Introduction
Cardiovascular disease in rabbits has historically been described as an uncommon disease with previous literature limited to occasional case reports and anecdotal information1–9 Rabbits have been used extensively as animal models for atherosclerosis, endocarditis, and drug-induced ardiomyopathies.10–12 A recent retrospective study3 on cardiovascular disease at a veterinary teaching hospital described an overall prevalence of 2.6%. The most common types of cardiovascular disease included cardiomyopathy followed by myocarditis and arteriosclerosis. Of those rabbits, 33.9% were diagnosed with congestive heart failure, indicating it may happen more common than previously thought.13 There are studies evaluating normal electrocardiograms,14,15 echocardiograms15 and blood pressure16,17 diagnostics in rabbits, which has aided in the diagnosis of cardiovascular disease in this species. While cardiovascular diagnostic tests in rabbits have been well described, there is a paucity of information regarding the treatment of cardiovascular disease in rabbits. A single study that evaluated IV administration of pimobendan to rabbits found a positive inotropic effect.18 However, the drug was continuously administered IV, which is not clinically performed in veterinary species.18 Currently, there are no pharmacokinetics or other pharmacodynamic studies evaluating drugs for the treatment and management of congestive heart failure in rabbits.
Pimobendan is a benzimidazole-pyridazinone derivative that is both a positive inotrope and vasodilator.19 It functions as a phosphodiesterase III inhibitor, which increases cyclic AMP concentrations and prolongs the action potential duration. This results in both venous and arterial vasodilation.20 It also increases the sensitivity of cardiac fibers to calcium, increases binding to troponin C, increases the release of calcium, and lowers the threshold of calcium for actin motility sliding.20–22
Pimobendan is labeled for the management of congestive heart failure in dogs in the US and Europe, but it is clinically used in variety of species. In the latest guidelines for the treatment of canine congestive heart failure, the American College of Veterinary Internal Medicine, pimobendan is recommended for both acute and chronic heart failure treatment in dogs.23 Various studies in dogs have proven its efficacy for cardiac conditions. It has been shown to extend survival time in Doberman Pinschers with preclinical dilated cardiomyopathy (DCM) and delay the onset of clinical signs.24 Pimobendan has also been shown to prolong the preclinical period in dogs with myxomatous mitral valve disease with echocardiographic and radiographic evidence of cardiomegaly by 15 months.25 In Irish Wolfhounds with preclinical cardiomyopathy or atrial fibrillation, pimobendan prolonged the time to onset of congestive heart failure or sudden death.26 Additionally, it has been shown to reduce the cardiac size and improve the outcome of dogs with myxomatous mitral valve disease and cardiomegaly.27
Pimobendan has been shown to be efficacious in other species as well. Cats with hypertrophic cardiomyopathy and congestive heart failure had a significantly prolonged survival time, compared with those not receiving pimobendan.28 In rats with pulmonary arterial hypertension, pimobendan resulted in improved echocardiographic parameters of systolic function.29 Healthy horses that received pimobendan IV had echocardiographic changes that indicated positive inotropic effects.30 Empiric recommended dosing in rabbits is 0.1 to 0.3 mg/kg orally every 12 to 24 hours.31 In a recent retrospective study13 in rabbits, the mean dosage was 0.375 ± 0.2 mg/kg administered 1 to 3 times a day. While this drug has been used clinically in rabbits, there are currently no studies evaluating the pharmacokinetics of this drug in this species.
The objective of this study was to determine the pharmacokinetics of pimobendan and the active metabolite, O-desmethylpimobendan (ODMP), in rabbits after a single oral dose as well as to evaluate for any adverse events. We hypothesized that rabbits would require higher doses compared to dogs and cats to sustain plasma concentrations reported to be effective in other species. Additionally, we hypothesized that no clinically relevant adverse effects would occur.
Materials and Methods
Animals
A total of 10 adult (age range, 8 months to 4 years) sexually intact (5 females and 5 males) New Zealand White rabbits (Oryctolagus cuniculus) were used for this study. Six rabbits were used for the first pilot study, 4 rabbits were used for the second pilot study, and 8 rabbits for the main study. Six rabbits used in the pilot study were used in the main study. The median body weight of the rabbits was 3.59 kg (range, 3.1 to 4.2 kg). All rabbits were transferred from a previous teaching protocol in which they received sedative drugs for student teaching of physical examinations. At least a 2-week washout period was used prior to the start of the pilot study and in between studies. The rabbits were determined to be healthy on the basis of full physical examination findings and PCV, with total protein concentration measurements obtained 1 week prior to the start of the study. The experimental protocol was approved by the Institutional Animal Care and Use Committee of the University of California-Davis.
All rabbits were housed individually for the duration of the study. Rabbits were moved to individual housing at least 2 days prior to drug administration. Each rabbit was fed commercial rabbit pellets (Laboratory Rabbit Diet HF #5236; LabDiet) and a large timothy hay cube (Kaytee Timothy Hay blend cubes) daily. Water was provided ad libitum via a water bottle. Absorbent pads were placed daily in the bottom of each cage to collect urine and feces. At least 1 week before the start of the pilot study, blood was collected from the rabbits (total volume, 3 to 6 mL/rabbit) to provide a blank sample, prior to drug administration for assay validation. Blood collected represented < 1% of the body weight of each rabbit.
