Immunohistochemistry frequently detects c-Kit expression in pulmonary small cell carcinoma and may help select clinical subsets for a novel form of chemotherapy.

The presence of c-Kit immunoreactivity in gastrointestinal stromal tumor (GIST), currently guides treatment with the selective c-Kit inhibitor STI571 (or Gleevec) in clinical trials and establishes a precedent of immunohistochemistry-guided treatment decisions. Thus, the optimization of detection conditions for c-Kit and the determination of its incidence in other malignancies have clinical bearing. Aims of our study were: 1) to determine the incidence of c-Kit expression in formalin-fixed paraffin-embedded tissue (FFPE) in pulmonary small cell carcinoma (SCC) and non small cell carcinoma (NSCC), pulmonary carcinoid, and malignant mesothelioma (MM); and 2) to test the feasibility of c-Kit determination using commercially available antibodies and routine immunohistochemical settings, comparing the performance of two commercially available antibodies, Dako and Santa Cruz. The Dako antibody detected positive stain in 10/22 SCC, 3/8 carcinoids, 1/57 NSCC (1/30 adenocarcinomas, 0/24 squamous cell carcinomas, 0/3 large cell undifferentiated carcinomas), and 7/33 MM. The Santa Cruz antibody detected c-kit in 8/22 SCC, 0/57 NSCC, 1/8 carcinoids, and 0/33 MM. HIER increased the performance of both antibodies. We conclude that c-Kit can routinely be detected in FFPE tissue with commercially available antibodies, and that the Dako anti-c-Kit has a higher sensitivity than the Santa Cruz antibody. C-Kit expression is common in SCC and carcinoids, very rare in NSCC, and infrequent in MM. The frequent c-Kit expression in SCC highlights that this molecule plays an important role in the biology of this malignancy, and that it could be targeted in subsets of patients for therapy with c-Kit inhibitors.