• View in gallery
    Figure 1—

    Illustration of the chemical structure of metformin.

  • View in gallery
    Figure 2—

    Mean plasma concentrations of metformin after IV and PO administration of 6 g to 4 adult horses. Oral administration was performed when horses were fed and also when feed was withheld for 12 hours. lg = Log.

  • 1.

    Bailey CJ, Turner RC. Metformin. N Engl J Med 1996;334:574579.

  • 2.

    DeFronzo RA. Pathogenesis of type 2 diabetes: implications for metformin. Drugs 1999;58:2930.

  • 3.

    Dunn CJ, Peters DH. Metformin. A review of its pharmacological properties and therapeutic use in non-insulin-dependent diabetes mellitus. Drugs 1995;49:721749.

    • Search Google Scholar
    • Export Citation
  • 4.

    Derosa G, D'Angelo A, Rogonesi PD, et al.Metabolic effects of pioglitazone and rosiglitazone in patients with diabetes and metabolic syndrome treated with metformin. Intern Med J 2007;37:7986.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 5.

    Moon RJ, Bascombe LA, Holt RI. The addition of metformin in type 1 diabetes improves insulin sensitivity, diabetic control, body composition, and patient well-being. Diabetes Obes Metab 2007;9:143145.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 6.

    Howlett HC, Bailey CJ. A risk-benefit assessment of metformin in type 2 diabetes mellitus. Drug Saf 1999;20:489503.

  • 7.

    Michels GM, Boudinot FD, Ferguson DC, et al.Pharmacokinetics of the antihyperglycemic agent metformin in cats. Am J Vet Res 1999;60:738742.

    • Search Google Scholar
    • Export Citation
  • 8.

    Pentikäinen PJ, Neuvonen PJ, Penttilä A. Pharmacokinetics of metformin after intravenous and oral administration to man. Eur J Clin Pharmacol 1979;16:195202.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 9.

    Treiber KH, Kronfeld DS, Hess TM, et al.Evaluation of genetic and metabolic predispositions and nutritional risk factors for pasture-associated laminitis in ponies. J Am Vet Med Assoc 2006;228:15381545.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 10.

    Frank N, Elliot SB, Brandt LE, et al.Physical characteristics, blood hormone concentrations, and plasma lipid concentrations in obese horses with insulin resistance. J Am Vet Med Assoc 2006;228:13831390.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 11.

    Kronfeld DS, Treiber KH, Hess TM, et al.Insulin resistance in the horse: definition, detection, and dietetics. J Anim Sci 2005;83(suppl E):E22E31.

    • Search Google Scholar
    • Export Citation
  • 12.

    Johnson PJ. The equine metabolic syndrome peripheral Cushing's syndrome. Vet Clin North Am Equine Pract 2002;18:271291.

  • 13.

    Frank N. Diagnosis of equine metabolic syndrome, in Proceedings. Annu Meet Am Coll Vet Intern Med Forum 2007;157159.

  • 14.

    Ralston SL. Insulin and glucose regulation. Vet Clin North Am Equine Pract 2002;18:295304.

  • 15.

    Sommardahl CS, Frank N, Elliott SB, et al.Effects of oral administration of levothyroxine sodium on serum concentrations of thyroid gland hormones and responses to injections of thyrotropin-releasing hormone in healthy adult mares. Am J Vet Res 2005;66:10251031.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 16.

    Vick MM, Sessions DR, Murphy BA, et al.Obesity is associated with altered metabolic and reproductive activity in the mare: effects of metformin on insulin sensitivity and reproductive cyclicity. Reprod Fertil Dev 2006;18:609617.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 17.

    Johnson PJ, Scotty NC, Wiedmeyer C, et al.Diabetes mellitus in a domesticated Spanish Mustang. J Am Vet Med Assoc 2005;226:584588.

  • 18.

    Firshman AM, Valberg SJ. Factors affecting clinical assessment of insulin sensitivity in horses. Equine Vet J 2007;39:567575.

  • 19.

    Ross MS. Determination of metformin in biological fluids by derivatization followed by high performance liquid chromatography. J Chromatogr 1977;133:408411.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 20.

