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Current and future aspect of immunotherapy for advanced renal cell carcinoma

Renal cell carcinoma (RCC) has been recognized as a cancer responsive to the activation of the immune system. In the 1980s, interferon-alpha and interleukin-2 were shown to prolong survival, but they were not widely used due to toxicities. Then, vascular endothelial growth factor receptor and mammalian target of rapamycin inhibitors demonstrated clinical benefits and became the principal treatment in the first- and second-line setting of metastatic RCC (mRCC). In recent years, the efficacy of immune checkpoint inhibitors (ICIs) is confirmed, either alone or in combination with ICI or antiangiogenic agents. ICI-based immunotherapies have now changed the landscape of treatment of mRCC. In this article, we will review the progress of immunotherapy in clear-cell mRCC.


introduction
In Taiwan, renal cell carcinoma (RCC) represents the majority of kidney cancers (>90%) with an estimated 1200 newly diagnosed cases in 2016. [1] Clear cell is the most common histological subtype (60%), which is characterized by increased angiogenesis through the activation of the vascular endothelial growth factor (VEGF) and the mammalian target of rapamycin pathways. [2] From 2005 to 2017, a number of antiangiogenic agents, mostly VEGF tyrosine kinase inhibitors (TKI), had been approved by the FDA for metastatic RCC (mRCC). [3,4] The prognosis of patients treated with VEGF TKI varied, with a median 2-year survival rate being 75%, 53%, and 7% in favorable-, intermediate-, and poor-risk groups assessed by the International mRCC Database Consortium (IMDC) criteria. [5] Although the first-line antiangiogenic therapies do benefit mRCC patients, the durable response is uncommon for intermediate/poor-risk patients. RCC has historically been recognized as an immunogenic cancer amenable to immune stimulation with interferon-alpha (IFN-α) and high-dose (HD) interleukin-2 (IL-2), which provides a durable complete response rate of 5%. [6][7][8] Recently, immune checkpoint inhibitors (ICIs), which block programmed death 1 (PD-1), PD-ligand 1 (PD-L1), or cytotoxic T-lymphocyte-associated antigen-4 (CTLA- 4), have been demonstrated to evoke effective anticancer immunity. Many studies have examined the role of these drugs as monotherapies and combination therapies in mRCC. Herein, we briefly review the progress of immunotherapy in the treatment of mRCC.

Interferon-alpha
The role of the immune system in mRCC was first explored through the use of immunoregulatory cytokines, including IFN-α and IL-2. [9][10][11] In a multicenter, randomized controlled trial, mRCC patients were randomly assigned to subcutaneous IFN-α or medroxyprogesterone acetate. The study met its primary endpoint, the overall survival (OS), which showed a 28% reduction in the risk of death in the IFN-α group (median OS: 8.5 months vs. 6 months, hazard ratio [HR]: 0.72; 95% confidence interval [CI]: 0.55-0.94; P = 0.017). [12] The objective response rate (ORR) also favored the IFN-α group (14% vs. 2%). Despite its efficacy, recent Cochrane review reveals that IFN-α monotherapy probably increases 1-year mortality compared with standard targeted therapies with sunitinib or temsirolimus (relative risk: 1.30; 95% CI: 1.13-1.51) and is associated with a higher incidence of Grade 3 or higher treatment-related adverse events (TRAEs) (risk ratio [RR]: 1.17; 95% CI: 1.03-1.32). [13] IFN-α had also been investigated in combination with other drugs to increase immune response. In the AVOREN and CALGB 90206 studies, the combination of IFN-α and bevacizumab significantly prolonged the progression-free survival (PFS) but not OS. [14][15][16] In addition, the combination increased the incidence of Grade 3 hypertension and overall adverse events. In the ARCC trial, the combination of IFN-α and temsirolimus did not significantly extend OS compared with temsirolimus or IFN-α alone. [17] As a result, IFN-α, either alone or combined with bevacizumab or temsirolimus, was not routinely used in treating mRCC patients.

