A model of orthostatic hypotension in the conscious monkey using lower body negative pressure

A non-human primate model for investigating drug-induced risk of orthostatic hypotension and sympathetic dysfunction: Preclinical correlate to a clinical test

INTRODUCTION

Drug induced orthostatic hypotension (OH) is an important clinical concern and can be an unexpected hurdle during drug development. OH is defined as an abnormal decrease in blood pressure (BP) triggered by a rapid postural change. The sympathetic nervous system is critical for controlling normal cardiovascular function and compensatory responses to changes in posture. Thus, OH can also serve as a surrogate indicator of sympathetic dysfunction. However, preclinical conscious models for investigating risk of OH and/or sympathetic dysfunction are lacking. Herein, we describe a conscious nonhuman primate (NHP) model which mimics the widely used clinical tilt table test for OH.

METHODS

Male, Cynomolgus NHPs (n = 7-8) implanted with radio-telemetry transmitters were placed in modified tilt chairs in a supine position. Subsequently, a 90° head up tilt was performed for 3 min followed by return to the supine position. BP and heart rate were continuously monitored. Test compounds were administered either intravenously or via oral gavage in a crossover design, with blood samples collected at the end of the each tilt to assess total drug concentrations.

RESULTS

Tilt responses were assessed following treatment with positive control compounds that cause sympathetic dysfunction; hexamethonium (ganglionic blocker) and prazosin (alpha-1 adrenergic receptor antagonist). Both compounds induced marked OH as evidenced by robust and sustained BP reduction in response to a head up tilt (decrease of 25-35 mmHg for hexamethonium, decrease of 21-44 mmHg for prazosin). OH incidence rates increased in a dose-dependent manner. OH incidences following treatment with minoxidil (vasodilator) were markedly lower to those observed with hexamethonium and prazosin indicating the role of sympathetic dysfunction in causing OH.

DISCUSSION

These data demonstrate that the NHP tilt test is a valuable model for investigating OH risk. This model fills an important preclinical gap for assessing such a safety concern and can be applied to programs where a sympathetic deficit and/or OH are anticipated or clinically observed.

Safety pharmacology in focus: new methods developed in the light of the ICH S7B guidance document

'Safety' continues to be a growth area in 'Pharmacology'. This issue of Journal of Pharmacological and Toxicological Methods is the third to be focused on methods development in the safety pharmacology area. The unusual nature of safety pharmacology mandates that methods development be done with, not only scientific validation, but also, adherence to the mandates of legislation to the forefront. This focused issue draws on a broad range of global safety pharmacology experts, many of whom operate in the industrial milieu. They have reviewed and updated current models, validated modifications, and have also introduced novel methodology important to the conduct of non-clinical safety pharmacology studies. The contributors were all active participants at the 5th Annual Safety Pharmacology Society (SPS) meeting held in Mannheim, Germany September 25-28, 2005. The publications presented here describe in vitro and in vivo pharmacological methods development that has been informed by the S7A regulatory guidance document for pre-clinical safety testing of drugs. While S7A describes the 'core battery' of methods used to characterize the safety pharmacology profile of a compound, the most recent news in Safety Pharmacology involves ratification of the related S7B safety guidance document. Unlike the past, S7B heralds a new era for the pharmaceutical industry since it now sets out how to address safety concerns of a new chemical entity (NCE) in relation to adverse actions on ventricular repolarization, a topic that has vexed industry and regulatory authorities for many years. Unsurprisingly there are many papers in the present issue that address this specific aspect of safety pharmacology. These include results from the Health and Environmental Sciences Institute of the International Life Sciences Institute (ILSI/HESI) initiative, in which non-clinical in vitro (hERG and Purkinje fiber) and in vivo (QT dog study) assays were found to be useful in the determination of drug-induced QT prolongation risk, and thus provide better characterization of a biomarker for the potential risk in humans for development of the torsades de pointes syndrome. However, safety methods development does not begin and end with ventricular repolarization. This focused issue also describes the re-evaluation and validation of a primate CNS model for evaluating orthostatic hypotension, and outlines a simple and rapid rodent object recognition task model that can be used to assess the amnesic potential of an NCE. Reviews of respiratory safety studies as well as both in vitro and in vivo aspects of cardiovascular function are also described. There are also papers that describe the pharmacology of vehicles and solvents used to solubilize study drugs and the applicability of voltage-sensitive dyes to optically record cardiac action potentials from single myocytes. Thus, this issue of the Journal of Pharmacological and Toxicological Methods remains a primary resource for industrial and academic pharmacologists interested in better understanding non-clinical safety pharmacology methods.