Background: The insulin-like growth factor-1 receptor (IGF-1R) pathway is known to play a role in the acquisition of resistance to epidermal growth factor receptor (EGFR)-specific tyrosine kinase inhibitors (TKIs) in non-small cell lung cancer (NSCLC). However, its exact role in TKI resistance has so far remained unclear. Here, we interrogated the hypothesis that the IGF-1R may serve as a biomarker for, and may play a role in, intrinsic resistance to the EGFR-specific TKI gefitinib in NSCLC.
Methods: Total-IGF-1R and phosphorylated (p)-IGF-1R expression levels were related to gefitinib sensitivity in 23 NSCLC cell lines. This sensitivity was re-evaluated after knocking down IGF-1R expression and after IGF-1R up-regulation through exogenous IGF-1 expression. The utility of IGF-1R expression as a predictive biomarker was also evaluated by immunohistochemistry (IHC) in 98 primary NSCLC samples from patients treated with gefitinib.
Results: Seventeen of the cell lines tested were resistant to gefitinib, whereas 3 cell lines were sensitive. The three remaining cell lines showed intermediate values. Thirteen resistant cell lines were found to be positive for total-IGF-1R expression, while all the sensitive cell lines were negative, resulting in a positive predictive value (PPV) of 81% for total-IGF-1R to predict resistance. Seven resistant cell lines exhibited high p-IGF-1R levels, whereas all 3 sensitive cell lines were negative for p-IGF-1R, resulting in a PPV of 100% for p-IGF-1R to predict resistance. Neither a knock-down of IGF-1R expression nor an activation of the IGF1-R pathway through exogenous IGF-1 expression affected gefitinib sensitivity. In primary NSCLC tissues, IGF-1R expression was found to be significantly higher in patients with progressive disease, i.e., showing gefitinib resistance, as compared to those with a complete or partial response.
Conclusions: IGF-1R acts as a predictor for resistance to gefitinib in NSCLC cell lines and NSCLC patients, but does not seem to play a role in the intrinsic resistance to this drug. High total-IGF-1R and p-IGR-1R levels may predict such a resistance. Since the underlying mechanism does not appear to be related to proliferation induction, alternative pathways should be explored.