The role of nitric oxide (NO) in tumour progression and metastasis is not clear, therefore the present work aimed to better characterise the effects of nitric oxide synthase (NOS) inhibition by L-N^ω-nitroarginine methyl ester (L-NAME) on primary tumour growth, pulmonary metastasis, inflammatory state and prostacyclin (PGI₂)/thromboxane A₂ (TXA₂) balance in a 4T1 murine model of breast cancer. To distinguish effects of NO deficiency on disease development, 4T1 cancer cells were administered orthotopically or intravenously to Balb/c mice. The systemic NO bioavailability, pulmonary inflammation and plasma levels of thromboxane B₂ (TXB₂) and 6-keto-prostaglandin F_1α (6-keto-PGF_1α) were assessed. The study shows that, in the orthotopic model of 4T1 breast cancer, L-NAME hampered primary tumour growth, reduced pulmonary metastases, delayed inflammatory response but did not alter biosynthesis of TXB₂ and 6-keto-PGF_1α as well as PGI₂/TXA₂ ratio in cancer-bearing mice. Interestingly, in the intravenous model of 4T1 breast cancer, NOS inhibition did not influence metastasis nor inflammation, but it increased both TXB₂ and 6-keto-PGF_1α biosynthesis without affecting PGI₂/TXA₂ ratio. In conclusion, in a 4T1 murine model of metastatic breast cancer, NO plays a major role in primary tumour development, while NO is not the key mediator of cancer cell extravasation to the lungs. Furthermore, NO-deficiency activates a PGI₂-dependent compensatory mechanism only in the intravenous model of 4T1 breast cancer.

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