Skin Cancer (SCC) Overview - Signaling Pathway. Diagnostics Marker. Targeted Therapy and Clinical Trials.

Fig.1 SCC signaling pathway. Targeted agents (listed in orange boxes) include those in clinical use (colored in green) and those in preclinical or early phase development (colored in red) for the treatment of SCC.

An Introduction to Skin Cancer (SCC)

Skin cancer is one of the most common cancers in the world. Non-melanoma skin cancer represents a group of cancers that slowly develop in the upper layers of the skin, which distinguishes these more common types of skin cancer from the more common and serious melanoma. This cancer affects more men than women and is more common in the elderly. Early symptoms usually show the appearance of a lump or discoloured patch on the skin. Cancerous lumps are red and firm and sometimes turn into ulcers, and cancerous patches usually are flat and scaly. Non-melanoma skin cancer mainly occurs on areas of skin that are regularly exposed to the sun including the face, ears, hands, shoulders, upper chest and back. Overexposure to ultraviolet (UV) light is the main cause of non-melanoma skin cancer. Other risk factors include a previous non-melanoma skin cancer, a family history of skin cancer, pale skin that burns easily, a large number of moles or freckles, taking medicine that suppresses your immune system, etc. There are two most common non-melanoma skin cancer types including basal cell carcinoma (BCC) and squamous cell carcinoma (SCC).

1 Main Signaling Pathways in SCC Therapy

1.1 TGF-β signaling cascade

The transforming growth factor-(TGF)-β family of polypeptides is a group of highly conserved proteins with a molecular weight of about 25 kDa. Upon activation of latent TGF-β, the ligand binds to the type II TGF-β receptor (TβR-II). The type I receptor (TβR-I) is then recruited into the ligand/TβR-II complex and phosphorylated and activated by the TbR-II kinase. Subsequently, the activated TβR-I receptor phosphorylates the receptor-associated Smads, Smad2, and Smad3 to form complexes with the common Smad, Smad4, and accumulate in the nucleus. This process then regulates gene expression and cellular responses along with coactivators and cell-specific DNA-binding factors. The TGF-β signaling pathway plays an important role in either the genesis and/or the progression of SCC in the epidermis as well as in the head-and-neck region. Besides, the activation of TGF-β signaling in established SCC clearly facilitates their progression to highly invasive and metastatic SCC. Thus, the analysis of TβR receptors potentially represents a powerful prognostic tool for the management of patients with SCC.

1.2 HGF/c-MET signaling cascade

Abnormal activation of the c-MET signaling pathway has been found in multiple carcinomas, including SCC, bladder, renal, cervical, colon, breast, ovary, lung, esophagus, gastric, and head and neck cancers. The potential genetic and functional abnormalities include gene amplification of c-MET/hepatocyte growth factor (HGF) gene loci, c-MET mutation, overexpression of c-MET and HGF, and autocrine or paracrine activation mediated by cytokines and growth factors. HGF/c-MET signaling involves in various aggressive malignant phenotypes, including increased cell proliferation and survival, epithelial-mesenchymal transition (EMT), invasion, angiogenesis and inflammation, and metastasis. HGF and its coregulated proinflammatory and proangiogenic growth factors, c-MET mutation, amplification and phosphorylation in tumor specimens, have been used as critical indicators for patient prognosis and responses to therapies targeting HGF/c-MET. Targeting HGF/c-MET and related signaling pathways are considered as important approaches in the development of anticancer drugs, and some small molecule inhibitors and biological antagonists have been investigated in clinical trials.

1.3 Epidermal growth factor receptor (EGFR) signaling cascade

The erythroblastic leukemia viral (v-Erb-b) oncogene homolog family of ERBB receptors consists of four transmembrane proteins, i.e. EGFR (ERBB1/HER1), ERBB2 (HER2/Neu), ERBB3 (HER3) and ERBB4 (HER4). Among these ERBB family members, and only EGFR, ERBB2 and ERBB3 are expressed in the skin. The EGFR regulates a variety of cellular and tissue functions in epithelia including cell division, survival, differentiation, and migration. EGFR signaling can be up-regulated in pathologies involving aberrant growth like squamous cancer, and promotes neoplastic progression. Many mechanisms related to the EGFR are regulated in normal and pathological processes. To date, some research has found important and surprising insights into the effects of EGFR-dependent signaling on cutaneous biology and carcinogenesis.

