Clioquinol: review of its mechanisms of action and clinical uses in neurodegenerative disorders

Clioquinol was produced as a topical antiseptic and marketed as an oral intestinal amebicide in 1934, being used to treat a wide range of intestinal diseases. In the early 1970s, it was withdrawn from the market as an oral agent because of its association with subacute myelo-optic neuropathy (SMON), a syndrome that involves sensory and motor disturbances in the lower limbs and visual changes. The first methods for determining plasma and tissue clioquinol (5-chloro-7-iodo-8-quinolinol) levels were set up in the 1970s and involved HPLC separation with UV detection, these were followed by a more sensitive GC method with electron capture detection and a gaschromatographic-massspectrometric (GC-MS) method. Finally, an HPLC method using electrochemical detection has proved to be as highly sensitive and specific as the GC-MS. In rats, mice, rabbits, and hamsters, clioquinol is rapidily absorbed and undergoes first-pass metabolization to glucuronate and sulfate conjugates; the concentrations of the metabolites are higher than those of free clioquinol. Bioavailabilty versus intraperitoneal dosing is about 12%. Dogs and monkeys form fewer conjugates. In man, single-dose concentrations are dose related, and the drug's half-life is 11-14 h. There is no accumulation, and the drug is much less metabolized to conjugates. Clioquinol acts as a zinc and copper chelator. Metal chelation is a potential therapeutic strategy for Alzheimer's disease (AD) because zinc and copper are involved in the deposition and stabilization of amyloid plaques, and chelating agents can dissolve amyloid deposits in vitro and in vivo. In general, the ability of clioquinol to chelate and redistribute metals plays an important role in diseases characterised by Zn, Cu, Fe dyshomeostasis, such as AD and Parkinson's disease, as it reduces oxidation and the amyloid burden. Zinc chelators may also act as anticancer agents. Animal toxicity studies have revealed species-specific differences in neurotoxic responses that are related to the serum levels of clioquinol and metabolites. This is also true in humans, who form fewer conjugates. The results of studies of Alzheimer patients are conflicting and need further confirmation. The potential therapeutic role of the two main effects of MPACs (the regulation of the distribution of metals and antioxidants) has not yet been fully explored.     

Subacute myelo-optic neuropathy (SMON). First neuro-pathological report outside Japan

A 72-year-old Caucasian woman developed degeneration of the spinal cord long tracts, polyneuropathy, and optic atrophy after chronic ingestion of clioquinol (200 mg/day) since she was 45. Diseases with a similar clinical picture, notably vitamin B12 deficiency (B12D), and subacute myelo-optic neuropathy (SMON), are discussed with regard to the pathologic findings. In our patient findings were different from those reported in B12D, but similar to those in SMON. If, as we believe, our patient was suffering from the same disease as the SMON described by Japanese authors, this is the first case reported outside Japan in which pathological verification has been obtained.     

SMON--a model of the iatrogenic disease]

