Age-related differences of QT interval and autonomic nervous system activity in female cynomolgus monkeys

Circadian Regulation of Cardiac Arrhythmias and Electrophysiology

Circadian rhythms in physiology and behavior are ≈24-hour biological cycles regulated by internal biological clocks (ie, circadian clocks) that optimize organismal homeostasis in response to predictable environmental changes. These clocks are present in virtually all cells in the body, including cardiomyocytes. Many decades ago, clinicians and researchers became interested in studying daily patterns of triggers for sudden cardiac death, the incidence of sudden cardiac death, and cardiac arrhythmias. This review highlights historical and contemporary studies examining the role of day/night rhythms in the timing of cardiovascular events, delves into changes in the timing of these events over the last few decades, and discusses cardiovascular disease-specific differences in the timing of cardiovascular events. The current understanding of the environmental, behavioral, and circadian mechanisms that regulate cardiac electrophysiology is examined with a focus on the circadian regulation of cardiac ion channels and ion channel regulatory genes. Understanding the contribution of environmental, behavioral, and circadian rhythms on arrhythmia susceptibility and the incidence of sudden cardiac death will be essential in developing future chronotherapies.

Reference intervals and method sensitivity for electrocardiology, hemodynamics, and body temperature parameters in healthy cynomolgus monkeys

In nonclinical studies, electrocardiograms (ECG) of cynomolgus monkey are recorded intermittently by external leads in manually restrained animals (snapshot recording) or continuously by jacketed external telemetry (JET) or implanted radiotelemetry transmitter in freely moving animals. With the implanted device, blood pressure and core body temperature can be monitored simultaneously. Despite the frequent use of cynomolgus monkeys in nonclinical safety pharmacology testing, few reference data are available for this species, comparisons of the ECG recording methods are limited, and power analyses are seldom conducted. In this study, pretreatment data were recorded from 406, 663, and 131 healthy experimentally naïve monkeys using the snapshot, JET, and implantable method, respectively, from 2019 to 2021. Reference intervals were determined for ECG, blood pressure, and body temperature parameters. Diurnal effects were observed in these parameters, with the exception of QRS and pulse pressure. The QRS, QT, and heart rate-corrected QTc intervals, as well as blood pressure, had a weak positive relationship with age and/or body weight. There were no sex differences in these parameters, and the country of origin only had minimal influences. Compared to telemetry, snapshot ECG data had shorter RR, PR, and QT intervals and longer QRS interval. The JET and implanted telemetry ECG data were comparable. Effect size analysis was conducted to estimate the method sensitivity for each parameter in common non-clinical study design scenarios. Snapshot recording, JET, and implanted telemetry were sensitive to detect 7-15 milliseconds of changes in QTc intervals in standard study designs, indicating these are powerful methods for assessment of QT prolongation in vivo.

Biology and postnatal development of organ systems of cynomolgus monkeys (Macaca fascicularis)

BACKGROUND

The cynomolgus macaque has become the most used non-human primate species in nonclinical safety assessment during the past decades.

METHODS

This review summarizes the biological data and organ system development milestones of the cynomolgus macaque available in the literature.

RESULTS

The cynomolgus macaque is born precocious relative to humans in some organ systems (e.g., nervous, skeletal, respiratory, and gastrointestinal). Organ systems develop, refine, and expand at different rates after birth. In general, the respiratory, gastrointestinal, renal, and hematopoietic systems mature at approximately 3 years of age. The female reproductive, cardiovascular and hepatobiliary systems mature at approximately 4 years of age. The central nervous, skeletal, immune, male reproductive, and endocrine systems complete their development at approximately 5 to 9 years of age.

CONCLUSIONS

The cynomolgus macaque has no meaningful developmental differences in critical organ systems between 2 and 3 years of age for use in nonclinical safety assessment.

Summary and Assessment of Studies on Cardiac Aging in Nonhuman Primates

Nonhuman primates (NHP) are important translational models for cardiac aging. To assess progress in this research area and to provide a reference for other investigators, we identified papers indexed in PubMed to determine what species, ages, outcomes, treatments, and approaches have been studied. Since 1983, 33 studies of cardiac aging in NHP have been published. Of these, 27 used species of macaque, 6 baboon, 1 vervet, 1 orangutan, and 1 marmoset (some studies were multispecies). Common research approaches were echocardiography, ECG, and histology of the left ventricle. Only 10 studies performed sex-based analyses. The average age of the oldest macaque studied was 26 y. The reported mean lifespan of macaques in captivity is around 30 y. The age of the oldest baboon studied was 24 y. Baboons in captivity are reported to live on average to 21 y. Twelve studies took a "life course" approach, studying animals of a wide range of ages from less than or equal to 10 y through the late teens to thirties, and employing analyses designed to show change over time. Keeping NHP into old age is a major challenge for biomedical research. The ideal design is to start monitoring in early life and to track how cardiac structure and function change with age. Important issues for future research are an increased focus on life-course approaches, investment in existing life-course NHP cohorts, better reporting of study sample characteristics, more molecular studies to identify genetic risk factors and mechanisms, attention to sex as a biological variable, a move away from descriptive reports to mechanistic studies, development of biomarkers to predict disease risk, and exploration of interventions that are implemented early in life to prevent or delay age-related disease later in life. Reducing exposure to early life adversity, identifying early-life biomarkers of aging and age-related disease, and early treatment can contribute to longer health span.

