Mechanism of valproic acid uptake by isolated rat brain microvessels

Chemotherapy in pediatric brain tumor and the challenge of the blood-brain barrier

BACKGROUND

Pediatric brain tumors (PBT) stand as the leading cause of cancer-related deaths in children. Chemoradiation protocols have improved survival rates, even for non-resectable tumors. Nonetheless, radiation therapy carries the risk of numerous adverse effects that can have long-lasting, detrimental effects on the quality of life for survivors. The pursuit of chemotherapeutics that could obviate the need for radiotherapy remains ongoing. Several anti-tumor agents, including sunitinib, valproic acid, carboplatin, and panobinostat, have shown effectiveness in various malignancies but have not proven effective in treating PBT. The presence of the blood-brain barrier (BBB) plays a pivotal role in maintaining suboptimal concentrations of anti-cancer drugs in the central nervous system (CNS). Ongoing research aims to modulate the integrity of the BBB to attain clinically effective drug concentrations in the CNS. However, current findings on the interaction of exogenous chemical agents with the BBB remain limited and do not provide a comprehensive explanation for the ineffectiveness of established anti-cancer drugs in PBT.

METHODS

We conducted our search for chemotherapeutic agents associated with the blood-brain barrier (BBB) using the following keywords: Chemotherapy in Cancer, Chemotherapy in Brain Cancer, Chemotherapy in PBT, BBB Inhibition of Drugs into CNS, Suboptimal Concentration of CNS Drugs, PBT Drugs and BBB, and Potential PBT Drugs. We reviewed each relevant article before compiling the information in our manuscript. For the generation of figures, we utilized BioRender software.

FOCUS

We focused our article search on chemical agents for PBT and subsequently investigated the role of the BBB in this context. Our search criteria included clinical trials, both randomized and non-randomized studies, preclinical research, review articles, and research papers.

FINDING

Our research suggests that, despite the availability of potent chemotherapeutic agents for several types of cancer, the effectiveness of these chemical agents in treating PBT has not been comprehensively explored. Additionally, there is a scarcity of studies examining the role of the BBB in the suboptimal outcomes of PBT treatment, despite the effectiveness of these drugs for other types of tumors.

Valproic acid uptake by bovine brain microvessel endothelial cells: role of active efflux transport

The basis for low brain permeability of valproic acid (VPA) appears to be the result of efflux transport at the blood-brain barrier (BBB); however, the identity of the putative efflux transporter has not been investigated. The objective of our studies was to determine whether the multidrug resistance-associated protein (MRP) might be involved in efflux transport of VPA. Brain microvessel endothelial cells (BMEC) were isolated from cow brains and grown to confluence. MRP messenger RNA (mRNA) in BMEC were verified by reverse transcriptase-polymerase chain reaction (RT-PCR). Functional activity was demonstrated using the steady-state retention of calcein and MRP inhibitors, indomethacin (IND) and probenecid (PRB). Probenecid (0.50 mM) and indomethacin (10 microM) produced a 26 and 13% ( P<0.05 ) elevation in steady-state cellular VPA uptake following a 30-min-incubation with tracer 3H-VPA and 30 microM cold VPA. In contrast, at higher concentrations of probenecid (2 mM) and indomethacin (500 microM), an 11 and 31% reduction in VPA uptake was observed. The biphasic pattern of VPA uptake suggested concurrent inhibition of uptake and efflux transporters by the inhibitor with differing sensitivities, i.e. the efflux transporter being more susceptible to inhibition than the influx transporter. Similar results were obtained in the MRP overexpressing cell line A549. Overall, the results suggest that MRP(s) is(are) involved in the efflux transport of VPA, but do not preclude the possible contribution(s) of other organic anion transporters. The findings also adds to the growing evidence that up-regulation of active drug efflux transporters at the BBB may contribute to the development of drug resistance to antiepileptic drug therapy.

Metabolic parameters of epilepsy: adjuncts to established antiepileptic drug therapy

Hughlings Jackson at the turn of the century defined epilepsy as a disorder originating in a "morbid nutrition" of the neuron. With the advances in modern neurochemistry, it is becoming increasingly clear that a chronic seizure predisposition or a lowering of the brain's discharge threshold can be demarcated by a number of biochemical markers. They include a tendency for an increased release of glutamate with or without GABAergic impairment, (intra)neural tissue alterations in water redistribution/osmolarity or other distortions of the cytoarchitecture, and an elevation of ionic calcium inside the cell. These changes are dominantly shared parameters of the seizure prone brain. Magnetic resonance spectroscopy (MRS) shows that cerebral levels of glutamate + glutamine (Glx) are increased interictally in epileptogenic regions in human partial epilepsy; other findings using this technique suggest damage to (cellular/mitochondrial) membranes, denoted by N-acetyl-aspartic acid (NAA) changes and a decreased energy capability. The merging of previous in vitro and ex vivo findings in neurophysiology and neurochemistry with magnetic resonance spectroscopy technology provides a powerful new methodology to interpret and to obtain clinical insight into the metabolic alterations that underlie an epileptogenic process. In this review some of these basic neurochemical and electrophysiological mechanisms are discussed. In addition, certain adjuncts to established antiepileptic drug therapy are suggested in the hope that over the long term they may help in correcting the primary metabolic deficits.

