In the last seven years, remarkable progress has transpired in the treatment of metastatic renal cell carcinoma (mRCC). Prior to 2005, therapy for advanced disease was limited to surgical resection and immunotherapy. Immunotherapy generally consisted of interferon-α, which induced short-lived responses and a median overall survival (OS) of 13 months.1
The introduction of the targeted therapies (TT) has transformed the therapeutic landscape for mRCC. These agents inhibit the hypoxia-inducible factor (HIF) axis either through the vascular endothelial growth factor (VEGF) pathway, which regulates tumor angiogenesis, or via the mammalian target of rapamycin (mTOR), a protein important in cell growth, survival, and response to hypoxia. There are now seven FDA-approved TT including the multi-tyrosine kinase receptor inhibitors (TKIs: sunitinib, pazopanib, axitinib, sorafenib), an antibody to the VEGF-A ligand (bevacizumab), and the mTOR inhibitors (temsirolimus, everolimus). In treatment-naïve patients, four regimens have been approved based on level 1 evidence from randomized controlled trials: sunitinib, bevacizumab + interferon-α in combination, pazopanib, and temsirolimus2-5
Given the absence of clinical biomarkers to direct selection, consideration of the trial design and patient populations in which the TT was proven effective can direct choice. Histology (clear cell vs non-clear cell), the type of prior therapies, and the Memorial Sloan-Kettering Cancer Center (MSKCC) prognostic stratification (good/intermediate vs poor)6
should be taken into account when selecting a particular therapy. While the phase III registration trials have not shown a statistically significant increase in survival with the exception of temsirolimus in poor risk patients, patients are clearly living longer with TT treatment compared with the immunotherapy era.1-5
Median OS is now >2 years with these agents, which represents a significant advance compared with a median 10 to 13 months in the immunotherapy era.6,7
The approved first-line VEGF inhibitors include the TKIs sunitinib and pazopanib as well as bevacizumab plus interferon-α. They achieve objective response rates (ORR) of more than 30% and median progression-free survivals (PFS) of 9 to 11 months in the treatment-naïve setting.4,5,8
Another investigational TKI, tivozanib, has shown promise. In a phase III comparator trial against sorafenib, tivozanib demonstrated a statistically significant improvement in median PFS of 11.9 months compared with 9.1 months (P
Second-line TKIs include sorafenib and axitinib (Table 1
). Sorafenib was the first TT to be approved in 2005. While its initial efficacy was not encouraging in the cytokine-naïve setting, it achieved approval given a median PFS of 5.5 months compared with placebo in a population of cytokine pre-treated patients.10
However, more recent phase III studies have used sorafenib as the comparator arm in the first- and second-line settings with greater than expected PFS results. This improvement suggests we understand better how to manage its toxicities after many years of experience. The potent selective VEGFR1-3 inhibitor, axitinib, has been approved in the second-line setting after demonstrating superiority over sorafenib with a median PFS of 6.7 months compared with 4.7 months in patients who had predominantly received one cytokine or sunitinib.11
Ongoing studies of axitinib in the treatment-naïve setting are of major interest.
The mTOR inhibitors temsirolimus and everolimus are sirolimus analogues, which inhibit mTOR by binding to FkBP-12. While assumed to have similar mechanistic actions, the two drugs have been studied in very different patient populations (Table 1
). The phase III trial of temsirolimus required poor-risk, treatment-naïve patients and demonstrated a statistically significantly superior OS of 7.3 months compared with interferon-α.3,12
Everolimus was evaluated in VEGF inhibitor-refractory patients.13
Median PFS was superior at 4.9 months compared with placebo. It did not show an OS benefit, likely reflecting the crossover effect of control patients receiving everolimus upon progression.14
Ongoing studies should elucidate the clinical benefit of the mTOR inhibitors in the treatment-naïve and favorable disease settings. Outstanding questions include whether mTOR inhibitors achieve their primary effect through mTOR, HIF axis inhibition, or both, and whether oral and intravenous administration is equivalent and thus, the agents interchangeable.