M. Rebuelto

Chronobiological Study of the Pharmacological Response of Rats to Combination Ketamine–Midazolam

Ketamine is commonly administered in combination with benzodiazepines to achieve surgical anaesthesia in rats. The aim of the present study was to analyze the pharmacological response of the combination ketamine–midazolam injected intraperitoneally at different times of day to rats. The study was conducted in July 2003, during the winter in the Southern hemisphere. Female prepuberal Sprague-Dawley rats synchronized to a 12 h light:12 h dark cycle (light, 07:00–19:00 h) were used as experimental animals. A combination treatment of ketamine (40 mg/kg) and midazolam (2 mg/kg) was administered to five different clock-time groups of rats (n = 7/group). Duration of the latency period, ataxia, loss-of-righting reflex (LRR), post-LRR ataxia, and total pharmacological response were assessed by visual assessment. Significant treatment-time differences were detected in the duration of LRR, post-LRR ataxia, and total pharmacological response duration. The longest pharmacological response occurred in rats injected during the light (rest) phase, and the shortest pharmacological response occurred in rats injected during the dark (activity) phase. Cosinor analysis documented circadian rhythmicity in the duration of post-LRR ataxia. The findings of the study indicate the duration of CNS-depression of the ketamine–midazolam combination exhibits treatment-time-dependent variation in the rat.

Treatment-time-dependent difference of ketamine pharmacological response and toxicity in rats.

Circadian rhythms impact many physiological functions that may affect drug pharmacological response. Ketamine is a dissociative agent commonly used for surgical anesthesia in rats. The aim of the present study was to analyze the central nervous system (CNS) depression and lethality of ketamine injected intraperitoneally at different times during the 24 h. The study was conducted in October 2001, spring in the Southern hemisphere. Female prepuberal Sprague–Dawley rats synchronized to a 12h light:12h dark cycle (light, 07:00h–19:00h) were studied. Ketamine (40 mg/kg) was administered to one of six different clock-time treatment groups (n=6–7 rats each). Duration of latency period, ataxia, loss of righting reflex (LRR), post-LRR ataxia, and total pharmacological response were determined by visual assessment. To investigate acute toxicity, ketamine lethal dose 50 (148.0 mg/kg) was also administered as a single injection to six different treatment-time groups of rats. Significant temporal differences and circadian rhythms were detected in drug-induced post-LRR ataxia and total pharmacological response duration. The longest pharmacological response occurred in rats injected during the light (rest) phase and the shortest response in the dark (activity) phase. No circadian rhythm was detected in acute toxicity. The study findings indicate that the duration of CNS depression of ketamine in rats exhibits circadian rhythmic variation.

Chronopharmacological study of cephalexin in dogs.

Recent studies have identified a 24 h rhythm in the expression and function of PEPT1 in rats, with significantly higher levels during the nighttime than daytime. Similarly, temporal variations have been described in glomerular filtration rate and renal blood flow, both being maximal during the activity phase and minimal during the rest phase in laboratory rodents. The aim of this study was to assess the hypothesis that the absorption of the first‐generation cephalosporin antibiotic cephalexin by dogs would be less and the elimination would be slower after evening (rest span) compared to morning (activity span) administration, and whether such administration‐time changes could impair the medications predicted clinical efficacy. Six (3 male, 3 female; age 4.83±3.12 years) healthy beagle dogs were studied. Each dog received a single dose of 25 mg/kg of cephalexin monohydrate per os at 10∶00 and 22∶00 h, with a two‐week interval of time between the two clock‐time experiments. Plasma cephalexin concentrations were determined by microbiological assay. Cephalexin peak plasma concentration was significantly reduced to almost 77% of its value after the evening compared to morning (14.52±2.7 vs. 18.77±2.8 µg/mL) administration. The elimination half‐life was prolonged 1.5‐fold after the 22∶00 h compared to the 10∶00 h administration (2.69±0.9 vs. 1.79±0.2 h). The area under the curve and time to reach peak plasma concentration did not show significant administration‐time differences. The duration of time that cephalexin concentrations remained above the minimal inhibitory concentrations (MIC) for staphylococci susceptiblity (MIC=0.5 µg/mL) was>70% of each of the 12 h dosing intervals (i.e., 10∶00 and 22∶00 h). It can be concluded that cephalexin pharmacokinetics vary with time of day administration. The findings of this acute single‐dose study require confirmation by future steady‐state, multiple‐dose studies. If such studies are confirmatory, no administration‐time dose adjustment is required to ensure drug efficacy in dogs receiving an oral suspension of cephalexin in a dosage of 25 mg/kg at 12 h intervals.

