The toxicology of aluminum in the brain: a review

Aluminum is environmentally ubiquitous, providing human exposure. Usual human exposure is primarily dietary. The potential for significant Al absorption from the nasal cavity and direct distribution into the brain should be further investigated. Decreased renal function increases human risk of Al-induced accumulation and toxicity. Brain Al entry from blood may involve transferrin-receptor mediated endocytosis and a more rapid process transporting small molecular weight Al species. There appears to be Al efflux from the brain, probably as Al citrate. There is prolonged retention of a fraction of Al that enters the brain, suggesting the potential for accumulation with repeated exposure. Al is a neurotoxicant in animals and humans. It has been implicated in the etiology of sporadic Alzheimer's disease (AD) and other neurodegenerative disorders, although this is highly controversial. This controversy has not been resolved by epidemiological studies, as only some found a small association between increased incidence of dementia and drinking water Al concentration. Studies of brain Al in AD have not produced consistent findings and have not resolved the controversy. Injections of Al to animals produce behavioral, neuropathological and neurochemical changes that partially model AD. Aluminum has the ability to produce neurotoxicity by many mechanisms. Excess, insoluble amyloid beta protein (A beta) contributes to AD. Aluminum promotes formation and accumulation of insoluble A beta and hyperphosphorylated tau. To some extent, Al mimics the deficit of cortical cholinergic neurotransmission seen in AD. Al increases Fe-induced oxidative injury. The toxicity of Al to plants, aquatic life and humans may share common mechanisms, including disruption of the inositol phosphate system and Ca regulation. Facilitation of Fe-induced oxidative injury and disruption of basic cell processes may mediate primary molecular mechanisms of Al-induced neurotoxicity. Avoidance of Al exposure, when practical, seems prudent.

Glomerular lesions in male rabbits treated with aluminium lactate: with special reference to microaneurysm formation

Novel glomerular lesions were seen in male rabbits after intravenous administration of aluminum lactate. Eight rabbits in the treated group were given 0.1 mmol/kg of aluminum lactate 5 days a week for 4 weeks. The control group of 8 rabbits was given 0.3 mmol/kg of sodium lactate by the same injection protocol. In the treated group, the mesangial cells in the glomerular tufts in 6 of 8 rabbits were distended with grayish blue granular material, which was identified by laser microprobe mass spectrometry and acid solochrome azurine stain as an aluminum compound. Other consistent findings in the glomeruli included microaneurysm in 6 of 8 rabbits and segmental sclerosis in 6 of 8 rabbits. Less frequently observed glomerular changes included crescent formation, necrosis with calcification, fibrosis of the Bowman's capsule, cystic dilation of the Bowman's space, and exudation of erythrocytes into the Bowman's space. The mechanism by which aluminum lactate induces the glomerular changes is not certain. However, the pathogenesis may involve the deposition of aluminum in the mesangial cells, resulting in mesangiolysis which in turn causes microaneurysm. The sclerotic change is interpreted as a sequela of microaneurysm. The findings suggest that aluminum induces glomerular lesions in rabbits. This may serve as a good animal model to study mesangiolysis and microaneurysm formation.

The hexadentate hydroxypyridinonate TREN-(Me-3,2-HOPO) is a more orally active iron chelator than its bidentate analogue

Bidentate hydroxypyridinone chelators effectively complex and facilitate excretion of trivalent iron. To test the hypothesis that hexadentate chelators are more effective than bidentate chelators at low concentrations, urinary and biliary Fe excretions were determined in Fe-loaded rats before and after administration of a bidentate chelator, Pr-(Me-3,2-HOPO), or its hexadentate analogue, TREN-(Me-3,2-HOPO). The bidentate chelator slightly increased biliary Fe excretion in Fe-loaded rats after IV (90 micromol/kg) and PO (90 or 270 micromol/kg) administration, but chelation efficiency did not exceed 1%. The hexadentate chelator markedly increased biliary Fe excretion, achieving overall chelation efficiencies of 14% after IV administration of 30 micromol/kg and 8 or 3% after PO (30 or 90 micromol/kg) administration. The hexadentate chelator was significantly more effective than the bidentate chelator after IV injection and oral dosing. In chelator-treated Fe-loaded or saline-injected rats, >90% of the excreted Fe was in the bile. Oral TREN-(Me-3,2-HOPO), given to non-Fe-loaded rats, did not appreciably change Fe output, indicating that there was little Fe depletion in the absence of Fe overload. These results support the hypothesis that greater Fe chelation efficiency can be achieved with hexadentate than with bidentate chelators at lower, and presumably safer, concentrations. The results also demonstrate that TREN-(Me-3, 2-HOPO) is a promising, orally effective, Fe chelator.

