New perspectives in squamous cell carcinoma of the lung

Enriqueta FELIP, MD, PhD
Vall d’Hebron University Hospital
Barcelona, SPAIN

New perspectives in squamous cell carcinoma of the lung

by E. Felip, Spain

Lung cancer is the leading cause of cancer death worldwide. Non–small cell lung cancer accounts for an estimated 80%-85% of lung cancers and can be divided into 2 predominant types, adenocarcinoma (≈50% of cases) and squamous cell carcinoma (≈30% of cases). All histological types of lung carcinoma are associated with smoking, the association being strongest for squamous and small-cell lung carcinoma. Squamous cell carcinoma tumors rarely harbor mutations in epidermal growth factor receptor (EGFR) or anaplastic lymphoma receptor tyrosine kinase (ALK) fusions. Targeted therapies that are effective against adenocarcinoma are ineffective or contraindicated in squamous cell carcinoma, for which treatment options are limited. Recently, molecular profiling of squamous cell carcinoma of the lung revealed targetable alterations, among them, fibroblast growth factor receptor (FGFR) amplifications, discoidin domain receptor tyrosine kinase 2 (DDR2) mutations, and phosphatidylinositol-4,5-bisphosphate3-kinase,catalyticsubunit α (PIK3CA) mutations/amplifications. At present, a number of clinical trials with FGFR inhibitors are ongoing in squamous cell carcinoma patients. Furthermore, exploration of immune checkpoints has resulted in novel immunotherapies designed to interrupt signaling through the programmed cell death protein 1 pathway in lymphocytes. Modulation of this pathway can restore antitumor immune responses and preliminary evidence clearly shows its activity in squamous cell carcinoma of the lung. This review briefly presents the standard treatment options available now for patients with stage IV squamous cell carcinoma and current knowledge of molecular alterations in this subgroup, focusing in particular on alterations with real potential to be therapeutically targeted.

Medicographia. 2015;37:256-262 (see French abstract on page 262))

Epidemiology of squamous cell carcinoma of the lung

Squamous cell carcinoma of the lung is the second most common lung cancer subtype. It is closely linked to smoking habit. Its pathogenesis seems to be different from that of adenocarcinoma.1 Well-differentiated squamous cell carcinoma characteristics in tissue show keratinization, intercellular bridges, and pearl formation (Figure 1). On immunohistochemical examination, p63 is found to be positive and thyroid transcription factor 1 (TTF-1) is negative.

The sequential pathogenesis is well described: bronchial epithelial cells exposed to smoking over an extensive period develop basal cell hyperplasia, squamous metaplasia, carcinoma in situ and, finally, squamous cell carcinoma. Squamous cell carcinoma of the lung is a heterogeneous entity. In a recent study, squamous cell carcinoma tumors were subclassified according to messenger RNA (mRNA) expression subtypes.2 That study shows that basaloid squamous cell carcinoma is a distinct histomolecular entity, allowing it to be recognized and distinguished from the non-basaloid type.

Treatment for stage IV squamous cell carcinoma of the lung

In advanced non–small cell lung cancer (NSCLC), histological subtyping has become an essential consideration in guiding treatment decision-making. For NSCLC patients with tumors harboring epidermal growth factor receptor (EGFR) mutation or anaplastic lymphoma receptor tyrosine kinase (ALK) rearrangement, treatment with an EGFR tyrosine kinase inhibitor (eg, gefitinib, erlotinib, or afatinib) or ALK inhibitor (eg, crizotinib), respectively, is recommended. The presence of EGFR mutations and ALK rearrangements is clearly related to adenocarcinoma subtype. The majority of guidelines suggest that there is no established need for EGFR or ALK testing in those patients with confident diagnosis of squamous cell carcinoma by the pathologist. Bevacizumab is contraindicated in patients with histological features of squamous cell carcinoma since a relevant rate of severe pulmonary hemorrhage has been reported with its use in this group of patients. Pemetrexed is also contraindicated in this group of patients because of worse survival outcomes reported for the cisplatin/pemetrexed combination when compared with the cisplatin/gemcitabine combination. For those reasons, in patients with stage IV squamous cell carcinoma, the standard treatment remains a platinum doublet without pemetrexed or bevacizumab.

Figure 1
Figure 1. Example of squamous cell carcinoma of the lung, which is p63 positive.

