Brain regional pharmacokinetics of 11C-labeled diphenylhydantoin: positron emission tomography in humans

Radiosynthesis of 11C-phenytoin Using a DEGDEE Solvent for Clinical PET Studies

Objective(s):

Phenytoin is an antiepileptic drug that is used worldwide. The whole-body pharmacokinetics of this drug have been extensively studied using ¹¹C-phenytoin in small animals. However, because of the limited production amounts that are presently available, clinical ¹¹C-phenytoin PET studies to examine the pharmacokinetics of phenytoin in humans have not yet been performed. We aimed to establish a new synthesis method to produce large amounts of ¹¹C-phenytoin to conduct human studies.

Methods:

[¹¹C] methane was produced using an in-house cyclotron by the ¹⁴N (p, α) ¹¹C nuclear reaction of 5 % of hydrogen containing 95 % of nitrogen gas. About 30 GBq of ¹¹C-methane was then transferred to a homogenization cell containing Fe₂O₃ powder mixed with Fe granules heated at 320 ⁰C to yield ¹¹C-phosgene. Xylene, 1,4-dioxane, and diethylene glycol diethyl ether (DEGDEE) were investigated as possible reaction solvents.

Results:

The ratio of ¹¹C-phenytoin radioactivity to the total ¹¹C radioactivity in the reaction vessel (reaction efficiency) was 7.5% for xylene, 11% for 1,4-dioxane, and 37% for DEGDEE. The synthesis time was within 45 min from the end of bombardment until obtaining the final product. The radioactivity produced was more than 4.1 GBq in 10 mL of saline at the end of synthesis. The specific activity of the product ranged from 1.7 to 2.2 GBq/μmol. The quality of the [¹¹C] phenytoin injection passed all criteria required for clinical use.

Conclusion:

The use of DEGDEE as a solvent enabled the production of a large amount of ¹¹C-phenytoin sufficient to enable PET studies examining the human pharmacokinetics of phenytoin.

Potential Therapeutic Applications of Adenosine A2A Receptor Ligands and Opportunities for A2A Receptor Imaging

Adenosine A_2A receptors (A_2A Rs) are highly expressed in the human striatum, and at lower densities in the cerebral cortex, the hippocampus, and cells of the immune system. Antagonists of these receptors are potentially useful for the treatment of motor fluctuations, epilepsy, postischemic brain damage, or cognitive impairment, and for the control of an immune checkpoint during immunotherapy of cancer. A_2A R agonists may suppress transplant rejection and graft-versus-host disease; be used to treat inflammatory disorders such as asthma, inflammatory bowel disease, and rheumatoid arthritis; be locally applied to promote wound healing and be employed in a strategy for transient opening of the blood-brain barrier (BBB) so that therapeutic drugs and monoclonal antibodies can enter the brain. Increasing A_2A R signaling in adipose tissue is also a potential strategy to combat obesity. Several radioligands for positron emission tomography (PET) imaging of A_2A Rs have been developed in recent years. This review article presents a critical overview of the potential therapeutic applications of A_2A R ligands, the use of A_2A R imaging in drug development, and opportunities and limitations of PET imaging in future research.

(11)C[double bond, length as m-dash]O bonds made easily for positron emission tomography radiopharmaceuticals

The positron-emitting radionuclide carbon-11 ((11)C, t1/2 = 20.3 min) possesses the unique potential for radiolabeling of any biological, naturally occurring, or synthetic organic molecule for in vivo positron emission tomography (PET) imaging. Carbon-11 is most often incorporated into small molecules by methylation of alcohol, thiol, amine or carboxylic acid precursors using [(11)C]methyl iodide or [(11)C]methyl triflate (generated from [(11)C]carbon dioxide or [(11)C]methane). Consequently, small molecules that lack an easily substituted (11)C-methyl group are often considered to have non-obvious strategies for radiolabeling and require a more customized approach. [(11)C]Carbon dioxide itself, [(11)C]carbon monoxide, [(11)C]cyanide, and [(11)C]phosgene represent alternative reactants to enable (11)C-carbonylation. Methodologies developed for preparation of (11)C-carbonyl groups have had a tremendous impact on the development of novel PET tracers and provided key tools for clinical research. (11)C-Carbonyl radiopharmaceuticals based on labeled carboxylic acids, amides, carbamates and ureas now account for a substantial number of important imaging agents that have seen translation to higher species and clinical research of previously inaccessible targets, which is a testament to the creativity, utility and practicality of the underlying radiochemistry.

