According to the European Commission, cancer was the second largest cause of death in the EU in 2021, with 1.1 million deaths annually [1]. Current cancer therapeutic approaches are not always as effective or long-lasting as expected, highlighting the need for more precise and efficient alternatives with minimal side effects. One such approach is photodynamic therapy (PDT), which uses photosensitizers (PS), compounds that become active only when exposed to light. PS-induced phototoxic reactions occur only in illuminated areas, reducing unwanted side effects. However, PS molecules are prone to photobleaching. Additionally, their activation typically depends on a red light, which has limited tissue penetration, complicating the treatment of deep-seated tumors. To solve these issues and enhance PS functionality, nanoparticles can be introduced as energy donors, thus shifting PS excitation to near-infrared (NIR) [2]. This study focuses on developing a therapeutic nanocomplex of PS Chlorin e6 (Ce6) loaded into the phospholipid/polyethylene glycol coating of upconverting nanoparticles (UCNP) for NIR-activated PDT.
Firstly, the surface of NaGdF\(_{4}\):Yb\(^{3+}\), Er\(^{3+}\)@NaGdF\(_{4}\):Yb\(^{3+}\), Nd\(^{3+}\) UCNPs were modified with 3 different mixtures of phosphatidylcholine and phosphoethanolamine-polyethylene glycol phospholipids (PLs). Ce6 encapsulation was achieved by mixing surface-coated UCNPs with concentrated Ce6 solution and washing excess Ce6 afterwards. The singlet oxygen generation efficiency of UCNPs-PLs-Ce6 was evaluated upon illumination with 980 nm and 808 nm lasers, using a singlet oxygen sensor green probe. Accumulation, cytotoxicity, and in vitro PDT were performed in colon cancer cell lines DLD-1 and HCT116.
The obtained spectroscopic data and average luminescence decay confirmed donor-acceptor pair formation and energy transfer between UCNPs and Ce6. Quantitative evaluation of singlet oxygen demonstrated the high effectiveness of UCNPs-PLs-Ce6, highlighting its promising therapeutic potential. Furthermore, a comparative study revealed significantly better photostability of Ce6 within the nanocomplex. In contrast, free Ce6 photobleached and degraded. Cellular studies showed UCNPs-PLs-Ce6 accumulation in the perinuclear region with maintained nanocomplex stability inside the cells. In vitro PDT upon 980 nm and 808 nm laser irradiation decreased cancer cell viability. Irradiation by 808 nm laser was continuously performed, while 980 nm laser needed fractionation in time due to thermal heating.
Overall, a therapeutic nanocomplex with extended PDT activation light from red to NIR and enhanced Ce6 photostability was developed. Photostable Ce6 enables improved therapeutic effects in vitro, while 808 nm laser light enables deeper tissue penetration and elimination of photothermal effects. This study not only contributes to fundamental knowledge but also demonstrates the potential of nano complexes in novel cancer therapy. This project has received funding from the Research Council of Lithuania (LMTLT), agreement No. S-MIP-22-31 and No. P-ST-24-258.