Current Pharmaceutical Design - Volume 22, Issue 38, 2016

With the aging population the occurrence of central nervous system diseases such as cancer, mental disorders and neurodegenerative diseases, is expected to increase and hence, the demand for effective drugs. However, the passage of drugs across the blood-brain barrier represents a major challenge in accomplishing efficient brain delivery of therapeutic agents. This highly efficient barrier is composed of a monolayer of capillary endothelial cells supported by pericytes and astrocytic end-feet, that together effectively shield the brain from the blood. The brain microvascular endothelial cells form a physical and metabolic barrier where paracellular and transcellular transport of molecules in and out of the brain is closely regulated, allowing nutrients to pass but preventing the entry of harmful neurotoxic substances, including drugs. For this purpose brain endothelial cells express efficient efflux pumps, such as ATP binding cassette (ABC) transporters, which limit the delivery of drugs into the brain. To treat the above-mentioned chronic central nervous system disorders, it is crucial to design compounds that can pass the blood-brain barrier and thus the ABC transporters. In order to achieve this, representative models of the blood-brain barrier with predictive validity are necessary. This review discusses the current in vitro and ex vivo model systems that are used to measure ABC transporter activity in order to study potential in vivo efficacy of blood-brain barrier-drug passage.

P-glycoprotein is an ATP-binding cassette transporter involved in drug absorption, distribution and excretion. It pumps a wide range of xenobiotic compounds out of the cells and plays a crucial role in Multi Drug Resistance. Moreover, recent studies have demonstrated that changes in P-gp function and/or expression at the blood brain barrier are implicated in the pathogenesis of neurological disorders such as therapy-refractory epilepsy, Alzheimer’s and Parkinson’s disease. In the last decades the studies have been addressed to the discovery of potent P-gp inhibitors able to revert pharmacoresistance and to the development of PET tracers to detect P-gp activity and expression for an early diagnosis and therapy monitoring of neurodegenerative disease. However, clinical trials have reported only limited success in reversing MDR and radiolabeled ligands were not actually useful to study differences of transporter function in different brain regions due to their low brain uptake. The difficulties into the discovery of new ligands is due to the use of different experimental assays, to the fact that P-gp is highly flexible protein with different binging sites and available crystallographic structures for the protein have inadequate resolution. To overcome these limitations research groups prefer computational approaches such as homology models in their structure-based design or ligand-based methodologies. A recent approach aimed to identify ligands which can interrupt ATP-binding and hydrolysis by P-gp, by interacting at the NBDs of the protein. In this review results from radiolabeled, substrates and inhibitors, for monitoring the activity and expression of P-gp, respectively, are presented.

ABC transporters at the human blood-brain barrier protect the brain against the entry of harmful compounds but may also limit (or prevent) the cerebral entry of therapeutic drugs (e.g. anti-epileptics, antidepressants and antipsychotics). The efflux function of these transporters may be impaired in neurodegenerative disorders like Alzheimer and Parkinson disease. For such reasons, there is much interest in modulation of the efflux function of ABC transporters and in the monitoring of this function with positron emission tomography (PET). The efflux function of P-glycoprotein, an important member of the ABC transporter family, can be quantified with the PET tracer (R)-[11C]verapamil, but the lipophilicity of this probe and the formation of radioactive metabolites which enter the brain complicate such measurements considerably. (R)-[11C]verapamil is also not very suitable for the detection of P-gp upregulation, as may occur in epilepsy or drug resistance. Current radiochemical efforts are therefore focused on the development of PET probes with improved characteristics, for example; capability to detect both up- and down regulation of transporter function and expression, a better metabolic profile (no brain-entering metabolites), reduced lipophilicity and a longer physical half-life (labeling with 18F instead of 11C).

P-glycoprotein function is associated with a number of neurodegenerative and psychiatric diseases as well as with pharmacoresistance to for example antiepileptic drugs. The ability to measure P-gp function in vivo would allow for an increased understanding of the mechanisms of disease and treatment. This review assesses the various approaches to in vivo quantification of P-gp function using currently available P-gp tracers and PET in humans. First, the use of compartment models, and their interpretation in terms of P-gp function at the blood-brain barrier, is discussed. Then, the methods that have been used to quantify PET data of the P-gp tracers [11C]verapamil, [11C]N-desmetyl-loperamide (dLop), [11C]laniquidar, [11C]phenytoin, [11C]tariquidar and [11C]elacridar are reviewed. In summary, the extraction of P-gp substrate PET tracers, which is their plasma to tissue rate constant K1 corrected for variations in regional cerebral blood flow, is generally considered to be the preferred measure of P-gp function.

