Pharmacokinetics and bioavailability of midazolam after intravenous, subcutaneous, intraperitoneal and oral administration under a chronic food-limited regimen: relating DRL performance to pharmacokinetics

Preclinical Bioavailability Assessment of a Poorly Water-Soluble Drug, HGR4113, Using a Stable Isotope Tracer

Drug solubility limits intravenous dosing for poorly water-soluble medicines, which misrepresents their bioavailability estimation. The current study explored a method using a stable isotope tracer to assess the bioavailability of drugs that are poorly water-soluble. HGR4113 and its deuterated analog, HGR4113-d7, were tested as model drugs. To determine the level of HGR4113 and HGR4113-d7 in rat plasma, a bioanalytical method using LC-MS/MS was developed. The HGR4113-d7 was intravenously administered to rats that were orally pre-administered HGR4113 at different doses; subsequently, the plasma samples were collected. HGR4113 and HGR4113-d7 were simultaneously determined in the plasma samples, and bioavailability was calculated using plasma drug concentration values. The bioavailability of HGR4113 was 53.3% ± 19.5%, 56.9% ± 14.0%, and 67.8% ± 16.7% after oral dosages of 40, 80, and 160 mg/kg, respectively. By eliminating the differences in clearance between intravenous and oral dosages at different levels, acquired data showed that the current method reduced measurement errors in bioavailability when compared to the conventional approach. The present study suggests a prominent method for evaluating the bioavailability of drugs with poor aqueous solubility in preclinical studies.

Bumetanide potentiates the anti-seizure and disease-modifying effects of midazolam in a noninvasive rat model of term birth asphyxia

Birth asphyxia and the resulting hypoxic-ischemic encephalopathy (HIE) are highly associated with perinatal and neonatal death, neonatal seizures, and an adverse later-life outcome. Currently used drugs, including phenobarbital and midazolam, have limited efficacy to suppress neonatal seizures. There is a medical need to develop new therapies that not only suppress neonatal seizures but also prevent later-life consequences. We have previously shown that the loop diuretic bumetanide does not potentiate the effects of phenobarbital in a rat model of birth asphyxia. Here we compared the effects of bumetanide (0.3 or 10 mg/kg i.p.), midazolam (1 mg/kg i.p.), and a combination of bumetanide and midazolam on neonatal seizures and later-life outcomes in this model. While bumetanide at either dose was ineffective when administered alone, the higher dose of bumetanide markedly potentiated midazolam's effect on neonatal seizures. Median bumetanide brain levels (0.47-0.53 µM) obtained with the higher dose were in the range known to inhibit the Na-K-Cl-cotransporter NKCC1 but it remains to be determined whether brain NKCC1 inhibition was underlying the potentiation of midazolam. When behavioral and cognitive alterations were examined over three months after asphyxia, treatment with the bumetanide/midazolam combination, but not with bumetanide or midazolam alone, prevented impairment of learning and memory. Furthermore, the combination prevented the loss of neurons in the dentate hilus and aberrant mossy fiber sprouting in the CA3a area of the hippocampus. The molecular mechanisms that explain that bumetanide potentiates midazolam but not phenobarbital in the rat model of birth asphyxia remain to be determined.

The loop diuretic torasemide but not azosemide potentiates the anti-seizure and disease-modifying effects of midazolam in a rat model of birth asphyxia

