Barbiturates antiseizure

Many CNS depressants have some liability for dependence. This is typically greater with barbiturates, but lesser with benzodiazepines, and perhaps nonexistent in many antiseizure medications. CNS depressants produce tolerance when administered chronically, where increasingly larger doses are required to sustain the same level of effect. Further, a cross-tolerance often develops, where the tolerance is generalized to other CNS depressants. For example, a person with an ethanol tolerance will also display some tolerance to barbiturates. The therapeutic index tends to decrease as tolerance increases, so that the difference between an effective and toxic dose diminishes. Thus, tolerance to CNS depressants is accompanied by a smaller safety margin. 

Amphetamine (Benzedrine). Amphetamine was synthesized in 1887. It was quickly found to be a potent stimulant with effects similar to cocaine, which had been discovered over 100 years before. In the subsequent years, amphetamine found a variety of uses. It was used to treat narcolepsy, Parkinson s disease, barbiturate overdose, bed wetting (enuresis), and obesity. It was also used to counteract the sedating effects of other drugs and medications including antiseizure medications and alcohol. 

Until 1990, approximately 16 antiseizure drugs were available, and 13 of them can be classified into five very similar chemical groups barbiturates. 

Antiseizure heterocyclic ring structure. The X varies as follows hydantoin derivatives, -N- barbiturates, -C-N- oxazolidinediones, -0- succinimides, -C-acetylureas, -NH2 (N connected to C2). Ri, R2/ and R3 vary within each subgroup. 

Aside from the bromides, phenobarbital is the oldest of the currently available antiseizure drugs. Although it has long been considered one of the safest of the antiseizure agents, the use of other medications with lesser sedative effects has been urged. Many consider the barbiturates the drugs of choice for seizures only in infants. 

The four derivatives of barbituric acid clinically useful as antiseizure drugs are phenobarbital, mephobarbital, metharbital, and primidone. The first three are so similar that they are considered together. Metharbital is methylated barbital, and mephobarbital is methylated phenobarbital both are demethylated in vivo. The pKas of these three weak acid compounds range from 7.3 to 7.9. Slight changes in the normal acid-base balance, therefore, can cause significant fluctuation in the ratio of the ionized to the un-ionized species. This is particularly important for phenobarbital, the most commonly used barbiturate, whose pKa is similar to the plasma pH of 7.4. 

Two prominent aspects of benzodiazepines limit their usefulness. The first is their pronounced sedative effect, which is unfortunate both in the treatment of status epilepticus and in chronic therapy. Children may manifest a paradoxical hyperactivity, as with barbiturates. The second problem is tolerance, in which seizures may respond initially but recur within a few months. The remarkable antiseizure potency of these compounds often cannot be realized because of these limiting factors. 

Mechanism of Action. Barbiturates are known to increase the inhibitory effects of GABA (see Chapter 6), and this effect is probably the primary way that these drugs decrease seizure activity. Barbiturates may also produce some of their antiseizure effects by inhibiting calcium entry into excitatory presynaptic nerve terminals and thereby decreasing the release of excitatory neurotransmitters such as glutamate.20... 

Barbiturates -barbital Phenobarbital, secobarbital Sedative-hypnotic (6), antiseizure [9], anesthetic (11]... 

Most of the sedative-hypnotics are capable of inhibiting the development and spread of epileptiform activity in the central nervous system. Some selectivity exists in that some members of the group can exert anticonvulsant effects without marked central nervous system depression (although psychomotor function may be impaired). Several benzodiazepines—including clonazepam, nitrazepam, lorazepam, and diazepam—are sufficiently selective to be clinically useful in the management of seizure states (see Chapter 24 Antiseizure Drugs). Of the barbiturates, phenobarbital and metharbital (converted to phenobarbital in the body) are effective in the treatment of generalized tonic-clonic seizures. 

Phenobarbital has selective antiseizure activity at low doses and has a long half-life suitable for maintenance treatment in seizure disorders (for characteristics of barbiturates, see sedative-hypnotics). Clonazepam is usually a backup drug in absence and myoclonic seizures it causes marked sedation at anticonvulsant doses. IV lorazepam and diazepam are both used in status epilepticus. 

Most barbiturates have antiseizure properties. The discussion below is limited to the two barbiturates that exert maximal antiseizure action at doses below those required for hypnosis, a property that determines their clinical utility as antiseizure agents. The pharmacology of barbiturates is considered in Chapter 16. 

There are two problems with regard to withdrawal from antiseizure drugs the effects of withdrawal itself and the need to continue suppression of seizures. Dose-tapering is an important principle in antiseizure drug withdrawal. As a rule, withdrawal from drugs used in absence seizures is easier than withdrawal from drugs used for partial and tonic-clonic seizures. Withdrawal is most difficult in patients who have been treated with barbiturates and benzodiazepines. The answer is (C). 

The barbiturates are substituted pyrimidine derivatives with an ureide configuration (Fig. 20.4). They are lipophilic weak acids (pKa 7-8) that are weii distributed into brain (see Appendix A for the respective pKa values). Although many barbiturates dispiay sedative-hypnotic activity (see Chapter 19), oniy a few have antiseizure properties. Paradoxically, many barbiturates cause convulsions at larger doses. The barbiturates clinically useful as AEDs are phenobarbital, mephobarbital, and primidone (Fig. 20.8). In laboratory animals, phenobarbital is effective by several tests in nontoxic doses. It is active against electrically induced seizures (MES), and it elevates the threshold for pentylenetetrazole stimulation. The mechanism of antiseizure action for the barbiturates... 

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