Effects of olanzapine and risperidone on glucose metabolism and insulin sensitivity in chronic schizophrenic patients with long-term antipsychotic treatment: a randomized 5-month study.

BACKGROUND: Comparisons of diabetic potential, glucose related metabolic levels, and insulin resistance between olanzapine and risperidone have produced variable results in cross-sectional and epidemiologic studies. Randomized prospective studies of metabolic effects during treatment with these drugs may provide results that are more informative.
METHOD: Hospitalized patients with chronic schizophrenia (DSM-IV), most of whom had been treated with multiple antipsychotics in the past, were randomly assigned to treatment with a single antipsychotic, olanzapine or risperidone, for a period of 5 months. At baseline and every treatment month thereafter, fasting glucose, insulin, insulin-related metabolic measures, and prolactin were assessed, and an oral glucose tolerance test (OGTT) was performed during baseline and months 1, 2, and 5 of treatment. Weight was assessed monthly, and waist and hip measures were taken at baseline and month 5. Data were analyzed on 23 patients randomly assigned to risperidone and 23 patients randomly assigned to olanzapine. The study was conducted from February 2003 to August 2007.
RESULTS: Most patients were overweight or obese at baseline (mean body mass index [BMI] = 29.4), but there were no differential drug effects on weight change and no differences between drug groups at the 5-month time point. There were no overall drug treatment differences in fasting glucose or glycohemoglobin or 2-hour glucose levels in OGTT and no differences between the two drug groups at the 5-month time point. There were no consistent drug treatment differences in the number of patients who developed borderline or diabetic glucose levels. Olanzapine-treated patients showed a significantly greater increase than risperidone-treated patients in a fasting measure of insulin resistance (P = .041), and olanzapine patients showed greater decreases in insulin sensitivity during OGTT (P = .023) compared to risperidone-treated patients. Olanzapine-treated patients had a significantly greater increase in 1-hour glucose and insulin levels during OGTT in subsequent months compared to baseline and greater increase in glucose and insulin area under the curve over time than the risperdone-treated patients. Prolactin levels decreased in olanzapine patients and increased in risperidone patients (P values approximately .02). There were no significant drug treatment differences in C-peptide levels or 2 indices proposed as measures of insulin secretion or beta-cell function (homeostasis model assessment of beta-cell function [HOMA-B], BIGTT-acute insulin response surrogate measure [BIGTT-AIR]). Changes in insulin resistance over time were not strongly related to changes in BMI or waist circumference during study drug treatment.
CONCLUSIONS: The increase in insulin levels during olanzapine treatment may compensate for the increase in insulin resistance and serve to reduce fasting and postprandial glucose levels. This may contribute to the lack of differences between olanzapine and risperidone in indices of diabetic or prediabetic glucose levels or glycohemoglobin. How many years this compensatory mechanism will persist needs further investigation. Periodic OGTT tests measuring glucose and insulin levels would be helpful in assessing the status of beta-cell insulin reserve in patients treated with olanzapine and other second-generation antipsychotics and assessing an individual patient's risk for conversion to type 2 diabetes. TRIAL REGISTRATION: clinicaltrials.gov Identifier NCT00287820. Copyright 2009 Physicians Postgraduate Press, Inc.

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