M Riedel 1 , H Kro¨nig 1 , MJ Schwarz 1 , RR Engel 1 ,C Sikorski 1 , K-U Ku¨ hn 2 , S Behrens 1 , H-J Mo¨ ller 1 ,M Ackenheil 1 and N Mu¨ ller 1 1 Psychiatric Hospital, Ludwig-Maximilian University Mu- nich, Germany; 2 Psychiatric Hospital, University of Bonn, Germany Correspondence: MJ Schwarz. E-mail: mschwarz@psy.med.uni-muenchen.de Supported by the Theodore and Vada Stanley Foundation. 1 Schwarz MJ et al. Brain Behav Immun 2001; 15: 340–370. 2 Schwarz MJ et al. Biol Psychiat 2000; 47: 29–33. 3 Hogg N, Leitinger B. J Leukoc Biol 2001; 69: 893–898. 4 Vora DK et al. Genomics 1994; 21: 473–477. 5 Macchioni P et al. Clin Exp Rheumatol 2000; 18: 553–558. 6 Nishimura M et al. Hum Immunol 2000; 61: 507–510. 7 Almawi WY et al. J Clin Endocrinol Metab 1999; 84: 1497–1502. 8 Gerli R et al. Trends Immunol 2001; 22: 72–77. 9 Torrey EF, Yolken RH. Brain Behav Immun 2001; 15: 401–410. No evidence for a direct effect of clozapine on fat-cell forma- tion and production of leptin and other fat-cell-derived factors Molecular Psychiatry (2003) 8, 258–259. doi: 10.1038/sj.mp.4001246 Atypical neuroleptics are known to induce substantial weight gain in a considerable proportion of patients within a few months of treatment. 1 Among the currently available atypical antipsychotic drugs clo- zapine appears to have the greatest weight-increasing potential. 2 To understand the underlying mechan- isms, we examined whether clozapine has an effect on human adipocyte development and specific functions of mature fat cells such as glucose transport, lipolysis, and production and secretion of leptin, interleukin-6 (IL-6) and plasminogen activator inhibitor-1 (PAI-1). The results of our in vitro study do not provide evidence for a direct effect of clozapine on these functions arguing that central rather than peripheral mechanisms may induce weight gain and the meta- bolic disturbances under clozapine treatment. Little is currently known about the mechanisms of weight gain by atypical neuroleptics. One common hypothesis is that patients experience a lower suppres- sion of appetite after meals indicating that the perception of satiety may be impaired. 1 Interestingly, clozapine was recently found to increase serum leptin levels within days after initiation of treatment. 3,4 To get more insight into the pathophysiology of clozapine- induced weight gain, we studied the effect of the drug on fat cell formation and selected metabolic and secretory functions of human adipose tissue. Adipose tissue samples (20–100 g) were obtained from mammary adipose tissue of young, healthy women (BMIo27 kg/m 2 ) undergoing elective mam- mary reduction surgery. Stromal cells from human adipose tissue were prepared by collagenase diges- tion as described recently 5 and were induced to undergo in vitro adipose differentiation in a serum- free hormone-supplemented medium. After 16–18 days in culture, most cells have developed into multilocular cells. Additionally, freshly isolated hu- man adipocytes were kept in suspension culture for 24 h as described recently. 6 To study whether clozapine is able to influence adipose differentiation, we added clozapine at con- centrations of 10 9 –10 5 M to adipocyte precursor cells for 3 and 16 days, respectively, in the presence of adipogenic factors. Clozapine had no significant effect on the percentage of newly developed fat cells as well as on the activity of glycerol-3-phosphate dehydrogenase, a marker enzyme of adipose differ- entiation, in comparison with control cells. In in vitro differentiated adipocytes, we measured 3 H-2-deoxy-d- glucose uptake in the absence and presence of 10 7 M Table 1 Effect of clozapine in fat-cell development and function in cultured human adipocytes. Only the results of the highest clozapine concentration are given as mean 7SD. n.s. = not significant Function No. of experiments Control Clozapine 10 5 M P-value % differentiated cells after 16 days in culture 3 1274 1076 n.s. GPDH activity (mU/mg protein) 3 217748 195740 n.s. Glucose transport Basal 4 100 a 9673 n.s. Insulin stimulated 4 178716 172718 n.s. Leptin Protein (ng/ml) 5 0.9670.38 0.9470.4 n.s. mRNA (copies/mg) 5 2.2  10 6 71.9  10 6 1.5  10 6 71.2  10 6 n.s. IL-6 protein (pg/ml) 5 115742 124746 n.s. PAI-1 protein (ng/ml) 5 34717 40728 n.s. a Defined as 100%. Scientific Correspondence 258 Molecular Psychiatry