Loop diuretics such as furosemide and bumetanide, which act by inhibiting the Na-K-2Cl cotransporter NKCC2 at the thick ascending limb of the loop of Henle, have been shown to exert anti-seizure effects. However, the exact mechanism of this effect is not known. For bumetanide, it has been suggested that inhibition of the NKCC isoform NKCC1 in the membrane of brain neurons may be involved; however, NKCC1 is expressed by virtually all cell types in the brain, which makes any specific targeting of neuronal NKCC1 by bumetanide impossible. In addition, bumetanide only poorly penetrates the brain. We have previously shown that loop diuretics azosemide and torasemide also potently inhibit NKCC1. In contrast to bumetanide and furosemide, azosemide and torasemide lack a carboxylic group, which should allow them to better penetrate through biomembranes by passive diffusion. Because of the urgent medical need to develop new treatments for neonatal seizures and their adverse outcome, we evaluated the effects of azosemide and torasemide, administered alone or in combination with phenobarbital or midazolam, in a rat model of birth asphyxia and neonatal seizures. Neither diuretic suppressed the seizures when administered alone but torasemide potentiated the anti-seizure effect of midazolam. Brain levels of torasemide were below those needed to inhibit NKCC1. In addition to suppressing seizures, the combination of torasemide and midazolam, but not midazolam alone, prevented the cognitive impairment of the post-asphyxial rats at 3 months after asphyxia. Furthermore, aberrant mossy fiber sprouting in the hippocampus was more effectively prevented by the combination. We assume that either an effect on NKCC1 at the blood-brain barrier and/or cells in the periphery or the NKCC2-mediated diuretic effect of torasemide are involved in the present findings. Our data suggest that torasemide may be a useful option for improving the treatment of neonatal seizures and their adverse outcome.

Effects of Chronic Oral Administration of Midazolam on Memory and Circadian Rhythms in Rats

Studies have shown the ability of benzodiazepine drugs to cause memory loss in animals and humans. Midazolam is a benzodiazepine commonly administered intravenously during surgical procedures because it reacts rapidly, causes anterograde amnesia, and has few side effects. It has also been used in palliative medicine where, among others, an oral route has been employed for chronic administration of the drug. The current study evaluated the effects of chronic orally administered midazolam on spatial working memory and procedural memory in control and experimental female rats over a three-week experimental period utilizing the Morris water maze. Sample and test run times to a submerged platform in the maze were recorded daily. In addition, activity wheels attached to each cage were employed to monitor daily circadian activity of the animals. Spatial working memory was not impaired in either group. However, procedural memory amnesia occurred in animals receiving the drug indicative of a consolidation or retrieval problem. Concerning circadian rhythms, a phase-shift was noted in experimental animals possibly indicating that time of day of drug administration is important. The findings of the present study could shed insight into altered reactions observed in humans who have received midazolam as a component of treatment in palliative medicine.

Brain pharmacokinetics and metabolism of the AMP-activated protein kinase selective inhibitor SBI-0206965, an investigational agent for the treatment of glioblastoma

PURPOSE

Emerging evidence suggests that 5' Adenosine Monophosphate-Activated Protein Kinase (AMPK), a key regulator of cellular bioenergetics, is a novel target for the treatment of glioblastoma (GBM), a lethal brain tumor. SBI-0206965, an aminopyrimidine derivative, is a potent AMPK inhibitor being investigated for the treatment of GBM. Here we characterized the systemic and brain pharmacokinetics (PK) and hepatic metabolism of SBI-0206965.

METHODS

We performed intracerebral microdialysis to determine brain partitioning of SBI-0206965 in jugular vein cannulated rats. We assessed systemic PK of SBI-0206965 in rats and C57BL/6 mice following oral administration. Employing human, mouse, and rat liver microsomes we characterized the metabolism of SBI-0206965.

RESULTS

SBI-0206965 is quickly absorbed, achieving plasma and brain extracellular fluid (ECF) peak levels within 0.25 - 0.65 h. Based on the ratio of C_max and AUC in brain ECF to plasma (corrected for protein binding), brain partitioning is ~ 0.6-0.9 in rats. However, the compound has a short elimination half-life (1-2 h) and low relative oral bioavailability (~ 0.15). The estimated in-vitro hepatic intrinsic clearance of SBI-0206965 in mouse, rat and human was 325, 76 and 68 mL/min/kg, respectively. SBI-0206965 metabolites included desmethylated products, and the metabolism was strongly inhibited by ketoconazole, a CYP3A inhibitor.

CONCLUSION

SBI-0206965 has adequate brain permeability but low relative oral bioavailability which may be due to rapid hepatic metabolism, likely catalyzed by CYP3A enzymes. Our observations will facilitate further development of SBI-0206965, and/or other structurally related molecules, for the treatment of GBM and other brain tumors.

