Upregulation of Nicotinic Acetylcholinergic Receptors in Human Lung Adenocarcinomas

Objectives: Development of novel and more sensitive methods for early diagnosis of lung cancer may be based on visualizing upregulated nicotinic acetylcholinergic receptors (nAChR) in cancer cells. Our previous results with A/J mice lung tumor model suggest that [¹⁸F]nifene PET is able to detect increases in α4β2[asterisk] nAChR receptor levels in the lungs resulting from tumor proliferation. We now report evaluation of α4β2[asterisk] and α7 nAChR in explanted human lung cancer specimens. These findings will help in translation of nAChR imaging probes in human lung cancer PET/CT studies.

Methods: We have carried out initial evaluation of the binding of [¹²⁵I]bungarotoxin (for α7) and [³H]cytisine (for α4β2[asterisk]) in human lung cancer specimens, 7 malignant adenocarcinomas of different grades and one squamous cell carcinoma specimen (4 male, 4 female, grades 1 to 3, all were found to be malignant). Lung sections (10 μm thick) were incubated with [¹²⁵I]bungarotoxin (0.01 μCi/cc) in Tris buffer containing bovine serum albumin for 2 hrs in r.t. and with [³H]cytisine (0.01 μCi/cc) in Tris buffer at 2 ^oC for 75 mins. For nonspecific binding nicotine (300 μM) was used. After washing and drying, the radiolabeled sections were placed on storage phosphor screens. The apposed phosphor screens were read and analyzed by OptiQuant acquisition and analysis program. Adjacent sections of the lung tissue were prepared for H&E staining. Additionally, normal rat brain horizontal slices were also included in the assay in order to confirm that binding of the two radiotracers is consistent with the distribution of α7 and α4β2[asterisk] nAChRs.

Results: The acquired images were analyzed with OptiQuant™ software where each region of interest (ROI) was measured in digital light units per mm² (DLU/ mm²). For reference rat brain sections were analyzed alongside the lung cancer specimens. Rat brain sections showed binding in the cortex and hippocampus consistent with α7 receptor binding by [¹²⁵I]bungarotoxin. Total binding in rat brain sections was reduced to 25% by nicotine consistent with previous observation. All lung cancer specimens showed displaceable [¹²⁵I]bungarotoxin binding demonstrating presence of and upregulation of α7 nAChRs. Smokers exhibited greater specific binding compared to nonsmokers. Average ratio of total binding to nonspecific in smokers was 4.23 whereas in nonsmokers the ratio was 1.50. Increases were lower in the case of [³H]cytisine suggesting that between the two receptor subtypes, a significantly greater upregulation of α7 subtype occurred compared to α4β2[asterisk] subtype. Smokers exhibited greater levels of [¹²⁵I]bungarotoxin binding compared to nonsmokers.

Conclusions: This preliminary study shows upregulation of nicotinic receptors in human non-small cell lung adenocarcinomas. This is consistent with our findings in A/J mice lung tumor model using [¹⁸F]nifene PET (in vivo tumor/nontumor = 1.5-2.0; ex vivo = 4). The human specimens are being tested with [¹⁸F]nifene (for α4β2) and [¹⁸F]ASEM (for α7) in order to test for consistency with the above findings and for potential translation to human PET studies of lung cancer. Relationship between tumor grade and α7 nAChR upregulation will require more specimens.