P-glycoprotein expression and function in patients with temporal lobe epilepsy: a case-control study

BACKGROUND

Studies in rodent models of epilepsy suggest that multidrug efflux transporters at the blood-brain barrier, such as P-glycoprotein, might contribute to pharmacoresistance by reducing target-site concentrations of antiepileptic drugs. We assessed P-glycoprotein activity in vivo in patients with temporal lobe epilepsy.

METHODS

We selected 16 patients with pharmacoresistant temporal lobe epilepsy who had seizures despite treatment with at least two antiepileptic drugs, eight patients who had been seizure-free on antiepileptic drugs for at least a year after 3 or more years of active temporal lobe epilepsy, and 17 healthy controls. All participants had a baseline PET scan with the P-glycoprotein substrate (R)-[(11)C]verapamil. Pharmacoresistant patients and healthy controls then received a 30-min infusion of the P-glycoprotein-inhibitor tariquidar followed by another (R)-[(11)C]verapamil PET scan 60 min later. Seizure-free patients had a second scan on the same day, but without tariquidar infusion. Voxel-by-voxel, we calculated the (R)-[(11)C]verapamil plasma-to-brain transport rate constant, K1 (mL/min/cm(3)). Low baseline K1 and attenuated K1 increases after tariquidar correspond to high P-glycoprotein activity.

FINDINGS

Between October, 2008, and November, 2011, we completed (R)-[(11)C]verapamil PET studies in 14 pharmacoresistant patients, eight seizure-free patients, and 13 healthy controls. Voxel-based analysis revealed that pharmacoresistant patients had lower baseline K1, corresponding to higher baseline P-glycoprotein activity, than seizure-free patients in ipsilateral amygdala (0·031 vs 0·036 mL/min/cm(3); p=0·014), bilateral parahippocampus (0·032 vs 0·037; p<0·0001), fusiform gyrus (0·036 vs 0·041; p<0·0001), inferior temporal gyrus (0·035 vs 0·041; p<0·0001), and middle temporal gyrus (0·038 vs 0·044; p<0·0001). Higher P-glycoprotein activity was associated with higher seizure frequency in whole-brain grey matter (p=0·016) and the hippocampus (p=0·029). In healthy controls, we noted a 56·8% increase of whole-brain K1 after 2 mg/kg tariquidar, and 57·9% for 3 mg/kg; in patients with pharmacoresistant temporal lobe epilepsy, whole-brain K1 increased by only 21·9% for 2 mg/kg and 42·6% after 3 mg/kg. This difference in tariquidar response was most pronounced in the sclerotic hippocampus (mean 24·5% increase in patients vs mean 65% increase in healthy controls, p<0·0001).

INTERPRETATION

Our results support the hypothesis that there is an association between P-glycoprotein overactivity in some regions of the brain and pharmacoresistance in temporal lobe epilepsy. If this relation is confirmed, and P-glycoprotein can be identified as a contributor to pharmacoresistance, overcoming P-glycoprotein overactivity could be investigated as a potential treatment strategy.

FUNDING

EU-FP7 programme (EURIPIDES number 201380).

Modeling of PET data in CNS drug discovery and development

Positron emission tomography (PET) is increasingly used in drug discovery and development for evaluation of CNS drug disposition and for studies of disease biomarkers to monitor drug effects on brain pathology. The quantitative analysis of PET data is based on kinetic modeling of radioactivity concentrations in plasma and brain tissue compartments. A number of quantitative methods of analysis have been developed that allow the determination of parameters describing drug pharmacokinetics and interaction with target binding sites in the brain. The optimal method of quantification depends on the properties of the radiolabeled drug or radioligand and the binding site studied. We here review the most frequently used methods for quantification of PET data in relation to CNS drug discovery and development. The utility of PET kinetic modeling in the development of novel CNS drugs is illustrated by examples from studies of the brain kinetic properties of radiolabeled drug molecules.

