Pharmacodynamics of selective inhibition of γ-secretase by avagacestat.

A hallmark of Alzheimer's disease (AD) pathology is the accumulation of brain amyloid ß-peptide (Aß), generated by γ-secretase-mediated cleavage of the amyloid precursor protein (APP). Therefore, γ-secretase inhibitors (GSIs) may lower brain Aß and offer a potential new approach to treat AD. As γ-secretase also cleaves Notch proteins, GSIs can have undesirable effects due to interference with Notch signaling. Avagacestat (BMS-708163) is a GSI developed for selective inhibition of APP over Notch cleavage. Avagacestat inhibition of APP and Notch cleavage was evaluated in cell culture by measuring levels of Aß and human Notch proteins. In rats, dogs, and humans, selectivity was evaluated by measuring plasma blood concentrations in relation to effects on cerebrospinal fluid (CSF) Aß levels and Notch-related toxicities. Measurements of Notch-related toxicity included goblet cell metaplasia in the gut, marginal-zone depletion in the spleen, reductions in B cells, and changes in expression of the Notch-regulated hairy and enhancer of split homolog-1 from blood cells. In rats and dogs, acute administration of avagacestat robustly reduced CSF Aß40 and Aß42 levels similarly. Chronic administration in rats and dogs, and 28-day, single- and multiple-ascending-dose administration in healthy human subjects caused similar exposure-dependent reductions in CSF Aß40. Consistent with the 137-fold selectivity measured in cell culture, we identified doses of avagacestat that reduce CSF Aß levels without causing Notch-related toxicities. Our results demonstrate the selectivity of avagacestat for APP over Notch cleavage, supporting further evaluation of avagacestat for AD therapy.

Pharmacokinetic-pharmacodynamic modeling of rifampicin-mediated Cyp3a11 induction in steroid and xenobiotic X receptor humanized mice.

The purpose of this study was to develop a mechanistic pharmacokinetic-pharmacodynamic (PK-PD) model to describe the effects of rifampicin on hepatic Cyp3a11 RNA, enzymatic activity, and triazolam pharmacokinetics. Rifampicin was administered to steroid and xenobiotic X receptor (SXR) humanized mice at 10 mg/kg p.o. (every day for 3 days) followed by triazolam (4 mg/kg p.o.) 24 h after the last dose of rifampicin. Rifampicin and triazolam concentrations and Cyp3a11 RNA expression and activity in the liver were measured over the 4-day period. Elevations in Cyp3a11 RNA expression were observed 24 h after the first dose of rifampicin, reaching a maximum (∼10 times baseline) after the third dose and were sustained until day 4 and began declining 48 h after the last rifampicin dose. Similar changes in enzymatic activity were also observed. The triazolam serum area under the curve (AUC) was 5-fold lower in mice pretreated with rifampicin, consistent with enzyme induction. The final PK-PD model incorporated rifampicin liver concentration as the driving force for the time-delayed Cyp3a11 induction governed by in vitro potency estimates, which in turn regulated the turnover of enzyme activity. The PK-PD model was able to recapitulate the delayed induction of Cyp3a11 mRNA and enzymatic activity by rifampicin. Furthermore, the model was able to accurately anticipate the reduction in the triazolam plasma AUC by integrating a ratio of the predicted induced enzyme activity and basal activity into the equations describing triazolam pharmacokinetics. In conjunction with the SXR humanized mouse model, this mathematical approach may serve as a tool for predicting clinically relevant drug-drug interactions via pregnane X receptor-mediated enzyme induction and possibly extended to other induction pathways (e.g., constitutive androstane receptor).

Pharmacokinetics and pharmacodynamics of gamma-hydroxybutyric acid during tolerance in rats: effects on extracellular dopamine.

gamma-Hydroxybutyrate (GHB) is a potent sedative/hypnotic and drug of abuse. Tolerance develops to GHB's sedative/hypnotic effects. It is hypothesized that GHB tolerance may be mediated by alterations in central nervous system pharmacokinetics or neurotransmitter response. Rats were dosed daily with GHB (548 mg/kg s.c. q.d. for 5 days), and sleep time was measured as an index of behavioral tolerance. Plasma and brain GHB pharmacokinetics on days 1 and 5 were monitored using blood and microdialysis sampling. Extracellular (ECF) striatal dopamine levels were measured by microdialysis as a pharmacodynamic endpoint of tolerance. Pharmacokinetic (PK)/pharmacodynamic (PD) modeling was performed to describe the plasma and brain disposition using an indirect response model with inhibition of dopamine synthesis rate to describe the pharmacodynamic response. GHB plasma and brain ECF concentration versus time profiles following acute or chronic exposure were not significantly different. GHB sedative/hypnotic tolerance was observed by day 5. Acute GHB administration resulted in a decrease in striatal ECF dopamine (DA) levels compared with baseline levels. GHB tolerance was reflected by a 60% decrease in dopamine area under the curve (effect and baseline): acute, 10.1 +/- 15.3% basal DA/min/10(-3) versus chronic, 4.73 +/- 1.49% basal DA/min/10(-3) (p < 0.05, n = 5; unpaired Student's t test). The PK/PD model revealed an increase in the IC50 following chronic exposure indicating decreased dopaminergic sensitivity toward the inhibitory effects of GHB. Our findings indicate that GHB pharmacokinetics do not contribute to behavioral tolerance; however, changes in neurotransmitter responsiveness may suggest specific neurochemical pathways involved in the development and expression of tolerance.

