Darolutamide does not interfere with OATP-mediated uptake of docetaxel.

The addition of darolutamide, an androgen receptor signalling inhibitor, to therapy with docetaxel has recently been approved as a strategy to treat metastatic prostate cancer. OATP1B3 is an SLC transporter that is highly expressed in prostate cancer and is responsible for the accumulation of substrates, including docetaxel, into tumours. Given that darolutamide inhibits OATP1B3 in vitro, we sought to characterise the impact of darolutamide on docetaxel pharmacokinetics. We investigated the influence of darolutamide on OATP1B3 transport using in vitro and in vivo models. We assessed the impact of darolutamide on the tumour accumulation of docetaxel in a patient-derived xenograft (PDX) model and on an OATP1B biomarker in patients. Darolutamide inhibited OATP1B3 in vitro at concentrations higher than the reported Cmax. Consistent with these findings, in vivo studies revealed that darolutamide does not influence the pharmacokinetics of Oatp1b substrates, including docetaxel. Docetaxel accumulation in PDX tumours was not decreased in the presence of darolutamide. Metastatic prostate cancer patients had similar levels of OATP1B biomarkers, regardless of treatment with darolutamide. Consistent with a low potential to inhibit OATP1B3-mediated transport in vitro, darolutamide does not significantly impede the transport of Oatp1b substrates in vivo or in patients. Our findings support combined treatment with docetaxel and darolutamide, as no OATP1B3 transporter based drug-drug interaction was identified.

Solute Carrier Transportome in Chemotherapy-Induced Adverse Drug Reactions.

Members of the solute carrier (SLC) family of transporters are responsible for the cellular influx of a broad range of endogenous compounds and xenobiotics. These proteins are highly expressed in the gastrointestinal tract and eliminating organs such as the liver and kidney, and are considered to be of particular importance in governing drug absorption and elimination. Many of the same transporters are also expressed in a wide variety of organs targeted by clinically important anticancer drugs, directly affect cellular sensitivity to these agents, and indirectly influence treatment-related side effects. Furthermore, targeted intervention strategies involving the use of transport inhibitors have been recently developed, and have provided promising lead candidates for combinatorial therapies associated with decreased toxicity. Gaining a better understanding of the complex interplay between transporter-mediated on-target and off-target drug disposition will help guide the further development of these novel treatment strategies to prevent drug accumulation in toxicity-associated organs, and improve the safety of currently available treatment modalities. In this report, we provide an update on this rapidly emerging field with particular emphasis on anticancer drugs belonging to the classes of taxanes, platinum derivatives, nucleoside analogs, and anthracyclines.

Vincristine Disposition and Neurotoxicity Are Unchanged in Humanized CYP3A5 Mice.

Previous studies have suggested that the incidence of vincristine-induced peripheral neuropathy (VIPN) is potentially linked with cytochrome P450 (CYP)3A5, a polymorphic enzyme that metabolizes vincristine in vitro, and with concurrent use of azole antifungals such as ketoconazole. The assumed mechanism for these interactions is through modulation of CYP3A-mediated metabolism, leading to decreased vincristine clearance and increased susceptibility to VIPN. Given the controversy surrounding the contribution of these mechanisms, we directly tested these hypotheses in genetically engineered mouse models with a deficiency of the entire murine Cyp3a locus [Cyp3a(-/-) mice] and in humanized transgenic animals with hepatic expression of functional and nonfunctional human CYP3A5 variants. Compared with wild-type mice, the systemic exposure to vincristine was increased by only 1.15-fold (95% confidence interval, 0.84-1.58) in Cyp3a(-/-) mice, suggesting that the clearance of vincristine in mice is largely independent of hepatic Cyp3a function. In line with these observations, we found that Cyp3a deficiency or pretreatment with the CYP3A inhibitors ketoconazole or nilotinib did not influence the severity and time course of VIPN and that exposure to vincristine was not substantially altered in humanized CYP3A5*3 mice or humanized CYP3A5*1 mice compared with Cyp3a(-/-) mice. Our study suggests that the contribution of CYP3A5-mediated metabolism to vincristine elimination and the associated drug-drug interaction potential is limited and that plasma levels of vincristine are unlikely to be strongly predictive of VIPN. SIGNIFICANCE STATEMENT: The current study suggests that CYP3A5 genotype status does not substantially influence vincristine disposition and neurotoxicity in translationally relevant murine models. These findings raise concerns about the causality of previously reported relationships between variant CYP3A5 genotypes or concomitant azole use with the incidence of vincristine neurotoxicity.