Ultrastructure of Purkinje cell perikarya and their dendritic processes in the rat cerebellar cortex in experimental encephalopathy induced by chronic application of valproate

Long-term intragastric administration of the antiepileptic drug sodium valproate (Vuprol Polfa) to rats for 1, 3, 6, 9 and 12 months, once daily at the effective dose of 200 mg/kg body weight showed morphological evidence of encephalopathy, manifested by numerous nonspecific changes within Purkinje cell perikarya and their dendritic processes. The first ultrastructural abnormalities appeared after 3 months. They became more severe in animals with longer survival and were most pronounced after 12 months. The changes were maintained both 1 and 3 months after drug withdrawal. Mitochondria of Purkinje cell perikarya were most severely affected. Damage to mitochondria was accompanied by disintegration and fragmentation of granular endoplasmic reticulum, dilation of channels and cisterns of Golgi apparatus, enlargement of smooth endoplasmic reticulum elements including submembranous cisterns, and accumulation of profuse lipofuscin deposits. Frequently, Purkinje cells appeared as dark ischemic neurones, with focally damaged cellular membrane and features of disintegration. Swollen Bergmann's astrocytes were seen among damaged Purkinje cells or at the site of their loss. The general pattern of submicroscopic alterations of Purkinje cell perikarya suggested severe disorders in several intercellular biochemical extents, including inhibition of oxidative phosphorylation and abnormal protein synthesis, both of which could lead to lethal damage. Ultrastructural abnormalities within dendrites were characterized by damage to elements of smooth endoplasmic reticulum, which was considerably enlarged, with formation of large vacuolar structures situated deep in the dendroplasm. Mitochondrial lesions and alterations in cytoskeletal elements--disintegration of microtubules or even their complete loss--were also observed. The general pattern of abnormalities within the organelles and cytoskeletal elements of dendritic processes in Purkinje cells in the VPA chronic experimental model imply that there are disturbances in detoxication processes. Furthermore these changes were irreversible, as they were maintained after drug withdrawal.

Uptake mechanism of valproic acid in human placental choriocarcinoma cell line (BeWo)

Valproic acid is an anticonvulsant widely used for the treatment of epilepsy. However, valproic acid is known to show fetal toxicity, including teratogenicity. In the present study, to elucidate the mechanisms of valproic acid transport across the blood-placental barrier, we carried out transcellular transport and uptake experiments with human placental choriocarcinoma epithelial cells (BeWo cells) in culture. The permeability coefficient of [3H]valproic acid in BeWo cells for the apical-to-basolateral flux was greater than that for the opposite flux, suggesting a higher unidirectional transport in the fetal direction. The uptake of [3H]valproic acid from the apical side was temperature-dependent and enhanced under acidic pH. In the presence of 50 microM carbonyl cyanide p-trifluoromethoxylhydrazone, the uptake of [3H]valproic acid was significantly reduced. A metabolic inhibitor, 10 mM sodium azide, also significantly reduced the uptake of [3H]valproic acid. Therefore, valproic acid is actively transported in a pH-dependent manner on the brush-border membrane of BeWo cells. Kinetic analysis of valproic acid uptake revealed the involvement of a non-saturable component and a saturable component. The Michaelis constant for the saturable transport (K(t)) was smaller under acidic pH, suggesting a proton-linked active transport mechanism for valproic acid in BeWo cells. In the inhibitory experiments, some short-chain fatty acids, such as acetic acid, lactic acid, propanoic acid and butyric acid, and medium-chain fatty acids, such as hexanoic acid and octanoic acid, inhibited the uptake of [3H]valproic acid. The uptake of [3H]valproic acid was also significantly decreased in the presence of 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid, salicylic acid and furosemide, which are well-known inhibitors of the anion exchange system. Moreover, p-aminohippuric acid significantly reduced the uptake of [3H]valproic acid. These results suggest that an active transport mechanism for valproic acid exists on the brush-border membrane of placental trophoblast cells and operates in a proton-linked manner.

