• View in gallery

    Observed plasma anti–factor Xa activity (reported as assay OD) versus time curves for TP (0.06 mg/kg, SC) and TT (0.20 mg/kg, SC) fondaparinux doses established in a 1-dose trial involving 2 healthy cats. The range of values when the TP dose was used is indicated with light gray shading. The range for the TT dose is shown by darker gray shading. The degree of overlap between the 2 ranges is indicated by the darkest shading.

  • View in gallery

    Predicted plasma concentration versus time curves for fondaparinux in 6 healthy cats after administration SC at 0.057 mg/kg (TP dose) and 0.171 mg/kg (TT dose). Both doses were determined on the basis of fondaparinux protocols in humans.

  • View in gallery

    Representative TEG tracings of a healthy cat that underwent a TT protocol (0.20 mg of fondaparinux/kg, SC, q 12 h for 7 days) as obtained before drug administration [baseline]), at peak plasma anti–factor Xa activity 2 hours after the last dose, 12 hours after the last dose, and 24 hours after the last dose. The x-axis represents time; the y-axis represents millimeters of deflection from 0 (midpoint).

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    Smith SA, Tobias AH, Jacob KA, et al. Arterial thromboembolism in cats: acute crisis in 127 cases (1992–2001) and long-term management with low-dose aspirin in 24 cases. J Vet Intern Med 2003; 17:7383.

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    Falconer L & Atwell R. Feline aortic thromboembolism. Aust Vet Pract 2003; 33:2032.

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    Smith SA, Tobias AH. Feline arterial thromboembolism: an update. Vet Clin North Am Small Anim Pract 2004; 34:12451271.

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    Arixtra [package insert]. Philadelphia: GlaxoSmithKline, 2010.

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    Bauer KA. Fondaparinux sodium: a selective inhibitor of factor Xa. Am J Health Syst Pharm 2001; 58(suppl 2):S14S17.

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    Gibaldi M & Perrier D. One compartment model. In: Gibaldi M, Perrier D, eds. Pharmacokinetics. 2nd ed. New York: Marcel Dekker Inc, 1982;143.

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    Brooks MB. Evaluation of chromogenic assay to measure the factor Xa inhibitory activity of unfractionated heparin in canine plasma. Vet Clin Pathol 2004; 33:208214.

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    Depasse F, Gerotziafas GT, Busson J, et al. Assessment of three chromogenic and one clotting assays for the measurement of synthetic pentasaccharide fondaparinux (Arixtra) anti-Xa activity. J Thromb Haemost 2004; 2:346348.

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    Marschner CB, Bjornvad CR, Kristensen AT, et al. Thromboelastography results on citrated whole blood from clinically healthy cats depend on modes of activation. Acta Vet Scand 2010; 52:38.

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    Hogan DF, Andrews DA, Green HW, et al. Antiplatelet effects and pharmacodynamics of clopidogrel in cats. J Am Vet Med Assoc 2004; 225:14061411.

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    Klein SM, Slaughter TF, Vail PT, et al. Thromboelastography as a perioperative measure of anticoagulation resulting from low molecular weight heparin: a comparison with anti-Xa concentrations. Anesth Analg 2000; 91:10911095.

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    Van De Wiele CM, Hogan DF, Green HW III, et al. Antithrombotic effect of enoxaparin in clinically healthy cats: a venous stasis model. J Vet Intern Med 2010; 24:185191.

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Dose determination of fondaparinux in healthy cats

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  • 1 Department of Veterinary Clinical Sciences, College of Veterinary Medicine, Purdue University, West Lafayette, IN 47907.
  • | 2 Department of Veterinary Clinical Sciences, College of Veterinary Medicine, Purdue University, West Lafayette, IN 47907.
  • | 3 Department of Veterinary Clinical Sciences, Faculty of Veterinary Science, University of Melbourne, Werribee, VIC 3030, Australia.
  • | 4 Department of Veterinary Clinical Sciences, College of Veterinary Medicine, Purdue University, West Lafayette, IN 47907.
  • | 5 Department of Veterinary Clinical Sciences, College of Veterinary Medicine, Purdue University, West Lafayette, IN 47907.
  • | 6 Department of Veterinary Clinical Sciences, College of Veterinary Medicine, Purdue University, West Lafayette, IN 47907.

