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Compendium November 2011 (Vol 33, Issue 11)

Pimobendan and Its Use in Treating Canine Congestive Heart Failure

by Danielle Bowles, BVSc, FANZCVS, Darren Fry, MA, VetMB, MRCVS, FANZCVS

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    Abstract

    Pimobendan, a calcium sensitizer and phosphodiesterase III inhibitor, has positive inotropic and vasodilatory properties. Its use in patients with naturally occurring congestive heart failure (CHF) has been studied in a number of blinded, randomized, multicenter clinical trials. It has been shown to improve quality of life, reduce heart insufficiency scores, and increase median survival times for patients with CHF due to dilated cardiomyopathy and myxomatous valvular disease. Although most studies have reported positive findings, some potential adverse effects have also been described. Studies are under way to further evaluate the effects of this novel positive inotrope and vasodilator in canine cardiac disease.

    Heart disease is common in veterinary patients. A potential consequence of severe heart disease in canine patients is the development of congestive heart failure (CHF). Traditionally, the main therapeutics used in CHF treatment have included diuretics, angiotensin-converting enzyme inhibitors (ACEIs), and the positive inotrope digoxin. However, over the past 20 years, interest in the use of positive inotropic agents other than digoxin in the treatment of CHF in human and veterinary patients has increased. Positive inotropes are pharmacologic agents that improve the contractility of cardiac muscle.1 A novel positive inotropic agent, pimobendan, has been developed for use in canine CHF. This article briefly reviews cardiac muscle physiology; describes the human and veterinary experiences with and knowledge regarding pimobendan; and describes the pharmacology and clinical recommendations regarding pimobendan use in canine CHF.

    Myocyte Structure, Physiology, and Contraction

    Cardiac muscle is composed of cardiomyocytes and a connective tissue matrix.2 The contractile unit of the cardiomyocyte (the sarcomere) is primarily composed of thin filaments (actin) and thick filaments (myosin). These filaments overlap, and proteins (troponin I [inhibitory], troponin C [calcium binding], troponin T [tropomyosin binding], and tropomyosin2,3) regulate myosin and  actin interaction based on cytosolic calcium levels, allowing contraction and relaxation to occur.2

    In diastole, the extracellular calcium level is higher than cytosolic levels and calcium is sequestered in the sarcoplasmic reticulum.2,4 In this resting state, tropomyosin prevents actin-myosin interaction and contraction. In systole, cytosolic calcium levels increase, and calcium binds to troponin C to allow contraction.4 Systole ends and diastole begins again as cytosolic calcium levels decrease.2

    Key Points

    • Pimobendan sensitizes cardiomyocytes to calcium rather than causing absolute increases in myocardium calcium concentration in systole, thereby reducing the risk of calcium-mediated proarrhythmic effects.
    • The labeled pimobendan dose is 0.1 to 0.3 mg/kg PO bid.
    • Pimobendan is an inodilator: it is a calcium sensitizer and phosphodiesterase inhibitor.
    • Pimobendan is approved for use in dogs with CHF secondary to DCM and MVD. It has been shown to improve quality of life, heart insufficiency scores, and overall mortality rates in canine patients with naturally occurring CHF.
    • Pimobendan has been shown to induce valvular lesions in patients with asymptomatic MVD, raising the question as to its use in asymptomatic MVD patients.
    • Further studies are under way to fully evaluate pimobendan use in dogs with naturally occurring heart disease secondary to MVD and DCM.

    These processes are regulated by intracellular messaging systems.5 Common messengers in this system include the nucleotides cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP).2,5,6 Phosphodiesterases (PDEs) are enzymes that are responsible for the degradation of these nucleotides. Inhibition of PDE prevents the degradation of cAMP and cGMP, thereby increasing their concentration. This affects cardiomyocyte and vascular smooth muscle contraction and relaxation.

