Phenobarbital tolerance and physical dependence: chronically equivalent dosing model

Pharmacokinetic analysis of a phenobarbital overdose treated with urinary alkalinization alone

Phenobarbital is a long-acting barbiturate used to treat alcohol withdrawal and epilepsy. Acute overdoses present with varying levels of central nervous system depression and large overdoses can be life threatening. Phenobarbital is an attractive candidate for enhanced elimination using urinary alkalinization given it is a weak acid with a long half-life and extensive urinary elimination. Limited human data exist regarding use of urine alkalinization for the treatment of phenobarbital overdose. We present a fourteen-year-old female who was treated with urinary alkalinization alone following an intentional ingestion of 3800 mg (84.4 mg/kg) of phenobarbital tablets. Urine drugs of abuse screening was preliminary positive for barbiturates and confirmed to be phenobarbital only. The initial serum phenobarbital concentration, drawn nine hours post-ingestion, was 97.4 mcg/ml (normal range 15-40 mcg/ml). Urinary alkalinization with sodium bicarbonate was started approximately 12 h post-ingestion and stopped at 72 h post-ingestion; clinical toxicity resolved by hospital day 5. The infusion was titrated to a urinary pH of greater than 7.5. Serial serum and urine phenobarbital measurements were obtained to determine elimination half-life and urinary excretion. The elimination half-life while undergoing urinary alkalinization was 81.3 h. Prior to initiation of urinary alkalinization, the urine phenobarbital concentration was 37 mcg/ml. Approximately 8.75 h after initiation, it was greater than 200 mcg/ml at a urine pH of 8.5. Urinary alkalinization appeared to augment urinary phenobarbital excretion, though with no discernible effect on elimination half-life and unclear clinical benefit. Further research is needed to better characterize the clinical effects of urinary alkalinization for phenobarbital overdose.

Alterations of Glucose Uptake and Protein Expression Related to the Insulin Signaling Pathway in the Brain of Phenobarbital-Addictive Rats by 18F-FDG PET/CT and Proteomic Analysis

Drug addiction is a chronic relapsing brain disease. Alterations of glucose uptake and metabolism are found in the brain of drug addicts. Insulin mediates brain glucose metabolism and its abnormality could induce brain injury and cognitive impairment. Here, we established a rat model of phenobarbital addiction by 90 days of dose escalation and evaluated addiction-related symptoms. We also performed ¹⁸F-fluorodeoxyglucose (FDG) positron emission tomography/computed tomography (PET/CT) to detect glucose uptake in the brain and proteomic analysis of the function of the differentially expressed (DE) proteins via bioinformatics in brain tissues by liquid chromatography coupled with tandem mass spectrometry (LC-MS/MS) on days 60 and 90 of phenobarbital or 0.5% carboxymethyl cellulose sodium (CMC-Na) (vehicle) administration. The results showed that phenobarbital-addictive rats developed severe withdrawal symptoms after abstinence and glucose uptake was significantly increased in the brain. Proteomics analysis showed that numerous DE proteins were enriched after phenobarbital administration, among which CALM1, ARAF, and Cbl proteins (related to the insulin signaling pathway) were significantly downregulated on day 60 but not day 90. However, SLC27A3 and NF-κB1 proteins (related to insulin resistance) were significantly upregulated on day 90 (data are available via ProteomeXchange with identifier PXD021101). Our data indicate that the insulin signaling pathway and insulin resistance may play a role in the development of phenobarbital addiction and brain injury, so the findings may have important clinical implications.