The distribution of aluminum into and out of the brain

The extent, rate and possible mechanism(s) by which aluminum enters and is removed from the brain are presented. Introduction of Al into systemic circulation as Al.transferrin, the predominant Al species in plasma, resulted in about 7 x 10(-5) of the dose in the brain 1 day after injection. This brain Al entry could be mediated by transferrin-receptor-mediated endocytosis (TfR-ME). When Al.citrate, the predominant small molecular weight Al species in blood plasma, is introduced systemically, Al rapidly enters the brain. The rate of Al.citrate brain influx suggests a more rapid process than mediated by diffusion or TfR-ME. The question has been raised: "Is the brain a 'one-way sink' for aluminum?". Clinical observations are a basis for this suggestion. Rat brain 26Al concentrations decreased only slightly from 1 to 35 days after systemic 26Al injection, in the absence or presence of the aluminum chelator desferrioxamine, suggesting prolonged brain Al retention. However, studies of brain and blood extracellular Al at steady state, using microdialysis, suggest brain Al efflux exceeds influx, suggesting carrier-mediated brain Al efflux. The predominant brain extracellular fluid Al species is probably Al.citrate. The hypothesis that brain Al efflux, presumably of Al.citrate, is mediated by the monocarboxylate transporter was tested and supported. Although some Al that enters the brain is rapidly effluxed, it is suggested that a fraction enters brain compartments within 24 h from which it is only very slowly eliminated.

Postmortem elevation in extracellular glutamate in the rat hippocampus when brain temperature is maintained at physiological levels: implications for the use of human brain autopsy tissues

Postmortem alterations in the neuronal cytoskeleton resemble some aspects of the cytoskeletal disruption associated with neurodegenerative disorders, and are also similar to those observed following ischemia and produced by excitotoxins in vivo and in vitro. This suggests the involvement of excitotoxic mechanisms during the postmortem interval. The purpose of this study was to determine if extracellular levels of glutamate are elevated postmortem. Extracellular levels of GABA and taurine were also monitored using in vivo microdialysis. These three amino acids were analyzed using high-performance liquid chromatography. When postmortem rat brain temperature cooled rapidly to near room temperature, dialysate concentrations of glutamate were not increased in the hippocampal CA1 region during a 2-h postmortem interval, although increased extracellular levels of GABA and taurine were observed. In contrast, maintenance of brain temperature at 37 degrees C resulted in a 12-to-40 fold elevation in extracellular glutamate levels 20-120 min postmortem. In addition, the elevation in dialysate taurine concentration was greater than that observed in rats in which postmortem brain temperature was not maintained. Excitatory amino acid antagonists, NBQX (2, 3-dihydroxy-6-nitro-7-sulfamoyl-benzo(F)quinoxaline) and MK-801 (dizocilpine, (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cylohepten-5, 10-imine hydrogen maleate blocked the additional elevation in taurine associated with maintaining brain at 37 degrees C, but had less robust effects against glutamate and GABA release. The results indicate that extracellular concentrations of glutamate, taurine and GABA increase in postmortem rat brain when physiologic temperatures are maintained, but that these increases are blunted when brain temperature decreases. After death, the human brain cools much more slowly than does the rat brain. Therefore, extracellular glutamate levels are likely to increase in the postmortem human brain and may contribute to excitotoxic neuronal damage occurring in the interval between death and autopsy.

Aluminum transport out of brain extracellular fluid is proton dependent and inhibited by mersalyl acid, suggesting mediation by the monocarboxylate transporter (MCT1)

