Evaluation of hypoxia in a feline model of head and neck cancer using ⁶⁴Cu-ATSM positron emission tomography/computed tomography

Human and feline head and neck squamous cell carcinoma (HNSCC) share histology, certain molecular features, as well as locally aggressive and highly recurrent clinical behavior. In human HNSCC, the presence of significant hypoxia within these tumors is considered an important factor in the development of a more aggressive phenotype and poor response to therapy. We hypothesized that feline head and neck tumors, particularly HNSCC, would exhibit hypoxia and that ⁶⁴Cu-diacetyl-bis(N4-methylthiosemicarbazone) (Cu-ATSM) positron emission tomography/computed tomography (PET/CT) would permit detection of intratumoral hypoxia. 12 cats with measureable head and neck tumors were given ⁶⁴Cu-ATSM and iodinated contrast for PET/CT scan. The presence or absence of hypoxia was also assessed using an intratumoral fluorescent life-time probe to quantitate pO₂ and pimonidazole immunohistochemical staining in biopsy specimens. In two cats, intratumoral O₂ and ⁶⁴Cu-ATSM uptake was measured before and after treatment with anti-angiogenic agents to determine the effect of these agents on hypoxia. Eleven of twelve feline tumors demonstrated significant ⁶⁴Cu-ATSM uptake, regardless of malignant or benign etiology. The presence (and absence) of hypoxia was confirmed using the fluorescent O₂ detection probe in nine tumors, and using pimonidazole staining in three tumors. Squamous cell carcinomas (HNSCC) demonstrated the highest degree of hypoxia, with T_max/M ratios ranging from 4.3 to 21.8. Additional non-neoplastic tissues exhibited ⁶⁴Cu-ATSM uptake suggestive of hypoxia including reactive draining lymph nodes, non-malignant thyroid pathology, a tooth root abscess, and otitis media. In two cats with HNSCC that received anti-vascular agents, the pattern of ⁶⁴Cu-ATSM uptake was altered after treatment, demonstrating the potential of the feline model to study the modulation of tumor oxygenation. Feline HNSCC serves as a clinically relevant model for the investigation of intratumoral hypoxia including its measurement, modulation and targeting