Simultaneous determination of GTS-21 and its metabolite in rat plasma by high-performance liquid chromatography using solid-phase extraction

Hydroxy metabolites of the Alzheimer's drug candidate 3-[(2,4-dimethoxy)benzylidene]-anabaseine dihydrochloride (GTS-21): their molecular properties, interactions with brain nicotinic receptors, and brain penetration

3-[(2,4-dimethoxy)benzylidene]-anabaseine dihydrochloride (DMXBA; GTS-21), an Alzheimer's drug candidate, selectively stimulates alpha7 nicotinic acetylcholine receptors. It rapidly enters the brain after oral administration and enhances cognitive behavior. Less than 1% of orally administered DMXBA is recovered in the urine. We report the identification and characterization of the major phase I metabolites of this drug candidate. Three hydroxy metabolites were generated in vitro by hepatic microsomal O-dealkylation of the two methoxy substituents on the benzylidene ring. They were also found in plasma of rats after oral administration, but at significantly lower concentrations relative to the parent compound. The metabolites displayed similar binding affinities and partial agonist potencies at rat brain alpha7 receptors. However, each displayed a higher efficacy than DMXBA for stimulating rat and human alpha7 receptors. Like DMXBA, the metabolites were weak antagonists at alpha4beta2 receptors. The predicted conformations of the metabolites were nearly identical with that of DMXBA. Ionization of the tetrahydropyridyl nitrogen was essential for high-affinity binding of DMXBA to the alpha7 receptor. The hydroxy metabolites were much more polar than DMXBA, derived from their experimentally estimated octanol/water partition coefficients, and they entered the brain much less readily than DMXBA. Their contributions to the behavioral effects of orally administered DMXBA, if any, would probably be very small during short-term administration. Benzylidene anabaseines pharmacologically similar to the hydroxy metabolites, but which enter the brain more readily, may provide greater stimulation of alpha7 receptors in the whole organism.

Safety, pharmacokinetics, and effects on cognitive function of multiple doses of GTS-21 in healthy, male volunteers

This study was designed to determine the safety, tolerability, pharmacokinetics and effects on cognitive function of GTS-21 in healthy, male volunteers. A total of 18 subjects were randomized to GTS-21 (25, 75 and 150 mg) or placebo administered three times daily (first 4 days, once on Day 5) for three, 5-day sessions. GTS-21 was well tolerated up to doses of 450 mg/day, with no clinically significant safety findings. C(max) and the area under the plasma concentration of GTS-21 and the metabolite 4-OH-GTS-21 increased in a dose-related fashion; although considerable intersubject variability occurred, it decreased with continued dosing. GTS-21 showed statistically significant enhancement of three measures of cognitive function (attention, working memory, episodic secondary memory) compared to placebo. A relationship between exposure to GTS-21 and the magnitude of the cognitive response was apparent, with maximal effect approached for doses between 75 and 150 mg three times a day. These data indicate that GTS-21 may represent a novel treatment for dementia.

Prediction of plasma concentration of GTS-21 in hairless rats following monolithic transdermal delivery

The transdermal therapeutic systems (TTS) usually achieve constant plasma concentration for an extended period of time. This is because a sufficient drug stored in the device can keep the constant concentration on the surface of the stratum corneum during the system application. When the drug molecules are not enough to provide the constant surface concentration, the rate of drug penetration decreases with time because of decreased supply of the drug molecules from the delivery device. This paper has proposed an empirical simple approach to predict the plasma concentration for such a TTS. A novel compound, GTS-21, for Alzheimers' disease currently under development was used as a model drug. In vivo and in vitro experiments were carried out in hairless rats. The in vivo plasma concentration-time profile in hairless rats following the application of TTS well agreed with the predicted profile based on the skin pharmacokinetic model together with the model parameters determined from the in vitro experiment.