The subacute myelo-optico-neuropathy (SMON) was hazard caused by clioquinol, an antiseptic, prescribed for the treatment of diarrhea and other bowel symptoms. Its overdosing and long-term taking led to the occurrence of SMON, for which physicians should be responsible. Clioquinol, originally a disinfectant powder for external use, was diverted later to a drug for internal use to sterilize the bowel where no intestinal absorption or action after absorption was expected. An annotation on the 6th Revision of the Japan Pharmacopoeia (1954) allowed irregular increase in its dosage depending on the severity of illness. An annotation on the 7th Revision (1961) ignoring the 6 papers published in the 1930's, 1940's or 1950's claimed that its metabolism was poorly known, yet neglected significant side effect and substantial absorption from the intestine. Its characterization as a superficial disinfectant helped the annotators be less interested in its absorption and its internal actions and side effects. Attention paid by clinicians to a polyneuropathy-like syndrome that complicated an uncontrollable hemorrhagic diarrhea (1958) and an encephalomyelitis or a paralysis of the lower half of the body associated with diarrhea or other bowel symptoms (1960, 1961) started the recognition of a new disease. During the dispute induced by the mass occurrence of the disease in several instances postmortem examination with neuropathologic expertise, especially of T. Tsubaki, Y. Toyokura and H. Tsukagoshi (1964), characterized SMON as a non-inflammatory new disease of the spinal cord, optic nerve and peripheral nerve with a pseudo-systemic degeneration of posterior and lateral columns and, therefore played a decisive role in establishing the truth of SMON. The discovery of the green hairy tongue (the tongue coated with green hairs) of SMON by T. Takasu, A. Igata and Y. Toyokura (January 1970) aroused researchers' interest in the green color of SMON and thereby began solving the cause of SMON. The discovery of the green urine in SMON patients by A. Igata, M. Hasebe and T. Tsuji (May 1970) especially facilitated the identification of the green substance in SMON that was achieved by M. Yoshioka and Z. Tamura (June 1970). The green color was derived from a chelate compound of clioquinol with ferric iron. The early epidemiological analysis related clioquinol taking to the occurrence of SMON well enough for the Japanese Government to take an administrative measure for the temporary suspension of selling clioquinol containing drugs and the postponing of their use (September 1970). Extensive and intensive multidisciplinary investigations conducted for the subsequent 20 months led to the conclusion by the SMON Investigation and Research Committee (Head: R. Kono) that the neurological disorders of patients who were diagnosed as SMON for the most part were caused by taking clioquinol (March 1972). Close clinical observation of patients opened a way to recognize a new disease and elucidate its cause. Expert specialized technical knowledge and skills established the firm knowledge of the new disease. The study of SMON began as a personal research and after its achievement was exposed to the public a great many investigators in different fields concerted efforts to solve problems. Both steps were indispensable for completing the study.     

Metals in our minds: therapeutic implications for neurodegenerative disorders

BACKGROUND: Abnormal interactions of copper or iron in the brain with metal-binding proteins (such as amyloid-beta peptide [Abeta] or neuromelanin) that lead to oxidative stress have emerged as important potential mechanisms in brain ageing and neurodegenerative disorders. Although a controlled study of desferrioxamine in Alzheimer's disease(AD) had some promising results, concerns about toxicity and brain delivery have limited trials of traditional chelators. The therapeutic significance of metal dysregulation in neurodegenerative disorders has remained difficult to test. RECENT DEVELOPMENTS: Clioquinol was identified as a prototype metal-protein-attenuating compound (MPAC). In a blinded and controlled 9 week study of a mouse model of AD, oral clioquinol decreased brain Abeta by 49% without systemic toxicity. The concentrations of copper and zinc in the brain rose by about 15% in mice treated with clioquinol. Two other studies in mice showed that the raising of brain copper concentrations through diet or genetics could lower amyloid load and increase survival. A recent placebo-controlled trial in 36 patients with AD showed that clioquinol (250-750 mg daily) reduced plasma concentrations of Abeta(1-42), raised plasma concentrations of zinc, and-in a subset with moderate dementia-slowed cognitive decline over 24 weeks. Two recent experiments also showed the neuroprotective effects of iron chelation in a mouse model of Parkinson's disease. WHERE NEXT?: The experimental and transgenic-animal studies of metal-protein interactions are convincing but do not provide conclusive answers either about causality or whether this strategy will protect against neurodegeneration in human beings. The finding that clioquinol could modulate plasma concentrations of amyloid and cognition in patients with AD needs to be interpreted cautiously, but is an important first step. Clioquinol was withdrawn because of concerns of its association with subacute myelo-optic neuropathy in Japan; therefore, any additional studies with this drug will likely be small and closely monitored proof-of-concept studies. The development of optimal second-generation MPACs is a desirable goal and may permit greater insights into the significance of metal-protein interactions across several neurodegenerative disorders.     

Subacute myelo‐optico‐neuropathy: Clioquinol intoxication in humans and animals

It remains a tragic event that some 10 000 individuals in Japan developed a unique neurologic disease, subacute myelo‐optico‐neuropathy (SMON). Many of the affected patients still suffer serious sequelae, such as dysesthesia and muscle weakness in the lower extremities, and loss or deficits in visual acuity. Neuropathologic studies on SMON patients and experimental reproduction of the disease in animals which had been administered clioquinol helped resolve the etiology of this disease. Common pathologic features seen in SMON patients and in dogs and cats chronically intoxicated with clioquinol were distal dominant axonopathy, mainly in the spinal long tracts and optic tracts. Particular abdominal symptoms present in patients after clioquinol ingestion could also be reproduced experimentally in dogs. SMON research in Japan may be worth reviewing for determining the etiology and preventing similar neurotoxic diseases in the future.     