PET and SPECT radiotracers to assess function and expression of ABC transporters in vivo

Adenosine triphosphate-binding cassette (ABC) transporters, such as P-glycoprotein (Pgp, ABCB1), breast cancer resistance protein (BCRP, ABCG2) and multidrug resistance-associated proteins (MRPs) are expressed in high concentrations at various physiological barriers (e.g. blood-brain barrier, blood-testis barrier, blood-tumor barrier), where they impede the tissue accumulation of various drugs by active efflux transport. Changes in ABC transporter expression and function are thought to be implicated in various diseases, such as cancer, epilepsy, Alzheimer's and Parkinson's disease. The availability of a non-invasive imaging method which allows for measuring ABC transporter function or expression in vivo would be of great clinical use in that it could facilitate the identification of those patients that would benefit from treatment with ABC transporter modulating drugs. To date three different kinds of imaging probes have been described to measure ABC transporters in vivo: i) radiolabelled transporter substrates ii) radiolabelled transporter inhibitors and iii) radiolabelled prodrugs which are enzymatically converted into transporter substrates in the organ of interest (e.g. brain). The design of new imaging probes to visualize efflux transporters is inter alia complicated by the overlapping substrate recognition pattern of different ABC transporter types. The present article will describe currently available ABC transporter radiotracers for positron emission tomography (PET) and single-photon emission computed tomography (SPECT) and critically discuss strengths and limitations of individual probes and their potential clinical applications.

The antiepileptic drug mephobarbital is not transported by P-glycoprotein or multidrug resistance protein 1 at the blood-brain barrier: a positron emission tomography study

Aim of this study was to determine whether the carbon-11-labeled antiepileptic drug [(11)C]mephobarbital is a substrate of P-glycoprotein (Pgp) and can be used to assess Pgp function at the blood-brain barrier (BBB) with positron emission tomography (PET). We performed paired PET scans in rats, wild-type (FVB) and Mdr1a/b((-/-)) mice, before and after intravenous administration of the Pgp inhibitor tariquidar (15mg/kg). Brain-to-blood AUC(0-60) ratios in rats and brain AUC(0-60) values of [(11)C]mephobarbital in wild-type and Mdr1a/b((-/-)) mice were similar in scans 1 and 2, respectively, suggesting that in vivo brain distribution of [(11)C]mephobarbital is not influenced by Pgp efflux. Absence of Pgp transport was confirmed in vitro by performing concentration equilibrium transport assay in cell lines transfected with MDR1 or Mdr1a. PET experiments in wild-type mice, with and without pretreatment with the multidrug resistance protein (MRP) inhibitor MK571 (20mg/kg), and in Mrp1((-/-)) mice suggested that [(11)C]mephobarbital is also not transported by MRPs at the murine BBB, which was also supported by in vitro transport experiments using human MRP1-transfected cells. Our results are surprising, as phenobarbital, the N-desmethyl derivative of mephobarbital, has been shown to be a substrate of Pgp, which suggests that N-methylation abolishes Pgp affinity of barbiturates.

Imaging of P-glycoprotein function and expression to elucidate mechanisms of pharmacoresistance in epilepsy

The issue of pharmacoresistance in epilepsy has received considerable attention in recent years, and a number of plausible hypotheses have been proposed. Of these, the so-called transporter hypothesis is the most extensively researched and documented. This hypothesis assumes that refractory epilepsy is associated with a localised over-expression of drug transporter proteins such as P-glycoprotein (Pgp) in the region of the epileptic focus, which actively extrudes antiepileptic drugs (AEDs) from their intended site of action. However, although this hypothesis has biological plausibility, there is no clinical evidence to support the assertion that AEDs are sufficiently strong substrates for transporter-mediated extrusion from the brain. The use of modern brain imaging techniques to determine Pgp function in patients with refractory epilepsy has started only recently, and may ultimately determine whether increased expression and function of Pgp or other efflux transporters are involved in AED resistance.

