Dichloroacetate and cerebral ischaemia therapeutics.

Brain ischaemia is a major medical problem which totally lacks meaningful therapeutic options. A drug that reduces morbidity and mortality associated with head injury and stroke would constitute a major medical breakthrough. Although many mechanistic approaches have been evaluated clinically for both stroke and head injury, none have yet to be proven successful. Dichloroacetate (DCA, Ceresine) is a small molecule that activates pyruvate dehydrogenase (PDH) and crosses the blood-brain barrier. PDH activation reduces neurotoxic lactic acidosis which always accompanies brain ischaemia. DCA shows substantial efficacy in a variety of models of stroke, pre-stroke, head or spinal cord injury. Agents that lower cerebral lactic acidosis have not yet been clinically evaluated in head injury and stroke, although DCA has been shown clinically to reduce ambient lactate concentrations in patients with such conditions. DCA has also been shown to be well-tolerated in these patients, and unlike many halogenated molecules, is not mutagenic. Since elevated brain lactate is correlated with poor outcome in both preclinical and clinical studies, an agent such as DCA may prove to reduce the brain injury associated with these disorders. Potential clinical applications of DCA include stroke, head injury, spinal cord injury, and chronic disorders such as congenital lactic acidosis (CLA) and mitochondrial lactic acidosis and stroke-like syndrome (MELAS).

Potential therapeutic applications of fructose-1,6-diphosphate.

Ischaemia-related tissue injury is the leading cause of death in developed countries. Drugs that can reduce ischaemic injury would be beneficial in treatment of myocardial infarction (MI), surgical trauma and stroke. Fructose-1,6-diphosphate (FDP) is a key intermediate in anaerobic glycolysis and is the product of the major regulatory enzyme in the pathway (phosphofructokinase). Preclinical and clinical data suggest that FDP has substantial cytoprotective effects in a variety of ischaemia-reperfusion injury scenarios. Evidence indicates that FDP has a direct effect on ATP pools, reduces ischaemia-induced tissue damage and has positive inotropic effects on heart function. The clinical data suggest that FDP may be a useful drug in a variety of ischaemic and inflammatory clinical settings where acute management of tissue injury is desired. Potential uses include: iv. administration for the reduction of ischaemic injury in sickle cell anaemia, bypass surgery, congestive heart failure, myocardial infarction, as well as organ preservation in transplants.

Regulation of adenosine concentration and cytoprotective effects of novel reversible adenosine deaminase inhibitors.

The physiological role of adenosine (Ado) is well known. Although a number of pharmacological attempts have been made to manipulate Ado concentrations in ischemic conditions in different tissues, none have been clinically accepted up to now, mostly due to insufficient elevation of Ado concentrations or unacceptable toxicity. In this study, we evaluated the biochemical and pharmacological actions of several novel erythro-9-(2-hydroxy-3-nonyl)adenine (EHNA) analogs to identify new reversible adenosine deaminase (ADA) inhibitors with potential clinical utility. In cell culture experiments, these compounds elevate cellular Ado concentrations under conditions of simulated ischemic stress but very little, if any, under normoxic conditions. Two compounds were selected for study: 9'-chloro-EHNA (CPC-405) and 9'-phthalimido-EHNA (CPC-406), which specifically inhibit ADA in cell-free preparations as well as in intact cells. CPC-405 and CPC-406 do not affect adenosine kinase activity, and they do not affect adenosine transport (influx). CPC-405 and CPC-406 are also more potent than EHNA in elevating adenosine release from human astrocytoma cells and bovine heart microvascular endothelial cells in 2-deoxyglucose-simulated ischemia or under anaerobic conditions. Inhibition of adenosine deaminase by CPC-405 or CPC-406, as well as the 2'-deoxyadenosine toxicity expressed in the presence of these ADA inhibitors, is reversed when the inhibitors are removed by washing the cells. In the isolated rat heart model of ischemia, these novel ADA inhibitors showed enhanced recovery of left ventricular end-diastolic pressure, left ventricular developed pressure, +dP/dtmax and -dP/dtmax. In the rat hippocampal slice model of hypoxia, these compounds also showed neuroprotective effects on CA1 hypoxic injury. In conclusion, these novel ADA inhibitors may represent clinically useful Ado elevating compounds that show cardioprotective, as well as neuroprotective, effects. Also, their potential for immunotoxicity, if any, appears to be transient in nature, representing an important clinical advantage compared with tight-binding ADA inhibitors such as deoxycoformycin.