Targeting OCT3 attenuates doxorubicin-induced cardiac injury.

Doxorubicin is a commonly used anticancer agent that can cause debilitating and irreversible cardiac injury. The initiating mechanisms contributing to this side effect remain unknown, and current preventative strategies offer only modest protection. Using stem-cell-derived cardiomyocytes from patients receiving doxorubicin, we probed the transcriptomic landscape of solute carriers and identified organic cation transporter 3 (OCT3) (SLC22A3) as a critical transporter regulating the cardiac accumulation of doxorubicin. Functional validation studies in heterologous overexpression models confirmed that doxorubicin is transported into cardiomyocytes by OCT3 and that deficiency of OCT3 protected mice from acute and chronic doxorubicin-related changes in cardiovascular function and genetic pathways associated with cardiac damage. To provide proof-of-principle and demonstrate translational relevance of this transport mechanism, we identified several pharmacological inhibitors of OCT3, including nilotinib, and found that pharmacological targeting of OCT3 can also preserve cardiovascular function following treatment with doxorubicin without affecting its plasma levels or antitumor effects in multiple models of leukemia and breast cancer. Finally, we identified a previously unrecognized, OCT3-dependent pathway of doxorubicin-induced cardiotoxicity that results in a downstream signaling cascade involving the calcium-binding proteins S100A8 and S100A9. These collective findings not only shed light on the etiology of doxorubicin-induced cardiotoxicity, but also are of potential translational relevance and provide a rationale for the implementation of a targeted intervention strategy to prevent this debilitating side effect.

MATE1 Deficiency Exacerbates Dofetilide-Induced Proarrhythmia.

Dofetilide is a rapid delayed rectifier potassium current inhibitor widely used to prevent the recurrence of atrial fibrillation and flutter. The clinical use of this drug is associated with increases in QTc interval, which predispose patients to ventricular cardiac arrhythmias. The mechanisms involved in the disposition of dofetilide, including its movement in and out of cardiomyocytes, remain unknown. Using a xenobiotic transporter screen, we identified MATE1 (SLC47A1) as a transporter of dofetilide and found that genetic knockout or pharmacological inhibition of MATE1 in mice was associated with enhanced retention of dofetilide in cardiomyocytes and increased QTc prolongation. The urinary excretion of dofetilide was also dependent on the MATE1 genotype, and we found that this transport mechanism provides a mechanistic basis for previously recorded drug-drug interactions of dofetilide with various contraindicated drugs, including bictegravir, cimetidine, ketoconazole, and verapamil. The translational significance of these observations was examined with a physiologically-based pharmacokinetic model that adequately predicted the drug-drug interaction liabilities in humans. These findings support the thesis that MATE1 serves a conserved cardioprotective role by restricting excessive cellular accumulation and warrant caution against the concurrent administration of potent MATE1 inhibitors and cardiotoxic substrates with a narrow therapeutic window.

Itraconazole-Induced Increases in Gilteritinib Exposure Are Mediated by CYP3A and OATP1B.

Gilteritinib, an FDA-approved tyrosine kinase inhibitor approved for the treatment of relapsed/refractory FLT3-mutated acute myeloid leukemia, is primarily eliminated via CYP3A4-mediated metabolism, a pathway that is sensitive to the co-administration of known CYP3A4 inhibitors, such as itraconazole. However, the precise mechanism by which itraconazole and other CYP3A-modulating drugs affect the absorption and disposition of gilteritinib remains unclear. In the present investigation, we demonstrate that pretreatment with itraconazole is associated with a significant increase in the systemic exposure to gilteritinib in mice, recapitulating the observed clinical drug-drug interaction. However, the plasma levels of gilteritinib were only modestly increased in CYP3A-deficient mice and not further influenced by itraconazole. Ensuing in vitro and in vivo studies revealed that gilteritinib is a transported substrate of OATP1B-type transporters, that gilteritinib exposure is increased in mice with OATP1B2 deficiency, and that the ability of itraconazole to inhibit OATP1B-type transport in vivo is contingent on its metabolism by CYP3A isoforms. These findings provide new insight into the pharmacokinetic properties of gilteritinib and into the molecular mechanisms underlying drug-drug interactions with itraconazole.

Kidney toxicity of the BRAF-kinase inhibitor vemurafenib is driven by off-target ferrochelatase inhibition.