    Mechanism of Action of Pimobendan

    Pimobendan (4,5-dihydro-6-[2-(4-methoxyphenyl)-1H-benzimidazole-5-yl]-5-methyl-3(2H)-pyridazinone) is a benzimidazole-pyridazinone derivative.7 It is a calcium sensitizer and a phosphodiesterase III (PDEIII) inhibitor.7 It has been termed an inodilator because of its dual action of positive inotropy and vasodilation.8

    The positive inotropic effects of pimobendan are thought to be due to its calcium-sensitizing effects, which cause increased calcium-troponin C interaction and thus increased actin-myosin cross-binding to produce a greater force of contraction.2 The vasodilatory effects of pimobendan are thought to be mediated through PDEIII inhibition. A PDEIII-mediated increase in cAMP results in vascular smooth muscle relaxation (vasodilation).

    Veterinary Use of Pimobendan

    Many of the early experimental studies for human pimobendan use involved animal models of human cardiac disease. The positive results in these studies led to further evaluation of pimobendan in dogs with dilated cardiomyopathy (DCM) or myxomatous valvular disease (MVD; TABLE 1).

    Studies of Myxomatous Valvular Disease and Dilated Cardiomyopathy

    Three studies evaluated the use of pimobendan in dogs with CHF secondary to MVD or DCM. The first was an open-label study9 to establish an effective dose; the second10 compared pimobendan with digoxin over a 4-week period. Most patients receiving pimobendan (88.9%9 and 86.7%10) showed an improvement in the New York Heart Association (NYHA; TABLE 2) heart scores. A 0.4- to 0.6-mg/kg/d dose was established to be safe and effective.

    Table 2. New York Heart Association (NYHA) Heart Insufficiency Score9

    Score

    Clinical Signs

    Class I (mild)

    No limitation in physical activity. Ordinary physical activity does not cause undue fatigue, palpitation, or dyspnea.

    Class II (mild)

    Slight limitation in physical activity. Comfortable at rest, but ordinary physical activity results in fatigue, palpitation, or dyspnea.

    Class III (moderate)

    Marked limitation in physical activity. Comfortable at rest, but less than ordinary physical activity causes fatigue, palpitation, or dyspnea.

    Class IV (severe)

    Cannot perform any physical activity without discomfort. Clinical signs seen at rest. If physical activity is undertaken, discomfort is increased.

    The PITCH study11,12 also showed an improvement in median survival in dogs with CHF secondary to MVD or DCM when treated with pimobendan (survival: 217 days) with or without benazepril compared with benazepril alone (survival: 42 days). There was no significant difference between patients receiving pimobendan without benazepril and those receiving both drugs. The major limitation of all three studies was that dogs with CHF due to different etiologies (DCM and MVD) were grouped together, so it was not clear whether the improvement in NYHA scores and median survival time was solely due to results in dogs with DCM or also reflected results in dogs with MVD.

    Studies of Myxomatous Valvular Disease

    In 2005, Smith et al13 published the first paper investigating pimobendan use only in dogs with CHF due to MVD. Two more papers were published in 2006 (VetSCOPE14) and 2008 (QUEST trial15) investigating pimobendan use in canine MVD patients. All three papers evaluated pimobendan in comparison with an ACEI (ramipril13 or benazepril14,16), and results were conflicting. Although the Smith paper showed no survival benefit for either treatment group, both the 56-day trial period and the optional long-term survival study published by the VetSCOPE group, as well as the QUEST trial, showed improved scores using the International Small Animal Cardiac Health Council (ISACHC; TABLE 3)14 or NYHA16 classification and improved median survival times in patients treated with pimobendan.

    Studies of Dilated Cardiomyopathy

    Two studies have evaluated the use of pimobendan in dogs with CHF due to DCM, and these studies have obtained the most profound results. Luis Fuentes and colleagues17 compared Doberman pinschers and American cocker spaniels with DCM, while O’Grady et al16 evaluated Doberman pinschers with DCM. In both studies, statistically significant improvements in the median survival time were seen in Doberman pinschers that were treated with pimobendan (329 and 130 days for the Luis Fuentes and O’Grady studies, respectively) compared with placebo (50 and 14 days for the Luis Fuentes and O’Grady studies, respectively).