Comparative Analysis of In Vitro and In Vivo Pharmacokinetic Parameters Related to Individual Variability of GTS-21 in Canine

To clarify the cause of the canine individual variability in plasma concentration after oral administration of GTS-21, we evaluated in vitro the metabolism to 4-OH-GTS-21 in liver microsomes of the same individuals from in vivo pharmacokinetic study. First, we applied to the Michaelis-Menten kinetic parameters to a dispersion model, and compared hepatic availability (F(H)) and hepatic clearance (CL(H)) values from in vitro with bioavailability (F), hepatic plasma flow (Q(PH)), and plasma clearance (CL(P)) values from in vivo. The ratios of CL(H) to Q(PH) were ranged 0.74 to 0.94, suggesting that GTS-21 is a hepatic plasma flow-limiting drug. A significant correlation of F(H) and F in the four dogs (r=0.995, p=0.005) indicates that the variability is predominantly caused by GTS-21 O4-demethylase activity. Second, we specified the cytochrome P450 (CYP) enzymes that are involved with the metabolism by chemical inhibition. alpha-Naphthoflavone, furafylline, quinidine, quinine, and troleandomycin significantly inhibited GTS-21 O4-demethylase activity. Thus CYP1A, CYP2D15, and CYP3A12 were involved with O4-demethylation. The variability in control activity decreased on addition of alpha-naphthoflavone and furafylline. Third, we quantified the contents of CYP1A and CYP3A12 by enzyme-linked immunosorbent assay. The content of CYP1A was consistent with GTS-21 O4-demethylase activity. We concluded that canine liver CYP1A causes the individual variability in GTS-21 plasma concentration after oral administration.

First-pass of GTS-21 on canine gut wall and liver determined by portal-systemic concentration difference

To clarify the cause of the canine individual variability of plasma concentration after oral administration of GTS-21 [(E)-3-(2,4-dimethoxybenzylidene)-3,4,5,6-tetra-hydro-2,3'-bipyridine dihydrochloride], we evaluated the absorption ratio (F(A)), intestinal availability (F(G)), and hepatic availability (F(H)). The bioavailability (F) was evaluated from the ratio of the area under the plasma concentration versus time curves after oral and intravenous administration. Three isoflurane anaesthetised dogs were fitted with an electromagnetic flow probe attached to the portal vein and cannulated through the portal and the femoral veins. After intraduodenal administration of GTS-21, both plasma concentrations were determined simultaneously. F(A) x F(G) was calculated from the portal-systemic concentration difference taking into consideration the blood-plasma partition ratio. F(A) was calculated from the residual drug contents of the small intestine. F(H) was calculated by dividing F by F(A) x F(G). The F values were 0.072, 0.021, and 0.037, indicating an individual variability of ca. threefold. The F(A) values were close to 1, and the F(G) values ranged from 0.449 to 0.461. Accordingly, the F(H) values were estimated at 0.170, 0.047, and 0.083. GTS-21 was completely absorbed but lost by first-pass effects of passage through the gut wall and liver. The first-pass effect of liver is larger than that of the gut wall, and dominates the individual variability in plasma concentration.

The brain alpha7 nicotinic receptor may be an important therapeutic target for the treatment of Alzheimer's disease: studies with DMXBA (GTS-21)

