Role of multidrug transporters in neurotherapeutics

P-glycoprotein: new insights into structure, physiological function, regulation and alterations in disease

The multidrug resistance phenomenon presents a major threat to the pharmaceutical industry. This resistance is a common occurrence in several diseases and is mediated by multidrug transporters that actively pump substances out of the cell and away from their target regions. The most well-known multidrug transporter is the P-glycoprotein transporter. The binding sites within P-glycoprotein can accommodate a variety of compounds with diverse structures. Hence, numerous drugs are P-glycoprotein substrates, with new ones being identified every day. For many years, the mechanisms of action of P-glycoprotein have been shrouded in mystery, and scientists have only recently been able to elucidate certain structural and functional aspects of this protein. Although P-glycoprotein is highly implicated in multidrug resistant diseases, this transporter also performs various physiological roles in the human body and is expressed in several tissues, including the brain, kidneys, liver, gastrointestinal tract, testis, and placenta. The expression levels of P-glycoprotein are regulated by different enzymes, inflammatory mediators and transcription factors; alterations in which can result in the generation of a disease phenotype. This review details the discovery, the recently proposed structure and the regulatory functions of P-glycoprotein, as well as the crucial role it plays in health and disease.

A Conformationally Gated Model of Methadone and Loperamide Transport by P-Glycoprotein

P-glycoprotein (Pgp) is a multidrug resistance transporter that limits the penetration of a wide range of neurotherapeutics into the brain including opioids. The diphenylpropylamine opioids methadone and loperamide are structurally similar, but loperamide has about a 4-fold higher Pgp-mediated transport rate. In addition to these differences, they showed significant differences in their effects on Pgp-mediated adenosine triphosphate (ATP) hydrolysis. The activation of Pgp-mediated ATP hydrolysis by methadone was monophasic, whereas loperamide activation of ATP hydrolysis was biphasic implying methadone has a single binding site and loperamide has 2 binding sites on Pgp. Quenching of tryptophan fluorescence with these drugs and digoxin showed competition between the opioids and that loperamide does not compete for the digoxin-binding site. Acrylamide quenching of tryptophan fluorescence to probe Pgp conformational changes revealed that methadone- and loperamide-induced conformational changes were distinct. These results were used to develop a model for Pgp-mediated transport of methadone and loperamide where opioid binding and conformational changes are used to explain the differences in the opioid transport rates between methadone and loperamide.

Insights Into the Molecular Mechanism of Triptan Transport by P-glycoprotein

The P-glycoprotein (Pgp) transporter reduces the penetration of a chemically diverse range of neurotherapeutics at the blood-brain barrier, but the molecular features of drugs and drug-Pgp interactions that drive transport remain to be clarified. In particular, the triptan neurotherapeutics, eletriptan (ETT) and sumatriptan (STT), were identified to have a >10-fold difference in transport rates despite being from the same drug class. Consistent with these transport differences, ETT activated Pgp-mediated ATP hydrolysis ∼2-fold, whereas STT slightly inhibited Pgp-mediated ATP hydrolysis by ∼10%. The interactions between them were also noncompetitive, suggesting that they occupy different binding sites on the transporter. Despite these differences, protein fluorescence spectroscopy revealed that the drugs have similar affinity to the transporter. NMR with Pgp and the drugs showed that they have distinct interactions with the transporter. Tertiary conformational changes probed by acrylamide quenching of Pgp tryptophan fluorescence with the drugs and a nonhydrolyzable ATP analog implied that the STT-bound Pgp must undergo larger conformational changes to hydrolyze ATP than ETT-bound Pgp. These results and previous transport studies were used to build a conformationally driven model for triptan transport with Pgp where STT presents a higher conformational barrier for ATP hydrolysis and transport than ETT.