Electrophysiological changes in the fasciculus gracilis of the cat following chronic clioquinol administration

Clioquinol was administered to cats for more than 200 days, in order to investigate the neural mechanisms underlying the sensory disturbances of subacute myelo-opticoneuropathy (SMON). Electrophysiological examination, carried out under urethane-chloralose anesthesia, revealed that there were 3 major abnormalities in the surface potentials of the nucleus gracilis evoked by sural nerve stimulation, i.e., a reduction in the peak-to-peak amplitude, prolongation of the N wave, and a reduction in P wave amplitude. The reduction in P-wave amplitude suggested suppression of presynaptic inhibition. This was confirmed by excitability tests of the presynaptic terminals of sural nerve fibers within the nucleus gracilis. Recordings of orthodromic volleys in the fasciculus gracilis, elicited by sural nerve stimulation, showed an increase in temporal dispersion. Increased temporal dispersion was also evident from recordings of antidromic volleys in the sural nerve. Peripheral axons of primary sensory neurons in the sural nerve and their terminals within the spinal cord showed no significant functional abnormalities in the chronic clioquinol cat. It is suggested that primary axons in the fasciculus gracilis near the nucleus gracilis are affected by chronic clioquinol administration.     

Intermittent transmission in the reciprocal inhibitory pathway from flexor muscle Ia afferents to extensor motoneurons in man

To know possible involvement of primary afferents in paresthesia of subacute myelo-optico-neuropathy (SMON), influence on polymodal receptor, a type of nociceptor, of its causal agent clioquinol (5-chloro-7-iodo-8-hydroxyquinoline) was tested using in vitro testis-superior spermatic nerve preparations of dogs. Exposure to clioquinol (greater than 1 microM) induced an abnormal bursting activity in polymodal receptors, and thereafter the receptors were sensitized to an algesic stimulus (hypertonic saline) and gained a new sensitivity to cold, while pure mechanoreceptors were not influenced. On the other hand, clioquinol glucuronide (100 microM), a detoxicated form, had no such effects. These results are consistent with several clinical observations of sensory aberrations, suggesting possible involvement of the abnormal activities of polymodal receptors in SMON paresthesia.     

Some observations on SMON from Bombay.

Nine patients in whom subacute myelo-opticoneuropathy (SMON) was diagnosed with varying degrees of confidence are discussed. The cases were discovered after a retrospective examination of our records for the period 1967-71, and a prospective search from March 1972 to date. Subacute myelopathy with predominant dysaesthesiae and greater involvement of the pyramidal tracts was seen more often than fully developed SMON. Subacute myelopathy was seen in six instances, opticomyelopathy in two and myeloneuropathy only once. Clioquinol could not be excluded as an aetiological agent. The difference in the reported prevalence of SMON between Japan and India is noted, and factors which may account for this difference are discussed. Problems related to the diagnosis of SMON outside Japan, and particularly in India, are stressed.     

Treatment of Alzheimer's disease with clioquinol

As heavy metal ions may be implicated in the formation of senile plaques in Alzheimer-afflicted brains, treatment with clioquinol was tested in 20 patients with Alzheimer's disease. Clioquinol is a chelator that crosses the blood-brain barrier and has greater affinity for zinc and copper ions than for calcium and magnesium ions. Treatment was given for 21 days at doses of 20 mg/day to 10 patients and 80 mg/day to another 10 patients. The study was blind to the dosages but included no controls. Cerebrospinal fluid (CSF) investigations revealed a significant increase at day 7 and a decrease at day 21 in Tau protein and growth-associated protein (GAP43). These proteins are increased in Alzheimer's disease and considered as rather stable markers. The initial increase may indicate a temporary cytotoxicity to the brain and/or an increased release into the CSF from stores in the tissue, possibly from senile plaques where the proteins are accumulated. The levels of CSF-Tau protein correlated positively and significantly with the serum levels of copper and also with the serum copper/zinc ratio. Clinical ratings showed slight improvement after 3 weeks treatment with clioquinol in this open study.     

