Immunohistochemistry with 3 different clones in anaplastic lymphoma kinase fluorescence in situ hybridization positive non-small-cell lung cancer with thymidylate synthase expression analysis: a multicentre, retrospective, Italian study

Introduction: ALK rearrangement is the only druggable oncogenic driver detectable by immunohistochemistry (IHC) not requiring further confirmation of positivity in accessing first-line specific inhibitors. ALK-positive patients experience clinical benefit from pemetrexed-based chemotherapy possibly due to lower thymidylate synthase (TS) levels. This study assesses agreement with three different ALK IHC clones in 37 FISH-positive NSCLC. TS expression by real time (RT)-PCR was compared with ALK FISH-negative cases. Materials and methods: 37 ALK FISH-positive NSCLC cases diagnosed between 2010 and 2015 in 7 Italian centres were investigated with ICH using three different anti-ALK antibodies (ALK1, 5A4 and D5F3). Staining for ALK1 and 5A4 was graded as 0+,1+,2+, and 3+, while the scoring for D5F3 was recorded as negative or positive. Proportion agreement analysis was done using Cohen's unweighted kappa (k). TS and β-actin expression levels were analysed by quantitative RT-PCR. Comparison between TS expression in ALK FISH-positive specimens and a control cohort of ALK FISH-negative ones was performed with the Mann-Whitney and Kruskal-Wallis tests. Results: Considering 2+ and 3+ as positive, the proportion of IHC agreement was 0.1691 (95% CI 0-0.4595) for ALK1/5A4, 0.1691 (95% CI 0-0.4595) for ALK1/D5F3, and 1 for D5F3/5A4. Considering 3+ as positive, it was 0.1543 (95% CI 0-0.4665) for ALK1/ 5A4, 0.0212 (95% CI 0-0.1736) for ALK1/D5F3, and 0.2269 (95% CI 0-0.5462) for 5A4/D5F3. Median TS expression was 6.07 (1.28-14.94) and ALK-positive cases had a significant lower TS expression than ALK-negative tumours (p = 0.002). Conclusions: IHC proved to be a reliable tool for the diagnosis of ALK-rearranged NSCLC. D5F3 and 5A4 clones have the highest percentage of agreement. TS levels are significantly lower in FISH-positive patients.