Dichloroacetate attenuates neuronal damage in a gerbil model of brain ischemia.

Dichloroacetate facilitated a reduction in brain lactate following ischemia in the gerbil. This treatment also improved high-energy metabolite and pyruvate dehydrogenase enzyme recovery. The purpose of this study was to determine the effect of dichloroacetate on ischemia-induced neuronal damage in the hippocampus of the gerbil. In adult male gerbils, carotid arteries were clamped bilaterally for 5 min. After ischemia, each gerbil was graded neurologically and received an ip injection of dichloroacetate (75 or 225 mg/kg) or an equal volume (5 mL/kg) of sodium acetate (66 mg/kg). On the following morning, gerbils received a second injection, and 3 d later were anesthetized and perfused intracardially. Brains were processed, and stained sections were analyzed for neuronal damage. Gerbils treated with 225 mg/kg dichloroacetate exhibited significantly less damage than the untreated group (p = 0.05, Dunn's test). Gerbils with a normal neurologic score evidenced no neuronal damage. Abnormal neurologic scores immediately after ischemia did not correlate with degree of neuronal damage observed 4 d later. These results indicate that neuronal damage is less in gerbils treated after ischemia with an appropriate dose of dichloroacetate. The lack of any histological evidence for an adverse effect of dichloroacetate in the controls supports the safety of this drug in this protocol. Normal neurologic scores immediately after ischemia can be used to identify gerbils mimicking ischemia in this model.

(3H)dipyridamole and (3H)nitrobenzylthioinosine binding sites at the human parietal cortex and erythrocyte adenosine transporter: a comparison.

We compared the binding sites of the adenosine transport inhibitors (3H)dipyridamole (DPR) and (3H)nitrobenzylthioinosine (NBI) in human parietal cortex and erythrocytes. In comparison with guinea pig (3H)DPR marked only slightly more binding sites than (3H)NBI with a Bmax of 1080 +/- 29 and 780 +/- 7 fmol/mg protein respectively in parietal cortex and 24288 +/- 2725 and 20875 +/- 1905 fmol/mg protein respectively in erythrocytes. NBI displaced (3H)DPR binding completely from its binding sites at about KD/2 concentrations in parietal cortex as well as erythrocytes with inhibition constants comparable to its dissociation constants. Lineweaver-Burke analysis in erythrocytes indicated a competitive inhibition of (3H)DPR binding by NBI. Pharmacological characterization of (3H)DPR binding sites in human erythrocytes is consistent with their localization on adenosine transporters. These findings provide evidence that as opposed to guinea pig (3H)DPR and (3H)NBI largely label binding sites to the same adenosine transporter in human erythrocytes and parietal cortex.

Inhibition by AICA riboside of gluconeogenesis in isolated rat hepatocytes.

