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Purpose: To measure fasted and fed gastric pH and gastric residence time (GRT) in Cynomolgus monkeys using Bravo radiotelemetry capsules., Methods: Continuous pH measurements were recorded with Bravo capsules, which were either attached to the monkeys' stomach or administered as free capsules. Meals (either slurry or standard), were administered at designated times with monkeys chair-restrained during slurry meal ingestion., Results: From the attached capsule studies, the fasted gastric pH (~1.9-2.2) was consistent among monkeys. Under fasted conditions, pH spikes were infrequently observed (once every 7.9 min to 3.6 h) with peaks reaching pH 9.4 and having short durations (<1 min). After feeding, the gastric pH rose quickly and remained alkaline for approximately 4.5-7.5 h before returning to baseline. Although significantly different (p < 0.05), there was overlap between the fasted (153 +/- 87 min) and fed (436 +/- 265 (slurry) and 697 +/- 193 (standard) min) GRT due to considerable inter- and intra-subject variability., Conclusions: Fasted gastric pH was similar between monkeys and literature human values. After a meal, the monkey gastric pH was elevated for a longer duration than that in human. The monkey GRT appears longer than that observed in human under both fasted and fed conditions, although this is likely dependent on the Bravo capsule size.

The purpose of this study was to measure the in vivo brain distribution of antihistamines and assess the influence of in vitro permeability, P-glycoprotein (Pgp) efflux, and plasma protein binding. Six antihistamines (acrivastine, chlorpheniramine, diphenhydramine doxylamine, fexofenadine, terfenadine) were selected based on previously reported in vitro permeability and Pgp efflux properties and dosed intravenously to steady-state plasma concentrations of 2-10 micromol/l in rats. Plasma and brain concentrations were measured by LC/MS/MS, and protein binding determined by ultrafiltration. Doxylamine, diphenhydramine and chlorpheniramine had brain-to-plasma concentration ratios of 4.34 +/- 1.26, 18.4 +/- 2.35 and 34.0 +/- 9.02, respectively. These drugs had high passive membrane permeability (>310 nm/s), moderate protein binding (71-84%) and were not Pgp substrates; features that yield high CNS penetration. In contrast, acrivastine and fexofenadine had low brain-to-plasma ratios of 0.072 +/- 0.014 and 0.018 + 0.002, consistent with low passive membrane permeability for both compounds (16.2 and 66 nm/s, respectively) and Pgp efflux. Finally, terfenadine had a brain-to-plasma ratio of 2.21 +/- 1.00 even though it underwent Pgp-mediated efflux (in vitro ratio = 2.88). Terfenadine's high passive permeability (285 nm/s) overcame the Pgp-mediated efflux to yield brain-to-plasma ratio >1. The brain-to-unbound plasma ratio was 22-fold higher suggesting that protein binding (96.3% bound) limited terfenadine's brain distribution. In conclusion, passive membrane permeability, Pgp-mediated efflux and/or high plasma protein binding influence the in vivo brain distribution of antihistamine drugs.

This study was conducted to compare the ability of two potential microdialysis perfusates to enhance the recovery of SB-265123, a lipophilic, highly protein-bound compound, both in vitro and in vivo. Initial in vitro experiments established that the recovery of SB-265123 by microdialysis using normal saline as a perfusate was poor (1.7%). Different concentrations of Intralipid and Encapsin also were evaluated in an identical in vitro setting, to determine enhancement of recovery. In vitro recovery was enhanced to approximately 24 and 65% with 5 and 20% Intralipid, and to approximately 59 and 62% with 5 and 20% Encapsin, respectively. A rat in vivo study was conducted with 20% Encapsin to confirm the in vitro observations. In the in vivo study, 75-80% recovery of free SB-265123 was achieved using 20% Encapsin as a perfusate. The results from this study indicate that for SB-265123, a lipophilic, highly protein-bound molecule, Encapsin is an efficient recovery enhancer in vitro. The results from this investigation further demonstrate that a recovery enhancer may be useful for in vivo applications, even with a compound that is highly bound to plasma protein., (Copyright 2002 John Wiley & Sons, Ltd.)

Membrane permeability and P-glycoprotein (Pgp) can be limiting factors for blood-brain barrier penetration. The objectives of this study were to determine whether there are differences in the in vitro permeability, Pgp substrate profiles, and physicochemical properties of drugs for central nervous system (CNS) and non-CNS indications, and whether these differences are useful criteria in selecting compounds for drug development. Apparent permeability (P(app)) and Pgp substrate profiles for 93 CNS (n = 48) and non-CNS (n = 45) drugs were determined by monolayer efflux. Calcein-AM inhibition assays were used to supplement the efflux results. The CNS set (2 of 48, 4.2%) had a 7-fold lower incidence of passive permeability values <150 nm/s compared with the non-CNS set (13 of 45, 28.9%). The majority of drugs (72.0%, 67 of 93) were not Pgp substrates; however, 49.5% (46 of 93) were positive in the calcein-AM assay when tested at 100 microM. The CNS drug set (n = 7 of 48, 14.6%) had a 3-fold lower incidence of Pgp-mediated efflux than the non-CNS drug set (n = 19 of 45, 42.2%). Analysis of 18 physicochemical properties revealed that the CNS drug set had fewer hydrogen bond donors, fewer positive charges, greater lipophilicity, lower polar surface area, and reduced flexibility compared with the non-CNS group (p < 0.05), properties that enhance membrane permeability. This study on a large, diverse set of marketed compounds clearly demonstrates that permeability, Pgp-mediated efflux, and certain physicochemical properties are factors that differentiate CNS and non-CNS drugs. For CNS delivery, a drug should ideally have an in vitro passive permeability >150 nm/s and not be a good (B --> A/A --> B ratio <2.5) Pgp substrate.

