A study of the pharmacokinetic interaction of istradefylline, a novel therapeutic for Parkinson's disease, and atorvastatin.

The effect of steady-state istradefylline, an agent for Parkinson's disease with P-glycoprotein and CYP3A inhibitory activity, on the pharmacokinetics of atorvastatin and its metabolites was evaluated in healthy volunteers. A single 40-mg dose of atorvastatin was administered to 20 subjects. After a 4-day washout, subjects received a single 40-mg atorvastatin dose following 40 mg istradefylline (n=16) or placebo (n=4) daily for 14 days. Plasma samples collected for 96 hours after atorvastatin administration, alone and in combination, were analyzed for atorvastatin, orthohydroxy atorvastatin, and parahydroxy atorvastatin. Istradefylline increased atorvastatin C(max) (53%), AUC(0-infinity) (54%), and t((1/2)) (27%); and increased AUC(0-infinity) for orthohydroxy atorvastatin (18%), but had no significant effect on its C(max) or t((1/2)); and had minimal effect on parahydroxy atorvastatin AUC(0-infinity). The lack of inhibition by istradefylline on metabolite systemic exposure, combined with increased atorvastatin systemic exposure, suggests a predominant P-glycoprotein inhibitory effect of istradefylline.

Pharmacokinetics and inflammatory fluid penetration of intravenous daptomycin in volunteers.

The lipopeptide antimicrobial daptomycin was administered intravenously at a dose of 4 mg/kg of body weight to seven healthy male volunteers. The concentrations of daptomycin in plasma, cantharidin-induced inflammatory fluid, and urine were measured by a microbiological assay. The mean +/- standard deviation peak concentrations in plasma and inflammatory fluid were 77.5 +/- 8.3 and 27.6 +/- 9.5 microg/ml, respectively; the mean terminal elimination half-lives were 7.74 and 13.2 h, respectively. The overall penetration of total drug into the inflammatory fluid (measured by ratio of the area under the concentration-time curve from 0 to 24 h for inflammatory fluid compared with that for plasma) was 68.4%. The mean urinary recovery over 24 h was 59.7%.

Disposition and toxicity of a mixed backbone antisense oligonucleotide, targeted against human cytomegalovirus, after intravitreal injection of escalating single doses in the rabbit.

The ocular disposition and toxicity of GEM132, a mixed backbone phosphorothioate oligonucleotide developed for the treatment of cytomegalovirus-induced retinitis, were studied in rabbits for 6 months following single intravitreal injection of 5, 20, or 100 microgram/eye (toxicity) and 3.7, 15.7, or 78.5 microgram/eye (disposition). Intraocular pressure, electroretinograms, and ophthalmoscopy were evaluated in the toxicity arm as well as gross and microscopic pathology at the termination of the study. Vitreous humor, retina, and the remaining ocular tissues were collected from all animals in the disposition arm. No toxicities were observed in the low-dose group. Intraocular pressure was transiently mildly increased in the mid- and high-dose groups; macroscopic findings were mild and infrequent. Changes in electroretinograms and histopathological findings attributed to GEM132 were observed by 4 weeks postdose in the high-dose group. Area under the curve values in all ocular tissues sampled were proportional to dose, suggesting GEM132 disposition exhibited first-order kinetics. Vitreous humor concentrations decreased in a multiphasic manner, consistent with rapid distribution. Polyacrylamide gel electrophoresis analysis of retinal extracts indicated that, at 4 weeks postdose, 90% of the radioactivity was associated with parent compound. At 8 weeks postdose, this had decreased to 70%, and subsequently to 50% at 21 weeks postdose. In retina, GEM132 reached concentrations >5 times IC(90) by 1 week postdose, with maximum concentrations 4 to 8 weeks postdose. Retinal concentrations of intact GEM132 then declined at a very slow rate. Microautoradiography suggested that radioactivity was distributed throughout the retinal layers, the largest amount being located in the middle layers.

A safety and pharmacokinetic study of a mixed-backbone oligonucleotide (GEM231) targeting the type I protein kinase A by two-hour infusions in patients with refractory solid tumors.

