Clinical & Experimental Metastasis 17: 299–306, 1999. © 1999 Kluwer Academic Publishers. Printed in the Netherlands. 299 Cure of colon cancer metastasis in rats with the new lipid A OM 174. Apoptosis of tumor cells and immunization of rats Nathalie Onier 1 , Sophie Hilpert 1 , Laurent Arnould 2 , Val´ erie Saint-Giorgio 1 , J. Gwynfor Davies 3 , Jacques Bauer 3 & Jean-François Jeannin 1 1 Laboratoire d’Immunologie et d’Immunoth´ erapie des Cancers de l’Ecole Pratique des Hautes Etudes (EPHE), INSERM U517, Universit´ e de Bourgogne, Facult´ e de M´ edecine, 7 boulevard Jeanne d’Arc, 21033, Dijon, France; 2 Laboratoire d’Anatomo-pathologie, Centre Georges-François Leclerc, Dijon, France; 3 OM Pharma, Meyrin, Switzerland Received 1 December 1998; accepted in revised form 21 April 1999 Key words: cancer immunotherapy, lipid A, in vivo, apoptosis, immunization Abstract The antitumoral effect of the new lipid A OM 174 was investigated in a model of colon cancer in rats. Peritoneal carcino- matosis were induced in BDIX rats by intraperitoneal injection of syngeneic PROb cancer cells. The treatment started 2 weeks later, when rats had macroscopic peritoneal nodules. An antitumoral effect was first obtained with OM 174 intraperitoneally injected, then an intravenous treatment was developed. When injected 15 times intravenously, at the dose of 1 mg/kg, 2 days apart, OM 174 induced the complete regression of tumors and hemorrhagic ascitis in 90% of the tumor- bearing rats, whereas all the untreated rats died of their tumors. To our knowledge, this treatment is the most effective ever applied to macroscopic tumors. Furthermore, the treatment induced the immunization of rats since the reinjection of PROb tumor cells in OM 174-cured rats did not cause the formation of new tumors while injection of another syngenic colon tumor cells did. Only in treated rats tumors were infiltrated with lymphocytes, macrophages and fibroblasts. The treatment did not increase necrosis but generated apoptotic areas. OM 174 was not directly toxic for tumor cells, and thus the observed effect involved the host-mediated antitumor reaction. Therefore we hypothesize that OM 174 therapy induces tumor cell apoptosis, stimulates the phagocytosis of apoptotic bodies and then activates immune system by antigen presentation. Introduction Colorectal cancer is the most frequent cause of cancer death in the industrialized countries. Excisional surgery is the only efficient treatment for this cancer, but most of patients die of metastases in the liver, the lung and/or the peritoneum. Many treatments have been tested but the results have been disappointing. The antitumor effect of lipopolysaccharides (LPS) has been known for a long time. At the beginning of the XVIIIth century, Deidier [1] reported observations showing that the infection in cancer patients could be concomitant with the remission of malignant diseases. About a century ago, Coley developed a cancer treatment with a mixture of bacterial tox- ins [2]. In 1943 Shear et al. found that the antitumor effect of Coley’s toxin was due to endotoxins or lipopolysaccharides [3], a major constituent of the outer membrane of Gram- negative bacteria. However, because of the LPS toxicity, cancer patients can only be treated with a few ng LPS per kg injected i.v. or μg per kg injected s.c., these doses being too Correspondence to: Jean-François Jeannin, Laboratoire d’Immunologie et d’Immunoth´ erapie des Cancers de l’Ecole Pratique des Hautes Etudes (EPHE), INSERM U517, Universit´ e de Bourgogne, Facult´ e de M´ edecine, 7 boulevard Jeanne d’Arc, 21033, Dijon, France. Fax: +33-380653930; E-mail: jfjeanni@satie.u-bourgogne.fr low to obtain a beneficial antitumor effect [4, 5]. The LPS antitumor effect is due to lipid A, the lipidic moiety of LPS [6, 7] which is a diphosphorylated diglucosamine structure substituted with fatty acids. The chemical structure of lipid A from Escherichia coli LPS has been known since 1984 [8, 9] making it feasible to prepare non-toxic derivatives of lipid A suitable for humans use. We previously studied the relationship between the struc- ture of synthetic derivatives of lipid A and their antitumor activity in an experimental model of colon cancer in rats [7]. This investigation was done in short-term experiments with i.p. injected lipids A. Using the same experimental model but in survival experiments, we report here the development of a i.v. treatment transposable in humans. OM 174 lipid A was able to cure 90% of rats bearing macroscopic tumors while all the untreated rats died of their tumors. We show tumor cell apoptosis, indirectly induced by OM 174, and the immunization of rats.