Chronopharmacokinetic Study of Gentamicin in Dogs

The purpose of this experiment was to determine whether the time of day of single intravenous doses of gentamicin affects the drugs pharmacokinetics in dogs maintained under a 12 h light (08:00 to 20:00 h), 12 h dark (20:00 to 08:00 h) cycle. Using a crossover design, 6 mixed‐breed male dogs received a single dose of 2 mg/kg of gentamicin at 8:00 or 20:00 h. Serial blood samples were collected and pharmacokinetic parameters were calculated following each timed dose. The concentration of the antibiotic was lower following the 08:00 h compared to the 20:00 h administration. When gentamicin was administered at 20:00 h, the initial concentration, mean residence time, and area under the disposition curve were significantly higher (p<0.05) and the apparent volume of distribution of the central compartment, apparent volume of distribution, apparent volume of distribution at steady‐state, and total body clearance (1.73±0.55 at 20:00 h versus 3.31±0.67 L/min/kg at 08:00 h) were significantly lower than for the 08:00 h administration (p<0.05). Our results show that the pharmacokinetics of gentamicin exhibits significant temporal variation when administered to dogs at different times of day.

Pharmacokinetics of erythromycin after the administration of intravenous and various oral dosage forms to dogs

The purpose of this study was to describe and compare the pharmacokinetic properties of different formulations of erythromycin in dogs. Erythromycin was administered as lactobionate (10 mg/kg, IV), estolate tablets (25 mg/kg p.o.) and ethylsuccinate tablets or suspension (20 mg/kg p.o.). After intravenous (i.v.) administration, the principal pharmacokinetic parameters were (mean +/- SD): AUC((0-infinity)) 4.20 +/- 1.66 microg x h/mL; C(max) 6.64 +/- 1.38 microg/mL; V(z) 4.80 +/- 0.91 L/kg; Cl(t) 2.64 +/- 0.84 L/h.kg; t((1/2)lambda) 1.35 +/- 0.40 h and MRT 1.50 +/- 0.47 h. After the administration of estolate tablets and ethylsuccinate suspension, the principal pharmacokinetic parameters were (mean +/- SD): C(max), 0.30 +/- 0.17 and 0.17 +/- 0.09 microg/mL; t(max), 1.75 +/- 0.76 and 0.69 +/- 0.30 h; t((1/2)lambda), 2.92 +/- 0.79 and 1.53 +/- 1.28 h and MRT, 5.10 +/- 1.12 and 2.56 +/- 1.77 h, respectively. The administration of erythromycin ethylsuccinate tablets did not produce measurable serum concentrations. Only the i.v. administration rendered serum concentrations above MIC(90) = 0.5 microg/mL for 2 h. However, these results should be cautiously interpreted as tissue erythromycin concentrations have not been measured in this study and, it is recognized that they can reach much higher concentrations than in blood, correlating better with clinical efficacy.

Antibiotic Treatment of Dogs and Cats during Pregnancy

The use of pharmacological agents in pregnant females poses a major clinical challenge due to the marked physiological changes that may modify the pharmacokinetics of drugs and to the potential effects on the fetus. The purpose of this paper is to review briefly our knowledge on the use of antibacterial drugs during pregnancy and to provide information for the judicious selection of an antimicrobial treatment for use in pregnant bitches and queens. The risk to the fetus is a result of the ability of a drug to reach the fetal circulation and to produce toxic effects. The placenta functions as a barrier that protects the fetus due to the presence of transporters and metabolising enzymes; however, during pregnancy, the presence and activity of both enzymes and transporters may change. Antimicrobial agents that have been shown to be safe for use during pregnancy include betalactams, macrolides, and lincosamides. Pharmacotherapy during pregnancy in all species may affect adversely the developing fetus; therefore, it should be avoided when possible.

Farmacocinética de la ampicilina en Alpacas (Lama pacos )

Ampicillin is a betalactamic antibiotic of frequent use in human and veterinary medicine. Ampicillin pharmacokinetics was determined in 6 alpacas after intra...

A Population Pharmacokinetic Approach to Describe Cephalexin Disposition in Adult and Aged Dogs

This study was conducted in order to characterize the pharmacokinetics of orally administered cephalexin to healthy adult and aged dogs, using a population pharmacokinetic approach. Two hundred and eighty-six cephalexin plasma concentrations obtained from previous pharmacokinetic studies were used. Sex, age, pharmaceutical formulation, and breed were evaluated as covariates. A one-compartment model with an absorption lag-time (Tlag) best described the data. The final model included age (adult; aged) on apparent volume of distribution (Vd/F), apparent elimination rate (ke/F), and Tlag; sex (female; male) on ke/F, and breed (Beagle; mixed-breed) on Vd/F. Addition of the covariates to the model explained 78% of the interindividal variability (IIV) in Vd/F, 36% in ke/F, and 24% in Tlag, respectively. Formulation did not affect the variability of any of the pharmacokinetic parameters. Tlag was longer, whereas Vd/F and ke/F were lower in aged compared to adult animals; in female aged dogs ke/F was lower than in male aged dogs; however, the differences were of low magnitude. Different disposition of cephalexin may be expected in aged dogs.

