RCC evokes an immune response, which has occasionally resulted in spontaneous and dramatic remissions. In an attempt to reproduce or accentuate this response, various immunotherapeutic strategies have been used, including nonspecific stimulators of the immune system, specific antitumour immunotherapy, adoptive immunotherapy, the induction of a graft-vs-tumour response via allogeneic haematopoietic stem cell transplantation, and the administration of partially purified or recombinant cytokines. Although many such therapies show hints of antitumour activity, the most consistent clinical results are with cytokines interleukin-2 (IL-2) and interferon-α (IFNα). In this review we examine the clinical results of immunotherapy in RCC, focusing on the clinical trials of IL-2 and IFNα. With the advent of novel therapies that target angiogenesis/signal transduction and offer more robust clinical benefit, the current role of immunotherapy in RCC will be explored, with emphasis on patient selection and combined strategies that aim to maximize the benefits of cytokine therapy.
IMMUNOBIOLOGY OF RCC
There are several inherent immune defects in RCC tumours, which support the notion that reversing this immunosuppression could be associated with an antitumour effect. RCC tumour infiltration by dendritic cells (DCs) is decreased in patients with RCC . RCC-derived tumour products have been shown to promote altered maturation and apoptosis of DCs . Furthermore, RCC-associated DCs manifest an immature/inactivated phenotype with impaired antigen-presenting capability . Thus, alterations in DC number and function present in patients with RCC might contribute to inherent immunodeficiency and tumour progression. In patients with RCC, T cell reactions to tumour-associated antigens are highly skewed toward immunosuppressive cytokine production by T helper cells, which represents a mechanism by which RCC can inhibit antitumour immunity . Relevant to invoking a therapeutic immune response, depletion of immunosuppressive T regulatory cells has been shown to enhance antitumour activity in mouse models of RCC and in humans with RCC receiving tumour RNA-transfected DC vaccines [5–7]. The full spectrum of RCC immunobiology is beyond the scope of this review and is adequately presented elsewhere [8–10]. Nonetheless, it is clear that the immune response is implicated in the pathogenesis of RCC and thus allows rational application of immunotherapy to promote an antitumour effect.
LOW-DOSE CYTOKINE THERAPY
IFNα is a naturally occurring glycoprotein produced in response to viral infections and foreign antigens. It has been investigated as an antitumour agent in various diseases, with postulated mechanisms of action including immunomodulation, antiproliferative activity and inhibition of angiogenesis. In advanced RCC both recombinant IFN-α2a (Roferon, Hoffmann-La Roche, Basel, Switzerland) and IFN-α2b (Intron A, Schering Plough International, Kenilworth, NJ) have been used. There is no clinically meaningful difference between these two IFNαs and thus the generic IFNα will be used to describe these data. Overall response rates of 10–15% were repeatedly demonstrated in large series. Responses are often delayed in onset, with the median time to response being ≈ 4 months. Most responses are partial, with a median response duration of 6–7 months; ≈ 2% of patients have complete responses, with occasional patients having a response persisting for >1 year.
Several randomized trials were done to investigate a possible survival benefit of IFNα in RCC; Table 1[11–13] is a summary of randomized trials investigating the effect of IFNα on overall survival (OS) in patients with metastatic RCC. One study randomized 350 patients with metastatic RCC to receive IFNα 10 million units (MU) 3 ×/week for 12 weeks or medroxyprogesterone (MPA) 300 mg daily for 12 weeks . Patients in each treatment arm were well balanced for prognostic characteristics in RCC, including performance status, time from first diagnosis of RCC to treatment, number of metastatic sites and nephrectomy status. This trial was closed at an interim analysis when the stopping boundary for a survival advantage of IFNα had been reached. Intent-to-treat analysis showed a significant OS advantage for patients randomized to IFNα, with a hazard ratio of 0.72 (P = 0.017). The median OS was 8.5 months in the IFNα arm and 6.0 months in the MPA arm.
Table 1. The use of IFNα in metastatic RCC
Response rate, % advantage for IFNα
OS impact, months advantage for IFNα (P)
MRC, Medical Research Council; Collab., collaborators; RCT, randomized controlled trial.