    Scheen AJ. Clinical pharmacokinetics of metformin. Clin Pharmacokinet 1996;30:359371.

  • 21.

    Sambol NC, Brookes LG, Chiang J, et al.Food intake and dosage level, but not tablet vs solution dosage form, affect the absorption of metformin HCl in man. Br J Clin Pharmacol 1996;42:510512.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 22.

    Stepensky D, Friedman M, Srour W, et al.Preclinical evaluation of pharmacokinetic-pharmacodynamic rationale for oral CR metformin formulation. J Control Release 2001;71:107115.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 23.

    Stepensky D, Friedman M, Raz I, et al.Pharmacokinetic-pharmacodynamic analysis of the glucose-lowering effect of metformin in diabetic rats reveals first-pass pharmacodynamic effect.

    • Search Google Scholar
    • Export Citation

Advertisement

Pharmacokinetics and bioavailability of metformin in horses

Jaime L. HustaceDepartment of Clinical Sciences, College of Veterinary Medicine, Oregon State University, Corvallis, OR 97331.

Search for other papers by Jaime L. Hustace in
Current site
Google Scholar
PubMed
Close
 DVM
,
Anna M. FirshmanDepartment of Clinical Sciences, College of Veterinary Medicine, Oregon State University, Corvallis, OR 97331.

Search for other papers by Anna M. Firshman in
Current site
Google Scholar
PubMed
Close
 BVSc, PhD
, and
John E. MataDepartment of Biomedical Sciences, College of Veterinary Medicine, Oregon State University, Corvallis, OR 97331.

Search for other papers by John E. Mata in
Current site
Google Scholar
PubMed
Close
 PhD

Abstract

Objective—To determine pharmacokinetics and oral bioavailability of metformin in healthy horses.

Animals—4 adult horses.

Procedures—6 g of metformin was administered 3 times IV and PO (fed and unfed) to each horse, by use of a crossover design, with a 1-week washout period between treatments. Plasma metformin concentration was determined via high-pressure liquid chromatography.

Results—Mean ± SD distribution half-life of metformin following IV administration was 24.9 ± 0.4 minutes with a volume of distribution of 0.3 ± 0.1 L/kg. Mean area under the curve was 20.9 ± 2.0 h·μg/mL for IV administration; PO administration resulted in area under the curves of 1.6 ± 0.4 h·μg/mL in unfed horses and 0.8 ± 0.2 h·μg/mL in fed horses. Bioavailability was determined to be approximately 7.1 ± 1.5% in unfed horses and 3.9 ± 1.0% in fed horses. The maximal concentration following PO administration in unfed horses was 0.4 ± 0.1 μg/mL with a time at maximal concentration of 0.9 ± 0.1 hours. In fed horses, maximal concentration was reduced to 0.3 ± 0.04 μg/mL with a time at maximal concentration at 1.3 ± 0.3 hours.

Conclusions and Clinical Relevance—The low bioavailability of metformin may explain the reported lack of clinical success in improving insulin sensitivity with metformin treatment in horses. Dosages and dose intervals previously used may have been insufficient to achieve plasma concentrations of drug comparable to the therapeutic range achieved in humans. Therefore, a larger and more frequently administered dose may be required to fully evaluate efficacy of metformin in horses.

Abstract

Objective—To determine pharmacokinetics and oral bioavailability of metformin in healthy horses.

Animals—4 adult horses.

Procedures—6 g of metformin was administered 3 times IV and PO (fed and unfed) to each horse, by use of a crossover design, with a 1-week washout period between treatments. Plasma metformin concentration was determined via high-pressure liquid chromatography.

Results—Mean ± SD distribution half-life of metformin following IV administration was 24.9 ± 0.4 minutes with a volume of distribution of 0.3 ± 0.1 L/kg. Mean area under the curve was 20.9 ± 2.0 h·μg/mL for IV administration; PO administration resulted in area under the curves of 1.6 ± 0.4 h·μg/mL in unfed horses and 0.8 ± 0.2 h·μg/mL in fed horses. Bioavailability was determined to be approximately 7.1 ± 1.5% in unfed horses and 3.9 ± 1.0% in fed horses. The maximal concentration following PO administration in unfed horses was 0.4 ± 0.1 μg/mL with a time at maximal concentration of 0.9 ± 0.1 hours. In fed horses, maximal concentration was reduced to 0.3 ± 0.04 μg/mL with a time at maximal concentration at 1.3 ± 0.3 hours.

