10616 Iron is required for the activity of ribonucleotide reductase and synthesis of DNA. Cellular surface transferrin receptors (TfR) are the principal transport protein for iron into cells. It has been reported that inhibition of TfR gene expression leads to the suppression of tumor cell growth in cell culture. In the present study, an antisense oligonucleotide targeted to the transferrin receptor gene was used to treat 4T1 mammary adenocarinoma in cell culture and a mouse model. The cytotoxicity of a 24-mer TfR antisense oligonucleotide was determined by 3[H]-thymidine incorporation in vitro. The sense and random sequence 24-mer oligonucleotides were used as controls. We found that TfR antisense inhibited mRNA expression of TfR and cell growth of the 4T1 cells in the dose-dependent manner. Mice bearing the 4T1 mammary adenocarcinoma were continuously treated for 14 days through subcutaneous implantation of Alzet osmotic mini-pumps. Tumor sizes were measured twice a week. Tumor necrosis and metastases were determined grossly at autopsy and by microscopy. Tumor metastases to lungs (all lung lobes) was scored by the following criteria: 0 = no metastasis, 1 = 1-2 foci/slide, 2 = 3-10 foci/slide, 3 = 11-20 foci/slide and 4 = >20 foci/slide. At 4, 7 and 10 days post-treatment, tumor sizes of the TfR antisense-treated group were significant smaller than control mice (p < 0.05, p < 0.05 and p < 0.05, respectively). At 14 days post-treatment, autopsy showed that 91.7% (11/12) of control mice had multiple metastatic foci on their lung surfaces, but only 16.7% (2/12) of TfR antisense-treated mice had metastatic tumor foci on their lung surfaces (p < 0.001). Primary tumor did not have significant necrosis in either group. Mean lung metastasis scores of 3.25 ± 1.06 in control group vs. 1.75 ± 1.49 in TfR antisense-treated group (p < 0.01) were determined by microscopy. In summary, TfR antisense inhibited cell growth in cell culture and primary tumor growth in mice, and reduced lung metastasis of the 4T1 mouse mammary adenocarcinoma. No significant financial relationships to disclose.