Benzodiazepines in the Management of Seizures and Status Epilepticus: A Review of Routes of Delivery, Pharmacokinetics, Efficacy, and Tolerability

Status epilepticus (SE) is an acute, life-threatening medical condition that requires immediate, effective therapy. Therefore, the acute care of prolonged seizures and SE is a constant challenge for healthcare professionals, in both the pre-hospital and the in-hospital settings. Benzodiazepines (BZDs) are the first-line treatment for SE worldwide due to their efficacy, tolerability, and rapid onset of action. Although all BZDs act as allosteric modulators at the inhibitory gamma-aminobutyric acid (GABA)_A receptor, the individual agents have different efficacy profiles and pharmacokinetic and pharmacodynamic properties, some of which differ significantly. The conventional BZDs clonazepam, diazepam, lorazepam and midazolam differ mainly in their durations of action and available routes of administration. In addition to the common intravenous, intramuscular and rectal administrations that have long been established in the acute treatment of SE, other administration routes for BZDs-such as intranasal administration-have been developed in recent years, with some preparations already commercially available. Most recently, the intrapulmonary administration of BZDs via an inhaler has been investigated. This narrative review provides an overview of the current knowledge on the efficacy and tolerability of different BZDs, with a focus on different routes of administration and therapeutic specificities for different patient groups, and offers an outlook on potential future drug developments for the treatment of prolonged seizures and SE.

Pharmacokinetic Comparison of Three Different Administration Routes for Topotecan Hydrochloride in Rats

Topotecan is actively used in clinic, with its primary use being in treatment of various types of cancer. The approved administration routes are oral and intravenous. The purpose of this study was to investigate and identify pharmacokinetic profiles of different administration routes. We conducted pharmacokinetic studies on three different routes of administration in rats. Five rats in each group received a single dose of 4 mg/kg of topotecan hydrochloride intravenously, orally, or subcutaneously, and the concentrations of lactone and total forms of the drug in plasma, urine, and feces were quantified. Various pharmacokinetic parameters were compared statistically. Plasma concentrations of both the lactone and total forms at elimination phase following subcutaneous administration, were two times higher than was seen with oral administration and 10 times higher than with intravenous administration. Subcutaneous administration of topotecan showed pharmacokinetic profiles similar to sustained release. In addition, subcutaneous administration showed bioavailability from 88.05% (for lactone form) to 99.75% (for total form), and these values were four–five times greater than those of oral administration. The results of this non-clinical study will not only provide greater understanding of the in vivo pharmacokinetics of topotecan, but also be useful for development of additional formulations and/or administration routes.

Midazolam impairs acquisition and retrieval, but not consolidation of reference memory in the Morris water maze

Amnesia is one of the most discussed properties of the benzodiazepine class of drugs. The effects of benzodiazepines on human memory are usually anterograde, while changes in retrograde memory functions were seldom reported. Such inconsistent findings have prompted numerous animal studies investigating the influences of these positive modulators of inhibitory neurotransmission on different stages of memory. Among the benzodiazepines, memory effects of midazolam are of special interest due to its many and varied clinical applications. The present Morris water maze study in adult male Wistar rats was performed in three experiments in which midazolam was administered at doses of 0.5, 1 and 2 mg/kg intraperitoneally, before or immediately after each of five daily learning sessions, with two trials in a session, as well as before the probe test. Midazolam impaired acquisition and subsequent retention of spatial learning of the position of the hidden platform even at a pre-training dose of 0.5 mg/kg. This low dose was not associated with impairment of the procedural component of learning, manifested by increased time spent in the periphery of the pool. The lack of midazolam effect on consolidation has not been confounded by the observed below-chance performance of the control group since our additional experiment using diazepam also administered immediately after each of five learning sessions has revealed a similar pattern of results. Finally, midazolam administered before the probe test impaired retrieval of reference memory at all tested doses. Hence, induction of retrograde, besides anterograde amnesia should be kept in mind as a possibility when midazolam is used in clinical settings.

