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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.
Synovial fluid and plasma kinetics of methylprednisolone and methylprednisolone acetate in horses following intra-articular administration of methylprednisolone acetate. Synovial fluid and plasma kinetics of methylprednisolone acetate (MPA) and methylprednisolone (MP) after a single intra-articular administration of MPA at a therapeutic dose (111 mg in toto) was measured in five horses. MPA was detected in synovial fluid for two to six days post injection and MP, which results from synovial MPA hydrolysis, was present in pharmacologically significant concentrations for 4.8 to 39 days, depending on the horse. MPA synovial concentration was maximal (289 +/- 284 micrograms/ml) at the first sampling time (2 h after administration) and MP synovial concentration was...
Gentamicin tissue concentrations in equine small intestine and large colon. Gentamicin sulfate (2.2 mg/kg of body weight, IV) was given to anesthetized horses. Jejunal and large colon tissue samples (1 g), serum, and urine were collected over a 4-hour period. Maximum gentamicin concentrations in serum (10.06 +/- 2.85 micrograms/ml) occurred at 0.25 hours after injection. Maximum gentamicin concentrations in the large colon (4.13 +/- 1.80 micrograms/ml) and jejunum (2.26 +/- 1.35 micrograms/ml) occurred in horses at 0.5 and 0.33 hours, respectively. Tissue concentrations decreased in parallel with serum concentrations and were still detectable at the end of the 4-hour ...
Phenylbutazone in the horse: a review. Phenylbutazone is an acidic, lipophilic, non-steroidal anti-inflammatory drug (NSAID). It is extensively metabolized in the horse. The metabolites so far identified, oxyphenbutazone, gamma-hydroxyoxyphenbutazone, account for some 25-30% of administered dose over 24 h. The plasma half-life of phenylbutazone and termination of its pharmacological action are determined primarily by its rate of hepatic metabolism. Phenylbutazone acts by inhibiting the cyclooxygenase enzyme system, which is responsible for synthesis of prostanoids such as PGE2. It appears to act on prostaglandin-H synthase and pros...
Gentamicin dosage in foals aged one month and three months. The absorption and disposition kinetics of gentamicin were compared at two dosage levels (2 and 4 mg/kg bodyweight [bwt]) in one- and three-month-old foals. Following intramuscular (im) injection of single 2 mg/kg bwt doses, the drug was absorbed rapidly and produced peak serum concentration (18.2 mu 5.3 +/- g/ml, n = 8) at 30 mins. Much wider variations were associated with the amount of drug absorbed and the serum gentamicin concentrations after administration at the higher dosage level. The half-life of gentamicin was similar in the one-month-old (3.7 +/- 1.7 h, n = 8) and three-month-old (...
Pharmacokinetics of amikacin in pony foals after a single intramuscular injection. Six healthy pony foals, from 2 to 11 days of age, were given a single IM injection of amikacin sulfate (250 mg/ml) at a dosage rate of 7 mg/kg of body weight. Serum amikacin concentrations were measured serially over a 24-hour period. The mean peak serum concentration was 14.7 micrograms/ml at 0.5 hour. The elimination rate constant for amikacin was 0.24/hour, the elimination half-life was 3.0 hours, and the apparent volume of distribution was 0.58 L/kg.
[14C]monensin balance in bile-fistulated ponies. To measure absorption of monensin or its metabolites and its elimination from the body, [14C]monensin sodium was given orally (1 mg/kg body wt) to two bile-fistulated ponies and iv (8.7 mg) to one bile-fistulated pony. For one orally-dosed pony, 4.7% of the 14C was eliminated in bile, 52% in feces, .7% in urine and 33% remained in the gastrointestinal (GI) tract after 3 d. Total 14C recovery was 90%. For the other orally-dosed pony, 18.3% of the 14C was eliminated in bile, 69% in feces, 1.7% in urine and 7% remained in the GI tract after 4 d. Total 14C recovery was 98%. For the iv-dosed pony, ...
Pharmacokinetics of probenecid and the effect of oral probenecid administration on the pharmacokinetics of cefazolin in mares. The pharmacokinetics and bioavailability of probenecid given IV and orally at the dosage level of 10 mg/kg of body weight to mares were investigated. Probenecid given IV was characterized by a rapid disposition phase with a mean half-life of 14.0 minutes and a subsequent slower elimination phase with a mean half-life of 87.8 minutes in 5 of 6 mares. In the remaining mare, a rapid disposition phase was not observed, and the half-life of the elimination phase was slower (172 minutes). The mean residence time of probenecid averaged 116 minutes for all 6 mares and 89.2 minutes for the 5 mares with...
