Topic:Biological Half-Life
Biological half-life refers to the time required for a substance to decrease by half in its concentration within the body. In horses, understanding the biological half-life of various substances, such as medications, nutrients, or toxins, is important for determining dosing schedules, withdrawal times, and potential effects on equine health. The biological half-life can vary significantly depending on the substance in question, as well as factors such as the horse's metabolism, age, and health status. This page compiles peer-reviewed research studies and scholarly articles that explore the biological half-life of different substances in horses, examining factors that influence these rates and their implications for veterinary medicine and equine management.
Pharmacokinetics of ascorbic acid in horses. The pharmacokinetics of ascorbic acid were studied in 29 horses after intravenous (iv), subcutaneous, intramuscular (im) and oral administration. Following iv injection of 5 and 10 g ascorbic acid, respectively, a biphasic decline of ascorbic acid serum levels was found, indicating that the vitamin distributes in the body according to a two-compartment open model. The apparent volume of distribution (average value for Vd(ss) = 0.6 litre/kg) was approximately equivalent to the volume of total body water. The terminal half-life of the biexponential serum level-time curve (t1/2 beta) varied betwe...
Determination of flunixin in equine plasma by reversed-phase liquid chromatography. Flunixin is determined in equine plasma by liquid chromatography on LiChrosorb RP-18 with 70% methanol in phosphate buffer pH 3.1 as the eluent, with detection at 284 nm. Plasma is deproteinized with methanol and the supernatant is then injected directly into the system. With a short pre-column (5 x 3 mm i.d.), which is replaced after 25-40 injections of sample, 420 plasma samples could be analysed on one analytical column. The detection limit in plasma is 0.30 micromol/l (89 ng/ml) and the method can be used in pharmacokinetic studies.
Phenylbutazone kinetics and metabolite concentrations in the horse after five days of administration. Phenylbutazone (PBZ) was administered (8.8 mg/kg of body weight) every 24 hours for 5 consecutive days, orally for the first 4 days and IV on day 5. The half-life (t 1/2) after this daily administration was 6.2 hours and the volume of distribution was 0.152 +/- 0.014 L/kg; the bioavailability after oral administration was 91.8 +/- 2.5%. The plasma concentration of PBZ at experimental hour (EH) 24 (24 hours after the 1st oral dose) was 1.7 +/- 0.39 micrograms/ml and increased to 4.2 +/- 0.29 micrograms/ml at EH 48 (24 hours after the 2nd oral dose). Values at EH 72, 96, and 120 (24 hours after ...
Mechanical properties of equine hoof wall tissue. The mechanical properties of pigmented equine hoof wall tissue were determined for samples taken from the inner and outer portions of the stratum medium of the toe. Two properties, the modulus of elasticity and proportional limit, which are measures of the rigidity and yield point, respectively, of the tissue, were studied for samples compressed in 3 orthogonal directions. All samples tested were anisotropic. Inner wall samples were less rigid and had a lower yield point than outer wall samples.
High-performance liquid affinity chromatography on silica-bound alcohol dehydrogenase. Horse liver alcohol dehydrogenase was immobilized on glycerylpropyl-silica (10 micron, 1000-A pores) activated with 2,2,2-trifluoroethanesulfonyl chloride (tresyl chloride). The coupling and activity yield was almost 100%. The coenzyme-binding sites were equivalent and virtually unaffected by the immobilization process, as judged from Scatchard plots and active-site titrations. The silica-bound enzyme, packed in steel columns, was integrated with HPLC equipment and then successfully used for chromatography of adenine nucleosides, adenine nucleotides, and triazine dyes. Dissociation constants w...