Conclusions and Clinical Relevance—The low bioavailability of metformin may explain the reported lack of clinical success in improving insulin sensitivity with metformin treatment in horses. Dosages and dose intervals previously used may have been insufficient to achieve plasma concentrations of drug comparable to the therapeutic range achieved in humans. Therefore, a larger and more frequently administered dose may be required to fully evaluate efficacy of metformin in horses.

Metformin (1,1-dimethylbiguanide) is an orally administered antihyperglycemic drug used commonly for management of type 2 diabetes mellitus in humans (Figure 1).1,2 It is used to control blood glucose concentrations by inhibiting hepatic gluconeogenesis and glycogenolysis and by enhancing peripheral tissue sensitivity to insulin.1,2 Metformin does not appear to affect the production of insulin by pancreatic β-cells and does not cause hypoglycemia when given at therapeutic concentrations.3–5 A recent study5 indicates that metformin also increases insulin sensitivity in patients with type 1 diabetes mellitus and helps prevent apoptosis of the pancreatic β-cells. Reported adverse effects of metformin treatment in humans include abdominal discomfort, inappetence, vomiting, and diarrhea.6 Metformin also causes severe lactic acidosis, which is now recognized to be rare and usually associated with high circulating drug concentrations, often in association with renal dysfunction.6,7 The drug is excreted unchanged by the kidneys in humans and cats, and normal kidney function is required for adequate clearance of the drug.7,8 To the authors’ knowledge, adverse effects of metformin use in horses are unknown.

Figure 1—
Figure 1—

Illustration of the chemical structure of metformin.

Citation: American Journal of Veterinary Research 70, 5; 10.2460/ajvr.70.5.665

Equine metabolic syndrome is a condition that results in abnormal distribution of adipose tissue, high circulating insulin and glucose concentrations, glucose intolerance, high plasma lipid concentrations, predisposition to laminitis, and infertility in mares.9,10 Horses with equine metabolic syndrome are commonly glucose intolerant and have insulin resistance.11 Insulin resistance is also thought to be involved in the pathogenesis of many other equine conditions such as pars intermedia dysfunction, diabetes mellitus, hyperlipemia, laminitis, endotoxemia, and osteochondrosis dissecans.10–16 In horses, further study is needed to understand the involvement of insulin resistance in these various diseases. Current treatment for horses with equine metabolic syndrome includes limiting them to diets with a low glycemic index and increasing exercise.11,12,16 In some horses, this approach is not sufficient to reduce blood concentrations of circulating glucose or insulin, especially in individuals that are unable to undertake sufficient exercise because of laminitis. Other treatments that have been proposed include alteration of the basal metabolic rate with thyroid hormone supplementation17 and increasing the sensitivity of peripheral tissues to insulin by administration of chromium picolinate.14,16 The use of metformin in horses with insulin resistance is particularly appealing because of its potentially beneficial method of action and low cost. In horses, there are anecdotal reports of metformin use in clinical cases, although efficacy is unknown. The drug has been evaluated in obese mares and was reported to improve insulin sensitivity after 30 days of treatment but not 60 days of treatment.18 In addition, conflicting results of efficacy at a dose of 3 g/d but not 6 or 9 g/d were obtained in that study.18 One case report15 of a Spanish Mustang with diabetes mellitus indicated that treatment with a combination of a low dose of 500 mg of metformin and glyburide (which stimulates insulin release from the pancreas) resulted in a decrease in basal glucose concentrations, although basal glucose concentrations are not a sensitive indicator of insulin action in horses.11,16 To the authors’ knowledge, there are no reports of metformin pharmacokinetics or bioavailability in horses from which to determine an appropriate dose regimen. Therefore, the purpose of the study reported here was to determine the pharmacokinetics of metformin after IV and PO administration and to assess oral bioavailability of the drug in healthy horses.