Extraction, radioiodination, and in vivo catabolism of equine fibrinogen. Equine fibrinogen was isolated and aliquots were stored frozen at -70 C before radiolabeling with 125I (half-life = 60.2 days; gamma = 35 keV, using monochloroiodine reagent. Radioiodination efficiencies were 49% to 53%, resulting in a labeled product with 98% protein-bound activity and 91% clottable radioactivity. In 6 equine in vivo investigations, plasma half-lives of 125I-labeled fibrinogen were from 4.1 to 5.2 days, corresponding to a mean daily plasma elimination rate of approximately 15%.
Rapid extraction, radioiodination, and in vivo catabolism of 125I-labeled fibrinogen in the horse. Two methods were analyzed for the rapid extraction of equine fibrinogen from fresh plasma, using ammonium sulfate-sodium phosphate buffer. Fibrinogen from each of these 2 methods was then radiolabeled with 125I (half-life = 60.2 days, gamma = 35 keV), using monochloroiodine reagent. Mean protein-bound activity was 98.5% and mean clottable radioactivity was 94.1%. Radiolabeled fibrinogen administered IV to 15 horses had an overall mean (+/- SD) plasma half-life of 4.95 +/- 0.44 days.
Effects of oral cimetidine on plasma concentrations of phenylbutazone in horses. Phenylbutazone was administered to six Thoroughbred horses in a cross-over study in which the horses received cimetidine pretreatment or no cimetidine pretreatment. Blood samples were collected at various times for 48 h after phenylbutazone administration and the plasma was analysed for phenylbutazone. Cimetidine pretreatment elevated phenylbutazone plasma concentrations during the first 8 h after phenylbutazone administration. The absorption rate, maximum phenylbutazone plasma concentrations and AUC were significantly greater with cimetidine pretreatment. The half-life of phenylbutazone did n...
Flunixin meglumine given in small doses: pharmacokinetics and prostaglandin inhibition in healthy horses. The pharmacokinetics and inhibition of prostaglandin synthesis in conscious horses given various dosages of flunixin meglumine were studied. Plasma concentrations of flunixin were measured by high-performance liquid chromatography, and serum thromboxane B2 and 6-keto prostaglandin F1 alpha were quantitated by radioimmunoassay. Within the dosage range studied, linear pharmacokinetics were achieved. After IV administration of flunixin (1.1 mg/kg, 0.25 mg/kg, 0.1 mg/kg), significant suppression of serum thromboxane generation persisted for 12, 4, and 3 hours, respectively. Repeated administration...
The organism as bioreactor. Interpretation of the reduction law of metabolism in terms of heterogeneous catalysis and fractal structure. Organisms and bioreactors are open, dissipative systems in steady state. They are functionally equivalent with respect to turnover and kinetics, and structurally analogous with respect to fractal organization and self-similar scaling. As heterogeneous catalytic systems both are governed by interaction of mass transport and reaction. The structural equivalent to turbulence in the reactor, yielding high efficiency, is the fractal folding and branching of the transport systems of the organism. Dimensionally and in terms of fractals, organisms and reactors are therefore area-volume hybrids. The ph...
Inhibitory effects of intravenous chloramphenicol sodium succinate on the disposition of phenylbutazone in horses. The effects of i.v. chloramphenicol sodium succinate on the pharmacokinetics of i.v. phenylbutazone in six healthy adult horses were investigated. Administration of chloramphenicol sodium succinate to mares reduced mean (+/- SD) phenylbutazone clearance from 0.600 +/- 0.222 to 0.339 +/- 0.123 ml/min per kg and increased mean (+/- SD) half life from 244 +/- 59.8 to 371 +/- 80.8 min and mean residence time from 333 +/- 86.2 to 533 +/- 124 min. The mean steady-state volume of distribution of phenylbutazone was unchanged, with mean (+/- SD) values of 187 +/- 28.9 ml/kg in control animals and 170 +...