GEM231 is a mixed-backbone oligonucleotide targeting the regulatory subunit alpha of type I protein kinase A, which plays an important role in growth and maintenance of malignancies. Preclinically, GEM231 inhibited human cancer xenografts either alone or synergistically with chemotherapeutic agents and has demonstrated an improved metabolic stability and safety profile compared to the first-generation compounds. Objectives of this study were to define the safety profile and pharmacokinetics of GEM231 administered as 2-h IV infusions twice weekly in patients with refractory solid tumors. Fourteen patients (13 evaluable for safety) received escalating doses of GEM231 at 20-360 mg/m2 (2.5-9 mg/kg). Tumor histologies included non-small cell lung cancer, renal cell cancer, sarcoma, and others. The plasma pharmacokinetics of GEM231 were linear and predictable. Maximum plasma concentration (Cmax) reached 50-70 microg/ml (8-13 microM) at dose 360 mg/m2 and 27-32 microg/ml at dose 240 mg/m2. The plasma half-life was about 1.5 h. The only clinical toxicities were transient grade I-II fever and fatigue at doses > or = 240 mg/m2. There was no treatment-related complement activation or thrombocytopenia at any dose level, except with the first dose in one patient who had pre-existing borderline thrombocytopenia. Transient activated partial thrombin time prolongation occurred at doses > or =160 mg/m2. Dose-limiting toxicities included transient activated partial thrombin time prolongation (one of three patients at 360 mg/m2) and cumulative reversible transaminase elevation (three of three patients at 360 mg/m2 and three of six patients at 240 mg/m2 during weeks 3-10). One patient with colon cancer had stabilization of a previously rising carcinoembryonic antigen. Thus, in this first clinical evaluation of a mixed-backbone oligonucleotide in cancer patients, high plasma concentrations of GEM231 were well tolerated without significant acute toxicities, but prolonged treatment was associated with reversible transaminitis. Although 240 mg/m2 by 2-h infusion twice weekly was safe for a 4-week treatment duration, alternative dosing schedules are being tested to minimize the cumulative toxicity, which will be essential to extend the duration of therapy at the highest GEM231 dose tested.

The disposition (ADME) of antisense oligonucleotides.

Antisense oligonucleotides hold great promise as novel therapeutic agents designed to specifically and selectively inhibit the production of various disease-related gene products. For efficacy to occur, the therapeutic entity must reach the site of action in amounts sufficient to produce the desired therapeutic effect. Pharmacokinetics is defined as the study of the time course of the absorption, distribution, metabolism and elimination (ADME) of drugs. An understanding of the pharmacokinetics of antisense oligonucleotides and, in particular, the mechanisms by which oligonucleotides accumulate in tissues and in cells are critical to understanding the limitations of this technology and ultimately the development of effective antisense therapeutics. This review summarizes the known observations on the pharmacokinetics of antisense oligonucleotides with particular attention to contributions (non-clinical and clinical) published during the interval 1997 to 2000. Principles underlying the ADME of oligonucleotides are presented at the beginning of each section.

Pharmacokinetics and tolerability of intravenous trecovirsen (GEM 91), an antisense phosphorothioate oligonucleotide, in HIV-positive subjects.

Trecovirsen, a 25-mer antisense phosphorothioate oligonucleotide targeted at the gag site of the HIV gene, was administered to HIV-positive volunteers as an i.v. infusion. Single doses ranged from 0.1 to 2.5 mg/kg in an ascending escalation in cohorts of 6 to 12 subjects. Plasma trecovirsen concentrations and pharmacokinetic parameters could be assessed at doses > or = 0.3 mg/kg. Peak plasma concentrations and AUC values increased disproportionately with increasing dose while elimination half-life increased and plasma clearance decreased, indicating a saturable process over this dose range. The only significant adverse event observed was an isolated, transitory increase in activated partial thromboplastin time at doses > or = 2.0 mg/kg that was related to plasma trecovirsen concentrations and is attributed to the polyanionic character of the molecule. Thus, trecovirsen administration was well tolerated in single i.v. doses up to 2.5 mg/kg.

Oligonucleotide transport in rat and human intestine ussing chamber models.