A multitude of disease and therapy related factors drive the frequent development of kidney disorders in cancer patients. Along with chemotherapy, the newer targeted therapeutics can also cause kidney dysfunction through on and off-target mechanisms. Interestingly, among the small molecule inhibitors approved for the treatment of cancers that harbor BRAF-kinase activating mutations, vemurafenib can trigger tubular damage and acute kidney injury. BRAF is a proto-oncogene involved in cell growth. To investigate the underlying mechanisms, we developed cell culture and mouse models of vemurafenib kidney toxicity. At clinically relevant concentrations vemurafenib induces cell-death in transformed and primary mouse and human kidney tubular epithelial cells. In mice, two weeks of daily vemurafenib treatment causes moderate acute kidney injury with histopathological characteristics of kidney tubular epithelial cells injury. Importantly, kidney tubular epithelial cell-specific BRAF gene deletion did not influence kidney function under normal conditions or alter the severity of vemurafenib-associated kidney impairment. Instead, we found that inhibition of ferrochelatase, an enzyme involved in heme biosynthesis contributes to vemurafenib kidney toxicity. Ferrochelatase overexpression protected kidney tubular epithelial cells and conversely ferrochelatase knockdown increased the sensitivity to vemurafenib-induced kidney toxicity. Thus, our studies suggest that vemurafenib-associated kidney tubular epithelial cell dysfunction and kidney toxicity is BRAF-independent and caused, in part, by off-target ferrochelatase inhibition.

Endogenous Biomarkers for SLC Transporter-Mediated Drug-Drug Interaction Evaluation.

Membrane transporters play an important role in the absorption, distribution, metabolism, and excretion of xenobiotic substrates, as well as endogenous compounds. The evaluation of transporter-mediated drug-drug interactions (DDIs) is an important consideration during the drug development process and can guide the safe use of polypharmacy regimens in clinical practice. In recent years, several endogenous substrates of drug transporters have been identified as potential biomarkers for predicting changes in drug transport function and the potential for DDIs associated with drug candidates in early phases of drug development. These biomarker-driven investigations have been applied in both preclinical and clinical studies and proposed as a predictive strategy that can be supplanted in order to conduct prospective DDIs trials. Here we provide an overview of this rapidly emerging field, with particular emphasis on endogenous biomarkers recently proposed for clinically relevant uptake transporters.

Androgen receptor signalling impairs docetaxel efficacy in castration-resistant prostate cancer.

Androgen receptor (AR) signalling drives neoplastic growth and therapy resistance in prostate cancer. Recent clinical data show that docetaxel combined with androgen deprivation therapy improves outcome in hormone-sensitive disease. We studied whether testosterone and AR signalling interferes with docetaxel treatment efficacy in castration-resistant prostate cancer (CRPC). We found that testosterone supplementation significantly impaired docetaxel tumour accumulation in a CRPC model, resulting in decreased tubulin stabilisation and antitumour activity. Furthermore, testosterone competed with docetaxel for uptake by the drug transporter OATP1B3. Irrespective of docetaxel-induced tubulin stabilisation, AR signalling by testosterone counteracted docetaxel efficacy. AR-pathway activation could also reverse long-term tumour regression by docetaxel treatment in vivo. These results indicate that to optimise docetaxel efficacy, androgen levels and AR signalling need to be suppressed. This study lends evidence for continued maximum suppression of AR signalling by combining targeted therapeutics with docetaxel in CRPC.

Role of Oatp2b1 in Drug Absorption and Drug-Drug Interactions.

The organic anion transporting polypeptide (OATP)2B1 is localized on the basolateral membrane of hepatocytes and is expressed in enterocytes. Based on its distribution pattern and functional similarity to OATP1B-type transporters, OATP2B1 might have a role in the absorption and disposition of a range of xenobiotics. Although several prescription drugs, including hydroxymethylglutaryl-coenzyme A-CoA reductase inhibitors (statins) such as fluvastatin, are OATP2B1 substrates in vitro, evidence supporting the in vivo relevance of this transporter remains limited, and most has relied on substrate-inhibitor interactions resulting in altered pharmacokinetic properties of the victim drugs. To address this knowledge deficit, we developed and characterized an Oatp2b1-deficient mouse model and evaluated the impact of this transporter on the absorption and disposition of fluvastatin. Consistent with the intestinal localization of Oatp2b1, we found that the genetic deletion or pharmacological inhibition of Oatp2b1 was associated with decreased absorption of fluvastatin by 2- to 3-fold. The availability of a viable Oatp2b1-deficient mouse model provides an opportunity to unequivocally determine the contribution of this transporter to the absorption and drug-drug interaction potential of drugs. SIGNIFICANCE STATEMENT: The current investigation suggests that mice deficient in Oatp2b1 provide a valuable tool to study the in vivo importance of this transporter. In addition, our studies have identified novel potent inhibitors of OATP2B1 among the class of tyrosine kinase inhibitors, a rapidly expanding class of drugs used in various therapeutic areas that may cause drug-drug interactions with OATP2B1 substrates.

