Renal Cell Cancer Overview - Signaling Pathway. Diagnostics Marker. Targeted Therapy and Clinical Trials.

Fig.1 RCC 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 RCC.

An Introduction to Renal Cell Cancer

Renal cell cancer (RCC) also known as hypernephroma, is a type of renal or kidney cancer. It represents the most common kind of renal tumor, the third malignancy within urological oncology. Normally, the kidneys play a vital role in body to help clear waste and regulate fluid balance, and the tubules in kidneys can help filter the blood, excrete waste and make urine. RCC often occurs upon the cancer cells growing in the tubules uncontrollably. As a fast-growing cancer, RCC can spread to other organs such as the lungs. Several risk factors could cause the RCC including tobacco smoking, exposure to asbestos or chemicals, dialysis treatment, hypertension, obesity, polycystic kidney disease and urinary tract infections. In the early stages of RCC, there are only few warning signs. As the disease progresses, various symptoms are emerging, including blood in the urine, fatigue, loss of appetite, weight loss, fever night sweats and excessive hair growth (in women). Currently, there are five standard treatment approaches are used in the RCC, including surgery, radiation therapy, chemotherapy, biologic therapy and targeted therapy.

1 Main Signaling Pathways in Renal Cell Cancer Therapy

1.1 Angiogenic signaling cascade

Kidney tumors are frequently characterized by hypoxic conditions due to local imbalance between oxygen (O2) supply and consumption like many solid tumors. Indeed, hypoxia and compensatory hyperactivation of angiogenesis are considered to be particularly important in RCC. Compared with other tumor types, the high vascularization property and VHL (key regulator of hypoxia response) mutation in renal tumor are specially associated with the pathogenesis of renal cancer. VHL combined with elongins B, C and Cullins (Cul2) to form ubiquitin ligase complex, which mediates the degradation of target protein induced by hypoxia. Hypoxic signaling is mediated by a small group of transcription factors called the hypoxia-inducible factors (HIFs). In turn, HIFs regulate the expression of more than 200 genes, which are involved in key pathways related to tumorigenesis, including angiogenesis, invasion and mitogenesis. Target proteins that regulated by HIFs include VEGF, PDGF, EGFR TGFα, HEF-1, GLUT1 and MUC1.

1.2 PI3K/AKT/mTOR signaling cascade

Protein kinase B (Akt) and mTOR play critical roles in main oncogenic processes including cell proliferation, survival and angiogenesis. Some mTOR inhibitors have showed promise for RCC in phase I clinical trials. Interaction between VEGF/PDGF and their receptor tyrosine kinases (VEGFR, PDGFR, KIT) on RCC tumor cells can activate PI3K and promote the generation of phosphatidylinositol-3,4,5-triphosphate (PIP3). Then the cytoplasmic kinase AKT is recruited by PIP3 to the cell membrane and activated by phosphorylation at two independent sites mediated by PDK1 and mTOR (TORC2), respectively. AKT activation inhibits apoptosis by phosphorylating and inactivating proapoptotic proteins such as procaspase 9, BAD, and apoptosis signal regulating kinase 1 (ASK1). Signaling from VEGF and PDGF through AKT also activates mTOR that functions as a component of two distinct complexes, TORC1 and TORC2. Akt, mTOR and p70S6K are constitutively activated in RCC. Negative regulation by the PTEN tumor suppressor gene could contribute to the Akt activation in RCC.

1.3 Wnt/β-catenin signaling cascade

Wnts are a family of secreted glycoproteins involved in regulating cell proliferation, differentiation and cell migration. The ultimate effector of canonical Wnt signaling is the transcriptional coactivator β-catenin, which is a key molecule in the pathogenesis of renal cancer. In normal cells, β-catenin is trapped in a highly processive enzyme complex, which includes casein kinase 1 (CK1), glycogen synthase kinase 3β (GSK3β), adenomatosis polyposis coli protein (APC) and axin. β-catenin can be phosphorylated by this complex and targeted for proteosomal degradation. Wnt regulates β-catenin to inhibit its phosphorylation, ubiquitination and degradation. β-catenin enters the nucleus and activates target genes such as the MYC oncogene. Wnt also can mediate its effect on cell growth and tumor promotion by activating the mTOR pathway. Wnt activates the mTOR pathway by inhibiting GSK3.