Several major antiepileptic drugs are substrates for human P-glycoprotein

One of the current hypotheses of pharmacoresistant epilepsy proposes that transport of antiepileptic drugs (AEDs) by drug efflux transporters such as P-glycoprotein (Pgp) at the blood-brain barrier may play a significant role in pharmacoresistance in epilepsy by extruding AEDs from their intended site of action. However, several recent in vitro studies using cell lines that overexpress efflux transporters indicate that human Pgp may not transport AEDs to any relevant extent. In this respect it has to be considered that most AEDs are highly permeable, so that conventional bi-directional transport assays as used in these previous studies may fail to identify AEDs as Pgp substrates, particularly if these drugs are not high-affinity substrates for Pgp. In the present study, we used a modified transport assay that allows evaluating active transport independently of the passive permeability component. In this concentration equilibrium transport assay (CETA), the drug is initially added at identical concentration to both sides of a polarized, Pgp-overexpressing cell monolayer instead of applying the drug to either the apical or basolateral side for studying bi-directional transport. Direct comparison of the conventional bi-directional (concentration gradient) assay with the CETA, using MDR1-transfected LLC cells, demonstrated that CETA, but not the conventional assay, identified phenytoin and phenobarbital as substrates of human Pgp. Furthermore, directional transport was determined for lamotrigine and levetiracetam, but not carbamazepine. Transport of AEDs could be completely or partially (>50%) inhibited by the selective Pgp inhibitor, tariquidar. However, transport of phenobarbital and levetiracetam was also inhibited by MK571, which preferentially blocks transport by multidrug resistance transporters (MRPs), indicating that, in addition to Pgp, these AEDs are substrates of MRPs. The present study provides the first direct evidence that several AEDS are substrates of human Pgp, thus further substantiating the transporter hypothesis of pharmacoresistant epilepsy.

Drug resistance in brain diseases and the role of drug efflux transporters

Resistance to drug treatment is an important hurdle in the therapy of many brain disorders, including brain cancer, epilepsy, schizophrenia, depression and infection of the brain with HIV. Consequently, there is a pressing need to develop new and more effective treatment strategies. Mechanisms of resistance that operate in cancer and infectious diseases might also be relevant in drug-resistant brain disorders. In particular, drug efflux transporters that are expressed at the blood-brain barrier limit the ability of many drugs to access the brain. There is increasing evidence that drug efflux transporters have an important role in drug-resistant brain disorders, and this information should allow more efficacious treatment strategies to be developed.

A comparison of central brain (cerebrospinal and extracellular fluids) and peripheral blood kinetics of phenytoin after intravenous phenytoin and fosphenytoin

Phenytoin (PHT) is a first-line drug in the treatment of status epilepticus. However, the parenteral PHT formulation is associated with administration difficulties and therefore fosphenytoin (FosPHT), a PHT pro-drug, has been developed. As the peripheral (blood) and central (cerebrospinal fluid [CSF] and brain extracellular fluid [ECF]) kinetic inter-relationship of PHT after i.v. FosPHT administration is unknown we sought to ascertain the relationship and to compare it to that of i.v. PHT. A freely behaving rat model, which allows for the concurrent and temporal sampling of blood (jugular vein), CSF (cisterna magna) and brain ECF (frontal cortex and hippocampus), was used. PHT and FosPHT were administered by i.v. infusion and blood, CSF and microdialysate samples collected at timed intervals up to 6 hours. The pharmacokinetic parameters in plasma of PHT after PHT and FosPHT (30 and 60 mg/kg) administration were indistinguishable. The PHT plasma free fraction (free/total concentration ratio) was 0.25-0.31 and 0.26-0.31 for PHT and FosPHT, respectively. Mean PHT Tmax values for CSF were 9-13 minutes. The equivalent values in the frontal cortex and hippocampal ECF were 29-34 minutes. Cmax values increased dose-dependently and were independent of whether PHT or FosPHT was administered. Furthermore the kinetic profiles of PHT for the frontal cortex and hippocampus were indistinguishable suggesting that PHT distribution in the brain is not brain region specific. Thus, overall, the central and peripheral kinetics of PHT are indistinguishable after PHT and FosPHT.

Functional neuroimaging with positron emission tomography

Epilepsy research using positron emission tomography (PET) has provided considerable new information about ictal and interictal dysfunctions in human epilepsy. Neuroreceptor mapping with PET ligands has revealed altered central benzodiazepine receptor and opiate receptor densities in partial epilepsies interictally, and regional increases in endogenous opioid peptide concentrations during absence seizures. Imaging of perfusion and glucose metabolism during cognitive processing has shown interictal abnormalities of regional activation in partial and generalized epilepsies. The diagnostically robust patterns of interictal glucose hypometabolism are not adequately explained by macrostructural and microstructural alterations in temporal lobe epilepsy. Current investigations of the pathophysiology of interictal hypometabolism must address ultrastructural and neurochemical factors. Clinical PET in presurgical evaluation of medically refractory epilepsies remains an active area of research, but remarkably little antiepileptic drug research has exploited PET techniques.