In recent studies using new compounds, some encouraging findings have been reported in patients with histological features of squamous cell carcinoma. In advanced NSCLC patients, the contribution of nab-paclitaxel, a 130-nm albuminbound (“nab”) form of paclitaxel designed to use endogenous albumin pathways to increase intratumoral concentrations of the active compound, has been analyzed. In a randomized trial, nab-paclitaxel achieved a higher response rate compared with paclitaxel when both were used in combination with carboplatin in the first-line treatment of NSCLC.3 In that trial, in the subgroup of patients with squamous cell carcinoma, nabpaclitaxel/ carboplatin achieved an overall response rate of 41% compared with 24% for paclitaxel/carboplatin. The molecular mechanisms explaining the antitumor activity of nabpaclitaxel/ carboplatin in patients with squamous cell carcinoma remain unknown.

Patients with treatment-naive advanced squamous cell carcinoma and good performance status generally receive a platinum- based doublet as standard therapy. Despite numerous clinical trials, the standard of care has remained the same for several decades. Necitumumab, a human anti-EGFR monoclonal antibody, has recently been analyzed in advanced squamous cell carcinoma patients in a phase 3 trial comparing cisplatin/gemcitabine with and without necitumumab (n=1093).4 Patients assigned to the necitumumab arm, compared with the platinum-based–therapy alone arm had a statistically significant improvement in overall survival (median overall survival: 11.5 months for the cisplatin/gemcitabine/ necitumumab arm versus 9.9 months for the cisplatin/gemcitabine arm; hazard ratio [HR] 0.84; P=0.0012).

In the second-line setting, ramucirumab, a monoclonal antibody against vascular endothelial growth factor receptor 2 (VEGFR2), has been analyzed in a phase 3 trial comparing docetaxel with ramucirumab or placebo in patients who had progressed after platinum-based therapy (n=1253).5 Patients with NSCLC of any histological type were eligible, and approximately 25% of patients enrolled had squamous NSCLC. Patients assigned to docetaxel and ramucirumab, compared with docetaxel and placebo, experienced a statistically significant improvement in overall survival (median overall survival: 10.5 months for the docetaxel/ramucirumab arm versus 9.1 months for the docetaxel/placebo arm; HR 0.86; P=0.023).

In the LUX-Lung 8 study (not an acronym), patients with squamous cell carcinoma following failure of first-line chemotherapy were randomized to receive afatinib or erlotinib. Median progression-free survival (PFS) was significantly higher for afatinib than erlotinib, both by independent central review (2.4 versus 1.9 months; P=0.0427) and by investigator review (2.7 versus 1.9 months; P=0.0053). Overall survival results from this study are pending.6

Promising results have also been observed with anti–programmed cell death protein 1 (PD-1) and anti–programmed death-ligand 1 (PD-L1) strategies, but these will not be discussed here.

In summary, the standard treatment for patients with stage IV squamous cell carcinoma who have good performance status remains a platinum doublet. In first-line treatment, recent studies have shown positive results with nab-paclitaxel/carboplatin and with the necitumumab/cisplatin/gemcitabine combination. In previously treated patients, randomized trials have shown positive results with the ramucirumab/docetaxel combination and with afatinib. These findings together with results achieved using anti–PD-1 and anti–PD-L1 strategies will change the treatment algorithm in patients with squamous cell carcinoma.

Molecular alterations in squamous cell carcinoma of the lung

In recent years, knowledge of the molecular pathogenesis of NSCLC has increased remarkably and brought about changes in the principles of treatment. However, these changes have been mainly limited to adenocarcinoma of the lung. Molecular genotyping of adenocarcinoma is currently the standard of care and includes analysis of EGFR and ALK, which are altered in approximately 20% of adenocarcinoma patients, for whom there are approved targeted therapies.