1.4 NF-κB signaling cascade

NF-κB is a protein complex that regulates expression of key genes required for innate and adaptive immunity, cell proliferation and survival, and lymphoid organ development. In humans, the NF-kB dimers comprises of five homologous subunits namely RelA/p65, c-Rel, RelB, p50/NF-kB1, and p52/NF-kB2. These proteins combine with each other to form homo- or heterodimeric complexes that are transcriptionally active. In the typical activation pathway, IκB inhibitory protein degradation occurs through phosphorylation by IκB kinase complex (catalytic subunits IKKα and IKKβ, and the regulatory scaffolding protein NEMO) at specific serine residues. As a result, free NF-κB dimer enters the nucleus and binds to κB enhancer sites in the DNA, and activates transcription of a series of genes involved in the immune and inflammatory response, cell growth, adhesion, metastasis, and apoptosis escape. During the skin cancer, NF-kB pathway plays an important role in both initial transformation steps and in progression of SCC.

SCC Diagnosis

2.1 Molecular Markers for SCC

UV radiation is harmful in prolonged exposure of the skin acting as a carcinogen, and cellular DNA is the major target for UVB radiation (290-320 nm). Chronic and excessive exposure to UV radiation can cause genomic and proteomic alterations at skin level, leading to immunosuppression favorable to the most common forms of skin cancer, BCC, SCC and melanoma. Genetic factors such as polymorphisms of the melanocortin 1 receptor gene can affect the skin’s sensitivity to UV and enhance cancer risk. The most important gene involved in the ultraviolet radiation effects is the p53 tumor suppressor gene, which plays an important role in apoptosis, cell proliferation, DNA differentiation and repairing process. Mutations of the p53 tumor suppressor gene trigger other mutations in cascade with the loss of control of aberrant cell growth, leading to the formation of cancer cells. In addition, several biomarkers, such as E-cadherin, Ki-67 and cyclin D1, have been shown to correlate with malignancy in non-melanoma skin cancer (NMSC). In SCC, the downregulation of E-cadherin in the primary lesion is closely related to the development of regional lymph node metastases. Ki-67 is a marker of the cell proliferation and a representative in fast and frequent recurrent aggressive tumors. cyclin D1 is an important regulator of the cell cycle that is essential in the development of skin cancer leading to the organization and abnormal differentiation of tissues. Other molecular markers in SCC include APC gene, NOTCH tumor suppressor genes, S100, pSTAT3, collagen VII (Col 7), etc.

2.2 Protein Markers for SCC

Upregulation of complement factor H (CFH) and factor H-like protein-1 (FHL-1) were identified significantly higher in tumors compared to normal skin. Immunohistochemistry analysis of CFH and FHL-1 in invasive SCCs showed a specific and stronger expression compared with in situ carcinoma and actinic keratoses. Thus, CFH and FHL-1 are considered as progression markers and potential therapeutic targets in skin SCCs. The cellular enzymatic portfolio is a good pool for emerging novel targets in SCC. For example, upregulation of MMP-7 expression has been found in cSCC that can activate heparin-binding epidermal growth factor-like growth factor (HB-EGF) promoting cellular proliferation. Thus, HB-EGF could be a target in advanced SCC diagnosis and therapy. Serine peptidase and their inhibitors (Serpins) are considered useful for biomarker monitoringing of SCC progression. Additionally, some other protein markers include EGFR, nuclear active IκB kinase (IKK), collagen XVII, integrin α6β4, laminin 332, etc.