The effects of intramuscular and intraperitoneal injections of benzo[a]pyrene on selected biomarkers in Clarias gariepinus

This study investigated the dose-dependent and time-course effects of intramuscular (i.m.) and intraperitoneal (i.p.) injection of benzo[a]pyrene (BaP) on the biomarkers EROD activity, GST activity, concentrations of BaP metabolites in bile, and visceral fat deposits (Lipid Somatic Index, LSI) in African catfish (Clarias gariepinus). Intraperitoneal injection resulted in 4.5 times higher accumulation of total selected biliary FACs than i.m. injection. Hepatic GST activities were inhibited by BaP via both injection methods. Dose-response relationships between BaP injection and both biliary FAC concentrations and hepatic GST activities were linear in the i.p. injected group but nonlinear in the i.m. injected fish. Hepatic EROD activity and LSI were not significantly affected by BaP exposure by either injection route. We conclude that i.p. is a more effective route of exposure than i.m. for future ecotoxicological studies of PAH exposure in C. gariepinus.

Stress-induced hyperthermia is reduced by rapid-acting anxiolytic drugs independent of injection stress in rats

BACKGROUND

Stress-induced hyperthermia (SIH) is the transient rise in body temperature after encountering a stressor. The SIH response can be blocked by administration of various anxiolytic drugs prior to inducing stress. However, a drug injection involves handling and injection stress and therefore induces a SIH response itself. In the standard SIH test, drugs are therefore injected 60 min before stress induction to allow injection-induced hyperthermia to decline. This makes it difficult to study putative anxiolytic compounds with a short half-life. The present study therefore aimed to compare the effects of standard (stressful) and stress-free anxiolytic drug administration on the subsequent SIH response with a 10-minute injection-stressor interval.

METHODS

Anxiolytic drugs with short half-lives (midazolam, 8-OH-DPAT, nicotine) were injected subcutaneously in rats using either a stressful (manual injection) or stress-free injection (subcutaneous cannula) method 10 min before novel cage stress. Body temperature and locomotor activity were measured using telemetric transmitters.

RESULTS

Stressful and stress-free drug administration resulted in comparable drug effects on the stress-induced hyperthermia and locomotor responses in rats.

CONCLUSION

The present study shows that both stressful and stress-free drug injection shortly before a stressor results in reproducible attenuation of the SIH response in rats. In rats, a short injection-stressor interval can therefore be applied using the SIH model, enabling the study of putative anxiolytic drugs with short half-lives.

Asymmetric Intestinal First-Pass Metabolism Causes Minimal Oral Bioavailability of Midazolam in Cynomolgus Monkey

Oral bioavailability of some drugs is substantially lower in cynomolgus monkeys than in various other species, including humans. In the present study, midazolam was used as a model drug to investigate the reason for the lower bioavailability in these monkeys. The bioavailability of midazolam after oral administration was minimal in monkeys and rats, being only 2.1 and 1.1%, respectively. In monkeys, this low bioavailability could not be explained simply in terms of a hepatic first-pass effect. To examine the roles of intestinal metabolism and transport, we evaluated apical-to-basal and basal-to-apical transport of midazolam, and the formation of metabolites in small intestinal tissues using an Ussing-type chamber. The values of mucosal extraction ratio were estimated to be 0.97, 0.93, and 0.89 during apical-to-basal transport in the upper, middle, and lower small intestine of monkeys, respectively, whereas the corresponding values for rats were close to zero, indicating that extensive metabolism of midazolam occurs, particularly in the upper region of the small intestine in monkeys, but not rats. Interestingly, formation of the metabolites was much greater during transport in the apical-to-basal direction than in the basal-to-apical direction, and this could be well explained by a mathematical model based on the assumption that extensive metabolism is associated with the uptake process of midazolam from the apical cell surface. Thus, we conclude that an asymmetric distribution of metabolic activity in the small intestine, leading to extensive metabolism during uptake from the apical cell surface, accounts for the minimal oral bioavailability of midazolam in cynomolgus monkeys.

