Far-field bioluminescent immune cell-mediated photodynamic therapy is limited by photon flux.

Photodynamic therapy (PDT) is a clinically established photochemical treatment in which light activation of photosensitizers (PS) generates reactive species, such as reactive oxygen species (ROS), that induce localized cytotoxicity. Beyond direct tumor ablation, PDT can modulate the tumor microenvironment, enhancing antitumor immunity and sensitizing tumors to combination therapies. Related sublethal regimens, termed photodynamic priming (PDP), aim to exploit these immunomodulatory effects. While deep tumors can be accessed using fiber-optic or endoscopic light delivery, extending PDT or PDP to disseminated metastatic disease would require numerous invasive insertions, limiting clinical practicality. Here, we examine immune cell-mediated light delivery, in which luciferase-expressing immune cells infiltrate tumors and act as internal bioluminescent sources for photodynamic activation. Although this strategy could enable PDT and PDP across widespread lesions without external illumination, it remains unclear whether far-field photon emission from luciferase is sufficient to activate PS, or whether near-field mechanisms such as bioluminescence resonance energy transfer are required. Using luciferase-engineered tumor-associated macrophages and verteporfin, we observed no detectable ROS generation or tumor cell cytotoxicity in vitro. Complementary Monte Carlo simulations incorporating tissue scattering and absorption show that achievable photon fluxes are several orders of magnitude below established thresholds for cytotoxic PDT. These findings quantitatively define the limits of far-field bioluminescence and highlight the need for near-field strategies for immune cell mediated PDT and PDP.