Light and Radiation Combo Doubles Survival in Rat Bladder Cancer Model

Bladder cancer remains one of the most common malignancies worldwide, ranking ninth among all cancers, and for patients with high-risk or muscle-invasive disease the standard of care often means radical removal of the bladder, a procedure with lasting consequences for quality of life. A new preclinical study published in Cancer Cell International now reports that a minimally invasive combination of repeated photodynamic therapy and ionizing radiation can dramatically improve outcomes in an orthotopic rat model of bladder cancer, doubling median survival when photodynamic therapy was used alone and producing nearly complete tumor regression when radiation was delivered beforehand. The findings, from a research team at Leipzig University led by Mandy Berndt-Paetz, offer a compelling case for organ-preserving multimodal treatment in a disease where such options remain urgently needed.

Photodynamic therapy, or PDT, works by delivering a photosensitizer, a compound that is otherwise non-toxic, into tumor tissue and then activating it with light of a specific wavelength. Upon illumination, the photosensitizer transfers energy to surrounding oxygen molecules, generating reactive oxygen species and singlet oxygen that destroy cells. Two reaction types are recognized: a Type I pathway that produces peroxides, hydroxyl radicals and superoxide ions through electron transfer cascades, and a Type II pathway that generates highly reactive singlet oxygen through direct energy transfer to molecular oxygen. Beyond direct tumor cell killing, PDT also damages tumor vasculature and triggers an acute inflammatory response that can activate both innate and adaptive immunity, with immunogenic cell death releasing damage-associated molecular patterns that further stimulate the immune system.

Despite this attractive mechanism, PDT has struggled to become a standard treatment for bladder cancer. Earlier photosensitizers such as photofrin and hematoporphyrin derivatives caused unwanted skin photosensitization when given systemically, while local application led to bladder wall fibrosis because of the high light intensities required. Perhaps the most fundamental limitation has been optical: conventional photosensitizers are activated at wavelengths of 630 to 690 nanometers, where light penetrates tissue to a depth of only about five millimeters, making it difficult to treat thicker, muscle-invasive tumors. The Leipzig team turned to tetrahydroporphyrin-tetratosylate, or THPTS, a water-soluble, positively charged near-infrared photosensitizer with an absorption maximum at 760 nanometers, a wavelength that allows tissue penetration of up to fifteen millimeters and therefore reaches tumors that earlier agents could not.

Before moving into animals, the researchers characterized the cellular effects of the combined approach in vitro. They cultured AY-27 rat bladder carcinoma cells, a line originally derived from a carcinogen-induced bladder tumor in Fischer rats, both as flat two-dimensional cultures and as three-dimensional spheroids that mixed tumor cells with primary rat bladder fibroblasts. These spheroids self-organized into bladder-like structures with an outer tumor cell layer surrounding an inner fibroblast core, better mimicking the architecture of real tumors. Treatment involved a single 4 Gy dose of X-ray irradiation followed one hour later by incubation with THPTS at concentrations ranging from 6.25 to 50 micromolar and illumination at 760 nanometers with a light dose of 10 joules per square centimeter.

The two culture formats told strikingly different stories. In two-dimensional cultures, the combination reduced cell viability by up to 90 percent after 72 hours, but response additivity analysis revealed the effects were subadditive rather than synergistic. In the three-dimensional spheroids, however, the combined treatment produced genuine synergism at nearly all tested concentrations, reducing metabolic activity by 39 percent after 72 hours while single therapies alone showed no significant effect. Mechanistic staining showed that PDT alone drove a pronounced increase in 4-hydroxynonenal, a marker of oxidative stress, while radiation elevated phospho-histone H2A.X, a marker of DNA damage; the combination strongly elevated both markers, particularly in the tumor cell layer. Crucially, the treatment significantly thinned the malignant outer layer of the spheroids while leaving the non-malignant stromal core untouched, underscoring the tumor selectivity of the approach. The authors note that the greater resistance of three-dimensional cultures to therapy makes the observed synergism there especially meaningful for predicting in vivo behavior.

The in vivo work used the well-characterized orthotopic AY-27 model, in which bladder tumors are induced in female F344 Fischer rats by intravesical instillation of tumor cells, a technique that had previously achieved a 100 percent tumor induction rate in the group’s hands. A preliminary study delivered a sobering lesson: a single session of PDT unexpectedly shortened survival compared with untreated controls, likely because a single illumination rapidly depletes oxygen in the tumor, reducing reactive oxygen species generation and leaving residual tumor cells to activate survival pathways. This finding echoed literature showing that fractionated PDT regimens improve long-term tumor control, and it motivated the team to design a repeated-treatment protocol for the main trial.

