Study Notes on Benzodiazepines

WHY WERE THE BENZODIAZEPINES (BZD) DEVELOPED?

• Benzodiazepines (BZD) emerged as an alternative to barbiturates (BTS) due to the potential toxicity associated with BTS.

• Despite concerns over safety being potentially overstated, the popularity of BZDs skyrocketed.

• By 1972, chlordiazepoxide (Librium) and diazepam (Valium) accounted for 49% of all prescriptions for psychoactive drugs, reflecting their widespread use.

• There were an estimated 3 million refills for Valium per month.

• Estimates suggest that 500 million people have used a BZD at least once in the last 30 years.

• BZDs are used in various capacities:

  • Anxiolytics: used for anxiety relief.

  • Antiseizure (antiepileptic): used to manage seizure disorders.

  • Hypnotics: used as sleep aids.

  • Amnesics: BZDs are preferred for blocking memory related to traumatic events (e.g., during invasive surgery) due to their effects at lower and safer doses.

VALIUM

• Common pharmacokinetic information regarding Diazepam (Valium):

  • Indication: Anxiolytic

  • Biotransformation Pathway: Oxidation

  • Elimination Half-Life: ranges between 20-70 hours.

PHARMACOKINETICS ACROSS THE BZD

• Benzodiazepines were developed as a class of structurally similar compounds, categorized into subgroups with distinct pharmacokinetic profiles.

• Absorption and Distribution:

  • BZDs are rapidly absorbed and effectively distributed via the bloodstream.

  • They can cross the blood-brain barrier (BBB) easily, primarily affecting central brain structures with very few peripheral binding sites.

• Clearance or Metabolism:

  • Some BZDs do not exert their action until metabolized into active metabolites.

  • For example:

  • Diazepam has a 24-hour half-life but is metabolized to nordiazepam, which has an 80-hour half-life, before eventually becoming oxazepam with an 8-hour half-life.

MECHANISMS OF ACTION FOR THE BZD

• Benzodiazepines act primarily as GABA agonists.

• Unlike barbiturates and ethanol, BZDs are described as “pure GABA” agonists because they lack the capacity to independently open Cl- channels, meaning their activity is directly dependent on GABA availability.

• Due to the predominance of binding sites located centrally, BZDs show minimal effects on autonomic function or lower brain functions.

• Illustration of a GABA Receptor:

  • Contains various binding sites:

  • Barbiturate site

  • Chloride site

  • GABA site

  • Picrotoxin site

  • Steroid site

  • Benzodiazepine site

PHARMACOLOGICAL EFFECTS OF THE BZDS

• Benzodiazepines produce several effects due to their action on specific brain targets:

  • Anxiolytic effects: Target the limbic system (specifically the amygdala).

  • Antiseizure effects: Exert influence in the cerebellum and hippocampus.

  • Hypnotic effects: Act on the reticular activating system to induce sleep.

  • Amnesic effects: Influence the cerebral cortex and hippocampus to impair memory recall.

ENDOGENOUS BZD AND ITS EFFECTS

• The search for endogenous benzodiazepines has led to the finding of beta carboline compounds (β-CCE).

• These compounds occupy BZD receptors and can displace other less potent BZDs, but they result in increased agitation and anxiety-like symptoms in subjects.

• Administration of low doses of β-CCE causes muscle tension, autonomic hyperreactivity, and reports of apprehension in humans.

• A small dose of a potent BZD effectively blocks the arousal caused by β-CCE, suggesting a competing endogenous anxiety mechanism operating at BZD sites.

• There are varying β-CCE types:

  • Type a compounds (causing arousal and anxiety - fight or flight response)

  • Type c compounds (exerting calming effects).

TOLERANCE, DEPENDENCE, AND ABUSE POTENTIAL OF BZDS

• Benzodiazepines have a natural limiting factor due to their mechanism of enhancing endogenous GABA, making them difficult to use lethally in suicide attempts.

• The therapeutic index for Midazolam (Versed) is recorded as 20,000:1, whereas it is significantly lower (7.8:1) for phenobarbital.

Development of Tolerance

• Evidence indicates the development of tolerance towards BZD effects through various studies, including the Geller-Seifter procedure testing in rats, showcasing behavioral changes in lever pressing as a function of drug dosage and environmental stimuli.

• Key Findings:

  • During the study, a reduction in lever pressing signifies anxiety influenced by shock.

  • Early administration of BZDs also suppressed behavior, but over time, tolerance emerged as the frequency of lever pressing increased across sessions.

  • Lucas and Griffiths (1982) observed withdrawals in baboons administered diazepam, induced by a BZD antagonist, showcasing withdrawal symptoms like retching, vomiting, lethargy, and depressed activity during abrupt cessation of the drug.

• Following chronic administration, diazepam was seen to stimulate measurable responses during withdrawal, posing safety concerns particularly in older adults, who demonstrate inefficient metabolism of BZDs leading to excessive sedation and mobility issues.

FINAL STUDIES AND FINDINGS

• Recent studies with human subjects indicate similar patterns in the elevation of anxiety responses when administering BZDs, including self-administered shock tasks revealing that:

  • Placebo group had a mean shock level selection of 4.85

  • 5 mg diazepam group selected a mean shock level of 6.12

  • 10 mg diazepam group selected a higher average shock level of 6.99

  • The percentage of participants using the highest shock level was 6.7% for placebo, 12.5% for 5 mg diazepam, and 40% for 10 mg diazepam demonstrating increased risk-related behaviors.

CONCLUSIONS

• Metabolic Tolerance: This phenomenon is particularly concerning in the elderly population, where the effective dose (ED50) for BZDs is markedly lower than that for younger individuals, leading to increased risks of side effects and excess sedation.

SUMMARY OF QUIZZES

• Quiz Questions:

1. The classic BZD is: Valium.

2. The BZDs are unique in that almost all receptors are located in the central nervous system.

3. The endogenous ligand for BZD receptors is beta carboline.

4. After repeated dosing with diazepam, administering a powerful antagonist will induce withdrawal.

5. Elderly BZD users show less efficient metabolism of BZDs than younger users.