Pharmacokinetics and bioavailability of cephalothin in horse mares. The pharmacokinetics and bioavailability of cephalothin given to 6 horse mares at a dosage level of 11 mg/kg of body weight IV or IM were investigated. The disposition of cephalothin given IV was characterized by a rapid disposition phase with a mean half-life of 2.89 minutes and a subsequent slower elimination phase with a mean half-life of only 14.7 minutes. The mean residence time of cephalothin was 10.6 +/- 2.11 minutes. The total plasma clearance of cephalothin averaged 13.6 ml/min/kg and was caused by metabolism and renal elimination. Renal clearance of cephalothin averaged 1.32 ml/min/k...
Disposition of sulfadimidine and its N4-acetyl and hydroxy metabolites in horse plasma. The plasma disposition of sulfadimidine (SDM) and its metabolites N4-acetylsulfadimidine (N4-SDM), 6-hydroxymethyl-4-methyl-pyrimidine (SCH2OH) and 5-hydroxy-4,6-dimethyl-pyrimidine (SOH), was studied in three horses following intravenous administration of SDM at dose levels of 20 and 200 mg/kg in cross-over trials. The percentages of N4-SDM (0.58-0.90%), SOH (0.83-6.75%) and SCH2OH (0.38-0.71%) in plasma, expressed as a percentage of the total sulfonamide concentration, were small and their plasma concentrations were parallel with SDM from 4 h following administration. At high doses (200 mg/k...
Combined pharmacokinetics of gentamicin in pony mares after a single intravenous and intramuscular administration. Healthy mature pony mares (n = 6) were given a single dose of gentamicin (5 mg/kg of body weight) IV or IM 8 days apart. Venous blood samples were collected at 0, 5, 10, 20, 30, and 45 minutes and at 1, 1.5, 2, 2.5, 3, 4, 6, 8, 10, 12, 18, 24, 30, 36, 40, and 48 hours after IV injection of gentamicin, and at 10, 20, 30, and 45 minutes and at 1, 1.5, 2, 2.5, 3, 4, 6, 8, 10, 12, 18, 24, and 30 hours after IM injection of gentamicin. Gentamicin serum concentration was determined by a liquid-phase radioimmunoassay. The combined data of IV and IM treatments were analyzed by a nonlinear least-square...
Pharmacokinetics of small doses of 3-methylindole given to horses. The pharmacokinetics of 3-methylindole (3MI) given orally in 2 doses (10 mg/kg and 20 mg/kg) to horses were determined. The pharmacokinetic plasma-concentration profiles for 3MI (10- and 20-mg/kg dosages) in horses were represented by a 2-compartment open model with first-order absorption, as determined by nonlinear least-squares regression analysis. Absorption of 3MI at both dosages was rapid. Comparisons of the peak plasma concentrations, the postdistribution half lives, total clearances, and areas under the curve of the plasma-concentration profiles between the 10- and the 20-mg/kg dosages ...
Measurement of the time between biosynthesis and surface excretion of sebaceous lipids in the horse. The time between the biosynthesis and excretion of sebum to the skin surface of the horse was examined by in vivo intradermal injection of [1-14C]acetate followed by periodic surface lipid collections. The radiolabelling of the major neutral lipid classes, equolides (giant ring omega-lactones, C32-C36) and cholesteryl esters, was evaluated by thin-layer chromatography and autoradiography. The distribution of radioactivity within the monounsaturated equolides was examined by oxidative fragmentation and evaluation of the products. A peak of radioactivity in the equolides and cholesteryl esters o...
Pharmacokinetics of gentamicin at steady-state in ponies: serum, urine, and endometrial concentrations. Gentamicin (GT) was administered IM to 6 healthy mature mare ponies at a dosage of 5 mg/kg of body weight every 8 hours for 7 consecutive days (total, 21 doses). Two venous blood samples were collected before (trough) and at 1 hour (peak) after the 5th, 10th, 14th, and 19th doses. An endometrial biopsy was done of each mare on days 4 and 7. On the 7th day, just before the 21st administration of GT, base-line blood samples were collected, and 22 blood samples were collected over a period of 48 hours after GT was given. The mares were catheterized on the 7th day, and urine was collected for 24 h...
Changes in the synovia after the intra-articular injection of sodium hyaluronate into normal horse joints and after arthrotomy and experimental cartilage damage. Sodium hyaluronate was injected into normal horse joints and joints that had undergone an arthrotomy and experimental cartilage damage. The elimination half-life for hyaluronic acid in normal joints was found to be approximately 96 h. The injection caused a non-significant increase (42%) in synovial fluid protein concentration and a fall in the intrinsic viscosity of the fluid. In the arthrotomy group the synovial fluid hyaluronic acid concentration fell after surgery but it was unaffected by the injection of sodium hyaluronate. An initial rise in the intrinsic viscosity of the synovial fluid ...