First pilot study—A pilot study was conducted to determine a dose to be used for the pharmacokinetic study. In this study, 6 rabbits were randomly assigned to receive pimobendan at 0.25 mg/kg (n = 2), 0.5 mg/kg (2), or 1 mg/kg (2) orally. Pimobendan (Vetmedin; Boehringer-Ingelheim) tablets (5-mg tablet) were crushed into a fine powder and mixed with 4.5 mL of a suspending vehicle (Ora-Plus; Paddock Laboratories Inc) by use of 2 Luer-lock, 12-mL syringes connected with a 2-way stopcock until it was a uniform suspension, resulting in a concentration of 0.77 mg/mL. The suspension was prepared a maximum of 1 hour prior to administration. The rabbits were manually restrained, and a syringe was placed within the mouth in between the incisors and premolars and administered over 30 seconds to allow swallowing of the entire volume. Blood samples were collected from the peripheral venipuncture (lateral or medial saphenous veins, cephalic veins, or jugular veins) immediately before (0 minutes) and 15, 30, and 45 minutes and at 1, 2, 4, 6, and 9 hours after drug administration. Less than 1% of body weight of blood of each animal was collected.
Second pilot study—Based on initial plasma concentrations from the first pilot study, a second pilot study was performed. Four rabbits received 7.5 mg of pimobendan as 1.5 tablets (5-mg tablets). The tablets were split into halves and pilled by use of a pill administrator (Pet Piller Professional Pack; H-Bar-S). This was followed by 3 to 6 mL of water in a syringe. Swallowing was verified after each half pill on the basis of palpation and swallowing motion of the rabbit. Blood samples were collected by peripheral venipuncture immediately before (0 minutes) and at 30 and 45 minutes and at 1, 2, 4, 6, 8, 12, and 24 hours after drug administration. Blood samples were handled in the same manner as described for the first pilot study.
Main study—The dose, timing of sample collection, and administration formulation was determined based on the pilot studies. There was a 3-month washout period between the pilot study and main study. All 8 rabbits received 7.5 mg of pimobendan (1.5 tablets that were 5 mg each) in a single day, correlated to a mean dose of 2.08 mg/kg. The tablets were crushed with a mortar and pestle into a fine powder. This was mixed with 10 mL of critical care feeding formula (Fine Grind Critical Care; Oxbow Enterprises Inc) and transferred to a catheter tip syringe. The last 3 mL of the syringe were filled with critical care formula that did not contain a drug. The drug was administered over 5 minutes. The catheter tip syringe was then filled with 5 mL of water to flush any remaining drug. Rabbits were not fasted, and food was offered following drug administration.
Blood samples (0.4 mL/sample) were collected by peripheral venipuncture before administration (time 0) and at 45 and 90 minutes and 3, 4, 6, 8, 12, 24, and 36 hours after administration. Collected blood samples were stored in heparinized tubes on ice for < 4 hours until centrifugation for 10 minutes at 3,800 X g. Plasma was separated immediately and stored in cryotubes at -80°C until analysis. At the end of the last time point, crystalloid fluids (25 to 30 mL/kg) were administered SC to each rabbit.
Monitoring—During all 3 parts of the study, rabbits were monitored for 48 hours before drug administration and 48 hours after drug administration. Daily food consumption (measured by weight of the diet), water consumption (measured in milliliters), body weight, and mentation were evaluated every 24 hours. Fecal output was measured by weighing fecal pellets. Urine output was evaluated on a scale of 1 to 4, with 1 indicating urine covering a fourth of the absorbent pad and 4 indicting urine covering the entire pad.
Pimobendan analysis—For the pilot studies, only pimobendan concentrations were measured. Plasma calibrators were prepared by dilution of the pimobendan working standard solutions (Toronto Research Chemicals) with drug free rabbit plasma to concentrations ranging from 0.05 to 500 ng/mL. Calibration curves and negative control samples were prepared fresh for each quantitative assay. In addition, quality control samples (plasma fortified with analyte at 3 concentrations within the standard curve) were included with each sample set as an additional check of accuracy.
Prior to analysis, 0.1 mL of plasma was diluted with 0.1 mL of water containing d3-pimobendan internal standard (Toronto Research Chemicals) at 100 ng/mL and 0.1 mL of 5% ammonium hydroxide in saturated sodium chloride solution. The samples were vortexed briefly and 3 mL of methyl tert-butyl ether was added to each plasma sample. The sample was then mixed by rotation for 20 minutes at 40 revolutions/min. After rotation, samples were centrifuged at 3,300 rpm (2,260 X g) for 5 minutes at 4°C and the top organic layer transferred to a glass tube, dried under nitrogen, and dissolved in 120 µL of 5% acetonitrile in water with 0.2% formic acid. The injection volume was 30 µL into the liquid chromatography tandem mass spectrometry (LC-MS/MS) system.
