms to predict the observed functional impacts. Thirty-five previously uncharacterized, rare, missense SLC10A1 variants were transiently expressed in human embryonic kidney 293 type T (HEK293T) cells. NCTP protein expression as well as uptake of substrates taurocholic acid (TCA) and rosuvastatin were assessed. Substrate-specific effects were observed for NTCP G191R, with TCA and rosuvastatin transport observed at 89 and 8% of wild-type (WT) uptake, respectively. Significantly reduced transport of TCA and rosuvastatin was observed for 19 variants (p < 0.05), with seven variants displaying decreased protein expression and marked reduction in transport of both substrates (0-13% of WT uptake, p < 0.0001). Performance of in silico algorithms to predict in vitro uptake, assessed using the area under the receiver operating characteristic curves (AUCROC), ranged from 0.69 to 0.97 and 0.72 to 0.84 for TCA and rosuvastatin uptake, respectively. In conclusion, we identified rare variants with significantly reduced NTCP expression and function. We demonstrated that no algorithm performed robustly enough to replace functional study in vitro, particularly given the broad substrate specificity of many pharmacogenes.

Drug interactions and pharmacogenetic factors contribute to variation in apixaban concentration in atrial fibrillation patients in routine care.

Factor Xa-inhibitor apixaban is an oral anticoagulant prescribed in atrial fibrillation (AF) for stroke prevention. Its pharmacokinetic profile is known to be affected by cytochrome P450 (CYP)3A metabolism, while it is also a substrate of the efflux transporters ATP-binding cassette (ABC)B1 (P-glycoprotein) and ABCG2 (breast cancer resistance protein, BCRP). In this study, we assessed the impact of interacting medication and pharmacogenetic variation to better explain apixaban concentration differences among 358 Caucasian AF patients. Genotyping (ABCG2, ABCB1, CYP3A4*22, CYP3A5*3) was performed by TaqMan assays, and apixaban quantified by mass spectrometry. The typical patient was on average 77.2 years old, 85.5 kg, and had a serum creatinine of 103.1 µmol/L. Concomitant amiodarone, an antiarrhythmic agent and moderate CYP3A/ABCB1 inhibitor, the impaired-function variant ABCG2 c.421C > A, and sex predicted higher apixaban concentrations when controlling for age, weight and serum creatinine (multivariate regression; R2 = 0.34). Our findings suggest that amiodarone and ABCG2 genotype contribute to interpatient apixaban variability beyond known clinical factors.

Fexofenadine and Rosuvastatin Pharmacokinetics in Mice with Targeted Disruption of Organic Anion Transporting Polypeptide 2B1.

Organic anion transporting polypeptide 2B1 (OATP2B1) is a widely expressed membrane transporter with diverse substrate specificity. In vitro and clinical studies suggest a role for intestinal OATP2B1 in the oral absorption of medications. Moreover, OATP2B1 is highly expressed in hepatocytes where it is thought to promote liver drug clearance. However, until now, a shortcoming of studies implicating OATP2B1 in drug disposition has been a lack of in vivo models. Here, we report the development of a knockout (KO) mouse model with targeted, global disruption of the Slco2b1 gene to examine the disposition of two confirmed mOATP2B1 substrates, namely, fexofenadine and rosuvastatin. The plasma pharmacokinetics of intravenously administered fexofenadine was not different between KO and wild-type (WT) mice. However, after oral fexofenadine administration, KO mice had 70% and 41% lower maximal plasma concentration (C max) and area under the plasma concentration-time curve (AUC0-last) than WT mice, respectively. In WT mice, coadministration of fexofenadine with grapefruit juice (GFJ) or apple juice (AJ) was associated with reduced C max by 80% and 88%, respectively, while the AUC0-last values were lower by 35% and 70%, respectively. In KO mice, AJ coadministration reduced oral fexofenadine C max and AUC0-last values by 67% and 59%, respectively, while GFJ had no effects. Intravenous and oral rosuvastatin pharmacokinetics were similar among WT and KO mice. We conclude that intestinal OATP2B1 is a determinant of oral fexofenadine absorption, as well as a target for fruit juice interactions. OATP2B1 does not significantly influence rosuvastatin disposition in mice. SIGNIFICANCE STATEMENT: A novel mouse model with targeted disruption of the Slco2b1 gene revealed that OATP2B1 is a determinant of oral absorption but not systemic disposition of fexofenadine, as well as a target of fruit juice interactions. Rosuvastatin oral and intravenous pharmacokinetics were not dependent on OATP2B1. These findings support the utility of the Slco2b1 KO mouse model for defining mechanisms of drug disposition at the intersection of in vitro and clinical pharmacology.