Abstract

Objective—To establish practical doses and administration frequencies of fondaparinux for cats that would approximate human therapeutic peak and trough plasma anti–factor Xa activities for thromboprophylaxis (TP) and thrombosis treatment (TT) protocols.

Animals—6 healthy adult purpose-bred cats.

Procedures—Dosage protocols for TP and TT were selected on the basis of a single compartment pharmacokinetic model incorporating data from humans but modified to account for the higher body weight–normalized cardiac output of cats. Fondaparinux was administered at 0.06 mg/kg, SC, every 12 hours (TP) for 7 days in one session, and 0.20 mg/kg, SC, every 12 hours (TT) for 7 days in another, with a minimum of 1 week separating the sessions. Plasma anti–factor Xa activity was measured before fondaparinux administration (day 1) and at 2 (peak) and 12 (trough) hours after drug administration on days 1 and 7. Platelet aggregation and thromobelastographic (TEG) parameters were also measured 2 hours after drug administration on day 7.

Results—Peak plasma anti–factor Xa activities on day 7 for TP (median, 0.59 mg/L; range, 0.36 to 0.77 mg/L) and TT (median, 1.66 mg/L; range, 1.52 to 2.00 mg/L) protocols were within therapeutic ranges for humans. However, only the TP protocol achieved trough anti–factor Xa activity considered therapeutic in humans (median, 0.19 mg/L; range, 0.00 to 0.37 mg/L) on day 7. There were significant changes in the TEG parameters at peak for the TT protocol, suggesting a hypocoagulable state. No significant changes in platelet aggregation were evident for either protocol.

Conclusions and Clinical Relevance—A fondaparinux dosage of 0.06 or 0.20 mg/kg, SC, every 12 hours, was sufficient to achieve a peak plasma anti–factor Xa activity in cats that has been deemed therapeutic in humans. This study provided preliminary data necessary to perform fondaparinux dose-determination and clinical efficacy studies.

Abstract

Objective—To establish practical doses and administration frequencies of fondaparinux for cats that would approximate human therapeutic peak and trough plasma anti–factor Xa activities for thromboprophylaxis (TP) and thrombosis treatment (TT) protocols.

Animals—6 healthy adult purpose-bred cats.

Procedures—Dosage protocols for TP and TT were selected on the basis of a single compartment pharmacokinetic model incorporating data from humans but modified to account for the higher body weight–normalized cardiac output of cats. Fondaparinux was administered at 0.06 mg/kg, SC, every 12 hours (TP) for 7 days in one session, and 0.20 mg/kg, SC, every 12 hours (TT) for 7 days in another, with a minimum of 1 week separating the sessions. Plasma anti–factor Xa activity was measured before fondaparinux administration (day 1) and at 2 (peak) and 12 (trough) hours after drug administration on days 1 and 7. Platelet aggregation and thromobelastographic (TEG) parameters were also measured 2 hours after drug administration on day 7.

Results—Peak plasma anti–factor Xa activities on day 7 for TP (median, 0.59 mg/L; range, 0.36 to 0.77 mg/L) and TT (median, 1.66 mg/L; range, 1.52 to 2.00 mg/L) protocols were within therapeutic ranges for humans. However, only the TP protocol achieved trough anti–factor Xa activity considered therapeutic in humans (median, 0.19 mg/L; range, 0.00 to 0.37 mg/L) on day 7. There were significant changes in the TEG parameters at peak for the TT protocol, suggesting a hypocoagulable state. No significant changes in platelet aggregation were evident for either protocol.