Comparative pharmacokinetics of intravenous cephalexin in pregnant, lactating, and nonpregnant, nonlactating goats

The aims of this study were to describe and compare the pharmacokinetics of a single dose of cephalexin (10 mg/kg) after its intravenous (i.v.) administration to five goats in three different physiological status: nonpregnant nonlactating (NPNL), pregnant (P) and nonpregnant lactating (L). Blood samples were collected at predetermined times, and plasma concentrations of cephalexin were measured by microbiological assay. Relevant pharmacokinetic parameters were calculated using noncompartmental analysis. Statistical comparison was performed applying the nonparametric anova. No significant differences were found for cephalexin pharmacokinetic parameters between the L and the NPNL group. Median V(dss) was significantly lower in pregnant goats (0.09 [0.07-0.10] L/kg) compared with NPNL goats (0.16 [0.14-0.49] L/kg). Median total Cl and V(dz) were significantly lower in pregnant goats (0.25 [0.19-0.29] L/h·kg and 0.11 [0.10-0.13] L/kg, respectively) than in lactating goats (0.40 [0.32-0.57] L/h·kg and 0.20 [0.14-0.23] L/kg, respectively). Median AUC(0-∞) was significantly higher in pregnant goats (37.79 [34.75-52.10] μg·h/mL) than in lactating goats (25.11 [17.44-31.14] μg·h/mL). Our study showed that even though some pharmacokinetic parameters of cephalexin are altered in pregnant and lactating goats, these differences are unlikely to be of clinical importance; therefore, no dose adjustment would be necessary during pregnancy and lactation.

Pharmacokinetics of ceftazidime after intravenous, intramuscular and subcutaneous administration to dogs

Ceftazidime, a third generation cephalosporin, is active againstGram negative bacteria, as Enterobacteriaceae (Escherichia coli,Proteusspp.,Klebsiellaspp.)andPseudomonasaeruginosa,andsomeGram positive cocci as staphylococci and streptococci. Modifica-tionsinthepharmacokineticsofantimicrobialagentsraiseamajorconcern as successful antimicrobial treatment depends on appro-priate plasma and tissue concentrations. Previous studies havedetermined that, for time-dependent killing antimicrobials, suchas beta-lactams, the major determinant for bactericidal activity isthe time for which serum drug concentrations remain above theminimum inhibitory concentration (T > MIC) for the offendingpathogen (Craig, 1998; Drusano, 2004; McKellar et al., 2004).Occasionally, parenteral administrations are considered inter-changeable by the veterinary practitioner. However, it is knownthat several factors, including route of administration, may alterthe rate and extent of drug absorption and disposition. We haverecentlyprovedthelackofinterchangeabilityoftheintramuscular(i.m.) and subcutaneous (s.c.) routes of administration forcephalexin in cows (Waxman et al., 2008).Ceftazidime is commercially available for intravenous (i.v.),i.m. and s.c. administration. Ceftazidime pharmacokinetics hasbeen well characterized in cats (Albarellos et al., 2008), calves(Soback & Ziv, 1989), cows (Rule et al., 1996), sheep (Rule et al.,1991) and dogs (Matsui et al., 1984; Kita et al., 1992; Mooreet al., 2000). However, to our knowledge, no studies have beenconducted to assess T > MIC calculated after the differentparenteral administrations in dogs. This study compares thepharmacokinetic behaviour and the efficacy predictor T > MICagainst P. aeruginosa of a single dose ceftazidime following i.v.,i.m. and s.c. administration to dogs. The aim was to determinewhether these administrations would provide T > MIC valuesthat would suggest similar clinical outcomes.Six female beagle adult dogs (body weight 15.61 ± 5.47 kg)were included in this study. All dogs were in good health, asdetermined by history, physical examination, haematologicaland biochemical tests and urinalysis. None of the dogs had beentreated with antibiotics in the previous 2 months or had ahistory of allergy to beta-lactams. The protocol was approved bythe Institutional Animal Care and Use Committee of theVeterinary Science School, University of Buenos Aires.Single doses of an aqueous solution of ceftazidime pentahy-drate (Ceftazidima Richet, Laboratorios Richet S.A., BuenosAires, Argentina) were administered by the cephalic vein(20 mg⁄kg), and by the i.m. (25 mg⁄kg) and s.c. (25 mg⁄kg)routes. Each dog received the three treatments following acrossover design with a 2-week washout period. Blood samples(2 mL) were collected from the jugular vein into heparinizedtubes at 0, 0.08, 0.16, 0.33, 0.5, 0.75, 1, 1.5, 2, 2.5, 3, 4, 6, 8,10, 12 and 24 h after ceftazidime administration. Samples wereimmediately centrifuged and the plasma was stored at )20 Cuntil assayed.Ceftazidime plasma concentrations were determined in tripli-cate by a microbiological assay (Bennet et al., 1966) usingEscherichia coli ATCC 25922 as test organism. Standard curves ofceftazidime were prepared in pooled canine plasma and runsimultaneously with test samples. The quantification limit was3.125 lg⁄mL. The correlation coefficient for the regression lineof the standard solution was 0.98. The within and between-daycoefficients of variation and the accuracy (bias) of the assay were<2.5%, 10% and 5%, respectively, in the range of observedconcentrations (from 3.125 to 200 lg⁄mL).A computer program (