Pharmacological and neuroethological studies of three antiepileptic drugs in the Genetic Audiogenic Seizure Hamster (GASH:Sal)

Epilepsy modeling is essential for understanding the basic mechanisms of the epileptic process. The Genetic Audiogenic Seizure Hamster (GASH:Sal) exhibits generalized tonic-clonic seizures of genetic origin in response to sound stimulation and is currently being validated as a reliable model of epilepsy. Here, we performed a pharmacological and neuroethological study using well-known and widely used antiepileptic drugs (AEDs), including phenobarbital (PB), valproic acid (VPA), and levetiracetam (LEV). The intraperitoneal administration of PB (5-20mg/kg) and VPA (100-300mg/kg) produced a dose-dependent decrease in GASH:Sal audiogenic seizure severity scores. The administration of LEV (30-100mg/kg) did not produce a clear effect. Phenobarbital showed a short plasmatic life and had a high antiepileptic effect starting at 10mg/kg that was accompanied by ataxia. Valproic acid acted only at high concentrations and was the AED with the most ataxic effects. Levetiracetam at all doses also produced sedation and ataxia side effects. We conclude that the GASH:Sal is a reliable genetic model of epilepsy suitable to evaluate AEDs.

Tolerance to the beneficial effects of prophylactic migraine drugs: a systematic review of causes and mechanisms

Loss of benefit of a previously effective treatment regimen, also known as tolerance, can be an important barrier to the successful preventive treatment of migraine. We undertook a systematic review of the literature to identify the prevalence and possible mechanisms of drug tolerance in migraine prophylaxis. Results demonstrate that the frequency of tolerance to prophylactic migraine treatment is unknown, but available data support an estimate that it occurs in 1-8% of patients receiving prophylaxis. Four broad types of tolerance were identified that are likely to be relevant to migraine prophylaxis. These are pharmacokinetic, pharmacodynamic, behavioral, and cross tolerance. The mechanisms that underlie these types of tolerance determine whether their effects can be overcome or minimized. For example, certain forms of tolerance may be affected by manipulation of environmental cues associated with drug administration, by the order in which drugs are used, and by the concomitant use of other medications. Many medications used for migraine prophylaxis exert their effects through the endogenous opioid system. The implications of this finding are explored, particularly the parallels between medication overuse headache and tolerance to migraine prophylaxis. Given the many ways in which tolerance to migraine medications may develop, in some ways it is not surprising that migraine-preventive drugs stop working; it is more surprising that in many cases they do not.

Sensitivity to the effects of sedative-hypnotics on motor performance: influence of task difficulty and chronic phenobarbital administration

The present investigation examined sensitivity to the effects of various sedative-hypnotics on motor performance in rats treated chronically with phenobarbital. Eight rats were trained to walk on a rotorod treadmill at low (8 r.p.m.) and high (24 r.p.m.) rotational speeds. Prior to the chronic regimen, phenobarbital, pentobarbital, amobarbital, diazepam and clonazepam produced dose-dependent impairments in motor performance at both speeds. During chronic treatment with phenobarbital (100 mg/kg/day), tolerance was conferred to the effects of all the drugs examined, as evidenced by rightward shifts in their dose-effect curves. For all drugs, the magnitude of this tolerance was generally consistent across the two speeds. Following a 6-week washout period, during which no drugs were administered, dose-effect curves for each drug shifted back toward their original (i.e. pre-chronic) positions. Under all conditions, the doses required for each drug to impair motor performance at the low speed were higher than those required to impair motor performance at the high speed. These data suggest that sensitivity to the motor-impairing effects of sedative-hypnotics is influenced by the difficulty of the behavioral task, but that task difficulty does not modulate the maximal extent to which tolerance and cross-tolerance are expressed.

