Glucose Metabolism in Patients With Schizophrenia Treated With Olanzapine or Quetiapine: A Frequently Sampled Intravenous Glucose Tolerance Test and Minimal Model Analysis

Objective: Clozapine and olanzapine treatment has been associated with insulin resistance in nonobese schizophrenia patients. Much less is known regarding other agents such as quetiapine. The objective of this study was to compare matched olanzapine- and quetiapine-treated schizophrenia patients and normal controls on measures of glucose metabolism.

Method: A cross-sectional comparison of quetiapine-treated and olanzapine-treated nonobese (body mass index < 30.0 kg/m²) schizophrenia subjects (DSM-IV) with matched normal controls using a frequently sampled intravenous glucose tolerance test and nutritional assessment was conducted from April 2002 to October 2004. Data from 24 subjects were included in the analysis (7 quetiapine, 8 olanzapine, 9 normal controls).

Results: There was a significant difference among groups for fasting baseline plasma glucose concentrations (p = .02), with olanzapine greater than normal controls (p = .01). The insulin sensitivity index (SI) differed significantly among groups (p = .039); olanzapine subjects exhibited significant insulin resistance compared to normal controls (p = .01), but there was no significant difference for quetiapine versus olanzapine (p = .1) or quetiapine versus normal controls (p = .40). SI inversely correlated with quetiapine dose (p = .0001) and waist circumference (p = .03) in quetiapine-treated subjects. Insulin resistance calculated by the homeostasis model assessment of insulin resistance (HOMA-IR) also differed significantly among groups (p = .03). The olanzapine group had a higher HOMA-IR level than normal controls (p = .01). There was a significant difference in glucose effectiveness (SG) among the groups (p = .049). SG was lower in the olanzapine group than in the quetiapine group (p = .03) and in the olanzapine group compared to normal controls (p = .049).

Conclusions: Our findings are consistent with our previous report that nonobese olanzapine – treated subjects showed insulin resistance, measured by both HOMA – IR and SI, and reduction in SG. Studies that include larger samples, unmedicated patients, and varying durations of antipsychotic exposure are necessary to confirm these results.