Physiologically based pharmacokinetic and pharmacodynamic modelling of alprazolam: Implications for anxiety and addiction.

AIMS: Alprazolam is an anxiolytic compound that can lead to psychological and physiological dependence especially with prolonged use. This study utilized physiologically based pharmacokinetic (PK) and pharmacodynamic (PD) modelling to further examine the underlying mechanisms of anxiety treatment and addiction. METHODS: Data and parameter values for this study were obtained from PubMed and DrugBank literature searches. The physiologically based PK models for alprazolam were developed using PK-Sim software and PD models were implemented with the MonolixSuite 2021R platform. RESULTS: After single administrations, peak unbound interstitial brain concentrations range from 4 to 33 nM for 0.25-2 mg-doses of the immediate-release form and 3-54 nM for 0.5-10-mg doses of the extended-release form. With repetitive administrations, peak concentration is 59 nM for a 2-mg alprazolam immediate-release dose and 122 nM for a 10-mg extended-release dose. Potentiation of EC10 GABA-gated currents from recombinant GABAA Rs composed of α1ß2γ2, α2ß3γ2 and α5ß3γ2 subunit combinations is 92, 150 and 75%, respectively, for an alprazolam concentration of 59 nM. The 10-90% rise times for the brain concentration-time profile following a single 1-mg immediate-release administration is 22.8 min and 3.8 h for a 3-mg extended-release administration. CONCLUSION: Unbound interstitial brain concentration-time profiles of alprazolam corresponded to changes in ß rhythm activity, peak saccade velocity, mood improvement, cognitive speed slowing and digit symbol substitution test scores. PD models for these endpoints suggest that alprazolam immediate-release maximal effects on cognitive slowing, cognitive impairment, sedation and mood improvement occur sequentially following the brain concentration-time profile.

Enhancement of α5-containing γ-aminobutyric acid type A receptors by the nonimmobilizer 1,2-dichlorohexafluorocyclobutane (F6) is abolished by the ß3(N265M) mutation.

BACKGROUND: Modulation of γ-aminobutyric acid type A receptors (GABAARs) by general anesthetics may contribute to their ability to produce amnesia. Receptors containing α5 subunits, which mediate tonic and slow synaptic inhibition, are co-localized with ß3 and γ2 subunits in dendritic layers of the hippocampus and are sensitive to low (amnestic) concentrations of anesthetics. Because α5 and ß3 subunits influence performance in hippocampus-dependent learning tasks in the presence and absence of general anesthetics, and the experimental inhaled drug 1,2-dichlorohexafluorocyclobutane (F6) impairs hippocampus-dependent learning, we hypothesized that F6 would modulate receptors that incorporate α5 and ß3 subunits. We hypothesized further that the ß3(N265M) mutation, which controls receptor modulation by general anesthetics, would similarly influence modulation by F6. METHODS: Using whole-cell electrophysiologic recording techniques, we tested the effects of F6 at concentrations ranging from 4 to 16 µM on receptors expressed in human embryonic kidney 293 cells. We measured drug modulation of wild-type α5ß3 and α5ß3γ2L GABAARs and receptors harboring the ß3(N265M) mutation. We also tested the effects of F6 on α1ß2γ2L receptors, which were reported previously to be insensitive to this drug when expressed in Xenopus oocytes. RESULTS: F6 enhanced the responses of wild-type α5ß3γ2L but not α1ß2γ2L receptors to low concentrations of GABA in a concentration-dependent manner. Receptors that incorporated the mutant ß3(N265M) subunit were insensitive to F6. When applied together with a high concentration of GABA, F6 blocked currents through α5ß3 but not α5ß3γ2L receptors. F6 did not alter deactivation of α5ß3γ2L receptors after brief high- concentration pulses of GABA. CONCLUSIONS: The nonimmobilizer F6 modulates GABAARs in a manner that depends on subunit composition and mode of receptor activation by GABA, supporting a possible role for α5-containing receptors in suppression of learning and memory by F6. Furthermore, common structural requirements indicate that similar molecular mechanisms may be responsible for the enhancing effects of F6 and conventional general anesthetics.

Pentobarbital produces activation and block of {alpha}1{beta}2{gamma}2S GABAA receptors in rapidly perfused whole cells and membrane patches: divergent results can be explained by pharmacokinetics.

Millimolar concentrations of the barbiturate pentobarbital (PB) activate gamma-aminobutyric acid (GABA) type A receptors (GABARs) and cause blockade reported by a paradoxical current increase or "tail" upon washout. To explore the mechanism of blockade, we investigated PB-triggered currents of recombinant alpha(1)beta(2)gamma(2S) GABARs in whole cells and outside-out membrane patches using rapid perfusion. Whole cell currents showed characteristic bell-shaped concentration dependence where high concentrations triggered tail currents with peak amplitudes similar to those during PB application. Tail current time courses could not be described by multi-exponential functions at high concentrations (> or =3,000 microM). Deactivation time course decayed over seconds and was slowed by increasing PB concentration and application time. In contrast, macropatch tail currents manifested eightfold greater relative amplitude, were described by multi-exponential functions, and had millisecond rise times; deactivation occurred over fractions of seconds and was insensitive to PB concentration and application time. A parsimonious gating model was constructed that accounts for macropatch results ("patch" model). Lipophilic drug molecules migrate slowly through cells due to avid partitioning into lipophilic subcellular compartments. Inclusion of such a pharmacokinetic compartment into the patch model introduced a slow kinetic component in the extracellular exchange time course, thereby providing recapitulation of divergent whole cell results. GABA co-application potentiated PB blockade. Overall, the results indicate that block is produced by PB concentrations sixfold lower than for activation involving at least three inhibitory PB binding sites, suggest a role of blocked channels in GABA-triggered activity at therapeutic PB concentrations, and raise an important technical question regarding the effective rate of exchange during rapid perfusion of whole cells with PB.