Blood brain barrier transport of aluminum citrate was assessed in rats by microdialysis of the jugular vein as well as the right and left frontal cortices. Previous studies (Allen et al., 1995. Evidence for energy-dependent transport of aluminum out of brain extracellular fluid. Toxicology 92, 193-202; Ackley and Yokel, 1997. Aluminum citrate is transported from brain into blood via the monocarboxylic acid transporter located at the blood-brain barrier. Toxicology 120, 89-97), and the current study, demonstrated that the steady-state brain-to-blood ratio of the unbound extracellular aluminum immediately surrounding the microdialysis probe is less than 1, suggesting the presence of a process other than diffusion across the blood brain barrier. It was speculated that a monocarboxylate transporter at the blood brain barrier was maintaining this ratio at less than 1 (Ackley and Yokel, 1997). Monocarboxylate transporters are proton co-transporters. Decreasing extracellular pH (increasing proton availability) increases monocarboxylate transport. After alkalinizing the dialysate perfusing a brain microdialysis probe (to pH = 10.2), the steady-state aluminum brain-to-blood ratio increased from 0.35 to 0.80. The addition of the proton ionophore, p-(trifluoromethoxy)phenylhydrazone (FCCP) (1 mM), to brain dialysate increased this ratio from 0.21 to 0.61. These increased ratios suggest that a proton-dependent process is removing Al from brain extracellular fluid. The monocarboxylate transporter is the only known proton-dependent transporter at the blood-brain barrier. There are two known isoforms of this transporter in the rodent, MCT1 and MCT2. Organomercurial thiol reagents, such as mersalyl acid, inhibit MCT1 but not MCT2. Mersalyl acid (50 mM) addition to brain dialysate increased the steady-state aluminum brain-to-blood ratio from 0.19 to 0.87, suggesting that MCT1 is at least partially mediating the efflux of aluminum from brain extracellular fluid.

An aluminum-induced increase in GFAP is attenuated by some chelators

Enhanced expression of glial fibrillary acidic protein (GFAP) has been shown to be associated with gliosis, a generic response of the CNS to neural injury. The effects of aluminum (Al) on regional GFAP concentrations were evaluated to determine potential sites of Al-induced neural injury. Rabbits received 20 Al (100 mumol/kg) or sodium lactate injections over 1 month. Frontal cortical GFAP increased (approximately twofold above control) in Al-loaded rabbits; whereas hippocampal and cerebellar GFAP concentrations were not affected. Frontal cortical synaptophysin, neurofilament 68, and myelin basic protein concentrations were then examined in an attempt to determine cell-specific targets of Al neurotoxicity. These proteins were not affected by Al. The ability of chelators to influence brain Al concentrations and the Al effect on GFAP were assessed. Desferrioxamine (DFO) and six 3-hydroxypyridin-4-ones (CPs) were given 12 times, over 1 month, to Al-loaded rabbits. CP24 significantly reduced brain Al. CP93, CP52, and CP24 significantly reduced frontal cortical GFAP. The data suggest an Al-induced gliosis consequent to subtle damage in the frontal cortex and a protective role of some chelators against this CNS injury.

Aluminum citrate is transported from brain into blood via the monocarboxylic acid transporter located at the blood-brain barrier

Aluminum citrate transport across the blood-brain barrier was assessed in rats by in vivo microdialysis. Microdialysis probes were implanted in the jugular vein as well as the left and right frontal cortex. It was demonstrated previously (Allen et al., 1995), in this study, that the steady-state aluminum citrate brain-to-blood-ratio (BBr) is less than 1, suggesting the presence of a process other than diffusion. The addition of 2,4-dinitrophenol (10 microM) to the dialysate perfusing a microdialysis probe in the brain increased the steady-state aluminum citrate brain-to-blood-ratio to a value (1.14) not significantly different from 1, suggesting the presence of an active transporter that is blocked by the metabolic inhibitor. The addition of valproic and pyruvic acid, as putative and known substrates for the monocarboxylic acid transporter, respectively, to brain dialysate (10 and 100 mM) had different outcomes. Valproic acid was ineffective at either concentration, whereas pyruvic acid (100 mM) significantly increased the aluminum citrate brain-to-blood-ratio from 0.19 to 0.31. Pyruvic acid (1 M in the dialysate) increased the aluminum citrate brain-to-blood-ratio to a value not different from unity, suggesting competition between aluminum citrate and pyruvic acid for transport. The only energy-dependent, pyruvic acid-inhibitable transporter is the monocarboxylic acid transporter. Theoretical, pharmacokinetic modeling suggests that the transporter producing an aluminum citrate brain-to-blood-ratio less than 1 is predominantly located at the blood-brain barrier, rather than at neuronal or glial cell membranes. We propose that the monocarboxylic acid transporter at the blood-brain barrier maintains a steady-state aluminum citrate brain-to-blood-ratio much less than 1.