Probenecid-inhibitable efflux transport of valproic acid in the brain parenchymal cells of rabbits: a microdialysis study

Delivery of valproic acid (VPA) to the human brain is relatively inefficient as reflected by a low brain-to-unbound plasma concentration ratio (< or =0.5) at steady state. Previous pharmacokinetic studies suggested that the unfavorable brain-to-plasma gradient is maintained by coupled efflux transport processes at both the brain parenchymal cells and blood-brain barrier (BBB); one or both of the efflux transporters are inhibitable by probenecid. The present study in rabbits utilized microdialysis to measure drug concentration in the brain extracellular fluid (ECF) of the cerebral cortex during steady-state i.v. infusion with VPA alone or with VPA plus probenecid. Probenecid co-infusion elevated VPA concentration in the brain tissue surrounding the tip of the microdialysis probe to a greater extent than in the ECF (230% versus 47%). Brain intracellular compartment (ICC) concentration was estimated. In control rabbits, the ICC concentration was 2.8+/-0.28 times higher than the ECF concentration. Probenecid co-infusion elevated the ICC-to-ECF concentration ratio to 4.2+/-0.44, which confirms the existence of an efflux transport system in brain parenchymal cells. The ECF-to-unbound plasma concentration ratio was well below unity (0.029), indicating an uphill efflux transport of VPA across the BBB. Co-infusion of probenecid did not have a significant effect on VPA efflux at the BBB as evidenced by a minimal change in the ECF-to-unbound plasma concentration ratio. This study suggests the presence of distinctly different organic anion transporters for the efflux of VPA at the parenchymal cells and capillary endothelium in the brain.

Carrier-mediated or specialized transport of drugs across the blood-brain barrier

In the drug development process, it remains a difficult task to regulate the entry of the drugs. However, recent progress in studies of the transporter-mediated influx and efflux of endogenous and exogenous compounds, including synthetic drugs, across the blood-brain barrier (BBB) is beginning to provide a rational basis for controlling drug distribution to the brain. This paper describes mechanisms established in the last decade for carrier-mediated influx and efflux of drugs and endocytosis of biologically active peptides across the BBB. The transport systems at the BBB described here are the uptake transporters for nutrients, such as amino acids and hexoses, monocarboxylates, amines, carnitine and glutathione and efflux transporters, such as P-glycoprotein and multiple organic anion transporters. Delivery of cationized peptides across the BBB via adsorptive-mediated endocytosis is also described. By utilizing such highly specific transport mechanisms, it should be possible to establish strategies to regulate the entry of candidate drugs, including peptides, into the brain.

Effect of valproate on the metabolism of the central nervous system

The effects of valproate on brain energy and lipid metabolism is reviewed. Increasing evidence suggests that valproate uses the monocarboxylic acid carrier in order to cross the blood brain barrier (BBB) and the neural cell plasma membranes. The uptake of valproate into the brain through this mechanism would compete with the uptake of energy precursors, such as the monocarboxylic acids 3-hydroxybutyrate, lactate or pyruvate and with some amino acids, but not with glucose. This could impair brain fuel utilization, specially during the neonatal period or childhood, when lactate or 3-hydroxybutyrate furnishes alternative substrates to glucose for the brain. It is concluded that valproate interference with energy metabolism may have implications for the therapeutic action of the drug, stressing the possibility that valproate-mediated alterations in brain lipid synthesis may contribute to the pharmacological action of the drug.

Saturable transport of valproic acid in rat choroid plexus in vitro

In order to obtain in vitro evidence for a specific transport system of valproic acid (VPA) at the blood-cerebrospinal fluid (CSF) interface, the uptake of VPA by isolated rat choroid plexus was investigated. The uptake clearance of [3H]VPA decreased with the increase of the unlabeled VPA concentration in the incubation medium. Kinetic analysis yielded an apparent Km of 10.0 mM, Vmax of 0.0871 mumol s-1 g-1 and Kns, a permeability coefficient of the nonsaturable component, of 6.85 microL s-1 g-1, indicating that both saturable and nonsaturable systems may contribute to VPA uptake by choroid plexus. Organic anions, penicillin G, p-aminohippurate, salicylate, and probenecid significantly inhibited VPA uptake by choroid plexus. We suggest that VPA translocation through choroidal membrane is partly operated by the organic anion transport system. A significant decrease of VPA uptake induced by 2,4-dinitrophenol, stilbenedisulfonate, and hypothermia (10 degrees C) indicates the involvement of an energy-dependent, carrier-mediated transport system. These results demonstrate that VPA is actively transported through the rat choroidal epithelium via a saturable system probably shared by organic anions.