Materials and Methods

Horses—Four healthy adult neutered horses (2 geldings and 2 ovariectomized mares) from the Oregon State University research herd aged 4, 10, 15, and 19 years of age and weighing (mean ± SD) 531.8 ± 21.3 kg were available for use in the study. Physical examinations and preliminary blood analyses including a serum biochemical profile and venous blood gas analysis were performed. All horses were considered healthy at the time of the study. The horses were brought into the Oregon State University Large Animal Clinic for acclimation 2 days before the trial and were housed in individual stalls in a climate-controlled barn. Diet included grass hay fed twice daily and ad libitum water for the duration of the study. The horses were housed and cared for in an accredited facility, and the study was approved according to the principles outlined by the Oregon State University Institutional Animal Care and Use Committee.

Study design—One day prior to the start of each treatment, an IV catheter was placed in the jugular vein of each horse to facilitate blood sample collection. Three treatments were administered to each horse in random order in a crossover design with a 1-week washout period between treatments. The 3 treatments were IV administration of metformin (6 g), PO administration of metformin (6 g) after feed withholding for 12 hours, and PO administration of metformin (6 g) with no feed withholding. For IV administration, 6 g of metformina was dissolved in 15 mL of saline (0.9% NaCl) solution and passed through a 0.2-μm filter.7,b In a previous study7 that used the same technique, the solution for IV use was analyzed by use of HPLC (before and after filtration) and found to contain 98% of the stated dose. Intravenous treatments were administered in 500 mL of saline solution over 5 minutes via the jugular catheter within 1 hour of compounding. The catheter was flushed thoroughly with heparinized saline solution after metformin administration. Oral treatments were administered by crushing metformin tabletsc and mixing with water and administering by mouth with a syringe within 1 hour after compounding.

Blood samples were collected at times 0, 0.5, 1, 2, 4, 6, 8, 10, 12, 14, and 24 hours after treatment. For the IV treatment, samples were also taken every 15 minutes for the first 4 hours. To preserve stability, plasma samples were obtained by centrifugation of blood within 1 hour after collection and immediately stored at −80°C for ≤ 3 weeks until shipment to a commercial laboratoryd for analysis of metformin concentrations via HPLC. Additional blood samples were collected for measurement of blood lactate concentrations at 0, 4, 8, 12, and 24 hours after IV metformin administration and placed in lithium heparin–containing tubes. Intravenous jugular catheters were removed at the end of each testing period.

Assay—Quantitative analyses for metformin in plasma were performed by a commercial laboratoryd with an HPLC procedure following derivatization with p-nitrobenzoyl chloride.19 In summary, an internal standard (buformin, an antidiabetic drug chemically related to metformin) was added to a 0.5-mL aliquot of specimen. Following the addition of sodium hydroxide and approximately 0.6 g of sodium chloride, the sample was mixed well to dissolve the salt. One milliliter of p-nitrobenzoyl chloride reagent (4 g of para-nitrobenzoyl chloride in 100 mL of acetonitrile) was added, the tube was capped, and the solution was mixed for 15 minutes and extracted with a mixed solvent of 30% ethyl acetate in n-butyl chloride. The solvent was removed and back extracted in 1N hydrochloric acid, which was made basic with sodium hydroxide, and again extracted in the mixed solvent. The solvent was evaporated to dryness, and the residue was reconstituted with mobile phase. Extracts were analyzed via HPLC with UV detection at 280 nm. To validate the assay in equine plasma, 6 concentrations of calibrators ranging from 0.05 to 50 μg/mL were included with each analytic batch, and 2 concentrations of control samples were included with each analytic batch. The between-run precision was 13.6% and 8.48% at 1.0 and 2.5 μg/mL, respectively. The lower limit of quantitation was 0.05 μg/mL, and the upper limit of quantitation was 50 μg/mL. The interassay accuracy was 95% at 1.0 μg/mL and 96% at 2.5 μg/mL. Intra-assay precision was 8.3%, 2.3%, and 5.2% at 0.05, 1.0, and 2.5 μg/mL, respectively. Intra-assay accuracy was 97%, 103%, and 94% at 0.05, 1.0, and 2.5 μg/mL, respectively.

