The search for potent anticancer drugs is still a huge challenge. The two most challenging aspects are on one hand to find drugs with minor side effects and on the other hand to find substances that are able to prevent growth of tumors that can not be successfully treated yet. Carnosine is a highly interesting candidate with regard to both aspects. Firstly, carnosine is a naturally occurring substance that is present in muscle and other organs. Secondly, experiments with cultured cells from non-curable malignant glioma demonstrated that carnosine is able to diminish proliferation of these cells. Just recently, it was demonstrated that carnosine inhibits glycolysis and ATP synthesis in tumor cells in culture . This observation is in contrast to the well-known fact that in normal cells carnosine improves both processes . The basic difference with regard to energy metabolism between tumor cells and normal cells have been described almost a century ago and are known as the Warburg effect . The molecular mechanisms of this effect are still not completely understood. In fact, the study of the mechanisms, on how carnosine affects normal and tumor cell energy metabolism differentially, may help to exploit the basic differences, and may suggest a way for the creation of a knowledge-based strategy to fight cancer without affecting normal cells . However, and first of all, it had to be asked whether carnosine can be administered as a drug. In the present study, we therefore investigated whether carnosine may in fact prevent growth of tumor cells in an established mouse model based on NIH3T3 cells expressing the oncogene HER2/neu [14, 22]. HER2/neu belongs to the human epidermal growth factor receptor family (HER) of tyrosine kinases. It was first described to be overexpressed in 25-30% of breast cancers, but its overexpression is also seen in subsets of gastric, esophagal and endometric cancers and in some cancers of the oropharynx, the lung and the bladder (for review see ). Just rececently, it was demonstrated that HER2 is also expressed in glioblastoma patient samples . In addition, Herceptin, a recombinant humanised anti-HER2/neu antibody, can induce cell death in cell lines derived from glioblastomas . In the mouse model employed in the present study, expression of HER2/neu is strongly required for tumor growth. Switching off expression by the administration of anhydrotetracycline results in rapid tumor regression. Therefore, it is tempting to speculate that carnosine also affects HER2/neu expression, but recent data more strongly indicates that carnosine exhibits its effect at the level of glycolysis, depleting ATP production, finally resulting in reduced proliferation . In the present work the anti proliferative effect of carnosine in vivo is demonstrated by the reduced growth of tumors in treated animals and by the smaller number of mitotic cells in treated tumors. One interesting challenge is now, how to enhance the local concentration of carnosine in hope to get a stronger effect on tumor growth. In fact, the carnosine concentration in the serum of treated animals after ~26 days of treatment and 24 hours after the last injection is not significantly different from the carnosine concentration in the serum of animals not treated with carnosine (data not shown). We also do not know whether the continuous administration of carnosine may induce carnosinase, the degrading enzyme, that in humans is known to be secreted from the liver into the serum . Of course, an intratumoral injection of carnosine might have been a possibility but this would have made the determination of tumor size in the experiments presented difficult. Considering a clinical application of carnosine, an intratumoral injection of carnosine or the implantation of wafers (e.g. into the resection cavity of a surgically excised tumor in the case of malignant gliomas), that continuously release carnosine, may be discussed. However, discussing a potential clinical application, we should not withhold, that two minor tumors developed in two carnosine treated mice of the first series and one minor tumor in two carnosine treated mice of the second series, close to the time, when the primary tumor already had reached its maximum size (data not shown). The additional tumors were very small and regressed after administration of anhydrotetracycline. Although they may have been initiated in the vicinity of the primary tumor by a small number of cells subcutaneously introduced by the injection, it cannot be ruled out, that they originated from cells that have migrated from the primary tumor. Whether carnosine may induce a migratory potential, is pure speculative and needs further experimental data.
One puzzling observation in the course of the experiments was the complete disappearance of one tumor in a mouse, that was continuously treated with carnosine. A careful examination of the mouse did not reveal any sign of an accidental injection that is usually detectable even after several days. In addition, regression needs several repeated injections and cannot just be caused by one accidental injection. Therefore, it must be concluded that the tumor regressed under the continuous treatment with carnosine. Although this tumor reappeared after 50 days of treatment, this observation is a highly interesting one with regard to a possible use of carnosine in human tumor therapy.