Microdialysis study of the neuropharmacokinetics of phenytoin in rat hippocampus and frontal cortex

Acute administration of phenytoin (PHT) is used in the treatment of status epilepticus, yet little is known about the neuropharmacokinetics of PHT in brain extracellular fluid (ECF), the pharmacodynamically relevant compartment. To characterize the neuropharmacokinetics of brain ECF PHT we implanted microdialysis probes in rat hippocampus and frontal cortex and placed a catheter in the internal jugular vein. PHT (50 or 100 mg/kg intraperitoneally, i.p.) was then administered, and microdialysate and serum samples were collected. PHT was rapidly absorbed, with a time to maximum concentration (Tmax) of approximately 20 min for serum concentrations. PHT rapidly entered the brain ECF compartment, with Tmax values similar to those of serum. In brain ECF, PHT concentrations then plateaued for 40-60 min despite decreasing serum concentrations. The area under the brain ECF concentration-time curve (AUC) was higher in hippocampus than frontal cortex. The possible mechanisms for these observations include entry of PHT into specific brain areas both across capillaries and through the cerebrospinal fluid (CSF), extensive binding of PHT in brain white matter, and differing blood flow in different brain regions.

Antiepileptic drug pharmacokinetics and neuropharmacokinetics in individual rats by repetitive withdrawal of blood and cerebrospinal fluid: phenytoin

The temporal pharmacokinetic (blood) and neuropharmacokinetic (cerebrospinal fluid, CSF) interrelationship of phenytoin was studied after acute and during chronic (up to 5 days) intraperitoneal administration of phenytoin (30, 50 or 100 mg/kg) using a new freely behaving rat model. After administration, phenytoin rapidly appeared in both serum (Tmax mean range 0.15-0.38 h) and CSF (Tmax mean range 0.9-1.4 h), suggesting ready penetration of the blood-brain barrier. However, transport across the blood-brain barrier may be rate limiting since whilst phenytoin concentrations rose dose dependently in serum, CSF concentrations did not. Further, the divergence between the blood and CSF compartments increased with chronic dosing. Cmax, AUC and t1/2 values for serum increased non-linearly, suggestive of accumulation kinetics. Based on these data, high initial phenytoin blood concentrations are essential if phenytoin entry into the brain is to be facilitated, and this may be important in studies of phenytoin in animal models of status epilepticus.

Drug distribution in dog brain studied by positron emission tomography

We used positron emission tomography to monitor the distribution of radioactivity in dog brain and muscle following i.v. administration of 11C-labelled antipyrine, imipramine, and quinidine. Twenty-five sequential scans of a transaxial slice of the head were performed within 90 min; radioactivity in plasma was measured in a gamma-counter. Following i.v. injection of [11C]antipyrine (50 mg kg-1; 9-68 mCi; n = 10), the decay of plasma activity was accompanied by rapid uptake in brain and variable uptake in muscle, immediately followed by a redistribution leading to equalization of the radioactivity in the tissues. Administration of [11C]imipramine (4 mg kg-1; 30-110 mCi; n = 8) was followed by a rapid build-up of a sustained gradient between high brain, and low plasma and muscle radioactivity. After i.v. injection of [11C]quinidine (1 mg kg-1; 11-87 mCi; n = 10), radioactivity in brain was low, with higher activity in plasma and muscle throughout the experiment. Positron emission tomography thus revealed for each drug a distinct pattern of distribution consistent with established properties of the compounds. This technique seems promising for the study of early drug distribution, notwithstanding certain limitations.

