RNA is a pro-apoptotic target of cisplatin in cancer cell lines and C. elegans.

Cisplatin not only targets DNA but also RNA. However, it is largely unknown whether platinated RNA (Pt-RNA) causes apoptosis and thus contributes to the cytotoxic effects of cisplatin. Consequently, cellular RNA was isolated from HepG2 and LS180 cells, exposed to cisplatin, and the resulting Pt-RNA (20 ng Pt/µg RNA) was transfected into these cancer cell lines or used to treat an apoptosis reporter Caenorhabditis elegans (C. elegans) strain (MD701, expressing CED-1::GFP). Cellular and molecular effects of Pt-RNA were evaluated by luminogenic caspase 3/7 assays, PCR array analysis, and fluorescence microscopy-based quantification of apoptosis in C. elegans gonads. Assuming RNA cross-linking (pseudo double-stranded RNA), the contribution of the Toll-like receptor 3 (TLR3, a sensor of double-stranded RNA) to apoptosis induction in cancer cell lines was investigated by pharmacological TLR3 inhibition and overexpression. In contrast to controls, Pt-RNA significantly enhanced apoptosis in C. elegans (2-fold) and in the cancer cell lines (2-fold to 4-fold). TLR3 overexpression significantly enhanced the pro-apoptotic effects of Pt-RNA in HepG2 cells. TLR3 inhibition reduced the pro-apoptotic effects of Pt-RNA and cisplatin, but not of paclitaxel (off-target control). Gene expression analysis showed that Pt-RNA (but not RNA) significantly enhanced the mRNA levels of nuclear factor kappa B subunit 2 and interleukin-8 in HepG2 cells, suggesting that Pt-RNA is a damage-associated molecular pattern that additionally causes pro-inflammatory responses. Together, this data suggests that not only DNA but also cellular RNA is a functionally relevant target of cisplatin, leading to pro-apoptotic and immunogenic effects.

Persister cell phenotypes contribute to poor patient outcomes after neoadjuvant chemotherapy in PDAC.

Neoadjuvant chemotherapy can improve the survival of individuals with borderline and unresectable pancreatic ductal adenocarcinoma; however, heterogeneous responses to chemotherapy remain a significant clinical challenge. Here, we performed RNA sequencing (n = 97) and multiplexed immunofluorescence (n = 122) on chemo-naive and postchemotherapy (post-CTX) resected patient samples (chemoradiotherapy excluded) to define the impact of neoadjuvant chemotherapy. Transcriptome analysis combined with high-resolution mapping of whole-tissue sections identified GATA6 (classical), KRT17 (basal-like) and cytochrome P450 3A (CYP3A) coexpressing cells that were preferentially enriched in post-CTX resected samples. The persistence of GATA6hi and KRT17hi cells post-CTX was significantly associated with poor survival after mFOLFIRINOX (mFFX), but not gemcitabine (GEM), treatment. Analysis of organoid models derived from chemo-naive and post-CTX samples demonstrated that CYP3A expression is a predictor of chemotherapy response and that CYP3A-expressing drug detoxification pathways can metabolize the prodrug irinotecan, a constituent of mFFX. These findings identify CYP3A-expressing drug-tolerant cell phenotypes in residual disease that may ultimately inform adjuvant treatment selection.

Quantification of Biologically Active DNA Alkylation in Temozolomide-Exposed Glioblastoma Cell Lines by Ultra-Performance Liquid Chromatography-Tandem Mass Spectrometry: Method Development and Recommendations for Validation.

Quantitative monitoring of biologically active methylations of guanines in samples exposed to temozolomide (TMZ) would be useful in glioblastoma research for preclinical TMZ experiments, for clinical pharmacology questions regarding appropriate exposure, and ultimately for precision oncology. The known biologically active alkylation of DNA induced by TMZ takes place on O6 position of guanines. However, when developing mass spectrometric (MS) assays, the possible signal overlap of O6-methyl-2'-deoxyguanosine (O6-m2dGO) with other methylated 2'-deoxyguanosine species in DNA and methylated guanosines in RNA must be considered. Liquid chromatography-tandem MS (LC-MS/MS) offers the analytical requirements for such assays in terms of specificity and sensitivity, especially when multiple reaction monitoring (MRM) is available. In preclinical research, cancer cell lines are still the gold standard model for in vitro drug screening. Here, we present the development of ultra-performance LC-MRM-MS assays for the quantification of O6-m2dGO in a TMZ-treated glioblastoma cell line. Furthermore, we propose adapted parameters for method validation relevant to the quantification of drug-induced DNA modifications.

