Colloidal semiconductor nanocrystals, namely quantum dots (QDs), have been at the heart of the nanomaterial research in bioimaging, biolabeling and drug delivery in recent decades due to their unique properties such as narrow emission bands, size-tuneable and strong fluorescence and excellent photostability [1, 2]. Specifically, Ag2S (AS) QDs are ideal for such applications as they are free of toxic heavy metals, have low solubility constant (Ksp=6.3x10-50), absorb in the visible/NIR region, emit in the medical imaging window and can be synthesized in aqueous media with various functional coatings, which renders them highly biocompatible [3-6]. Furthermore, AS QDs have been reported as strong photosensitizers to initiate photothermal therapy (PTT) at long-wavelengths. PTT is a highly local therapy, which may be evaluated as a single or complementary treatment method, and there are several studies indicating improved therapeutic outcome of combined PTT/chemotherapy [5]. One limiting factor in using aqueous AS QDs for this approach is the inability to load hydrophobic drugs to aqueous QDs.

Albumin is the most abundant protein in blood plasma and an emerging drug carrier for chemotherapeutic agents [7]. The first oncology approved albumin-based drug delivery system, Abraxane® (ABX), is an albumin-bound form of hydrophobic chemotherapy drug paclitaxel (PTX) [7, 8].

Inspired by ABX, we developed bovine serum albumin (BSA) coated AS QDs for combinational chemo and photothermal therapy of breast cancer. Previously developed by our group [9], glutathione (GSH) coated AS QDs were further functionalized with BSA via standard amidation chemistry using EDC/sulfo-NHS protocol, which provided small and strongly luminescent QDs. This is a tremendous improvement over AS QDs directly synthesized in BSA solution in the literature. Then, hydrophobic PTX was loaded onto these particles. AS-BSA particles were ca. 10 nm, much smaller than Abraxane (130 nm), and have strong emission in the medical imaging window (905 nm). The photothermal therapy potential of the AS-BSA particles were first investigated in the solution at 808 nm laser irradiation (700 mW), which caused an approximately 10oC temperature increase at 750 μg/mL [Ag] concentration. Next, in-vitro cytotoxicity studies were conducted with and without laser irradiation, using SKBR3 and MDA-MB-231 breast cancer cell lines. Additionally, L929 cell line was used to show the non-toxic nature of the QDs for healthy cells. AS-BSA particles showed no cytotoxicity on these three cell lines after 24h of incubation in the range of 5-75 μg/mL [Ag] concentration, as determined by MTT assay. The dark toxicity of AS-BSA-PTX was comparable to free PTX (20-300 ng/mL), indicating chemotherapy potential of the nanoparticles in the dark treatment. Cells treated with AS-BSA and then exposed to 10 min irradiation at 808 nm (700 mW/cm2) lost a significant amount of viability, supporting the in-vitro PTT potential of the QDs. Furthermore, 2-fold increase in toxicity was achieved with the combination therapy using AS-BSA-PTX+laser in SKBR3 cells. The difference between monotherapies and the combination therapy was not as dramatic in the case of MDA-MB-231 cells; however, the cell viability was still reduced more than the dark at the same PTX concentrations. At the 75 μg/mL [Ag] concentration, near complete cell death was achieved for both cell lines with combination therapy. Overall, these particles have great potential as a chemo-photothermal therapy agent, which benefits from the general safety, strong NIR emission and PTT potential.