Challenges with the precise prediction of ABC-transporter interactions for improved drug discovery

Introduction: Given that membrane efflux transporters can influence a drug’s pharmacokinetics, efficacy and safety, identifying potential substrates and inhibitors of these transporters is a critical element in the drug discovery and development process. Additionally, it is important to predict the inhibition potential of new drugs to avoid clinically significant drug interactions. The goal of preclinical studies is to characterize a new drug as a substrate or inhibitor of efflux transporters.

Areas covered: This article reviews preclinical systems that are routinely utilized to determine whether a new drug is substrate or inhibitor of efflux transporters including in silico models, in vitro membrane and cell assays, and animal models. Also included is an examination of studies comparing in vitro inhibition data to clinical drug interaction outcomes.

Expert opinion: While a number of models are employed to classify a drug as an efflux substrate or inhibitor, there are challenges in predicting clinical drug interactions. Improvements could be made in these predictions through a tier approach to classify new drugs, validation of preclinical assays, and refinement of threshold criteria for clinical interaction studies.     

Interactions of pesticides with membrane drug transporters: implications for toxicokinetics and toxicity

Introduction: Drug transporters are now recognized as major actors of pharmacokinetics. They are also likely implicated in toxicokinetics and toxicology of environmental pollutants, notably pesticides, to which humans are widely exposed and which are known to exert various deleterious effects toward health. Interactions of pesticides with drug transporters are therefore important to consider.

Areas covered: This review provides an overview of the interactions of pesticides with membrane drug transporters, i.e. inhibition of their activity, regulation of their expression, and handling of pesticides. Consequences for toxicokinetics and toxicity of pesticides are additionally summarized and discussed.

Expert opinion: Some pesticides belonging to several chemical classes, such as organochlorine, pyrethroid, and organophosphorus pesticides, have been demonstrated to interact with various uptake and efflux drug transporters, including the efflux pump P-glycoprotein (P-gp) and the uptake organic cation transporters (OCTs). This provides proof of the concept that pesticide–transporter relationships merit attention. More extensive and systematic characterization of pesticide–transporter relationships, possibly through the use of in silico methods, is however likely required. In addition, consideration of transporter polymorphisms, pesticide mixture effects, and realistic pesticide concentrations reached in humans may help better define the in vivo relevance of pesticide–transporter interactions in terms of toxicokinetics and toxicity.     

Clinical reappraisal of the influence of drug-transporter polymorphisms in epilepsy

Introduction: Although novel antiepileptic drugs (AEDs) have been recently released, the issue of drug resistance in epileptic patients remains unsolved and largely unpredictable.

Areas covered: We aim to assess the clinical impact of genetic variations that may influence the efficacy of medical treatment in epilepsy patients. Indeed, many genes, including genes encoding drug transporters (ABCB1), drug targets (SCN1A), drug-metabolizing enzymes (CYP2C9, CYP2C19), and human leucocyte antigen (HLA) proteins, may regulate the mechanisms of drug resistance in epilepsy. This review specifically focuses on the ABC genes, which encode multidrug resistance-associated proteins (MRPs) and may reduce the blood–brain barrier penetration of anticonvulsant AEDs.

Expert opinion: Drug resistance remains a crucial problem in epilepsy patients. Pharmacogenomic studies may improve our understanding of drug responses and drug resistance by exploring the impact of gene variants and predicting drug responses and tolerability.     

Piperine-mediated drug interactions and formulation strategy for piperine: recent advances and future perspectives

Introduction: Piperine has various pharmacological effects and can modulate the functional activity of metabolic enzymes and drug transporters. Consequently, there is a great interest in the application of piperine as an alternative medicine or bioavailability enhancer.

Areas covered: This review deals with the effects of piperine on metabolizing enzymes and drug transporters. It provides the readers with an update on transporter-mediated and also metabolic enzyme-mediated piperine-drug interactions, with emphasis on its in vivo implications. This article also encompasses recent advances in the formulation approaches and technologies for optimizing the delivery of piperine.

Expert opinion: Piperine can influence the pharmacokinetics of coadministered drugs, which may result in a therapeutically beneficial or adverse effect. Given that piperine inhibits or stimulates the activity of metabolic enzymes and transporters depending on the treatment conditions, the clinical significance of piperine-drug interactions should be assessed by varying the dose, dosing frequency, and the duration of treatment. In particular, better understanding the clinical relevance of piperine-drug interactions based on long-term assessments will provide a strong basis for the feasibility and applicability of piperine as a bioenhancer or a health-promoting agent. The development of effective formulations is also critical to facilitate the therapeutic applications of piperine.     