An integrative approach to cisplatin chronic toxicities in mice reveals importance of organic cation-transporter-dependent protein networks for renoprotection.

Cisplatin (CDDP) is one of the most important chemotherapeutic drugs in modern oncology. However, its use is limited by severe toxicities, which impair life quality after cancer. Here, we investigated the role of organic cation transporters (OCT) in mediating toxicities associated with chronic (twice the week for 4 weeks) low-dose (4 mg/kg body weight) CDDP treatment (resembling therapeutic protocols in patients) of wild-type (WT) mice and mice with OCT genetic deletion (OCT1/2-/-). Functional and molecular analysis showed that OCT1/2-/- mice are partially protected from CDDP-induced nephrotoxicity and peripheral neurotoxicity, whereas ototoxicity was not detectable. Surprisingly, proteomic analysis of the kidneys demonstrated that genetic deletion of OCT1/2 itself was associated with significant changes in expression of proinflammatory and profibrotic proteins which are part of an OCT-associated protein network. This signature directly regulated by OCT consisted of three classes of proteins, viz., profibrotic proteins, proinflammatory proteins, and nutrient sensing molecules. Consistent with functional protection, CDDP-induced proteome changes were more severe in WT mice than in OCT1/2-/- mice. Laser ablation-inductively coupled plasma-mass spectrometry analysis demonstrated that the presence of OCT was not associated with higher renal platinum concentrations. Taken together, these results redefine the role of OCT from passive membrane transporters to active modulators of cell signaling in the kidney.

Mitigation of acute kidney injury by cell-cycle inhibitors that suppress both CDK4/6 and OCT2 functions.

Acute kidney injury (AKI) is a potentially fatal syndrome characterized by a rapid decline in kidney function caused by ischemic or toxic injury to renal tubular cells. The widely used chemotherapy drug cisplatin accumulates preferentially in the renal tubular cells and is a frequent cause of drug-induced AKI. During the development of AKI the quiescent tubular cells reenter the cell cycle. Strategies that block cell-cycle progression ameliorate kidney injury, possibly by averting cell division in the presence of extensive DNA damage. However, the early signaling events that lead to cell-cycle activation during AKI are not known. In the current study, using mouse models of cisplatin nephrotoxicity, we show that the G1/S-regulating cyclin-dependent kinase 4/6 (CDK4/6) pathway is activated in parallel with renal cell-cycle entry but before the development of AKI. Targeted inhibition of CDK4/6 pathway by small-molecule inhibitors palbociclib (PD-0332991) and ribociclib (LEE011) resulted in inhibition of cell-cycle progression, amelioration of kidney injury, and improved overall survival. Of additional significance, these compounds were found to be potent inhibitors of organic cation transporter 2 (OCT2), which contributes to the cellular accumulation of cisplatin and subsequent kidney injury. The unique cell-cycle and OCT2-targeting activities of palbociclib and LEE011, combined with their potential for clinical translation, support their further exploration as therapeutic candidates for prevention of AKI.

Loss of SLCO1B3 drives taxane resistance in prostate cancer.

BACKGROUND: Both taxanes, docetaxel and cabazitaxel, are effective treatments for metastatic castration-resistant prostate cancer (mCRPC). However, resistance to taxanes is common. Our objective was to investigate mechanisms of taxane resistance in prostate cancer. METHODS: Two docetaxel-resistant patient-derived xenografts (PDXs) of CRPC were established (PC339-DOC and PC346C-DOC) in male athymic nude mice by frequent intraperitoneal administrations of docetaxel. Next-generation sequencing was performed on PDX tissue pre- and post-docetaxel resistance and gene expression profiles were compared. [(14)C]-docetaxel and [(14)C]-cabazitaxel uptake assays in vitro and cytotoxicity assays were performed to validate direct involvement of transporter genes in taxane sensitivity. RESULTS: Organic anion-transporting polypeptide (SLCO1B3), an influx transporter of docetaxel, was significantly downregulated in PC346C-DOC tumours. In accordance with this finding, intratumoural concentrations of docetaxel and cabazitaxel were significantly decreased in PC346C-DOC as compared with levels in chemotherapy-naive PC346C tumours. In addition, silencing of SLCO1B3 in chemo-naive PC346C resulted in a two-fold decrease in intracellular concentrations of both taxanes. Overexpression of SLCO1B3 showed higher sensitivity to docetaxel and cabazitaxel. CONCLUSIONS: The SLCO1B3 determines intracellular concentrations of docetaxel and cabazitaxel and consequently influences taxane efficacy. Loss of the drug transporter SLCO1B3 may drive taxane resistance in prostate cancer.