Conclusions and Clinical Relevance—A fondaparinux dosage of 0.06 or 0.20 mg/kg, SC, every 12 hours, was sufficient to achieve a peak plasma anti–factor Xa activity in cats that has been deemed therapeutic in humans. This study provided preliminary data necessary to perform fondaparinux dose-determination and clinical efficacy studies.

Cardioembolic events in cats are an uncommon but severe consequence of cardiac disease that occur with similar frequency within the major categories of myocardial disease.1 Chamber dilatation and endothelial injury provide the conditions for thrombus formation, whereas an underlying hypercoagulable state likely increases the chance of a cardioembolic event. Because most cardiac diseases are uncurable, anti-thrombotic drugs have become the empirical mainstay in the primary and secondary prevention of cardioembolic events in affected individuals.2,3

Fondaparinux sodium is a novel antithrombotic drug used in the prevention and treatment of thromboembolic events in humans. The molecule, which is a synthetic pentasaccharide, is a highly specific inhibitor of factor Xa, with no effects on factor IIa (thrombin) or platelet function.4 In humans, fondaparinux is administered SC, has a bioavailability of 100%, and is excreted unchanged in the urine.4 Some physicians consider it an ideal anti-thrombotic agent because of its selective targeting of factor Xa, a pharmacokinetic profile that permits once-daily administration, and its lack of cross-reactivity in humans with heparin-induced thrombocytopenia.5 The high bioavailability and lack of requirement for drug monitoring and frequent administration may make fondaparinux an attractive alternative to UFH and LMWH for the prevention of cardioembolic disease in cats.

The purpose of the study reported here was to establish practical doses and administration frequencies of fondaparinux in cats that would approximate human therapeutic peak and trough plasma anti–factor Xa activities for TP and TT protocols. A secondary objective was to examine the effect of fondaparinux on TEG and platelet function.

Materials and Methods

Animals—Six healthy adult sexually intact purpose-bred cats (3 males and 3 females) were used in the study. The cats were determined to be healthy through physical examination, fecal examination for intestinal parasites, FeLV and FIV testing, and heartworm antibody testing. Each was housed individually, with fresh water available at all times. A dry food considered balanced and complete for adult cats was offered twice a day. A light cycle of 12 hours of light and 12 hours of dark was used, and environmental enrichment was provided through multiple toys and play interaction with the investigators. There were no periods of food withholding during the study. The Purdue Animal Care and Use Committee approved the research protocol.

Fondaparinux—Fondaparinux sodiuma is available commercially in 1-dose syringes containing 2.5, 5.0, 7.5, and 10 mg dissolved in saline (0.9% NaCl) solution, all with a concentration of 12.5 mg/mL. Because of the small body size of cats used in the study, multiple 10-mg syringes were emptied into a sterile bottle and diluted with sterile saline solution to a final concentration of 1 mg/mL to allow accurate dose administration prior to each trial. Given the absence of data on fondaparinux in cats, the therapeutic range of plasma anti–factor Xa activity in humans was used as a guide for determining the administration protocols to use in cats.

Pharmacokinetic model—Values for fondaparinux pharmacokinetic parameters in humans4 were used to estimate values for cats by use of the principles of allometric scaling.6 Because fondaparinux is cleared from the body principally by renal filtration, time was scaled as the ratio of the mean human resting heart rate (70 beats/min) to that of cats (150 beats/min), resulting in an estimate of elimination half-life in cats of 9.3 hours, compared with 20 hours in humans. The volume of distribution in cats was presumed to be similar to that of humans, normalized for body weight (ie, 0.143 L/kg). Bioavailability was presumed to be 100%. The desired trough and peak plasma concentrations were set at 0.1 and 0.4 mg/L, respectively, for TP and 0.1 and 1.2 mg/L, respectively, for TT on the basis of therapeutic concentrations in humans4 (trough and peak concentrations, 0.14 to 0.19 mg/L and 0.39 to 0.50 mg/L, respectively, for TP and 0.46 to 0.62 mg/L and 1.20 to 1.26 mg/L, respectively, for TT). The absorption rate after SC administration in cats was presumed to be 10-fold faster than the estimated elimination rate. A 1-compartment open model was chosen. The desired loading dose to achieve the target maximal concentration after SC administration was then calculated by use of standard formulae to be 0.057 mg/kg for TP and 0.171 mg/kg for TT.7 This method was used to estimate the initial doses that would be used in the study.