Cardiac changes in epileptic baboons with high-frequency microburst VNS therapy: A pilot study

The epileptic baboon provides a natural model of idiopathic generalized epilepsy and sudden unexpected death in epilepsy (SUDEP). We sought to evaluate autonomic differences, including heart rate (HR), heart rate variability (HRV) and corrected QT-duration (QTc) between two epileptic (EB1, EB2) and one control (CB) baboon, and the autonomic effects of high-frequency (HF) microburst Vagal Nerve Stimulation (VNS) Therapy in the epileptic baboons. At baseline, EB2's HR was increased over both EB1 and CB, and EB1's HRV was decreased compared to the others. QTc-intervals were significantly prolonged in both epileptic baboons. EB1 became free of generalized tonic-clonic seizures (GTCS) with VNS therapy, whereas EB2's GTCS were reduced by a third. HR decreased in both epileptic baboons, but while HRV improved in EB1, it decreased in EB2. EB2 succumbed to SUDEP after 9 months. This pilot study demonstrates abnormalities in HR, HRV and QTc-intervals in epileptic baboons. HF VNS Therapy demonstrated different effects on HRV in the two epileptic baboons, which, in addition to persistent GTCS and elevated HR, may have contributed to SUDEP risk in EB2. Future studies are needed to establish normative values for HRV and determine variability of HR, HRV and QTc-intervals in epileptic baboons.

Establishment of a new formula for QT interval correction using a large colony of cynomolgus monkeys

The demand for monkeys for medical research is increasing, because their ionic mechanism of repolarization is similar to that of humans. The QT interval is the distance between the Q wave and T wave, but this interval is affected by heart rate. Therefore, QT correction methods are commonly used in clinical settings. However, an accurate correction formula for the QT interval in cynomolgus monkeys has not been reported. We assessed snapshot electrocardiograms (ECGs) of 353 ketamine-immobilized monkeys, including aged animals, and contrived a new formula for the corrected QT interval (QTc) as a marker of QT interval prolongation in cynomolgus monkeys. Values for QTc were calculated using the formula [QTc] = [QT] / [RR]ⁿ, along with several other formulas commonly used to calculate QTc. We found that the optimal exponent of the QT interval corrected for heart rate, n, was 0.576. The mean value of QTc in healthy monkeys determined using the new formula was 373 ± 31 mm, and there were no significant differences between the sexes. Other ECG parameters were not significantly different between the sexes and there were no age-related effects on QTc. Prolongation of QTc to over 405 ms, as calculated by the new formula, was observed in 50 monkeys with underlying diseases. Additionally, all monkeys with QTc above 440 ms by the new formula had some underlying disease. The results resemble those in humans, suggesting that the new QTc formula could be useful for diagnosis of QT interval prolongation in cynomolgus monkeys.

Usefulness of simultaneous and sequential monitoring of glucose level and electrocardiogram in monkeys treated with gatifloxacin under conscious and nonrestricted conditions

Drug-induced cardiac electrophysiological abnormalities accompanied by hypoglycemia or hyperglycemia increase the risk for life-threatening arrhythmia. To assess the drug-induced cardiotoxic potential associated with extraordinary blood glucose (GLU) levels, the effect of gatifloxacin (GFLX) which was frequently associated with GLU abnormality and QT/QTc prolongations in the clinic on blood GLU and electrocardiogram (ECG) parameters was investigated in cynomolgus monkeys (n=4) given GFLX orally in an ascending dose regimen (10, 30, 60 and 100 mg/kg). Simultaneous and sequential GLU and ECG monitoring with a continuous GLU monitoring system and Holter ECG, respectively, were conducted for 24 h under free-moving conditions. Consequently, GFLX at 30 and 60 mg/kg dose-dependently induced a transient decrease in GLU without any ECG abnormality 2–4 h postdose. Highest dose of 100 mg/kg caused severe hypoglycemia with a mean GLU of <30 mg/dL, accompanied by remarkable QT/QTc prolongations by 20–30% in all animals. In contrast, hyperglycemia without QT/QTc prolongations was noted 24 h after dosing in one animal. A close correlation between GLU and QTc values was observed in animals treated with 100 mg/kg, suggesting that GFLX-induced hypoglycemia enhanced QT/QTc prolongations. Furthermore, the 24-h sequential GLU monitoring data clearly distinguished between GFLX-induced GLU abnormality and physiological GLU changes influenced by feeding throughout the day. In conclusion, the combined assessment of continuous GLU and ECG monitoring is valuable in predicting the drug-induced cardio-electrophysiological risk associated with both GLU and ECG abnormalities.

