Plasma terminal half-life

Synthesis of highly potent novel anti-tubercular isoniazid analogues with preliminary pharmacokinetic evaluation

Thirty two novel isoniazid analogues were prepared by one-pot three component condensations of isoniazid (INH), 3-mercaptopropionic acid and various aryl/heteroaryl aldehydes. The synthesized compounds were evaluated for their anti-TB activity against Mycobacterium tuberculosis H37Rv (MTB) and cytotoxicity. Among the compounds, compound N-(2-(4-(benzyloxy) phenyl)-4-oxo-1,3-thiazinan-3-yl) isonicotinamide (17) inhibited MTB with MIC of 0.12 μM and was three times more potent than INH. The main pharmacokinetic parameters after intravenous administration (10 mg/kg body weight) in male Wistar rats viz. t(½), K(el), mean plasma clearance and mean volume of distribution were found to be 1.14±0.20 h, 0.62±0.10 h(-1), 22.48±0.16 mL/kg/min and 1.99±0.49 L, respectively. The systemic absorption was slow after oral administration (50 mg/kg body weight). The peak plasma concentration was found to be 1.31±0.06 μg/mL attained in 3 h. The bioavailability was found to be 16.7%.

A reappraisal of current dosing strategies for intravenous fosfomycin in children and neonates

The rising incidence of multi-drug resistant bacterial pathogens has renewed interest in the long-known antibacterial fosfomycin. Not least because of its low toxicological potential, there is good clinical experience with intravenous fosfomycin for various Gram-positive and Gram-negative infections in the treatment of children and neonates. However, the current dosing recommendations for intravenous fosfomycin vary widely in paediatric patients. In the present review, we summarized available plasma pharmacokinetic data derived from neonates or children following intravenous administration of fosfomycin. Subsequently, we used this information for recalculation of different dosing strategies and simulated a variety of clinically applied dosing regimens. The percentage of time above the minimal inhibitory concentration (T>MIC) was calculated for each dosing strategy, as this pharmacokinetic-pharmacodynamic parameter was shown to be most predictive of antimicrobial and clinical success of fosfomycin treatment. Our data corroborate the current practice of selecting the dosage of intravenous fosfomycin primarily on the basis of bodyweight and age in paediatric patients. As with other 'time-dependent' antibacterials, a dosing interval of 6-8 hours should be preferred over 12 hours except for immature neonates. Given a T>MIC target of 40-70%, currently recommended dosing strategies appear to be insufficient in children aged 1-12 years, if pathogens with MICs of ≥32 mg/L are suspected and subjects are presenting with normal renal function. Likewise, the lowest recommended daily dose for neonates and infants (aged up to 12 months) of 100 mg/kg bodyweight of fosfomycin should be considered only for pre-term neonates with a postmenstrual age below 40 weeks.

The effects of different ages and dosages on the plasma disposition and hair concentration profile of ivermectin following pour-on administration in goats

The effects of different ages and dosages on the plasma disposition and hair concentration profile of ivermectin following pour-on administration in goats. J. vet. Pharmacol. Therap.34, 70-75. The effects of different ages and dosages on the plasma disposition and hair degradation of ivermectin (IVM) were investigated following pour-on administration in goats. Twenty-eight female Saanen goats allocated into two groups of 14 animals according to their ages as young (5-6 months old) and old (12-24 months old) groups. Each age group was divided into two further of seven goats and administered pour-on formulation of IVM topically at the in recommended dosage rate of 0.5 mg/kg bodyweight The recommended cattle dosages rate of 0.5 mg/kg or at the higher dosage of 1.0 mg/kg. Blood samples were collected at various times between 1 h and 40 days. In addition, hair samples (>0.01 g) were collected using tweezers from the application sites and far from application sites of the all animals throughout the blood sampling period. The plasma and hair samples were analyzed by high performance liquid chromatography (HPLC) using fluorescence detection following solid and liquid phase extractions, respectively. Dose- and age-dependent plasma disposition of IVM were observed in goats after pour-on administration. In addition, relatively high concentration and slow degradation of IVM in hair samples collected from the application site and far from the application site were observed in the present study. The differences between young and old goats are probably related to differences in body condition and/or lengths of haircoat. The systemic availability of IVM following pour-on administration is relatively much lower than after oral and subcutaneous administrations but the plasma persistence was prolonged. Although, the longer persistence of IVM on hairs on the application site may prolong of efficacy against ectoparasites, the poor plasma availability could result in subtherapeutic plasma concentrations, which may confer the risk of resistance development in for internal parasites after pour-on administration in goats.