Interleukin-2
IL-2 is a cytokine predominantly secreted by antigen-stimulated CD4 + T-cells. The binding of IL-2 and its receptor not only leads to the activation and differentiation of antigen-specific T-cells but also potentiates the cytotoxic activity of natural killer cells. [18] Fyfe et al. reported the efficacy of HD IL-2 in mRCC patients when HD IL-2 was administrated by 15-min intravenous (IV) infusion of 600,000 or 720,000 IU/kg every 8 h for 5 days per cycle. [7] The ORR was 14%, including a durable complete response rate of 5%. Nevertheless, HD IL-2 was associated with severe toxicities, with the need of vasopressor support in more than 50% of patients and treatment-related death in 4% of patients. Given the toxicities of HD IL-2, low-dose (LD) IL-2 was studied as monotherapy or in combination with IFN-α. [19,20] Although LD IL-2-based therapies demonstrated equivalent OS and less toxicities, they exhibited a lower response rate (10% vs. 20%) and a shorter duration of response compared with HD IL-2. Hence, HD IL-2 was approved by the FDA in 1992 in treating mRCC patients who had good performance status and preserved organ functions.

Ipilimumab (cytotoxic T-lymphocyte-associated antigen-4 inhibitor)
The interaction between CTLA-4 on T-cells and its ligand, the B-7 family of molecules, on antigen-presenting cells, inhibits proliferation and functions of T-cells. [21] Ipilimumab, a monoclonal antibody blocking CTLA-4, had been used in mRCC treatment. In a Phase II study reported by Yang et al., patients received HD ipilimumab (all doses at 3 mg/kg) or LD ipilimumab (a loading dose of 3 mg/kg with subsequent doses of 1 mg/kg) every 3 weeks. [22] The ORR was 12.5% and 4.7% in HD and LD ipilimumab, respectively, and some patients who failed prior IL-2 therapy still responded. The incidence of Grade 3 or worse TRAE was much higher in the HD ipilimumab group (43% vs. 14%). Notably, there was an association between autoimmune side effects and ORR (30% vs. 0% in patients with or without side effects).

Nivolumab (programmed death 1 inhibitor)
PD-1 is an inhibitory receptor expressed by T-cells, and its blockade can overcome the immune tolerance of cancers. Nivolumab had been evaluated in an early Phase I study at a dose of 1 mg/kg or 10 mg/kg every 2 weeks. Nine of thirty-three mRCC patients had a response (27%), and the response was durable. [23] In a Phase II study, a total of 168 clear-cell mRCC patients who failed previous VEGF inhibitors were randomly assigned to nivolumab (0.3 mg/kg, 2 mg/kg, or 10 mg/kg every 3 weeks). [24] About 70% of the patients received more than one prior systemic therapy. The ORR was almost the same in different dosing schedules (20%). The median PFS was 2.7, 4, and 4.2 months, and the median OS was 18.2, 25.5, and 24.7 months, respectively. The most common adverse event was fatigue (22%-35%), and Grade 3 or 4 TRAE occurred in 11% of the patients.

Atezolizumab (programmed death-ligand 1 inhibitor)
Atezolizumab was studied in a Phase I trial, which enrolled a total of 70 mRCC patients (clear-cell type = 63; nonclear-cell type = 7). [30] More than 80% of the patients were of intermediate/poor risk and had failed at least one prior line of systemic therapy. The ORR was 15%, which differed on the basis of immune cell (IC) PD-L1 status (18% vs. 9% in IC PD-L1-positive and negative patients, respectively). The median PFS was 5.6 months, and the median OS was 28.9 months. According to these findings, atezolizumab was viewed as a promising anticancer therapy and studied in subsequent combination trials.

Pembrolizumab (programmed death 1 inhibitor)
Pembrolizumab had been evaluated in treatment-naïve mRCC patients. In the KEYNOTE-427 study, a total of 110 clear-cell mRCC patients were included, and pembrolizumab monotherapy (200 mg every 3 weeks) demonstrated an ORR of 33.6%. [31] The subgroup analysis showed that an ORR was higher in the intermediate/poor-risk patients than in the favorable-risk patients (37.3% vs. 27.5%). Grade 3 or higher TRAE occurred in 18.2% of the patients, and one patient had Grade 5 pneumonitis. KEYNOTE-427 demonstrated the efficacy and tolerable toxicities of single-agent ICI in the frontline treatment of mRCC.