Pharmacokinetic studies of theophylline in horses. The pharmacokinetics of theophylline were determined in Standardbred trotters after single intravenous and oral administration. A bi-exponential equation was fitted to the intravenous data and a tri-exponential equation to the oral data. The biological half-life of theophylline was found to be 14.8 h, the volume of distribution 1.02 l/kg and the total plasma clearance 0.86 ml/kg/min. The oral absorption of the drug was complete (bioavailability 108%) and rapid (absorption half-life 0.4 h).
Pharmacokinetics and bioavailability of cefazolin in horses. The pharmacokinetics and bioavailability of cefazolin given (IV, IM) to horses at the dosage of 11 mg/kg were investigated. The disposition of cefazolin given by IV route was characterized by a rapid disposition phase with a half-life of 5 to 10 minutes and a subsequent slower elimination phase with a half-life of 35 to 46 minutes. The total plasma clearance of cefazolin averaged 5.51 ml/min/kg and was due mainly to renal clearance (5.39 ml/min/kg) of unchanged drug. The volume of distribution at steady-state averaged 188 ml/kg. Plasma protein binding of cefazolin at a concentration of 10 micr...
[Chronopharmacokinetics of phenylbutazone in the horse. Application to antidoping control]. Chronopharmacokinetics of intravenous phenylbutazone in the horse was studied with the aim of antidoping control. Among parameters studied, the single one which seemed to depend on circadian rhythm was the elapsed time between the injection and the plasmatic peak. There was no relationship between the injection time and the both parameters: half-life and time required to reach the forensic level of 4 micrograms/ml. This later, and oxyphenbutazone/phenylbutazone ratio, should depend on individual factors. Therefore, the injection time should not be a main parameter for the phenylbutazone evalua...
Effect of feeding on the fate of orally administered phenylbutazone, trimethoprim and sulphadiazine in the horse. Phenylbutazone, sulphadiazine and trimethoprim were administered to three horses on two occasions, recently fed and unfed, and the effect of feeding on the pharmacokinetics of the three drugs assessed. The mean peak concentrations of phenylbutazone and trimethoprim were reduced by feeding by 34 and 75 per cent, respectively. The pharmacokinetics of sulphadiazine were not significantly affected.
Pharmacokinetics and bioavailability of theophylline in horses. The pharmacokinetics and bioavailability of theophylline in horses were investigated following both intravenous and intragastric administration of aminophylline solutions at doses corresponding to 15 and 10 mg/kg theophylline base. A rapid distributive phase with a half-life of approximately 15-30 min was followed by a slower elimination half-life averaging 15-17 h. The apparent volume of distribution averaged 850-900 ml/kg. Theophylline, administered as aminophylline solution, was both rapidly and completely absorbed from the equine digestive tract. Based on the bioavailability and dispositio...
Population distributions of phenylbutazone and oxyphenbutazone after oral and i.v. dosing in horses. Experiments to determine the residual plasma concentrations of phenylbutazone and its metabolites found in horses racing on a 'no-race day medication' or 24-h rule were carried out. One dosing schedule (oral-i.v.) consisted of 8.8 mg/kg (4 g/1000 lbs) orally for 3 days, followed by 4.4 mg/kg (2 g/1000 lbs) intravenously on day 4. A second schedule consisted of 4.4 mg/kg i.v. for 4 days. The experiments were carried out in Thoroughbred and Standardbred horses at pasture, half-bred horses at pasture, and in Thoroughbred horses in training. After administering the i.v. schedule for 4 days to Thor...
Effect of tyrosine modification on the biological and immunological properties of equine chorionic gonadotropin. The tyrosine residues of equine chorionic gonadotropin have been nitrated with tetranitromethane and the resulting effects on the biological and immunological activities of the hormone studied. All of the tyrosine residues in equine chorionic gonadotropin were found to react with tetranitromethane when a 100-fold molar excess of reagent was used or with an 8.6 molar excess in the presence of 5 M guanidine hydrochloride. Complete nitration abolished the biological activities and decreased the immunological activity of the hormone. The nitration of one tyrosine residue resulted in the loss of 70...
