Stephan Hungerbühler
Pharmacokinetics of unfractionated heparin in dogs with a decreased glomerular filtration rate caused by chronic renal insufficiency
The objective of this study was to examine the influence of a decreased renal function caused by chronic renal insufficiency (CRI) on the pharmacokinetics of unfractionated heparin (UFH) in dogs. Thirty-two dogs with a reduced glomerular filtration rate (GFR: <1,25 ml/kg/min) due to chronic renal insufficiency and twenty dogs with normal GFR (3–6 ml/kg/min) were included in this study. Four groups of dogs received either 500 International Units (I.U.)UFH/kilogram body mass (kg BM) subcutaneously (control [group 1]: n=10; CRI [group 2]: n=10) or 50 I.U.(UFH)/kg BM intravenously (control [group 3]: n=10; CRI [group 4]: n=22). From all dogs included in this study, the renal function was determined using the GFR and urea and creatinine concentrations one day before the heparin injection. In dogs receiving subcutaneous heparin (groups 1 and 2), blood samples were taken before the heparin injection as well as 15 and 30 minutes and 1, 2, 3, 4, 6, 8, 10, 12 and 24 hours after the injection. In dogs receiving intravenous heparin (group 3 and 4), blood samples were taken before the heparin injection as well as 2, 5, 10, 15 and 30 minutes and 1, 2, 3, 4, 6 and 8 hours after the injection. The experiments were performed using a commercial sodium heparin (Liquemin®, Hoffman La Roche) produced from porcine intestinal mucosa. The plasma heparin-activity, activated thromboplastin time (aPTT), thrombin time (TT), antithrombin-(AT)-activity, plasma albumin concentration, platelet count and haematocrit were measured at all time points. The plasma heparin-activity was measured using a factor Xa-dependent chromogenic substrate assay and the values were corrected to eliminate the dilution by anticoagulant (citrate). The aPTT and TT were measured using coagulometric methods, the AT-activity using a chromogenic substrate assay, albumin, creatinine and urea concentrations were determined using photometric assays, the platelet count and haematocrit using an automatic blood cell counter.
After subcutaneous injection of UFH, dogs with CRI showed significantly lower maximum plasma heparin-activity values ([Cmax]; CRI: 0.54 I.U./ml; 0.34–1.25 I.U./ml [median;
minimum–maximum]; control: 0.96 I.U./ml; 0.41–1.65 I.U./ml; Mann-Whitney-test:
p=0.0481) and significantly higher total clearance results ([Cltot]; CRI: 1.27 ml/min/kg; 0.48–
1.91 ml/min/kg; control: 0.65 ml/min/kg; 0.48–1.78 ml/min/kg; Mann-Whitney-test:
p=0.0444) than dogs with normal renal function. The time of the maximum plasma heparin-activity ([Tmax]; CRI: 274 min; 209–423 min; control: 293 min; 206–435 min; Mann-Whitney-test: p=0.3812) and the half-live ([T50]; CRI: 190 min; 146–295 min; control: 203 min; 144–304 min; Mann-Whitney-test: p=0.3251) were not significantly different between the groups.
After intravenous injection, dogs with CRI showed significantly lower Cmax values (CRI: 0.34 I.U./ml; 0.23–0.67 I.U./ml; control: 0.71 I.U./ml; 0.54–0.85 I.U./ml; Mann-Whitney-test:
p<0.0001), higher Cltot values (CRI: 4.1 ml/min/kg; 0.94–13.5 ml/min/kg; control: 1.50 ml/min/kg; 1.11–2.41 ml/min/kg; Mann-Whitney-test: p=0.0002) and shorter T50 values (CRI:
23.4 min; 8.5–62.0 min; control: 32.8 min; 21.1–40.2 min; Mann-Whitney-test: p=0.0092) when compared to dogs with normal renal function.
Ratio-values were calculated for aPTT and TT as qoutients of actual results and baseline values (before UFH-application). After subcutaneous injection, the maximum ratio (aPTT:
CRI: 2.1x [median]; control: 2.2x; Wilcoxon-test: p>0.4500; TT: CRI: 2.4x; control: 3.6x;
Wilcoxon-test: p=0.5708) was similar in the different groups. After intravenous injection, the maximum ratio in dogs with CRI was signifcantly lower in comparison to dogs with normal renal function (aPTT: CRI: 1.9x; control: 2.4x, Wilcoxon-test: p=0.0463; TT: CRI: 2.9x;
control: 1.9x; Wilcoxon-test: p=0.0124). The rank correlation coefficient according to Spearman (rs) for the relationship between plasma heparin-acivity and aPTT- or TT-ratio was between 0.77 and 0.85 (p=0.001), without significant differences related to the clotting test or the renal function status.
The AT-activity was significantly lower in dogs with CRI than in dogs with normal renal function (analysis of variance [ANOVA]: p<0.05). The AT-activity decreased significantly in dogs with normal renal function after subcutaneous UFH-application (by 13 % of the baseline value) and showed less significant changes in the other groups. The platelet count showed no significant differences between the different groups (ANOVA: p>0.05), neither after
subcutaneous nor after intravenous heparin injection. ANOVA showed significant differences between time points after subcutaneous injection (p=0.0026) reflecting minor declines of the platelet count at different times. The haematocrit was significantly lower in dogs with CRI in comparison to dogs with normal kidney function (ANOVA: p<0.05). Clear differences of the ANOVA between different times (p<0.05) reflected the fact that minor declines of the haematocrit in comparison to the baseline value occurred in different groups at various times.
Possible causes for the surprising results, i.e. higher Cltot and shorter T50 in dogs with decreased GFR, include an increased elimination of UFH by an activated mononuclear phagocyte system and endothelial cells as well as an increased heparin-neutralizing activity by increased plasma concentrations of platelet-factor-(PF 4)-concentrations or an increased binding to heparin-binding proteins such as fibrinogen or other acute-phase-proteins (e.g.
vibronectin) and, finally, an increased permeability of the glomerular basal membrane for UFH molecules. The lower haematocrit in dogs with CRI causes a dilution of the plasma heparin-activity and therefore probably is the main reason for the lower Cmax. The lower ratios of aPTT and TT observed in dogs suffering from CRI after intravenous injection of heparin are caused by the lower plasma heparin-activities. The lower AT-activity and haematocrit in dogs with CRI are well known consequences of CRI. The decrease of AT-activity which was observed in part of the groups is directly caused by heparin, because the results are corrected regarding possible dilution effects based on alterations of albumin concentration. The minor decreases of platelet count and haematocrit can be explained by the multiple blood collections.
The results of this study indicate that standard treatment protocols for UFH may lead to subtherapeutic plasma heparin-activities in dogs with CRI. With regard to clarification of the cause for the increased elimination of UFH in dogs with CRI, it would be of interest to study PF 4-, ß-thromboglobulin and acute-phase-protein-concentrations, to investigate the function of the mononuclear phagocyte system as well as to study the effect of repeated injections of UFH in these patients.