The concentration of pimobendan was measured in plasma in positive mode (LC-MS/MS+). Quantitative analysis was performed on a triple quadrupole mass spectrometer (TSQ Altis; Thermo Scientific) coupled with a liquid chromatography system (Vanquish; Thermo Scientific). Detection and quantification were conducted by use of selective reaction monitoring of the initial precursor ion for pimobendan (m/z, 335.1) and d3-pimobendan (m/z, 338.1). The response of the product ions for pimobendan (m/z, 224.1, 250.1) and d3-pimobendan (m/z, 227.1, 322.2) was plotted, and peaks at the proper retention time were integrated by use of software (Quanbrowser; Thermo Scientific). The software was used to generate calibration curves and quantitate pimobendan in all samples by linear regression. A weighting factor of 1/X was used for all calibration curves.
Pimobendan working solutions were the same as described above and were used to quantitate the ODMP metabolite. The calibrator and sample extraction procedure and the instrument used for analysis were the same as described above for pimobendan. Detection and quantification were conducted with selective reaction monitoring of the initial precursor ion for ODMP (m/z, 321.2) and the product ions for pimobendan (m/z, 235.9, 305.0). Chromatography conditions and quantitation were the same as described above for pimobendan.
The response for pimobendan was linear and gave correlation coefficients of 0.99 or better. The intraday and interday precision and accuracy of the assay were determined by assaying quality control samples in replicates (n = 6) for pimobendan. Accuracy was reported as percent nominal concentration and precision was reported as percent relative SD. For pimobendan, intraday accuracy was 96% for 0.15 ng/mL, 104% for 40 ng/mL, and 115% for 250 ng/mL. Interday accuracy was 102% for 0.15 ng/mL, 104% for 40 ng/mL, and 106% for 250 ng/mL. Intraday precision was 2% for 0.15 ng/mL, 4% for 40 ng/mL, and 3% for 250 ng/mL. Interday precision was 4% for 0.15 ng/mL, 4% for 40 ng/mL, and 4% for 250 ng/mL. The technique was optimized to provide a limit of quantitation of 0.05 ng/mL and a limit of detection of approximately 0.01 ng/mL for pimobendan.
Pharmacokinetic analysis—Pharmacokinetic analyses was performed by use of a pharmacokinetic computer software program (WinNonlin version 6.4; Pharsight) and noncompartmental modeling. Peak concentrations (Cmax) as well as time to peak plasma concentrations (tmax) were determined for the parent compound by visual inspection of the concentration-time data. Other parameters including area under the plasma concentration curve (AUC), terminal half-life (t1/2λ), and time of last measurable plasma concentration were determined (Stata version 15.1/IC; StatCorp LP). The AUC from time 0 to infinity (AUC0–∞) was determined with the linear up–log down trapezoidal rule.
Results
Pilot studies—Plasma pimobendan concentrations were not detectable at time 0 for any portion of the study. In the first pilot study evaluating 0.25-, 0.5-, and 1-mg/kg doses of pimobendan in a liquid suspension, concentrations were detected at all times points and pimobendan was still present at 9 hours after administration. The mean plasma concentrations for the pilot studies are reported (Figure 1).
Mean ± SD plasma pimobendan concentrations in New Zealand White rabbits (Orcytolagus cuniculi) for 9 and 24 hours following oral administration of 0.25 mg/kg (solid line; n = 2), 0.5 mg/kg (dotted line; 2), 1 mg/kg (short and long dashed line; 2) in oral suspension formulation, and 2 mg/kg in tablet form (long dashed line; 4) during 2 pilot studies. Six rabbits received the oral suspension and then 4 rabbits received the tablet form of the drug after a 4-week washout period. The y-xis is logarithmically scaled. Time 0 was the time of drug injection.
Citation: American Journal of Veterinary Research 83, 4; 10.2460/ajvr.21.03.0032
For the second pilot study, pimobendan tablets were administered at 7.5 mg/rabbit. The median weight of the 4 rabbits used for this investigation was 3.7 kg (range 3.4 to 4.2 kg), correlating with a mean dose for 2 mg/kg each (range. 1.78 to 2.2 mg/kg). Initially, 6 rabbits were enrolled in the second pilot study; however, 2 rabbits died of acute aspiration pneumonia following oral tablet administration of pimobendan. They both presented with hypersalivation, open-mouth breathing, and marked dyspnea and died within 2 hours of administration despite veterinary intervention. On necropsy, intraluminal esophageal foreign material that resembled pimobendan tablets was found. The mean plasma concentrations for the 4 rabbits that completed the pilot study are reported (Figure 1). Due to the presence of pimobendan in the plasma during the last time point of the first pilot study, blood collection was extended to 24 hours for the second pilot study.
Main study—All rabbits remained healthy for the main investigation of the study. Animals maintained body weight and had no changes in food or water intake, urination, and defecation. One animal had a decrease in fecal output and food intake on a single day, but the animal had tipped the food bowl over for half of the day and resumed a normal appetite and defecation the following day. The day following venipuncture, 2 of 8 rabbits had erythema of the lateral saphenous region with return to normal the following day.