Phenoxymethyl penicillin in the horse: an alternative to parenteral administration of penicillin. This preliminary study evaluated phenoxymethyl penicillin (Penicillin V) as an alternative to parenteral administration of penicillin in horses. Penicillin V was administered orally to five horses at two different doses and plasma levels of the drug were determined at timed intervals. The results were evaluated by regression analysis. Following the administration of penicillin V at a dose of 66,000 IU/kg or 110,000 IU/kg, the mean peak plasma levels obtained were 1.55 micrograms/mL and 2.34 micrograms/mL respectively. A plasma level two to four times above the minimal inhibitory concentration ...
A pharmacokinetic study of digoxin in the horse. Digoxin was administered orally and intravenously to seven healthy adult mares and geldings in two separate trials. At a dose of 44 microgram digoxin/kg body weight, the oral study was characterized by an absorption phase with a mean (+/- 1 standard deviation) peak serum digoxin concentration of 2.21 ng/ml (+/- 0.45) at a mean of 2.29 h (+/- 1.52) after administration. A second rise in serum digoxin concentration started about 6-8 h after administration and extended to about 20 h after administration. The mean bioavailability (F) was 23.38% (+/- 5.96). At a dose of 22 microgram digoxin/kg body...
Warfarin pharmacokinetics in the horse. The pharmacokinetics of racemic warfarin were studied in 6 adult horses. After IV administration, the plasma concentration of warfarin showed a biphasic decline in time. Analysis of the data, according to 2-compartment kinetics, revealed the following constants: biological half-life was 13.3 hours, apparent volume of distribution was 0.46 L X kg-1, body clearance was 25.3 ml X hour-1 X kg-1. Warfarin was bound (91.5%) to plasma proteins. Unchanged warfarin was not detected in the urine. Absorption from the gastrointestinal tract was almost complete. Concentrations of warfarin in tissue were ex...
Pharmacokinetics of phenytoin (diphenylhydantoin) in horses. The pharmacokinetics of the anti-convulsant phenytoin were investigated in clinically healthy horses after oral (p.o.) and intravenous (i.v.) administration. A single dose of phenytoin (8.8 mg/kg body weight) was given i.v. as a bolus to nine horses and one horse received 13.2 mg/kg. A two-compartment open model was used to describe the disposition of phenytoin. Four of the horses that received an i.v. dose (three at 8.8 mg/kg and one at 13.2 mg/kg) were then given the same dose 3 days later by the oral route. Phenytoin achieved a peak concentration in serum within 1-4 h after p.o. administrat...
The pharmacology of furosemide in the horse. V. Pharmacokinetics and blood levels of furosemide after intravenous administration. Studies were undertaken to determine blood levels of furosemide in horses after 0.5- and 1.0-mg/kg doses administered iv. Analyses indicated that the pharmacokinetic parameters were dose independent and best described by a three-compartment open model. The alpha-, beta-, and gamma-phase half-lives of 5.6, 22.3, and 158.5 min, respectively, were observed after the 0.5-mg/kg dose. Similarly, the respective half-lives after the 1.0-mg/kg dose were 5.8, 24.1, and 177.2 min. After a 0.5-mg/kg dose of furosemide, population frequency distributions were evaluated at 1 hr and 4 hr post-drug administra...
Pharmacokinetics and protein binding of morphine in horses. Morphine could be detected in horses dosed with 0.1 mg of drug/kg of body weight for up to 48 hours in blood and 144 hours in urine. This dose of morphine elicited no observable effects and is a suggested analgesic dose. Computer analysis revealed that a 3-compartment open system was the best fitting model with a serum half life (t1/2(beta)) of 87.9 minutes and a urine t1/2(beta) of 101.1 minutes. Binding to equine serum proteins was linear over a drug concentration range of 3.88 X 10(-5)M to 3.50 X 10(-8)M and averaged 31.6%. In RBC-partitioning experiments, 78.1% of the drug was found in the...