Crohn's Disease Is Associated with Decreased CYP3A4 and P-Glycoprotein Protein Expression.

Cytochrome P450 (CYP) 3A4 and P-glycoprotein (P-gp) have broad substrate overlap and are involved in the metabolism and transport of nearly 50% of currently prescribed medications. In the intestine, CYP3A4 and P-gp are coexpressed in the enterocytes at the intestinal villous tip and act in a coordinated manner to limit drug and xenobiotic oral bioavailability prior to further metabolism and disposition in the liver. Crohn's disease (CD), a form of inflammatory bowel disease, introduces a transmural intestinal insult that disrupts the intestinal barrier function; it therefore has the potential to affect intestinal drug metabolism and transport. We hypothesized that individuals with CD have reduced intestinal expression of CYP3A4 and P-gp. We obtained intestinal biopsy samples from individuals with and without CD and quantified the expression of CYP3A4 and P-gp. When we carried out Western analysis for protein expression, we observed a significant reduction in ileal (45% decrease) and colonic (78% decrease) CYP3A4 protein expression in subjects with CD compared with those without. Similarly, an 85% reduction in colonic P-gp protein expression was seen in the CD patients. Our data highlight important and novel findings pertaining to CD-associated changes to the intestinal expression of CYP3A4 and P-gp that are of relevance to better predict substrate drug dosing for patients with CD.

CYP2D6 genotype and endoxifen plasma concentration do not predict hot flash severity during tamoxifen therapy.

PURPOSE: Tamoxifen is frequently prescribed to prevent breast cancer recurrence. Tamoxifen is a prodrug and requires bioactivation by CYP2D6. Tamoxifen use is often limited by adverse effects including severe hot flashes. There is paucity of prospectively collected data in terms of CYP2D6 genotype and measured tamoxifen, 4-hydroxytamoxifen and endoxifen concentrations in relation to hot flash severity during tamoxifen therapy. METHODS: We conducted a longitudinal prospective study of breast cancer patients on tamoxifen (n = 410). At each visit, blood samples were collected, and patients completed a standardized hot flash survey (n = 1144) that reflected hot flash severity during the 7 days prior to the visit. Plasma concentrations of tamoxifen, 4-hydroxytamoxifen, and endoxifen were measured using liquid chromatography-tandem mass spectrometry and genotyping was carried out for CYP2D6. A linear mixed-effects regression analysis assessed the association of covariates in relation to the hot flash severity score (HFSS). RESULTS: Median age at first assessment was 50 years with 61.9% of patients considered peri-menopausal. Most patients (92.2%) experienced hot flash symptoms with 51.0% having low HFSS (0-4) and 7.32% experiencing HFSS > 25. Age was significantly associated with hot flash severity, with patients aged 45-59 more likely to have higher HFSS. Neither duration of tamoxifen therapy nor observed tamoxifen, endoxifen and 4-hydroxy tamoxifen plasma concentration predicted hot flash severity. Genetic variation in CYP2D6 or CYP3A4 was not predictive of hot flash severity. CONCLUSIONS: Hot flash severity during tamoxifen therapy can not be accounted for by CYP2D6 genotype or observed plasma concentration of tamoxifen, 4-hydroxytamoxifen, or endoxifen.

Letrozole concentration is associated with CYP2A6 variation but not with arthralgia in patients with breast cancer.

