Withdrawal assessment following subchronic oral ketamine administration in Cynomolgus macaques

Recently there is increased regulatory interest in the assessment of physical dependence and withdrawal as part of the safety assessment for novel therapeutic entities. Choosing appropriate and sensitive parameters to detect withdrawal syndromes, and relevant positive control comparator drugs that can be administered in the same manner as the test agent, are critical study design elements. Pilot studies to determine the effects of oral ketamine in cynomolgus monkeys during, and following cessation of treatment, were explored. Detailed behavioral observations (both remote and interactive), food consumption, and body weight and temperature, were assessed during the dose-ranging, repeat dose (5 or 14 days), and withdrawal phases (3 or 5 days). Doses explored during dose-ranging included 20, 40, 100, or 200 mg/kg ketamine; subsequent withdrawal assessments were conducted following repeat dosing of 150 mg/kg. In the 14-day dosing study, exposure to ketamine and norketamine was assessed following 8 days of dosing. Administration of 150 mg/kg ketamine produced decreased activity, loss of balance, ataxia, hunched posture, nystagmus, lateral recumbence, and changes in alertness levels during dosing phases. When ketamine was withdrawn, increased reactivity, increased activity, and stereotypic behaviors were demonstrated that were absent during baseline or the dosing phase of the studies.