Short-term oral 3-hydroxypyridin-4-one dosing increases aluminum excretion and partially reverses aluminum-induced toxicity in the rabbit independent of chelator lipophilicity

The objectives of the present study were to determine the efficacy and toxicity of repeated oral administration of 3-hydroxypyridin-4-one (HP) chelators in a rabbit model of aluminum (Al) accumulation and toxicity, and the influence of chelator lipophilicity on these effects. Efficacy was assessed as chelator-induced Al mobilization and excretion and reversal of Al accumulation and Al-induced toxicity. Chelator-induced toxicity was assessed by multiple measures. Six HPs were given orally 12 times over 1 month to Al-loaded rabbits, which had significant elevation of Al in most tissues and evidence of Al-induced nephrotoxicity, osteomalacia, and anemia. Intravenous desferrioxamine (DFO), the current chelator of choice for the treatment of Al-overload and toxicity, was included as a positive control. All six HPs and DFO demonstrated efficacy evidenced by significantly greater urinary and biliary Al elimination after the twelfth dose than seen in saline-treated controls. All of the HPs were more effective than DFO. Chelator-induced urinary Al excretion accounted for 58-98% of total (urinary plus biliary) Al excretion. Chelator-facilitated Al excretion was nearly complete within 12 hr, demonstrating a fairly short duration of action in rabbits with intact renal function. HP treatments did not consistently affect tissue concentrations of Al or other metals. However, there was a trend toward chelator-induced reduction of Al-induced nephrotoxicity. The influence of HP lipophilicity was limited to a positive correlation between HP x Al lipophilicity and biliary Al output and a negative correlation between HP and HP x Al lipophilicity and reduction of Kupffer cell Al. Little toxicity was evident after repeated oral HP dosing. Adrenal weight increased after treatment with several HPs. There was a decrease in testes weight after several HPs, which is consistent with an antiproliferative effect. More frequent dosing and/or a longer duration of HP treatment might produce greater reversal of the Al-induced toxicity and perhaps reveal more adverse effects than seen in this study. There was a lack of profound toxicity during this short-term study. The 1,2-dimethyl (CP20) and 1,2-diethyl (CP94) HPs, which have been the most extensively studied HPs, were the least effective of the HPs examined. These results encourage the further investigation of other HPs as oral alternatives to DFO for the treatment of Al accumulation and toxicity.

Intraneuronal aluminum potentiates iron-induced oxidative stress in cultured rat hippocampal neurons

Aluminum can facilitate Fe-mediated oxidative injury, which may contribute to Al neurotoxicity. It has been reported that Al potentiates Fe-induced oxidative stress in cultured granule cells, suggesting a mechanism for Al facilitation of Fe-mediated oxidative injury. However, the relationship of intracellular Al concentration to Fe-induced oxidative stress has not been reported. In the present study, neuronal oxidative stress and survival were investigated. Embryo rat hippocampal neuron cultures were treated with Al2(SO4)3 and/or FeSO4. An ionophore, A23187, was utilized to facilitate cellular Al uptake. Intraneuronal Al concentration was ascertained by laser microprobe mass spectrometry (LMMS). Neuronal oxidative stress was measured by confocal laser scanning microscopy, using 2,7-dichlorofluorescin diacetate (DCFH-DA) as a probe. The study showed that neuronal Al uptake was facilitated by the ionophore and that an increase of intraneuronal Al concentration potentiated Fe-induced oxidative stress and neuronal death. The results indicate that Al potentiation of Fe-induced oxidative stress might contribute to Al facilitation of oxidative injury, and thus to Al neurotoxicity.

Hippocampal acetylcholine increases during eyeblink conditioning in the rabbit

The classically conditioned rabbit nictitating membrane reflex (NMR) is modulated by the septohippocampal cholinergic system. Disruption of this system retards NMR acquisition. Aluminium (Al) is a neurotoxin that interferes with hippocampal acetylcholine (ACh) synthesis and release. Using microdialysis, this study tested the hypothesis that NMR acquisition in the rabbit is associated with hippocampal ACh release. This was conducted by measuring ACh release in control and A1-intoxicated rabbits during NMR training. NMR training consisted of four sessions of 100 conditioning trials/session in a delay paradigm. The percentage of conditioned responses (CRs) increased with each conditioning session for both groups, although percent CRs was significantly greater in the control group. Acetylcholine release in the ventral hippocampus increased significantly over baseline in the control group during the second and third conditioning sessions. In the Al-intoxicated group, ACh release did not increase significantly during any conditioning session. A separate group of rabbits was pseudoconditioned, receiving the same conditioning stimuli, although explicitly unpaired. This group did not acquire the CR. Acetylcholine release did not significantly increase during any conditioning session, suggesting that the increase in ACh release observed in the control group was not merely a product of conditioning stimuli presentation. The lack of increased ACh release in the Al-intoxicated rabbits was associated with a CR acquisition deficit. The results of this study are consistent with a role of hippocampal cholinergic function in NMR acquisition in the rabbit.