Oxaliplatin-induced neurotoxicity is dependent on the organic cation transporter OCT2.

Oxaliplatin is an integral component of colorectal cancer therapy, but its clinical use is associated with a dose-limiting peripheral neurotoxicity. We found that the organic cation transporter 2 (OCT2) is expressed on dorsal root ganglia cells within the nervous system where oxaliplatin is known to accumulate. Cellular uptake of oxaliplatin was increased by 16- to 35-fold in cells overexpressing mouse Oct2 or human OCT2, and this process was associated with increased DNA platination and oxaliplatin-induced cytotoxicity. Furthermore, genetic or pharmacologic knockout of Oct2 protected mice from hypersensitivity to cold or mechanical-induced allodynia, which are established tests to assess acute oxaliplatin-induced neurotoxicity. These findings provide a rationale for the development of targeted approaches to mitigate this debilitating toxicity.

Lysosomal Sequestration Determines Intracellular Imatinib Levels.

The intracellular uptake and retention (IUR) of imatinib is reported to be controlled by the influx transporter SLC22A1 (organic cation transporter 1). We recently hypothesized that alternative uptake and/or retention mechanisms exist that determine intracellular imatinib levels. Here, we systematically investigate the nature of these mechanisms. Imatinib uptake in cells was quantitatively determined by liquid chromatography-tandem mass spectrometry. Fluorescent microscopy was used to establish subcellular localization of imatinib. Immunoblotting, cell cycle analyses, and apoptosis assays were performed to evaluate functional consequences of imatinib sequestration. Uptake experiments revealed high intracellular imatinib concentrations in HEK293, the leukemic cell lines K562 and SD-1, and a gastrointestinal stromal tumor cell line GIST-T1. We demonstrated that imatinib IUR is time-, dose-, temperature-, and energy-dependent and provide evidence that SLC22A1 and other potential imatinib transporters do not substantially contribute to the IUR of imatinib. Prazosin, amantadine, NH4Cl, and the vacuolar ATPase inhibitor bafilomycin A1 significantly decreased the IUR of imatinib and likely interfere with lysosomal retention and accumulation of imatinib. Costaining experiments with LysoTracker Red confirmed lysosomal sequestration of imatinib. Inhibition of the lysosomal sequestration had no effect on the inhibition of c-Kit signaling and imatinib-mediated cell cycle arrest but significantly increased apoptosis in imatinib-sensitive GIST-T1 cells. We conclude that intracellular imatinib levels are primarily determined by lysosomal sequestration and do not depend on SLC22A1 expression.

Rare versus common variants in pharmacogenetics: SLCO1B1 variation and methotrexate disposition.

Methotrexate is used to treat autoimmune diseases and malignancies, including acute lymphoblastic leukemia (ALL). Inter-individual variation in clearance of methotrexate results in heterogeneous systemic exposure, clinical efficacy, and toxicity. In a genome-wide association study of children with ALL, we identified SLCO1B1 as harboring multiple common polymorphisms associated with methotrexate clearance. The extent of influence of rare versus common variants on pharmacogenomic phenotypes remains largely unexplored. We tested the hypothesis that rare variants in SLCO1B1 could affect methotrexate clearance and compared the influence of common versus rare variants in addition to clinical covariates on clearance. From deep resequencing of SLCO1B1 exons in 699 children, we identified 93 SNPs, 15 of which were non-synonymous (NS). Three of these NS SNPs were common, with a minor allele frequency (MAF) >5%, one had low frequency (MAF 1%-5%), and 11 were rare (MAF <1%). NS SNPs (common or rare) predicted to be functionally damaging were more likely to be found among patients with the lowest methotrexate clearance than patients with high clearance. We verified lower function in vitro of four SLCO1B1 haplotypes that were associated with reduced methotrexate clearance. In a multivariate stepwise regression analysis adjusting for other genetic and non-genetic covariates, SLCO1B1 variants accounted for 10.7% of the population variability in clearance. Of that variability, common NS variants accounted for the majority, but rare damaging NS variants constituted 17.8% of SLCO1B1's effects (1.9% of total variation) and had larger effect sizes than common NS variants. Our results show that rare variants are likely to have an important effect on pharmacogenetic phenotypes.