Cyclosporine A (CsA) affects the pharmacodynamics and pharmacokinetics of the atypical antipsychotic amisulpride probably via inhibition of P-glycoprotein (P-gp)

The importance of P-glycoprotein (P-gp) in the pharmacokinetics of amisulpride and the effects of a P-gp inhibitor cyclosporine A (CsA) was investigated both, in vitro and in vivo. In vitro and in vivo results indicated amisulpride as a substrate of P-gp. Amisulpride was not metabolized by rat liver microsomes. Open field behavior showed time dependent abolishment in locomotion by amisulpride (50 mg kg(-1)). Co-administration of CsA (50 mg kg(-1)) resulted in a higher and significantly longer antipsychotic effect (24 h after drug administration). Accordingly, the area under concentration-time curve in serum and brain was higher in CsA co-treated rats (13.5 vs. 29.8 micromol h l(-1) for serum and 2.16 vs 2.98 micromol h l(-1) for brain tissue) while renal clearance was not affected. These results pointed to a pharmacokinetic drug interaction between CsA and amisulpride most likely caused by inhibition of P-gp.

Effects of continuous ingestion of green tea or grape seed extracts on the pharmacokinetics of midazolam

Limited systematic data on herb-drug interaction are available, despite many opportunities to concomitant use of herb with prescribed drugs. We investigated the effects of 15 herbal extracts in dietary supplements on CYP2C9, CYP2D6 and CYP3A4 activities in human liver microsomes. Strong inhibition of these CYP activities was found by the addition of green tea extracts (GTE) or grape seed extracts (GSE) in vitro. To examine the effects of these extracts on CYP3A activities in vivo, the pharmacokinetics of midazolam (MDZ) was analyzed in rats. Although single treatments with these extracts had negligible effects, 1 week of treatment with them resulted in a significant increase in the ke of intravenously administered MDZ, indicating the induction of CYP3A in the liver. In contrast, 1 week of treatment with GTE, but not GSE, caused a significant increase in the C(max) and AUC(0-infinity) of orally administered MDZ without change in the t(1/2), suggesting a reduction in CYP3A activity in the small intestines. These studies indicate that subchronic ingestion of GTE or GSE may alter the pharmacokinetics of MDZ, and the effects of GTE on CYP3A activity appear opposite between liver and small intestine, which could not be predicted from in vitro experiments.

In vitro, pharmacokinetic, and pharmacodynamic interactions of ketoconazole and midazolam in the rat

Interactions of midazolam and ketoconazole were studied in vivo and in vitro in rats. Ketoconazole (total dose of 15 mg/kg intraperitoneally) reduced clearance of intravenous midazolam (5 mg/kg) from 79 to 55 ml/min/kg (p < 0.05) and clearance of intragastric midazolam (15 mg/kg) from 1051 to 237 ml/min/kg (p < 0.05), increasing absolute bioavailability from 0.11 to 0.36 (p < 0.05). Presystemic extraction occurred mainly across the liver as opposed to the gastrointestinal tract mucosa. Midazolam increased electroencephalographic (EEG) amplitude in the beta-frequency range. Ketoconazole shifted the concentration-EEG effect relationship rightward (increase in EC(50)), probably because ketoconazole is a neutral benzodiazepine receptor ligand. Ketoconazole competitively inhibited midazolam hydroxylation by rat liver and intestinal microsomes in vitro, with nanomolar K(i) values. At a total serum ketoconazole of 2 microg/ml (3.76 microM) in vivo, the predicted reduction in clearance of intragastric midazolam by ketoconazole (to 6% of control) was slightly greater than the observed reduction in vivo (to 15% of control). However, unbound serum ketoconazole greatly underpredicted the observed clearance reduction. Although the in vitro and in vivo characteristics of midazolam in rats incompletely parallel those in humans, the experimental model can be used to assess aspects of drug interactions having potential clinical importance.

