Principles of photothermal therapy
Photothermal therapy relies on agents that convert near-infrared radiation into heat, producing either moderate hyperthermia at around 42-45 degrees, useful as a sensitizer to radiotherapy and chemotherapy, or ablation at temperatures above 50 degrees, with cell death through necrosis and coagulation. The NIR-I window, between roughly 700 and 900 nm, offers fair tissue penetration, while NIR-II, beyond 1000 nm, benefits from lower scattering and a higher maximum permissible exposure limit, with a gain in depth. Selectivity depends almost entirely on where the transducer ends up, which makes the carrier as decisive as the absorber.
Photothermal transducers differ in extinction coefficient, conversion efficiency and behavior under irradiation. Gold nanorods, which can be tuned by changing their aspect ratio, have high efficiencies but may undergo morphological reshaping under intense pulses, with loss of the absorption peak, whereas materials such as polydopamine and carbon derivatives are more stable but less intense. The choice of absorber should be made according to the route of administration and the spectral window of the available source.
The role of fibroin in building the agent
Fibroin acts on several levels. As a colloidal stabilizer, it coats and disperses gold nanostructures, such as nanorods and nanoshells, as well as polydopamine, transition metal sulfides and selenides, and graphene derivatives, reducing aggregation and surface toxicity. In some protocols, tyrosine residues take part in the reduction and in situ growth of metal nanoparticles, allowing synthesis in water under mild conditions. As an injectable matrix, fibroin hydrogels can hold the agent at the intratumoral site, making repeated sessions possible with a single administration.
A further advantage is the possibility of co-loading cytotoxic or immunomodulatory drugs, whose release can be accelerated by local heating through changes in the secondary structure and permeability of the matrix, giving spatiotemporal control over the combined effect.
Intratumoral distribution is shaped by high interstitial pressure and a dense extracellular matrix, which hinder diffusion. Injectable fibroin hydrogels that gel in situ sidestep part of the problem, because they create a localized depot and allow high doses exactly where they are needed, and temperature-sensitive formulations can be injected as a liquid and solidify on contact with tissue. Gradual biodegradation of the matrix releases the absorbers over time, so the kinetics of their release and elimination must be evaluated.
Design parameters and critical issues
Photothermal conversion efficiency, stability under repeated irradiation, light dose and power density need to be reported rigorously, since comparisons between systems are often undermined by incompatible protocols. Tumor thickness, perfusion that cools the target, and the non-uniform distribution of the agent produce thermal gradients that leave undertreated margins, a well-known source of recurrence. Monitoring with thermography or photoacoustic imaging makes it possible to close the control loop.
On the biological side, hyperthermia induces heat shock proteins that can confer thermotolerance, and ablation can release tumor antigens with potential immunogenic effects, a phenomenon that paves the way for combinations with checkpoint inhibitors.
Thermal dosimetry is the aspect that most separates a demonstration from a therapy. Numerical models based on the bioheat equation, calibrated with in vivo thermographic measurements, make it possible to predict the temperature distribution and to plan power, time and optical fiber placement. The concept of equivalent thermal dose at 43 degrees, which integrates temperature and duration, is the standard tool for comparing different protocols.
Toward clinical trials
The obstacles are the clearance and persistence of non-biodegradable inorganic nanomaterials, the scalability of synthesis, and the limited depth reachable by external sources. The most plausible indications concern superficial tumors or those accessible endoscopically or interstitially, where light can be delivered through optical fibers.
Therapeutic combinations and safety
The most plausible clinical value of photothermal therapy lies in combinations. Hyperthermia increases perfusion and vascular permeability, sensitizes cells to radiotherapy and to drugs such as cisplatin and doxorubicin, and can amplify the immune response when paired with immunotherapy. Fibroin matrices that allow the absorber and the drug to be co-localized can exploit these synergies with a single administration, reducing systemic exposure.
On the safety front, one must consider thermal damage to adjacent healthy tissue, the long-term toxicity of inorganic absorbers, which may accumulate in the reticuloendothelial system, and the reproducibility of treatment across patients with different anatomy and perfusion. Defining safety margins, real-time monitoring systems and patient selection criteria is a prerequisite for any early-phase clinical trial.