Dose determination trial—To ensure responsible use of resources and limit possible adverse reactions in the cats, 4 of the 6 cats were randomly chosen by coin toss to receive only 1 dose of fondaparinux. Two cats received 0.06 or 0.10 mg of fondaparinux/kg, SC, which represented a low- and high-dose TP protocol, respectively, and 2 received 0.20 or 0.28 mg/kg, SC, which represented a low- and high-dose TT protocol, respectively. These doses were chosen to represent the range of the estimated doses obtained by use of the pharmacokinetic model. Fondaparinux was administered through a 22-gauge needle into the subcutaneous tissue along the dorsal aspect of the thorax between the scapulae. The unshaven skin was prepared by wiping with isopropyl alcohol prior to needle insertion. Plasma anti–factor Xa activity was measured by use of an assayb prior to drug administration (baseline) and at 2, 4, 8, 12, and 24 hours afterward. From these data, peak and trough activities were determined and the doses and administration frequency for TP and TT protocols for use in the long-term administration study were established.

Repeated dose trial—Data from the 1-dose trial were used to develop TP and TT administration protocols. To this end, fondaparinux for the TP protocol was administered to all 6 cats at 0.06 mg/kg, SC, every 12 hours for 7 days by use of the same injection technique described for the 1-dose trial. Plasma anti–factor Xa activity was measured prior to drug administration and at 2 and 12 hours after administration on days 1 and 7 of treatment to approximate peak and trough values, respectively. An additional blood sample was collected 24 hours after administration of the final fondaparinux dose to confirm elimination of the drug. In a similar fashion, a repeated-dose TT administration regimen (0.20 mg of fondaparinux/kg, q 12 h) was evaluated in all cats for 7 days, with plasma anti–factor Xa activity measured at the same time points. Additional blood samples were collected 12, 24, and 48 hours after administration of the final dose to confirm drug elimination. There was at least a 7-day washout period between the TP and TT trials for each cat.

Blood collection and handling—Cats were gently restrained during blood collection by means of standard manual techniques. Blood samples were collected via jugular venipuncture with a 23-gauge winged collection setc fitted with a needle at the distal end. Once blood flow was observed, the distal needle was inserted into blood-collecting tubes, with the first 1 to 2 mL of blood discarded and the remaining blood collected directly into 3.2% sodium citrate to a final ratio of blood to citrate of 9:1. Plasma samples were obtained for anti–factor Xa activity by centrifugationd at 3,000 × g at 4°C for 10 minutes and were stored at −80°C until analysis. A total of 2.7 mL of blood was collected to perform the TEG and platelet aggregation tests, and an additional 2.7 mL of blood was collected to determine anti–factor Xa activity. Including the initial volume of discarded blood, ≤ 7 mL of blood was collected from any 1 cat at any point in the study.

Evaluation of plasma anti–factor Xa activity—The frozen plasma samples were shipped overnight to the Comparative Coagulation Laboratory at Cornell University on dry ice. There, plasma anti–factor Xa activity was measured within 1 month after sample collection by use of a 1-step competitive inhibition chromogenic assaye as described elsewhere.8 Unlike UFH or LMWH treatment, a concentration curve in which fondaparinux rather than UFH was used as the indicator in pooled feline plasma was required to determine anti–factor Xa activity with fondaparinux treatment. For this reason, 10 mL of blood was collected prior to drug administration and plasma was obtained in the same manner for determination of plasma anti–factor Xa activity in each of the 6 cats. This assay method and calibrators for LMWH have been shown to be suitable to measure fondaparinux in human plasma.9