In the main experiment, 40 rats with established bladder tumors were divided into four groups of ten: untreated controls, PDT alone, radiation alone, and radiation followed by PDT. Treatments were given three times at seven-day intervals, starting 14 days after tumor inoculation. Each session involved irradiation of the lower abdomen with 8.5 Gy from an orthovoltage X-ray source, immediately followed by transurethral instillation of 100 micromolar THPTS into the bladder for two hours and then laser illumination at 760 nanometers through a glass fiber. The radiation schedule was carefully calibrated so that its biologically effective dose on late-responding normal bladder tissue matched that of current human bladder cancer radiotherapy regimens, while leaving room for additional antitumor effect from the PDT component.

The survival results were remarkable. Rats receiving three cycles of PDT alone showed a doubling of median overall survival to 70 days compared with 36.5 days in untreated controls, a difference the authors believe is the first reported 100 percent survival increase from local repeated PDT monotherapy in bladder cancer in vivo. Even more striking, neither the radiation-only group nor the combination group reached the study’s termination criteria during the observation period, meaning every animal in those cohorts survived without clinical symptoms. Histopathology added crucial nuance: while radiation alone left three of eight bladders with viable tumors, the combination therapy produced a treatment response in every tumor examined, achieving complete regression in seven of eight rats, with only one residual tumor measuring 1817 micrometers at its base compared with tumors exceeding 3100 micrometers after radiation alone.

Immunohistochemistry for CD45-positive leukocytes revealed that PDT triggered a significant local immune response, with increased leukocyte accumulation at both the invasive tumor margin and the tumor center compared with untreated controls. In the combination-treated animals, large numbers of strongly CD45-positive immune cells were observed beneath tumor regression zones, though statistical comparison was impossible because so many tumors had regressed completely. The authors suggest that pairing this multimodal focal therapy with immune checkpoint inhibitors could further amplify its inherent immunogenic effects, and they point toward future refinements including systemic photosensitizer delivery to reach deeper tumor regions, alternative excitation sources such as X-ray-activated scintillating nanoparticles, and photosensitizers favoring less oxygen-dependent Type I reactions to overcome the rapid oxygen depletion that limits solid tumor treatment.

The study is not without limitations. The biologically effective dose calculations relied on reference alpha-beta ratios from the literature rather than values determined specifically for AY-27 cells or rat bladder tissue, and the trial lacked a control group of tumor-free rats treated with the combination to assess off-target toxicity. Roughly half of the bladders showing complete tumor regression displayed moderate histopathological changes, including urothelial damage, edema, fibrosis and inflammation. Nevertheless, the treatments were overall well tolerated, with 33 of 40 animals completing the protocol as planned. If the results translate to the clinic, the authors propose the approach could benefit patients with high-grade non-muscle-invasive bladder cancer, for whom bladder removal is currently recommended, and potentially even those with non-metastatic muscle-invasive tumors up to fifteen millimeters thick, offering a genuinely organ-preserving alternative built on two technologies, near-infrared light and fractionated radiation, that are already mainstays of modern oncology.

Subject of Research: Combined photodynamic therapy and ionizing radiation for organ-preserving treatment of bladder cancer in an orthotopic rat model

Article Title: Enhanced survival through repeated photodynamic therapy and almost complete tumor regression by prior radiation therapy in an orthotopic rat bladder cancer model

Article References: Berndt-Paetz, M., Nürnberger, S., Gonsior, S., Pączek-Hippe, E., Patties, I., Weimann, A., Michalik, R., Neuhaus, J., & Glasow, A. (2026). Enhanced survival through repeated photodynamic therapy and almost complete tumor regression by prior radiation therapy in an orthotopic rat bladder cancer model. Cancer Cell International, 26(1), Article 315. https://doi.org/10.1186/s12935-026-04465-2

Image Credits: AI Generated

DOI: 10.1186/s12935-026-04465-2

Keywords: photodynamic therapy, THPTS, bladder cancer, ionizing radiation, near-infrared photosensitizer, orthotopic rat model, tumor regression, synergistic cytotoxicity, organ-preserving therapy, immune cell recruitment, Enhanced, survival