UNINTENTIONAL OVERDOSE WITH INTRATHECAL ZICONOTIDE

Steven G. Charapata MD, Research Medical Center, Kansas City, MO; David Ellis MD, PhD, Elan Pharmaceuticals, South San Francisco, CA Ziconotide is a novel, N‐type, voltage‐sensitive calcium channel (VSCC) blocker, with well‐documented efficacy as an intrathecal (IT) analgesic. Ziconotide has been administered to over 1000 chronic pain patients in nine clinical trials. Over 350 patients have been on ziconotide IT therapy for more than three months in a long‐term safety and tolerability study. Common adverse events for ziconotide include dizziness, nausea, nystagmus, abnormal gait, constipation, urinary retention, somnolence, postural hypotension, vomiting, confusion and abnormal vision. Ziconotide adverse events are recognizable, reversible and manageable, by dose adjustment and slow dose titration. Case reports of unintentional overdose in six chronic pain patients treated with IT ziconotide are presented. These unintentional overdoses were attributable to pump programming or dilution errors; none were lethal. The patient who received the highest overdose was administered 31 mcg/hr over 24 hours, or nearly 750 mcg ziconotide, total. This hourly dose rate is 300‐fold the current recommended initial dose rate of 0.1 mcg/hr. This patient was sedated, but arousable; vital signs were stable and patient had no change in blood pressure. His symptoms resolved within 24 hours. His Visual Analog Score of Pain Intensity (VASPI) was reduced from 82 at baseline to 2.5 at the end of the titration period. The patient elected to continue in the long‐term IT ziconotide study. The other 5 cases of inadvertent overdose were less severe, with dose rate at 5 mcg/hr or less. Associated adverse events also resolved within 24‐hours of discontinuing ziconotide infusion. Unlike an unintentional overdose with IT morphine, which slows respiration and could potentially lead to hypoxia, coma or death; ziconotide does not produce respiratory depression. No tolerance to the analgesic effect of ziconotide, or withdrawal symptoms after discontinuation of the drug have been reported. Ziconotide has a wide margin of safety as an IT analgesic.