ABSTRACT:Thirty-two structurally diverse drugs used for the treatment of various conditions of the central nervous system (CNS), along with two active metabolites, and eight non-CNS drugs were measured in brain, plasma, and cerebrospinal fluid in the P-glycoprotein (P-gp) knockout mouse model after subcutaneous administration, and the data were compared with corresponding data obtained in wild-type mice. Total brain-to-plasma (B/P) ratios for the CNS agents ranged from 0.060 to 24. Of the 34 CNS-active agents, only 7 demonstrated B/P area under the plasma concentration curve ratios between P-gp knockout and wild-type mice that did not differ significantly from unity. Most of the remaining drugs demonstrated 1.1-to 2.6-fold greater B/P ratios in P-gp knockout mice versus wild-type mice. Three, risperidone, its active metabolite 9-hydroxyrisperidone, and metoclopramide, showed marked differences in B/P ratios between knockout and wild-type mice (6.6-to 17-fold). Differences in B/P ratios and cerebrospinal fluid/ plasma ratios between wild-type and knockout animals were correlated. Through the use of this model, it appears that most CNSactive agents demonstrate at least some P-gp-mediated transport that can affect brain concentrations. However, the impact for the majority of agents is probably minor. The example of risperidone illustrates that even good P-gp substrates can still be clinically useful CNS-active agents. However, for such agents, unbound plasma concentrations may need to be greater than values projected using receptor affinity data to achieve adequate receptor occupancy for effect.Active transport mechanisms as determinants of drug absorption, distribution, and clearance have been the focus of considerable research effort over the past decade. Of the numerous transporter proteins recently investigated, the one for which the greatest amount of knowledge exists is P-glycoprotein (MDR1). Originally described as a transporter involved in imparting drug resistance to tumor cells, P-glycoprotein has been demonstrated to be important in reducing absorption of drugs from the intestinal lumen, in active secretion of drugs into urine and bile, and in extrusion of drugs from vital organs such as the brain and reproductive tissues (Troutman et al., 2002). As such, P-glycoprotein-mediated transport has become an important issue in the discovery and development of new drugs. For example, new compounds that are promising with regard to target receptor/ enzyme activity can be severely hampered in their ability to elicit pharmacological effects in vivo should they be good substrates for P-glycoprotein, especially if the route of administration is intended to be oral or the target tissues is one rich in P-glycoprotein activity. Furthermore, the potential for drug-drug interactions arises in the event that the P-glycoprotein substrate is coadministered with another agent that can inhibit P-glycoprotein.Several models have been developed to assess drugs as P-glycoprotein substrates. In vitro models have included the Caco...