Single-dose pharmacokinetics of oxytetracycline and penicillin G in tammar wallabies (Macropus eugenii)

The pharmacokinetics of oxytetracycline and penicillin G was investigated in tammar wallabies (Macropus eugenii). Groups of eight healthy tammar wallabies were administered i.v. oxytetracycline hydrochloride (40 mg/kg), i.m. long-acting-oxytetracycline (20 mg/kg), i.v. sodium penicillin G (30 mg/kg), or i.m. procaine/benzathine penicillin G (30 mg/kg). Plasma concentrations of oxytetracycline were determined using high-performance liquid chromatography. Pharmacokinetic parameters were comparable to those reported for eutherians of equivalent size and suggest that the practice of adjusting allometrically scaled doses to account for the lower metabolic rate of marsupials may not be valid. Long-acting oxytetracycline and penicillin G both demonstrated depot effects. However, the plasma concentrations achieved question the therapeutic efficacy of the long-acting preparations.

Clinical interpretation of pharmacokinetic and pharmacodynamic data in zoologic companion animal species

The treatment and prevention of pain in zoologic companion animals is difficult because of the lack of data available on the safety and efficacy of analgesics. Pharmacokinetic (PK)-pharmacodynamic (PD) studies integrate changes in drug concentrations and changes in the drug's effect. All experimental studies assessing the PDs of analgesics have limitations in animals, but the data provided by experimental studies are valuable in designing dosages. Placebo-controlled, randomized, and blinded clinical trials provide the best PK and PD data, but are rarely performed in major veterinary species because of the number of animals required for the study, lack of preliminary PK and PD data in a given species, species-specific differences in PK and PD, and ethical and toxicologic concerns. The usefulness and limitations as well as considerations for interpreting PK, PD, and controlled clinical studies are discussed. An example of allometric analysis of buprenorphine in mammals is also included.

Determination of bromadiolone residues in fox faeces by LC/ESI-MS in relationship with toxicological data and clinical signs after repeated exposure

In many countries, the fox (Vulpes vulpes), predator of small mammals, is particularly affected by anticoagulant rodenticides such as bromadiolone due to secondary poisoning. Nevertheless, to date, no method of exposure monitoring is applicable in the field over large areas, and no toxicological data are available concerning sensitivity of foxes to bromadiolone. The aim of this work was to compare excretion kinetics of bromadiolone in fox faeces with clinical and haemostatic effects after repeated exposure to intoxicated voles. A sensitive method for the quantification of bromadiolone excretion in fox faeces and plasma was developed, using liquid chromatography combined with electrospray ionisation mass spectrometry (LC/ESI-MS). The LoD was 0.9microg/kg and 0.15microg/L, and the LoQ was 3.0microg/kg and 0.5microg/L, in faeces and in plasma, respectively. Four captive foxes were fed for 2 or 5 days with water voles (Arvicola terrestris Sherman) spiked with bromadiolone at concentrations close to those measured in the field. Faeces and blood were collected for bromadiolone titration, and blood-clotting tests were performed to monitor fox health daily during 10 days and then every 3-4 days until the end of the experiment (D28). Then, after euthanasia, a complete necropsy was performed, and levels of bromadiolone residues in the liver were determined. Bromadiolone residues were detected in faeces 15h after the first exposure. They increased dramatically during the exposure period and then gradually decreased, but they remained detectable at the end of the experiment, i.e., 26 days after the last exposure. Bromadiolone residues in plasma showed a similar pattern but were no longer detectable 7-24 days after the last exposure. Two foxes presented very severe external haemorrhages, requiring the administration of the antidote vitamin-K1. Bromadiolone residues in faeces and their relationships with exposure and other direct-markers that were measured are discussed. Liver residues and the toxicity data of our study will help to interpret data from fox carcasses collected by wildlife disease surveillance networks. These findings provide a basis for programs aiming to monitor the exposure of wild fox populations to bromadiolone using non-invasive methods based on standard sampling and analysis of residues in faeces.