Pimobendan administration in critical care feeding formulation was successful in all rabbits and was given over a 5-minute period. The median body weight of the rabbits in the main study of the investigation was 3.6 kg (range, 3.1 to 4.2 kg), correlating with a mean dose of 2.08 mg/kg. After drug administration, all rabbits had quantifiable pimobendan plasma concentrations at all time points evaluated, except at time point 0 and at 36 hours, in which no rabbits had detectable concentrations. One rabbit reached very high plasma concentrations of pimobendan at 45 minutes (104 ng/mL) but then had the lowest plasma concentrations by 6 hours that remained low for the remainder of the time points. This animal was removed for further noncompartmental analysis because of the disparity between this animal’s concentrations and the remainder of the animals. The mean ± SD plasma concentrations of pimobendan were determined (Figure 2). Noncompartmental pharmacokinetic parameters were summarized (Table 1). Plasma ODMP was quantifiable starting at the first time point (45 minutes) and measured in most rabbits until 24 hours. At 36 hours, plasma concentrations were detectable in only 3 rabbits and were below 0.25 ng/mL. The mean maximum concentration of ODMP occurred at 1.5 hours following administration of pimobendan and was 19.46 ng/mL.
Mean ± SD of pimobendan (black circles) and O-desmethylpimobendan (ODMP; white circles) following administration of 7.5 mg of pimobendan/rabbit (mean, 2.08 mg/kg) in a critical care feeding formulation during the main study (n = 8). Each rabbit received 1.5 tablets of pimobendan crushed in 10 mL of a critical care feeding formulation that was administered over 5 minutes. There was a 3-month washout period between the pilot study and main study. At 36 hours, pimobendan concentrations were below the lower limit of quantification in all rabbits. The y-xis is logarithmically scaled. Time 0 was the time of drug injection.
Citation: American Journal of Veterinary Research 83, 4; 10.2460/ajvr.21.03.0032
Noncompartmental pharmacokinetics of plasma drug concentrations following oral administration of 7.5 mg of pimobendan/rabbit (mean, 2.08 mg/kg) of pimobendan in a critical care feeding formulation to 7 New Zealand White rabbits (Orcytolagus cuniculus).
| Pharmacokinetic parameters | Pimobendan |
|---|---|
| Cmax (ng/mL) | 15.7 ± 7.54 |
| tmax (h) | 2.79 ± 1.25 |
| t1/2λ (h) | 3.54 ± 1.31 |
| AUC0–∞ (h•ng/mL) | 65.11 ± 18.85 |
| AUC%extrap (%) | 0.78 ± 0.65 |
| Cl/F (mL/h/kg) | 3,2557 ± 7720 |
AUC = Area under the plasma concentration curve. AUC%extrap = AUC from the last measured time extrapolated to infinity. AUC0–∞ = AUC from time 0 to infinity. Cl/F = Clearance relative to bioavailability. Cmax = Peak concentration. t1/2λ = Terminal half-life.
Nonparametric superposition—The results of superposition of pimobendan at a dose of 2 mg/kg in a critical care feeding formula were determined every 24 hours and at every 12-hour dosing intervals (Figure 3). Based on this superposition, < 5% drug accumulation is expected with every 12-hour dosing interval, as mean Cmax differed by only 0.3 ng/mL and trough concentrations by 0.5 ng/mL, compared with every 24-hour dosing.
Plasma concentrations of pimobendan following pharmacokinetic modeling of administration of 7.5 mg of pimobendan/rabbit (mean, 2.08 mg/kg) in a critical care feeding formulation given every 24 hours (A) or every 12 hours (B) for 4 days. The y-axis is logarithmically scaled. Time 0 was the time of drug administration.
Citation: American Journal of Veterinary Research 83, 4; 10.2460/ajvr.21.03.0032
Discussion
In this study, the pharmacokinetics of a single dose of pimobendan (2.08 mg/kg) in rabbits was evaluated. Rabbits had detectable concentrations of pimobendan for up to 24 hours and of ODMP for up to 24 to 36 hours. Rabbits achieved plasma concentrations that are considered therapeutic in other species. Although pimobendan has been used empirically in rabbits, the pharmacokinetics have not previously been reported.
The rabbits in this study had a relatively large SD and variable plasma concentrations of pimobendan. This has also been observed in dogs with oral dosing.32 Individual variability should be considered with clinical dosing of this drug in rabbits. In this study, the mean ± SD of Cmax of pimobendan (15.7 ± 7.54 ng/mL) was lower than that in dogs and cats of previous studies.32–35 For example, in dogs that received a nonaqueous solution of pimobendan at 0.26 mg/kg, the Cmax was 18.6 ng/mL, and in cats that received the pimobendan tablet, the Cmax was 29.9 ng/mL in one study and 34 ng/mL in another.33–35 However, the Cmax in rabbits is higher than what is cited for dogs in the manufacturer‘s information of the licensed reference product, which is only 3.09 ± 0.76 ng/mL following oral administration of 0.25 mg/kg.36 The manufacturer’s information describes plasma concentrations in dogs as undetectable by 4 hours after administration.