A large decrease in brain nicotinic receptor levels occurs in Alzheimer's disease, relative to muscarinic and other receptors. Neurons possessing high affinity nicotinic receptors seem particularly vulnerable. The low affinity nicotinic receptors which selectively bind alpha-bungarotoxin are not significantly affected. The major nicotinic receptor subtype which binds this toxin is a homo-oligomer composed of alpha7 subunits. Due to its exceptionally high calcium ion selectivity, this particular receptor can be considered as a ligand-gated calcium channel. Alpha7 receptors are found in regions of the brain which are important for cognition, including cerebral cortex and hippocampus. Hippocampal receptors are largely confined to GABAergic interneurons. Alpha7 receptors seem less likely than alpha4-beta2 receptors to be up-regulated in number and down-regulated in function as a result of chronic agonist exposure. A family of nicotinic agonists based upon the marine animal toxin anabaseine have been synthesized and investigated. One of these compounds, DMXBA [3-(2,4-dimethoxybenzylidene)-anabaseine; code name GTS-21] has displayed promising characteristics during phase I clinical tests. In the rat DMXBA is selectively agonistic upon alpha7 nicotinic receptors. In addition it is a moderately potent antagonist at alpha4-beta2 receptors. DMXBA enhances a variety of cognitive behaviors in mice, monkeys, rats and rabbits. It also displays neuroprotective activity upon cultured neuronal cells exposed to beta-amyloid or deprived of NGF. The compound is much less toxic than nicotine and does not affect autonomic and skeletal muscle systems at doses which enhance cognitive behavior. Phase I clinical tests indicate that large doses can be safely administered orally without adverse effects. Psychological tests on healthy young male subjects indicate a positive effect of DMXBA on some measures of cognition. While DMXBA is a much weaker partial agonist on human alpha7 receptors than upon rat alpha7 receptors, its 4-hydroxy metabolite has been shown to have excellent efficacy on both receptors. Thus, some of the physiological and behavioral effects of GTS-21 may be due to the actions of this primary metabolite.

Metabolism and disposition of GTS-21, a novel drug for Alzheimer's disease

1. GTS-21, a novel drug for Alzheimer's disease, is currently under clinical development. In the current study, the metabolism and disposition of GTS-21 have been evaluated in rat and dog after single oral and intravenous administration. 2. Following oral administration of [14C]GTS-21 to rat, radioactivity was primarily excreted in the faeces (67%) via the bile with possible enterohepatic circulation. Urinary excretion of radioactivity in rat and dog was 20 and 19% respectively. 3. GTS-21 was rapidly and extensively absorbed after oral administration and rapidly cleared from plasma. The maximum concentration ratio of GTS-21 to total radioactivity in plasma was low, indicating first-pass or pre-systemic biotransformation. 4. In rat, GTS-21 showed linear pharmacokinetics over doses ranging from 1 to 10 mg/kg with an absolute bioavailability of 23%. In dog, the absolute bioavailability was 27% at an oral dose of 3 mg/kg. 5. GTS-21 was O-demethylated to yield compounds that were then subject to glucuronidation. Three of the metabolites in rat urine were isolated and characterized as 4-OH-GTS-21, 4-OH-GTS-21 glucuronide and 2-OH-GTS-21 glucuronide. The major urinary metabolites were 4-OH-GTS-21 glucuronide and 2-OH-GTS-21 glucuronide. 6. In vitro chemical inhibition of cytochrome P450 in human liver microsomes indicated that CYPIA2 and CYP2E1 were the isoforms primarily responsible for the O-demethylation of GTS-21, with some contribution from CYP3A.

Simultaneous determination of a novel anticancer drug, TAS-103, and its N-demethylated metabolite in monkey plasma by high-performance liquid chromatography using solid-phase extraction

A simple and rapid method for the analysis of a novel anticancer drug, TAS-103, and its metabolite demethyl-TAS-103 in monkey plasma has been developed. This method is based on high-performance liquid chromatography with visible detection at 460 nm after solid-phase extraction with a Sep-Pak Vac PS-2 cartridge. The extraction recoveries of each compound, including the internal standard TAS-1-1018, were from 88 to 102%. The quantitation limit of each compound was 5.0 ng/ml in 0.5 ml of plasma. The coefficients of variation for each compound ranged from 0.9 to 4.9%, and relative errors for each compound ranged from -3.8 to 4.6%. Both compounds in monkey plasma were stable at -80 degrees C for 39 days and the extracts were stable at ambient temperature for 24 h. This method has been demonstrated to be useful for the pharmacokinetic study of TAS-103 in monkey plasma after intravenous administration.