Cellular and intestinal absorption of naked oligonucleotides (ONs) is limited and still remains a developmental challenge. A previous report in the literature suggests that ON absorption occurs via a paracellular mechanism. The aim of this study was to test this hypothesis using rat and human intestine in a Ussing chamber and in Caco-2 cells. Transport of a (35)S-labelled mixed backbone ON (MBO) across human or rat intestinal tissue or across Caco-2 cells was measured after a 2-h incubation in the presence or absence of increasing MBO concentrations or with uptake inhibitors and enhancers. MBO intestinal absorption was compared with an internal standard, mannitol. (35)S-MBO demonstrated very little absorption (<1%) across rat and human intestinal tissues. Transport appeared to be unsaturable up to 500 microM, and relatively insensitive to compounds that opened tight junctions or inhibited P-glycoprotein. However, preliminary studies with Caco-2 cells suggest a possible saturable mechanism at higher ON concentrations. Confocal fluorescence microscopy studies show that fluorescein isothiocyanate (FITC)-MBO was internalized into intestinal cells. Although some differences in ON transport were observed as a function of the transport model, MBO transport was mostly consistent with a transcellular, rather than a paracellular, absorption mechanism.

On the antipyrine test in laboratory animals. Studies in the dog and monkey.

The antipyrine (AP) test has been challenged in species other than humans on the grounds that, in some nonhuman species, particularly on induction, hepatic blood flow may become as prominent a factor in AP clearance as hepatic metabolism. Therefore, we investigated in dogs and monkeys the disposition of AP to determine how well AP serves as a model drug to indicate changes in rates of hepatic clearance. After administration of an oral solution of AP (5 mg/kg) to control dogs, the percentage of the dose absorbed was 98%, based on urinary and fecal excretion of AP and its metabolites. Despite complete AP absorption, absolute bioavailability of AP was 78 +/- 12% under basal conditions, suggesting that AP does undergo some degree of presystemic elimination, approximately 22%. After PB administration of 20 mg/kg/day for 9 days, po, AP bioavailability decreased to 60 +/- 14%. The systemic clearance of AP increased from 9.4 +/- 2.3 ml/min/kg under basal conditions to 27.5 +/- 4.6 ml/min/kg following PB. PB decreased mean plasma AP half-life from 71.5 min under basal conditions to 27.7 min, and mean hepatic blood flow increased from 0.49 liters/min to 0.63 liters/min. Induction doubled the hepatic extraction ratio for AP to 0.4 from 0.2 under basal conditions. In beagle dogs after PB pretreatment, 97% of the total systemic clearance of AP was estimated to be due to enhanced hepatic AP metabolism, only 3% to increased hepatic blood flow. Therefore, for dogs under both basal and induced conditions it is concluded that AP clearance reflects predominantly hepatic AP metabolism, being negligibly influenced by hepatic blood flow.(ABSTRACT TRUNCATED AT 250 WORDS)

Pregnancy-specific changes of antipyrine pharmacokinetics correlate inversely with changes of estradiol/progesterone plasma concentration ratios.

Antipyrine pharmacokinetics as well as estradiol and progesterone concentrations were measured in plasma of 11 healthy pregnant women during the first two trimesters and again in the same patients 6 to 20 weeks after interruption of pregnancy. During pregnancy, the clearance of antipyrine increased and its half-life decreased in all but two instances. A significant inverse relationship was found between the degree of pregnancy-specific changes of plasma clearance of antipyrine and estradiol-progesterone plasma concentration ratios: high estradiol-progesterone ratios during pregnancy corresponded to decreased plasma clearance of antipyrine; low ratios of the two hormones during pregnancy corresponded to increased plasma clearance of antipyrine. The current results indicate that the balance between circulating estrogens and progesterone may be an appropriate indicator for pregnancy-specific changes of a particular hepatic drug metabolism during pregnancy.

Evaluation of drug absorption and presystemic metabolism using an in situ intestinal preparation.

An in situ rat intestinal preparation was modified to include portal and jugular venous blood collection techniques as well as sampling from the intestinal lumen. Viability could be maintained for 3 h. The utility of the preparation was examined by studying the disposition of four model drugs, each with differing characteristics with respect to absorption and presystemic metabolism. Haloperidol (4-[4-(4-chlorophenyl)-4-hydroxy-1-piperidinyl]-1-(4-fluorophenyl)-1- butanone), a reference compound used for model development, disappeared from the intestinal lumen with a half-life of 14 +/- 3 min. When the antiarthritic agent, tolmetin sodium (sodium 1-methyl-5-(4-methylbenzoyl)-1H-pyrrole-2-acetate dihydrate), was studied in the preparation, it was rapidly absorbed (t1/2 for disappearance from the intestinal lumen = 8 min), achieved plasma concentrations comparable to in vivo data, and underwent little presystemic elimination. In contrast, fenoctimine sulfate (4-(diphenylmethyl)-1-[(octylimino)methyl]piperidine sulfate), an antisecretory compound, disappeared more slowly from the intestinal lumen (t1/2 = 60 min), was present in portal plasma, but was not detected in systemic plasma. Extensive hepatic first-pass elimination of fenoctimine was evident. Tolmetin glycine amide (N-([1-methyl-5-(4-methylbenzoyl)-1H-pyrrol-2-yl]acetyl)glycine), a tolmetin prodrug, disappeared from the intestinal lumen very slowly (t1/2 approximately 3 h) compared with the other agents tested. It was determined that this drug was being hydrolyzed presystemically to tolmetin by the intestinal mucosa and the liver. These results establish the utility of this intestinal preparation for studying drug absorption and presystemic elimination.