Molecular prediction of clinical response to anti-PD-1/anti-PD-L1 immune checkpoint inhibitors: New perspectives for precision medicine and mass spectrometry-based investigations.

Monoclonal antibodies (mAbs) acting as immune checkpoint inhibitors (ICIs) are among the most frequently used immunotherapies in oncology. However, precision medicine approaches to adapt the treatment to the patient are still poorly exploited. Given the risk of severe adverse reactions, predicting patient eligibility for ICI therapy represents a great asset for precision medicine. Today, the extended panel of mass spectrometric approaches, accompanied by newly developed sample preparation methods is a strategy of choice for responder and non-responder stratification on a molecular basis, and early detection of resistance. In this perspective article, we review the biodisposition of mAbs, the interest in molecular stratification of patients treated with these mAbs, and the possible analytical strategies to achieve this goal, with a major emphasis on mass spectrometric approaches.

Regulation of PXR Function by Coactivator and Corepressor Proteins: Ligand Binding Is Just the Beginning.

The pregnane X receptor (PXR, NR1I2) is a nuclear receptor which exerts its regulatory function by heterodimerization with the retinoid-X-receptor α (RXRα, NR2B1) and binding to the promoter and enhancer regions of diverse target genes. PXR is involved in the regulation of drug metabolism and excretion, metabolic and immunological functions and cancer pathogenesis. PXR activity is strongly regulated by the association with coactivator and corepressor proteins. Coactivator proteins exhibit histone acetyltransferase or histone methyltransferase activity or associate with proteins having one of these activities, thus promoting chromatin decondensation and activation of the gene expression. On the contrary, corepressor proteins promote histone deacetylation and therefore favor chromatin condensation and repression of the gene expression. Several studies pointed to clear cell- and ligand-specific differences in the activation of PXR. In this article, we will review the critical role of coactivator and corepressor proteins as molecular determinants of the specificity of PXR-mediated effects. As already known for other nuclear receptors, understanding the complex mechanism of PXR activation in each cell type and under particular physiological and pathophysiological conditions may lead to the development of selective modulators with therapeutic potential.

Acquired ABC-transporter overexpression in cancer cells: transcriptional induction or Darwinian selection?

Acquired multidrug resistance (MDR) in tumor diseases has repeatedly been associated with overexpression of ATP-binding cassette transporters (ABC-transporters) such as P-glycoprotein. Both in vitro and in vivo data suggest that these efflux transporters can cause MDR, albeit its actual relevance for clinical chemotherapy unresponsiveness remains uncertain. The overexpression can experimentally be achieved by exposure of tumor cells to cytotoxic drugs. For simplification, the drug-mediated transporter overexpression can be attributed to two opposite mechanisms: First, increased transcription of ABC-transporter genes mediated by nuclear receptors sensing the respective compound. Second, Darwinian selection of sub-clones intrinsically overexpressing drug transporters being capable of extruding the respective drug. To date, there is no definite data indicating which mechanism truly applies or whether there are circumstances promoting either mode of action. This review summarizes experimental evidence for both theories, suggests an algorithm discriminating between these two modes, and finally points out future experimental approaches of research to answer this basic question in cancer pharmacology.

In vitro evidence suggesting that the toll-like receptor 7 and 8 agonist resiquimod (R-848) unlikely affects drug levels of co-administered compounds.