The role of the placenta in drug transport and fetal drug exposure

Introduction: It was assumed for decades that the human placenta serves as a barrier, protecting the fetus from exposure to xenobiotics circulating in the mother. The thalidomide disaster completely reversed this concept. The study of the human placenta is therefore critical to understanding the mechanisms by which xenobiotics reach the fetus and exert their effects.

Areas covered: This review describes mechanisms by which drugs interact with the human placenta, and experimental methods to study these interactions in humans.

We have selected three areas of current clinical interest, where the placenta exhibits critical role in drug transport: The ABC transporters, the placental handling of cancer therapeutic drugs and the interaction between the placenta and immunoglobulins.

Expert commentary: The optimal model to predict drug transfer and transport from the mother to the fetus is the isolated human placental lobule perfused in vitro. Unlike subcellular preparations or tissue homogenates, data obtained from a perfused intact tissue, where structural integrity and cell-cell organization are maintained, more closely reflect the in vivo situation. Moreover, confounding metabolic and physiologic influences are minimized and the experimental conditions can be controlled. It is important to remember that due to significant differences in the function of the placenta in the first two months (histiotrophic nutrition) and later in pregnancy (hemotrophic nutrition) there might be differences in the transplacental transfer of drugs. While most of our knowledge comes from studies on term placentae, we are in need of studies on young placenta that functions during the period of organogenesis.     

Advances and challenges in hereditary cancer pharmacogenetics

Introduction: Cancer pharmacogenetics usually considers tumor-specific targets. However, hereditary genetic variants may interfere with the pharmacokinetics of antimetabolites and other anti-cancer drugs, which may lead to severe adverse events.

Areas covered: Here, the impact of hereditary genes considered in drug labels such as thiopurine S-methyltransferase (TPMT), UDP-glucuronosyltransferase 1A1 (UTG1A1) and dihydropyrimidine dehydrogenase (DPYD) are discussed with respect to guidelines of the Clinical Pharmacogenetics Implementation Consortium (CPIC). Moreover, the association between genetic variants of drug transporters with the clinical outcome is comprehensively discussed.

Expert opinion: Precision therapy in the field of oncology is developing tremendously. There are a number of somatic tumor genetic markers that are indicative for treatment with anti-cancer drugs. By contrast, for some hereditary variants, recommendations have been developed. Although we have vast knowledge on the association between drug transporter variants and clinical outcome, the overall data is inconsistent and the predictability of the related phenotype is low. Further developments in research may lead to the discovery of rare, but functionally relevant single nucleotide polymorphisms and a better understanding of multiple genomic, epigenomic as well as phenotypic factors, contributing to drug response in malignancies.     

Microbiota-derived uremic retention solutes: perpetrators of altered nonrenal drug clearance in kidney disease

Introduction: Scientific interest in the gut microbiota is increasing due to improved understanding of its implications in human health and disease. In patients with kidney disease, gut microbiota-derived uremic toxins directly contribute to altered nonrenal drug clearance. Microbial imbalances, known as dysbiosis, potentially increase formation of microbiota-derived toxins, and diminished renal clearance leads to toxin accumulation. High concentrations of microbiota-derived toxins such as indoxyl sulfate and p-cresol sulfate perpetrate interactions with drug metabolizing enzymes and transporters, which provides a mechanistic link between increases in drug-related adverse events and dysbiosis in kidney disease.

Areas covered: This review summarizes the effects of microbiota-derived uremic toxins on hepatic phase I and phase II drug metabolizing enzymes and drug transporters. Research articles that tested individual toxins were included. Therapeutic strategies to target microbial toxins are also discussed.

Expert commentary: Large interindividual variability in toxin concentrations may explain some differences in nonrenal clearance of medications. Advances in human microbiome research provide unique opportunities to systematically evaluate the impact of individual and combined microbial toxins on drug metabolism and transport, and to explore microbiota-derived uremic toxins as potential therapeutic targets.     