5-Amino-4-imidazolecarboxamide (AICA) riboside, the nucleoside corresponding to AICA ribotide (AICAR or ZMP), an intermediate of the de novo pathway of purine biosynthesis, was found to exert a dose-dependent inhibition on gluconeogenesis in isolated rat hepatocytes. Production of glucose from lactate-pyruvate mixtures was half-maximally inhibited by approximately 100 microM and completely suppressed by 500 microM AICA riboside. AICA riboside also inhibited the production of glucose from all other gluconeogenic precursors investigated, i.e., fructose, dihydroxyacetone, and L-proline. Measurements of intermediates of the glycolytic-gluconeogenic pathway showed that AICA riboside provoked elevations of triose phosphates and fructose-1,6-bisphosphate and decreases in fructose-6-phosphate and glucose-6-phosphate. The effects of AICA riboside persisted when the cells were washed 10 min after its addition but were suppressed by 5-iodotubercidin, an inhibitor of adenosine kinase. AICA riboside provoked a dose-dependent buildup of normally undetectable Z nucleotides. After 20 min of incubation with 500 microM AICA riboside, ZMP, ZTP, and ZDP reached 3, 0.3, and 0.1 mumol/g cells, respectively. Concentrations of ATP were not significantly modified by addition of up to 500 microM AICA riboside when the cells were incubated with lactate-pyruvate but decreased with fructose or dihydroxyacetone. The activity of rat liver fructose-1,6-bisphosphatase was inhibited by ZMP with an apparent Ki of 370 microM. It is concluded that AICA riboside exerts a suppressive effect on gluconeogenesis because it provokes an accumulation of ZMP, which inhibits fructose-1,6-bisphosphatase.(ABSTRACT TRUNCATED AT 250 WORDS)

Multidrug resistance in MCF-7 human breast cancer cells is associated with increased expression of nucleoside transporters and altered uptake of adenosine.

The rate of adenosine uptake and the corresponding expression of nucleoside transporters were studied in several MCF-7 human breast-cancer cell lines that express different levels of multidrug resistance (MDR). Kinetic studies of adenosine transport in these cell lines revealed that the mean apparent Km and Vmax values for the nucleoside transporters increased with increasing MDR. The apparent Km and the apparent Vmax of Adriamycin-resistant (ADR10) cell lines were respectively 3.2- and 1.8- fold those of Adriamycin-sensitive wild-type (WT) cells (P less than 0.001). A partially revertant cell line (ADR10rev) that was derived from the ADR10 line and was partially sensitive to Adriamycin exhibited apparent Km and Vmax parameters that lay between those of the ADR10 and WT cells (P less than 0.001 vs ADR10 cells; P less than 0.05 vs WT cells). ADR10 cell membranes bound greater than 4 times more of the nucleoside transporter blockers [3H]-nitrobenzylthioinosine [( 3H]-NBI) and [3H]-dipyridamole [( 3H]-DPR) than did WT cell membranes per unit protein (P less than 0.0001). Scatchard analysis revealed a 2-3 times greater density for nucleoside transporters in ADR10 membranes as compared with those in WT membranes. ADR10rev membranes bound less [3H]-NBI and [3H]-DPR than did ADR10 membranes (P less than 0.001), but they bound more of the blockers than did WT membranes (P less than 0.05). A 2.5-h exposure to 200 nM phorbol-12,13-dibutyrate (PDBu), which activates protein kinase C (PKC) and induces WT cells to exhibit a 4-fold increased transient MDR phenotype, increased the apparent Km of WT cells for adenosine transport by greater than 2 times (P less than 0.001) to a value close to that found for the ADR10 cells. An identical exposure of ADR10 cells to PDBu produced no significant effect. The apparent Km of ADR10rev cells was increased 1.4 times by a 2.5-h PDBu exposure. None of the cell lines were affected by a 2.5-h exposure to 200 nM phorbol-13,10-diacetate (PDA), a much less active phorbol, or vehicle. These results suggest that MDR in MCF-7 cells is associated with changes in nucleoside transport, including both the number of transporters and their rate of transport, and that such changes can be partially mimicked by stimulation of PKC.

Adenosinergic modulation of homocysteine-induced seizures in mice.

