Possible involvement of cationic-drug sensitive transport systems in the blood-to-brain influx and brain-to-blood efflux of amantadine across the blood-brain barrier

Mechanistic basis for multidrug resistance and collateral drug sensitivity conferred to the malaria parasite by polymorphisms in PfMDR1 and PfCRT

Polymorphisms in the Plasmodium falciparum multidrug resistance protein 1 (pfmdr1) gene and the Plasmodium falciparum chloroquine resistance transporter (pfcrt) gene alter the malaria parasite’s susceptibility to most of the current antimalarial drugs. However, the precise mechanisms by which PfMDR1 contributes to multidrug resistance have not yet been fully elucidated, nor is it understood why polymorphisms in pfmdr1 and pfcrt that cause chloroquine resistance simultaneously increase the parasite’s susceptibility to lumefantrine and mefloquine—a phenomenon known as collateral drug sensitivity. Here, we present a robust expression system for PfMDR1 in Xenopus oocytes that enables direct and high-resolution biochemical characterizations of the protein. We show that wild-type PfMDR1 transports diverse pharmacons, including lumefantrine, mefloquine, dihydroartemisinin, piperaquine, amodiaquine, methylene blue, and chloroquine (but not the antiviral drug amantadine). Field-derived mutant isoforms of PfMDR1 differ from the wild-type protein, and each other, in their capacities to transport these drugs, indicating that PfMDR1-induced changes in the distribution of drugs between the parasite’s digestive vacuole (DV) and the cytosol are a key driver of both antimalarial resistance and the variability between multidrug resistance phenotypes. Of note, the PfMDR1 isoforms prevalent in chloroquine-resistant isolates exhibit reduced capacities for chloroquine, lumefantrine, and mefloquine transport. We observe the opposite relationship between chloroquine resistance-conferring mutations in PfCRT and drug transport activity. Using our established assays for characterizing PfCRT in the Xenopus oocyte system and in live parasite assays, we demonstrate that these PfCRT isoforms transport all 3 drugs, whereas wild-type PfCRT does not. We present a mechanistic model for collateral drug sensitivity in which mutant isoforms of PfMDR1 and PfCRT cause chloroquine, lumefantrine, and mefloquine to remain in the cytosol instead of sequestering within the DV. This change in drug distribution increases the access of lumefantrine and mefloquine to their primary targets (thought to be located outside of the DV), while simultaneously decreasing chloroquine’s access to its target within the DV. The mechanistic insights presented here provide a basis for developing approaches that extend the useful life span of antimalarials by exploiting the opposing selection forces they exert upon PfCRT and PfMDR1.

Edoxaban and the Issue of Drug-Drug Interactions: From Pharmacology to Clinical Practice

Edoxaban, a direct factor Xa inhibitor, is the latest of the non-vitamin K antagonist oral anticoagulants (NOACs). Despite being marketed later than other NOACs, its use is now spreading in current clinical practice, being indicated for both thromboprophylaxis in patients with non-valvular atrial fibrillation (NVAF) and for the treatment and prevention of venous thromboembolism (VTE). In patients with multiple conditions, the contemporary administration of several drugs can cause relevant drug-drug interactions (DDIs), which can affect drugs' pharmacokinetics and pharmacodynamics. Usually, all the NOACs are considered to have significantly fewer DDIs than vitamin K antagonists; notwithstanding, this is actually not true, all of them are affected by DDIs with drugs that can influence the activity (induction or inhibition) of P-glycoprotein (P-gp) and cytochrome P450 3A4, both responsible for the disposition and metabolism of NOACs to a different extent. In this review/expert opinion, we focused on an extensive report of edoxaban DDIs. All the relevant drugs categories have been examined to report on significant DDIs, discussing the impact on edoxaban pharmacokinetics and pharmacodynamics, and the evidence for dose adjustment. Our analysis found that, despite a restrained number of interactions, some strong inhibitors/inducers of P-gp and drug-metabolising enzymes can affect edoxaban concentration, just as it happens with other NOACs, implying the need for a dose adjustment. However, our analysis of edoxaban DDIs suggests that given the small propensity for interactions of this agent, its use represents an acceptable clinical decision. Still, DDIs can be significant in certain clinical situations and a careful evaluation is always needed when prescribing NOACs.