Data analysis—Metformin concentration–time data following IV and PO administrations were analyzed by use of noncompartmental analysis and estimated by use of a computer program.e The Cmax and time of maximal concentration were determined via observation. The AUC was calculated by use of the linear trapezoidal method. Volume of distribution, elimination rate constant, mean residence times, and clearance were estimated by use of noncompartmental analysis with IV input and first-order output from the plasma concentration–time data collected during IV treatment from 15 to 120 minutes. To determine the bioavailability of metformin, the absorption of the drug after PO administration was compared with the concentration-time relationship (AUC) in the bloodstream after it was injected IV. Bioavailability of orally administered metformin was reported as a fraction of the concentration reached after IV administration (F = AUCoral/AUCIV). Bioavailability was determined with and without feed restriction. Values are reported as mean ± SD.

Results

Results of the pharmacokinetic analysis (Table 1) revealed that, following IV administration, metformin was rapidly eliminated from the plasma (Figure 2). The concentration half-life was quantifiable for > 5 half-lives. The mean distribution half-life of metformin following IV administration was 24.9 ± 0.4 minutes with a mean residence time of 27.5 ± 2.1 minutes. The mean AUC was 20.9 ± 2.0 h·μg/mL for IV administration, whereas PO administration resulted in AUCs of 1.6 ± 0.4 h·μg/mL in unfed horses and 0.8 ± 0.2 h·μg/mL in fed horses. The AUCs determined for each horse were used to estimate the bioavailability, and the mean bioavailability of metformin was approximately 7.1 ± 1.5% in unfed horses and 3.9 ± 1.0% without feed withholding. This estimate was based on the plasma data collected from fed and unfed horses, compared with IV infusion for those samples with quantifiable amounts of metformin. Quantitative data were obtained for samples collected up to 8 hours following administration. The maximal concentration of metformin achieved with the 6-g dose following PO administration was 0.4 ± 0.1 μg/mL in unfed horses. The Cmax was reduced to 0.3 ± 0.04 μg/mL in fed horses; however, this was not significantly different from values in unfed horses.

Table 1—

Pharmacokinetic variables (mean ± SD) for metfor-min after IV and PO administration of 6 g of metformin to 4 healthy adult horses. Oral administration was performed when horses were fed (Fed) and when feed was withheld for 12 hours (Unfed).

VariableIVUnfedFed
Cmax (μg/mL)0.4 ± 0.10.3 ± 0.04
Tmax (h)0.9 ± 0.11.3 ±0.3
C0 (μg/mL)40.4 ± 16.72
Vd (L/kg)141.3 ± 6.0
Cl (mL/min)5,720 ± 524
AUC (h·μg/mL)20.9 ± 2.01.6 ± 0.40.8 ± 0.2
F(%)7.1 ±1.53.9 ±1.0
kel (1/min) t1/2 (min)0.04 ± 0.002
t1/2 (min)24.9 ± 0.4

— = Not Applicable. Tmax = Time at maximal concentration. Vd = Volume of distribution. Cl = Clearance. F = Fractional bioavailability. kel = Elimination rate constant. t1/2 = Half-life.

Figure 2—
Figure 2—

Mean plasma concentrations of metformin after IV and PO administration of 6 g to 4 adult horses. Oral administration was performed when horses were fed and also when feed was withheld for 12 hours. lg = Log.

Citation: American Journal of Veterinary Research 70, 5; 10.2460/ajvr.70.5.665

Adverse effects of treatment—Mild signs of abdominal discomfort (pawing) and loose feces, characterized as an increased water content with loss of fecal ball structure with no change in fecal consistency, were observed in 2 horses within 4 hours after IV administration of metformin. Signs resolved within 15 minutes after walking the horses. No clinically evident adverse effects were observed after PO administration of the drug. Blood lactate activity remained within reference range (0.5 ± 0.1 mmol/L) after IV administration in all horses studied.