Another study randomized 160 patients with advanced, progressive RCC to receive IFNα 18 MU 3 ×/week plus vinblastine 0.1 mg/kg i.v. for 3 weeks or the same dose and schedule of vinblastine alone . The primary endpoint of this trial was OS, with 80% power to detect a difference in median OS of 12 vs 8 months. Patients in each treatment arm were well balanced for characteristics known to be prognostic for OS in RCC. There was a significant OS advantage for the IFNα arm, with a median OS of 15.8 months, vs 8.8 months for the vinblastine arm (P = 0.005). There were also significant differences in overall response rates (16.5% vs 2.4%; P = 0.003), complete response rates (8.9% vs 1.2%) and median time to disease progression (3 vs 2 months; P < 0.001), all favouring the IFNα arm.
In addition, a recent meta-analysis reviewed 53 randomized controlled trials involving 6117 patients treated between 1995 and 2004 with IL-2 or IFNα for metastatic RCC . There were four trials (644 patients in all) that randomized patients to IFNα vs other than IFNα as the control arm, including the two largest trials noted above. The weighted median improvement in survival with IFNα vs control was 3.8 months (P = 0.007) with an odds ratio for death at 1 year of 0.56 for IFNα (95% CI 0.40–0.77). There was no evidence of dose–response relationship and no correlation between response rate and OS.
IL-2 is an important member of a class of glycoproteins that regulate lymphocyte function and growth. It is produced in response to infection and essential for discriminating between self and foreign antigens. Although the mechanism of action of IL-2 is not completely understood, antitumour effects in murine models have been linked to the direct killing of tumour cells by activated T cells and natural killer cells [14,15].
Efforts to enhance the therapeutic index of IL-2 have led to the investigation of regimens involving lower doses of IL-2 and different routes of administration. Approaches have included outpatient s.c. therapy, and low/intermediate dose i.v. therapy. Although many initial reports were encouraging, long-term follow-up data and expanded phase II data have generally been lacking. Several studies have examined the effectiveness and toxicity of outpatient s.c. administered IL-2 [16–18]; in one series of 27 outpatients, daily s.c. IL-2 resulted in a 22% response rate (two complete and four partial responses) . Two patients had ongoing complete responses of 19 and 17 months at the time of publication in 1992; however, no additional information was reported. In another report describing results of s.c. IL-2 therapy, only one of 16 patients had a partial remission. Sleijfer et al. extensively studied a regimen in which each IL-2 dose is 10% (72 000 IU/kg) of the standard high-dose i.v. dose. The initial report on the first 65 patients with metastatic RCC described a 15% response rate .
A large randomized trial using both low-dose IL-2 and IFNα showed a similar clinical benefit for each cytokine and failed to show an OS benefit for the combined low-dose cytokines vs single-agent, with significantly more toxicity in the combined arm . No prospective randomized trial has reported any benefit of adding any non-cytokine agents to cytokines. More recent data from the French Immunotherapy Group studied the impact of low-dose cytokine therapy on survival in patients with intermediate prognosis as defined by previous studies with these cytokines . Patients with untreated metastatic RCC and a Karnofsky performance status of ≥ 80 and more than one site of metastatic disease, excluding hepatic metastases, were randomized to receive MPA, s.c. IFNα, s.c. IL-2 or combined cytokine therapy. In all, 492 patients were randomized and the treatment groups were well balanced for predictors of response and survival. Patients receiving IL-2 vs other than IL-2 were compared, as were patients receiving IFNα vs other than IFNα regimens in a 2 × 2 factorial design. There was no significant difference among the treatment arms. The inclusion of cytokine monotherapy arms with MPA in this analysis does not allow for an adequately powered comparison of cytokine vs non-cytokine therapy in these patients. Given the abundant data about survival benefit with IFNα alone, these results must be viewed cautiously. Furthermore, these results do not preclude a role for low-dose cytokines in properly selected patients of good prognosis or in combination with other agents.
High-dose bolus IL-2 was approved by the Food and Drug Administration in 1992 for its ability to produce durable tumour responses in a small percentage of patients with metastatic RCC. However, this regimen was associated with substantial toxicity and limited efficacy, which narrowed its application to highly selected patients treated at specialized centres [22–24]. In recent years, the relative merits of these low- and high-dose IL-2 regimens was clarified by the results of randomized trials (Table 2) [20,25,26]. More significantly, laboratory investigations associated with this clinical research suggest that the potential exists for identifying predictors of response (or resistance) and thus enhancing the ability to apply IL-2 therapy to those most likely to benefit.