4ß-hydroxycholesterol as an endogenous CYP3A marker in cancer patients treated with taxanes.

AIM: Taxanes are anti-cancer agents used to treat several types of solid tumours. They are metabolized by cytochrome P450 (CYP) 3A, displaying a large pharmacokinetic (PK) variability. In this study, we evaluated the endogenous CYP3A4 marker 4ß-hydroxycholesterol (4ß-OHC) as a potential individual taxane PK predictor. METHODS: Serum 4ß-OHC and cholesterol concentrations were determined in 291 paclitaxel and 151 docetaxel-treated patients, and were subsequently correlated with taxane clearance. RESULTS: In the patients treated with paclitaxel, no clinically relevant correlations between the 4ß-OHC or 4ß-OHC : cholesterol ratio and paclitaxel clearance were found. In the patients treated with docetaxel, 4ß-OHC concentration was weakly correlated with docetaxel clearance in males (r = 0.35 P = 0.01, 95% CI 0.08, 0.58). Of the 10% patients with taxane outlier clearance values, 4ß-OHC did correlate with docetaxel clearance in males (r = 0.76, P = 0.03, 95% CI 0.12, 0.95). CONCLUSION: There was no clinical correlation between paclitaxel clearance and the CYP3A4 activity markers 4ß-OHC or the 4ß-OHC : cholesterol ratio. A weak correlation was observed between 4ß-OHC and docetaxel clearance, but only in males. This endogenous CYP3A4 marker has limited predictive value for taxane clearance in patients.

Uptake carriers and oncology drug safety.

Members of the solute carrier (SLC) family of transporters are responsible for the cellular influx of a broad range of endogenous compounds and xenobiotics in multiple tissues. Many of these transporters are highly expressed in the gastrointestinal tract, liver, and kidney and are considered to be of particular importance in governing drug absorption, elimination, and cellular sensitivity of specific organs to a wide variety of oncology drugs. Although the majority of studies on the interaction of oncology drugs with SLC have been restricted to the use of exploratory in vitro model systems, emerging evidence suggests that several SLCs, including OCT2 and OATP1B1, contribute to clinically important phenotypes associated with those agents. Recent literature has indicated that modulation of SLC activity may result in drug-drug interactions, and genetic polymorphisms in SLC genes have been described that can affect the handling of substrates. Alteration of SLC function by either of these mechanisms has been demonstrated to contribute to interindividual variability in the pharmacokinetics and toxicity associated with several oncology drugs. In this report, we provide an update on this rapidly emerging field.

Quantification of the aromatase inhibitor letrozole and its carbinol metabolite in mouse plasma by UHPLC-MS/MS.

A liquid chromatography - electrospray ionization-mass spectrometry (LC-ESI-MS) method was developed for the quantification of letrozole, a third-generation aromatase inhibitor, and its main carbinol metabolite (CM) in support of murine pharmacokinetic studies. Using polarity switching, simultaneous ESI-MS measurement of letrozole and CM was achieved in positive and negative mode, respectively. The assay procedure involved a one-step protein precipitation and extraction of all analytes from mouse plasma requiring only 5 µL of sample. Separation was optimized on an Accucore aQ column with gradient elution at a flow rate of 0.4 mL/min in 5 min. Two calibration curves per day over four consecutive measurement days showed satisfactory linear responses (r2 > 0.99) over concentration ranges of 5-1000 ng/mL and 20-2000 ng/mL for letrozole and CM, respectively. No matrix effect was found, and the mean extraction recoveries were 103-108 % for letrozole and 99.8-107 % for CM. Precision and accuracy within a single run and over four consecutive measurement days were verified to be within acceptable limits. Application of the developed method to preclinical pharmacokinetic studies in mice receiving oral letrozole at a dose 1 or 10 mg/kg revealed that the systemic exposure to letrozole was dose-, formulation-, and strain-dependent. These findings may inform the future design of preclinical studies aimed at refining the pharmacological profile of this clinically important drug.