Nivolumab plus ipilimumab
Given the efficacy in melanoma, the combination of nivolumab and ipilimumab was evaluated in patients with mRCC. In the Phase I CheckMate 016 study, mRCC patients were randomized to tri-weekly nivolumab 3 mg/kg plus ipilimumab 1 mg/kg (N3I1), nivolumab 1 mg/kg plus ipilimumab 3 mg/ kg (N1I3), or nivolumab 3 mg/kg plus ipilimumab 3 mg/ kg (N3I3) for four doses followed by maintenance bi-weekly nivolumab 3 mg/kg until disease progression or intolerable toxicity. [32] At the time of the analysis, the N3I3 group was censored because of dose-limiting TRAE, and a total of 47 patients in N3I1 and N1I3 groups were included for the evaluation. The ORR was 40.4% in both the arms, and the 2-year OS was 67.3% and 69.6% in the N3I1 and N1I3 arms, respectively. Patients in the N3I1 group had a higher complete response rate (11.1% vs. 0%) and a lower incidence of Grade 3 or more TRAE (38.3% vs. 61.7%).
In view of the promising results of CheckMate016, the N3I1 regimen was compared with sunitinib (50 mg daily for 4 weeks every 6 weeks) in treatment-naïve clear-cell mRCC patients in the randomized Phase III CheckMate 214 study. [33] The co-primary endpoints were OS, ORR, and PFS among IMDC intermediate/poor-risk patients. At a median follow-up of 25.2 months, a total of 1096 patients were enrolled, and three-fourth of them were of intermediate/poor risk. In intermediate/poor-risk patients, the median OS was not reached in the N3I1 combination group versus 26 months in the sunitinib group (HR: 0.63, P < 0.001). The ORR was 42% versus 27% (P < 0.001), and the complete response rate was 9% versus 1%, respectively. The median PFS did not differ significantly between the two groups (11.6 months vs. 8.4 months, HR: 0.82, P = 0.03, not met the prespecified level of 0.009). The incidence of Grade 3 or 4 TRAE was 46% in the N3I1 group and 63% in the sunitinib group. TRAE leading to drug discontinuation was documented in 22% and 12% of the patients, respectively. In exploratory analyses of favorable-risk  Table 1]. [26] coMbination of iMMunE chEckpoint inhibitors and antiangiogEnic agEnts VEGF plays an important role in vascular normalization, which facilitates the infiltration of ICs into the tumor microenvironment. [34] The combination of PD-1 blockade and antiangiogenesis was shown to have synergistic antitumor effects. [35] There have been several Phase I/II trials evaluating combination therapy with ICI and antiangiogenic agents, showing a much higher ORR. [4] Based on these promising results, several randomized Phase III trials compared the efficacy of combination therapy with sunitinib in the first-line setting of mRCC [ Table 1].