Table 2. Selected randomized trials of cytokine therapy in metastatic RCC
Response rate, %
Durable CR, %
P, OS difference
FIG, French Immunotherapy Group; NCI SB, National Cancer Institute Surgery Branch; CWG, Cytokine Working Group; HD, high-dose; LD, low-dose; Ci.v., continuous i.v. infusion; NS, not statistically significant; CR, complete response; NR, not reported. *The durable CR rate was 7 vs 0, favoring HD IL-2 (unpublished data)
The French Immunotherapy Group conducted a large-scale, phase III randomized trial comparing intermediate-dose IL-2 administered by continuous i.v. infusion plus s.c. IFNα with either IL-2 or IFNα administered alone . The three treatment groups (425 patients in all) were well balanced for known predictors of response and survival. The response rate and 1-year event-free survival were significantly greater for the combined IL-2 and IFNα arm than for either of the single-agent arms, although there was no significant difference in OS among the three groups. Notably, there were responses in only 6.5% and 7.5% of patients receiving IL-2 or IFNα alone, respectively, with only 2.9% and 6.1% of these patients still responding at the week 25 evaluations. Although there was more antitumour activity with the combination arm, this was largely due to the rather limited activity of the single-agent regimens. The comparative efficacy of an intermediate-dose combination of IL-2 and IFNα and high-dose IL-2 alone remained to be established.
The National Cancer Institute Surgery Branch investigators performed a randomized trial comparing standard high-dose i.v. bolus IL-2 and a low-dose i.v. bolus IL-2 regimen developed by Yang et al.. After randomizing 117 patients, a third arm was added involving s.c. IL-2 administered according to the regimen described by Sleijfer et al.. The results were analysed and reported according to groups that were concurrently randomized. Among the 306 patients concurrently assigned to either high- or low-dose i.v. IL-2, the response rate was significantly higher with high-dose therapy (21% vs 13%), with a trend towards more durable responses. The response was more durable in patients who received the high-dose i.v. IL-2 than in those who received the low-dose i.v. IL-2; there were no differences in OS. Although toxicities were also significantly greater in the high-dose group (particularly hypotension), there were no deaths attributable to IL-2 in either arm and patient assessments of quality of life were found to be roughly equivalent. Among the patients concurrently assigned to either s.c. IL-2 or high-dose i.v. IL-2, the response rate was higher with high-dose i.v. IL-2 (21% vs 10%) but the difference was of borderline statistical significance (P = 0.048). Once again, there were no differences in OS.
In an effort to determine the value of outpatient s.c. IL-2 and IFNα relative to high-dose i.v. IL-2 the Cytokine Working Group (CWG) performed a phase III trial in which patients were randomized to receive either outpatient IL-2 and IFNα every 6 weeks or standard high-dose inpatient IL-2 every 12 weeks . In all, 193 patients were enrolled and 192 were evaluable for toxicity and tumour response. The response rate for high-dose IL-2 was 23%, vs 10% for IL-2/IFNα (P = 0.018). Eight patients had a complete response on high-dose IL-2, vs three on low-dose IL-2/IFNα. The median response duration was 24 months for high-dose IL-2 and 15 months for IL-2/IFNα (P = 0.18). The median OS was 17.5 and 13 months (P = 0.12), favouring high-dose IL-2. Ten patients on high-dose IL-2 were progression-free at 3 years, vs three on IL-2/IFNα (P = 0.08). Of note, responses to high-dose IL-2 were seen with equal frequency across the stratification criteria, while low-dose IL-2/IFNα appeared to produce fewer responses in patients with liver and/or bone metastases and in those who had not had a previous nephrectomy to remove the primary tumour. For patients with bone or liver metastases (P = 0.001) or primary in place (P = 0.04) survival was better with high-dose IL-2 than with IL-2/IFNα, while there were no significant survival differences between the treatments for patients who had had a previous nephrectomy or who were without bone or liver metastases (Fig. 1). The explanation for this unanticipated result is not readily apparent. It is possible that higher serum or tissue IL-2 levels are needed either to overcome the immune suppression associated with greater tumour burden, or to activate T cells in sites of disease other than lung and soft tissue. Consequently, it seems that patients with liver or bone metastases or unresected primaries represent a group of patients who seem to require a more intensive IL-2 regimen to achieve clinical benefit, whereas the impact of dose is less critical in patients with resected primaries and tumour confined to lung.
Taken together, these studies suggest that high-dose i.v. bolus IL-2 is better in terms of response rate and possibly response quality than regimens involving low-dose cytokines (Table 2). The superiority of high-dose IL-2 is particularly apparent in patients with tumour metastases in immune-sequestered sites, such as liver or bone, or who have their primary tumour in place, or who fall into the intermediate- or poor-risk groups defined by the French Immunotherapy Group. High-dose i.v. IL-2 remains a standard of care for appropriately selected patients with access to such therapy, given its ability to produce responses. However, as with IFNα, the toxicity and limited efficacy of high-dose i.v. IL-2 therapy mandates additional efforts at better defining the patients for whom this therapy is appropriate, and exploring combined therapy to maximize benefit.