Resiquimod (R-848) is an immune response modifier activating toll-like receptor 7 and 8. Its potential to cause pharmacokinetic interactions with concurrently administered drugs is unknown. To study the time course of the effect of resiquimod in LS180 cells as a model for intestinal tissue, luciferase-based reporter gene assays and reverse transcription polymerase chain reaction were used to investigate whether resiquimod affects the activities of nuclear factor kappa B (NF-ĸB), pregnane x receptor (PXR) or the transcription of selected central genes for drug disposition (cytochrome P-450 isozyme 3A4 (CYP3A4), CYP1A1, UDP-glucuronosyltransferase 1A1 (UGT1A1), ATP-binding cassette transporters ABCC2, ABCB1). Its impact on the activities of organic anion transporting polypeptides 1 or 3 (OATP1B1/3), breast cancer resistance protein (BCRP), P-glycoprotein (P-gp) or CYP3A4 was evaluated using fluorescence- or luminescence-based activity assays. Resiquimod irrelevantly increased NF-ĸB activity after 2 h (1 µM: 1.07-fold, P = 0.0188; 10 µM: 1.09-fold, P = 0.0142), and diminished it after 24 h (1 µM: 0.64-fold, P < 0.0001; 10 µM: 0.68-fold, P < 0.0001) and 30 h (10 µM: 0.68-fold, P = 0.0003). Concurrently, PXR activity after 24 h was marginally increased by 10 µM (1.05-fold, P = 0.0019). Resiquimod did not alter mRNA expression levels, activities of uptake or efflux transporters, or CYP3A4 activity. Given the marginal effects on NF-ĸB, PXR, expression levels of selected PXR target genes, and activities of important drug transporters and CYP3A4 in vitro, resiquimod is not expected to cause major pharmacokinetic drug-drug interactions in vivo.

Approaching Sites of Action of Temozolomide for Pharmacological and Clinical Studies in Glioblastoma.

Temozolomide (TMZ), together with bulk resection and focal radiotherapy, is currently a standard of care for glioblastoma. Absorption, distribution, metabolism, and excretion (ADME) parameters, together with the mode of action of TMZ, make its biochemical and biological action difficult to understand. Accurate understanding of the mode of action of TMZ and the monitoring of TMZ at its anatomical, cellular, and molecular sites of action (SOAs) would greatly benefit precision medicine and the development of novel therapeutic approaches in combination with TMZ. In the present perspective article, we summarize the known ADME parameters and modes of action of TMZ, and we review the possible methodological options to monitor TMZ at its SOAs. We focus our descriptions of methodologies on mass spectrometry-based approaches, and all related considerations are taken into account regarding the avoidance of artifacts in mass spectrometric analysis during sampling, sample preparation, and the evaluation of results. Finally, we provide an overview of potential applications for precision medicine and drug development.

Approaching sites of action of drugs in clinical pharmacology: New analytical options and their challenges.

Clinical pharmacology is an important discipline for drug development aiming to define pharmacokinetics (PK), pharmacodynamics (PD) and optimum exposure to drugs, i.e. the concentration-response relationship and its modulators. For this purpose, information on drug concentrations at the anatomical, cellular and molecular sites of action is particularly valuable. In pharmacological assays, the limited accessibility of target cells in readily available samples (i.e. blood) often hampers mass spectrometry-based monitoring of the absolute quantity of a compound and the determination of its molecular action at the cellular level. Recently, new sample collection methods have been developed for the specific capture of rare circulating cells, especially for the diagnosis of circulating tumour cells. In parallel, new advances and developments in mass spectrometric instrumentation now allow analyses to be scaled down to the cellular level. Together, these developments may permit the monitoring of minute drug quantities and show their effect at the cellular level. In turn, such PK/PD associations on a cellular level would not only enrich our pharmacological knowledge of a given compound but also expand the basis for PK/PD simulations. In this review, we describe novel concepts supporting clinical pharmacology at the anatomical, cellular and molecular sites of action, and highlight the new challenges in mass spectrometry-based monitoring. Moreover, we present methods to tackle these challenges and define future needs.

Reporter cell assay-based functional quantification of TNF-α-antagonists in serum - a proof-of-principle study for adalimumab.

Quantification of therapeutic antibodies is commonly based on physico-chemical assays such as enzyme-linked immunoabsorption assays (ELISA) and lately on mass spectrometry. However, the functional integrity of evaluated immunoglobulins is yet not assessed. Consequently, a commercially available reporter cell line was used to quantify the functional concentration of the anti-tumor necrosis factor alpha (TNF-α) antibody adalimumab present in serum of a healthy beagle dog treated with 3 mg intravenous adalimumab (Humira®). HEK-Blue™-hTLR3 cells express a secreted alkaline phosphatase under the control of a nuclear factor kappa B (NF-κB) response element. Its enzymatic activity can be recorded using colorimetry, which reports activity of extracellular NF-κB stimuli such as TNF-α. Using an adalimumab concentration-response calibration curve, the functional concentration of serum adalimumab was estimated to be 4.9 ± 1.4 µg/ml, which was in excellent agreement with ELISA results (4.8 µg/ml). The obtained data suggest that this simple, easy-to-handle reporter cell assay can be used for the functional quantification of adalimumab present in samples from in vitro or pre-clinical in vivo experiments. Moreover, this assay could be used in vitro to compare the pharmacodynamics of adalimumab biosimilars or different anti-TNF-α compounds, respectively.