The recovery of fondaparinux inhibitory activity in plasma samples was evaluated in ex vivo experiments. The stock fondaparinux was first added to an aliquot of feline plasma to achieve a concentration of 2.5 μg/mL, then diluted through a concentration range to a final fondaparinux concentration of 0.125 μg/mL. The fondaparinux-spiked plasma samples were assayed in duplicate to determine linearity and dilutional agreement of the LMWH test method. Results for the spiked samples and treated cats were expressed in units of LMWH anti–factor Xa per milliliter of plasma, compared with results for the assay calibrator,f and as residual factor Xa percentage, as described elsewhere.9 The residual values were calculated on the basis of raw data (change in OD per minute) by comparing the observed sample change in OD per minute with that of pooled feline plasma containing no fondaparinux (test OD/baseline OD × 100).

Recovery of anti–factor Xa activity in feline plasma was similar to that reported for human plasma, with assayed LMWH anti–factor Xa activity of 1 U/mL corresponding to 1 μg of fondaparinux/mL and detectable anti–factor Xa activity at a concentration of 0.125 μg of fondaparinux/mL. There was a strong negative linear relationship between the recovered anti–factor Xa activity and absorbance change throughout the range of 2.5 to 0.125 μg of fondaparinux/mL.

TEG—Thromboelastographyg was used to evaluate the effect of fondaparinux on the coagulation cascade.10 The citrated whole blood samples were allowed to rest for 30 minutes at room temperature (22°C) after collection. Citrated whole blood samples were recalcitrated with 0.2M calcium chloride, and TEG tracings were obtained without the use of any coagulation activator within 2 hours after sample collection. Four parameters were recorded for analysis: reaction time (R), clot formation time (K), α angle, and MA.

For the TP and TT dose trials, the TEG assays were performed prior to drug administration (baseline) and 2 hours after drug administration on day 7, when plasma anti–factor Xa concentrations were presumed to have reached peak steady-state activity. Additional samples were collected at 12, 24, and 48 hours after the last dose was administered in the TT protocol trial.

Platelet aggregation—Whole blood platelet aggregation was performed by use of citrated blood samples as previously described.11 Collagenh (5 μg/mL) and ADPh (5μM) were used as agonists, and maximal platelet aggregation was recorded. Similar to the TEG evaluation, for both the TP and TT protocol trials, platelet aggregation was performed prior to drug administration (baseline) and 2 hours after drug administration on day 7 when plasma anti–factor Xa concentrations were presumed to have reached peak steady-state activity.

Statistical analysis—Peak and trough anti–factor Xa activities from the repeated administration trial are reported as median and range and are compared with the ranges deemed therapeutic in humans for TP and TT. Changes in median values for the TEG parameters R, K, α angle, MA, and maximal platelet aggregation were evaluated by use of the Wilcoxon signed rank test. Baseline values were compared with peak steady-state activity for the TP protocol. For the TT protocol, similar comparisons were made with additional comparisons between baseline values and values for blood samples obtained 12, 24, and 48 hours after drug administration on day 7. Values of P < 0.05 were considered significant. Statistical analyses were performed by use of a commercial statistical software program.i

Results

Dose determination—The observed pharmacokinetic profiles for the TP and TT protocols for each of the 4 evaluated cats were similar, with peak plasma anti–factor Xa concentrations within the human therapeutic range 2 hours after SC fondaparinux administration and lower than the therapeutic range by 12 hours after administration (Figure 1). On the basis of these data, the lower doses for TP (0.06 mg/kg) and TT (0.20 mg/kg) were selected for the long-term administration study, with an administration frequency of every 12 hours.