THE FUTURE OF CYTOKINE THERAPY IN METASTATIC RCC
Recently, therapies targeted against vascular endothelial growth factor and other molecular targets, e.g. mammalian target of rapamycin (mTOR), have produced robust clinical effects in this disease [27–29]. As such, the role of cytokines has been critically re-examined. It is clear that further investigation into the proper selection of patients and combined therapy will be required to optimize the benefits of cytokine therapy in metastatic RCC.
PATHOLOGICAL AND MOLECULAR PREDICTORS OF RESPONSE TO IL-2
Responses to immunotherapy are most frequent in patients with RCC of clear cell histology [30,31]. This observation was detailed in a retrospective analysis of pathology specimens obtained from 231 patients who had received IL-2 therapy on CWG clinical trials . For patients with primary tumour specimens available for review, the response rate to IL-2 was 21% (30 of 146) for patients with primary tumours of clear cell histology, compared to 6% for patients with other than clear cell histology (one responder of 17). Among the patients with clear cell RCC a response to IL-2 was also associated with the presence of alveolar features, and the absence of papillary or granular features. The response rate in patients whose primary tumours had ‘good’ predictive features (e.g. >50% alveolar and no granular or papillary features) was 39% (14 of 36). In addition, patients with primary tumours containing ‘intermediate’ predictive features (e.g. alveolar but not papillary features and <50% granular features) had a response rate of 19% (15 of 77). Patients with tumours containing ‘poor’ predictive features (e.g. >50% granular or any papillary features) had a response rate of 3% (one of 33). The median survival for all patients with clear cell tumours by risk group was 2.87, 1.36 and 0.87 years, respectively (P < 0.001). As a result of these data, it might be appropriate for patients whose primary tumour is of other than clear cell histology or of clear cell histology but with ‘poor’ predictive features to forego IL-2-based treatment altogether. However, given that even in the most favourable predictive group, over half the patients failed to respond to IL-2 therapy, additional investigations into tumour-associated predictors of responsiveness to IL-2 are necessary.
Some investigators have begun to examine tumour tissue to identify immunohistochemical markers that might predict the outcomes of patients with RCC. Carbonic anhydrase IX (CAIX) has been identified as one potential marker. Bui et al. used a monoclonal antibody designed to detect CAIX expression to analyse immunohistochemically paraffin-embedded RCC specimens. They showed that >90% of RCC express CAIX and that its expression decreases with advancing stage. In their analysis, there was high CAIX expression in primary tumours in 79% of patients and it was associated with improved survival and possibly response to IL-2-based therapy. In addition, all long-term responders to IL-2-based treatment had high CAIX expression. In that study, low CAIX expression was associated with a worse outcome for patients with locally advanced RCC and was an independent predictor of outcome in patients with metastatic disease.
Building on this work, Atkins et al. performed a nested case-control study within the larger cohort of patients whose pathology was analysed. CAIX expression levels were correlated with response to IL-2, pathological risk category and survival. As in the report by Bui et al., the percentage of CAIX-positive tumour cells was used to separate those with high (>85%) and low (≤85%) expression. Of 66 selected patients, 27 (41%) had responded to IL-2-based regimens, with 20 (30%) remaining alive at a median follow-up of 2.6 years; 24 (36%), 31 (47%) and 11 (17%) were classified into good-, intermediate-, or high-risk groups according to the pathology model described above. Forty-one specimens (62%) had high CAIX expression. Of 27 responding patients, 21 (78%) had high CAIX expression, vs 20/39 (51%) nonresponders (odds ratio 3.3, P = 0.04). The median survival was 3 and 1 year for high and low CAIX expression, respectively (P = 0.04). Even though there was a tumour response in six patients with low CAIX staining, survival was >5 years only in the patients with high CAIX-expressing tumours. High CAIX staining was associated with better pathology features but remained an independent predictor of response. For example, in patients within the intermediate pathology group, all nine responders had high CAIX expression, vs only 11 of 22 nonresponders. A two-compartment model was proposed in which one group of patients with either good pathology or intermediate pathology and high CAIX expression contained 26 of 27 (96%) responders, compared with only 18 of 39 (46%) nonresponders (odds ratio 30; P < 0.01). There was also a significant survival benefit for this group (P < 0.01).