Pharmacokinetic/pharmacodynamic simulations were performed to assess the feasibility of central nervous system (CNS) drug delivery via endogenous transporters resident at the blood-brain barrier (BBB). Pharmacokinetic models were derived for intravenous bolus dosing of a hypothetical drug in the absence or presence of an endogenous, competing transport inhibitor. These models were linked to CNS pharmacodynamic models where the effect sites were either cell surface receptors or intracellularly localized enzymes. The response of the dependent parameter, the duration of effect (t(dur)), was examined in relationship to changes in the independent parameters, i.e. dose, elimination rate constant (k(e1)), BBB transport parameters (K(m1) and V(max1)) and EC(50) (effective concentration that elicits a 50% response). As expected, t(dur) increased with (a) increases in drug doses, (b) decreases in k(e1) or (c) decreases in EC(50), irrespective of the effect site. Surprisingly, endogenous transport inhibition produced decreases in drug terminal half-life and corresponding decreases in t(dur). Interestingly, t(dur) was independent of assigned transporter K(m) and V(max) when the dose/EC(50) ratio (dose/EC(50)) was >1 (irrespective of endogenous transport inhibition), but highly dependent on K(m1) and V(max1) when dose/EC(50) was (a) <1 (no endogenous transport inhibition) or (b) equal to 1 (with endogenous transport inhibition). Oral input of the endogenous transport inhibitor produced a decrease in t(dur) when the dose/EC(50) range was 0.1-10. These simulations highlight that (a) systemic pharmacokinetic and BBB transport parameters influence t(dur), (b) drug terminal half-life is inversely related to circulating levels of endogenous inhibitors, and (c) oral ingestion of endogenous transport inhibitor(s) reduces t(dur). Overall, these simulations provide insight for the feasibility of rational CNS drug design/delivery via endogenous transporters., (Copyright 2000 John Wiley & Sons, Ltd.)

Blood-brain barrier (BBB) transport of essential polar substrates is mediated by specific, carrier-mediated transport proteins. The BBB transport mechanisms for polar compounds with terminal guanidino functional groups (R-NHC(NH)NH(2)) are not well defined. The goal of the present work was to investigate the BBB transport mechanism(s) for terminal guanidino substrates using an in situ brain perfusion technique. Brain region radiotracer influx clearance (Cl(in)) was calculated for representative guanidino substrates, [14C]L-arginine, [14C]aminoguanidine and [14C]guanidine, in the presence or absence of excess terminal guanidino analogues. The Cl(in) for [14C]L-arginine (0.21+/-0.0094 cm(3)/min/g wet brain weight, mean+/-S.E.M., n=four rats) was significantly decreased by 1000x concentrations of unlabeled L-arginine, N(G)-methyl-L-arginine, N(G)-,N(G)-dimethyl-L-arginine and N(G)-amino-L-arginine by approximately 83% (P<0.01; n=4-5), whereas 1000x concentrations of nitro-L-arginine, aminoguanidine and guanidine were without effect. In contrast, the respective Cl(in) of [14C]aminoguanidine and [14C]guanidine (0.0085+/-0.00039 and 0.015+/-0.0015 cm(3)/min/g, n=4, respectively) were not significantly decreased by 1000x concentrations of unlabeled aminoguanidine or guanidine. The Cl(in) values for all [14C]guanidino probes were significantly greater (P<0. 05) from that of [3H]inulin, a marker of cerebrovascular blood volume. These data suggest that the hydrophilic guanidino cations aminoguanidine and guanidine penetrate the BBB by a minor diffusional process with no appreciable transport via saturable processes. In contrast, BBB penetration of L-arginine occurs via the saturable basic amino acid transporter that has specificity for amino acid analogues possessing cationic terminal guanidino groups.

Aminoguanidine has potential pharmacologic utility for diabetes and nitric oxide - mediated inflammation. Because aminoguanidine is positively charged at physiologic pH (pK(a) approximately 10), it is unlikely that simple diffusion is a predominant mechanism for cellular penetration. This study sought to determine the transport processes by which aminoguanidine, a cationic compound, traverses across cellular membranes. In cultured opossum kidney (OK) cell monolayers, aminoguanidine transport involved both saturable and non-saturable diffusion processes. At passage numbers below 67, the observed V(max) and K(m) for saturable influx were significantly lower than that observed at passages greater than 79 (V(max): low passage, 21.2+/-7.8 pmol/(min*mg protein), n=3; versus high passage, 129.7+/-24.3 pmol/(min*mg protein), n=3, P<0.05; K(m): low passage, 23.7+/-10.8 microM, n=3; versus high passage, 101.7+/-5.6 microM, n=3, P<0.05; mean+/-S.E.M.). Nonsaturable processes were not statistically different (k(ns): low passage, 1.6+/-0.1 pmol/(min*mg protein*microM), n=3; high passage, 1.1+/-0.2 pmol/(min*mg protein*microM) n=3). Saturable influx was temperature dependent, and independent of ATP energy, sodium gradients or changes in membrane potential. Other organic cations competitively inhibited and trans-stimulated saturable influx. Aminoguanidine influx was increased in the presence of an outwardly-directed proton gradient and was inhibited in the presence of an inwardly-directed proton gradient. Correspondingly, aminoguanidine efflux was trans79) express a saturable, bi-directional carrier-mediated process to transport aminoguanidine across cellular membranes.