The effect of zolpidem on operant behavior and its relation to pharmacokinetics after intravenous and subcutaneous administration: concentration-effect relations

We characterized the effects of i.v. and s.c. zolpidem (1-8 mg/kg) under a differential reinforcement of low-rate schedule (i.e. DRL 45 s) in 3-hour sessions. Both behavioral and pharmacokinetic-pharmacodynamic analyses were used with the intent to compare the effects of zolpidem with those of benzodiazepines reported previously under the same behavioral paradigm. Zolpidem increased the shorter-response [inter-response times (IRTs)<45 s] rate and decreased the reinforcement rate in a dose- and time-related fashion. The behavioral profiles of zolpidem were mainly similar to those of benzodiazepines, except zolpidem produced far fewer shorter IRT responses. Pharmacokinetically, zolpidem decays biexponentially with distributional and terminal elimination half-lives of 5.2 and 42 min, respectively. The absorption rate constant and absolute bioavailability for s.c. zolpidem were 0.083/min and of 84.1%, respectively. The pharmacodynamic parameters for the reinforcement rate, an index of timing performance, were determined by integration of behavioral and pharmacokinetic profiles in a between-subject design using the effect-linked inhibitory sigmoidal E(max) model. The pharmacokinetic-pharmacodynamic analysis revealed that the potency of zolpidem (concentration required to produce 50% maximal effects, IC(50)) in disrupting the timing performance was 0.129 microg/ml. The pharmacodynamic estimates of zolpidem were compared to our previous results for benzodiazepines.

High-performance liquid chromatographic determination of cocaine and its metabolites in serum microsamples with fluorimetric detection and its application to pharmacokinetics in rats

A sensitive, selective and simple HPLC method with fluorimetric detection is described for quantitating cocaine and its three metabolites in rat serum microsamples (50 microl). Chromatographic separation is achieved on a Hypersil BDS C18 column (100X2.1 mm, 5 microm) with an isocratic mobile phase consisting of methanol-acetonitrile-25.8 mM sodium acetate buffer, pH 2.6, containing 1.0 x 10(-4) M tetrabutylammonium phosphate (14:10:76, v/v/v). The detection limit (0.5 ng/ml) for all the compounds, using direct fluorometric detection operated at excitation and emission wavelengths of 230 and 315 nm, respectively, was approximately five-times lower than that of using a UV detector operated at 235 nm. The effects of ratio of 2-propanol to chloroform in extraction solvents on the recovery and precision for cocaine and its metabolites were systematically examined. The method was used to study the pharmacokinetics of cocaine after administration of intravenous 2 mg/kg and oral 20 mg/kg doses.

Intravenous and oral clozapine pharmacokinetics, pharmacodynamics, and concentration-effect relations: acute tolerance

We examined the pharmacokinetics and pharmacodynamics of intravenous (1-5 mg/kg) and oral clozapine (2.5-10 mg/kg) in rats (terminal half-life=81.8 min; oral bioavailability=5.32%). Both dose- and concentration-effect relations of clozapine were characterized. Clozapine's effects were similar to those of benzodiazepines because of the similarity in effect-time profiles between the two classes of drugs. The IC(50) value increased as a function of dose; consequently, clozapine's relative potency decreased linearly with the logarithm of AUC((0-infinity)), or bioavailable dose regardless of route of administration. The IC(50) is an index for the sensitivity of behavioral performance to clozapine; relative potency provides an index for estimating the extent of acute tolerance. As IC(50) increases, relative potency decreases, and consequently, acute tolerance increases. Our results demonstrated that greater acute tolerance was observed for i.v. clozapine than for p.o. clozapine; however, clozapine exhibited a single concentration-effect relation across dose and route of administration after correcting for relative potencies.