Homocysteine thiolactone (HTL) elicits seizures in mice at a dose of 850 mg/kg (95-100% of animals) with an average latency time of 19.5 min. These seizures are reversed by both 5' N-ethylcarboximide adenosine (NECA) and flunitrazepam, with respective ED50 doses of 0.025 and 0.20 mg/kg. NECA was approximately four-fold more potent as an inhibitor of HTL-induced seizures than of seizures induced by pentylenetetrazol (PTZ, 75 mg/kg). Flunitrazepam was equipotent in both seizure paradigms. The purine precursor 5-amino-4-imidazole carboxamide riboside, (AICAr), although virtually ineffective against PTZ-induced seizures at doses greater than 1 g/kg, was able to inhibit HTL-induced seizures with an ED50 of approximately 350 mg/kg. The anticonvulsant effect of AICAr was dose and time dependent. The anticonvulsant potency of AICAr was increased by simultaneous administration of the adenosine uptake blocker Mioflazine, whereas the central nervous system (CNS)-impermeable adenosine uptake blocker dipyridamole had no effect. The ability of AICAr to permeate the blood-brain barrier (BBB) is limited (less than 1%) and may explain its low potency as an anticonvulsant. AICAr also has very low potency at brain adenosine A1 and A2 receptors as well as adenosine uptake sites (IC50 greater than 10(-3) M), suggesting that its anticonvulsant properties are not mediated by direct action at these sites. The results indicate that AICAr does have frank anticonvulsant effects and further suggest that HTL-induced seizures may represent a useful paradigm for evaluation of adenosinergic agents. AICAr or more potent derivatives thereof may represent a new class of anticonvulsants with the ability to target seizure foci selectively.

Adenosinergic approaches to stroke therapeutics.

Basic neuroscience research findings during the past five years have established a clear relationship between the excitatory amino acid (EAA) neurotransmitters (glutamic and aspartic acid) and various pathological states. A major mechanism of neural tissue degeneration following cerebral ischemia, and perhaps other neurodegenerative diseases, seems to involve overactivity of the EAA system in brain. This process is called delayed excitotoxicity and it has become a focal point for the design of new drugs that inhibit its course (EAA receptor blockers). Very recently it has been shown that it is possible to block delayed excitotoxicity using adenosine A1 receptor agonists which inhibit EAA release pre-synaptically. This approach is very effective in reducing post-stroke neurological damage in a number of animal models and has certain advantages when compared to the EAA receptor blocker strategy. Adenosine agonists not only inhibit excitotoxicity but they also block granulocyte activation and the capillary no-reflow phenomenon which results. An additional adenosinergic approach involves brain permeable adenosine uptake blockers which would serve to increase adenosine levels somewhat selectively at ischemic foci thereby inhibiting EAA release. The adenosinergic approach to stroke therapeutics may be a potentially effective strategy for new drug development in neurology, and may have general applicability to other neurodegenerative disease states where excitotoxicity is being implicated.

Late ontogenetic development of adenosine A1 receptor coupling to associated G-proteins in guinea pig cerebellum but not forebrain.

The ontogenetic profile of [3H]forskolin and [3H]cyclohexyladenosine [( 3H]CHA) binding sites in guinea pig forebrain and cerebellum was investigated. G-protein interactions of these binding sites were also examined by analyzing 5'-guanylylimidodiphosphate (Gpp(NH)p) interactions with [3H]CHA and [3H]forskolin binding. In forebrain, similar binding characteristics of [3H]CHA and [3H]forskolin binding are observed between the developmental stages E36 (the earliest time point studied) through to adult (P28, the latest time point studied), although transient increased binding of both ligands is observed just prior to birth. Scatchard analysis of binding isotherms reveal that this transient rise just prior to birth is due to an increase in the number of binding sites (Bmax) with little or no change in receptor affinity (Kd). In contrast, in cerebellum both [3H]CHA and [3H]forskolin binding remains at a relatively low level until just prior to birth when a dramatic increase of binding of both ligands is observed which continues to increase up to P28. Scatchard analysis of binding isotherms reveal that such changes in binding of both ligands are largely due to increases in Bmax and not Kd, although Scatchard analysis of [3H]CHA binding to cerebellar E51 membranes reveals an absence of higher affinity [3H]CHA binding sites. Gpp(NH)p did not affect [3H]forskolin binding. Gpp (NH)p displacement profiles of [3H]CHA binding reveal a maximum (adult) inhibition of [3H]CHA binding (approximately 80% displacement) at all time points (E36 through P28) in forebrain membranes, but not in cerebellar membranes. In cerebellum, displacement of [3H]CHA binding by Gpp(NH)p is much greater after birth than before birth.(ABSTRACT TRUNCATED AT 250 WORDS)

Cerebral ischemia in gerbils: postischemic administration of cyclohexyl adenosine and 8-sulfophenyl-theophylline.

