Study Objectives: Despite aggressive interventions such as vasopressors and highdose insulin, many patients with amlodipine toxicity succumb to refractory shock. Amlodipine-induced shock is unique in that its mechanism of action is theorized to occur not only through L-type calcium channel blockade, but also via release of nitric oxide (NO) in the peripheral vasculature. Methylene blue, a NO scavenger, has been used clinically in amlodipine-induced refractory shock with its efficacy studied in only a single rat model. We designed a randomized, two-armed porcine study comparing methylene blue to norepinephrine therapy in amlodipine toxicity. Here, we describe the preparation of the study drug and a three-pig pilot study to define a toxic dose of amlodipine. Methods: Amlodipine preparation: 250 commercially obtained 10 mg amlodipine tablets were ground and mixed with 300 mL of dimethylsulfoxide (DMSO). This mixture was sonicated for 15 minutes, with the resulting product separated using vacuum filtration. This solution was then centrifuged and the supernatant collected. This was then diluted and placed in an ultraviolet-visible (UV/vis) spectrophotometer. The concentration was measured by determining absorption at 360 nm and then compared to a previously derived absorption versus concentration plot.Pilot study: Three pigs were sedated, instrumented, and
monitored according to an established porcine model that has been used for similar studies at our institution. Based on previous experience with porcine models of poison-induced shock, we administered a 2 mg/kg bolus of amlodipine followed by an infusion of 0.3 mg/kg/hr. The dose was adjusted in the subsequent pigs if a predefined point of toxicity was reached too quickly or not at all during the five-hour study period. Throughout the study period, hemodynamic and laboratory parameters were monitored. Results: Amlodipine preparation: From a total of 2500 mg of amlodipine tablets, our procedure produced 300 mL of amlodipine dissolved in DMSO with a concentration confirmed by UV/vis spectroscopy of 6.9 mg/mL, an 83% yield. Pilot study: The first pig developed hypotension and death within fifteen minutes of the bolus infusion. We thus eliminated the bolus and initiated a more conservative drug infusion rate of 0.25 mg/kg/hour, increasing the infusion every 20 minutes up to 1 mg/kg/hour. This pig lived until the end of the five-hour protocol, but only displayed mild evidence of amlodipine toxicity. The DMSO solvent was found to depolymerize the polyethylene and polyvinylchloride intravenous tubing and much of the drug leaked. For the third pig, the drug was infused through a more durable polytetrafluoroethylene line. The same initial infusion rate was used, increasing the dose until desired effect. This animal had hemodynamic patterns consistent with our expectation of amlodipine toxicity using infusion rates of 2 to 5.5 mg/kg/hour. Conclusion: We piloted a porcine model of amlodipine toxicity for the purpose of studying a novel antidote. Amlodipine can be reliably extracted from tablets in a DMSO solution using a vacuum filtration procedure and concentrations can be confirmed using UV/vis spectroscopy. IV line and catheter material must be considered when using DMSO as a solvent, as many plastics are not compatible with DMSO infusion. An infusion rate of 2 to 5.5 mg/kg/hour in a porcine model, without an initial bolus, will likely produce expected amlodipine toxicity.