Benzodiazepines

WHY WERE THE BENZODIAZEPINES (BZD) DEVELOPED?

  • Benzodiazepines (BZD) emerged as a safer alternative to barbiturates (BTS), addressing potential toxicity associated with BTS.

  • The notion of BZD safety was potentially overstated; nonetheless, BZD usage surged.

  • In 1972, chlordiazepoxide (Librium) and diazepam (Valium) constituted 49% of all psychoactive drug prescriptions.

  • Valium had 3 million refills per month, indicating massive consumption.

  • An estimated 500 million people used a BZD at least once over the previous 30 years.

  • Primary uses of BZD include:

    • Anxiolytics: for anxiety management

    • Antiseizure: as antiepileptics

    • Hypnotics: serving as sleep aids

    • Amnesics: utilized for their memory-blocking effects at lower doses, particularly for traumatic events such as surgery.

VALIUM AND PHARMACOKINETIC SUMMARY OF THE BENZODIAZEPINES

  • Drug Information Table 16.1:

    • Diazepam - Anxiolytic

    • Initial Biotransformation Pathway: Oxidation

    • Elimination Half-Life: 20-70 hours

    • Desmethyldiazepam, Oxazepam, Temazepam, Clorazepate - Anxiolytics

    • Initial Biotransformation Pathway: Oxidation

    • Elimination Half-Life: Varies (36-96 hours)

    • Alprazolam (Xanax) - Anxiolytic

    • Initial Biotransformation Pathway: Oxidation

    • Elimination Half-Life: 8-15 hours

    • Lorazepam - Anxiolytic

    • Initial Biotransformation Pathway: Conjugation

    • Elimination Half-Life: 10-20 hours

    • Bromazepam, Clobazam, Triazolam, Nitrazepam, Quazepam, Midazolam - Various pharmacokinetics fall under anxiolytics and hypnotics with differing half-lives.

PHARMACOKINETICS OF THE BENZODIAZEPINES

  • Benzodiazepines are a class of structurally similar compounds that exhibit a range of pharmacokinetic profiles:

    • Absorption and Distribution:

    • Rapidly absorbed and effectively distributed in the bloodstream.

    • Classified into groups based on pharmacokinetics.

    • Blood-Brain Barrier (BBB):

    • Cross the BBB easily, indicating limited peripheral binding sites, primarily affecting central nervous system structures.

    • Clearance/Metabolism:

    • Unique in that some BZD require biotransformation into active metabolites to exert effects. For example:

      • Diazepam has a half-life of 24 hours but metabolizes into nordiazepam, which has an 80-hour half-life, and further into oxazepam with an 8-hour half-life.

MECHANISMS OF ACTION FOR THE BENZODIAZEPINES

  • Unlike barbiturates and ethanol, benzodiazepines are characterized as “pure GABA” agonists.

  • BZD do not have the secondary capability to open Cl⁻ channels independently, thus their action is limited by GABA availability.

  • Predominantly affecting central nervous system sites leads to minimal or no impact on peripheral nerve function.

PHARMACOLOGICAL EFFECTS OF THE BENZODIAZEPINES

  • Anxiolytic Effects:

    • Act upon the limbic system, specifically influencing the amygdala to reduce anxiety.

  • Antiseizure Effects:

    • Function through effects in the cerebellum and hippocampus mitigating seizure activity.

  • Hypnotic Effects:

    • Affecting the reticular activating system facilitates sleep induction.

  • Amnesic Effects:

    • Influence on cerebral cortex and hippocampus aids in memory blockage.

ENDOGENOUS BENZODIAZEPINES

  • Research into endogenous benzodiazepines revealed compounds like beta carbolines (β-CCE).

  • Activity of β-CCE:

    • β-CCE can occupy BZD receptors, displacing less potent BZD.

    • Induces agitation and hyperactivity in subjects responding to its presence.

    • Low doses of β-CCE result in muscle tension and autonomic hyperreactivity in humans, indicating an anxiogenic response.

    • Conversely, administration of a potent BZD can block β-CCE induced arousal, suggesting an endogenous arousal mechanism might exist.

  • Two distinct classes of β-CCE observed:

    • Type A: Arousing, induces fight or flight responses.

    • Type C: Calming and induces relaxation.

  • Research Example:

    • Stephens et al. (1987) conducted tests on rats regarding drinking behavior when administered β-CCE antagonists or agonists, indicating varying levels of anxiety through behavioral changes.

TOLERANCE, DEPENDENCE, AND ABUSE POTENTIAL OF BENZODIAZEPINES

  • Safety of BZD:

    • Limited by their mechanism of enhancing endogenous GABA, resulting in relative ineffectiveness during suicide attempts.

    • Therapeutic Index for midazolam (Versed): 20,000:1, compared to phenobarbital's ratio of 7.8:1.

  • Development of Tolerance Studies:

    • Conducted using the Geller-Seifter procedure with rats, analyzing behavior under varying schedules of reinforcement and punishment.

  • Findings from Studies:

    • BZD administration correlated with altered behavior indicative of tolerance formation.

    • Withdrawal symptoms observed with abrupt cessation following chronic diazepam administration.

  • Additional Research:

    • Berman et al. (2005) likened self-administered shock responses in humans to show similar anxiogenic behaviors when tested with different doses of diazepam.

  • Elderly Consideration:

    • Notable challenge with metabolic tolerance development of BZD in elderly individuals, typically requiring lower effective dosages due to compromised metabolism.

    • The Effective Dose (ED50) for elderly is significantly lower than that for younger individuals due to their ineffective metabolism of BZD.