The tmax in rabbits following administration of 2 mg/kg (2.79 ± 1.25 hours) is later than results of a study32 in dogs, which reported a tmax of 54 ± 36.7 minutes and a study33 in cats of 42 minutes. Additionally, the t1/2λ in this study of 3.54 ± 1.31 hours was longer than in both dogs (0.9 hours) and cats (0.7 hours).32–34 The rabbits in this study received a dose of about 2 mg/kg, which is 8 times as high as the dose used in the dog study and may explain the differences in tmax and t1/2λ. Due to the lack of an IV route of administration in this study, clearance could not be directly measured.
The pharmacokinetic and pharmacodynamic relationship of pimobendan has been described for dogs, cats, and humans. Interestingly, the duration of pharmacodynamic changes appears to outlast the plasma concentrations, with relatively low plasma concentrations being present at 8 hours in dogs and cats (< 1 ng/mL).33,34 In humans, there is a relatively short elimination half-life and cardiac function parameters are improved for < 8 hours.37 Plasma concentrations of pimobendan that correlate with the maximum cardiovascular effects in dogs that received 0.27 mg/kg of nonaqueous suspension range from 4 to 15 ng/mL at 2 hours and from 0.9 to 2 ng/mL at 4 hours.34 In the aforementioned study in dogs, substantial effects were lost within 8 hours. In humans, the estimated concentration corresponding with 50% of the maximal effect was 6.54 ± 1.35 ng/mL.38 Older studies that use the chewable pimobendan tablets (Vetmedin) found a more prolonged maximum rise in left ventricular pressure, a measurement of contractility, that lasted for 8 hours after dosing with the time to maximum increase being shorter with higher doses.39 While there is not a direct correlation between the plasma concentration and pharmacodynamic effect, the rabbits in this study had a mean ± SD concentration of pimobendan of 11.95 ± 10 ng/mL at 1.5 hours and 10.35 ± 3.49 ng/mL at 4 hours, which may be therapeutic if rabbits have a similar pharmacodynamic response to pimobendan as dogs.
Rabbits in this study produced measurable plasma concentrations of the active metabolite of pimobendan, ODMP. While reported values are notably relative concentrations, the maximum concentration of ODMP in rabbits (19.46 ng/mL) after administration of 7.5 mg (2.08 mg/kg) of pimobendan was similar to dogs (16.2 ng/mL) and cats (16.5 ng/mL) following administration of 5 mg (0.27 mg/kg) and 1.25 mg (0.28 mg/kg), respectively.33,34 The time when this maximum concentration occurred was also similar to that described in dogs and cats. While the overall pimobendan concentrations in plasma in rabbits were lower than those in dogs and cats, rabbits produced similar plasma concentrations of the active metabolite ODMP at the dose used in the main study. Due to expense and lack of a commercially available reference standard, metabolite concentrations were calculated relative to the parent compound; therefore. pharmacokinetic parameters were not calculated. Concentrations of the metabolite were reported simply to aid in comparison with other species studied.
In dogs, plasma concentrations were below the limit of quantification at 8 hours for pimobendan and at 12 hours for ODMP. In rabbits, pimobendan was still detectable in 7 of 8 rabbits at 24 hours and was below the level of quantification at 36 hours. Additionally, ODMP was still detected at low concentrations in 4 of 8 rabbits at 36 hours. Based on these results, administration of 2 mg/kg every 12 hours may be needed in healthy rabbits to maintain concentrations above target plasma concentrations and appears to have < 5% drug accumulation based on pharmacokinetic modeling. However, it may be safer and likely still effective to administer 2 mg/kg every 24 hours in sick rabbits or those with reduced organ function until further pharmacodynamic and multidose studies are performed.
The lower plasma concentrations, variability, and longer t1/2λ in rabbits may be due to the unique gastrointestinal anatomy and physiology in rabbits. Rabbits are hindgut fermenting species and rely on regular food consumption for adequate gastrointestinal health and physiology. Because of this, healthy rabbits always have food material within their stomach and intestines.40 In dogs, food may decrease the bioavailability when pimobendan is administered as an aqueous solution.36 Rabbits have also been hypothesized to have a lower absorption of some drugs, such as meloxicam, because of the persistent presence of food within the gastrointestinal tract.41 Studies evaluating rabbits as a model for pharmacokinetic studies have shown there is a prolonged retention of particles in the stomach, compared with other species.42 Therefore, the presence of food within the stomach in rabbits may have led to delayed absorption of pimobendan, compared with omnivorous or carnivorous species in which fasting may be possible. However, since an IV formulation was not used in this study, no direct comparisons of the bioavailability between species can be made.