Adenosine agonists have now been shown by several laboratories to have profound neuroprotective effects when administered either pre- or postischemia. In an effort to determine whether these effects are centrally mediated, the effects of the non-brain-permeable adenosine receptor antagonist 8-sulfophenyl-theophylline (8-SPTH) on cyclohexyladenosine (CHA) -mediated protection was determined. Both survival and neurologic outcome were assessed in gerbils following 30 minutes of bilateral carotid occlusion. A dose of 2 mg/kg of CHA 5 minutes postreperfusion resulted in highly significant increases in survival relative to saline injected controls. Administration of doses of 8-SPTH sufficient to normalize the hypotension observed with CHA resulted in the same degree of postischemic protection. Similar results were obtained when neurologic status was evaluated. The results indicate that the neuroprotective effects of CHA are apparently centrally mediated.

Adenosinergic modulation of caffeine-induced c-fos mRNA expression in mouse brain.

The adenosine receptor antagonist, caffeine, transiently induced proto-oncogene c-fos mRNA in mouse brain in a dose-dependent fashion. In situ hybridization revealed that caffeine-induced c-fos expression was high in caudate-putamen and olfactory tubercle at both subconvulsive and convulsive doses. The pattern of c-fos mRNA distribution following caffeine administration differs from that reported after seizures induced by electroconvulsive shock (ECS) or other chemical convulsants, and closely parallels the distribution of adenosine A2 receptors. Furthermore, the potent adenosine A2 receptor agonist, 5'-N-ethylcarboxamide adenosine (NECA) blocked caffeine-induced c-fos expression whereas the adenosine A1 receptor ligand, N6-cyclohexyladenosine (CHA), had no effect. This study suggests that the caffeine-induced expression of c-fos mRNA may be mediated by the adenosine A2 receptor in mouse brain.

C-fos mRNA expression following electrical-induced seizure and acute nociceptive stress in mouse brain.

A single electroconvulsive shock (ECS) induced a rapid and transient expression of c-fos mRNA in mouse brain. In earclipped sham controls, low but significant expression of c-fos mRNA was also observed. These data suggest that c-fos mRNA may be transiently induced by seizure activity as well as much more subtle and qualitative different stimuli, such as the acute nociceptive stress associated with earclipping.

Protective effect of cyclohexyladenosine on adenosine A1-receptors, guanine nucleotide and forskolin binding sites following transient brain ischemia: a quantitative autoradiographic study.

The effects of an i.p. administration of cyclohexyladenosine (CHA) have been examined upon ischemic brain damage in gerbils. Ischemia was induced for 20 min by occlusion of both carotid arteries, and CHA was administered 5 min after recirculation at a dose of 2 mg/kg. Animals were sacrificed either 1, 3 or 6 days after ischemia and their brains were used for examination of cell morphology and quantitative autoradiography. In animals subject to ischemia, the deterioration of the laminar organization of the hippocampus was associated with a significant decrease in adenosine A1-receptors (labeled with [3H]CHA), G-protein (labeled with [3H]forskolin). The treatment with CHA considerably improved the morphological preservation of cells in the CA1 region of the hippocampus and prevented the reduction in the specific binding of all radioligands. Adenosine, its analogues and other substances modulating adenosine receptors may thus provide new therapeutic approaches to the treatment of ischemia-induced brain injury.

Differential effects of acute and repeated electrically and chemically induced seizures on [3H]Nimodipine and [125I]omega-conotoxin GVIA binding in rat brain.

