Immune priming before intravesical bacillus Calmette-Guérin in non-muscle-invasive bladder cancer: lessons and perspectives from the RUTIVAC-1 trial

Intravesical bacillus Calmette-Guérin (BCG) therapy remains the cornerstone of treatment for patients with non-muscle-invasive bladder cancer (NMIBC). The standard treatment paradigm consists of a 6-week induction course, followed by maintenance therapy for 1 year in intermediate-risk and up to 3 years in high-risk (HR) disease (1,2). Nevertheless, the response to BCG is heterogeneous and influenced by a complex interplay of tumor- and patient-specific factors, as well as treatment-related variables, including completion of maintenance therapy and, in some reports, the specific BCG strain administered (1-4). Despite adherence to guideline-recommended therapy, disease recurrence occurs in up to 40% of patients with NMIBC within 2 years, ultimately resulting in BCG-unresponsive disease (5). This clinical entity is characterized by aggressive biological behavior and poor oncologic outcomes, with 5-year progression rates exceeding 25% (6).

As such, improving the therapeutic efficacy of intravesical BCG remains a critical unmet need to reduce treatment failure and improve long-term outcomes. One proposed strategy has involved combining BCG with a systemic immune checkpoint inhibitor to counteract tumor immune-evasion mechanisms mediated through the programmed death-1 (PD-1)/programmed death-ligand 1 (PD-L1) axis. Three randomized trials, POTOMAC, CREST, and ALBAN, have evaluated BCG combined with systemic durvalumab, sasanlimab, and atezolizumab, respectively, in BCG-naïve patients with HR-NMIBC. Across these studies, the addition of checkpoint inhibitors resulted in only a numerical improvement in event-free survival compared with BCG alone, without a consistent or clinically meaningful reduction in progression rates (7). Importantly, these modest efficacy signals came at the cost of substantially increased toxicity, with overall and grade ≥3 treatment-related adverse events occurring approximately four-fold more frequently with combination therapy than with BCG alone. When coupled with higher treatment costs and increased logistical complexity, these findings limit the practicality of widespread adoption of systemic checkpoint inhibition in the BCG-naïve setting.

These limitations underscore the need for alternative, biologically informed strategies to enhance BCG efficacy while preserving its favorable risk-benefit profile. Intravesical BCG functions as an immunotherapy, exerting its antitumor effects through the induction of potent local innate and adaptive immune responses that engage multiple interacting immunologic pathways and diverse immune cell populations within the tumor microenvironment (8). Although the full complexity of these pathways has yet to be fully elucidated, leveraging immune-priming strategies represents a biologically plausible approach to enhance the therapeutic activity of intravesical BCG. Immune priming refers to the process by which an initial exposure to a pathogen, vaccine, or immunologic stimulus enhances the immune system’s ability to mount a more effective response upon subsequent encounters with the same or related antigens (9).

Building on the concept of immune priming in BCG-based therapy, one strategy that has been explored in NMIBC is the use of a double-induction BCG regimen, in which an initial induction course serves as a priming exposure for a subsequent second induction. In a phase II clinical trial, 75 patients with HR-NMIBC received two induction courses of BCG (12 intravesical instillations) without maintenance therapy (10). After a median follow-up of 4.4 years, the study demonstrated a favorable 5-year recurrence-free survival (RFS) rate of 69% [95% confidence interval (CI): 58–81%] (10). Notably, this finding is comparable to the approximately 60% 5-year RFS reported in the Southwest Oncology Group (SWOG)-8507 trial, in which patients received a single induction course followed by 3 years of maintenance BCG, recognizing that SWOG-8507 was conducted in the early 2000s using different risk stratification criteria and a potentially lower baseline recurrence risk than contemporary high-risk cohorts (11). Importantly, no long-term toxic effects were reported, although detailed data on short-term or grade-specific adverse events were not provided. The regimen also required fewer total BCG instillations compared with standard induction plus maintenance schedules (12 vs. 27) and demonstrated a durable response, with only three bladder cancer-related deaths over five years, corresponding to a cancer-specific survival of 97%, a finding of particular relevance in the context of ongoing global BCG shortages. Despite these encouraging findings, the double-induction approach has several important limitations. The available evidence is derived from a single, nonrandomized phase II study without a control arm, precluding definitive conclusions regarding efficacy. In addition, increased cumulative exposure to BCG may lead to greater local and systemic BCG-related toxicity, potentially compromising treatment adherence, while ongoing global BCG shortages raise concerns regarding the feasibility of intensified dosing strategies (12,13). Lastly, the absence of immune correlative analyses precludes determination of whether the observed benefit reflects true immune priming or nonspecific antigenic exposure that could contribute to immune exhaustion.