The metabolism of fenclofenac in the horse. 14C-Fenclofenac (2-(2'-4'-dichlorophenoxy)-phenylacetic acid) was administered orally to horses, and urinary metabolites investigated by chromatography. Fenclofenac was rapidly absorbed and eliminated, with a plasma half-life (t1/2) of 2.3 h, with 83.2 and 85.8% of the dose being recovered in the urine in 0-24 h. The major urinary metabolite was the ester glucuronide (58.8, 70.0% dose), and evidence is presented that this metabolite undergoes a structural rearrangement to give beta-glucuronidase-resistant isomers. The other 14C-labelled components in horse urine were unchanged fenclofenac (13....
Trimethoprim-sulfadiazine in the horse: serum, synovial, peritoneal, and urine concentrations after single-dose intravenous administration. Six healthy adult mares were given a single IV injection of trimethoprim (TMP)-sulfadiazine (SDZ) at a dosage rate of 2.5 mg of TMP/kg of body weight and 12.5 mg of SDZ/kg. Serum, synovial, peritoneal, and urine TMP-SDZ concentrations were measured serially over a 48-hour period. The highest measured mean concentrations of TMP and SDZ were found in the first (0.5 hour) sample of serum, synovial fluid, and peritoneal fluid. The mean peak concentrations of TMP and SDZ averaged 4.37 micrograms/ml and 21.81 micrograms/ml for serum, 2.95 micrograms/ml and 15.31 micrograms/ml for synovial fluid, and...
The pharmacokinetics, plasma protein binding and time response relationships of 2-amino-5-phenyl-2-oxazolin-4-one (pemoline) in the horse. The disposition kinetics of pemoline after iv and oral administration of 2.4 mg/kg of the drug were studied. The elimination half-life was 39.4 hr. The mean volume of distribution was 1.5 liters/kg indicating extensive tissue distribution and sequestration for an amphoteric drug. Plasma protein binding determined by in vitro equilibrium dialysis was concentration dependent. The mean binding capacity was found to be 0.80 mu-mol/g, an apparent dissociation constant of 3.73 X 10(-5) molar, and a total plasma protein concentration of 64.7 g/liter. The mean systemic availability by oral administrat...
Pharmacokinetics of erythromycin in foals and in adult horses. The pharmacokinetic parameters of erythromycin in foals were determined following intravenous administration of 5.0 mg/kg to animals aged 1, 3, 5 and 7 weeks. The distribution of the drug was described by a two-compartment open model, and no significant differences were observed between coefficients on which the parameters were based. Pharmacokinetic values were also determined for four mares given 5.0 mg/kg intravenously and for six 10-12-week-old foals given 20.0 mg/kg intravenously. The half-life of erythromycin for all groups of animals (foals less than 7 weeks, mares, foals 10-12 weeks) w...
Cortisol (hydrocortisone) disappearance rate and pathophysiologic changes after bilateral adrenalectomy in equids. Six ponies and 1 horse were bilaterally adrenalectomized (BADX). The survival time of 2 of the 7 animals after BADX was 24 and 72 hours without supplemental corticosteroids. The cause of death was not related to the surgical technique. The biological half-life of cortisol (hydrocortisone) was estimated to be 2.1 +/- 0.6 hours. The disappearance of cortisol in the horse was found to be biphasic, composed of redistribution and elimination phases. Pathophysiologic changes (ie, increased serum sodium and chloride, increased PCV, and decreased serum potassium) similar to those seen in other species...
Sodium retention and cortisol (hydrocortisone) suppression caused by dexamethasone and triamcinolone in equids. Three ponies and 1 horse were bilaterally adrenalectomized (BADX). The initial hypoadrenal episode after BADX was reversed with 20 mg of dexamethasone (DXM) IM (n = 2) or 20 mg of triamcinolone (TMC) IM (n = 2). Nine hypoadrenal crises were reversed with 20 mg of DXM given IM (n = 4) or 20 mg of TMC given IM (n = 5). Sodium and chloride retention and potassium excretion were documented based on changes in serum electrolytes and urinary excretion. Eight intact adult horses were randomly assigned to 2 groups to study the effects of a single IM injection of DXM (0.044 mg/kg of body weight) or TMC...