3 Targeted Therapy for SCC

Targeted therapy represents a very exciting era in the treatment of cancer diseases. Several molecular pathways are deregulated and activated in SCC making this disease attractive for targeted molecular therapies. Such as Cetuximab, a monoclonal antibody targeting the epidermal growth factor receptor, improves the overall survival when combined with radiation therapy or chemotherapy. Some novels agents targeting different molecular pathways in SCC are currently under development. Among them, dual (EGFR/HER2) or pan-human epidermal growth factor receptor inhibitors and drugs that target the MET receptor and NF-κB pathway have shown either interesting preclinical activity or promising preliminary clinical efficacy. Here, we summarize the potential targets and new drugs developed that have been used in recent, ongoing and future clinical trials to try to improve the clinical outcomes of this disease (Table1-8).

3.1 SCC therapy for EGFR pathway

Inhibition of the EGFR pathway can be achieved with low molecular weight tyrosine kinase inhibitors (TKIs) or with specific monoclonal antibodies (MoAbs). TKIs can target the EGFR tyrosine kinase intracellularly and inhibit phosphorylation and downstream signaling pathways, such as the two main compounds, erlotinib and gefitinib. The most studied antibody is cetuximab that is a chimeric IgG1 MoAb specifically binding to the EGFR with high affinity, blocking ligand-induced EGFR phosphorylation. Panitumumab and zalutumumab are two completely humanized anti-EGFR MoAbs under investigation. Nimotuzumab is another humanized MoAbs against EGFR with intermediate affinity, which has shown some activity in some solid tumors. Lapatinib is an oral small molecule that acts as a reversible inhibitor of both the EGFR and HER-2 tyrosine kinases. BIBW 2992 (afatinib) is an orally bioavailable irreversible inhibitor of both EGFR and HER-2 kinases.

Table 1 Clinical trials of EGFR tyrosine kinase inhibitor erlotinib

According to statistics, a total of 5 erlotinib projects targeting SCC EGFR tyrosine kinase are currently in clinical stage, of which 3 are recruiting and 2 are not recruiting.

Table 2 Clinical trials of EGFR tyrosine kinase inhibitor gefitinib

According to statistics, a total of 1 gefitinib project targeting SCC EGFR tyrosine kinase is currently in clinical stage and not recruiting.

Table 3 Clinical trials of EGFR inhibitor cetuximab

According to statistics, a total of 7 cetuximab projects targeting SCC EGFR are currently in clinical stage, of which 3 are recruiting and 4 are not recruiting.

Table 4 Clinical trials of EGFR inhibitor Panitumumab

According to statistics, a total of 6 Panitumumab project targeting SCC EGFR are currently in clinical stage, of which 3 are recruiting and 3 are not recruiting.

Table 5 Clinical trials of EGFR inhibitor Lapatinib

According to statistics, a total of 29 Lapatinib projects targeting SCC EGFR are currently in clinical stage, of which 6 are recruiting and 23 are not recruiting.

Table 6 Clinical trials of EGFR inhibitor BIBW 2992

According to statistics, a total of 1 BIBW 2992project targeting SCC EGFR is currently in clinical stage and is recruiting.

3.2 SCC therapy for HGF/c-MET pathway

Different strategies are developed to inhibit the HGF/MET pathway and include MoAbs currently, which target either MET, or HGF and small molecule tyrosine kinase inhibitors. PF-2341066 (crizotinib) is a MET TKI showed the anti-tumor growth in a preclinical animal model. All these data support the investigation of MET inhibitors in SCC in monotherapy or in combination with anti-EGFR.

Table 7 Clinical trials of MET inhibitor PF-2341066

According to statistics, a total of 1 PF-2341066 project targeting SCC MET is currently in clinical stage and is recruiting.

3.3 SCC therapy for NF-κB pathway

Bortezomib is the first proteasome inhibitor that can inhibit NF-kB activation and proliferation, induces apoptosis in SCC cell and to be selected for clinical development as an anticancer agent. It has been demonstrated to be highly effective in the treatment of hematologic malignancies. Bortezomib has also been studied and shown anti-tumor activity for a number of solid tumor types, including SCC.

Table 8 Clinical trials of NF-κB inhibitor Bortezomib

According to statistics, a total of 1 Bortezomib project targeting SCC NF-κB are currently in clinical stage and is not recruiting.