Michael K. Pugsley

Principles of Safety Pharmacology

Safety Pharmacology is a rapidly developing discipline that uses the basic principles of pharmacology in a regulatory‐driven process to generate data to inform risk/benefit assessment. The aim of Safety Pharmacology is to characterize the pharmacodynamic/pharmacokinetic (PK/PD) relationship of a drugs adverse effects using continuously evolving methodology. Unlike toxicology, Safety Pharmacology includes within its remit a regulatory requirement to predict the risk of rare lethal events. This gives Safety Pharmacology its unique character. The key issues for Safety Pharmacology are detection of an adverse effect liability, projection of the data into safety margin calculation and finally clinical safety monitoring. This article sets out to explain the drivers for Safety Pharmacology so that the wider pharmacology community is better placed to understand the discipline. It concludes with a summary of principles that may help inform future resolution of unmet needs (especially establishing model validation for accurate risk assessment). Subsequent articles in this issue of the journal address specific aspects of Safety Pharmacology to explore the issues of model choice, the burden of proof and to highlight areas of intensive activity (such as testing for drug‐induced rare event liability, and the challenge of testing the safety of so‐called biologics (antibodies, gene therapy and so on.).

Predicting drug-induced changes in QT interval and arrhythmias: QT-shortening drugs point to gaps in the ICHS7B Guidelines

The regulatory guidelines (ICHS7B) recommending inhibition of the delayed rectifier K+ current (IKr), carried by human ether‐a‐go‐go‐related gene (hERG) channels in cardiac cells (the hERG test), as a ‘first line’ test for identifying compounds inducing QT prolongation, have limitations, some of which are outlined here.

Scientific review and recommendations on preclinical cardiovascular safety evaluation of biologics.

Biological therapeutic agents (biologicals), such as monoclonal antibodies (mAbs), are increasingly important in the treatment of human disease, and many types of biologicals are in clinical development. During preclinical drug development, cardiovascular safety pharmacology studies are performed to assess cardiac safety in accord with the ICH S7A and S7B regulations that guide these studies. The question arises, however, whether or not it is appropriate to apply these guidelines, which were devised primarily to standardize small molecule drug testing, to the cardiovascular evaluation of biologicals. We examined the scientific literature and formed a consensus of scientific opinion to determine if there is a rational basis for conducting an in vitro hERG assay as part of routine preclinical cardiovascular safety testing for biologicals. We conclude that mAb therapeutics have very low potential to interact with the extracellular or intracellular (pore) domains on hERG channel and, therefore, are highly unlikely to inhibit hERG channel activity based on their targeted, specific binding properties. Furthermore, mAb are large molecules (>140,000 Da) that cannot cross plasma membranes and therefore would be unable to access and block the promiscuous inner pore of the hERG channel, in contrast with typical small molecule drugs. Consequently, we recommend that it is not appropriate to conduct an in vitro hERG assay as part of a preclinical strategy for assessing the heart rate corrected QT interval (QTc) prolongation risk of mAbs and other types of biologicals. It is more appropriate to assess QTc risk by integrating cardiovascular endpoints into repeat-dose general toxicology studies performed in an appropriate non-rodent species. These recommendations should help shape future regulatory strategy and discussions for the cardiovascular safety pharmacology testing of mAbs as well as other biologicals and provide guidance for the preclinical cardiovascular evaluation of such agents.

Molecular characterisation of the interactions between olmesartan and telmisartan and the human angiotensin II AT1 receptor

Whereas some angiotensin II (Ang II) type 1 receptor blockers (ARBs) produce surmountable antagonism of AT1 receptors, others such as olmesartan and telmisartan display varying degrees of insurmountability. This study compared the molecular interactions of olmesartan and telmisartan with the human AT1 receptor, using well characterised in vitro methods and model systems.

A Review of the Structural and Functional Features of Olmesartan Medoxomil, An Angiotensin Receptor Blocker

The angiotensin II (A-II) type 1 (AT1) receptor-mediated effects of A-II play a key role in the pathophysiology of hypertension. Effective inhibition of A-II is provided by the latest class of antihypertensive medications, the AT1 receptor blockers (ARBs). These orally available agents were developed around a common imidazole-based structural core. The most recent member of this drug class to be approved by the Food and Drug Administration, olmesartan medoxomil, contains unique features that may explain its clinical efficacy. Key structural elements of olmesartan medoxomil include a hydroxyalkyl substituent at the imidazole 4-position and a hydrolyzable ester group at the imidazole 5-position. Inter- and intramolecular hydrogen bonding involving these groups may contribute to the potentiation of antagonist activity. After oral administration, olmesartan medoxomil is deesterified in the intestinal tract to produce the active metabolite olmesartan, which undergoes no additional metabolic change. The marked antihypertensive efficacy of olmesartan medoxomil may result from a unique pharmacological interaction of the drug with the AT1 receptor, resulting in a potent, long-lasting, dose-dependent blockade of A-II. This review article characterizes the structural features of olmesartan that may be responsible for its clinical efficacy. Inferential pharmacological studies compare and contrast the effects of olmesartan to those of other ARBs in comparable preclinical animal models.

