Antimicrobial photodynamic inactivation (aPDI) and antimicrobial blue light (aBL) are considered low-risk treatments for the development of bacterial resistance and/or tolerance due to their multitargeted modes of action. In this study, we assessed the development of Staphylococcus aureus tolerance to these phototreatments. Reference S. aureus USA300 JE2 was subjected to 15 cycles of both sub-lethal aPDI (employing an exogenously administered photosensitizer (PS), i.e., rose Bengal (RB)) and sub-lethal aBL (employing endogenously produced photosensitizing compounds, i.e., porphyrins). We demonstrate substantial aPDI/aBL tolerance development and tolerance stability after 5 cycles of subculturing without aPDI/aBL exposure (the development of aPDI/aBL tolerance was also confirmed with the employment of clinical MRSA and MSSA strain as well as other representatives of Gram-positive microbes, i.e. Enterococcus faecium and Streptococcus agalactiae). In addition, a rifampicin-resistant (RIF^R) mutant selection assay showed an increased mutation rate in S. aureus upon sub-lethal phototreatments, indicating that the increased aPDI/aBL tolerance may result from accumulated mutations. Moreover, qRT-PCR analysis following sub-lethal phototreatments demonstrated increased expression of umuC, which encodes stress-responsive error-prone DNA polymerase V, an enzyme that increases the rate of mutation. Employment of recA and umuC transposon S. aureus mutants confirmed SOS-induction dependence of the tolerance development. Interestingly, aPDI/aBL-tolerant S. aureus exhibited increased susceptibility to gentamicin (GEN) and doxycycline (DOX), supporting the hypothesis of genetic alterations induced by sub-lethal phototreatments. The obtained results indicate that S. aureus may develop stable tolerance to studied phototreatments upon sub-lethal aPDI/aBL exposure; thus, the risk of tolerance development should be considered significant when designing aPDI/aBL protocols for infection treatments in vitro and in clinical settings.

We formulated and analyzed a novel nanoformulation of the anticancer drug cisplatin (Cis) with C₆₀ fullerene (C₆₀+Cis complex) and showed its higher toxicity toward tumor cell lines in vitro when compared to Cis alone. The highest toxicity of the complex was observed in HL-60/adr and HL-60/vinc chemotherapy-resistant human leukemia cell sublines (resistant to Adriamycin and Vinculin, respectively). We discovered that the action of the C₆₀+Cis complex is associated with overcoming the drug resistance of the tumor cell lines through observing an increased number of apoptotic cells in the Annexin V/PI assay. Moreover, in vivo assays with Lewis lung carcinoma (LLC) C57BL/6J male mice showed that the C₆₀+Cis complex increases tumor growth inhibition, when compared to Cis or C₆₀ fullerenes alone. Simultaneously, we conducted a molecular docking study and performed an Ames test. Molecular docking specifies the capability of a C₆₀ fullerene to form van der Waals interactions with potential binding sites on P-glycoprotein (P-gp), multidrug resistance protein 1 (MRP-1), and multidrug resistance protein 2 (MRP-2) molecules. The observed phenomenon revealed a possible mechanism to bypass tumor cell drug resistance by the C₆₀+Cis complex. Additionally, the results of the Ames test show that the formation of such a complex diminishes the Cis mutagenic activity and may reduce the probability of secondary neoplasm formation. In conclusion, the C₆₀+Cis complex effectively induced tumor cell death in vitro and inhibited tumor growth in vivo, overcoming drug resistance likely by the potential of the C₆₀ fullerene to interact with P-gp, MRP-1, and MRP-2 molecules. Thus, the C₆₀+Cis complex might be a potential novel chemotherapy modification.

Unusual thermodynamic properties of C₆₀ fullerene complexation with antibiotic doxorubicin were observed and explained.

The specific features of structural self-organization of C₆₀ fullerene and antitumor drug cisplatin (Cis) in physiological solution (0.9% NaCl) have been investigated.