Alterations in neuronal transport but not blood-brain barrier transport are observed during gamma-hydroxybutyrate (GHB) sedative/hypnotic tolerance.

PURPOSE: To investigate if gamma-Hydroxybutyrate (GHB) tolerance is mediated by alterations in GHB systemic pharmacokinetics, transport (blood brain barrier (BBB) and neuronal) or membrane fluidity. MATERIALS AND METHODS: GHB tolerance in rats was attained by repeated GHB administration (5.31 mmol/kg, s.c., QD for 5 days). GHB sedative/hypnotic effects were measured daily. GHB pharmacokinetics were determined on day 5. In separate groups, on day 6, in situ brain perfusion was performed to assess BBB transport alterations; or in vitro studies were performed (fluorescence polarization measurements of neuronal membrane fluidity or [3H]GABA neuronal accumulation). RESULTS: GHB sedative/hypnotic tolerance was observed by day 5. No significant GHB pharmacokinetic or BBB transport differences were observed between treated and control rats. Neuronal membrane preparations from GHB tolerant rats showed a significant decrease in fluorescence polarization (treated-0.320 +/- 0.009, n = 5; control-0.299 +/- 0.009, n = 5; p < 0.05). [3H]GABA neuronal transport Vmax was significantly increased in tolerant rats (2,110.66 +/- 91.06 pmol/mg protein/min vs control (1,612.68 +/- 176.03 pmol/mg protein/min; n = 7 p < 0.05). CONCLUSIONS: Short term GHB administration at moderate doses results in the development of tolerance which is not due to altered systemic pharmacokinetics or altered BBB transport, but might be due to enhanced membrane rigidity and increased GABA reuptake.

Neuroinflammatory role of prostaglandins during experimental meningitis: evidence suggestive of an in vivo relationship between nitric oxide and prostaglandins.

Nitric oxide (NO) and prostaglandins are inflammatory mediators produced during meningitis. The purpose of the present study was to pharmacologically inhibit cyclooxygenase-2 (COX-2) and inducible NO synthase (iNOS) to 1) explore the prostaglandin contribution to blood-cerebrospinal fluid barrier permeability alterations and 2) elucidate the in vivo concentration relationship between prostaglandin E2 (PGE2) and NO during experimental meningitis. Intracisternal injection of lipopolysaccharides (LPSs, 200 microg) induced neuroinflammation. Rats were dosed with nimesulide (COX-2 inhibitor), aminoguanidine (iNOS inhibitor), or vehicle. Evans blue was used to assess blood-cerebrospinal fluid barrier permeability. Meningeal NO and cerebrospinal fluid PGE2 were assayed using conventional methods. (Results are expressed as mean +/- S.E.M. of 5-9 rats/group.) Nimesulide failed to prevent blood-cerebrospinal fluid barrier disruption [cerebrospinal fluid Evans blue (micrograms per milliliter): control, 0.22 +/- 0.22*; LPS, 11.58 +/- 0.66; LPS + nimesulide, 10.58 +/- 0.86; *p < 0.05; ANOVA]. Although nimesulide decreased PGE2 (picograms per microliter; p < 0.01) in LPS + nimesulide rats (13.9 +/- 1.96) versus LPS + vehicle (73.8 +/- 12.4), meningeal NO production (picomoles/30 min/10(6) cells; p < 0.01) increased unexpectedly in LPS + nimesulide rats (439 +/- 47) versus LPS + vehicle rats (211 +/- 31). In contrast, aminoguanidine inhibited meningeal NO (picomoles/30 min/10(6) cells; p < 0.005) in LPS + aminoguanidine (111 +/- 20) versus LPS (337 +/- 48) but had no effects (p > 0.05) on PGE2. The in vivo relationship between PGE2 and NO was mathematically described by a biphasic, bell-shaped curve (r2 = 0.42; n = 27 rats; p < 0.0001). Based on these results, inhibition of prostaglandin synthesis not only fails to prevent blood-cerebrospinal fluid barrier disruption during neuroinflammation and but also promotes increased meningeal NO production. The in vivo concentration relationship between PGE2 and NO is biphasic, suggesting that inhibition of COX-2 alone may promote NO toxicity through enhanced NO synthesis.