Multiple oral dosing of valacyclovir in horses and ponies

The aim of the current study was to investigate whether multiple oral dosing of valacyclovir could result in plasma concentrations exceeding the EC(50)-value of acyclovir against equine herpesvirus 1 (EHV1) during the majority of the treatment period. Additionally, we wanted to determine the concentration of acyclovir in nasal mucus and cerebrospinal fluid (CSF). Valacyclovir was administered to four horses and two ponies, three times daily, at a dosage of 40 mg/kg, for four consecutive days. Blood was collected prior to each administration and 1 h after dosing. Nasal mucus samples and CSF were collected once during treatment; 1 h after the last administration. This dosage regimen resulted in plasma concentrations that were higher than the EC(50)-value of 1.7 microg/mL, i.e. EC(50) of an isolate highly susceptible to acyclovir, for 80% of the treatment period; and higher than the EC(50)-value of 3.0 microg/mL, i.e. EC(50) of an isolate less susceptible to acyclovir, for 60% of the treatment period. Concentration in nasal mucus samples and CSF was 0.36-1.17 microg/mL and 0.11-0.23 microg/mL, respectively. This study illustrates that multiple dosing of valacyclovir may result in a therapeutic benefit as plasma concentrations could be maintained above the EC(50)-value of acyclovir against EHV1 for more than 50% of the treatment period. Acyclovir could be detected in both nasal mucus samples and CSF. However, these concentrations were lower than the EC(50).

Strategies to prolong the plasma residence time of peptidedrugs

Well established as well as recently developed strategies to prevent the fast clearance of peptide drugs from the circulation are reviewed.

Influence of the injection site on the pharmacokinetics of amoxicillin after intramuscular administration of a conventional and a long-acting formulation in sheep

The pharmacokinetics of amoxicillin (AMX) were investigated in sheep following intravenous (i.v.) and intramuscular (i.m) injection, comparing two different drug formulations, a conventional and a long-acting AMX-trihydrate suspension. For the i.m. application two different injections sites, the neck area and the hind limb were used to identify possible differences in the kinetic parameters related to the site of injection. A three-compartment open model could best describe AMX disposition after i.v. administration. Data analysis after i.m. administration of the conventional suspension at both injection sites revealed the occurrence of a flip-flop phenomenon, clearly indicating that absorption of AMX is the rate-limiting step of its overall disposition. A moderate effect of the injection site was observed with a tendency for the neck area to be advantageous, mainly in terms of rate rather than extent of absorption. Injection of the long-acting formulation led to a focal depot formation, thus yielding lower but remarkably prolonged serum AMX levels reflected in the respective terminal half-lives. The concentration-time profile of AMX after administration of the long-acting formulation was less affected by the injection site, but the low serum levels justify its use only in cases in which a high susceptibility of the involved bacterial population is confirmed.

Peripheral distribution of amoxicillin in sheep and influence of local inflammation

The pharmacokinetics of amoxicillin (AMX) in blood serum (SBS) and tissue cage fluid (TCF) was studied in sheep. Four tissue cages, prepared from silicone rubber tubing, were subcutaneously inserted in the neck area (two on each side) of the experimental animals and AMX was administered both intravenously (IV) and intramuscularly (IM) at the dose rate of 15mg/kg bodyweight. The impact of local inflammation on AMX distribution in TCF was studied after intra-cavity injection of a lambda carrageenan solution in one of the two tissue cages used after each administration. In contrast to the three-compartment AMX disposition after IV injection, two-compartment, absorption-limited pharmacokinetics was observed after IM administration. Non-inflamed and inflamed TCF data revealed, in all cases, the attainment of low, but prolonged concentrations and absence of an inflammation-induced effect on AMX penetration into and elimination from TCF.