Elucidating the beneficial effects of melphalan, adriamycin, and corticoids in combination with bortezomib against multiple myeloma in vitro.

Combining bortezomib with other anti-cancer drugs or glucocorticoids is more efficient in multiple myeloma than bortezomib alone. However, the molecular mechanism of this beneficial effect is largely unknown. To investigate the effects of these compounds on bortezomib's anti-proliferative potency and its intracellular accumulation and potency to inhibit the chymotrypsin-like proteasomal subunit, seven myeloma cell lines were investigated after exposure to bortezomib alone or either combined with adriamycin plus dexamethasone (PAD regimen) or melphalan plus prednisolone (VMP regimen), respectively. PAD or VMP combinations did not alter cellular bortezomib uptake. However, PAD and VMP regimens increased bortezomib's chymotrypsin-like subunit inhibitory potency. This likely originates from indirect proteasome modulation, because adriamycin, dexamethasone, melphalan, or prednisolone did not inhibit this subunit when used alone. Strikingly, the anti-proliferative potency of bortezomib was not enhanced but slightly lowered in some cell lines when used in combinations. Adriamycin, dexamethasone, melphalan, or prednisolone can enhance bortezomib's chymotrypsin-like subunit inhibitory potency, likely by mechanisms indirectly influencing proteasome functionality.

Methadone against cancer: Lost in translation.

Recently, the opioid analgesic d,l-methadone has gained much attention as a potential antineoplastic compound, considerably triggered through lay press and media. In consequence, physicians and pharmacists are currently confronted with numerous patients willing to use d,l-methadone against their malignancies. Well-performed in vitro and in vivo models have in fact shown pro-apoptotic effects of d,l-methadone or other opioids, but also proliferation-stimulating properties. Moreover, the mechanisms of proposed opioid-stimulated apoptosis are incompletely described or contradicting. Finally, the receptors mostly responsible for induction of apoptosis by d,l-methadone remain unclear as contributions of both µ-opioid receptors, Fas cell death receptors, toll-like receptors, N-Methyl-d-aspartate receptors and opioid growth factor receptors were suggested. Such ambiguity prevents rational application of d,l-methadone or patient stratification to enhance beneficial antineoplastic effects. From a clinical point of view, d,l-methadone and other opioids might in fact prolong survival, but such effects likely originate from their analgesic and neuro-psychotropic properties and, thus, improvements of quality of life. Crucial obstacles to the administration of d,l-methadone are incomplete knowledge about its systemic disposition, highly variable pharmacokinetics, profound drug-drug- or drug-disease interaction and QT-prolongation potential. This article summarizes and rates the pharmacological basis of d,l-methadone as an antineoplastic agent and puts its administration in clinical oncology into perspective. Despite enthralling experimental findings about d,l-methadone-mediated apoptosis in cancerous cells or tissues, clinicians should realize the current lack of evidence for the use of d,l-methadone as an antineoplastic agent. Its administration against cancer pain is, however, tenable, albeit restricted to certain clinical situations.

The pregnane X receptor (PXR) and the nuclear receptor corepressor 2 (NCoR2) modulate cell growth in head and neck squamous cell carcinoma.

Head and neck squamous cell carcinoma (HNSCC) is the sixth most frequent cancer worldwide. The pregnane X receptor (PXR) is a nuclear receptor regulating several target genes associated with cancer malignancy. We here demonstrated a significant effect of PXR on HNSCC cell growth, as evidenced in PXR knock-down experiments. PXR transcriptional activity is more importantly regulated by the presence of coactivators and corepressors than by PXR protein expression. To date, there is scarce information on the regulation of PXR in HNSCC and on its role in the pathogenesis of this disease. Coactivator and corepressor expression was screened through qRT-PCR in 8 HNSCC cell lines and correlated to PXR activity, determined by using a reporter gene assay. All cell lines considerably expressed all the cofactors assessed. PXR activity negatively correlated with nuclear receptor corepressor 2 (NCoR2) expression, indicating a major role of this corepressor in PXR modulation and suggesting its potential as a surrogate for PXR activity in HNSCC. To test the association of NCoR2 with the malignant phenotype, a subset of three cell lines was transfected with an over-expression plasmid for this corepressor. Subsequently, cell growth and chemoresistance assays were performed. To elucidate the mechanisms underlying NCoR2 effects on cell growth, caspase 3/7 activity and protein levels of cleaved caspase 3 and PARP were evaluated. In HNO97 cells, NCoR2 over-expression decreased cell growth, chemoresistance and increased cleaved caspase 3 levels, caspase activity and cleaved PARP levels. On the contrary, in HNO124 and HNO210 cells, NCoR2 over-expression increased cell growth, drug resistance and decreased cleaved caspase 3 levels, caspase activity and cleaved PARP levels. In conclusion, we demonstrated a role of PXR and NCoR2 in the modulation of cell growth in HNSCC. This may contribute to a better understanding of the highly variable HNSCC therapeutic response.