Hydroxylation and glucuronidation of various xenobiotics by hepatic microsomes from the fetal lamb, pregnant ewe and human fetus.

The ability of microsomes isolated from liver of pregnant ewes and their fetuses at near term to catalyze the biotransformation of benzo[a]pyrene, hexobarbital, meperidine, methadone and morphine was investigated. Cytochromes P-450 and b5, NADPH and NADH cytochrome c reductase, methadone and meperidine N-demethylase and morphine glucuronyltransferase activities were detected in microsomes from both maternal and fetal livers. Fetal hepatic microsomes however, lacked the ability to catalyze the hydroxylation of hexobarbital and benzo[a]pyrene.

Plasma protein binding of bepridil.

The binding of the calcium-channel blocking agent, bepridil HCl (Vascor), to plasma proteins was investigated using radiolabeled bepridil and equilibrium dialysis. Greater than 99.7% of added bepridil-14C was found to freshly collected human plasma. The binding was characterized by a saturable high-affinity site (KD = 32 ng/mL = 87 nM) on alpha1-acid glycoprotein (AAG) or on an AAG-human serum albumin complex and lower affinity binding sites on albumin and other plasma macromolecules. Bepridil that is not bound to plasma proteins is extensively distributed into erythrocytes as evidenced by a red blood cell to free drug distribution coefficient of 71 +/- 7. Despite this high value, the blood to plasma ratio of bepridil averaged only 0.67 in humans, indicating that most of the circulating drug is bound to plasma proteins. Bepridil protein binding was not affected by additions of nonesterified fatty acids. Free fractions of bepridil were enhanced by addition of verapamil, nifedipine, diltiazem, disopyramide, and warfarin but only at concentrations above those achieved clinically. Bepridil was also displaced by the plasticizer, tris-(2-butoxyethyl)phosphate. Plasma obtained from a small number of angina patients prior to bepridil administration showed no differences in ability to bind bepridil compared with plasma obtained from healthy subjects.

High levels of methylxanthines in chocolate do not alter theobromine disposition.

Theobromine disposition was measured twice in 12 normal men, once after 14 days of abstention from all methylxanthines and once after 1 week of theobromine (6 mg/kg/day) in the form of dark chocolate. Mean theobromine t 1/2, apparent volume of distribution, and clearance after abstinence from all methylxanthines were 10.0 hours, 0.76 L/kg, and 0.88 ml/min/kg. High daily doses of chocolate for 1 week did not change these values. After subjects abstained from methylxanthines, urinary radioactivity over 72 hours after a single, oral dose of [8-14C]theobromine consisted of 42% 7-methylxanthine, 20% 3-methylxanthine, 18% theobromine, 10% 7-methyluric acid, and 10% 6-amino-5[N-methylformylamino]-1-methyluracil. A week of daily theobromine consumption in the form of dark chocolate also did not alter this urinary profile of theobromine and its metabolites. Although these results might appear to differ from other reports of inhibition of theobromine elimination after five consecutive daily doses of theobromine in aqueous suspensions, both the rate and extent of absorption of theobromine in chocolate were less than that of theobromine in solution. Relative bioavailability of theobromine in chocolate was 80% that of theobromine in solution. This reinforces the fundamental principle that both the metabolic and the therapeutic consequences of a particular chemical can differ when that chemical is given in the pure compared with the dietary form.

Theobromine kinetics and metabolic disposition.