Atezolizumab plus bevacizumab
The activity of atezolizumab plus bevacizumab in treatment-naïve mRCC patients was studied in the Phase II IMmotion 150 trial. [36] A total of 305 patients was randomized to a combination of atezolizumab and bevacizumab, atezolizumab, or sunitinib alone. In PD-L1-positive populations, the combination group had improved PFS (14.7 months vs. 7.8 months in sunitinib, HR: 0.64; 95% CI: 0.38-1.08; P = 0.095) and response rate (46% vs. 27%). However, there was no difference in PFS and ORR in the intention-to-treat (ITT) population. Forty percent of the patients with combination therapy had Grade 3 or 4 TRAE (57% in the sunitinib group), and only 9% of the patients discontinued therapy due to treatment-related toxicities. In the exploratory biomarker analyses, the high expression of angiogenesis gene signature was related to better ORR (46% vs. 9%, P < 0.001) and longer PFS (19.5 months vs. 3.7 months, HR: 0.31; 95% CI: 0.18-0.55; P < 0.001) in the sunitinib group. In addition, the high expression of effector T-cell gene signature (T eff High ) correlated with better ORR (49% vs. 16%, P = 0.002) and longer PFS (21.6 months vs. 5.6 months, HR: 0.50; 95% CI, 0.30-0.86; P = 0.011) in the combination group. Given that myeloid differentiation was related to immunosuppressive activity, its effects on treatment efficacy were also investigated. Atezolizumab monotherapy had worse PFS in the T eff High Myeloid High tumors compared with the T eff High Myeloid Low tumors (HR: 3.82; 95% CI: 1.7-8.6; P = 0.001). Interestingly, the outcome significantly improved after the addition of bevacizumab (HR: 0.25; 95% CI: 0.10-0.60; P = 0.002), suggesting that bevacizumab could possibly reverse the immune escape mechanism of myeloid-mediated immunosuppression.
On the basis of promising results in IMmotion 150 study, the Phase III IMmotion 151 trial evaluated the efficacy of combination therapy (atezolizumab [1200 mg] plus bevacizumab [15 mg/kg] IV every 3 weeks) versus sunitinib (50 mg orally once daily, 4 weeks on, 2 weeks off) in the first-line setting of mRCC. [27] This study enrolled 915 patients with clear-cell or sarcomatoid histology. Co-primary endpoints were investigator-assessed PFS in the PD-L1-positive population and OS in the ITT population. In PD-L1-positive patients, the PFS was longer (11.2 months vs. 7.7 months, HR: 0.74; 95% CI: 0.57-0.96; P = 0.0217) and the ORR was higher (43% vs. 34%) in the combination group than in the sunitinib group. In the subgroup analysis for PFS, patients with sarcomatoid differentiation had longer survival time (HR: 0.46; 95% CI: 0.28-0.78). For the other co-primary endpoints of OS in the ITT population, there was no difference between the two groups at the second interim analysis (33.6 months vs. 34.9 months, HR: 0.93; 95% CI: 0.76-1.14; P = 0.4751). Forty percent of the patients in the combination group and 54% of the patients in the sunitinib group had Grade 3 or 4 TRAE, with 5% and 8% of patients leading to treatment discontinuation, respectively.

Pembrolizumab plus axitinib
Atkins et al. reported a Phase Ib study which evaluated the combination of pembrolizumab and axitinib in mRCC patients. [37] A total of 52 patients were enrolled, and the regimen showed an extremely high ORR of 73% (complete response rate: 8%). Grade 3 or worse TRAE occurred in 65% of the patients, the most frequent being hypertension (23%), diarrhea (10%), and fatigue (10%). Due to the inspiring results, subsequent randomized Phase III study was conducted.
In the KEYNOTE-426 study, a total of 861 treatment-naïve clear-cell mRCC patients were randomly assigned to receive pembrolizumab (200 mg intravenously every 3 weeks for up to 35 weeks) plus axitinib (5 mg orally twice daily), or sunitinib monotherapy (50 mg orally once daily, 4 weeks on, 2 weeks off). [28] The primary endpoints were OS and PFS in the ITT population. After a median follow-up of 12.8 months, pembrolizumab plus axitinib showed an increased 12-month survival rate (89.9% vs. 78.3%, HR: 0.53; 95% CI: 0.38-0.74; P < 0.0001), longer PFS (15.1 months vs. 11.1 months, HR 0.69; 95% CI, 0.57-0.84; P < 0.001), and a higher ORR (59.3% vs. 35.7%, P < 0.001). The benefit of combination therapy was shown across all IMDC-risk groups and different PD-L1 expression status. Grade 3 or higher adverse events of any cause occurred in 75.8% and 70.6% of the patients in the combination arm and sunitinib arm, respectively. The combination arm led to the discontinuation of both the drugs in 10.7% of the patients and dose reduction of axitinib in 20.3%. On the other hand, the sunitinib arm led to the discontinuation in 13.9% of the patients and dose reduction in 30.1%. Given the promising results, the FDA approves pembrolizumab plus axitinib for first-line treatment of advanced RCC in April 2019.