Aluminum toxicokinetics

In this study of the toxicokinetics of aluminum we have examined some of the fundamental issues that currently define our understanding of the toxicology of aluminum in humans. There is a vast literature on this subject, and it was not our aim to review this literature but to use it to develop our understanding of the toxicokinetics of aluminum and to identify critical and unresolved issues related to its toxicity. In undertaking this task we have chosen to define the term toxicokinetics to encompass those factors that influence both the lability of aluminum in a body and the sites at which aluminum is known to accumulate, with or without consequent biological effect. We have approached our objective from the classical pharmacological approach of ADME: the absorption, distribution, metabolism, and excretion of aluminum. This approach was successful in identifying several key deficits in our understanding of aluminum toxicokinetics. For example, we need to determine the mechanisms by which aluminum crosses epithelia, such as those of the gastrointestinal tract and the central nervous system, and how these mechanisms influence both the subsequent transport and fate of the absorbed aluminum and the concomitant nature and severity of the biological response to the accumulation of aluminum. Our hope in highlighting these unresolved issues (summarized in Table 1) is that they will be addressed in future research.

Prevention and treatment of aluminum toxicity including chelation therapy: status and research needs

The prevention and treatment of aluminum (Al) accumulation and toxicity are reviewed. Recommendations to further our understanding of desferrioxamine (deferoxamine, DFO) treatment and to develop more effective chelation approaches are provided. Reduction of Al accumulation and toxicity may benefit end-stage renal disease (ESRD) patients and perhaps those suffering from specific neurodegenerative disorders as well as workers with Al-induced neurocognitive disorders. The clearance of Al may be increased by extracorporeal chelation, renal transplantation, perhaps complexation with simple ligands such as silicon (Si), and systemic chelation therapy. The abilities of extracorporeal chelation and Si to reduce Al accumulation require further evaluation. Although it may not be possible to design Al-specific chelators, chelators with greater Al selectivity are desired. Aluminum-selective chelation might be achieved by targeted chelator distribution or by the use of adjuvants with the chelator. The ability of carboxylic acids to facilitate Al elimination, under specific conditions, warrants further study. Desferrioxamine does not produce significant biliary Al excretion. A chelator with this property may be useful in ESRD patients. The necessity for an Al chelator to distribute extravascularly to be effective is unknown and should be determined to guide the selection of alternatives to DFO. The lack of oral efficacy and occasional side effects of DFO encourage identification of orally effective, safer Al chelators. The bidentate 3-hydroxypyridin-4-ones are currently the most encouraging alternatives to DFO. They have been shown to increase urinary Al excretion in rats and rabbits, but to have toxicity comparable to, or greater than, DFO. Their toxicity may relate to incomplete metal complexation. The ability of orally effective chelators to increase absorption of chelated metal from the gastrointestinal (Gl) tract needs to be evaluated. Orally effective, safe Al chelators would be of benefit to peritoneal dialysis patients and those with neurodegenerative disorders, if Al chelation therapy is indicated. The reduction of Alzheimer's disease (AD) progression and the reversal of Al-induced behavioral deficits and neurofibrillary tangles by DFO encourage further study of Al chelation therapy for selected neurodegenerative disorders.

Status and future concerns of clinical and environmental aluminum toxicology

A wide range of toxic effects of aluminum (Al) have been demonstrated in plants and aquatic animals in nature, in experimental animals by several routes of exposure, and under different clinical conditions in humans. Aluminum toxicity is a major problem in agriculture, affecting perhaps as much as 40% of arable soils in the world. In fresh waters acidified by acid rain, Al toxicity has led to fish extinction. Aluminum is a very potent neurotoxicant. In humans with chronic renal failure on dialysis, Al causes encephalopathy, osteomalacia, and anemia. There are also reports of such effects in certain patient groups without renal failure. Subtle neurocognitive and psychomotor effects and electroencephalograph (EEG) abnormalities have been reported at plasma Al levels as low as 50 micrograms/L. Infants could be particularly susceptible to Al accumulation and toxicity, reduced renal function being one contributory cause. Recent reports clearly show that Al accumulation occurs in the tissues of workers with long-term occupational exposure to Al dusts or fumes, and also indicate that such exposure may cause subtle neurological effects. Increased efforts should be directed toward defining the full range of potentially harmful effects in humans. To this end, multidisciplinary collaborative research efforts are encouraged, involving scientists from many different specialties. Emphasis should be placed on increasing our understanding of the chemistry of Al in biological systems, and on determining the cellular and molecular mechanisms of Al toxicity.