Several studies have attempted to distinguish lung adenocarcinoma from squamous cell carcinoma at the molecular level and, recently, genetic alterations in squamous cell carcinoma have been described. The most prevalent mutations found in lung cancer are those within the tumor protein 53 gene (TP53). In addition to TP53 mutation, adenocarcinoma tumors often contain loss-of-function mutations in other tumor suppressor genes, such as serine/threonine kinase 11 (LKB1/STK11), neurofibromin 1 (NF1), cyclin-dependent kinase inhibitor 2A (CDKN2A), SWI/SNF-related, matrix-associated, actin-dependent regulator of chromatin, subfamily a, member 4 (SMARCA4), and kelch-like ECH-associated protein 1 [KEAP1].7 Squamous cell carcinoma tumors have a different spectrum of inactivated tumor suppressors, which partly overlap with those seen in adenocarcinoma tumors: TP53 mutation is observed in the majority of squamous cell carcinoma tumors and other mutations can include inactivation of CDKN2A, phosphatase and tensin homolog (PTEN), KEAP1, MLL2 (also known as KMT2D, [lysine] (K)-specific methyltransferase 2D), major histocompatibility complex, class I, A (HLA-A), nuclear factor, erythroid 2-like 2 (NFE2L2), Notch 1 (NOTCH1), and retinoblastoma 1 (RB1).8,9

A large next-generation sequencing (NGS) analysis of 178 squamous cell carcinoma tumors performed by the Cancer Genome Atlas Research Network identified a mean of 360 exome mutations, 323 altered copy number segments, and 165 genomic rearrangements per tumor.10 The mean somatic mutation rate of 8.1 mutations per megabase observed in this squamous cell carcinoma study was higher than that previously reported for a number of other cancer types. The spectrum of somatic copy number alteration in squamous cell carcinoma was mostly similar to that found in adenocarcinoma with the notable exception of selective amplification of a region on chromosome 3q. The evidence of a unique clinical and pathologic character of squamous cell carcinoma was confirmed in this study, which identified SRY (sex determining region Y)-box 2 (SOX2) amplification, NFE2L2 and KEAP1 mutations, phosphatidylinositol 3-kinase (PI3K) pathway changes, FGFR1 amplification, and discoidin domain receptor tyrosine kinase 2 (DDR2) mutations not encountered, or relatively rarely so, in adenocarcinoma.

In summary, the Cancer Genome Atlas Project report has identified a number of potential therapeutic targets in patients with squamous cell carcinoma that need to be investigated in clinical trials, among them fibroblast growth factor receptor 1 (FGFR1) amplifications, discoidin domain receptor 2 (DDR2) mutations, and PI3K alterations. Molecular alterations described in squamous cell carcinoma tumors of the lung that may well be targetable are FGFR1 amplification; DDR2 mutation; mesenchymal-epithelial transition factor (MET) mutation/ amplification; phosphatidylinositol-4,5-bisphosphate 3- kinase, catalytic subunit α(PIK3CA) mutation/amplification; PTEN loss; SOX2 amplification; AKT1 mutation; and TP53 mutation. These are discussed individually here.

The FGFR signaling pathway normally contributes to the physiological processes of tissue repair, hematopoiesis, angiogenesis, and embryonic development.11 There are 4 members in the FGFR family, and 22 known fibroblast growth factor (FGF) ligands. Activation of this pathway leads to enhanced growth of NSCLC cells. Furthermore, FGFR1 amplification has been identified as a potential relevant target in squamous cell carcinoma of the lung. Weiss et al reported that 22% of 153 samples of squamous cell carcinoma tissue were found to have FGFR1 amplification by fluorescence in situ hybridization (FISH) analysis.12 Dutt et al identified FGFR1 amplification in 21% of 57 squamous cell carcinoma samples and in 3% of adenocarcinomas of the lung by single-nucleotide polymorphism array analysis.13 Both studies showed that cell growth in models harboring FGFR1 amplification depends on FGFR1. The percentage of stage IV squamous cell carcinoma patients with FGFR1 amplification tends to be around 20%.14 In a recent systematic review including 13 eligible studies analyzing FGFR1 in a total of 1798 squamous cell carcinoma patients, 19% of them were found to have FGFR1 amplification.15 Pearson’s correlation analysis suggested that smoking status was highly correlated with FGFR1 amplification.

Targeting the FGF/FGFR signaling pathway is among the strategies being explored in squamous NSCLC. That strategy is supported by the growth-promoting effects of the FGF signaling pathway observed in preclinical studies, as well as the finding that FGF/FGFR-related aberrations may be more common in squamous versus adenocarcinoma and other histological types.16

Table I
Table I. Fibroblast growth factor receptor (FGFR) inhibitors in clinical trials in non–small cell lung cancer.