Discussion

Results of the present study revealed marked differences in the pharmacokinetics of metformin in horses, compared with humans. In humans, metformin has a half-life of 6.2 hours as judged on the basis of a 3-compartment open model with initial elimination half-life estimated to be 1.7 to 3.0 hours and a terminal elimination of 9 to 17 hours.8 In the present study, metformin was rapidly eliminated and no longer detectable in plasma after 4 hours. Following IV administration in humans, the drug is completely excreted unchanged in urine with a renal clearance of (mean ± SD) 454 ± 47 mL/min.8 Our estimates suggested that the clearance of metformin in horses is > 10 times that in humans and may, in part, explain the low plasma concentrations following PO administration.

Results of a human study20 indicate an absolute oral bioavailability of 40% to 60%, with gastrointestinal absorption complete within 6 hours of ingestion. In rats and humans, the rate of absorption is slower than the rate of renal elimination, which results in what has been described as a flip-flop pharmacokinetic model.8,21 Furthermore, an inverse relationship between the dose ingested and the relative absorption has been observed as doses are increased,20,21 suggesting the involvement of an active, saturable absorption process that we hypothesize may be lacking in horses. On the basis of our results, it is possible that the rate of absorption and clearance of metformin may be higher in horses than in humans, which may in part account for the drug's low bioavailability. Low fractional bioavailability, as measured in the study reported here, can be caused by either low absorption or high excretion. Additional studies, including analysis of urinary excretion data, are needed to determine which of these is responsible for the low bioavailability of metformin in horses. Despite the rapid clearance from plasma, the maximal concentrations reached were close to the reported therapeutic range for humans of 0.5 to 1 μg/mL.20 Our data suggested that dosages and administration intervals used in earlier equine studies were insufficient to achieve plasma concentrations of drug comparable to the therapeutic range achieved in humans. More information regarding metformin pharmacodynamics and pharmacokinetics in horses may be obtained by use of a larger sample size.

Low bioavailability only partially explains the lack of success with administration of metformin in horses. Metformin is reported to have a substantial first-pass pharmacodynamic effect that occurs in the liver and gastrointestinal wall in rats, and PO administration of metformin provides greater clinical response than IV administration in humans.22,23 These pharmacodynamic and pharmacokinetic effects are hypothesized to be linked to the therapeutic effects of treatment because stronger, sustained glucose-lowering responses are observed when metformin is administered orally, compared with the equivalent dose administered IV.22 On the basis of the results of the study reported here, we hypothesize that the lack of clinical success of treatment with metformin in horses may be attributable to a combination of low bioavailability and increased rate of elimination. Although the horses used in this study had a wide age range, we did not detect a significant difference in bioavailability in young versus old horses. Therefore, it is apparent that a study designed to measure efficacy of metformin in horses may require larger doses and more frequent administration or perhaps a sustained-release formulation.

Abbreviations

AUC

Area under the curve

Cmax

Maximal concentration

HPLC

High-pressure liquid chromatography

a.

Metformin-HCl, Spectrum Chemical MFG Corp, Gardena, Calif.

b.

MCE syringe filter, Fisher Scientific, Pittsburgh, Pa.

c.

Metformin-HCl, 1 gram tablets, Apotex Corp, Weston, Fla.

d.

NMS Labs, Willow Grove, Pa.

e.

WinNonlin Professional, version 5.2, Pharsight Corp, Cary, NC.

References

  • 1.

    Bailey CJ, Turner RC. Metformin. N Engl J Med 1996;334:574579.

  • 2.

    DeFronzo RA. Pathogenesis of type 2 diabetes: implications for metformin. Drugs 1999;58:2930.

  • 3.

    Dunn CJ, Peters DH. Metformin. A review of its pharmacological properties and therapeutic use in non-insulin-dependent diabetes mellitus. Drugs 1995;49:721749.

    • Search Google Scholar
    • Export Citation
  • 4.

    Derosa G, D'Angelo A, Rogonesi PD, et al.Metabolic effects of pioglitazone and rosiglitazone in patients with diabetes and metabolic syndrome treated with metformin. Intern Med J 2007;37:7986.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 5.

