Effect of Short Term Atypical Antipsychotic Administration Compared to Placebo on Hepatic Insulin Extraction

Overview

Within the past 20 years, there has been a striking increase in the incidence of obesity 1;2, type 2 diabetes mellitus (T2DM) 3-5, and cardiovascular diseases (CVD) in the schizophrenic population 6-8 . Large NIH-funded trials indicate that the prevalence of metabolic syndrome is twice to three times greater in schizophrenic patients on a specific class of drug termed the "atypical antipsychotics" (AAPs), of which olanzapine is an example, as compared to matched controls 8. Identification of the pathophysiological mechanisms contributing to metabolic disease in schizophrenic patients on AAPs has been hampered by the inability to differentiate underlying disease from treatment-emergent complications. In addition, despite falling within the same drug class, different AAPs exhibit differential associations with metabolic disease. Olanzapine is one of the AAPs associated with the greatest weight gain and degree of metabolic impairments.

The increased incidence of T2DM and CVD with the AAPs has been assumed a consequence of weight gain. However, accumulating evidence of weight-independent effects derived from in vitro and rodent studies suggests a direct effect of some AAPs on the pancreatic B-cell and the liver. We have recently completed the first of two studies supported by an NIH grant which provides evidence for a direct effect of olanzapine on metabolism, independent of weight gain or psychiatric disease. Findings from the first study indicate that short-term administration (9-days) of olanzapine compared to aripiprazole, another AAP as well as placebo dramatically increases post-prandial insulin levels in healthy control subjects independent of weight gain; providing evidence of tissue specific effects of the drug in humans. We also found that the increase in insulin was not accompanied by an increase in plasma C-peptide concentrations suggesting that olanzapine may decrease hepatic insulin extraction. Decreases in hepatic insulin extraction can be mediated through muscarinic blockade and in fact, data from our laboratory demonstrates that atropine (the muscarinic antagonist) inhibits hepatic insulin extraction in humans. Blockade of muscarinic receptors is consistent with the receptor binding profile of olanzapine which shows a higher receptor antagonism for muscarinic receptors compared to other AAPs. In addition, we also found significant increases in glucagon-like peptide 1 (GLP-1) and decreases in insulin sensitivity following olanzapine administration compared to aripiprazole and placebo. The proposed study follows up on these findings with a study designed to investigate the mechanisms contributing to the olanzapine-induced post-prandial hyperinsulinemia and GLP-1. The overall hypothesis of this study is that olanzapine blocks peripheral muscarinic receptors, leading to a compensatory increase in vagal efferent activation which contributes to an increase in insulin secretion.