1.4 HGF/MET signaling cascade

Hepatocyte growth factor (HGF) widely exists in kidney tissue, and the expression and activity of HGF and its receptor c-MET have been found associated with papillary RCC. Activating point mutations in MET are found in sporadic papillary RCC and most papillary RCCs show without mutation of MET. Some Met inhibitors are used in clinical trials for the treatment of RCC and other cancers by inhibiting the autophosphorylation of c-MET, blocking interaction between HGF and c-MET and the downstream signaling cascade of activated c-MET. HGF binding to MET leads to phosphorylation of MET and causes the recruitment of adapter proteins and activation of the Ras/MAPK and PI3K/ AKT effector pathways to promote RCC growth and metastasis.

Renal Cell Cancer Diagnosis

2.1 Molecular Markers for Renal Cell Cancer

Molecular markers in RCC may be categorized based on the source of the marker (tissue, serum or urine based), the clinicopathological significance/biochemical structure (histological type of RCC, progression, survival, response to therapy, hypoxia inducible pathway, proliferation, cell cycle regulation, cell adhesion, immunogens, apoptosis, enzymes, and CD based, proteinaceous, glycoprotein and DNA based markers) and diagnosis vs prognosis. Ferritin is a serum based biomarker in RCC that correlates with RCC stage. NMP-22 is a protein can be measured to evaluate a patient at risk for RCC. Neopterin is a pteridine intermediate metabolite in the biopterin synthetic pathway that has been investigated as a serum marker for supportive immunotherapy as well as for the evaluation of the response to immunotherapy in RCC. CAIX has been investigated as an independent prognostic factor for survival in patients with metastatic RCC. Other molecular markers include Ki-67, p53, p21, TATI, TPS, BCL-2, etc.

2.2 Protein Markers for Renal Cell Cancer

Numerous markers have been examined in RCC, including CAIX, CAXII, gelsolin, phosphatase and tensin homolog deleted on chromosome 10 (PTEN), epithelial cell adhesion molecule (EpCAM), CD10, p53, sodium-potassium adenosine triphosphatase subunits, vimentin, Ki-67, CXC chemokine receptor-4, VEGF ligands, VEGF receptors, androgen receptors, bcl-2, β-catenin, cadherin-6, CA-125 protein, epithelial membrane antigen, CD44, insulin-like growth factor-1, caveolin-1, and cyclin A. Immunohistochemical analysis of Ki-67, p53, gelsolin, CAIX, CAXII, PTEN, EpCAM, and vimentin was performed on a custom tissue microarray representing all stages of localized and metastatic RCC. Ki-67 is a nuclear antigen that is selectively expressed in proliferating tumor cells. In RCC, p53 mutations involve in the cellular proliferation and decrease in apoptosis. Gelsolin contributes to sever actin during cell motility. CAIX and CAXII overexpression can leads to a VHL mutation in 75% of sporadic clear-cell RCCs. PTEN plays a role in cellular migration, proliferation, and apoptosis. EpCAM often is highly expressed on the cell surface of most carcinomas. Vimentin is an independent predictor of poor prognosis in RCC. Increased expression of Ki-67, p53, vimentin, and gelsolin are related to worse survival, whereas the inverse was true for CAIX, PTEN, CAXII, and EpCAM. Met, gelsolin, p53, and Met*CAIX are significant predictors of survival and are used to create a prognostic model.

3 Targeted Therapy for Renal Cell Cancer

A variety of molecular mechanisms involved in the pathogenesis of RCC provide promising ways for targeted therapy. Major components of cell signaling pathways, such as the angiogenic signaling pathway, PI3K/AKT/mTOR, Wnt/β-catenin and HGF/MET signaling pathways, are altered in RCC cells by oncogenes through overexpression or mutation, leading to dysregulated cell signaling and cell proliferation. 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 (Table1-7).