PURPOSE: The aromatase inhibitor (AI) letrozole is a first-line drug in the adjuvant treatment of breast cancer in postmenopausal women. Adherence to AI therapy, including letrozole, remains problematic due to the development of debilitating AI-induced arthralgia. Letrozole is metabolized in the liver by CYP2A6. It remains unknown if plasma letrozole levels or CYP2A6 genetic variation is associated with the development of arthralgia. METHODS: We enrolled 126 female breast cancer patients initiated on letrozole therapy and prospectively collected blood samples at baseline and two follow-up time points to determine letrozole plasma concentrations and CYP2A6 genotype. At each visit, participants completed two validated questionnaires to assess the severity of arthralgia symptoms. RESULTS: More than half (55%) of patients experienced a significant increase in their arthralgia symptoms after initiation of treatment. The clinical variables of body mass index (P = 0.0003) and age (P = 0.0430) were negatively and positively associated with plasma letrozole concentrations, respectively. CYP2A6 genotype was significantly associated with letrozole levels (P < 0.0001), and increased plasma letrozole levels were observed in patients with CYP2A6 reduced-function genotypes. Plasma levels of letrozole and CYP2A6 genotype were not significantly associated with a change in pain score from baseline. CONCLUSIONS: CYP2A6 genotype was a significant predictor of letrozole plasma levels, but was not associated with the development of arthralgia.

Apixaban and Rosuvas--tatin Pharmacokinetics in Nonalcoholic Fatty Liver Disease.

There is little known about the impact of nonalcoholic fatty liver disease (NAFLD) on drug metabolism and transport. We examined the pharmacokinetics of oral apixaban (2.5 mg) and rosuvastatin (5 mg) when administered simultaneously in subjects with magnetic resonance imaging-confirmed NAFLD (N = 22) and healthy control subjects (N = 12). The area under the concentration-time curve to the last sampling time (AUC0-12) values for apixaban were not different between control and NAFLD subjects (671 and 545 ng/ml × hour, respectively; P = 0.15). Similarly, the AUC0-12 values for rosuvastatin did not differ between the control and NAFLD groups (25.4 and 20.1 ng/ml × hour, respectively; P = 0.28). Furthermore, hepatic fibrosis in NAFLD subjects was not associated with differences in apixaban or rosuvastatin pharmacokinetics. Decreased systemic exposures for both apixaban and rosuvastatin were associated with increased body weight (P < 0.001 and P < 0.05, respectively). In multivariable linear regression analyses, only participant weight but not NAFLD, age, or SLCO1B1/ABCG2/CYP3A5 genotypes, was associated with apixaban and rosuvastatin AUC0-12 (P < 0.001 and P = 0.06, respectively). NAFLD does not appear to affect the pharmacokinetics of apixaban or rosuvastatin.

Contribution of Organic Anion-Transporting Polypeptides 1A/1B to Doxorubicin Uptake and Clearance.

The organic anion-transporting polypeptides represent an important family of drug uptake transporters that mediate the cellular uptake of a broad range of substrates including numerous drugs. Doxorubicin is a highly efficacious and well-established anthracycline chemotherapeutic agent commonly used in the treatment of a wide range of cancers. Although doxorubicin is a known substrate for efflux transporters such as P-glycoprotein (P-gp; MDR1, ABCB1), significantly less is known regarding its interactions with drug uptake transporters. Here, we investigated the role of organic anion transporting polypeptide (OATP) transporters to the disposition of doxorubicin. A recombinant vaccinia-based method for expressing uptake transporters in HeLa cells revealed that OATP1A2, but not OATP1B1 or OATP1B3, and the rat ortholog Oatp1a4 were capable of significant doxorubicin uptake. Interestingly, transwell assays using Madin-Darby canine kidney II cell line cells stably expressing specific uptake and/or efflux transporters revealed that OATP1B1, OATP1B3, and OATP1A2, either alone or in combination with MDR1, significantly transported doxorubicin. An assessment of polymorphisms in SLCO1A2 revealed that four variants were associated with significantly impaired doxorubicin transport in vitro. In vivo doxorubicin disposition studies revealed that doxorubicin plasma area under the curve was significantly higher (1.7-fold) in Slco1a/1b-/- versus wild-type mice. The liver-to-plasma ratio of doxorubicin was significantly decreased (2.3-fold) in Slco1a/1b2-/- mice and clearance was reduced by 40% compared with wild-type mice, suggesting Oatp1b transporters are important for doxorubicin hepatic uptake. In conclusion, we demonstrate important roles for OATP1A/1B in transporter-mediated uptake and disposition of doxorubicin.