Abbreviations: FGFR, fibroblast growth factor receptor; FGF, fibroblast growth factor; PDGFR, platelet-derived growth factor receptor; VEGFR, vascular endothelial growth factor receptor.

A number of FGFR inhibitors are now being analyzed in clinical trials in NSCLC patients (Table I). AZD4547, an FGFR1 inhibitor, is under clinical investigation in recurrent squamous cell carcinoma of the lung, both as a monotherapy and in combination with docetaxel in a phase 1/2 trial. The results of the phase 1 expansion of AZD4547 in patients with previously treated stage IV FGFR1-amplified squamous cell carcinoma of the lung were presented at the 2014 meeting of the American Society of Clinical Oncology (ASCO).17 Of 15 treated patients, 7 had FISH ratios ≥ 2.8 (high amplification), and a total of 14 patients were evaluable for tumor response: 1 with partial response, 4 with stable disease, and 9 with disease progression. The partial response was observed in a patient with high FGFR1 amplification. The authors concluded that AZD4547 has modest overall activity in patients with FGFR1-amplified stage IV squamous cell carcinoma of the lung, but did not meet the primary efficacy end point required for study continuation. BGJ398 is a pan-FGFR inhibitor being investigated in a dose-escalation trial for FGFR-amplified malignancies. The results with BGJ398 in a subgroup of squamous cell carcinoma patients with FGFR1 amplification treated in a phase 1 study were also presented at ASCO 2014.18 Of 26 evaluable patients, 4 (15.4%) achieved lasting partial response and 9 (34.6%) had stable disease. The results showing promising activity in squamous cell carcinoma patients with FGFR1 amplification have encouraged further development of BGJ398 in FGFR1-amplified squamous cell carcinoma patients. Dovitinib (TKI258) is an inhibitor of FGFR3, and potentially other FGFRs, which is currently being tested in patients with advanced NSCLC or colorectal cancer and in patients with squamous cell carcinoma tumors of the lung. Nintedanib is being studied in phase 2 studies in patients with FGFR1-amplified NSCLC. BAY1163877, a pan-FGFR inhibitor, is being analyzed in a study in patients with FGFR-amplified cancers. GSK3052230, an antagonist of FGFR receptors, is a fusion protein composed of the extracellular domain of FGFR1 and the crytallizable fragment (Fc) of immunoglobulin G1 (IgG1). It is currently being examined in a phase 1 study both as a monotherapy and in combination with chemotherapy in squamous cell carcinoma patients with alterations in FGF receptors.

DDR2 is a tyrosine kinase receptor that plays a role in cell adhesion, proliferation, and extracellular remodeling after binding of collagen, an endogenous ligand. In a study by Ford et al, it was reported that upregulation of DDR2 was related to prolonged disease-free and overall survival of patients with NSCLC, especially those with squamous cell carcinoma.19

Hammerman et al examined 290 squamous cell carcinoma tissue samples and found the frequency of DDR2 mutations to be 3.8%.20 DDR2 mutations are driving molecular alterations, whose activation was inhibited by treatment with dasatinib, a multikinase inhibitor. They also presented details of a squamous cell lung cancer patient harboring a DDR2 kinase domain mutation who responded to dasatinib and erlotinib treatment. A phase 2 trial is currently examining dasatinib in NSCLC patients with DDR2 mutation.

MET, also known as hepatocyte growth factor receptor (HGFR), is a proto-oncogene with relevant implications in NSCLC.21 MET mutations have been observed in about 3% of treatmentnaive NSCLC patients, mainly within exons 2 and 14, outside of the kinase domain.22 Mutations in MET are observed more often in smokers. The MET mutation N375S was detected as a germline mutation in a high proportion of East Asian tissue samples, and was correlated to the incidence of squamous cell carcinoma.23

MET amplification occurs in 2% to 4% of NSCLC patients.24 Such amplifications occur at equal frequencies in squamous and adenocarcinomas with or without EGFR or Kirsten rat sarcoma viral oncogene homolog (KRAS) mutations.25 Lung cancer cells with MET amplification are highly sensitive to MET inhibitors. A recent case report showed a major partial response to crizotinib (a dual MET/ALK inhibitor) in a patient with stage IV squamous cell carcinoma of the lung with MET amplification determined by FISH.26