3.1 Renal cell cancer therapy for angiogenic pathway

Sunitinib represents a highly potent, selective inhibitor of multiple receptor tyrosine kinases, including VEGFR-1, VEGFR-2, VEGFR-3, PDGFR-α, PDGFR-β, c-kit, and Flt-3. Preclinical data showed that sunitinib has antitumor activity by both inhibition of angiogenesis and direct antiproliferative effects. Based on the highly statistically significant improvement in progression-free survival (PFS) and its tolerability, sunitinib is considered as a new standard of care for the first-line treatment of good- and intermediate-risk metastatic clear cell RCC. Other studies combining sunitinib with IFN α, bevacizumab, and temsirolimus are evaluating. Bevacizumab is a recombinant monoclonal antibody that targeting VEGF-A has demonstrated activity against metastatic RCC in several clinical trials. Other angiogenesis, VEGFR, PDGFR and c-kit tyrosine kinase inhibitors include Lenalidomide, Pazopanib, Axitinib, Vatalanib. Lapatinib is an EGFR and erb-2 tyrosine kinase inhibitor is been used in the treatment of RCC and is evaluated in Phase II clinical trial.

Table 1 Clinical trials of receptor tyrosine kinases inhibitor Sunitinib

According to statistics, a total of 53 Sunitinib projects targeting RCC receptor tyrosine kinases are currently in clinical stage, of which 21 are recruiting and 32 are not recruiting.

Table 2 Clinical trials of VEGF-A inhibitor bevacizumab

According to statistics, a total of 22 bevacizumab projects targeting RCC VEGF-A are currently in clinical stage, of which 11 are recruiting and 11 are not recruiting.

Table 3 Clinical trials of receptor tyrosine kinases inhibitor Pazopanib

According to statistics, a total of 29 Pazopanib projects targeting RCC receptor tyrosine kinases are currently in clinical stage, of which 10 are recruiting and 19 are not recruiting.

Table 4 Clinical trials of receptor tyrosine kinases inhibitor Axitinib

According to statistics, a total of 32 Axitinib projects targeting RCC receptor tyrosine kinases are currently in clinical stage, of which 14 are recruiting and 18 are not recruiting.

3.2 Renal cell cancer therapy for PI3K/AKT/mTOR pathway

Sorafenib is an oral, bi-aryl urea molecule that was developed as a c-Raf and b-Raf kinase inhibitor but was also found to inhibit several receptor tyrosine kinases, including VEGFR-1, VEGFR-2, VEGFR-3, PDGFR-β, Flt3, and c-kit. A phase II clinical trial evaluated the effects of sorafenib in patients with a variety of cancers including RCC. Median PFS from randomization was significantly longer with sorafenib than placebo. Sorafenib significantly prolonged median PFS compared with placebo, and median survival improvement was preliminarily reported. Benefit was evident across all subsets evaluated. Temsirolimus and everolimus are orally administered mTOR inhibitors used for targeted therapy in metastatic renal cell carcinoma (mRCC). Clinical trials in patients with mRCC showed that treatment with temsirolimus or everolimus prolonged PFS relative to the placebo in patients with mRCC that had progressed on other targeted therapies.

Table 5 Clinical trials of receptor tyrosine kinases inhibitor Sorafenib

According to statistics, a total of 7 Sorafenib projects targeting RCC receptor tyrosine kinases are currently in clinical stage, of which 3 are recruiting and 4 are not recruiting.

Table 6 Clinical trials of mTOR inhibitor Temsirolimus

According to statistics, a total of 11 Temsirolimus projects targeting RCC mTOR are currently in clinical stage, of which 7 are recruiting and 4 are not recruiting.

Table 7 Clinical trials of mTOR inhibitor everolimus

According to statistics, a total of 20 everolimus projects targeting RCC mTOR are currently in clinical stage, of which 10 are recruiting and 10 are not recruiting.