Pharmacodynamics of tolerance development to the anesthetic and anticonvulsant effects of phenobarbital in rats

In this study, the potential development of tolerance towards the anesthetic and anticonvulsant effects of phenobarbital after chronic administration to rats was investigated, differentiating between dispositional and functional tolerance. Chronic exposure to phenobarbital by daily ip injections of 20 or 100 mg/kg for 14 days caused a 30% increase in clearance within 1 week. No changes occurred in the total amounts of phenobarbital and para-hydroxyphenobarbital excreted into urine and feces. Pharmacodynamic effects were quantitated after 2 weeks of treatment. The anesthetic effect was measured by slow iv infusion of phenobarbital until onset of loss of righting reflex (LRR), followed by measurement of drug concentrations in serum (both total and free), brain, and cerebrospinal fluid (CSF). With a phenobarbital dose of 100 mg/kg daily, CSF concentrations at the onset of LRR significantly increased from 136 +/- 12 to 176 +/- 21 mg/L (p less than 0.001), indicating that functional tolerance developed for the anesthetic effect. Protection of phenobarbital against convulsions induced by pentylenetetrazol (PTZ), as measured by the elevation of the PTZ plasma threshold concentration necessary to elicit seizures at the EC50 of phenobarbital, was not altered. The discrepancy observed in the development of functional adaptation of the CNS demonstrates that different mechanisms of action are reflected in the different measures of the anesthetic and anticonvulsant effects of phenobarbital that were utilized.

Cerebral glucose utilization in rat brain during phenobarbital withdrawal

The phenobarbital withdrawal syndrome in rats is characterized by tremors, arched back, weight loss and hyperactivity. This syndrome is shown to be associated with both general and localized increases in cerebral glucose utilization. An increase in glucose utilization (significant at the P less than or equal to 0.001 level) was observed in 72% of the 57 structures examined. Increases in glucose utilization of greater than or equal to 180% of control values were noted in structures associated with the motor system (columns in the frontal sensorimotor cortex, globus pallidus, dentate nucleus of the cerebellum and ovoid areas in the cerebellar vermis), thalamic nuclei (lateral and posterior), dorsal lateral geniculate, mammillary body, cingulate cortex, locus ceruleus, and cerebellar flocculus and paraflocculus. The structures showing the greatest increase in glucose utilization were cerebellar paraflocculus (257% of control), columns in the frontal sensorimotor cortex (247% of control) and ovoid areas in the cerebellar vermis (223% of control). Areas of the brain that have been described as cell body areas for serotonergic (raphe), noradrenergic (locus ceruleus), dopaminergic (substantia nigra, zona compacta) and GABAergic (globus pallidus) neurons also showed increases in glucose utilization. The pattern of cerebral glucose utilization accompanying the phenobarbital withdrawal syndrome in rats contrasts with that for morphine withdrawal and exhibits both similarities and differences with respect to ethanol withdrawal.

Intensity of the withdrawal syndrome varies with duration of pentobarbital administration

The effect of three dosage schedules on the expression of a withdrawal syndrome indicative of physical dependence on pentobarbital was determined in male Sprague-Dawley rats. Rats were prepared with an intraperitoneal cannula and were continuously infused with either saline (control) or pentobarbital sodium, using an escalating drug dosage schedule, for either 5 (PB-5), 13 (PB-13) or 20 (PB-20) days. Final doses reached were 500 mg/kg/day (PB-5) and 1000 mg/kg/day (PB-13). PB-20 rats reached 1000 mg/kg/day on day 13 and were maintained at this dose for an additional 7 days. Body weight, water consumption and assessment of CNS depression were obtained daily. Following the last day of pentobarbital infusion all rats were infused with saline for a 72-hour drug-free period. Water consumption, body weight and assessment of overt behavioral signs indicative of a drug withdrawal syndrome were obtained at specific times during the drug-free period. PB-5 rats showed little evidence of withdrawal while PB-20 rats demonstrated the greatest degree of withdrawal. Peak withdrawal scores were observed to be 1, 3.8 and 5 for PB-5, PB-13, and PB-20, respectively. Withdrawal scores for group PB-13 and PB-20 were found to be significantly greater than either control or PB-5 but were not significantly different from each other. Body weight for PB-13 and PB-20 mice declined slightly (nonsignificant) during the drug-free period while a significant decrease (40% decline) in water consumption was demonstrated by 24 hours of this period.(ABSTRACT TRUNCATED AT 250 WORDS)