LY-450139 is a ␥-secretase inhibitor shown to have efficacy in multiple cellular and animal models. Paradoxically, robust elevations of plasma amyloid-␤ (A␤) have been reported in dogs and humans after administration of subefficacious doses. The present study sought to further evaluate A␤ responses to LY-450139 in the guinea pig, a nontransgenic model that has an A␤ sequence identical to that of human. Male guinea pigs were treated with LY-450139 (0.2-60 mg/kg), and brain, cerebrospinal fluid, and plasma A␤ levels were characterized at 1, 3, 6, 9, and 14 h postdose. Low doses significantly elevated plasma A␤ levels at early time points, with return to baseline within hours. Higher doses inhibited A␤ levels in all compartments at early time points, but elevated plasma A␤ levels at later time points. To determine whether this phenomenon occurs under steadystate drug exposure, guinea pigs were implanted with subcutaneous minipumps delivering LY-450139 (0.3-30 mg/kg/day) for 5 days. Plasma A␤ was significantly inhibited at 10 -30 mg/kg/day, but significantly elevated at 1 mg/kg/day. To further understand the mechanism of A␤ elevation by LY-450139, H4 cells overexpressing the Swedish mutant of amyloid-precursor protein and a mouse embryonic stem cell-derived neuronal cell line were studied. In both cellular models, elevated levels of secreted A␤ were observed at subefficacious concentrations, whereas dose-responsive inhibition was observed at higher concentrations. These results suggest that LY-450139 modulates the ␥-secretase complex, eliciting A␤ lowering at high concentrations but A␤ elevation at low concentrations.The pathological accumulation of amyloid-␤ peptide into dense core plaques in the brains of Alzheimer's disease patients is the ultimate target of multiple disease-modifying drug discovery efforts. One strategy that has entered the clinic is the use of a ␥-secretase inhibitor to reduce central A␤ production. Preclinically, multiple ␥-secretase inhibitors have demonstrated central and peripheral A␤-lowering activity in transgenic mouse lines overexpressing human mutant amyloid precursor protein (Dovey et al., 2001;Cirrito et al., 2003;Lanz et al., 2003Lanz et al., , 2004Wong et al., 2004;, as well as nontransgenic species (Anderson et al., 2005;Best et al., 2006;El Mouedden et al., 2006). Whereas acute treatment of old, plaque-bearing mice should have little immediate impact on plaque load (insoluble A␤), these inhibitors have been shown to inhibit A␤ in CSF (Lanz et al., 2003;Barten et al., 2005) and interstitial fluid (Cirrito et al., 2003) similarly in both plaque-free and plaque-bearing mice. In addition, plasma A␤ has been shown to be reduced similarly by ␥-secretase inhibition in both young and old Tg2576 mice (Lanz et al., 2003;Barten et al., 2005). These findings indicate that despite the presence or absence of insoluble A␤ plaques, these compounds had similar potency in reducing soluble, secreted A␤ in young and old transgenic mice.The ability of plasma and CSF A␤ to track pharmacologic...

ABSTRACT:The pharmacokinetic properties of drugs may be altered by kinetic deuterium isotope effects. With specifically deuterated model substrates and drugs metabolized by aldehyde oxidase, we demonstrate how knowledge of the enzyme's reaction mechanism, species differences in the role played by other enzymes in a drug's metabolic clearance, and differences in systemic clearance mechanisms are critically important for the pharmacokinetic application of deuterium isotope effects. Ex vivo methods to project the in vivo outcome using deuterated carbazeran and zoniporide with hepatic systems demonstrate the importance of establishing the extent to which other metabolic enzymes contribute to the metabolic clearance mechanism. Differences in pharmacokinetic outcomes in guinea pig and rat, with the same metabolic clearance mechanism, show how species differences in the systemic clearance mechanism can affect the in vivo outcome. Overall, to gain from the application of deuteration as a strategy to alter drug pharmacokinetics, these studies demonstrate the importance of understanding the systemic clearance mechanism and knowing the identity of the metabolic enzymes involved, the extent to which they contribute to metabolic clearance, and the extent to which metabolism contributes to the systemic clearance.

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Atypical antipsychotic treatment has been associated with serious metabolic adverse events, such as glucose dysregulation and development of type 2 diabetes. As part of our studies on possible underlying mechanisms, we investigated the acute effects of various typical and atypical antipsychotics on plasma glucose and insulin in FVB/N mice, a strain that showed a more pronounced hyperglycemic response to clozapine than C57BL/6 and CD-1 mice. Acute administration of high doses of clozapine, olanzapine, quetiapine, perphenazine, or chlorpromazine significantly increased plasma glucose by 100%-140% above basal levels without significant effects on insulin levels. In contrast, risperidone reduced plasma glucose (-30%) and markedly enhanced plasma insulin levels. Doses of ziprasidone that gave 50-fold higher free plasma concentrations than therapeutic plasma levels, as well as high doses of aripiprazole and haloperidol, did not significantly alter either glucose or insulin levels. Clozapine- and olanzapine-induced hyperglycemia occurred at free plasma concentrations that were within, or one order of magnitude above, the range of therapeutic plasma levels. Pretreatment with either the ganglionic blocker hexamethonium, or the alpha(2) adrenergic receptor antagonist yohimbine, blocked the clozapine- and chlorpromazine-induced increase in glucose levels. Taken together, these results suggest that typical and atypical antipsychotics with known metabolic liability produce acute hyperglycemia in mice and that this effect is likely driven by activation of the sympathetic autonomic nervous system via a central mechanism.