Abnormal activity of polymodal receptors induced by clioquinol (5-chloro-7-iodo-8-hydroxyquinoline)

To know possible involvement of primary afferents in paresthesia of subacute myelo-optico-neuropathy (SMON), influence on polymodal receptor, a type of nociceptor, of its causal agent clioquinol (5-chloro-7-iodo-8-hydroxyquinoline) was tested using in vitro testis-superior spermatic nerve preparations of dogs. Exposure to clioquinol (>1 μM) induced an abnormal bursting activity in polymodal receptors, and thereafter the receptors were sensitized to an algesic stimulus (hypertonic saline) and gained a new sensitivity to cold, while pure mechanoreceptors were not influenced. On the other hand, clioquinol glucuronide (100 μM), a detoxicated form, had no such effects. These results are consistent with several clinical observations of the sensory aberrations, suggesting possible involvement of the abnormal activities of polymodal receptors in SMON paresthesia.     

Elevated plasma concentrations of homocysteine in antiepileptic drug treatment

PURPOSE: Homocysteine is an experimental convulsant and an established risk factor in atherosclerosis. A nutritional deficiency of vitamin B6, vitamin B12, or folate leads to increased homocysteine plasma concentrations. During treatment with carbamazepine (CBZ), phenytoin, or phenobarbital, a deficiency in these vitamins is common. The objective of the study was to test the hypothesis that antiepileptic drug (AED) treatment is associated with increased homocysteine plasma concentrations. METHODS: A total of 51 consecutive outpatients of our epilepsy clinic receiving stable, individually adjusted AED treatment and 51 sex- and age-matched controls were enrolled in the study. Concentrations of total homocysteine and vitamin B6 were measured in plasma; vitamin B12 and folate were measured in the serum of fasted subjects. RESULTS: Patients and controls differed significantly in concentrations of folate ( 13.5+/-1.0 vs. 17.4+/-0.8 nM and vitamin B6 (39.7+/-3.4 vs. 66.2+/-7.5 nM), whereas serum concentrations of vitamin B12 were similar. The homocysteine plasma concentration was significantly increased to 14.7+/-3.0 microM in patients compared with controls (9.5+/-0.5 microM; p < 0.05, Wilcoxon rank-sum test). The number of patients with concentrations of >15 microM was significantly higher in the patient group than among controls. The same result was obtained if only patients with CBZ monotherapy were included. Patients with increased homocysteine plasma concentrations had lower folate concentrations. CONCLUSIONS: These data support the hypothesis that prolonged AED treatment may increase plasma concentrations of homocysteine, although the alternative explanation that increased homocysteine plasma concentrations are associated with the disease and not the treatment cannot be completely excluded at the moment.     

Effects of long-term phenytoin treatment on brain weight and zinc and copper metabolism in rats

The effects of phenytoin (PHT) treatment on brain weights and the zinc (Zn) and copper (Cu) concentrations in liver, kidney, and five parts of the brain have been studied in rats. After 32 wk of treatment (daily doses 72–88 mg/kg body weight), significantly reduced brain weights were found in rats sacrificed during treatment, but not in those sacrificed after 14 d of abstinence. The weight reduction mainly seemed to affect cortex, but cerebellum was also influenced. The PHT treatment during 18 wk did not significantly reduce the brain weights. At the end of treatment, significantly increased serum Cu concentrations were found, as well as decreased Zn levels in the liver and low Cu levels in the kidney. No large alterations were found in the trace element concentrations of different brain regions. The PHT treatment for 32 wk induced physical dependence, recorded as convulsions. It is suggested that PHT through a chelate binding with Zn and Cu interferes with the metabolism of the trace elements and the drug may cause a Zn deficiency. The observed decrease of the brain weights may have some parallel to the mental side effects of the drug observed during chronic epilepsy therapy.     