[3H]Nimodipine and high-affinity [125I]omega-conotoxin GVIA (CgTX) binding were investigated in membranes from rat cerebral cortex, cerebellum, and hippocampus after electrically and chemically induced seizures. Animals were decapitated 30 min after a single electroconvulsive shock (ECS) or lidocaine-induced seizure and 24 h after the last of 10 once-daily ECS or six once-daily lidocaine-induced seizures. After a single ECS, [3H]nimodipine and [125I]CgTX binding sites decreased in cerebral cortex (by 10% and 17%, respectively). A downregulation of [3H]nimodipine binding sites in hippocampus occurred after single and repeated lidocaine-induced seizures (by 24% and 11%, respectively), whereas [125I]CgTX binding remained unaltered. An earlier report on changes in [3H]nitrendipine binding after chronic ECS in cortex and hippocampus was not confirmed.

Mouse brain c-fos mRNA distribution following a single electroconvulsive shock.

The regional distribution of c-fos mRNA in the mouse brain has been investigated by in situ hybridization autoradiography after seizures induced by an acute electroconvulsive shock (ECS). ECS led to a widespread induction of the proto-oncogene c-fos in the brain, with highest concentrations in discrete areas within the limbic system and also in the hypothalamus and cerebellum. The mild stress of sham treatment in earclipped animals induced a weaker and qualitatively different pattern of c-fos mRNA expression involving the cortex, hippocampus, and cerebellum. These data suggest the usefulness of c-fos in situ hybridization as a marker of neuronal stimulation and in mapping a range of effects from a mild stress to the robust changes of an electroconvulsive seizure.

Electroconvulsive shock (ECS) and the adenosine neuromodulatory system: effect of single and repeated ECS on the adenosine A1 and A2 receptors, adenylate cyclase, and the adenosine uptake site.

The effect of a single electroconvulsive shock (ECS) (30 min and 24 h after treatment) and repeated ECS (10 once-daily) on the adenosine neuromodulatory system was investigated in rat cerebral cortex, cerebellum, hippocampus, and striatum. The present study examined the adenosine A1 receptor using N6-[3H]cyclohexyladenosine ([3H]CHA), the A2 receptor using 5'-N-[3H]ethylcarboxyamidoadenosine ([ 3H]NECA), adenylate cyclase using [3H]forskolin, and the adenosine uptake site using [3H]nitrobenzylthioinosine ([3H]NBI). At 30 min after a single ECS, the Bmax of the [3H]NBI binding in striatum was increased by 20%, which is in good agreement with the well-known postictal adenosine release. The Bmax of [3H]forskolin binding in striatum and cerebellum was increased by 60 and 20%, respectively. In contrast to earlier reported changes following chemically induced seizures, [3H]CHA binding was not altered postictally. At 24 h after a single ECS, there were no changes for any ligand in any brain region. Following repeated ECS, there was a 20% increase of [3H]CHA binding sites in cerebral cortex, which lasted for at least 14 days after the last ECS. [3H]Forskolin binding in hippocampus and striatum was 20% lowered 24 h after 10 once-daily ECS but had already returned to control levels 48 h after the last treatment. Evidence is provided that the upregulated adenosine A1 receptors are coupled to guanine nucleotide binding proteins and, furthermore, that this upregulation is not paralleled by an increase in adenylate cyclase activity as labeled by [3H]forskolin.

Pyruvate dehydrogenase interactions with peripheral-type benzodiazepine receptors.

Although peripheral-type benzodiazepine recognition sites have been demonstrated in the brain of various species, the precise identity and function of the peripheral benzodiazepine receptor have not been established yet. In light of the recent demonstration of the mitochondrial localization of this receptor and its potential role in intermediary metabolism, we investigated the relationship between the benzodiazepines and the enzyme pyruvate dehydrogenase (PDH), a component of the mitochondrial membrane. The results obtained in the present study demonstrate a specific interaction between PDH and the ligands for the peripheral-type type benzodiazepine receptor, which might account for their effects on cell growth and differentiation.