    Table 3. International Small Animal Cardiac Health Council (ISACHC) Classification14

    Score

    Description

    Subclassification

    Class I

    Asymptomatic: Heart disease is detectable, but patient does not show clinical signs of heart failure.

    IA: Heart disease is present, but patient shows no evidence of compensation (volume or pressure overload ventricular hypertrophy).


    IB: Heart disease is present with radiographic or echocardiographic evidence of compensation.

    Class II

    Mild to moderate: Clinical signs of heart failure present at rest or with minimal exercise. Quality of life is affected. Clinical signs include mild dyspnea, exercise intolerance, tachypnea, and ascites. Home treatment indicated.

     

    Class III

    Advanced: Clinical signs of severe heart disease are obvious even at rest, with marked dyspnea, exercise intolerance, and hypoperfusion at rest. The most severe cases present with cardiogenic shock. Death or severe debilitation likely without treatment.

    IIIA: Home care is possible.


    IIIB: Hospitalization is necessary.

    One limitation of the Luis Fuentes study was that the placebo group contained a higher percentage of patients with atrial fibrillation than the pimobendan group (three of five dogs and one of five dogs, respectively). Atrial fibrillation has been reported as a negative prognostic factor in Doberman pinschers with DCM, although the affected Doberman in the Luis Fuentes study that received pimobendan survived 37 weeks compared with the 1 to 13 weeks previously reported.18 This limitation was not seen in the O’Grady paper because atrial fibrillation was an exclusion criterion.16

    No survival advantage was noted in the cocker spaniel group; however, only four of the 10 patients in this group reached the study end point, and of these, only one patient died of cardiac disease. Due to the small sample size and short evaluation period, further evaluation in cocker spaniels was recommended.

    Studies of Asymptomatic Myxomatous Valvular Disease

    Three studies19–21 have been published evaluating the effects of pimobendan on asymptomatic MVD in dogs. None of these studies assessed survival data, and conflicting findings were seen between the studies. The first study examined the effects of pimobendan in patients with experimentally induced asymptomatic MVD and showed a significant decrease in mitral regurgitation in patients receiving 0.25 mg/kg of pimobendan twice daily.19 The second evaluated spontaneous asymptomatic MVD in a colony of beagles.20 All patients were euthanized at the completion of the 512-day study period. Heart murmur grade, mitral valve thickness, and nodular changes were all increased in patients receiving pimobendan at standard doses (0.25 mg/kg PO bid). This finding prompted a recommendation that an echocardiographic analysis be performed before, and regularly throughout, pimobendan treatment in MVD patients.

    The third paper21 evaluated patients with ISACHC IB asymptomatic MVD. At 30 days, patients receiving pimobendan (0.2 to 0.3 mg/kg bid) had an improvement in systolic function, as measured by increased injection fraction and left ventricular internal diameter in systole, compared with control patients, but this change was not sustained over the 6-month study period. These patients did not develop hypotension during the study period.

    Pimobendan Versus Angiotensin-Converting EnzymeInhibitor Therapy

    A recent study22 evaluated the effects of pimobendan and furosemide on the renin-angiotensin-aldosterone system (RAAS) in healthy dogs. Pimobendan did not  activate the RAAS, and it did not prevent activation of this system associated with furosemide treatment. This finding suggests that patients receiving furosemide in conjunction with pimobendan should also be concurrently treated with an ACEI to facilitate angiotensin-converting enzyme inhibition.