Ultrastructural study of a muscle biopsy from a patient with subacute myelo-optic neuropathy

Summary A man, aged 46, who had been taking Clioquinol in high doses for a long period, developed a characteristic neurological syndrome of subacute myelo-optic neuropathy rather abruptly. Electron microscopical examination of the muscle biopsy, obtained five months after the onset of the disease, revealed severe degenerative changes of the presynaptic nerve endings and some unique paracrystalline inclusions in the sole plate region. The latter may represent the morphological expression of the toxic agent which is held responsible for the subacute myelo-optic neuropathy.

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Zusammenfassung Ein 46j?hriger Mann, welcher über lange Zeit hohe Dosen von Clioquinol einnahm, entwickelte recht pl?tzlich charakteristische neurologische Ausf?lle, entsprechend einer subakuten myelooptischen Neuropathie (SMON). Bei der elektronenoptischen Untersuchung einer Muskelbiopsie, die 5 Monate nach Beginn der Erkrankung entnommen worden war, zeigten sich ausgepr?gte degenerative Ver?nderungen der pr?synaptischen Nervenendigungen und vereinzelte parakristalline Einschlüsse in der Endplattenregion, die einmalig erscheinen. Letzteres k?nnte das morphologische Substrat der toxischen Substanz darstellen, die für den SMON verantwortlich gemacht wird.

     

Clinical analysis of longstanding subacute myelo-optico-neuropathy: sequelae of clioquinol at 32 years after its ban

One thousand and thirty-one longstanding patients with subacute myelo-optico-neuropathy (SMON; 275 males, 756 females; mean age +/- S.D., 72.9 +/- 9.6 years; age at onset 37.6 +/- 9.8 years; duration of illness 35.3 +/- 4.0 years) were examined in 2002, 32 years after banning of clioquinol. At onset, 66.7% of patients were unable to walk, and 4.7% complete blindness. At present time, about 41% of patients were still difficult to walk independently, including 15.8% of completely loss of locomotion. One point six percent of patients were in complete blindness and 5.8% had severe visual impairment. The majority (95.6 - 97.7%) of patients exhibited sensory disturbances including superficial and vibratory sensations and dysesthesia. Dysautonomia was observed as leg hypothermia in 79.8%, urinary incontinence in 60.7%, and bowel disturbance in 95.3%. As complication, high incidence was revealed with cataract (56.2%), hypertension (40.2%), vertebral disease (35.5%), and limb articular disease (31.5%). These results indicate the serious sequelae of clioquinol intoxication, SMON.     

Cerebral Cortical Cholesterol Changes in Cobalt-Induced Epilepsy

The present study was designed to elucidate cerebral cortical cholesterol changes observed earlier in cobalt-induced epilepsy in the rat. This model is produced by insertion of cobalt metal rods into the cerebral cortex. Seven days after implantations of cobalt (or other metals), measurements of cerebral cortical lipid concentrations were made in the direct area of metal implantation (lesion area) and in the nonnecrotic tissue immediately adjacent to the lesion site (adjacent area). The cortical concentration of free cholesterol decreased and the concentrations of cholesterol esters (CE) greatly increased in the adjacent area of rats implanted with epileptogenic metals (cobalt and nickel) but not in those implanted with nonepileptogenic metals (copper and stainless steel). Differences in the fatty acid compositions of CE from plasma and those from cobalt-lesioned cortex indicated that the accumulated CE originated in the brain rather than from the plasma CE pool. A time course study revealed that changes in the cortical concentrations of free and esterified cholesterol precedes the first appearance of epileptiform activity as measured by electrocorticography. The reduction of free cholesterol levels most likely reflects increased cholesterol metabolism in the cobalt-lesioned cortex. A major portion of this free cholesterol appears to be converted to CE, but not to bile acid or neutral sterol (potential products of cholesterol metabolism). The concentration of free fatty acids, the other substrate of CE formation, was decreased in the lesion site. The increased metabolism of cholesterol could not be explained by changes in the activities of CE synthetase and hydrolase(s). Since cholesterol in the brain is found almost exclusively in membranes, it is likely that these sterol changes reflect membrane alterations or disintegration. However, the exact relationship of these changes to the epileptogenic effect of cobalt remains unclear.     