Preparation of ergoloid mesylate submicron emulsions for enhancing nasal absorption and reducing nasal ciliotoxicity

The aim of this investigation was to prepare ergoloid mesylate submicron emulsions (EMSEs) for enhancing nasal absorption of drug and reducing nasal ciliotoxicity. Following intranasal administrations of EMSE and ergoloid mesylate solution (EMS) and intravenous administration of EMS to rats separately at the dose of 2 mg kg(-1), the levels of EM in blood and the cerebrospinal fluid (CSF) were evaluated by microdialysis method. The nasal ciliotoxicity was evaluated by using in situ toad palate model. The absolute bioavailability and the AUC in the CSF following intranasal administration of EMSE (56.3 +/- 5.3%, AUC(CSF) 28,594 +/- 5680 ng ml(-1) min) were statistically higher than those after intranasal administration of EMS (47.4 +/- 3.5%, AUC(CSF) 19,870 +/- 2247 ng ml(-1) min). No significant difference was found for the value of the brain drug direct transport percentage (DTP%) or the drug targeting efficiency (DTE) between the group receiving EMSE and the group receiving EMS. In conclusion, EMSE exhibited higher nasal absorption of EM in rats and significantly lower nasal ciliotoxicity whereas no greater brain-targeting efficiency in comparison with EMS.

Pharmacokinetics of a controlled-release liposome-encapsulated hydromorphone administered to healthy dogs

The purpose of the study was to assess the pharmacokinetics of liposome-encapsulated (DPPC-C) hydromorphone administered intravenously (IV) or subcutaneously (SC) to dogs. A total of eight healthy Beagles aged 12.13 +/- 1.2 months and weighing 11.72 +/- 1.10 kg were used. Dogs randomly received liposome encapsulated hydromorphone, 0.5 mg/kg IV (n = 6), 1.0 mg/kg (n = 6), 2.0 mg/kg (n = 6), or 3.0 mg/kg (n = 7) SC with a 14-28 day washout between trials. Blood was sampled at serial intervals after drug administration. Serum hydromorphone concentrations were measured using liquid chromatography with mass spectrometry. Serum concentrations of hydromorphone decreased rapidly after IV administration of the DPPC-C formulation (half-life = 0.52 h, volume of distribution = 12.47 L/kg, serum clearance = 128.97 mL/min/kg). The half-life of hydromorphone after SC administration of DPPC-C formulation at 1.0, 2.0, and 3.0 mg/kg was 5.22, 31.48, and 24.05 h, respectively. The maximum serum concentration normalized for dose (C(MAX)/D) ranged between 19.41-24.96 ng/mL occurring at 0.18-0.27 h. Serum hydromorphone concentrations fluctuated around 4.0 ng/mL from 6-72 h after 2.0 mg/kg and mean concentrations remained above 4 ng/mL for 96 h after 3.0 mg/kg DPPC-C hydromorphone. Liposome-encapsulated hydromorphone (DPPC-C) administered SC to healthy dogs provided a sustained duration of serum hydromorphone concentrations.

Pharmacokinetics and pharmacodynamics of ramipril and ramiprilat in healthy cats

The pharmacokinetics of ramipril and its active metabolite, ramiprilat, was determined in cats following single and repeated oral doses of ramipril (Vasotop tablets) (once daily for 9 days) at dose rates of 0.125, 0.25, 0.5 and 1.0 mg/kg. The pharmacodynamic effects were assessed by measuring plasma angiotensin-converting enzyme (ACE) activity. Maximum ramipril concentrations were attained within 30 min following a single dose and declined rapidly (concentrations were below the limit of quantification 4 h after treatment). Peak ramiprilat concentrations were detected at approximately 1.5 h. The apparent terminal half-life (t((1/2)beta)) was > or =20 h irrespective of the dose. Ramiprilat accumulated in plasma (ratio of accumulation 1.3 to 1.9 depending on the dose rate) following repeated administration. Steady-state conditions were attained after the second dose. Excretion was predominant in faeces (87%) and to a lesser extent in urine (11%). The rate and extent of absorption of ramipril as well as its conversion to ramiprilat were not significantly influenced by the presence of food in the gastrointestinal tract. Plasma-ACE activity was almost completely abolished 0.5-2.0 h after treatment, irrespective of the dose rate. Significant inhibition of ACE activity of 54.7 to 82.6% (depending on the dosage) was still present 24 h after treatment. Treatment was well-tolerated in all cats. Ramipril at a dose rate of 0.125 mg/kg once daily produces significant and long-lasting inhibition of ACE activity in healthy cats. The appropriateness of this dosage regime needs to be confirmed in diseased cats.