In the US, currently the only commercially available formulation of pimobendan is a beef- flavored chewable tablet. Other formulations exist in other countries such as oral capsules and liquid formulation as well as an IV formulation.32 The authors elected to evaluate the chewable tablet instead of other formulations, as this is the commercially available formulation for patients in the United States at this time and therefore the most clinically applicable. The chewable tablet, however, is very challenging to administer to an herbivorous species due to the size and beef flavor. Additionally, we elected to use the commercial tablet opposed to the bulk chemical or compound of pimobendan because studies have shown that the bioavailability was lower when using the bulk chemical. A study of Hispaniolan Amazon parrots (Amazona ventralis) compared a suspension made from the bulk chemical with the commercial tablets and found that the bulk chemical reached much lower plasma concentrations.43 While using the pimobendan tablet to formulate a liquid suspension in our first pilot study, the most concentrated suspension that could be made was 0.77 mg/mL because of the need of a large volume of suspending vehicle. The resulting suspension was challenging to administer in the study due to the large volume required, with the largest dose (1 mg/kg) requiring approximately 5 mL of liquid to be administered. For the required dose for the main study of 2 mg/kg, it would have resulted in a mean volume of 10 mL/rabbit, which was thought to not be practical or safe to administer due to concerns for aspiration in rabbits unwilling to swallow it. While another formulation of the drug may have been preferable to obtain more accurate pharmacokinetic parameters, the authors elected to use the tablets mixed with critical care formula, as this was deemed the most clinically relevant administration route with the current formulation available. However, as previous studies have used a variety of formulations of pimobendan, direct comparison between them is cautioned.
There was a significant adverse event in the second pilot study, resulting in 2 fatalities secondary to aspiration pneumonia and esophageal foreign material. The decision to administer the tablets in half pieces followed by water was based on previous study33 models and the assumption that rabbits would chew the tablets and swallow them similar to other species. Further tablets were also not administered to each rabbit until swallowing had been felt on cervical palpation. No further rabbits were enrolled in this pilot study once the fatalities occurred. Ptyalism following oral administration has been reported in dogs and cats with both a tablet (cats) and nonaqueous solution (dogs).34,35 However, choking and aspiration pneumonia have not been previously reported as an adverse event following pimobendan administration in dogs, cats, or rabbits. The tablets are in a chewable form and are relatively large. Despite splitting the tablets into smaller pieces, use of a pill administrator, encouraging chewing, and administering water following administration, complications still occurred in our study. Based on this, the administration of tablets is not recommended and may result in complications, including death. The administration of tablets crushed in a critical care feeding formula, however, resulted in no adverse events and was well tolerated.
No other adverse events were reported in this study. However due to the scope of the study and limitations in staffing and equipment, electrocardiograms, blood pressure measurements, and continuous heart rates were not obtained. Reported adverse effects in cats include vomiting, diarrhea, constipation, anorexia, agitation, and rare arrhythmias.33–35 Other adverse effects reported in dogs include decreased platelet counts, elevated serum alkaline phosphatase, diarrhea, tachycardia, hypo or hypertension, and ST segment depression.39,44,45 The drug also can cause a dose dependent increase in mitral valve thickening, left ventricular endocardial thickening and subendocardial necrosis with supratherapeutic dosing in dogs.39 In dogs, the occurrence of arrythmias does not appear to be associated with pimobendan.39 No adverse effects have been reported clinically in rabbits to the authors’ knowledge.13
Limitations of this study include a lack of an IV formulation for comparison and to evaluate bioavailability. This study utilized a single dose of a drug; therefore, it is unknown whether drug accumulation may occur with multiple doses. Due to the scope of the study, additional monitoring parameters and a pharmacodynamic component were not performed. Additionally, young healthy rabbits were used in this study. Patients with cardiovascular disease and congestive heart failure may have hypoperfusion of organs such as the stomach, small intestine, kidney, and liver, which ran result in changes in absorption and drug pharmacokinetics such as drug clearance and volume of distribution.46,47 Therefore, the dose used in this study may need to be adjusted for diseased rabbits.
The results of this study suggested that pimobendan at a dose of 2 mg/kg in a critical care feeding formulation resulted in plasma concentrations that correlated with therapeutic effects in other species studied. While rabbits reached lower plasma concentrations of pimobendan than some studies in dogs and cats, they produced similar concentrations of the active metabolite ODMP. Rabbits had a delayed time to maximum concentration, compared with other species of mammals studied, likely due to their unique gastrointestinal anatomy. For administration of the chewable tablet, the use of a critical care feeding formulation is recommended to improve compliance and safety. Future pharmacodynamic and multidose studies are required to make appropriate recommendations regarding dosing frequency.
Acknowledgments
Funded by the Center for Companion Animal Health, School of Veterinary Medicine, University of California, Davis, CA.
References
- 1. ↑
Orcutt C, Malakoff R. Cardiovascular disease. In: Quesenberry K, Orcutt C, Mans C, Carpenter JW, eds. Ferrets, Rabbits and Rodents, Clinical Medicine and Surgery. 4th ed. Saunders; 2020:250–257.
- 2.
Schuhmann BHK. Cardiac disease in pet rabbits. [German] Herzerkrankungen bei Kaninchen. Wien Tierarztl Monatsschr. 2014;101(9-10):197–205.
- 3. ↑
Di Girolamo N, Palmieri C, Baron Toaldo M, et al. First description of partial atrioventricular septal defect in a rabbit. J Exot Pet Med. 2018;27(4):5–9.