    Summary

    Treatment with pimobendan in Doberman pinschers with CHF due to DCM has resulted in a marked survival advantage. Further studies are needed to determine if pimobendan treatment is beneficial in American cocker spaniels and other breeds with CHF due to DCM. Although not as dramatic as the DCM results, a survival advantage has also been demonstrated in patients with CHF due to MVD. Long-term survival study data are needed before a recommendation to use pimobendan in asymptomatic MVD or DCM patients can be made. While most studies have reported no significant adverse effects, findings from one study20 suggest that the use of pimobendan in asymptomatic MVD dogs may exacerbate the disease, and further studies are necessary to evaluate this.

    Currently, pimobendan is approved in the United States for the management of canine CHF due to atrioventricular valvular insufficiency or DCM, with concurrent therapy for CHF as appropriate. The recommended label dose is 0.1 to 0.3 mg/kg PO bid, given on an empty stomach at least 1 hour before food.

    The Human Experience With Pimobendan

    An early trial23 of the use of a pure PDEIII inhibitor, milrinone, found that patients with CHF had improved morbidity but a dramatic increase in mortality from all causes, including cardiovascular disease, compared with patients receiving a placebo. The findings from this study led to great reservations about the use of PDEIII inhibitors in people with CHF.

    Two subsequent studies24,25 used pimobendan as a positive inotropic agent, with conflicting results. The first study24 failed to show improved quality of life or a survival benefit in patients receiving pimobendan. There was a trend toward higher mortality in patients receiving pimobendan; however, it failed to reach statistical significance. The second study25 showed a positive effect on quality of life, and an increase in mortality was not seen. Nevertheless, the results from these studies raised enough suspicion in the human cardiology field to result in pimobendan being removed from human trials. Currently, pimobendan is only licensed for use in people with CHF in Japan.

    Adverse Reactions

    Numerous studies have shown that pimobendan is well tolerated in dogs with CHF.10,12–16,18 Adverse effects seen include a mild positive chronotropic effect and mild, dose-dependent gastrointestinal signs (vomiting, diarrhea, and soft stools).10,13,14 The potential hypotensive21 or proarrhythmic properties of pimobendan have not yet been documented in veterinary patients.

    Conclusion

    Pimobendan is a novel inodilator that has been shown to improve quality of life, heart insufficiency scores, and overall mortality rates in canine patients with naturally occurring CHF secondary to DCM and MVD when used in conjunction with conventional CHF treatment (e.g., furosemide, ACEI, antiarrhythmic medication). A treatment benefit for asymptomatic patients with DCM or MVD has not yet been established. Further studies are needed and are under way to assess the long-term effects of pimobendan in patients with naturally occurring CHF and with asymptomatic DCM or MVD.

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    References

    1. Fuentes VL, Corcoran B, French A, et al. A double blinded, randomized, placebo-controlled study of pimobendan in dogs with dilated cardiomyopathy. J Vet Intern Med 2002;16:255-261.

    2. Normal clinical cardiovascular physiology. In: Kittleson MD, Kienle RD. Small Animal Cardiovascular Medicine. St. Louis, MO: Mosby; 1998:11-35.

    3. Matthew L, Katz SD. Calcium sensitizing agents in heart failure. Drugs Aging 1998;12(3):191-204.

    4. Perrone SV, Kaplinsky EJ. Calcium sensitizer agents: a new class of inotropic agents in the treatment of decompensated heart failure. Int J Cardiol 2005;103:248-255.

    5.  Boswell-Smith V, Spina D, Page CP. Phosphodiesterase inhibitors. Br J Pharmacol 2006;147:S2552-S2557.

    6. Omori K, Kotera J. Overview of PDEs and their regulation. Circ Res 2007;100:309-332.

    7. Pouleur H, Gurne O, Hanet C, et al. Effects of pimobendan (UD-CG- 115) on the contractile function of the normal and “postischaemic” canine myocardium. J Cardiovasc Pharmacol 1988;11:100-106.

    8. Tissier R, Chetboul V, Moraillon R, et al. Increased mitral valve regurgitation and myocardial hypertrophy in two dogs with long term pimobendan therapy. Cardiovasc Toxicol 2005;5(1):43-52.