Avelumab plus axitinib
Avelumab, an anti-PD-L1 monoclonal antibody, had been studied in combination with axitinib in treating mRCC patients in the first-line setting. The JAVELIN Renal 100 study is an open-label, dose-finding, and dose-expansion Phase Ib study. [38] In the dose-finding phase, patients received 5 mg axitinib orally twice daily for 7 days, followed by the combination therapy with 10 mg/kg avelumab intravenously every 2 weeks and 5 mg axitinib orally twice daily. At the cutoff date, 32 of the 55 patients (58%) had a confirmed response. Grade 3 or higher TRAE occurred in 58% of the patients, the most common being hypertension (29%), increased the level of alanine aminotransferase, amylase, and lipase (7%), and palmar-plantar erythrodysesthesia syndrome (7%). These findings led to the subsequent Phase III JAVELIN Renal 101 study, which enrolled a total of 886 clear-cell mRCC patients who did not receive previous systemic treatment. [29] Patients were randomized to receive avelumab plus axitinib or sunitinib monotherapy. The primary endpoints were PFS and OS among PD-L1-positive population. The combination therapy significantly prolonged PFS in PD-L1-positive population (13.8 months vs. 7.2 months, HR: 0.61; 95% CI: 0.47-0.79; P < 0.001) and ITT population (13.8 months vs. 8.4 months, HR: 0.69; 95% CI: 0.56-0.84; P < 0.001). Among the patients with PD-L1-positive tumors, the ORR was also higher in patients receiving combination therapy (55.2% vs. 25.5%). The benefits of PFS and ORR were demonstrated regardless of MSKCC/IMDC-risk groups. At a median follow-up of 11.6 months and 10.7 months for OS in the two arms, 13.7% and 15.2% of the patients had died, respectively (HR: 0.82; 95% CI: 0.53-1.28; P = 0.38). The Grade 3 or worse TRAE was around 71% in both the arms. Based on these findings, the FDA approved avelumab in combination with axitinib for the first-line treatment of patients with advanced RCC in May 2019. Other combinations of ICI and antiangiogenic agents are underway and summarized in Table 2.

novEl agEnts in rEnal cEll carcinoMa
In addition to ICI and the combination of antiangiogenetic drugs, many novel agents have been developed in mRCC. AM0010 is a pegylated IL-10, which induces IFN-γ and suppresses the transforming growth factor-β (TGF-β). [39] The combination of AM0010 with pembrolizumab or nivolumab demonstrated an ORR of 40%. [40,41] NKTR-214 is a pegylated IL-2 prodrug, which interferes with the binding to IL-2 receptor but preserves the binding to IL-2 receptor-beta. This modification results in a five-hundred-fold increase of IL-2 concentration within tumors and preferentially stimulates effector T-cells with minimal effects on regulatory T-cells. [42] NKTR-214 combined with nivolumab had been studied in the PIVOT study, which showed an ORR of 54%. [43] A number of agents including inhibitors of indoleamine 2,3-dioxygenase, TGF-β, lymphocyte-activated gene 3, adenosine, and glutamine are under the investigation in RCC. [44] Many trials evaluating the combination of novel agents and ICI are ongoing [ Table 2].

futurE considErations
Despite the promising efficacy of ICI and anti-VEGF combination therapies, some issues need to be considered. First, the high incidence of TRAE is a concern. The I Avelumab + gemcitabine in sarcomatoid RCC CSF1R: Colony-stimulating factor 1 receptor, HDAC: Histone deacetylase, ICI: Immune checkpoint inhibitor, IL-2: Interleukin 2, JAK1: Janus kinase 1, NSCLC: Nonsmall cell lung cancer, PI3K: Phosphoinositide 3-kinase, RCC: Renal cell carcinoma, mRCC: Metastatic RCC, UC: Urothelial carcinoma, VEGF: Vascular endothelial growth factor identification of predictive biomarkers is urgently required to balance the benefit and toxicities. Second, whether we should treat mRCC patients with ICI and anti-VEGF agents or in sequence in first-line setting remains elusive. Third, there are patients who have primary or acquired resistance after the treatment of ICI. Further investigation to potentiate the efficacy of current immunotherapy agents is mandatory. On the other hand, adjuvant cytokines had been studied to prevent a recurrence, which revealed no additional benefit. [45][46][47] Nevertheless, given the success of newer immunotherapy agents, there are ongoing clinical trials to evaluate the role of ICIs in the adjuvant setting.

conclusions
The landscape of the treatment of mRCC has shifted from anti-VEGF therapy to ICI and their combination with antiangiogenic agents. Further studies about biomarker identification, drug sequencing, and research of innovate combinations are needed. With the advance of immunotherapy-based treatment, the survival time of mRCC is expected to be longer and the potential cure rate to be higher.

Financial support and sponsorship
Nil.

Conflicts of interests
There are no conflicts of interests.