Post-injection delays in experimental chambers, but not in home cages, produce both sensitization and tolerance of operant behaviour to midazolam: relation to pharmacokinetics

The effects of post-injection delay time and environmental context on behaviour after subcutaneous administration of 3 mg/kg midazolam were investigated under a differential reinforcement of low rate schedule (i.e. DRL 45 s) in 3 h sessions. Post-injection delays were varied (0-120 min) for two groups of rats placed in either the experimental chamber (group 1) or home cage (group 2) during the pre-session delay times. Midazolam increased shorter-response (inter-response times < 45 s) rates and decreased reinforcement rates in a time-related manner. Reinforcement rate-time profiles were also integrated with parallel pharmacokinetics. Post-injection delays in either environment yielded performances that mirrored the pharmacokinetic profile operative at the corresponding time-delay points. At higher concentrations (> 0.12 microg/ml) the pharmacokinetics of midazolam largely determined the behavioural effects in both groups, regardless of post-injection delays. However, at lower drug concentrations, longer post-injection delays (> 60 min) in the experimental chamber produced both sensitization and tolerance, as measured by greater increases in shorter-response rates and a more rapid return of the reinforcement rate, respectively. Interaction of the discriminative stimulus effects of midazolam with the context probably alters the magnitude of behavioural effects when the delay occurs in the experimental chambers, whereas no such interaction is present in group 2. The DRL schedule with post-injection delays in experimental chambers provides a useful behavioural paradigm for studying both sensitization and tolerance.

Dose independent pharmacokinetics of caffeine after intravenous administration under a chronic food-limited regimen

Several studies have shown that caffeine follows non-linear pharmacokinetics in both rats and humans. Recent data have demonstrated that caffeine may following linear pharmacokinetics when administered orally and intraperitoneally to food-limited rats. In this study the pharmacokinetics of caffeine was analyzed following intravenous (i.v.) administration to rats under a food-limited regimen. Four rats were administered four doses of caffeine and a standard dose of the caffeine metabolites, paraxanthine, theobromine, and theophylline. Caffeine pharmacokinetic parameters were dose independent following intravenous doses ranging from 1 to 20 mg/kg. Furthermore, the caffeine area under the curve (AUC) increased linearly as a function of dose. The mean fraction of caffeine converted to paraxanthine, theobromine, and theophylline was 16%, 16%, and 7%, respectively. The linear pharmacokinetics demonstrated in the present study may be attributed to the induction of hepatic metabolism under a chronic food-limited regimen.

Determination of zolpidem in serum microsamples by high-performance liquid chromatography and its application to pharmacokinetics in rats

A single-solvent extraction step high-performance liquid chromatographic method is described for quantitating zolpidem in rat serum microsamples (50 microl). The separation used a 2.1 mm I.D. reversed-phase OD-5-100 C18 column, 5 microm particle size with an isocratic mobile phase consisting of methanol-acetonitrile-26 mM sodium acetate buffer (adjusted to pH 2.0 with 40% phosphoric acid) containing 0.26 mM tetrabutylammonium phosphate (13:10:77, v/v/v). The detection limit was 3 ng/ml for zolpidem using an ultraviolet detector operated at 240 nm. The recovery was greater than 87% with analysis performed in 12 min. The method is simple, rapid, and applicable to pharmacokinetic studies of zolpidem after administering two intravenous bolus doses (1 and 4 mg/kg) in rats.

Pharmacokinetic determinants of cocaine's differential effects on locomotor and operant behavior

Dose-response, effect-time and concentration-effect relations of intravenous cocaine (1-4 mg/kg) were investigated on contingency-controlled [fixed-ratio (FR) 70 performance] and unconditioned (locomotor activity) behaviors. Cocaine dose-response curves exhibited decreasing rates of response under the FR 70 schedule but increasing locomotor activity in a dose-related fashion. Effect-time profiles confirmed that these changes were time-dependent and provided additional clarity by mirroring the biexponential decay of cocaine concentrations with time. The duration of action of cocaine was comparatively shorter on locomotor activity than on FR performance. We integrated effect-time profiles of the two behaviors with concentration-time profiles simulated from our previously published pharmacokinetic parameters to derive cocaine's pharmacodynamic parameters. Classical inhibitory Emax and sigmoidal Emax models were used to describe cocaine's effects on FR performance and locomotor activity, respectively. Simultaneous pharmacokinetic-pharmacodynamic modeling reveals evidence of acute tolerance to cocaine in locomotor activity, as indicated by decreasing potency with dose, but not in contingency-controlled behavior.