    Moon RJ, Bascombe LA, Holt RI. The addition of metformin in type 1 diabetes improves insulin sensitivity, diabetic control, body composition, and patient well-being. Diabetes Obes Metab 2007;9:143145.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 6.

    Howlett HC, Bailey CJ. A risk-benefit assessment of metformin in type 2 diabetes mellitus. Drug Saf 1999;20:489503.

  • 7.

    Michels GM, Boudinot FD, Ferguson DC, et al.Pharmacokinetics of the antihyperglycemic agent metformin in cats. Am J Vet Res 1999;60:738742.

    • Search Google Scholar
    • Export Citation
  • 8.

    Pentikäinen PJ, Neuvonen PJ, Penttilä A. Pharmacokinetics of metformin after intravenous and oral administration to man. Eur J Clin Pharmacol 1979;16:195202.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 9.

    Treiber KH, Kronfeld DS, Hess TM, et al.Evaluation of genetic and metabolic predispositions and nutritional risk factors for pasture-associated laminitis in ponies. J Am Vet Med Assoc 2006;228:15381545.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 10.

    Frank N, Elliot SB, Brandt LE, et al.Physical characteristics, blood hormone concentrations, and plasma lipid concentrations in obese horses with insulin resistance. J Am Vet Med Assoc 2006;228:13831390.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 11.

    Kronfeld DS, Treiber KH, Hess TM, et al.Insulin resistance in the horse: definition, detection, and dietetics. J Anim Sci 2005;83(suppl E):E22E31.

    • Search Google Scholar
    • Export Citation
  • 12.

    Johnson PJ. The equine metabolic syndrome peripheral Cushing's syndrome. Vet Clin North Am Equine Pract 2002;18:271291.

  • 13.

    Frank N. Diagnosis of equine metabolic syndrome, in Proceedings. Annu Meet Am Coll Vet Intern Med Forum 2007;157159.

  • 14.

    Ralston SL. Insulin and glucose regulation. Vet Clin North Am Equine Pract 2002;18:295304.

  • 15.

    Sommardahl CS, Frank N, Elliott SB, et al.Effects of oral administration of levothyroxine sodium on serum concentrations of thyroid gland hormones and responses to injections of thyrotropin-releasing hormone in healthy adult mares. Am J Vet Res 2005;66:10251031.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 16.

    Vick MM, Sessions DR, Murphy BA, et al.Obesity is associated with altered metabolic and reproductive activity in the mare: effects of metformin on insulin sensitivity and reproductive cyclicity. Reprod Fertil Dev 2006;18:609617.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 17.

    Johnson PJ, Scotty NC, Wiedmeyer C, et al.Diabetes mellitus in a domesticated Spanish Mustang. J Am Vet Med Assoc 2005;226:584588.

  • 18.

    Firshman AM, Valberg SJ. Factors affecting clinical assessment of insulin sensitivity in horses. Equine Vet J 2007;39:567575.

  • 19.

    Ross MS. Determination of metformin in biological fluids by derivatization followed by high performance liquid chromatography. J Chromatogr 1977;133:408411.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 20.

    Scheen AJ. Clinical pharmacokinetics of metformin. Clin Pharmacokinet 1996;30:359371.

  • 21.

    Sambol NC, Brookes LG, Chiang J, et al.Food intake and dosage level, but not tablet vs solution dosage form, affect the absorption of metformin HCl in man. Br J Clin Pharmacol 1996;42:510512.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 22.

    Stepensky D, Friedman M, Srour W, et al.Preclinical evaluation of pharmacokinetic-pharmacodynamic rationale for oral CR metformin formulation. J Control Release 2001;71:107115.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 23.

    Stepensky D, Friedman M, Raz I, et al.Pharmacokinetic-pharmacodynamic analysis of the glucose-lowering effect of metformin in diabetic rats reveals first-pass pharmacodynamic effect.

    • Search Google Scholar
    • Export Citation

Contributor Notes

Address correspondence to Dr. Hustace.