Figure 1—
Figure 1—

Observed plasma anti–factor Xa activity (reported as assay OD) versus time curves for TP (0.06 mg/kg, SC) and TT (0.20 mg/kg, SC) fondaparinux doses established in a 1-dose trial involving 2 healthy cats. The range of values when the TP dose was used is indicated with light gray shading. The range for the TT dose is shown by darker gray shading. The degree of overlap between the 2 ranges is indicated by the darkest shading.

Citation: American Journal of Veterinary Research 73, 4; 10.2460/ajvr.73.4.556

Repeated dose trial—Median (range) plasma anti–factor Xa activities were summarized (Table 1). On day 1 and 7, the median peak anti–factor Xa activities for the TP (0.30 to 0.50 mg/L) and TT (1.10 to 1.30 mg/L) protocols achieved concentrations that approximated therapeutic concentrations in humans (Figure 2). The median trough anti–factor Xa activity approximated human therapeutic concentrations (0.10 to 0.19 mg/L) for the TP protocol, but the median for the TT protocol was less than the human therapeutic range (0.40 to 0.65 mg/L).

Figure 2—
Figure 2—

Predicted plasma concentration versus time curves for fondaparinux in 6 healthy cats after administration SC at 0.057 mg/kg (TP dose) and 0.171 mg/kg (TT dose). Both doses were determined on the basis of fondaparinux protocols in humans.

Citation: American Journal of Veterinary Research 73, 4; 10.2460/ajvr.73.4.556

Table 1—

Median (range) plasma anti–factor Xa activities (mg/L) in 6 healthy sexually intact adult purpose-bred cats before fondaparinux administration (baseline) and at various points afterward for a 7-day TP and TT protocol, compared with values considered therapeutic for humans at the same time points.

Measurement pointFeline TPHuman TPFeline TTHuman TT
Baseline0.00 (0.00–0.00)0.00 (0.00–0.00)
Day 1
   2 h0.56* (0.54–0.64)1.58* (1.13–2.08)
   12 h0.14* (0.06–0.33)0.23 (0.14–0.33)
Day 7
   2 h0.59* (0.36–0.77)0.39–0.501.66* (1.52–2.00)1.20–1.26
   12 h0.19* (0.00–0.37)0.14–0.190.16 (0.11–0.35)0.46–0.62
   24 h0.04 (0.00–0.34)0.16 (0.09–0.21)
   48 hNM0.05 (0.00–0.06)

Indicated value is within target range of plasma anti–factor Xa activity considered therapeutic in humans.

— = Not applicable. NM = Not measured.

Fondaparinux was administered at 0.06 mg/kg, SC, every 12 hours (TP) for 7 days in one session, and 0.20 mg/kg, SC, every 12 hours (TT) for 7 days in another, with a minimum of 1 week between sessions.

TEG—Values of TEG parameters and the median percentage change in these values were summarized (Table 2). With the TP protocol, there was no significant effect on any measured parameter in the study cats, compared with baseline values. With the TT protocol, the median R and K were significantly increased and α angle and MA were significantly decreased, which suggests a hypocoagulable profile (Figure 3). Analysis of the blood samples collected 12 hours after the last fondaparinux dose was administered on day 7 revealed a persistent significant decrease in MA. This effect on MA diminished with drug washout and was not significant by 24 hours after the last dose was administered.

Figure 3—
Figure 3—

Representative TEG tracings of a healthy cat that underwent a TT protocol (0.20 mg of fondaparinux/kg, SC, q 12 h for 7 days) as obtained before drug administration [baseline]), at peak plasma anti–factor Xa activity 2 hours after the last dose, 12 hours after the last dose, and 24 hours after the last dose. The x-axis represents time; the y-axis represents millimeters of deflection from 0 (midpoint).

Citation: American Journal of Veterinary Research 73, 4; 10.2460/ajvr.73.4.556

Table 2—

Results of TEG and platelet aggregation testing for the cats in Table 1.