    9. Justus C, Kleeman R, Schmidt H. Clinical efficacy and tolerance of Vetmedin over the dose range of 0.2–0.6 mg/kg/day in dogs with congestive heart failure. Boehringer Ingelheim Vetmedica GmbH Internal Rep. no. UDC 6821 UDC 9201. Unpublished data.

    10. Kleeman R, LeBobinnec G, Bruyere D, et al. Clinical efficacy of Vetmedin in comparison to digoxin for the treatment of CHF in dogs. Proc Fourth Eur Cong Fed Eur Companion Anim Vet Assoc, 1998.

    11. Lombard CW. Clinical experience with pimobendan (Vetmedin®). Proc Basic Cardiovasc Sci Meet, 2000.

    12. Lombard CW. Pimobendan in congestive heart failure. ACVIM Forum, 2003.

    13. Smith PJ, French AT, Van-Israel N, et al. Efficacy and safety of pimobendan in canine heart failure caused by myxomatous mitral valve disease. J Small Anim Pract 2005;46:121-130.

    14. Lombard CW, Jons O, Bussadori CM. Clinical efficacy of pimobendan vs benazepril for the treatment of acquired atrioventricular valvular disease in dogs. J Am Anim Hosp Assoc 2006;42:249-261.

    15. Haggstrom J, Boswood A, O’Grady M, Jons O. Effect of pimobendan on survival in dogs with congestive heart failure due to myxomatous mitral valve disease. J Vet Intern Med 2008;22:1124-1135.

    16. O’Grady MR, Minors SL, O’Sullivan ML, Horne R. Effect of pimobendan on case fatality rate in Doberman pinschers with congestive heart failure caused by dilated cardiomyopathy. J Vet Intern Med 2008;22:897-904.

    17. Luis Fuentes V, Kleemann R, Justus C, et al. The effect of novel inodilator pimobendan (Vetmedin®) on the heart failure status in cocker spaniels and Doberman pinschers with idiopathic dilated cardiomyopathy. Proc BSAVA Cong, 1998.

    18. Calvert CA, Pickus CW, Jacobs GJ, Brown J. Signalment, survival and prognostic factors in Doberman pinchers with end stage cardiomyopathy. J Vet Intern Med 1997;11:323-326.

    19. Kanno N, Kuse H, Kawasaki M, et al. Effects of pimobendan for mitral valve regurgitation in dogs. J Vet Med Sci 2007;69(4):373-377.

    20. Chetboul V, Lefebvre HP, Sampedrano CC, et al. Comparative adverse cardiac effects of pimobendan and benazepril monotherapy in dogs with mild degenerative mitral valve disease: a prospective, controlled, blinded and randomized study. J Vet Intern Med 2007;21:742-753.

    21. Ouellet M, Belanger MC, Difruscia R, et al. Effect of pimobendan on echocardiographic values in dogs with asymptomatic mitral valve disease. J Vet Intern Med 2009;23:258-263.

    22. Sayer MB, Atkins C, Fujii Y, et al. Acute effect of pimobendan and furosemide on the circulating renin-angiotensin-aldosterone system in healthy dogs. J Vet Intern Med 2009;23:1003-1006.

    23. Packer M, Carver JR, Rodeheffer RJ, et al. Effect of oral milrinone on mortality in severe chronic heart failure. New Engl J Med 1991;325(21):1468-1475.

    24. Lubsen J. Effect of pimobendan on exercise capacity in patients with heart failure: main results from the pimobendan in congestive heart failure (PICO) trial. Heart 1996;76:223-231.

    25. The EPOCH study group. Effects of pimobendan on adverse cardiac events and physical activities in patients with mild to moderate chronic heart failure: the effects of pimobendan on chronic heart failure study (EPOCH study). Circ J 2002;66:149-157.

    References »

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