Possible contribution of acetylamrinone and its enhancing effects on platelet aggregation under shear stress conditions in the onset of thrombocytopenia in patients treated with amrinone.

BACKGROUND: Thrombocytopenia is recognized as one of the most common complications when the patients with severe heart failure are treated with cardiotropic phosphodiesterase (PDE)-3 inhibitors. To understand the mechanism of the onset of this complication, we focused on the effects of various PDE-3 inhibitors and its stable metabolite of acetylamrinone on platelet aggregation occurring under physiological shear stress conditions. METHOD: Blood specimens were obtained from eight apparently healthy adult donors. Platelet-rich plasma was separated after anticoagulation by citrate. The effects of PDE-3 inhibitors of amrinone and olprinone, as well as the stable metabolite of the former of acetylamrinone, on platelet aggregation induced by its exposure to a shear rate of 1200 and 10,800 s(-1) were determined by optically modified cone-plate viscometer. RESULTS: Both olprinone and amrinone inhibited platelet aggregation at 10,800 s(-1) in a dose-dependent manner with the IC(50) value of 14 +/- 1 and 61 +/- 8 microM (mean +/- S.D.), respectively, while amrinone significantly inhibited platelet aggregation at 1200 s(-1) only at highest concentration tested (100 microM). Contrary to the effects shown with PDE-3 inhibitors, acetylamrinone did not inhibit platelet aggregation at all. Moreover, it even enhanced the aggregation at 1200 s(-1) when used with 5 microM. CONCLUSIONS: Our results demonstrate possible contribution of the enhancing effects of acetylamrinone on platelet aggregation occurring under blood flow conditions, which reduced the platelet count when occurring in real circulation, to the higher incidence of thrombocytopenia in patients treated with amrinone.

Pharmacokinetics of amrinone in neonates and infants.

OBJECTIVE: To evaluate the pharmacokinetics of amrinone and its metabolites in neonates and infants after reconstructive surgery for congenital heart disease. DESIGN: Prospective study. SETTING: Pediatric intensive care unit in a university hospital. PARTICIPANTS: Fifteen neonates aged less than 1 month with transposition of the great arteries and 14 infants aged 2 to 6 months with complete atrioventricular septal defect. INTERVENTIONS: Amrinone, loading dose of 2 mg/kg, was administered before weaning from cardiopulmonary bypass, followed by a maintenance infusion of 7.5 microg/kg/min. MEASUREMENTS AND MAIN RESULTS: Blood samples to determine plasma concentrations of amrinone, N-acetylamrinone, and N-glycolylamrinone were drawn before amrinone administration, frequently after the loading dose, every 6 hours during the maintenance infusion, and until 48 hours after the end of the infusion. Amrinone clearance was 2.4 +/- 0.9 mL/kg/min in neonates and 3.2 +/- 1.2 mL/kg/min in infants (p < 0.05). The volume of distribution at steady-state was smaller (p < 0.05) in neonates than in infants. The elimination half-life of amrinone was 10.7 +/- 6.7 hours in neonates and 6.1 +/- 1.4 hours in infants (p < 0.05). There was a linear correlation between the clearance of amrinone and the body surface area (r = 0.67; p < 0.05). The ratio of the plasma concentration of N-acetylamrinone to that of amrinone did not differ between neonates and infants. CONCLUSIONS: Amrinone is eliminated at a slower rate in neonates than in infants. The rate of acetylation of amrinone appears to be similar; the differences in the elimination capacity of amrinone are mainly due to the immature renal function in neonates.

Hemodynamic effects of N-acetylamrinone in a porcine model of group B streptococcal sepsis.

High plasma concentrations of N-acetylamrinone, a primary metabolite of amrinone, are measured in some children during prolonged amrinone infusion. The purpose of this investigation was to determine if N-acetylamrinone has direct hemodynamic effects independent of amrinone. Twenty neonatal piglets received an infusion of 6 x 10(9) colony-forming units/kg of group B Streptococcus to induce sepsis. Subsequently, they were divided into 1 of 3 groups and received a 1-hr infusion of either normal saline (N = 4); 8 mg/kg amrinone, followed by 20 micrograms/kg/min (N = 9); or 8 mg/kg N-acetylamrinone, followed by 20 micrograms/kg/min (N = 7). Hemodynamic measurements and arterial/venous blood-gas determinations were obtained every 30 min during the study. Systemic vascular resistance and pulmonary vascular resistance were calculated. One milliliter of blood was obtained every 30 min during drug administration to determine plasma amrinone and N-acetylamrinone concentrations. The mean amrinone plasma concentrations measured at 30 and 60 min during the infusion time in the group receiving amrinone were 8.8 +/- 1.1 and 6.9 +/- 0.7 micrograms/ml, respectively. These animals experienced a significant decrease in mean pulmonary artery pressure and pulmonary vascular resistance, compared with saline controls after a 30-min infusion of amrinone. The mean N-acetylamrinone plasma concentrations measured at 30 and 60 min during the N-acetylamrinone infusion were 7.3 +/- 0.8 and 5.7 +/- 0.6 micrograms/ml, respectively. There was no difference between any hemodynamic parameter measured in these animals, compared with saline controls at any time during the infusion. We conclude that amrinone, but not N-acetylamrinone, causes pulmonary vasodilation in a porcine model of sepsis and that the parent drug is the sole active component in amrinone.