The pharmacokinetics and blood-brain barrier permeation of the chelators 1,2 dimethly-, 1,2 diethyl-, and 1-[ethan-1'ol]-2-methyl-3-hydroxypyridin-4-one in the rat

The 3-hydroxypyridin-4-ones (HPs) are iron and aluminum chelators. Their ability to enter the brain had not previously been directly determined. To determine whether they cross the blood-brain barrier (BBB), three HPs possessing a wide range of lipophilicity were examined: 1-[ethan-1'ol]-2-methyl-HP (CP40), 1,2-dimethyl-HP (CP20, L1, deferiprone), and 1,2-dimethyl-HP (CP94, EL1NEt). Their pharmacokinetics were determined in rats to establish dosing parameters for microdialysis studies of BBB permeation. Studies were then conducted with microdialysis probes in the blood, frontal cortex, and lateral ventricle to determine the rate and extent of HP BBB permeability. All three HPs were detectable in brain dialysate samples collected 0-7 min after HP injection, demonstrating rapid entry into the brain. The extent of unbound distribution (an indicator of the mechanism of BBB permeation) was 0.9 and 1.2 for the frontal cortex and lateral ventricle for CP20, and was 1.1 and 1.6 for CP94, suggesting diffusion across the BBB. The extent of unbound distribution of CP40 was 0.2 for both the frontal cortex and lateral ventricle, suggesting the presence of a transporter moving it out of brain extracellular fluid. Introduction of cyanide into the brain did not affect the brain to blood CP40 ratio, suggesting that the transporter is not energy-dependent. Both CP94 and CP40 caused death due to respiratory failure, whereas CP20 did not. The ability of less toxic bidentate HP chelators, such as CP20, to enter the brain may enable their use in the treatment of metal-induced diseases and iron-facilitated oxidative injury involving the central nervous system.

Aluminum facilitation of iron-mediated lipid peroxidation is dependent on substrate, pH and aluminum and iron concentrations

It has been suggested that aluminum (Al) plays a role in neurological disorders. The mechanism of its neurotoxicity has not been established. Brain lipid peroxidation (LP) contributes to neurodegeneration. There have been conflicting reports concerning the Al effect on LP. In the present study, LP of three Folch Fractions from bovine brain and five pure phospholipids was determined in the presence of varying concentrations of iron (Fe) and Al at pH 5.5 and 7.4. Lipid peroxidation was measured as thiobarbituric acid reactive substances. Iron initiated LP, whereas Al did not. However, Al significantly facilitated Fe-mediated LP of bovine brain Folch Fractions I and III, bovine brain-derived phosphatidylserine, and egg yolk phosphatidylcholine. Bovine brain phosphatidylserine was the most susceptible substrate among the lipids tested. Aluminum facilitation of LP was Al and Fe concentration dependent. The peroxidation was greater at pH 5.5 than 7.4. There was no significant Al effect with Folch Fraction V, bovine brain-derived phosphatidylethanolamine, phosphatidylcholine, or sphingomyelin. This study confirmed the ability of Al to facilitate Fe-mediated LP and identified the substrates, pH, and Al concentrations favoring the peroxidation. A potential mechanism for Al facilitation of Fe-mediated LP is proposed.

The 3-hydroxypyridin-4-ones more effectively chelate aluminum in a rabbit model of aluminum intoxication than does desferrioxamine

This study was conducted to assess the influence of lipophilicity on the in vivo aluminum (Al) chelation activity of 3-hydroxypyridin-4-ones (HPs). Chelation activity was evidenced as increased Al elimination in an animal model of Al accumulation and toxicity. The subjects were Al-loaded rabbits. A non-Al-loaded group was included to characterize the rabbit model of Al intoxication. Eight HPs and desferrioxamine (DFO), the drug currently used to treat Al intoxication, were studied. Chelation activity was determined from quantitative biliary and urinary Al excretion and serum Al determinations conducted for 24 hr after DFO or HP intravenous administration, compared with saline. Toxicity was evaluated by observation, blood biochemistry assays, hematological evaluation, gross necropsy, and histopathological assessment of the liver. Al loading produced nephrotoxicity, hepatotoxicity, and anemia. Each of the chelators mobilized Al into serum. The efficiency of Al chelation, calculated from 24-hr biliary plus urinary Al output, ranged from 2.8 to 11.7% for the HPs, compared with 2.1% for DFO. Urinary Al excretion accounted for 78-98% of total Al excretion. Nearly all of the chelator-facilitated Al excretion occurred within 8 hr of dosing. Al chelation efficacy did not correlate with HP or HP Al lipophilicity; however, increasing HP lipophilicity increased the biliary fraction of the excreted Al. There was no evidence for toxicity after HP dosing, other than the previously shown ability of one of the HPs to produce seizures. The greater chelation efficacy of the HPs than DFO provides advantages over DFO. The lack of toxicity after a single dose of all but the most lipophilic HP encourages their further evaluation as orally effective chelators.

