Comparison of ANP and BNP Granular Density in Atria of Rats After Physiological and Pathological Hypertrophy

Atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP) are cardiac hormones located in atria granules. Both peptides respond to cardiac pressure and volume dynamics and accordingly serve as translation biomarkers for the clinical treatment of heart failure. Serum ANP and BNP play central secretary roles in blood pressure and cardiac output regulation and have proven utility as differential biomarkers of cardiovascular proficiency and drug-induced maladaptation, yet both peptides are impervious to exercise-induced hypertrophy. We employed immunoelectron microscopy to examine the effects of 28 days of chronic swim exercise or administration of a PPARγ agonist on atrial granules and their stored natriuretic peptides in Sprague Dawley rats. Chronic swimming and drug treatment both resulted in a 15% increase in heart weight compared with controls, with no treatment effects on perinuclear granule area in the left atria (LAs). Drug treatment resulted in larger size granules with greater BNP density in the right atria. Comparing swimming and PPARγ agonist treatment effects on ANP:BNP granule density ratios between atrial chambers revealed a shift toward a greater proportion of ANP than BNP in LAs of swim-trained rats. These data suggest a distinction in the population of ANP and BNP after chronic swim or PPARγ that makes it a novel metric for the differentiation of pathological and physiological hypertrophy.

Evaluation of Cardiac Toxicity Biomarkers in Rats from Different Laboratories

There is a great need for improved diagnostic and prognostic accuracy of potential cardiac toxicity in drug development. This study reports the evaluation of several commercially available biomarker kits by 3 institutions (SRI, Eli Lilly, and Pfizer) for the discrimination between myocardial degeneration/necrosis and cardiac hypertrophy as well as the assessment of the interlaboratory and interplatform variation in results. Serum concentrations of natriuretic peptides (N-terminal pro-atrial natriuretic peptide [NT-proANP] and N-terminal pro-brain natriuretic peptide [NT-proBNP]), cardiac and skeletal troponins (cTnI, cTnT, and sTnI), myosin light chain 3 (Myl3), and fatty acid binding protein 3 (FABP3) were assessed in rats treated with minoxidil (MNX) and isoproterenol (ISO). MNX caused increased heart-to-body weight ratios and prominent elevations in NT-proANP and NT-proBNP concentrations detected at 24-hr postdose without elevation in troponins, Myl3, or FABP3 and with no abnormal histopathological findings. ISO caused ventricular leukocyte infiltration, myocyte fibrosis, and necrosis with increased concentrations of the natriuretic peptides, cardiac troponins, and Myl3. These results reinforce the advantages of a multimarker strategy in elucidating the underlying cause of cardiac insult and detecting myocardial tissue damage at 24-hr posttreatment. The interlaboratory and interplatform comparison analyses also showed that the data obtained from different laboratories and platforms are highly correlated and reproducible, making these biomarkers widely applicable in preclinical studies.

Natriuretic Peptides as Cardiovascular Safety Biomarkers in Rats: Comparison With Blood Pressure, Heart Rate, and Heart Weight

Cardiovascular (CV) toxicity is an important cause of failure during drug development. Blood-based biomarkers can be used to detect CV toxicity during preclinical development and prioritize compounds at lower risk of causing such toxicities. Evidence of myocardial degeneration can be detected by measuring concentrations of biomarkers such as cardiac troponin I and creatine kinase in blood; however, detection of functional changes in the CV system, such as blood pressure, generally requires studies in animals with surgically implanted pressure transducers. This is a significant limitation because sustained changes in blood pressure are often accompanied by changes in heart rate and together can lead to cardiac hypertrophy and myocardial degeneration in animals, and major adverse cardiovascular events (MACE) in humans. Increased concentrations of NPs in blood correlate with higher risk of cardiac mortality, all-cause mortality, and MACE in humans. Their utility as biomarkers of CV function and toxicity in rodents was investigated by exploring the relationships between plasma concentrations of NTproANP and NTproBNP, blood pressure, heart rate, and heart weight in Sprague Dawley rats administered compounds that caused hypotension or hypertension, including nifedipine, fluprostenol, minoxidil, L-NAME, L-thyroxine, or sunitinib for 1-2 weeks. Changes in NTproANP and/or NTproBNP concentrations were inversely correlated with changes in blood pressure. NTproANP and NTproBNP concentrations were inconsistently correlated with relative heart weights. In addition, increased heart rate was associated with increased heart weights. These studies support the use of natriuretic peptides and heart rate to detect changes in blood pressure and cardiac hypertrophy in short-duration rat studies.

Integrated Approach to Early Detection of Cardiovascular Toxicity Induced by a Ghrelin Receptor Agonist

Cardiovascular (CV) safety concerns are among the leading causes of compound attrition in drug development. This work describes a strategy of applying novel end points to a 7-day rodent study to increase the opportunity to detect and characterize CV injury observed in a longer term (ie, 28 days) study. Using a ghrelin receptor agonist (GSK894281), a compound that produces myocardial degeneration/necrosis in rats after 28 days at doses of 0.3, 1, 10, or 60 mg/kg/d, we dosed rats across a range of similar doses (0, 0.3, 60, or 150 mg/kg/d) for 7 days to determine whether CV toxicity could be detected in a shorter study. End points included light and electron microscopies of the heart; heart weight; serum concentrations of fatty acid-binding protein 3 (FABP3), cardiac troponin I (cTnI), cardiac troponin T (cTnT), and N-terminal proatrial natriuretic peptide (NT-proANP); and a targeted transcriptional assessment of heart tissue. Histologic evaluation revealed a minimal increase in the incidence and/or severity of cardiac necrosis in animals administered 150 mg/kg/d. Ultrastructurally, mitochondrial membrane whorls and mitochondrial degeneration were observed in rats given 60 or 150 mg/kg/d. The FABP3 was elevated in rats given 150 mg/kg/d. Cardiac transcriptomics revealed evidence of mitochondrial dysfunction coincident with histologic lesions in the heart, and along with the ultrastructural results support a mechanism of mitochondrial injury. There were no changes in cTnI, cTnT, NT-proANP, or heart weight. In summary, enhancing a study design with novel end points provides a more integrated evaluation in short-term repeat dose studies, potentially leading to earlier nonclinical detection of structural CV toxicity.