Structure-activity relationships of milrinone analogues determined in vitro in a rabbit heart membrane Ca(2+)-ATPase model.

The cardiac activity of a series of analogues of the positive inotropic bipyridines amrinone (5-amino-[3,4'-bipyridin]-6(1H)-one) and milrinone (2-methyl-5-cyano-[3,4'-bipyridin]-6(1H)-one) was evaluated in vitro in a rabbit myocardial membrane Mg(2+)-dependent, Ca(2+)-stimulable adenosine triphosphatase (Ca(2+)-ATPase) model and structure-activity relationships were compared for nine closely related derivatives. In the present studies, a 5-bromo analogue of milrinone stimulated myocardial membrane Ca(2+)-ATPase significantly (10(-7) M; P < 0.001 vs control, with 67% of the activity of milrinone), whereas a 2'-methyl-2H-milrinone derivative was inactive. Although amrinone was inactive in this assay, its 2-methyl analogue was stimulatory. However, analogues lacking a 2-substituent (with or without a 5-cyano group) or with the 3-N position blocked by a methyl group did not stimulate myocardial membrane Ca(2+)-ATPase activity. Structural data for these bipyridines show that those with either a 2- or 2'-methyl substituent have a twist conformation, whereas those without are nearly planar. Activity data reveal that those bipyridines with a nonplanar conformation are more active in the Ca(2+)-ATPase assay. Further study of milrinone analogues with a 2'-methyl substituent shows that even though the effect on the twist angle is equivalent to that of 2-methyl substitution, these analogues are less potent. Data for this series reveal that the prerequisites for Ca(2+)-ATPase stimulation include not only a 2-methyl to maintain a twist conformation but also a free 3-N position and a 5-substituent. This model for optimal activity in the myocardial membrane Ca(2+)-ATPase system differs from those proposed for phosphodiesterase enzyme receptor recognition only in the requirement for a nonplanar molecule. We have previously shown that milrinone, but not amrinone, shares structural homology with thyroxine and was able to stimulate myocardial membrane Ca(2+)-ATPase activity in a manner similar to the thyroid hormone. Additionally, milrinone, but not amrinone, was an effective competitor for thyroxine binding to the serum transport protein transthyretin. Analysis of the milrinone-transthyretin crystal complex confirms the structural homology between milrinone and thyroid hormone which is not shared by amrinone. Modeling studies of the binding interactions of milrinone analogues indicate that the 2-desmethylmilrinone analogue, the most inhibitory analogue, lacks the hydrophobic contacts present in milrinone in its transthyretin-bound complex.(ABSTRACT TRUNCATED AT 400 WORDS)

Amrinone and N-acetylamrinone assay in human plasma using solid-phase extraction and reversed-phase chromatography.

A simple and sensitive liquid chromatographic assay for simultaneous quantitation of amrinone and N-acetylamrinone in human plasma was developed. The method involves extraction of samples via activated solid-phase extraction Bond Elut C18 disposable columns, followed by chromatographic separation on a reversed-phase phenyl column using isocratic condition and UV detection. The assay can measure concentrations of both compounds over the range 0.075-10 micrograms ml-1. The injection interval is 11 min. The inter-day relative standard deviation (RSD) for replicate analysis of spiked samples is less than 10% and the accuracy more than 94% for both compounds over the standard curve range. The assay has been successfully applied to pharmacokinetic studies in humans.

Amrinone-associated thrombocytopenia: pharmacokinetic analysis.