A new ECG measure (RSh) for detecting possible sodium channel blockade in vivo in rats

A new electrocardiographic (ECG) measure for detecting possible sodium channel blocking actions of drugs in anaesthetized rats is described. The conventional measures for sodium channel blockers are increased QRS width and/or P-R prolongation, however, these are limited in their sensitivity. This new measure, RSh, is the height from the peak of the R wave to the bottom of the S wave; it is more sensitive to known sodium channel blocking agents than conventional measures. This was shown by comparing the ECG effects of sodium channel blocking class I antiarrhythmic drugs from the three subclasses lidocaine (Ia), quinidine (Ib), and flecainide (Ic). In each case, RSh increased before changes could be detected in QRS or P-R. With tetrodotoxin and quinacainol, a new class I agent, changes in RSh correlated directly with previously reported changes in dV/dtmax of intracellular potentials recorded in vivo from epicardial cells. Representatives from antiarrhythmic classes II, III, and IV were also tested and only changed RSh when they had known sodium channel blocking properties at high doses. Other physiological maneuvers for altering heart rate, such as changing vagal activity, administration of catecholamines, or direct right atrial pacing, did not alter RSh. Thus RSh is a useful in vivo measure for the detection of possible class I antiarrhythmic actions. It has the advantages of being sensitive, selective, easy to measure, and involving minimal preparation.

The ‘overly-sensitive’ heart: sodium channel block and QRS interval prolongation

Cardiac safety remains of paramount importance in the development of successful clinical drug candidates. Great progress has been made recently in understanding liabilities associated with delayed ventricular repolarization (manifest as QT prolongation) and in predicting (thus avoiding) drugs that delay repolarization based on application of strategic preclinical assays. Following the advances made in clinical electrophysiological monitoring and conduct of thorough QT studies, focus is now shifting towards monitoring of additional drug‐induced effects, particularly on ventricular conduction (measured as changes in the QRS interval on the ECG) as part of evolving clinical thorough ECG studies. In this issue of the British Journal of Pharmacology, a study by Harmer et al. proposes provisional safety margins for QRS prolongation in man based on retrospective clinical data and a single in vitro approach to assess potency of block of cardiac sodium current (hNav1.5), the ionic current responsible for ventricular conduction (observed as QRS prolongation). The present commentary places their study in context with evolving preclinical cardiac electrophysiological safety assessments, along with discussions focused on ensuring the proper ‘translation’ of preclinical findings with potential clinical concerns. Given the extant limitations and uncertainties of presently available data, as well as our limited understanding of the pro‐arrhythmic potential associated with these changes, due caution should be applied when considering the proposed in vitro‐based margins for drug‐induced QRS prolongation measured clinically. Additional validation with multiple preclinical models and more rigorous clinical safety studies will be necessary to substantiate these recommended margins.

Sodium channel-blocking properties of spiradoline, a kappa receptor agonist, are responsible for its antiarrhythmic action in the rat

Spiradoline (U-62,066E), a selective kappa (kappa) receptor agonist, was examined for actions on the cardiovascular system and on myocardial ionic currents in rats. We initially characterized cardiac, hemodynamic, and antiarrhythmic actions of spiradoline in isolated perfused rat hearts and pentobarbital-anesthetized rats. Electrophysiologic studies in isolated myocytes were used to elucidate the mechanism for changes observed in vivo in the ECG, as well as for antiarrhythmic actions against electrical and ischemia-induced arrhythmias. In isolated rat hearts, spiradoline reduced heart rate and cardiac contractility and increased the PR interval and QRS width of the ECG in a concentration-dependent manner. In anesthetized rats, spiradoline dose-dependently reduced blood pressure and heart rate and prolonged the PR interval and QRS width. At slightly higher doses, it increased the QaT interval of the ECG. RSh, an index of sodium channel blockade in the rat, also was dose-dependently increased. Electrical stimulation of the left ventricle suggested that spiradoline may exert its antiarrhythmic action by blockade of myocardial sodium currents. The electrophysiologic actions of spiradoline on sodium currents, the transient outward (i(to)) and sustained plateau potassium (ik(sus)) currents were studied in isolated cardiac rat myocytes by whole-cell patch-clamp techniques. Spiradoline (15-500 microM) reduced peak sodium current in a rapid, reversible, and concentration-dependent manner; it also increased the rate of decay of I(to) and reduced the amplitude of Ik(sus). At a concentration of 150 microM, spiradoline produced a 24 +/- 2 mV hyperpolarizing shift in sodium current inactivation kinetics but did not alter activation processes. Spiradoline showed both tonic and frequency-dependent components of sodium current block. Thus spiradoline produced its antiarrhythmic actions via sodium channel blockade in myocardial tissue, although higher doses also block potassium currents. This combined ion channel-blocking property may be of added clinical benefit in the setting of myocardial ischemia.