Complementary to intensified intravesical schedules, homologous immune priming may also be achieved through systemic exposure to BCG prior to intravesical therapy. Preclinical murine studies have demonstrated that subcutaneous BCG administered before intravesical therapy induces BCG-specific T cells, enhances their trafficking to the bladder, increases local immune-cell infiltration, and amplifies inflammatory responses within the bladder microenvironment (14). A pilot study evaluated the feasibility of administering percutaneous BCG 21 days before intravesical BCG therapy in patients with high-grade NMIBC. At 3 months, 11 of 13 patients (85%) remained disease-free. Percutaneous BCG was well tolerated, with no severe adverse events reported. In vitro analyses further demonstrated that immune priming enhanced activation of natural killer (NK) and Gamma delta T lymphocytes (γδ T cells), key innate lymphocyte populations implicated in BCG-mediated antitumor activity, and increased their cytotoxicity against bladder cancer cell lines (15). However, despite these encouraging early findings, preliminary results from the large, three-arm randomized SWOG S1602 trial, which included 984 treatment-naïve patients with HR-NMIBC, failed to demonstrate a significant long-term benefit of systemic BCG priming (16). This trial compared intravesical TICE BCG with intravesical Tokyo-172 BCG (arms 1 and 2) and evaluated intradermal BCG priming prior to Tokyo-172 intravesical induction (arm 3). Five-year high-grade recurrence-free survival (HG-RFS) rates did not differ significantly between study arms, with 64% in patients receiving intravesical Tokyo-172 alone and 63% in those undergoing intradermal priming prior to Tokyo-172 intravesical induction, suggesting no additional benefit from systemic priming (16,17).

To extend immune-priming strategies beyond homologous BCG exposure, Buisan et  al. evaluated a heterologous prime-boost approach using the RUTI vaccine, a poly-antigenic formulation composed of fragmented, purified, heat-inactivated Mycobacterium tuberculosis bacilli, designed to enhance immune responses to intravesical BCG in patients with HR-NMIBC (18). In RUTIVAC-1, a randomized double-blind phase I trial, 40 patients received either two subcutaneous doses of the non-live RUTI vaccine or placebo prior to standard BCG induction. RUTI priming induced robust systemic immune activation before BCG exposure, characterized by marked expansion of activated CD4⁺ and CD8⁺ T-cell populations. In contrast, placebo recipients demonstrated an early increase in regulatory T cells (Tregs), a suppressive immune shift that was largely absent in the RUTI-treated group. In addition, RUTI vaccination elicited a broader and more polyfunctional cytokine response, including enhanced interferon-γ and interleukin-2 (IL-2) production. Although not powered for efficacy, exploratory follow-up suggested favorable trends in 5-year recurrence-free and progression-free survival among RUTI-treated patients. The vaccine was well tolerated, with only mild injection-site reactions reported. A limitation of the RUTIVAC-1 trial, however, was the limited scope of immune phenotyping, which focused primarily on peripheral T-cell subsets and did not evaluate innate immune populations, functional immune states, or immune responses within the bladder tumor microenvironment (18).

Despite these encouraging immunologic signals, the clinical interpretability of the immune findings from the RUTIVAC-1 trial remains limited. The study was small and insufficiently powered to assess the impact of immune priming on oncologic endpoints. Moreover, baseline risk factors were unevenly distributed between the treatment arms, a limitation explicitly acknowledged by the authors, which further undermines confidence in the exploratory recurrence-free and progression-free survival analyses.

In the RUTIVAC-1 trial and similar studies evaluating immune-priming strategies, future investigations would benefit from a comprehensive assessment of immune infiltration within the bladder mucosa of patients receiving the RUTI vaccine or similar agents, correlated with parallel analyses of peripheral T-cell subsets. The RUTIVAC-1 trial primarily focused on changes in peripheral T-cell subsets (18), while several immune populations known to play central roles in BCG responsiveness, including innate immune cells, NK cells, and γδ T cells, were not evaluated, despite their established contributions to cytotoxic activity, antigen presentation, and interferon-γ-mediated antitumor immunity (8). In the absence of comprehensive mucosal immune profiling, it remains uncertain whether RUTI priming meaningfully enhances the local immune cascade through which BCG exerts its antitumor effects.