Aluminum chelation by 3-hydroxypyridin-4-ones in the rat demonstrated by microdialysis

The ability and site of the metal-chelating 3-hydroxypyridin-4-ones (HPs) to mobilize aluminum (Al) was assessed in Al-loaded rats using microdialysis. Four HPs with greatly varying lipophilicity were studied. One week after Al loading, microdialysis probes were implanted in the liver, a jugular vein, and the frontal cortex. An HP was given iv followed by continuous microdialysis for 5 h. Al concentrations in dialysates from the liver increased rapidly and were consistently greater than from blood, suggesting that liver was a primary site of Al chelation. Brain dialysate Al concentrations remained low, suggesting little Al chelation in the brain and little distribution of the Al HP complex into the brain. Al concentrations were determined in the main organs/tissues of a separate group of Al-loaded rats, and the percentage of the total Al body burden in each organ/tissue was calculated. The skeletal system and liver had 57 and 28% of the Al body burden, consistent with the liver as a primary site of Al chelation. The HPs chelate extravascular Al and have been shown by others to be orally active. They warrant further investigation as Al chelators.

HPLC quantitation of a very hydrophilic 3-hydroxypyridin-4-one chelator using a simple separation procedure and the baseline file subtraction method

The lack of a method to isolate very hydrophilic 3-hydroxypyridin-4-ones (HPs) from blood has prevented determination of their pharmacokinetics. The objective of this study is to develop method to quantitate these compounds. A simple sample preparation method coupled with high-performance liquid chromatography is used to quantitate 1-[ethan-1-ol]-2-methyl-3- hydroxypyridin-4-one, a very hydrophilic HP, in plasma. Plasma proteins are precipitated by trichloroacetic acid. The baseline file subtraction method is used to improve the resolution of this HP in the presence of interfering chromatographic peaks that could not be resolved from the HP by the methods investigated. The method is used to determine the pharmacokinetics of this HP in rabbits. The precision of the pharmacokinetic results is comparable or better than the results obtained from seven more lipophilic HPs that were separated by a published method. The new method is slightly modified and used in a study of the pharmacokinetics of this HP in the rat, and precision is comparable with results obtained with two more lipophilic HPs determined by the published method. Baseline file subtraction is useful when other methods cannot be used to adequately resolve a hydrophilic analyte from coeluting interfering substances.

Delayed elevation of platelet activating factor in ischemic hippocampus

We used in vivo microdialysis to define the chronological relationship between release of thromboxane and platelet activating factor (PAF) into the extracellular space of ischemic hippocampus. The thromboxane level peaked after 20 min of postischemic reperfusion, followed by a delayed PAF response 120 min later. We conclude that cerebral ischemia causes delayed elevation of PAF in the extracellular space, long after the immediate synthesis and release of thromboxane metabolites.

Selective adherence of a sucralfate-tetracycline complex to gastric ulcers: implications for the treatment of Helicobacter pylori

The adherence of a sucralfate-tetracycline complex to gastric ulcers and to nearby non-ulcer sites was determined in the rabbit antrum. Persistent gastric ulcers were produced by a previously described method. The presence of the complex was assessed 1 and 4 h after dosing. Drug adherence was determined by quantitation of aluminum in stomach wall biopsies. Significantly more aluminum adhered to ulcer sites than to nearby non-ulcer sites. Adherence of the complex did not significantly decrease from 1 to 4 h. The complexation of tetracycline to sucralfate did not alter the selective adherence of sucralfate to gastric ulcers, providing a mechanism of ulcer site-selective drug delivery in the treatment of Helicobacter pylori gastric ulcer disease.

Evidence for energy-dependent transport of aluminum out of brain extracellular fluid