Ticarcillin administration to the equine: Intrauterine and intramuscular. Serum levels of ticarcillin disodium, a semi-synthetic penicillin (Beecham Laboratories, Bristol, Tennessee, 37620), were measured at various time intervals up to and including 24 h after intrauterine and intramuscular administration in adult female horses. Three separate studies were conducted in Part I: in the first and second studies, serum levels were measured after intrauterine administration of 1 and 3 g of ticarcillin, respectively, and in the third study, levels were measured after intramuscular administration of 6 g of ticarcillin. In Part II, serum levels of ticarcillin were measured...
Studies on prolactin 48: isolation and properties of the hormone from horse pituitary glands. Isolation of prolactin from equine pituitary glands has been described. It has a potency of 42 IU/mg in the pigeon crop-sac test and consists of 199 amino acids. The hormone has only four half-cystine residues in contrast to other mammalian prolactins which have six residues. From NH2-terminal sequence analysis and amino acid composition of cyanogen bromide fragments, the NH2-terminal disulfide loop is missing in the equine prolactin molecule. Circular dichroism spectra indicate that the alpha-helical content of equine prolactin appears to be lower (50%) than that found in the ovine hormone (6...
Pharmacology, pharmacokinetics, and behavioral effects of caffeine in horses. Caffeine (4 mg/kg) was given by rapid IV injection to 4 horses. Plasma concentrations of the drug peaked at 10 micrograms/ml and decreased rapidly at first, and then more slowly, with an apparent beta-phase half-life of 18.2 hours. Urinary concentrations of caffeine were remarkably consistent at about 3 times plasma values of the drug. Caffeine was detectable in both plasma and urine of the horses for up to 9 days after dosing. After oral administration, caffeine was absorbed poorly with an apparent bioavailability of 39%. Although blood concentrations of caffeine peaked rapidly after oral adm...
Serum concentrations of penicillin in the horse after administration of a variety of penicillin preparations. Twelve geldings were used to evaluate serum levels of penicillin following the intravenous (iv) and intramuscular (im) injection of benzylpenicillin, im procaine penicillin and im fortified benzathine penicillin, at a variety of dose rates. Blood samples were collected up to 7 h after iv and im benzylpenicillin, up to benzathine penicillin. Results were analysed using an analysis of variance and regression analyses were used to calculate the time taken for the serum penicillin concentration to decline to 0.5 micrograms/ml. As a result of the experiments the following dosage regimes could be re...
Pharmacodynamic and pharmacokinetic properties of diazepam in horses. The cardiopulmonary, behavioral, and pharmacokinetic properties of diazepam were determined in horses. Heart rate, cardiac output, mean pulmonary artery, aortic and right atrial blood pressures, respiratory rate, and arterial pH and blood gas values did not change after IV diazepam (0.05, 0.1, 0.2, 0.4 mg/kg) administration. Increasing doses of diazepam resulted in signs of muscle weakness including fixed stance, muscle fasciculations of the head, neck, and thorax muscles, ataxia, and then recumbency. Dosages of diazepam exceeding 0.2 mg/kg produced mild sedation. Behavioral changes persisted ...
Gentamicin sulfate in the horse: serum, synovial, peritoneal, and urine concentrations after single dose intramuscular administration. Ten healthy adult mares were given a single intramuscular dose (2.2 mg/kg) of gentamicin sulfate. Over a 48-h period, gentamicin concentrations were measured serially in the serum of all ten mares and in synovial fluid, peritoneal fluid, and urine of six of the mares. The mean peak serum gentamicin concentration was 5.73 micrograms/ml at 1 h. Gentamicin was detected in synovial fluid and peritoneal fluid, with mean peak gentamicin concentrations of 2.41 micrograms/ml and 3.92 micrograms/ml, respectively, observed at 2 h. These concentrations declined in parallel with serum concentrations and w...