A randomized,single-blind,Phase I trial (INVICTAN-1) assessing the bioequivalence and safety of BI 695502, a bevacizumab biosimilar candidate,in healthy subjects

Objectives: This Phase I trial (INVICTAN®-1) evaluated three-way bioequivalence and safety of BI 695502 a bevacizumab biosimilar candidate, and reference product bevacizumab from two sources (US-approved Avastin®, Genentech; EU-approved Avastin, Roche).

Methods: Healthy male subjects (N = 91) were randomized 1:1:1 to receive a single intravenous infusion of 1 mg/kg of BI 695502 or US- or EU-approved Avastin. An interim analysis was planned when ~50% of subjects were evaluable for the primary end point to determine if the prespecified criteria for bioequivalence were achieved; if demonstrated, the study could be stopped early. The primary end point was area under the concentration–time curve (AUC) of the analyte in plasma from time zero extrapolated to infinity (AUC_0–∞). Other pharmacokinetic (PK) parameters, safety, and in vitro binding affinity were also evaluated.

Results: The interim analysis demonstrated three-way bioequivalence for all comparisons. The confidence intervals around the geometric mean ratios of the primary and secondary PK parameters were within the predefined acceptance ranges. Study drugs were well tolerated with no clinically relevant differences in safety.

Conclusion: BI 695502 and US- and EU-approved Avastin showed three-way bioequivalence with similar safety profile.

Clinical trial registration: NCT01608087.     

Inflammation and pharmacokinetics: potential implications for HIV-infection

Introduction: The physiological changes accompanying inflammation may alter the pharmacokinetics (PK) of certain medications. Individuals infected with HIV have chronically elevated inflammatory markers despite viral suppression following effective antiretroviral therapy (ART), as well as age-related inflammation. Understanding the potential clinical implications of inflammation on the PK of medications is important for understanding dose-response relationships and necessitates future research.

Areas covered: An extensive literature search was carried out using PubMed and associated bibliographies to summarize the current state of knowledge regarding altered PK in response to inflammation and its application to the field of HIV.

Expert opinion: Preclinical and clinical studies show that inflammation leads to a downregulation of certain drug metabolizing enzymes and both up and down regulation of transporters depending on the transporter and cell type. Decreased gastric acidity, fluid shifts, and plasma protein alterations also occur with inflammation, leading to potential absorption, distribution, and clearance changes. More research is needed including controlled PK studies to address the clinical relevance of these observations, especially in the aging HIV-infected population. Results from future studies will enable us to better predict drug concentrations in individuals with inflammation, in line with efforts to provide personalized pharmacotherapy in our healthcare system.     

Hepatic transporter drug-drug interactions: an evaluation of approaches and methodologies

Introduction: Drug-drug interactions (DDIs) continue to account for 5% of hospital admissions and therefore remain a major regulatory concern. Effective, quantitative prediction of DDIs will reduce unexpected clinical findings and encourage projects to frontload DDI investigations rather than concentrating on risk management (‘manage the baggage’) later in drug development. A key challenge in DDI prediction is the discrepancies between reported models.

Areas covered: The current synopsis focuses on four recent influential publications on hepatic drug transporter DDIs using static models that tackle interactions with individual transporters and in combination with other drug transporters and metabolising enzymes. These models vary in their assumptions (including input parameters), transparency, reproducibility and complexity. In this review, these facets are compared and contrasted with recommendations made as to their application.

Expert opinion: Over the past decade, static models have evolved from simple [I]/k_i models to incorporate victim and perpetrator disposition mechanisms including the absorption rate constant, the fraction of the drug metabolised/eliminated and/or clearance concepts. Nonetheless, models that comprise additional parameters and complexity do not necessarily out-perform simpler models with fewer inputs. Further, consideration of the property space to exploit some drug target classes has also highlighted the fine balance required between frontloading and back-loading studies to design out or ‘manage the baggage’.     

Preventable statin adverse reactions and therapy discontinuation. What can we learn from the spontaneous reporting system?

Background: Statin treatment is often associated with poor adherence, which may be due to the onset of adverse drug reactions (ADRs). We investigated on potential risk factors related to preventable cases of statin-induced ADRs and to the discontinuation of statin therapy.

Methods: We performed a study using the database of Italian spontaneous reporting. The target population for the preventability assessment was all patients with suspected statin-induced ADRs deriving from Campania Region (a territory of Southern Italy) between 2012 and 2017. Additionally, a local sentinel surveillance site involving General Practitioners was selected to countercheck in routine clinical practice the role of ADRs for statin discontinuation.

