One of the main reasons of resistance to EGFR tyrosine kinase inhibitors (TKIs) is that there are alternative mechanisms for persistent activating EGFR downstream signaling, including both RAS/Erk and PI3K/Akt kinase pathways. Therefore, simultaneous inhibition of both pathways would be necessary to reduce tumor cell survival more effectively. One of the candidate combinations is concurrent use of EGFR-TKIs and statins, which are irreversible inhibitors of 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase and have been used to treat hypercholesterolemia through blocking the mevalonate biosynthesis pathway. Beside the cholesterol lowering effect, statins have been shown to induce apoptosis in several tumor types. It affects the synthesis of other products of the mevalonate pathway such as isoprenoids, which are used as substrates for prenylation. Attachment of isoprenoids to RAS proteins facilitates their anchoring to the cell membrane where they carried out their roles. By interrupting the biosynthesis of mevalonate, statins inhibit activation of RAS and downstream signaling cascades, including the RAF/MEK/ERK and PI3K/AKT, which play critical roles in regulation of cell survival and proliferation. Therefore, it seems to be a promising therapeutic approach overcoming tumor resistance to EGFR-TKIs, which is associated with RAS activation.
According to the recent clinical result of phase II trial, a randomized phase II study of gefitinib with or without simvastatin in previously treated patients with advanced NSCLC conducted by Han et al.37 gefitinib plus simvastatin combination produced higher response rates than gefitinib alone in patients with non-adenocarcinoma (5/13 [39%] v 1/13 [8%], P=0.06). This finding suggests that simvastatin may enhance sensitivity to gefitinib in non-adenocarcinoma that is relatively resistant to gefitinib. Moreover, by Mantha et al.35 demonstrated that the combination of gefitinib and lovastatin showed significant synergic cytotoxic effects in vitro in a total of 16 squamous cell carcinomas, NSCLC, and colon carcinoma cell lines. Of special interest, these cell lines did not possess the activating mutations of EGFR, which confer increased sensitivity to gefitinib. Nevertheless, combining lovastatin with gefitinib induced more significant inhibition of AKT activation than either agent alone. Additionally, lovastatin significantly enhanced the sensitivity to gefitinib treatment regardless PTEN loss in glioblastoma cell lines. These results suggest that statins can augment EGFR inhibition.
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