- 4.
Lord B, Devine C, Smith S. Congestive heart failure in two pet rabbits. J Small Anim Pract. 2011;52(1):46–50.
- 5.
Vörös K, Seehusen F, Hungerbühler S, Meyer-Lindenberg A, von der Hoeh N. Ventricular septal defect with aortic valve insufficiency in a New Zealand White rabbit. J Am Anim Hosp Assoc. 2011;47(4):e42–e49. doi:10.5326/jaaha-ms-5498
- 6.
Shell LG, Saunders G. Arteriosclerosis in a rabbit. J Am Vet Med Assoc. 1989;194(5):679–680.
- 7.
Künzel F, Kolm U, Bohler A, Url A. Mitral regurgitation due to papillary muscle dysfunction in a pet rabbit. A case report. Tieräerztliche Praxis Ausgabe K Kleintiere/Heimtiere. 2005;33(6):423–430.
- 8.
Martel-Arquette A, Tjostheim SS, Miller J, Carlson J, Mans C. Aortocavitary fistula secondary to vegetative endocarditis in a rabbit. J Vet Cardiol. 2019;21:49–56.
- 9. ↑
Martin MW, Darke PG, Else RW. Congestive heart failure with atrial fibrillation in a rabbit. Vet Rec. 1987;121(24):570–571.
- 10. ↑
Yanni AE. The laboratory rabbit: an animal model of atherosclerosis research. Lab Anim. 2004;38(3):246–256.
- 11.
Hershberger E, Coyle EA, Kaatz GW, Zervos MJ, Rybak MJ. Comparison of a rabbit model of bacterial endocarditis and an in vitro infection model with simulated endocardial vegetations. Antimicrob Agents Chemother. 2000;44(7):1921–1924.
- 12. ↑
Gava FN, Zacche E, Ortiz EM, et al. Doxorubicin induced dilated cardiomyopathy in a rabbit model: an update. Res Vet Sci. 2013;94(1):115–121.
- 13. ↑
Ozawa S, Guzman D, Keel K. Clinical and pathological findings in rabbits (oryctolagus cuniculus) with cardiovascular disease: 59 cases (2001–2018). J Am Vet Med Assoc. 2021;259(7):764–776.
- 14. ↑
Lord B, Boswood A, Petrie A. Electrocardiography of the normal domestic pet rabbit. Vet Rec. 2010;167(25):961–965.
- 15. ↑
Turner Giannico A, Ayres Garcia DA, Lima L, De Lara FA. Determination of normal echocardiographic, electrocardiographic, and radiographic cardiac parameters in the conscious New Zealand Wite Rabbit. J Exot Pet Med. 2015;24(2):223–234.
- 16. ↑
Harvey L, Knowles T, Murison PJ. Comparison of direct and Doppler arterial blood pressure measurements in rabbits during isoflurane anaesthesia. Vet Anaesth Analg. 2012;39(2):174–184.
- 17. ↑
Sato K, Chatani F, Sato S. Circadian and short-term variabilities in blood pressure and heart rate measured by telemetry in rabbits and rats. J Auton Nerv Syst. 1995;54(3):235–246.
- 18. ↑
Hagemeijer F. Calcium sensitization with pimobendan: pharmacology, haemodynamic improvement, and sudden death in patients with chronic congestive heart failure. Eur Heart J. 1993;14(4):551–566.
- 19. ↑
Verdouw PD, Hartog JM, Duncker DJ, Roth W, Saxena PR. Cardiovascular profile of pimobendan, a benzimidazole-pyridazinone derivative with vasodilating and inotropic properties. Eur J Pharmacol. 1986;126(1-2):21–30.
- 20. ↑
Fujimoto S, Matsuda T. Effects of pimobendan, a cardiotonic and vasodilating agent with phosphodiesterase inhibiting properties, on isolated arteries and veins of rats. J Pharmacol Exp Ther. 1990;252(3):1304–311.
- 21.
Bohm M, Morano I, Pieske B, et al. Contribution of cAMP-phosphodiesterase inhibition and sensitization of the contractile proteins for calcium to the inotropic effect of pimobendan in the failing human myocardium. Circ Res. 1991;68(3):689–701.
- 23. ↑
Atkins C, Bonagura J, Ettinger S, et al. Guidelines for the diagnosis and treatment of canine chronic valvular heart disease. J Vet Intern Med. 2009;23(6):1142–1150.
- 24. ↑
Summerfield NJ, Boswood A, O’Grady MR, et al. Efficacy of pimobendan in the prevention of congestive heart failure or sudden death in Doberman Pinschers with preclinical dilated cardiomyopathy (the PROTECT Study). J Vet Intern Med. 2012;26(6):1337–1349.
- 25. ↑
Boswood A, Haggstrom J, Gordon SG, et al. Effect of pimobendan in dogs with preclinical myxomatous mitral valve disease and cardiomegaly: the epic study—a randomized clinical trial. J Vet Intern Med. 2016;30(6):1765–1779.