Results: In total, 34 of 655 (5.19%) regional cases were preventable and among detected risk factors 90.0% was related to healthcare professionals’ practices and 10.0% to patient behaviour. In 81.4% (533/655) of cases, statin therapy was discontinued due to ADRs, mainly classified as not serious and associated with a positive prognosis. These results were also confirmed in the active sentinel site.

Conclusions: Our findings suggest an inappropriate use of statins among the identified preventable cases and a potential inappropriate statin discontinuation due to ADRs. These factors may be useful for targeting interventions to improve statin adherence.     

Taurine prevents ultraviolet B induced apoptosis in retinal ganglion cells

Background: Compatible osmolytes accumulation is an active resistance response in retina under ultraviolet (UV) radiation and hypertonicity conditions.

Objective: The purpose of this research is to investigate the protective role of taurine on retina under UVB radiation.

Methods: Osmolytes transporters were measured by quantitative realtime PCR. Osmolytes uptake was estimated by radioimmunoassay. Cell viability was calculated by MTT assay. Cell apoptosis was measured by flow cytometry analysis.

Results: Hypertonicity accelerated osmolytes uptake into retinal ganglion cells (RGCs) including taurine, betadine, and myoinositol. UVB radiation increased osmolytes transporter expression and osmolytes uptake. In addition, osmolyte taurine remarkably prevented UVB radiation induced cell apoptosis in RGCs.

Conclusions: The effect of compatible osmolyte taurine on cell survival rate may play an important role in cell resistance and adaption to UVB exposure.     

The role of membrane transporters in ovarian cancer chemoresistance and prognosis

Introduction: Ovarian cancer has the highest mortality rate of all cancers in women. There is currently no effective method for early diagnosis, limiting the precision of clinical expectations. Predictions of therapeutic efficacy are currently not available either. Specifically, the development of chemoresistance against conventional chemotherapy poses a fundamental complication. Some membrane transporters have been reported to influence chemoresistance, which is often associated with a poor prognosis.

Areas covered: The aim of this article is to review the existing information about membrane transporters and their role in both ovarian cancer chemoresistance and its outcomes. We then highlight limitations of current methodologies and suggest alternatives providing avenues for future research.

Expert opinion: Membrane transporters play an important role in development of chemoresistance and affect prognosis of ovarian cancer patients; however, due to variations in methodology and in patient populations, their specific roles have yet to be clarified. For further evaluation of the clinical utility of membrane transporters, it is essential to validate results and improve methods for marker assessment across laboratories. A promising area for future research is to identify the genetic variability in potential markers in peripheral blood. These markers would then stratify patients into defined groups for optimal intervention.     

Corneal perforation: another side effect of nicorandil

Context: Nicorandil is an antianginal drug used for 20 years in Japan and introduced in France in 1994. Since 1997, side effects such as mucocutaneous ulcerations have regularly been reported.

Objective: To describe the first case of a patient with a spontaneous corneal perforation associated with mucocutaneous ulcerations while taking Nicorandil.

Materials and methods: A 81-year-old patient, with no past history of ocular disease but a long past history of cardiovascular disease, presented with a spontaneous paracentral corneal perforation. This was consecutive to 5 months of recurrent keratoconjunctivitis and mucocutaneous ulcerations resistant to conventional therapy. (He was taking nicorandil for 5 years.) A penetrating keratoplasty was performed in emergency.

Results: Inflammatory and infectious causes of spontaneous corneal perforation were ruled out. After initial uneventful post-operative wound healing, an epithelial ulcer appeared on the graft. Dermatologists suggested the iatrogenic role of nicorandil and the drug was discontinued. Both mucocutaneous and corneal ulcerations resolved rapidly.

Discussion: Although mucocutaneous ulcerations have been attributed several times to nicorandil, this is, to our knowledge, the first major corneal damage due to this antianginal drug. Timing, pattern of illness, absence of other aetiology, recurrence of epithelial ulceration on the corneal graft and its spontaneous healing after nicorandil discontinuation make it highly apparent probable that nicorandil was directly involved in this corneal perforation.

Conclusion: Ophthalmologists and dermatologists should be aware of the risk of severe but reversible corneal ulcerations in patients treated with nicorandil. A pharmacovigilance warning statement should be compulsory.     