Some pharmacokinetic aspects of the lipophilic terfenadine and zwitterionic fexofenadine in humans

Fexofenadine, an active metabolite of the second-generation histamine H1 receptor antagonist (antihistamine) terfenadine, does not have the disadvantage of QT prolongation. In addition, unlike first-generation antihistamines, it is associated with few CNS adverse effects. Chemically, fexofenadine has a zwitterionic structure that makes it an interesting molecule for use as an oral drug. Fexo-fenadine has negligible hepatic metabolism in humans, and is recovered mainly in the faeces in an unchanged form after oral administration. The absolute oral bioavailability of fexofenadine in humans is not known because of a lack of studies of intravenous administration of this agent. Its apparent elimination half-life (t1/2) ranges from 3 to 17 hours and is highly dependent on study design, i.e. the length of blood sampling. This large discrepancy might be associated with a 'flip-flop' phenomenon caused by slow absorption of the zwitterionic molecule. This review summarises the available literature related to the absorption, elimination and excretion of fexofenadine and terfenadine. Based on these data, the volume of distribution, t1/2 and oral bioavailability of fexofenadine in humans are estimated. Understanding these pharmacokinetic aspects of this drug might be very useful for medicinal chemists utilising fexofenadine/terfenadine as an example for designing zwitterionic compounds to combat cardiotoxicity and other issues related to basic and lipophilic molecules.

Estimation of absolute oral bioavailability of moxidectin in dogs using a semi-simultaneous method: influence of lipid co-administration

Moxidectin is a long-acting anthelmintic drug for which little is known about its kinetic behaviour in dogs and its oral absolute bioavailability has never been reported. We studied the pharmacokinetics of moxidectin in dogs, with a special emphasis on oral bioavailability and the influence of lipid co-administration, by using a semi-simultaneous method of administration. Ten Beagle dogs were dosed orally and then intravenously (i.v.) with 0.2 mg/kg moxidectin. The oral application was conducted with or without corn oil co-administration. Moxidectin concentration-time profiles in plasma were analysed using a compartmental modelling approach, designed to fit the oral and i.v. kinetic disposition curves simultaneously. In contrast to what happens in other species, our study indicates that the bioavailability of orally given moxidectin in dogs is nearly total (90.2 +/- 7.4%), and is not enhanced by lipid co-administration. The clearance, the volume of distribution, the mean residence time and the terminal half-life were similar to what was already described for other species. Finally our trial suggests that the body condition (degree of obesity) is likely to be a major determinant of moxidectin kinetics in dogs because of its modulation of the volume of distribution that indirectly controls the terminal half-life of the drug.

Pharmacokinetic studies on tobramycin in horses

The objective of the study was to evaluate the pharmacokinetics of tobramycin in plasma and urine in the horse (n = 7) after intravenous administration of a dose of 4 mg/kg b.w. Plasma tobramycin concentrations were assayed microbiologically and by means of HPLC analyses. Pharmacokinetic parameters, calculated on the basis of concentrations determined with the microbiological assay were not statistically different from those obtained when data from HPLC analysis were used, but the microbiological assay was more sensitive in the detection of low plasma and urine values. The values of the total body clearance (Cl(B)) were 101.4 +/- 30.1 and 130.0 +/- 49.9 mL/kg/h, respectively. The overall extraction ratio was 2.9%. The determined capacity of elimination of tobramycin in horses was similar to those for other aminoglycosides. Within 24 h after treatment, 57.6 +/- 12.2% of injected antibiotic was excreted in the urine.

Decrease in Plasma Concentrations of Antiangiogenic Agent TSU-68 ((Z)-5-[(1,2-Dihydro-2-oxo-3H-indol-3-ylidene)methyl]-2,4-dimethyl-1H-pyrrole-3-propanoic acid) during Oral Administration Twice a Day to Rats