Stoichiometry and conditional stability constants of Cu(II) or Zn(II) clioquinol complexes; implications for Alzheimer's and Huntington's disease therapy

Successful trials with 5-chloro-7-iodo-8-hydroxyquinoline (clioquinol, CQ) for Alzheimer's disease treatment prompted renewed interest in assessing whether its therapeutic action is related to the coordination of neurotoxic trace metals, such as Cu(II) and Zn(II). We now report conditional stability constants (K(C')) for CQ Cu(II) and Zn(II) complexes measured in a biological buffer containing Ca(II) and Mg(II) ions. UV-vis spectroscopy and polarography evidenced a 1:2 stoichiometry of Cu(II) and Zn(II) CQ complexes; the K(C')s calculated were: Cu(CQ)(2) 1.2x10(10), and Zn(CQ)(2) 7.0x10(8)M(-2); the CQ affinity for Cu(II) is at least an order of magnitude higher than for Zn(II). To test the possible functional relevance of the Cu(II) CQ complexes in the brain, we bioassayed free Cu(II) concentration by the metal-induced inhibition of ATP-gated currents of the P2X(4) receptor, a predominant brain P2X receptor. CQ reduced concentration-dependently the Cu(II) inhibition of the ATP-gated currents. In view that the stability constant of CQ for Zn(II) is similar to that of Abeta-amyloid for Zn(II), and the fact that CQ may form complexes with Cu(II), even in the presence of competing ions, the present results highlight that the formation of Cu(II) CQ complexes in the brain may act by diminishing free Cu(II) concentrations modifying thereby brain excitability, or favoring the degradation of beta-amyloid plaques or huntingtin, rather than through a specific effect of CQ itself.     

Distribution of trace elements in the brain of EL (epilepsy) mice

The association of essential trace elements with epileptic seizures is poorly understood. On the basis of the evidences that the release of zinc from the brain of epilepsy (EL) mice, an animal model of genetically determined epilepsy, is enhanced by the induction of seizures and that alteration of zinc homeostasis is responsive to susceptibility to seizures, the distribution of trace elements in the brain was studied using EL mice and ddY mice, which form the genetic background for the inbred EL mice. The multitracer technique was applied to determine the distribution of trace elements. Twenty-four hours after intravenous injection of the multitracer, the concentration of 65Zn and 56Co in the brain of untreated EL mice was higher than in ddY mice, while the concentration of 65Zn and 56Co in the brain was decreased in seized EL mice. 75Se concentration in the hippocampus, cerebral cortex and cerebellum of untreated EL mice was lower than in ddY mice, while 75Se concentration in the hippocampus was increased in seized EL mice. 83Rb, an element of homologous series to potassium, concentration in the hippocampus and cerebral cortex of untreated EL mice was lower than in ddY mice, and 83Rb concentration in the cerebral cortex was decreased in seized EL mice. The movement of zinc, cobalt and selenium in the brain may be altered by enhancement of susceptibility to seizures. These results suggest that alteration of homeostasis of zinc, cobalt and selenium in the brain may be involved in the susceptibility, development or termination of seizures in EL mice.     

Serum Vitamins and Heavy Metals in Blood and Urine, and the Correlations among Them in Parkinson's Disease Patients in China

Background: Some heavy metals are suspected to be pathogenic and some vitamins protective against Parkinson's disease (PD), and the interaction between heavy metals and vitamins could be associated with the pathophysiology of PD. Methods: Subjects comprised PD patients and sex- and age-matched controls recruited from an outpatient clinic in China. Morning blood and urine samples were used to measure concentrations of metals and vitamins. Results: The serum iron, whole-blood manganese, urine iron and copper levels were significantly higher in the PD patients than in the controls. The correlation coefficient between serum and urine concentrations of iron in the PD patients was significant. The serum vitamin E/urine copper ratio was significantly lower in the PD patients than in the controls. Serum vitamin E was negatively correlated with serum copper and was positively correlated with urine copper in the PD patients. Serum vitamin B(12) was positively correlated with serum zinc in the PD patients and was negatively correlated with urine zinc in the controls. Conclusions: Excessive intake of iron and copper, accumulation of manganese, vitamin E/copper imbalance in intake, and vitamin B(12) decrease by zinc deficiency in the body might be involved in the etiology of PD.