Resistance to antiepileptic drugs (AED) remains a major problem in clinical epileptology. This pharmacoresistance is independent of the choice of AEDs. Different hypotheses have been proposed to explain the neurobiological basis for pharmacoresistance in epilepsy. The transporter hypothesis is the mostly investigated theory. Hereby, overexpression of multidrug efflux transporters, such as P-glycoprotein (Pgp), at the blood-brain-barrier (BBB) is thought to be involved in pharmacoresistance in epilepsy by extruding AEDs from their target site. Accumulating evidence supports an overexpression of Pgp in pharmacoresistant epilepsy. Molecular Imaging studies provide unique opportunities for the in-vivo study of the transporter hypothesis in the central nervous system (CNS). Several studies demonstrated that positron emission tomography (PET) with [11C]-radiolabled Pgp substrates is a promising tool for in vivo investigation of Pgp function at the rat, monkey and human BBB. Quantification of Pgp over activity in epilepsy patients by in vivo imaging could be highly useful because altered treatment strategies or novel AED could then be applied.

Alzheimer’s disease is a neurodegenerative disorder and the most common form of dementia. One of the pathological hallmarks of the disease is amyloid deposition in the brain. The major cause of amyloid deposition in sporadic Alzheimer’s disease is thought to be decreased brain clearance of amyloid. There is compelling preclinical evidence that the blood-brain barrier, a structure that maintains homeostasis in the central nervous system and protects the brain from harmful substances, plays an important role in amyloid clearance. Indeed, several dedicated transporter systems are present at the blood-brain barrier which may have a role in brain amyloid clearance, such as P-glycoprotein (P-gp). In vitro experiments and animal studies indicated increased amyloid deposition when P-gp was eliminated by pharmacological blockade or by genetic modification. And as decreased P-gp expression has been found in AD brains, P-gp became more and more a suspect. Using an imaging technique called positron emission tomography, P-gp transporter function was found to be decreased in Alzheimer’s disease patients compared to healthy controls, further establishing the important role of P-gp in the pathogenesis of the disease. In this review, we summarize what is now known about P-gp in Alzheimer’s disease pathology, as these transporters may provide a novel target for therapeutic strategies.

ATP-binding cassette (ABC) transporters are a huge family of ATP-dependent transmembrane proteins whose main function is exporting or importing substances or molecules through the cell membranes, plasma cell membrane, or inner membranes in organelles. They fulfill these functions by maintaining cell integrity, metabolism, and homeostasis. They are expressed in a variety of tissues as they transport numerous essential compounds including lipids and other signaling molecules. ABC transporters became widely studied since the discovery of their ability to carry a multitude of xenobiotics, including therapeutic drugs, and in light of the fact that they represent a hurdle for the treatment of resistant cancers. In contrast, the role of ABC transporters in neurological diseases like Alzheimer`s and Parkinson`s, depression, schizophrenia, and epilepsy remains controversial and their mechanism of action in these pathologies remains elusive, thus hindering the implementation of therapies aimed at modulating the functions of these transporters. To date, a number of natural and synthetic compounds are known to act as inhibitors, substrates, and even inducers of these transporters, being able to modulate their expression and/or function; however, their implication as therapeutic agents is far from reaching wide clinical utilization. This review highlights the importance of overcoming the challenges posed by ABC transporters in drug development.

Capillary endothelial cells in the brain express P-glycoprotein (P-gp), which works as a functional blood-brain barrier (BBB). P-gp pumps out multiple types of molecules from the brain parenchyma into the blood. Therefore, altered P-gp function at the BBB will change the concentrations of therapeutic drugs in the central nervous system (CNS) and hence impact the toxicity and efficacy of CNS drugs. Positron emission tomography (PET) is the only way to non-invasively measure P-gp function in the living human brain. PET imaging of P-gp function was first demonstrated in 1998 with the substrate tracer racemic [11C]verapamil. Since then, several drug interaction studies and proof-of-concept studies regarding drug resistance have been performed with P-gp PET imaging. Although preclinical findings have been very positive regarding the possibilities and importance of P-gp PET imaging, very few studies have shown the clinical relevance of P-gp PET imaging in different disorders of the brain. This review summarizes the pharmacological studies with PET using substrate tracers and emphasizes the importance of PET imaging to understand the mechanism of action of CNS drugs.

