CXCR4 and head and neck squamous cell carcinoma: In contrast, our CXCR4-targeted nanotoxins, administered at a repeated dosage schedule, did not induce acute systemic toxicity (at the end of treatment) nor long-term toxicity (4 weeks after treatment), in the HNSCC mouse model, displaying undetectable markers of toxicity in plasma, unaltered cell blood count and lack of histological alterations in non-tumor organs, at a dosage that achieves a potent antitumor effect.