Distribution of diphenylhydantoin in the brain during experimental status epilepticus of the cat

The distribution of diphenylhydantoin (PHT) (40 mg/kg i.p.) in the brain was investigated in cats with convulsive generalized (group 1) and focal penicillin-induced status epilepticus (group 2), and in controls. A significant increase in the amount of PHT entering the brain during the convulsive status was found, with peak brain levels at 45 min (12 +/- 3.2 micrograms/g vs. 6.0 +/- 0.8 in normal cats, P less than 0.05). In the focal status brain concentrations of PHT reached levels intermediate between controls and group 1 cats. At 15 min, elevated blood levels of the drug were paralleled by increased concentrations in the brain, whereas at 30 and 45 min other factors, such as changes in cerebral blood flow, cerebral pH, vascular resistance, metabolic derangement and blood-brain barrier disruption were presumably responsible for the altered brain PHT uptake. The relevance of these data to clinical practice is discussed, in relation to the treatment of human status epilepticus and the potentially neurotoxic effects of the drug.

Positron emission tomography: human brain function and biochemistry

Positron emission tomography (PET) is an analytical imaging technique that provides a way of making in vivo measurements of the anatomical distribution and rates of specific biochemical reactions. This ability of PET to measure and image dynamic biochemistry builds a bridge between the basic and clinical neurosciences founded on the commonality of the types of measurements made. Clinical findings with PET in humans are suggesting hypotheses that can be tested rigorously in the basic science laboratory.

Kinetics and displacement of [11C]RO 15-1788, a benzodiazepine antagonist, studied in human brain in vivo by positron tomography

The brain regional distribution and kinetics of RO 15-1788, a benzodiazepine (BZD) antagonist labeled with 11C was studied by time-of-flight positron tomography after intravenous injection in four normal human volunteers. In two control studies, there was a high uptake of [11C]RO 15-1788 in gray matter structures initially (brain/blood ratio approximately 3), and subsequent retention that was highest in cerebral cortex, a structure known to have a high density of BZD receptors in vitro. Variation in tissue kinetics of [11C]RO among different gray matter structures may, however, suggest regional differences in binding characteristics or environment of BZD receptors. In two displacement studies, unlabeled RO 15-1788 was injected ten minutes after the radioligand: there was an immediate and marked washout of [11C]brain radioactivity that reached 70% in the occipital cortex with a 0.05 mg/kg dose (indicating a high specific to non-specific binding ratio) but was less prominent with a 0.01 mg/kg dose. These data suggest that [11C]RO 15-1788 may be useful for in vivo mapping of human brain BZD receptors using positron tomography.

The use and impact of positron computed tomography scanning in epilepsy

Through the effective combination of instrumentation, tracer kinetic principles, and radiopharmaceuticals, positron computed tomography (PET) allows for the analytic, noninvasive measurement of local tissue physiology in humans. A large number of studies have already been performed in patients with epilepsy using 18F-fluorodeoxyglucose (FDG) to measure local cerebral glucose utilization. In patients with complex partial epilepsy who are candidates for surgery, hypometabolic zones have been seen consistently (70%) in the interictal state. These areas of hypometabolism have been related to electroencephalographic findings, surgical pathology, and clinical symptomatology. The complex anatomical and pathophysiological investigation of these hypometabolic zones is discussed. Ictal studies of patients with partial seizures have demonstrated a much more variable metabolic pattern which usually consists of hypermetabolism relative to baseline or interictal studies. Generalized epilepsy produced by electroconvulsive shock and petit mal epilepsy have been studied using FDG to estimate glucose metabolism. These studies demonstrated hypermetabolism in the ictal state, relative to interictal or postictal scans, but with a more generalized pattern than ictal studies of partial seizures. Methodological problems in the study of epilepsy with PET are discussed in detail. The investigation of interictal hypometabolism through animal models of epilepsy and quantitative autoradiography is described as a means to understand the human PET results. The impact and future direction of PET studies in epileptic populations will probably employ the use of behavioral, pharmacological, and electrophysiological maneuvers to provide more specific details about the fundamental pathophysiological mechanisms of specific aspects of epilepsy. These techniques may allow for a truly pathophysiological classification system for the common and unusual types of epilepsy, and through this classification system improve the therapeutic and prognostic clinical approach to patients.