Significance of the expression of MRP1 and MRP2 in peripheral blood mononuclear cells of children with intractable epilepsy

Intactable epilepsy (IE) is relatively common in pediatric epilepsy. The resistance mechanism of IE has been previously investigated. Multidrug-resistant associated protein 1 (MRP1) and MRP2 are associated with drug transport. The aim of the present study was to investigate the expression of MRP1 and MRP2 in peripheral blood mononuclear cells of children with IE. Fifty outpatient or inpatient children were included in the study as the experimental group. Additionally, 50 children with epilepsy controlled by anti-epileptic drugs (AEDs) and 50 healthy children without epilepsy, who served as the control group, were included in the present study. Expression of MRP1 and MRP2 in the peripheral blood mononuclear cells of children in all the groups was detected using RT-PCR and western blot analysis. The results showed that the relative expression of MRP1 and MRP2 mRNA in the peripheral blood mononuclear cells of children with IE (MRP1, 0.795±0.042; MRP2, 0.804±0.023) was higher than that in epilepsy controlled by AEDs (MRP1, 0.682±0.030; MRP2, 0.675±0.021) and healthy children without epilepsy (MRP1, 0.665±0.031; MRP2, 0.654±0.029) (P<0.01). The mean relative expression of MRP1 and MRP2 protein in the peripheral blood mononuclear cells of children with IE (MRP1, 2.027±0.034; MRP2, 1.902±0.021) was higher than that in children with epilepsy controlled by AEDs (MRP1, 1.131±0.042; MRP2, 1.086±0.027) and healthy children without epilepsy (MRP1, 1.093±0.023; MRP2, 1.045±0.018) (P<0.01). The difference in the MRP1 and MRP2 mRNA and protein expression between the children with epilepsy controlled by AEDs and healthy children without epilepsy was not statistically significant (P>0.05). In conclusion, a higher expression of MRP1 and MRP2 in the peripheral blood mononuclear cells of children with IE may be relevant to the drug-resistant mechanism of IE.

Systems biology approaches to adverse drug effects: the example of cardio-oncology

Increased awareness of the cardiovascular toxic effects of chemotherapy has led to the emergence of cardio-oncology (or onco-cardiology), which focuses on screening, monitoring and treatment of patients with cardiovascular dysfunctions resulting from chemotherapy. Anthracyclines, such as doxorubicin, and HER2 inhibitors, such as trastuzumab, both have cardiotoxic effects. The biological rationale, mechanisms of action and cardiotoxicity profiles of these two classes of drugs, however, are completely different, suggesting that cardiotoxic effects can occur in a range of different ways. Advances in genomics and proteomics have implicated several genomic variants and biological pathways that can influence the susceptibility to cardiotoxicity from these, and other drugs. Established pathways include multidrug resistance proteins, energy utilization pathways, oxidative stress, cytoskeletal regulation and apoptosis. Gene-expression profiles that have revealed perturbed pathways have vastly increased our knowledge of the complex processes involved in crosstalk between tumours and cardiac function. Utilization of mathematical and computational modelling can complement pharmacogenomics and improve individual patient outcomes. Such endeavours should enable identification of variations in cardiotoxicity, particularly in those patients who are at risk of not recovering, even with the institution of cardioprotective therapy. The application of systems biology holds substantial potential to advance our understanding of chemotherapy-induced cardiotoxicity.

Role of P-glycoprotein in refractoriness of seizures to antiepileptic drugs in Lennox-Gastaut syndrome

Mechanism of seizure refractoriness to antiepileptic drugs in children with Lennox-Gastaut syndrome is not known. Efflux of antiepileptic drugs due to increased expression/function of P-glycoprotein, a multidrug efflux transporter protein on the cell surface is a proposed mechanism. The authors studied the expression/function of P-glycoprotein on peripheral blood mononuclear cells of 29 children with Lennox-Gastaut syndrome, 23 children with other epilepsies, and 19 healthy children. The authors found a higher P-glycoprotein expression/function in Lennox-Gastaut syndrome, a higher percent positive cells as compared to children with other epilepsy (P < 0.001) and to healthy controls (P = 0.012), higher P-glycoprotein expression as compared to healthy controls (P = 0.003), a higher total P-glycoprotein expression (relative florescence intensity × percent positive cells) as compared to children with other epilepsies (P < 0.001) and healthy controls (P < 0.001), and a higher P-glycoprotein function as compared to children with other epilepsies (P = 0.001) and healthy controls (P = 0.002). These findings may explain seizure refractoriness to anti-epileptic drugs in Lennox-Gastaut syndome.