- 26. ↑
Vollmar AC, Fox PR. Long-term outcome of irish wolfhound dogs with preclinical cardiomyopathy, atrial fibrillation, or both treated with pimobendan, benazepril hydrochloride, or methyldigoxin monotherapy. J Vet Intern Med. 2016;30(2):553–559.
- 27. ↑
Boswood A, Gordon SG, Häggström J, et al. Longitudinal analysis of quality of life, clinical, radiographic, echocardiographic, and laboratory variables in dogs with preclinical myxomatous mitral valve disease receiving pimobendan or placebo: the EPIC study. J Vet Intern Med. 2018;32(1):72–85.
- 28. ↑
Reina-Doreste Y, Stern JA, Keene BW, et al. Case-control study of the effects of pimobendan on survival time in cats with hypertrophic cardiomyopathy and congestive heart failure. J Am Vet Med Assoc. 2014;245(5):534–539.
- 29. ↑
Yoshiyuki R, Nakata TM, Fukayama T, et al. Pimobendan improves right ventricular myocardial contraction and attenuates pulmonary arterial hypertension in rats with monocrotaline-induced pulmonary arterial hypertension. J Med Ultraon (2001). 2014;41(2):173–180.
- 30. ↑
Afonso T, Giguére S, Rapoport G, Barton MH, Coleman AE. Cardiovascular effects of pimobendan in healthy mature horses. Equine Vet J. 2016;48(3):352–356.
- 31. ↑
Fisher P, Graham J. Rabbits. In: Carpenter J, ed. Exotic Animal Formulary. 5th ed. Elsevier; 2018:494–531.
- 32. ↑
Bell ET, Devi JL, Chiu S, Zahra P, Whittem T. The pharmacokinetics of pimobendan enantiomers after oral and intravenous administration of racemate pimobendan formulations in healthy dogs. J Vet Pharmacol Ther. 2016;39(1):54–61.
- 33. ↑
Yata M, McLachlan AJ, Foster DJ, Hanzlicek AS, Beijerink NJ. Single-dose pharmacokinetics and cardiovascular effects of oral pimobendan in healthy cats. J Vet Cardiol. 2016;18(4):310–325.
- 34. ↑
Yata M, McLachlan AJ, Foster DJ, Page SW, Beijerink NJ. Pharmacokinetics and cardiovascular effects following a single oral administration of a nonaqueous pimobendan solution in healthy dogs. J Vet Pharmacol Ther. 2016;39(1):45–53.
- 35. ↑
Hanzlicek AS, Gehring R, KuKanich B, et al. Pharmacokinetics of oral pimobendan in healthy cats. J Vet Cardiol. 2012;14(4):489–496.
- 37. ↑
Chu K-M, Shieh S-M, Hu OY-P. Pharmacokinetics and pharmacodynamics of enantiomers of pimobendan in patients with dilated cardiomyopathy and congestive heart failure after single and repeated oral dosing. Clin Pharmacol Ther. 1995;57(6):610–621.
- 38. ↑
Chu K-M, Hu OY-P, Shieh S-M. Cardiovascular effect and simultaneous pharmacokinetic and pharmacodynamic modeling of pimobendan in healthy normal subjects. Drug Metab Dispos. 1999;27(6):701–709.
- 39. ↑
Boehringer Ingelheim Vetmedica I. Freedom of Information Summary: Vetmedin. NADA 141–273. 2007.
- 40. ↑
Smith SM. Gastrointestinal physiology and nutrition. In: Quesenberry KE, Carpenter JW, Christoph M, eds. Rabbits and Rodents, Clinical Medicine and Surgery. 4th eds. Saunders; 2020:162–173.
- 41. ↑
Turner PV, Chen CH, Taylor MW. Pharmacokinetics of meloxicam in rabbits after single and repeat oral dosing. Comp Med. 2006;56(1):63–67.
- 42. ↑
Kararli TT. Comparison of the gastrointestinal anatomy, physiology, and biochemistry of humans and commonly used laboratory animals. Biopharm Drug Dispos. 1995;16(5):351–380.
- 43. ↑
Guzman DS, Beaufrere H, KuKanich B, et al. Pharmacokinetics of single oral dose of pimobendan in Hispaniolan Amazon parrots (Amazona ventralis). J Avian Med Surg. 2014;28(2):95–101.
- 44. ↑
Reinker LN, Lee JA, Hovda LR, Rishniw M. Clinical signs of cardiovascular effects secondary to suspected pimobendan toxicosis in five dogs. J Am Anim Hosp Assoc. 2012;48(4):250–255.
- 45. ↑
Kitzen JM, Lynch JJ, Driscoll EM, Lucchesi BR. Cardiac electrophysiologic and hemodynamic activity of pimobendan (UD-CG 115 BS), a new inotropic agent. J Pharmacol Exp Ther. 1988;244(3):929–939.
- 46. ↑
Giallourakis CC, Rosenberg PM, Friedman LS. The liver in heart failure. Clin Liver Dis. 2002;6(4):947–967.
- 47. ↑
Shammas FV, Dickstein K. Clinical pharmacokinetics in heart failure. An updated review. Clin Pharmacokinet. 1988;15(2):94–113.