TSU-68 ((Z)-5-[(1,2-dihydro-2-oxo-3H-indol-3-ylidene)methyl]-2,4-dimethyl-1H-pyrrole-3-propanoic acid) is a new drug under investigation that inhibits receptor tyrosine kinases involved in tumor angiogenesis. In clinical pharmacokinetic studies, lower plasma concentrations of orally administered TSU-68 are observed after the second dose given within 12 h after the first dose. We examined the cause of this observation through in vivo and ex vivo approaches using rats in which a rapid decrease in the exposure was shown as in humans. In rats, the area under the concentration-time curve after the second dose was decreased to 26% of that after the first dose during administration of TSU-68 (200 mg/kg) twice a day. Plasma clearance of TSU-68 intravenously administered 12 h after oral administration was 1.5-fold higher and the half-life was 2-fold shorter compared with those after the single intravenous administration. The amount of absorbed TSU-68, as indicated by the radioactivity totally excreted in the bile and urine following oral administration of [(14)C]TSU-68, was unchanged by the prior oral administration. These results demonstrate that administered TSU-68 causes an increase in its elimination but not a decrease in its absorption after the subsequent administration. Furthermore, rat liver taken 12 h after administration of TSU-68 exhibited 6-fold higher activity of its microsomal oxidase than untreated liver. This result suggests that TSU-68 induced its own oxidative metabolism (i.e., autoinduction). In conclusion, the decrease in plasma concentrations of TSU-68 during the administration twice a day to rats was due to the rapid autoinduction. The same mechanism is probably at work in the clinical setting.

Long-term Follow-up of Blood Lead Levels and Haematological and Biochemical Parameters in Heifers that Survived an Accidental Lead Poisoning Episode

Four of 10 heifers that survived an episode of lead poisoning caused by an old broken battery in the pasture were periodically monitored from days 14 to 205 after the episode, with the aim of (i) determining the time-course of lead clearance from blood; and (ii) assessing whether the exposed cattle showed changes in haematological parameters or serum markers of hepatic and renal function (e.g. serum creatinine level) that might be useful as indicators of subclinical toxicity. On the first sampling day, lead levels in blood varied between 0.478 and 0.758 mg/l. The half-life of lead in blood was rather variable, ranging from 68 to 266 days. Despite the high blood lead levels, no significant changes were detected in haematological parameters or hepatic/renal markers, all of which remained within normal ranges. In conclusion, these cattle that survived an episode of lead poisoning may constitute a risk for human consumers, but do not appear to have suffered severe health consequences themselves.

Evidence of a flip-flop phenomenon in acamprosate pharmacokinetics: anin vivo study in rats

The pharmacokinetics of acamprosate were examined in the rat after oral and intravenous administration in order to detect the possible presence of a flip-flop phenomenon. Rats received 9.3 or 73.3 mg/kg of the drug as an intravenous bolus. The same doses were orally administered via gastric intubation. Plasma samples were taken from the jugular vein for determination of acamprosate concentration by liquid scintillation counting. The drug content was also quantified in urine and faeces. The acamprosate bioavailability was close to 20%, the amount recovered in the faeces being around 80% of the administered dose. The terminal slope of the oral plasma curve was significantly lower than that obtained after intravenous administration of the drug at both doses tested (p<2 x 10(-6) in both cases). Moreover, the downward slope after oral administration (lambda2=0.006 +/- 0.001 min(-1)) practically coincided with the first-order absorption rate constant, previously reported by us, obtained using an in situ rat gut technique. It is concluded that the acamprosate absorption rate is considerably slower than its elimination rate so that the drug exhibits flip-flop pharmacokinetics after oral administration. The lower intrinsic first-order absorption rate constant, ka, is responsible for this phenomenon.

Pharmacokinetics and pharmacokinetic/pharmacodynamic relationships for angiotensin-converting enzyme inhibitors

The pharmacokinetic (PK) properties and the pharmacokinetic/pharmacodynamic (PK/PD) relationships for the angiotensin-converting enzyme (ACE) inhibitors (ACEIs), such as enalaprilat, benazeprilat, imidaprilat and ramiprilat, differ from those of conventional drugs. This is because of their immediate and saturable binding to an ACE pool which is partly circulating (and contributing to the measured plasma concentration), and partly noncirculating (tissular), being anchored to the endothelium of vessels and not measurable by the analytical technique. A physiologically based model is required to allow appropriate interpretation of the different phases of the disposition curve of ACEI. The protracted terminal phase observed for all ACEIs is not a conventional elimination phase but a phase dependent on ACEI dissociation from ACE. In contrast, the phase which reflects ACEI elimination (and which is interpreted as a distribution phase for a conventional drug) has a short half-life, thus explaining the absence of drug accumulation during repeated dosing and mild kidney failure. ACE inhibition is the surrogate endpoint generally selected for establishing a PK/PD relationship and for simulating dosage regimen scenarios in order to decide on the appropriate dosage regimen for ACEIs.