Willuhn et al., observed that habitual cocaine use was correlated with reductions in D2/D3 receptors linked to decreased cue activation in occipital cortex and cerebellum. Dopamine agonist therapy maintains dopamine function and is a relapse prevention tactic focused on psychoactive drug and behavioral addictions. Medication Assisted Treatment (MAT) with emphasis on glutaminergic medications fails in the long-term treatment of Reward Deficiency Syndrome Behaviors (RDS). While the careful use of “dopamine antagonist-therapy” short-term is supported, the research-based concept of “dopamine agonist therapy” in long-term is proposed. Neurogenetics and epigenetics are important in understanding treatment response and clinical outcomes. The neuro–mechanisms involving “dopamine homeostasis” are key to understanding recovery from drug and non–drug addictive behaviors. For example, patients who carry the DRD2 A1 allele (30-40 less D2 receptors) should consider Neuronutriant–Amino-Acid therapy (KB220 variants) a prevention modality. DRD2 A1 allele carriers show amplified striatal function of L-amino acid decarboxylase, prior to dopamine biosynthesis. Another example is the effect of Acute Tyrosine Phenylalanine Depletion (ATPD) on decision-making and reward found carriers with amino-acid deficiency (ATPD). They experienced attenuated reward and reduced decision-making ability as quantified by Iowa Gambling Task (IGT). Future research should be directed at asking the question; Would “dopamine agonist therapy” using KB220 variants reduce methylation and increase acetyl groups to enhance DRD2 expression especially in DRD2 A1 allele carriers and lead to increased dopamine function and a reduction of drug and non-drug seeking behaviors?

Stenosis of the critical blood vessels, which occurs in a variety of cardiovascular and cerebrovascular diseases, is one of leading causes of death in the world. Vascular stenosis will significantly alter the hemodynamic features in the vessel. Hemodynamic shear stress, one of the most important physical parameters of blood flow, will be dramatically elevated at the stenotic site. When platelets flow through the constricted site, they will sense these abnormally high shear stresses, and then respond by activating, sticking to the vascular wall, and aggregating at these sites. The shear-dependent platelet activation inspired a novel targeting platform—shear stress activated drug targeting delivery. The shear-activated drug delivery systems preferentially release their content under elevated shear stress, providing a novel approach to cure various diseases, in particular, cardiovascular diseases. In this review, we, on one hand, introduced the features of hemodynamic shear stress under both physiological and pathological conditions. On the other hand, we summarized the carriers displaying sensitivity to shear stress, such as liposomes, aggregations, gels, emulsions, in addition to the factors affecting the mechanical properties of them. Lastly, the clinical applications and prospects of this novel drug targeting strategy were discussed. It is hoped that, with a better understanding of shear stress-sensitive carriers and their targeted principle, a novel targeted drug delivery strategy will be one day applied in the clinics of the future.

Streptokinase (SK) is an efficient thrombolytic agent that dissolves fibrin blood clots with clinical efficiency comparable to the high priced drug, tissue plasminogen activator (tPA). However, being of bacterial origin, its major drawbacks are its potentially high antigenicity, and relatively short circulating half-life (approximately 10-15 min). In the present investigation, an attempt has been made to address both these shortcomings by site-specific pegylation, and to obtain longer lasting thrombolytics, which are consistent with clinical requirements. Therefore, we employed available three-dimensional structural information on SK to carry out site-specific cysteine incorporation at 'optimal’ surfaceexposed sites within all the three domains in streptokinase followed by pegylation with 20KDa PEG groups, and screening for biologically active variants. Interestingly, some of these SK PEG-conjugates exhibited considerably subdued immunereactivity along with enhanced in-vitro proteolytic stability profiles and extended circulating in-vivo half-lives (2 to 20-fold compared to that of native unconjugated SK) depending upon location and number of PEG-groups per molecule obtained in homogeneous form. The obtained results are a promising approach for favorably modulating immune-reactivity and half-life by cysteine- specific PEGylation of SK to achieve therapeutic attributes desirable for the treatment of different circulatory disorders, such as ischemic stroke, myocardial infarction and pulmonary embolism.

Background: Delirium is commonly observed among elderly surgical subjects during the postoperative period, and the incidence of postoperative delirium (POD) in elderly patients (≥65 years) ranges widely from 10% to 70%. This study aimed at investigating potential serum predicative factors for POD in elderly patients after open abdominal surgery. Methods: 140 subjects scheduled to undergo elective gastrointestinal tumor resection via laparotomy from March, 2013 to May, 2015 were enrolled in this study. Participants enrolled were evaluated for delirium and delirium severity on preoperative day, postoperative days 2 and 3. Non-fasting blood samples were collected in the morning on the day before surgery for the detection of serum cytokines expressions by enzyme-linked immunesorbent assays (ELISA). Results: A total of 140 patients were finally enrolled in this study and 36 of them occurred POD, with a POD incidence of 25.7%. In comparison with non-POD group, the serum levels of insulin-like growth factor-1 (IGF-1) demonstrated significantly decreased of patients in POD group (P<0.01). The receiver operating characteristic (ROC) curve analysis revealed that serum IGF-1 levels as potential predicative factor for POD with the area under the ROC curve (AUC) values of 0. 805, with 95 % confidence interval (CI) of 0.719-0.891 (P<0.001). Univariate and multivariate logistic regression analysis resulted in serum IGF-1 level as a non-invasive predicative factor for POD (OR=2.52, 95%CI: 1.19-5.43, P=0.019). Conclusion: Our results showed that serum IGF-1 level was a potential predicative biomarker for POD among patients undergoing elective gastrointestinal tumor resection via laparotomy.