  TPTT
ParameterBaseline2 h2 h12 h24 h48 h
TEG
   R (min) Value4.05 (2.85 to 5.10)4.13 (3.85 to 7.55)9.40* (5.25 to 19.30)6.38 (2.65 to 7.35)3.68 (2.50 to 5.45)2.70 (2.35 to 4.20)
   Change from baseline (%)NM−0.39 (−6.00 to 164.91)142.57 (5.00 to 343.68)40.20 (−2.59 to 123.68)−22.19 (−24.00 to 45.33)−5.26 (−53.00 to 12.00)
   K (min) Value0.98 (0.80 to 1.40)2.00 (1.20 to 3.35)4.50* (2.10 to 16.55)1.85 (1.20 to 2.30)1.20 (0.08 to 2.50)1.10 (0.90 to 2.05)
   Change from baseline (%)NM105.66 (15.38 to 175.00)434.38 (121.05 to 1,555.00)69.23 (−14.59 to 155.56)19.17 (−15.79 to 137.50)22.22 (12.50 to 46.43)
   α angle (°) Value75.20 (70.40 to 79.30)58.13 (44.95 to 73.60)34.20* (11.25 to 42.00)57.84 (47.40 to 73.45)62.85 (55.50 to 71.25)63.90 (60.00 to 71.70)
   Change from baseline (%)NM−22.72 (−37.01 to −2.19)−55.73 (−85.01 to −43.89)−22.93 (−37.47 to 4.33)−16.27 (−30.01 to −3.53)−9.58 (−20.84 to −9.23)
   MA (mm) Value68.65 (67.30 to 72.20)64.98 (58.35 to 74.85)50.08* (32.60 to 62.05)61.69* (58.65 to 70.25)64.88 (62.95 to 71.25)65.30 (59.70 to 67.80)
   Change from baseline (%)NM−5.32 (−18.62 to 10.40)−28.00 (−52.96 to −7.80)−9.78 (−13.96 to −2.70)−4.46 (−12.81 to 4.78)−9.56 (−11.95 to −5.44)
Platelet aggregation
   Amplitude for collagen (Ω)24.75 (23.50 to 41.50)26.25 (19.50 to 30.00)25.75 (23.50 to 33.00)NMNMNM
   Amplitude for ADP (Ω)20.00 (18.00 to 35.50)19.75 (12.50 to 26.00)22.00 (18.50 to 24.50)NMNMNM

Data are median (range). Positive values indicate percentage increase, and negative values indicate percentage decrease.

Indicated value is significantly (P < 0.05) different from baseline value.

K = Clot formation time. NM = Not measured. R = Reaction time.

Platelet aggregation—No significant change in maximal platelet aggregation was evident with either fondaparinux protocol at peak plasma anti–factor Xa activity on day 7 (Table 2).

Discussion

In the present study, a TP (0.06 mg/kg, SC, q 12 h) and TT (0.20 mg/kg, SC, q 12 h) protocol for fondaparinux administration achieved median peak plasma anti–factor Xa activities in healthy cats that were higher than the upper limit of the human therapeutic range. However, median plasma anti–factor Xa activity remained within the human therapeutic range throughout the 12-hour treatment period for the TP protocol only. This could suggest that more frequent administration than every 12 hours is necessary to maintain a plasma anti–factor Xa concentration within the human therapeutic range for a feline TT protocol.

Platelet aggregation measured at peak plasma anti–factor Xa activity was not appreciably affected by the TP or TT protocols as expected. However, the TEG tracings revealed significant changes in R, K, α angle, and MA, suggesting hypocoagulability of the treated cats' blood had occurred with the TT protocol.

Values of R and K are inversely correlated with plasma clotting factor activity.12 As a specific factor Xa inhibitor, it is not unexpected for fondaparinux to also affect the TEG parameters that are related to plasma clotting factors. However, the MA is principally related to fibrinogen and platelet function, so the reason fondaparinux, with no appreciable antiplatelet effect, would have an impact on the MA is unclear. In fact, the TEG changes persisted after plasma anti–factor Xa activity decreased to lower than the human therapeutic range and the maximal anti–factor Xa activity achieved for the TP protocol. This could suggest that the TEG changes were dose dependent and that anti–factor Xa activity was not the only variable involved in those changes. Similarly, it has been demonstrated that some of the antithrombotic effect of the LMWH enoxaparin in cats is not attributable to anti–factor Xa activity.13 Additional study in this area is needed.