Safety pharmacology investigations in toxicology studies: An industry survey

INTRODUCTION The Safety Pharmacology (SP) Society (SPS) conducted an industry survey in 2012 in an attempt to define current industry practices as they relate to inclusion of safety pharmacology (SP) endpoints into Toxicology studies. METHODS A total of 361 participants from Asia (9.1%), Europe (19.4%) and North America (71.4%) responded to the survey. The preponderance of respondents were toxicologists (53.2%) followed by safety pharmacologists (27.2%) and scientists involved in the conduct of both disciplines (19.6%). Most participants (58.6%) were from pharmaceutical companies employing more than 500 employees. RESULTS A majority (68.2%) reported having experience in designing, performing or interpreting the SP component of a study when performed as part of a toxicology study. Some participants (42.0%) had submitted data to a regulatory agency where ICHS7 studies were performed as part of a toxicology study rather than as a standalone study. When comparing species that were used in studies in which SP was added to toxicology studies, canines were the most frequently reported animals used for new chemical entities (NCE) whereas non-human (NH) primates were the most frequent for the assessment of biological agents. The most frequent primary motivator for adding ICHS7 SP endpoints to regulatory toxicology studies was to generate additional data to allow for determination of an integrated risk assessment thereby testing Confidence in Safety (CIS) to better manage and/or mitigate risk. The current ability to add safety pharmacology endpoints into regulatory toxicology studies was used to address a specific concern (by 42.1% of respondents) to allow management of risk more effectively (36.8%) or to generate data that contributes to cessation of the progression of a compound (21.1%). For an NCE, SP measurements in toxicology studies were conducted in addition to standalone SP studies (by 40.6% of respondents) or in addition/instead of standalone safety pharmacology studies (by 39.8% of respondents). For biological agents, a majority (74.3%) indicated SP measurements in toxicology were conducted instead of standalone studies as outlined in the ICHS6 guideline while inclusion of SP endpoints in toxicology studies for biological agents in addition to standalone studies was reported by only 25.7% of the respondents. DISCUSSION The survey highlights that obtaining regulatory agreement for the proposed combined SP/Tox study designs may be useful before study conduct in some cases. Respondents suggest that such discussion could occur at the pre-IND meeting before the IND/CTA enabling program.

Evaluation of drug-induced QT interval prolongation in animal and human studies: a literature review of concordance

Evaluating whether a new medication prolongs QT intervals is a critical safety activity that is conducted in a sensitive animal model during non‐clinical drug development. The importance of QT liability detection has been reinforced by non‐clinical [International Conference on Harmonization (ICH) S7B] and clinical (ICH E14) regulatory guidance from the International Conference on Harmonization. A key challenge for the cardiovascular safety community is to understand how the finding from a non‐clinical in vivo QT assay in animals predicts the outcomes of a clinical QT evaluation in humans. The Health and Environmental Sciences Institute Pro‐Arrhythmia Working Group performed a literature search (1960–2011) to identify both human and non‐rodent animal studies that assessed QT signal concordance between species and identified drugs that prolonged or did not prolong the QT interval. The main finding was the excellent agreement between QT results in humans and non‐rodent animals. Ninety‐one percent (21 of 23) of drugs that prolonged the QT interval in humans also did so in animals, and 88% (15 of 17) of drugs that did not prolong the QT interval in humans had no effect on animals. This suggests that QT interval data derived from relevant non‐rodent models has a 90% chance of predicting QT findings in humans. Disagreement can occur, but in the limited cases of QT discordance we identified, there appeared to be plausible explanations for the underlying disconnect between the human and non‐rodent animal QT outcomes.