t-Darpp stimulates protein kinase A activity by forming a complex with its RI regulatory subunit.

t-Darpp is the truncated form of the dopamine- and cAMP-regulated phosphoprotein of 32kDa (Darpp-32) and has been demonstrated to confer resistance to trastuzumab, a Her2-targeted anticancer agent, via sustained signaling through the phosphatidylinositol-4,5-bisphosphate 3-kinase (PI3K)/Akt pathway and activation of protein kinase A (PKA). The mechanism of t-Darpp-mediated PKA activation is poorly understood. In the PKA holoenzyme, when the catalytic subunits are bound to regulatory subunits RI or RII, kinase activity is inhibited. We investigated PKA activity and holoenzyme composition in cell lines overexpressing t-Darpp (SK.tDp) or a T39A phosphorylation mutant (SK.tDpT39A), as well as an empty vector control cell line (SK.empty). We also evaluated protein-protein interactions between t-Darpp and PKA catalytic (PKAc) or regulatory subunits RI and RII in those cell lines. SK.tDp cells had elevated PKA activity and showed diminished association of RI with PKAc, whereas SK.tDpT39A cells did not have these properties. Moreover, wild type t-Darpp associates with RI. Concurrent expression of Darpp-32 reversed t-Darrp's effects on PKA holoenzyme state, consistent with earlier observations that Darpp-32 reverses t-Darpp's activation of PKA. Together, t-Darpp phosphorylation at T39 seems to be crucial for t-Darpp-mediated PKA activation and this activation appears to occur through an association with RI and sequestering of RI away from PKAc. The t-Darpp-RI interaction could be a druggable target to reduce PKA activity in drug-resistant cancer.

Bortezomib, carfilzomib and ixazomib do not mediate relevant transporter-based drug-drug interactions.

In order to optimize the clinical application of an increasing number of proteasome inhibitors, investigations into the differences between their respective pharmacodynamic and pharmacokinetic profiles, including their ability to act as a perpetrator in drug-drug interactions, are warranted. Therefore, in the present in vitro study, it was investigated whether bortezomib, carfilzomib and ixazomib are able to alter the expression, and/or the activity, of specific drug transporters generally relevant for pharmacokinetic drug-drug interactions. Through induction experiments, the current study demonstrated that the aforementioned three proteasome inhibitors do not induce mRNA expression of the transporter genes ATP binding cassette (ABC)B1, C1, C2 and G2 in the LS180 cell line, which was used as a model for systemic induction. By contrast, in certain myeloma cell lines, ixazomib provoked minor alterations in individual transporter gene expression. None of the proteasome inhibitors tested relevantly inhibited drug transporters within the range of physiological plasma concentrations. Taken together, transporter-based drug-drug interactions are unlikely to be a primary concern in the clinical application of the tested compounds.

Impact of enzalutamide and its main metabolite N-desmethyl enzalutamide on pharmacokinetically important drug metabolizing enzymes and drug transporters.