Metabolism and kinetics of a single oral dose of 30 microCi 8-14C-theobromine with 10 mg/kg theobromine sodium acetate were studied in six healthy, nonmedicated, nonsmoking men after 14 days' abstention from all methylxanthine sources. Identification and quantitation of metabolites in plasma and urine both by HPLC and by thin-layer chromatography coupled with radiography indicated that theobromine was predominant in plasma. For urine, both methods identified theobromine as well as 7-methylxanthine, 7-methyluric acid, 3-methylxanthine, 6-amino-5[N-methylformylamino]-1-methyluracil, and a small amount of 3,7-dimethyluric acid as the metabolites of theobromine. All administered radioactivity was recovered in urine and no polar metabolites could be detected. Analysis of the urinary excretion data by the sigma-minus method allowed calculation of the apparent first-order rate constants for production of 7-methylxanthine, 7-methyluric acid, 3-methylxanthine, 3,7-dimethyluric acid, and 6-amino-5[N-methylformylamino]-1-methyluracil.

Localized and systemic effects of environmental ammonia in rats.

The purpose of this study was to determine if environmental ammonia is absorbed through the lungs of rats into the blood and, in turn, exerts an effect on blood pH, blood gases, and hepatic drug metabolizing enzyme activity. In phase 1 of the study, rats with surgically implanted aortic cannulas were exposed to varying environmental ammonia concentrations (15 to 1157 ppm). Blood pH, pCO2, pO2, and blood ammonia concentrations were measured at 0, 8, 12, and 24 hours post-exposure. In phase 2, hepatic microsomal enzyme activity (ethylmorphine-N-demethylase and cytochrome P-450) was determined after a 3-day and 7-day exposure to varying environmental ammonia concentrations (4 to 714 ppm). No significant changes were found in blood pH, pCO2, or the histologic appearance of the lungs or trachea. The pO2 and liver microsomal enzymes had only minor changes. The blood ammonia concentration increased significantly (p less than or equal to 0.05) in a linear fashion with increasing environmental ammonia concentrations, indicating pulmonary absorption of ammonia. These levels also declined over time at higher concentrations, suggesting that compensation was occurring. Low environmental ammonia concentrations (less than 100 ppm) produced extremely small changes in blood ammonia concentration, and they had no measurable effects on other parameters examined in the study. These findings suggest that environmental ammonia concentrations found in animal holding rooms may cause minimal adverse effects in healthy rats.

Influence of different forms of tobacco intake on nicotine elimination in man.

In each of three separate experiments mean plasma nicotine t1/2 beta was slightly, but statistically significantly, shorter in habituated compared to naive cigarette smokers. Two of these experiments involved nicotine administration by smoking a cigarette containing a standardized amount of nicotine, whereas in the third experiment nicotine was injected intravenously as a single tracer dose of 14C-nicotine. In contrast to cigarette smokers, naive and habituated snuff dippers had similar mean plasma nicotine t1/2 beta. Habituated pipe smokers tended to have very slightly, but not statistically significantly, shorter plasma nicotine t1/2 beta S than naive pipe smokers. These distinctions are related to special features that characterize each form of tobacco intake.

Smoking-induced changes in nicotine disposition: application of a new HPLC assay for nicotine and its metabolites.

A sensitive, rapid high-pressure liquid chromatographic assay was developed to compare the disposition of an intravenous dose of 14C-nicotine in normal, carefully matched smokers and nonsmokers. The elimination half-lifes of nicotine and cotinine were shorter in smokers than in nonsmokers. Also consistent with an inductive effect of smoking was the increased nicotine elimination rate constant in smokers, but smoking induced more complex kinetic changes: nicotine volume of distribution was diminished in smokers, whereas nicotine clearance and area under the concentration-time curve were unchanged. The presence of nicotine and its principal metabolites in a morning specimen of urine obtained from nonsmokers before 14C-nicotine administration suggests ubiquitous, passive exposure to and absorption of chemicals present in cigarette smoke.

Drug disposition during pregnancy.

This review examines the literature on drug disposition during human pregnancy. Drugs are considered on the basis of their mode of excretion and hepatic extraction ratio in order to assess the general effects of pregnancy on drug disposition. Pregnancy increases clearance of drugs whose excretion is predominantly renal and related to creatinine clearance but fails to change clearance of parenterally administered drugs that are extensively metabolized by liver and have a high hepatic extraction ratio. The data are unclear as to how pregnancy influences the disposition of drugs extensively metabolized by liver with a low to intermediate hepatic extraction ratio.