Statin intolerance – a question of definition

Introduction: Statin therapy is the backbone of pharmacologic therapy for low-density lipoproteins cholesterol lowering and plays a pivotal role in cardiovascular disease prevention. Statin intolerance is understood as the inability to continue using a statin to reduce individual cardiovascular risk sufficiently, due to the development of symptoms or laboratory abnormalities attributable to the initiation or dose escalation of a statin. Muscle symptoms are the most common side effects observed.

Areas covered: The main aim of this article is to present a review on published definitions of statin intolerance. In addition, a brief review on clinical aspects and risk factors of statin intolerance is provided and features for a common definition for statin intolerance are suggested.

Expert opinion: A definition of statin intolerance by major drug regulatory agencies is not available. In clinical studies, different definitions are chosen and results are not comparable; different medical associations do not agree on one common definition. There is an unmet need to establish a common definition of statin intolerance to ensure an appropriate clinical use of this important drug class. Further work is required to develop a consensus definition on statin intolerance that could have significant positive impact on both research and clinical management.     

Empirical estimation of under-reporting in the U.S. Food and Drug Administration Adverse Event Reporting System (FAERS)

Background: To examine how closely reporting rates in the FDA Adverse Event Reporting System (FAERS) reflect expected rates of known adverse drug events (ADEs).

Methods: We selected three groups of drugs to reflect hypothesized variation in sensitivity to reporting, including statins, biologics, and narrow therapeutics index drugs (NTI). The numbers of ADEs in FAERS were divided by utilization estimates from ambulatory health care data (NAMCS/NHAMCS) to calculate a reported proportion. One sample z-test for proportions compared the proportion of ADEs reported to an expected ADE proportion derived from drug labels, reference databases, and peer-reviewed papers.

Results: The majority of drug-ADE pairs showed significant under-reporting. For example, roughly 0.01% to 44% of statin events were reported (z-test p < 0.0001). Biological (0.002% to >100%) and NTI (20% to >100%) drugs had relatively higher reporting rates. Roughly 20% to 33% of the minimum number of expected serious events were reported with biologics and NTI drugs.

Conclusions: This study supports previous evidence of under-reporting of ADEs in spontaneous reporting data. But, under-reporting varies considerably by the type of drug and the type of ADEs, and this variability in under-reporting should be considered when interpreting safety signals.     

Pharmacokinetics–pharmacodynamics of antifungal agents in the central nervous system

Introduction: Mortality from invasive fungal disease involving the central nervous system (CNS) is excessive. Achieving therapeutic drug concentrations at the site of infection within the CNS is always difficult and its evaluation is complex due to anatomical barriers and variable pathophysiological lesions.

Areas covered: This review provides an updated summary of the CNS PK of antifungal therapies. It considers factors that influence the success of antifungal regimens for CNS infection as well as preclinical and clinical data that quantify antifungal pharmacokinetics (PK) in the CNS. Furthermore, it presents state-of-the-art technologies to enhance the clinical use of existing antifungal drugs, and introduces novel antifungal drugs in development.

Expert opinion: The antifungal drugs currently available are either suboptimal, or are being used suboptimally, for CNS disease. Therapeutic drug monitoring is mandatory to enhance their effectiveness. Novel drugs in development may offer more efficacious options. In all cases, contemporary technologies to assess CNS PK offer the opportunity to enhance our understanding and use of antifungal drugs for CNS fungal disease.     

Taurine reduces blue light-induced retinal neuronal cell apoptosis in vitro

Background: The massive uptake of organic compatible osmolytes is a self-protective response to multiple stressors.

Objective: This study aimed to determine the protective effects of the osmolyte taurine against blue light-induced apoptosis in retinal neuronal cells in vitro.

Methods: Real-time PCR was used to measure osmolyte transport. Radioimmunoassays were performed to measure osmolyte uptake. Cell Counting Kit-8 assays were conducted to measure cellular viability. Flow cytometry analysis was used to measure apoptosis.

Results: Compared with normotonic stress, hypertonic stress-induced uptake of osmolytes, including betaine, myoinositol, and taurine, into the retinal neuronal cells. Blue light increased osmolyte transporter mRNA expression together with osmolyte uptake. Furthermore, taurine significantly suppressed blue light-induced retinal neuronal cell apoptosis.

Conclusion: The compatible osmolyte taurine may have an important role in cell resistance to blue light and cell survival.