That this analysis was enriched for responding patients makes it inappropriate to report response rates. However, if this model were applied to an unselected population of patients with RCC receiving IL-2 therapy, about half of patients would be estimated in each risk group, and the response rate would be 35–40% for the good-risk group and <5% for the poor-risk group. Although this model and these assumptions require prospective validation, it highlights the potential for using pathological and molecular features of the tumour to identify the best patients to receive IL-2 therapy. Additional studies to explain these preliminary observations and correlate results with previously described clinical features are necessary.
FUTURE CLINICAL TRIALS
This year, the CWG will launch the high-dose IL-2 ‘Select’ Trial, the primary objective of which is to determine prospectively if the predictive model proposed by Atkins et al. can identify a group of patients with advanced RCC who are significantly more likely to respond to high-dose IL-2-based therapy (‘good’ risk) than a historical, unselected patient population . New factors (including baseline immune function, immunohistochemical markers and gene expression patterns) that might be associated with response to high-dose IL-2 therapy will also be explored, in an attempt to more narrowly limit the application of IL-2 to those patients most likely to benefit. If this cannot be accomplished, the application of high-dose IL-2 therapy in patients with metastatic RCC will probably be narrowed further.
IFNα is also being further investigated; combination studies with some of the newer targeted agents such as bevacizumab, sunitinib and sorafenib are underway. Further, patient selection based on tumour biology is being explored. For example, a recent clinical trial of IFNα and the cyclooxygenase-2 inhibitor in metastatic RCC patients, identified maximum cyclooxygenase-2 expression in the primary tumour as potentially identifying patients responsive to this combined therapy . Although this initial trial was only hypothesis-generating, it serves as an example of the type of patient selection/combination trial that might allow for continued utility of cytokines in metastatic RCC.
In an effort to apply the clinical activity observed with cytokines to patients with earlier stages of disease, various of adjuvant trials have been reported. The Eastern Cooperative Oncology Group completed a trial comparing adjuvant IFNα with observation in patients with high-risk resected RCC. Eligible patients were to be T3b-c, T4 and/or N1-N3. Patients were randomly assigned to receive either a year of IFNα or routine observation. With a minimum follow-up of 36 months and a mean of 68 months overall, there was no statistically significant difference in disease-free survival between the treatment arms . A similar study by the European Organisation for the Research and Treatment of Cancer also showed no benefit for the adjuvant administration of IFNα.
The CWG performed a trial randomly assigning patients who satisfied these high-risk staging criteria (stage T3b-4, N1-3 or resected metastatic disease) to either a single cycle of high-dose IL-2 or observation (with IL-2-based therapy at the time of recurrence) . This study took several years to accrue 69 patients and ultimately was closed early after an interim analysis determined that the anticipated 30% improvement in disease-free survival for the patients receiving high-dose IL-2 could not be achieved. Thus, there is currently no evidence to support the use IFNα or IL-2 in the adjuvant setting in patients with high-risk renal cancer. These studies were compromised by the inability to clearly define a population at high risk of recurrence, the increasing lack of availability of such high-risk patients, and mostly by the limited antitumour effect of cytokines.
Metastatic RCC has long been a testing ground for novel immunotherapy. Several such approaches, including vaccination and allogeneic bone marrow transplantation, have been tested over the past two decades. Vaccination therapy showed the generation of potentially relevant immune responses, although any clinical benefit and objective responses are not consistent [38–40]. Allogeneic bone marrow transplant attempts to induce a graft-vs-tumour effect in the patient through the transfer of a sibling’s bone marrow stem cells. Although initial reports were encouraging, further clinical trials highlighted the potential toxicity and limited applicability of this approach [41,42]. Active investigation into immunotherapeutic approaches in metastatic RCC are still being pursued, although it is clear that such approaches must now be clinically developed, accounting for the clinical effectiveness and widespread use of targeted therapy.
RCC has long been considered an immunologically influenced malignancy and thus served as a platform for the clinical testing of anticancer immunotherapy. Based on known defects in immune function in RCC, several immunotherapeutic approaches have been investigated in this disease. The nonspecific cytokines IL-2 and IFNα have had the most testing, producing modest benefits for the entire cohort of unselected patients, and dramatic and durable complete responses in a few patients. Additional immunotherapeutic strategies have been tested in metastatic RCC, but definitive evidence of clinical benefit is lacking. The advent and clinical effect of targeted therapy in RCC does not eliminate the potential utility of cytokine therapy in RCC, but rather requires a rational refinement of this therapy through patient selection and combined regimens to maximize the clinical effect.