κ-Opioid receptor activation suppresses triple-negative breast cancer progression by inducing urea cycle dysfunction and mitochondrial damage in vitro and in mice

BACKGROUND: Triple-negative breast cancer (TNBC) is an aggressive malignancy with limited therapeutic options. Opioids are commonly used for perioperative and chronic pain management in patients with cancer. Notably, the κ-opioid receptor (KOR) has shown antitumour potential, but its therapeutic impact in TNBC is unknown. METHODS: We analysed KOR expression in TNBC patient cohorts and tissue microarrays. The effects of the KOR agonist U69593 were assessed on key malignant phenotypes in vitro, including apoptosis and mitochondrial integrity. We then validated its impact on tumour growth and metastasis using subcutaneous and haematogenous mouse models. RESULTS: KOR expression was downregulated in TNBC tissue (n=60, P=0.0004), and higher KOR levels were associated with more favourable patient prognosis (n=45 low vs 15 high, P=0.03). The KOR agonist U69593 suppressed TNBC progression both in vitro, by inhibiting cell proliferation, migration, and invasion while inducing apoptosis, and in vivo, by suppressing tumour growth and metastasis. U69593 induced severe mitochondrial damage, evidenced by loss of mitochondrial membrane potential and depleted intracellular adenosine triphosphate. Metabolomic profiling revealed that this was linked to disruption of the urea cycle, leading to the accumulation of toxic ammonia. Restoring urea cycle function by overexpression of the key enzyme ornithine transcarbamylase rescued TNBC cells from U69593-induced apoptosis and abrogated its antitumour effects. CONCLUSIONS: These findings demonstrate that the KOR agonist U69593 restrains TNBC progression by disrupting urea cycle function and compromising mitochondrial integrity, highlighting the potential of dual-action therapeutics that combine effective analgesia with direct antitumour activity.