Amrinone-associated thrombocytopenia is thought to result from nonimmune-mediated peripheral platelet destruction. Platelet destruction may be a concentration-dependent toxic effect of amrinone or its principal metabolite N-acetylamrinone. Eighteen children receiving amrinone after heart surgery were prospectively evaluated to correlate the pharmacokinetics of amrinone and N-acetylamrinone with thrombocytopenia. Amrinone and N-acetylamrinone plasma concentrations were determined by HPLC during loading, infusion, and terminal elimination, with concurrent monitoring of platelet counts. Thrombocytopenia developed in eight patients (platelet count, 66 +/- 17 x 10(9) platelets/L [mean +/- SD]). Peak and steady-state amrinone plasma concentration, amrinone total dose, duration of amrinone exposure, and amrinone area under curve (AUC) were similar between patients with and without thrombocytopenia. N-Acetylamrinone peak concentration, steady-state concentration, N-acetylamrinone AUC, and ratio of N-acetylamrinone to amrinone were greater in patients with thrombocytopenia. This association suggests that N-acetylamrinone, and not amrinone, may be the mediator of thrombocytopenia in children receiving amrinone.

New cardiotonic agents related to amrinone: synthesis of 1,2-dihydro-5-arylpyridin-2-ones.

For development of new cardiotonic agents a series of 5-aryl-3,4-dihydropyridin-2(1H)-ones, related to amrinone have been prepared from methylquinolines, 2-arylacetic acid or 3-arylethanones by direct aminomethylenation and subsequent condensation-cyclization with malonamide and cyanacetamide in classic basic media or phase-transfer catalysis, in good to excellent yields. Preliminary pharmacological assays have shown that these compounds, especially 5h and 5i, have a remarkable cardiotonic effect and present a selective inhibition of PDE-III/PDE-I isolated from cat heart.

5-Acyl-6-aryl-4-nitro-3(2H)pyridazinones and related 4-amino compounds: synthesis and pharmacological evaluation.

Several 4-nitro- and 4-amino-5-acyl-6-aryl-3(2H)pyridazinones were prepared and their in vitro and ex vivo antiaggregatory properties were evaluated. 4-Nitro derivatives 3 generally showed good activity in vitro towards arachidonic acid (AA)-induced human blood platelet aggregation. The 4-amino compound 4a, which has weak in vitro activity, exhibited antiplatelet activity, particularly on adenosine dephosphate (ADP)-induced aggregation ex vivo in rabbit. Moreover, the same compound was shown to be active in platelet-activating factors (PAF)-induced rat paw hyperalgesia and to be endowed with low acute oral toxicity. The 4-amino derivatives 4a-m and the other pyridazinones 5-9 administered orally to rats were also found to be more potent antiinflammatory agents than acetyl salicylic acid (ASA). Compounds 3a and 4a, tested in vitro on lipopolysaccharide (LPS)-stimulated rat peritoneal macrophages, were seen to be active in the inhibition of prostaglandin E2 (PGE2) production and interleukin-1 activity. Structure-activity relationship studies in the series of antiaggregating pyridazinones 3 have shown the primary importance of the nitro and acetyl substituents at positions 4 and 5, respectively. Hydrophobic substituents at position 2 were also required for better activity.

Pyridazine analogues of biologically active compounds. Part 5: Novel potential cardiotonics of the amrinone type.

Preparation of a series of novel pyridazine derivatives structurally related to bipyridine cardiotonics, starting from 4-methylpyridazine or 4-acetylpyridazine, respectively, is described. As observed with compounds 8, 11 and 16, an enhancement of in vitro cardiotonic activity is associated with the replacement of one or both pyridine subunit(s) in amrinone or milrinone by a 1,2-diazine system.

Effect of the acetylator phenotype on amrinone pharmacokinetics.

Ten healthy male subjects were phenotyped with isoniazid for their acetylator status and then received intravenous amrinone at a dose of 75 mg during a period of 10 minutes. Blood samples were drawn at specified times during a 24-hour period after dosing. Plasma concentrations of amrinone were determined by a specific HPLC method. The plasma concentration data were fitted to a biexponential model by nonlinear regression. The mean apparent first-order elimination t1/2 for amrinone in the slow acetylators was 4.4 hours, whereas it was 2.0 hours in the fast acetylators (P less than 0.05). There was little difference in the volume of distribution at steady state. Clearance was lower in the slow acetylators, 16.6 L/hr, than in the fast acetylators, 37.2 L/hr (P less than 0.05). The AUC was higher for the slow acetylators, 4.96 micrograms X hr X ml-1, than for the fast acetylators, 2.20 micrograms X hr X ml-1 (P less than 0.01). Concentrations of amrinone and its N-acetyl metabolite in the urine from each volunteer were determined. The ratio of N-acetylamrinone to amrinone was calculated and, as expected, the fast acetylators had a higher ratio than did the slow acetylators (P less than 0.01).