Despite these constraints, the RUTIVAC-1 trial is notable for its conceptual novelty. Unlike homologous prime-boost approaches using repeated exposure to live BCG, RUTI represents a heterologous priming strategy, which may fundamentally alter the quality of subsequent immune responses. By delivering fragmented mycobacterial antigens in a non-live formulation while avoiding persistent stimulation by live mycobacteria, heterologous priming may preferentially reinforce T helper 1 (TH1)-polarized immunity while mitigating regulatory T-cell expansion. In contrast, repeated homologous priming with BCG alone could theoretically promote compensatory immune regulation, including accumulation of Tregs, potentially blunting the net antitumor benefit. This distinction is noteworthy in light of the negative results of homologous systemic BCG priming observed in SWOG S1602 (16,17).

Thus, efforts to optimize BCG therapy must consider the complex interplay between BCG-induced immune activation and tumor immune-evasion mechanisms. Within this dynamic environment, certain immune cell populations may paradoxically contribute to immune suppression, cytotoxic T-cell silencing, or dysregulated inflammation (8). These converging pathways diminish effective antitumor immunity and may ultimately facilitate tumor persistence or progression (Figure 1). Identifying biomarkers of BCG resistance is therefore essential and should be tailored to tumor-specific molecular features. Ideally, once clinically applicable biomarkers or multi-omic classifiers are established, clinicians will be able to determine upfront whether BCG is an appropriate therapy for a given tumor profile, and, where relevant, whether adding immune priming could enhance therapeutic efficacy. This approach may prove more rational than attempting to optimize BCG in tumors inherently unlikely to benefit based on their biological signatures.

Figure 1 Interplay between BCG-activated immunity and tumor immune evasion mechanisms. Some elements of this figure were adapted from Servier Medical Art (https://smart.servier.com), licensed under the Creative Commons Attribution 4.0 International License (CC BY 4.0; https://creativecommons.org/licenses/by/4.0/). BCG, bacillus Calmette-Guérin; CD, cluster of differentiation; NK, natural killer; Tregs, regulatory T cells; γδ T cells, gamma delta T lymphocytes.

Given this mechanistic complexity, the bladder tumor microenvironment poses significant challenges for systemic immune-priming strategies designed to enhance BCG responsiveness. Whether priming can reliably amplify the full spectrum of BCG-mediated antitumor pathways, while avoiding the unintended promotion of immune-evasion mechanisms, remains an important unanswered question.

Finally, these observations raise several unresolved questions regarding host immune competence and the mechanisms underlying durable responses to BCG. In particular, whether systemic immune-priming strategies can meaningfully enhance BCG efficacy in elderly patients or those with baseline immunosuppression remains unclear. Notably, intravesical BCG continues to demonstrate clinical activity in populations traditionally considered immunosenescent or immune-compromised, suggesting that BCG may engage immune pathways that are distinct from, or partially independent of, classical adaptive immune capacity. Elucidating why BCG retains antitumor activity in these settings, and whether immune priming can selectively amplify beneficial innate or trained immunity responses without exacerbating immune exhaustion or toxicity, represents an important area for future investigation.

Collectively, the available data on immune priming prior to intravesical BCG therapy underscore the critical need for a well-powered, prospective, randomized trial specifically evaluating heterologous RUTI priming before BCG induction, incorporating comprehensive tumor-level immune profiling alongside robust clinical endpoints. Until such evidence is available, immune-priming strategies, whether homologous or heterologous, should remain investigational, and no changes to current clinical management can be justified.

Acknowledgments

None.

Footnote

Provenance and Peer Review: This article was commissioned by the editorial office, AME Clinical Trials Review. The article has undergone external peer review.

Peer Review File: Available at https://actr.amegroups.com/article/view/10.21037/actr-26-0068/prf

Funding: None.

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://actr.amegroups.com/article/view/10.21037/actr-26-0068/coif). M.A.O. reports receiving consulting fees from Fidia Farmaceuticals, Theralase, and Urogen; honoraria from Merck; and serves on the Data Safety Monitoring Board for Combat Medical. The other authors have no conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.

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