In the treatment of cats with cardioembolic disease, an ideal antithrombotic drug has yet to be identified. Over the past decades, there has been research into the pharmacokinetics and pharmacodynamics of antiplatelet drugs (acetylsalicylic acid and clopidogrel) and anticoagulant drugs (warfarin, UFH, and LMWH). A safe and effective drug that requires infrequent administration and does not require monitoring would be ideal for use in cats. Given its small molecular size and enhanced ability to increase the affinity of antithrombin for factor Xa (300-fold), compared with UFH and LMWH, fondaparinux has higher bioavailability and a prolonged half-life in humans, allowing for once-daily SC administration.4 However, the present study demonstrated that cats require treatment every 12 hours with fondaparinux to maintain plasma anti–factor Xa activity within the human therapeutic range for TP. This difference was not unexpected because fondaparinux is excreted unchanged in the urine,4 where excretion is directly related to glomerular filtration rate, which in turn, is directly related to heart rate.

Given that adult cats have higher mean heart rates than humans, it is not surprising that cats require more frequent fondaparinux administration, as was both anticipated and then demonstrated in pharmacokinetic modeling. The small difference between the allometric prediction and the experimentally measured elimination of fondaparinux in cats may be due to interspecies differences in renal drug clearance. Variables such as plasma protein binding, the relative percentage of cardiac output that is devoted to renal blood flow, and renal tubular secretion or resorption of fondaparinux in cats warrant investigation.

In the present study, a TP protocol of 0.06 mg/kg, SC, every 12 hours approximated the plasma anti–factor Xa activity in humans considered effectively treated with fondaparinux. A TT protocol of 0.20 mg/kg, SC, every 12 hours approximated only peak anti–factor Xa activity in humans. We can make no conclusion as to the degree of anti–factor Xa activity achieved with fondaparinux that would be effective for clinical antithrombotic treatment in cats, although the present study did yield the data necessary to perform dose-confirmation and clinical efficacy studies of fondaparinux in cats.

ABBREVIATIONS

LMWH

Low molecular-weight heparin

MA

Maximum amplitude

OD

Optical density

TEG

Thromboelastography

TP

Thromboprophylaxis

TT

Thrombosis treatment

UFH

Unfractionated heparin

a.

Arixtra, GlaxoSmithKline, Research Triangle Park, NC.

b.

Provided by Dr. Marjory Brooks, Comparative Coagulation Laboratory, Cornell University, Ithaca, NY.

c.

Standard winged blood collection set with multi-sample luer adapter 23-gauge × 3/4 inch 12-inch tubing, product No. DBM1–23G, Kawasumi Laboratories America Inc, Tampa, Fla.

d.

Model GR422, Jouan, Winchester, Va.

e.

Rotachrom Heparin, Diagnostica Stago Inc, Parsippany, NJ.

f.

STA Calibrator HPBM/LMWH, Diagnostica Stago, Parsippany, NJ.

g.

TEG 5000 Thromboelastograph Hemostasis System, TEG Analytical Software (TAS), version 4.2.3, Haemoscope Corp, Niles, Ill.

h.

Chrono-Log Corp, Havertown, Pa.

i.

Stata, version 11.1, StataCorp, College Station, Tex.

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Contributor Notes

Supported by the American College of Veterinary Internal Medicine Cardiology Research Committee.

Presented in abstract form at the American College of Veterinary Internal Medicine Forum, Anaheim, Calif, June 2010.

The authors thank Dr. George Moore for statistical advice and analysis.

Address correspondence to Dr. Hogan (hogandf@purdue.edu).