Determination of clozapine and its metabolite, N-desmethylclozapine, in serum microsamples by high-performance liquid chromatography and its application to pharmacokinetics in rats

A single solvent extraction step high-performance liquid chromatographic method is described for quantitating clozapine and its metabolite, N-desmethylclozapine, in rat serum microsamples (50 microl). The separation used a 2.1-mm I.D. reversed-phase Symmetry C18 column with an isocratic mobile phase consisting of methanol-acetonitrile-28.6 mM sodium acetate buffer, pH 2.6 (10:20:70, v/v/v). The detection limit was 2.5 ng/ml for all the compounds using an ultraviolet detector operated at 230 nm. The method was used to study the pharmacokinetics of clozapine after an intravenous bolus dose (2.5 mg/kg).

Caffeine has similar pharmacokinetics and behavioral effects via the i.p. and p.o. routes of administration

Caffeine administered intraperitoneally (i.p.) or orally (p.o.) decreased the reinforcement rate and increased the nonreinforced response rate in a dose-related fashion under a differential reinforcement of low rate schedule (DRL 45-s) in 3-h sessions. These effects were similar following both routes of caffeine administration. The parallel pharmacokinetics for i.p. and p.o. caffeine were each determined and related to the respective effects of caffeine on reinforcement rate. Serum caffeine concentrations were similar across the session after the absorption phase for a given dose. Consequently, the effect remained in approximately the same range within a dose, and no single dose possessed a full concentration-effect relation for the two routes. The effects of i.p. and p.o. caffeine on reinforcement rate plateaued at doses higher than 40 mg/kg, which produced a serum caffeine concentration of approximately 25 microg/ml regardless of the route of administration. The EC50 values were 7.34 and 9.93 microg/ml for i.p. and p.o. caffeine, respectively. This study as well as our previous studies demonstrated that the i.p. route is dependable for studying caffeine dose response relations but not for studying other drugs (e.g., midazolam). The possible mechanism accounting for this difference is discussed.

Pharmacokinetic-pharmacodynamic modeling of the coexistence of stimulatory and sedative components for midazolam

Midazolam increased the shorter-response rate and decreased the reinforcement rate of a contingency-controlled timing behavior--a differential-reinforcement-of-low-rate 45-s schedule. The responding rate changes observed were immediately interpretable as functions of midazolam concentration during a 3-h session--a period for investigating the onset, peak, and disappearance of midazolam effect--in rats. That the midazolam pharmacokinetic-pharmacodynamic model was a direct application of our alprazolam pharmacokinetic-pharmacodynamic model implies that both drugs exhibit similar pharmacological effects. The two peaks of the shorter-response rate increases produced by midazolam were modeled as a stimulation-sedation model that consisted of two opposing effect-link sigmoidal Emax functions. The stimulation-sedation model suggested that midazolam possesses both stimulatory and sedative effects in a continuous but sequential fashion, and hypothesizes the coexistence of stimulation and sedation components for midazolam; this model may help delineate possible mechanisms for rebound side effects and of tolerance in humans. The reinforcement rate was, then, an index for evaluating the deficit in timing performance.

Determination of cocaine and its metabolites in serum microsamples by high-performance liquid chromatography and its application to pharmacokinetics in rats

A single-solvent extraction step high-performance liquid chromatographic method is described for quantitating cocaine and its three metabolites in rat serum microsamples (50 microl). The separation used a 2.1-mm I.D. reversed-phase Brownlee C18 column with an isocratic mobile phase consisting of methanol-acetonitrile-25.8 mM sodium acetate buffer, pH 2.2, containing 1.29-10(-4) M tetrabutylammonium phosphate (12.5:10:77.5, v/v/v). The detection limit was 2.5 ng/ml for all the compounds using an ultraviolet detector operated at 235 nm. The method was used to study the pharmacokinetics of cocaine after an intravenous (i.v.) bolus dose (4 mg/kg).