Polymorphism of ABCB1/MDR1 C3435T in children and adolescents with partial epilepsy is due to different criteria for drug resistance - preliminary results

Background

The diagnosis of “drug resistance” in epilepsy can be defined and interpreted in various ways. This may be due to discrepant definitions of drug resistance to pharmacotherapy.

The aim of our study was to investigate the relationship between C3435T polymorphism of the MDR1 gene and drug resistance in epilepsy with the consideration of 4 different criteria for qualification to groups sensitive and resistant to applied pharmacotherapy.

Material/Methods

Evaluation of C3435T polymorphism of MDR1/ABCB1 gene was conducted on a group of 82 white children and young adolescents up to 18 years old. While qualifying the patients to the group of sensitive or drug resistant, the following 4 definitions of drug resistance were applied: the ILAE’s, Appleton’s, Siddiqui’s, and Berg’s.

Results

A detailed analysis of genotypes of the MDR1 gene did not show any significant discrepancies between the groups of patients resistant and sensitive to antiepileptic drugs (AEDs) in 4 consecutive comparisons taking into consideration various criteria of sensitivity and resistance to pharmacotherapy.

Conclusions

The obtained results clearly confirm the lack of a connection between the occurrence of drug-resistant epilepsy and C435T polymorphism of the MDR1 gene irrespective of the definition of drug resistance applied to the patient.

Epilepsy: Indian perspective

There are 50 million people living with epilepsy worldwide, and most of them reside in developing countries. About 10 million persons with epilepsy are there in India. Many people with active epilepsy do not receive appropriate treatment for their condition, leading to large treatment gap. The lack of knowledge of antiepileptic drugs, poverty, cultural beliefs, stigma, poor health infrastructure, and shortage of trained professionals contribute for the treatment gap. Infectious diseases play an important role in seizures and long-term burden causing both new-onset epilepsy and status epilepticus. Proper education and appropriate health care services can make tremendous change in a country like India. There have been many original researches in various aspects of epilepsy across India. Some of the geographically specific epilepsies occur only in certain regions of our country which have been highlighted by authors. Even the pre-surgical evaluation and epilepsy surgery in patients with drug-resistant epilepsy is available in many centers in our country. This article attempts to provide a complete preview of epilepsy in India.

FGF-2 protects cardiomyocytes from doxorubicin damage via protein kinase C-dependent effects on efflux transporters

AIMS

The anti-cancer anthracycline doxorubicin (DOX) increases the risk of cardiac damage, indicating a need to protect the heart and still allow the benefits of drug treatment. Fibroblast growth factor-2 (FGF-2) is cardioprotective against ischaemia-reperfusion injury. Our aim is to investigate: (i) the ability of FGF-2 to protect against DOX-induced cardiomyocyte damage and (ii) the contribution of efflux drug transport to any increase in injury-resistance.

METHODS AND RESULTS

Neonatal rat cardiomyocyte damage was assessed by measuring cell death markers and lactate dehydrogenase (LDH) activity in the culture medium. LDH activity was increased significantly after incubation with 0.5 μM DOX for 24 h in the absence but not presence of 10 nM FGF-2; this beneficial effect of FGF-2 was blocked by tyrosine kinase (FGF) receptor inhibition. An increase in efflux drug transporter RNA levels was also detected after FGF-2 treatment in the presence of DOX. The beneficial effect of FGF-2 against cell damage and increased transporter RNA levels were blunted with protein kinase C (PKC) inhibition. Finally, FGF-2 stimulated efflux transport of calcein and DOX, and treatment with efflux transporter inhibitors significantly attenuated the protective effect of FGF-2 from DOX-induced injury.

CONCLUSION

Administered FGF-2 increases resistance to DOX-induced cardiomyocyte damage, by a mechanism dependent on PKC as well as regulation of efflux transporter production and/or function.