Fluoromethylhistidine inhibits tumor growth without producing depletion of endogenous histamine G. Cricco, N. Engel, M. Croci, C. Davio, G. Martı ´n, C. Fitzsimons, R. Bergoc and E. Rivera Laboratorio de Radioiso ´topos, Facultad de Farmacia y Bioquı ´mica, Universidad de Buenos Aires, Junin 956 PB, 1113 Buenos Aires, Argentina Introduction Experimental mammary carcinomas induced by N-nitroso- N-methylurea (NMU) administration express H 1 and H 2 histamine receptors with an atypical linkage to signal transducers [1]. In these tumors, histamine regulates cell growth as well as expression of histidine decarboxylase (HDC) mRNA and histamine content [2]. We postulate that one of the main actions of endogenous histamine is to behave as an autocrine growth factor [3]. The in vivo treatment of tumor bearing rats with the specific HDC inhibitor fluoromethylhistidine (MFMH) resulted in a marked inhibition of tumor growth as reported for other experimental tumors [4], but histological findings were consistent with a high histamine con- centration in the tumor microenvironment rather than with histamine depletion. On this basis, we further investigated the mechanism through which MFMH inhibits tumor growth and cell proliferation in NMU-induced mammary carcinomas. Materials and methods A single cellular suspension of tumor cells was cultured in soft agar under serum free medium conditions, as described [3]. Histamine, H 1 agonist 2-(3-fluorophenyl)histamine and HDC specific inhibitor MFMH were tested in the final concentrations: 0.01, 0.1, 1, 10 and 100 mM. Tumor slices of 50–100 mg were incubated in Hanks’ balanced salt solution at 37 C, in humidified 5%CO 2 /air atmosphere [2]. MFMH was added at 10 ¹4 M. At different time points, tissue samples and incubation media were processed for histamine content determination by radio- immunoassay (Immunotech, France), as described [2]. Twenty Sprague-Dawley rats were randomized into four groups; half were injected with the carcinogen (NMU) and the other two remained as controls. Animals received daily 50 mg/kg of MFMH by i.p. injection or saline. Blood was collected with EDTA and plasma was diluted 1/100 with PBS before being included in the RIA for histamine determination. Results and discussion The in vivo administration of MFMH produced a marked inhibition of tumor growth. Histopathological studies revealed a high degree of necrosis, moderate number of intact mast cells and increased angiogenesis. All these features were consistent with a high histamine concentration in the tumor microenvironment [5]. Plasma from MFMH treated rats showed a marked increase in histamine levels. This increase was specific for tumor bearing rats, since in control animals MFMH did not modify plasma histamine levels (Fig. 1). It has been reported that treatment of humans with MFMH produces a decrease in blood and urine histamine levels [6]. A rapid decrease in histamine content has been demonstrated in several mouse tissues, except in skin [7]. In NMU-induced carcinomas, histamine content was not significantly modified by MFMH, but histamine release from tumor cells was enhanced as shown by the increase in histamine concentration in the incubation media (Fig. 2). These data indicate that the marked inhibition on tumor growth produced by MFMH administration is not mediated by the depletion of tumor histamine endogenous content. In the NMU-induced tumors, histamine at concentrations of over 100 nM as well as the H 1 agonist, produced a dose Supplement 1 (1997) S59–S60  Birkha ¨user Verlag, Basel, 1997 1023-3830/97/010S59-02 $ 1.50+0.20/0 Inflammation Research Correspondence to: E. Rivera Fig. 1. Histamine plasma levels of rats with NMU-induced tumors (squares) or control animals (circles) treated with MFMH 50 mg/kg or saline for five days. Values at day 10 represent histamine levels after discontinuity of treatment. Curve data correspond to mean  SD of a representative rat from each experimental group. Identical results were obtained for the remaining animals. 