The new wave of research in the epilepsies

In addition to yielding new routes of navigation, the workshops from which the material in this supplement comes developed a conceptual blueprint for priority challenges in epilepsy research. All participants called attention to the ultimate goal, namely, understanding the mechanisms of human epilepsies. And, foremost to achieving this goal is the search for polymorphisms of restriction endonuclease patterns in monogenic forms of epilepsies in an attempt to localize the abnormal gene, or genes, to a specific chromosome. In human temporal lobe epilepsy, a priority challenge is to record paroxysmal depolarization shifts in hippocampal slices in vitro, slices excised from the known site of epileptogenicity. Parallel experiments exploring biochemical membrane abnormalities in neuronal and glial membranes isolated from the hippocampal seizure focus are especially valuable. Together with genetic studies using restriction-fragment-length polymorphisms, these experiments should distinguish between the respective contributions of genetic and environmental factors in multifactorial forms of partial epilepsies, such as temporal lobe epilepsy. In the genesis and spread of human temporal lobe epilepsy, the role of kindling and the mirror focus must be resolved. Recent successful applications of positron emission tomography, single-photon-emission computed tomography, and nuclear magnetic resonance computed tomography show promise in finally constructing the ion transport pathways, neurotransmitter systems, and metabolic processes within the functioning brains of epileptic patients.

Positron emission tomographic study of affective disorders: problems and strategies

Because anatomical studies of psychiatric disorders in humans have been largely unsuccessful, and because pharmacological interventions in patients with mental illness can be analyzed by means of biochemical assay techniques, positron emission tomography (PET) provides an exciting new means to study mental illness. Using fluorine-18-labeled fluorodeoxyglucose and PET to measure local cerebral glucose utilization, the investigators examined patients with affective disorders and normal age-matched controls. Patients with unipolar depression were studied in a drug-free baseline state following short-term administration of methylphenidate and in a euthymic state in long-term follow-up. Patients with bipolar disorders were studied in either the manic or depressed phases of illness and again in a euthymic state in long-term follow-up. Age-matched controls were studied both with and without methylphenidate administration. The results demonstrate that metabolic subgrouping of patients may be possible. Mood changes produced by pharmacological agents resulted in changes in the patterns of metabolism when compared with the baseline state. Problems associated with the use of PET to study mental illness fall into the categories of patient classification (diagnosis, state, trait, ambient conditions, and mental processes) and data analysis (anatomical and statistical). These problems, the limitations of the present techniques, and strategies for their improvement are discussed. Despite these difficulties, PET should prove to be a fruitful means of exploring the human biochemical abnormalities associated with mental illness.

The use of positron emission tomographic scanning in epilepsy

Positron emission tomography (PET) with fluorine-18-labeled fluorodeoxyglucose (18FDG) has demonstrated the epileptogenic lesion in partial epilepsy to be hypometabolic interictally . This finding is useful for localizing the area of resection when surgical therapy is contemplated. 18FDG scans during partial seizures show increased metabolism in areas of ictal onset and spread and in other regions of decreased metabolism that could reflect postictal effects. In the generalized epilepsies, petit mal absences and generalized convulsions induced by electroconvulsive shock therapy (ECT) are associated with global hypermetabolism, while global hypometabolism is seen in the postictal period following ECT. More information about the factors that influence the interictal hypometabolic zone in partial epilepsy should improve the diagnostic value of this finding for presurgical localization and perhaps also for the evaluation of other therapeutic regimens. New techniques for more dynamic PET studies with improved resolution, combined with computerized electroencephalographic analysis, should allow more accurate interpretation of ictal, as well as interictal, phenomena. Application of PET technology to other paroxysmal disorders may provide a basis for new diagnostic classifications that have therapeutic and prognostic value and may allow clearer differentiation among epileptic phenomena, myoclonus, and movement disorders. More clinical and animal research is needed, however, before we can delineate fundamental mechanisms of human epilepsy from PET data. To this end, it is now possible to use combined multidisciplinary parallel approaches in patients and animals to define specific aspects of epileptic disorders clinically, to intensively investigate them with experimental models in the animal laboratory, and to verify the relevance of these experimental results by returning to clinical studies.

Positron emission tomography of the brain: new possibilities for the investigation of human cerebral pathophysiology

In the foregoing an overview of positron emission tomography has been presented. Its theoretical, technical, and methodological implications, as well as its clinical applications have been outlined. The emphasis has been on the quantitative aspects of the method and its usefulness is investigating normal and pathological functions of brain tissue. Although the potential of this new research technique is obvious, many theoretical and practical difficulties still need to be solved. Nevertheless it provides an opportunity to bridge the gap between basic experimental research and clinical medicine.