Recent Advances in Photodynamic Therapy for Deep-Seated Tumors with the Aid of Nanomedicine

Abstract

Photodynamic therapy (PDT) works through photoactivation of a specific photosensitizer (PS) in a tumor in the presence of oxygen. PDT is widely applied in oncology to treat various cancers as it has a minimally invasive procedure and high selectivity, does not interfere with other treatments, and can be repeated as needed. A large amount of reactive oxygen species (ROS) and singlet oxygen is generated in a cancer cell during PDT, which destroys the tumor effectively. However, the efficacy of PDT in treating a deep-seated tumor is limited due to three main reasons: Limited light penetration depth, low oxygen concentration in the hypoxic core, and poor PS accumulation inside a tumor. Thus, PDT treatments are only approved for superficial and thin tumors. With the advancement of nanotechnology, PDT to treat deep-seated or thick tumors is becoming a reachable goal. In this review, we provide an update on the strategies for improving PDT with nanomedicine using different sophisticated-design nanoparticles, including two-photon excitation, X-ray activation, targeting tumor cells with surface modification, alteration of tumor cell metabolism pathways, release of therapeutic gases, improvement of tumor hypoxia, and stimulation of host immunity. We focus on the difficult-to-treat pancreatic cancer as a model to demonstrate the influence of advanced nanomedicine in PDT. A bright future of PDT application in the treatment of deep-seated tumors is expected.

Keywords: hypoxia; metal–organic framework (MOF); pancreatic cancer; photodynamic therapy (PDT); photosensitizer.

Figure 1

The principle of photodynamic reaction and its basic limitations applied in deep-seated tumor treatment. PS: Photosensitizer, TME: Tumor microenvironment.

Figure 2

Strategies for improving photodynamic therapy with nanoparticles. NPs: Nanoparticles.

Figure 3

Photosensitizer activation through one-photon excitation and two-photon excitation (TPE). The TPE requires two photons of approximately half the energy compared to the energy needed in one-photon excitation.

Figure 4

Nanoparticles can act as a tumor targeted carrier to deliver photosensitizers to a deep-seated tumor with additional functions. US: Ultrasound, MRI: Magnetic resonance imaging, CT: Computed tomography, PET: Positron emission tomography, PA: Photoacoustic.

Figure 5

Various medical gases with unique functions could be delivered into the tumor site by nanocarriers to enhance PDT efficacy in tumor treatment.