Ion channels, long QT syndrome and arrhythmogenesis in ageing

Ageing is associated with increased prevalences of both atrial and ventricular arrhythmias, reflecting disruption of the normal sequence of ion channel activation and inactivation generating the propagated cardiac action potential. Experimental models with specific ion channel genetic modifications have helped clarify the interacting functional roles of ion channels and how their dysregulation contributes to arrhythmogenic processes at the cellular and systems level. They have also investigated interactions between these ion channel abnormalities and age-related processes in producing arrhythmic tendency. Previous reviews have explored the relationships between age and loss-of-function Na_v 1.5 mutations in producing arrhythmogenicity. The present review now explores complementary relationships arising from gain-of-function Na_v 1.5 mutations associated with long QT3 (LQTS3). LQTS3 patients show increased risks of life-threatening ventricular arrhythmias, particularly after 40 years of age, consistent with such interactions between the ion channel abnormailities and ageing. In turn clinical evidence suggests that ageing is accompanied by structural, particularly fibrotic, as well as electrophysiological change. These abnormalities may result from biochemical changes producing low-grade inflammation resulting from increased production of reactive oxygen species and superoxide. Experimental studies offer further insights into the underlying mechanisms underlying these phenotypes. Thus, studies in genetically modified murine models for LQTS implicated action potential recovery processes in arrhythmogenesis resulting from functional ion channel abnormalities. In addition, ageing wild type (WT) murine models demonstrated both ion channel alterations and fibrotic changes with ageing. Murine models then suggested evidence for interactions between ageing and ion channel mutations and provided insights into potential arrhythmic mechanisms inviting future exploration.

Establishment of reference values for complete blood count and blood gases in cynomolgus monkeys (Macaca fascicularis)

Cynomolgus monkeys are closely related to humans phylogenetically, and this has resulted in their widespread use as a preclinical model. Hematological data with regard to these monkeys are thus important. Although reference values for blood components and sex hormones have been established for cynomolgus monkeys, those for arterial blood gases have not. The arterial blood gases quickly reflect respiratory and circulatory dynamics, and are thus useful for animal management and safe general anesthesia and surgical operations. Furthermore, since O₂ is transported by RBC, CBC and blood gases are closely related. The present study aimed to establish reference values for arterial blood gases and CBC in cynomolgus monkeys over a wide age range. Blood gases and CBC of arterial blood, collected from 41 female and 21 male anesthetized monkeys, were measured. Age correlated with RBC, HGB and HCT in the CBC. Values differed significantly between males and females in pCO₂, CO₂ concentration, MCV and MCH. The pH of blood was equivalent to that of humans and pCO₂ was more stable, whereas MCV and MCH were lower than those in humans. Erythrocytes were smaller and less pigmented than in other Macaca species. Several relationships between gender and age, and blood gases and CBC were identified in cynomolgus monkeys. In conclusion, these reference values will be useful as markers for veterinary applications and in the care and maintenance of these animals.

Power spectral analysis of heart rate variability in cynomolgus monkeys in safety pharmacology studies: comparative study with beagle dogs

INTRODUCTION

Power spectral analysis of heart rate variability is a tool known to provide information of interest on the autonomic control of heart rate in human. However, its use and its conditions of application and interpretation for safety purposes are not well defined for cardiovascular safety pharmacology studies. Likewise, data of power spectral analysis of heart rate variability in cynomolgus monkeys, a species often appropriate for use as second non rodent species in preclinical safety programmes, are not available. This study was designed to evaluate the relevance of this biomarker in this non human primate species, and to compare results with those from beagle dogs under the conditions of safety evaluation studies.

METHODS

Power spectral analysis of heart rate variability was performed on data collected in both species by telemetry following a standard design for cardiovascular safety pharmacology studies. Various pharmacological agents were tested in order to compare the profile of responses in both species after modifying the autonomic nervous balance.

RESULTS

Heart rate variability in cynomolgus monkeys is mainly driven by the parasympathetic nervous system as in beagle dogs although vagal tone is less than in dogs. Power spectral analysis of heart rate variability allows detection of interaction with the autonomic nervous system in both species in all investigated situations, i.e. sympatholytic/sympathomimetic and parasympatholytic/parasympathomimetic drug induced effects. However, due to species difference in the autonomic control of heart rate, cynomolgus monkeys are likely to be more sensitive than beagle dogs for assessment of sympatholytic properties.

DISCUSSION

This study confirms that power spectral analysis of heart rate variability from data derived from ECG recordings in telemetry studies is applicable in cardiovascular safety pharmacology studies and may provide relevant information about possible interaction with the autonomic nervous system when new drug entities are evaluated in either species. However, interspecies differences in autonomic control must be taken into account when interpreting possible drug effects.