In silico and in vitro evaluation of brain penetration properties of selected nootropic agents

Aim: Predictive ( in silico) data suggested that nootropic supplements may penetrate the blood–brain barrier (BBB). We evaluated, in vitro, the ability of nootropics to enter the brain based on the high throughput screening (HTS) measurement of interactions with the P-gp efflux transporter and physicochemical properties and correlated these data with the in silico predictions. Methods & results: The software predicted that piracetam, docosahexaenoic acid (DHA), amantadine and thioflavin-T can best penetrate the BBB. The lipophilicity of these compounds may be predicted by measuring the critical micelle concentration (CMC). DHA and verapamil demonstrated high lipophilicity. DHA, verapamil and phosphatidylserine (PS) may be good substrates of the P-gp transporter. Conclusion: Permeability of nootropics may be successfully predicted by high throughput screening-lead optimization assay technologies.

Efflux proteins at the blood-brain barrier: review and bioinformatics analysis

1. Efflux proteins at the blood-brain barrier provide a mechanism for export of waste products of normal metabolism from the brain and help to maintain brain homeostasis. They also prevent entry into the brain of a wide range of potentially harmful compounds such as drugs and xenobiotics. 2. Conversely, efflux proteins also hinder delivery of therapeutic drugs to the brain and central nervous system used to treat brain tumours and neurological disorders. For bypassing efflux proteins, a comprehensive understanding of their structures, functions and molecular mechanisms is necessary, along with new strategies and technologies for delivery of drugs across the blood-brain barrier. 3. We review efflux proteins at the blood-brain barrier, classified as either ATP-binding cassette (ABC) transporters (P-gp, BCRP, MRPs) or solute carrier (SLC) transporters (OATP1A2, OATP1A4, OATP1C1, OATP2B1, OAT3, EAATs, PMAT/hENT4 and MATE1). 4. This includes information about substrate and inhibitor specificity, structural organisation and mechanism, membrane localisation, regulation of expression and activity, effects of diseases and conditions and the principal technique used for in vivo analysis of efflux protein activity: positron emission tomography (PET). 5. We also performed analyses of evolutionary relationships, membrane topologies and amino acid compositions of the proteins, and linked these to structure and function.

Involvement of a proton-coupled organic cation antiporter in the blood-brain barrier transport of amantadine

The blood-to-brain transport of amantadine, a weak N-methyl-d-aspartate (NMDA) antagonist, has been shown previously to participate in the cationic drug-sensitive transport system across the mouse blood-brain barrier (BBB). The purpose of the present study was to characterize the influx transport system by means of both an in situ mouse brain perfusion technique and in vitro studies using rat immortalized brain capillary endothelial cells (GPNT). The observed concentration-dependent initial uptake rate of [(3) H]amantadine suggested the involvement of a carrier-mediated transport mechanism. The normal uptake at physiological pH 7.4 was decreased by 72.9% in acidic perfusate, while it was increased by 35.3% in alkaline perfusate. These results suggest that pH-dependent transport is regulated by utilizing an oppositely directed proton gradient as a driving force. In addition, the [(3) H]amantadine uptake was moderately inhibited by the adamantane structural analogs (rimantadine and memantine) and other cationic drugs (pyrilamine, clonidine, nicotine, etc.), but not by substrates or inhibitors of the well-characterized organic cation transporters (tetraethylammonium, l-carnitine and choline). A similar inhibition pattern was observed between the in vivo studies and the in vitro experiments. These results indicate that the influx transport for amantadine across the BBB involves a proton-coupled organic cation antiporter. Copyright © 2016 John Wiley & Sons, Ltd.