In a randomized, controlled trial published in 2014, intravenous sodium nitroprusside was shown to be effective in further reducing positive and negative symptoms of schizophrenia in patients taking a number of antipsychotics including chlorpromazine, haloperidol, olanzapine, risperidone and quetiapine. The same researchers based in Brazil and Canada have published two case reports of patients on clozapine who safely received intravenous sodium nitroprusside (1). The patients, both men, were 22 and 33 years old, and they had persistent positive symptoms of psychosis despite receiving clozapine at adequate dose and duration. Serum clozapine levels were not reported.
The men received nitroprusside according to the same protocol published in JAMA Psychiatry: an infusion of 0.5 microgram per kilogram per minute for four hours. In both cases, the improvements in positive and negative symptoms as measured by the Positive and Negative Syndrome Scale became apparent within hours and lasted for days.
The report does not mention cardiovascular parameters, but in a personal communication, the investigators said that at this dose, nitroprusside has little effect on blood pressure in normotensive people despite treatment with antipsychotics that can reduce blood pressure. The two patients did not receive further infusions because of concern about toxicity with repeated doses of nitrous prusside, which transiently produces small amounts cyanide; however, toxicity is rare with doses less than 5 micrograms per kilogram per minute. With infusions lasting more than 24 hours or in patients with renal insufficiency, accumulation of thiocyanate may occur, which can cause delirium (2). The risk of such toxic events appears to be minimal in low-dose nitroprusside treatment in appropriately selected patients.
This treatment has promise for clozapine-resistant schizophrenia, a severe disease with no well-established treatments except possibly electroconvulsive therapy (ECT). Randomized controlled trial in clozapine-resistant schizophrenia, also called ultra-resistant or super-refractory schizophrenia, are warranted.
1. Maia-de-Oliveira JP, Belmonte-de-Abreu P, Bressan RA, Cachoeira C, Baker GB, Dursun SM, Hallak JE. Sodium nitroprusside treatment of clozapine-refractory schizophrenia. J Clin Psychopharmacol. 2014;34:761-763.
2. Michel T, Hoffman BB. Treatment of myocardial ischemia and hypertension. In: Brunton L, Chabner B, Knollman B, eds. Goodman & Gilman’s The Pharmacological Basis of Therapeutics. 12th ed. New York, NY: McGraw-Hill Co., 2011 (online version).