Aluminum (Al) can cause CNS toxicity. The mechanism of its blood-brain barrier (BBB) permeation is poorly understood. In this study, microdialysis was used to determine extracellular fluid (ECF) unbound aluminum distribution between frontal cortex (FC) and blood during steady-state aluminum concentrations. The brain/blood aluminum ratio was determined. Over a 16-fold range of aluminum concentrations (dosed as aluminum citrate), brain/blood aluminum ratios were 0.10-0.15, consistently and significantly < 1. Aluminum diffusion cannot account for these results, suggesting the presence of a carrier that moves aluminum out of brain extracellular fluid. These aluminum brain/blood ratios (BBRs) were not significantly different over the range of concentrations studied, suggesting an inability to saturate the carrier. Brain/blood aluminum ratios obtained with four aluminum-hydroxypyridinones were also significantly < 1 (0.1-0.3), and were generally significantly different among themselves and from the aluminum citrate BBR. Movement of a BBB permeability marker from blood into brain extracellular fluid suggested partial BBB opening. The aluminum BBRs obtained (all << 1), in the presence of a partially opened BBR, suggest an efficient carrier moving aluminum out of brain ECF. Addition of cyanide to the brain microdialysis probe solution significantly increased the Al (citrate) BBR to 1. These results suggest the presence of an efficient, energy-dependent carrier that removes aluminum from brain ECF, either into brain cells or blood.

A phase I trial of 5-day continuous infusion cisplatin and interferon alpha

Combination therapy of cisplatin with interferon alpha (IFN) has been shown in several in vitro as well as in vivo models to be synergistic. In order to decrease toxicity seen with cisplatin, 5-day continuous infusions, in place of bolus administration, have been introduced. This led us to investigate the combination of 5-day continuous infusion cisplatin with repeated IFN dosing in a phase I cisplatin dose escalation study. A group of 17 patients were enrolled in this trial. The maximum tolerated dose (MTD) of cisplatin was 20 mg/m2 per day when combined with 3 x 10(6) units IFN given three times a week. The dose-limiting toxicities seen included thrombocytopenia, leukopenia, and nausea and vomiting. Pharmacokinetic analyses of free (unbound or ultrafilterable) platinum revealed that the decay curve fitted a monoexponential model. Pharmacokinetic parameters of cisplatin were found to correlate with toxicity. Both increases in the maximum concentration of cisplatin achieved (Cpmax) as well as the area-under-the-curve (AUC) for free platinum, correlated with the incidence of nausea and vomiting (both acute and delayed) and hematological toxicities (leukopenia and thrombocytopenia). None of the patients exhibited significant changes in renal function while on this study. The free platinum levels were higher than found in similar studies evaluating comparable cisplatin infusions alone. The enhanced toxicities seen in this trial may be explained by the results of an in vitro study using human plasma spiked with cisplatin and IFN that revealed decreased protein binding of cisplatin by 2.5-3.0-fold. Of the 17 patients treated, two non-small cell lung cancer patients obtained a partial response and one malignant melanoma patient obtained complete resolution of a malignant pleural effusion. Considering the acceptable toxicity seen in this trial, we recommend phase II trials be conducted with continuous infusion cisplatin with IFN in the treatment of non-small cell lung cancer.

Studies of aluminum neurobehavioral toxicity in the intact mammal

1. Aluminum (Al) has been implicated in neurotoxic syndromes in several conditions, including Alzheimer's disease (AD). The developmental stage of the mammalian brain most susceptible to Al was determined in rabbits systematically exposed to Al during the prenatal, postnatal, or second month or for 1 month as adults or as aged subjects. Eyeblink reflex classical conditioning showed an Al-induced learning deficit only in the adult and aged rabbits. 2. 4-Aminopyridine, which was reported to improve learning in AD subjects, attenuated the Al-induced learning deficit. 3. Conditioned eyeblink acquisition is slower in AD subjects than controls, supporting the Al-loaded rabbit as a model of some AD effects. 4. To determine if the Al-loaded rabbit modeled the AD cholinergic deficit, acetylcholine (Ach) overflow was measured in rabbit hippocampus using microdialysis. Aluminum pretreatment reduced basal and potassium-stimulated Ach overflow compared to controls. 5. Acetylcholine overflow increased as control rabbits acquired the conditioned eyeblink reflex, then subsequently decreased, although conditioned eyeblink performance continued. In contrast, Al-loaded rabbits showed a delay in conditioned eyeblink acquisition and greatly attenuated Ach overflow. The Al-induced attenuation of Ach overflow may contribute to the Al-induced learning deficit. 6. Brain Al entry was studied using microdialysis of blood, brain, and lateral ventricle. Aluminum rapidly entered the brain and lateral ventricle. Frontal cortical Al was greater than lateral ventricular Al, suggesting that Al primarily enters the brain through the cerebral microvasculature. 7. The brain/blood Al ratio was always significantly less than 1. This ratio was influenced by the Al form administered, brain site and animal species. Thus, there appears to be an active process moving Al out of brain extracellular fluid (ECF). 8. Brain and blood dialysate Ach concentrations were not different after cyanide addition to the dialysate, supporting the conclusion that an active process moves Al out of brain ECF.