Enzalutamide is a new drug against castration-resistant prostate cancer. Recent data indicate profound induction of drug metabolizing enzymes (e.g. cytochrome P450 isoenzyme (CYP) 3A4) but comprehensive in vitro data on other CYP enzymes, drug conjugating enzymes or drug transporters is scarce. Moreover, the mechanisms of induction are poorly investigated and the effects of the active metabolite N-desmethyl enzalutamide are unknown. Using LS180 cells as an induction model and quantitative real-time reverse transcription polymerase chain reaction, our study demonstrated a concentration-dependent induction of CYP1A1, CYP1A2, CYP3A5, CYP3A4, UGT1A3, UGT1A9, ABCB1, ABCC2 and ABCG2 mRNA. Induction of CYP3A4 and ABCB1 was confirmed by Western blot analysis and is likely mediated by activation of the nuclear receptor pregnane x receptor, elucidated by a luciferase-based reporter gene assay. Enzalutamide's main active metabolite N-desmethyl enzalutamide exhibited only weak induction properties. mRNA expression of UGT2B7 was suppressed by enzalutamide and its metabolite. Both compounds are apparently not transported by P-glycoprotein (P-gp) or breast cancer resistance protein (BCRP). N-desmethyl enzalutamide more potently inhibited important drug transporters (P-gp, BCRP, OATPs) than enzalutamide. Taken together, the pharmacokinetics of concurrently administered drugs is likely altered during enzalutamide therapy. Levels of metabolically (mainly CYP3A4) eliminated drugs are expected to be decreased, whereas the abundance of compounds with solely transporter-determined pharmacokinetics (P-gp, OATPs) is likely enhanced.

Cellular effect and efficacy of carfilzomib depends on cellular net concentration gradient.

PURPOSE: The cellular interrelation between intracellular concentrations of unbound carfilzomib, a second-generation proteasome inhibitor, and subsequent proteasome inhibition and effect on cell viability are unknown and were evaluated for two different exposure regimens: A high dose bolus regime of 500 nM for 1 h followed by 47 h in drug-free media vs. 48-h continuous exposure to 10 nM. METHODS: Eight multiple myeloma cell lines were exposed to either one of the two exposure regimens. We quantified the intracellular unbound carfilzomib fraction up to 48 h with a new ultra-performance liquid chromatography coupled to tandem mass spectrometry (UPLC/MS/MS) method. Intracellular concentrations were compared to simultaneously determined cell viability (AlamarBlue® assay) and proteasomal subunit activity (ProGlo™ assay). RESULTS: Within the first 10 min, the proportional intracellular enrichment of unbound carfilzomib was higher (313 nM; 62.6%) for the exposure to 500 nM compared to 10 nM (1.93 nM; 19.3%). However, after 1 h, an intracellular/extracellular concentration equilibrium was reached with both settings. At low exposure concentrations, drug removal after 1 h diminished carfilzomib efficacy. Moreover, proteasomal activity recovered when exposed to 10 nM for 48 h. However, when exposure concentration was high (500 nM) proteasome inhibition was complete and sustained even with drug removal after 1 h. CONCLUSIONS: We demonstrated that the carfilzomib concentration gradient determines cellular uptake kinetics. The uptake kinetics in turn affects binding, saturation, and activity of the proteasome. Together, these data underscore the importance of steep concentrations for the in vitro efficacy of carfilzomib.

Differential effects of the enantiomers of tamsulosin and tolterodine on P-glycoprotein and cytochrome P450 3A4.

The pregnane X receptor (PXR) is a transcription factor regulating P-glycoprotein (P-gp; ABCB1)-mediated transport and cytochrome P450 3A4 (CYP3A4)-mediated metabolism of xenobiotics thereby affecting the pharmacokinetics of many drugs and potentially modulating clinical efficacy. Thus, pharmacokinetic drug-drug interactions can arise from PXR activation. Here, we examined whether the selective α1-adrenoreceptor blocker tamsulosin or the antagonist of muscarinic receptors tolterodine affect PXR-mediated regulation of CYP3A4 and of P-gp at the messenger RNA (mRNA) and protein level in an enantiomer-specific way. In addition, the effect of tamsulosin and tolterodine on P-gp activity was evaluated. We used quantitative real-time PCR, gene reporter assay, western blotting, rhodamine efflux assay, and calcein assay for determination of expression, activity, and inhibition of P-glycoprotein. The studied compounds significantly and concentration-dependently increased PXR activity in the ABCB1-driven luciferase-based reporter gene assay. We observed much stronger induction of ABCB1 mRNA by S-tamsulosin as compared to the R or racemic form. R or racemic form of tolterodine and R-tamsulosin concentration-dependently increased P-gp protein expression; the latter also enhanced P-gp efflux function in a rhodamine-based efflux assay. R-tamsulosin and all forms of tolderodine slightly inhibited P-gp. The effect on CYP3A4 expression followed the same pattern but was much weaker. Taken together, tamsulosin and tolterodine are demonstrated to interfere with P-gp and CYP3A4 regulation in an enantiomer-specific way.