PIK3CA amplification/mutation
PIK3CA, located on chromosome 3q26, encodes a class I PI3K α catalytic subunit (p110α). The PI3K-AKT pathway plays a critical role in survival and growth of diverse cancer cells. Both copy number gain and mutations of PI3K are found in lung cancer. The copy number gain of PI3K is found in 33.1% of squamous cell carcinomas, in 6.2% of adenocarcinomas, and in 4.7% of small-cell lung cancers.27 Mutations of PI3K are described in 16% of squamous cell carcinoma of the lung.10,14,28

In a recent manuscript, the authors analyzed PIK3CA mutation in tumor tissue from 1144 NSCLC patients and identified 42 (3.7%) patients with mutations in exons 9 and 20.29 These mutations were found more often in squamous cell carcinoma (8.9%) than in adenocarcinoma (2.9%; P<0.001) histological types. The most common PIK3CA mutation was exon 9 E545K. The majority of patients (57%) had additional oncogenic aberrations.

Inhibitors of this pathway are in development and clinical trials using a number of PI3K inhibitors are ongoing in patients with squamous cell carcinoma of the lung.

PTEN loss
PTEN, a tumor suppressor gene located on chromosome 10q23, encodes a lipid phosphatase inhibiting the PI3K-AKT pathway. Loss of PTEN activates PI3K-AKT signaling. Inactivation of PTEN by somatic PTEN deletions, mutations, and epigenetic mechanisms is found in many cancers. Reduction or loss of PTEN expression has been reported in up to 70% of NSCLC patients, in both adenocarcinoma and squamous cell carcinoma histological types.30 PTEN mutations, occurring in approximately 5% of lung cancers, are significantly associated with squamous cell rather than adenocarcinoma histological types.31 Patients with lung cancer showing PTEN loss may be more sensitive to inhibitors of the PI3K pathway; clinical trials of PI3K inhibitors for lung cancer patients with PTEN-loss tumors are ongoing.

SOX2 amplification
Amplification within chromosome 3q26 is a common genetic alteration found in squamous cell carcinoma of the lung. SOX2 is a candidate oncogene present in this locus and amplification of SOX2 has been reported in about 20% of squamous cell carcinoma cases.32 SOX2 is a transcriptional factor that plays an important role in the regulation of stem cell function and the development of lung epithelium. Bass et al showed that inhibition of SOX2 suppressed cell growth.33 Nevertheless, subsequent studies have confirmed that SOX2 amplification is not enough for carcinogenesis to occur and additional mutations of downstream effectors are needed to develop a cancer.

AKT1 mutation
E17K somatic mutation of AKT1, located on chromosome 14q32, activates the protein kinase continuously. This mutation is known to be present in about 1% of squamous cell carcinomas and appears to be nonoverlapping with other driver mutations.34 AKT inhibitors are currently being studied in a number of trials. At present, the role of AKT1 mutations in the selection of therapy is yet to be established.

TP53 mutation
TP53, a tumor suppressor gene, is located on chromosome 17p13 and encodes a protein functioning mainly as a tran scriptional factor that regulates the transcription of genes related to cell cycle arrest, apoptosis, and DNA repair. TP53 mutations are found in more than half of NSCLC cases and in about 65% of squamous cell carcinomas.7,10 Approximately 75% of mutations are missense mutations and lead to loss of function as a transcription factor.

The mutations are affected by smoking. In a substantial number of tumors, wild-type p53 is inactivated by overexpression or amplification of MDM2 proto-oncogene, E3 ubiquitin protein ligase (MDM2), which ubiquinates p53 and marks it for degradation. Currently, the presence or absence of TP53 mutation has no impact on the selection of therapy.


Recent randomized trials have produced positive findings in regard to new treatment options for patients with stage IV squamous cell carcinoma in both the first-line and second-line settings, including necitumumab and ramucirumab, among others. These compounds, together with anti–PD-1 and anti– PD-L1 strategies, will probably change the treatment algorithm in stage IV squamous cell carcinoma of the lung. In addition, recent studies have identified several potential therapeutic targets including FGFR1 amplifications, DDR2 mutations, and PI3K pathway alterations. Clinical trials with inhibitors of these pathways are ongoing, FGFR inhibitors being, at present, at the most developed stage.

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Keywords: discoidin domain receptor; fibroblast growth factor receptor; non–small cell lung cancer; squamous cell carcinoma; targeted therapy