Recently, the number of deaths related to cancer is growing worldwide [1]. The elevated tumor tissue resistance, as well as lower efficiency and numerous side effects of conventional therapies also affect the increasing number of oncological patients; thus, the development of new therapeutic procedures and drugs is necessary. The use of natural bioactive compounds could be a suitable option since many of their biomedical properties were reported previously [2]. Their natural origin, interactive potential [3], and positive effects on the reversal of multidrug resistance in cancer cells [4] could be considered as an advantage in the medical field. Using combination of different natural compounds to increase overall therapeutic effects of treatment is becoming a trend in medicine and pharmacology [5]. Based on that, the possibility of using secondary metabolites in combined therapy for intentional increase of the treatment effectiveness and reduction of side effects should be further investigated.

Photodynamic therapy belongs to the methods enabling a combined interaction of therapeutics or therapeutic approaches in medicine. This concept includes the usage of a photosensitizer that can produce a higher amount of reactive oxygen species (ROS) in the presence of molecular oxygen and the radiation of a certain wavelength [6]. An increased ROS level can disturb the redox balance and induce the oxidative damage of several cell components. This may potentially lead to a programmed cell death, as the essential parts of the cells are damaged, and the repair mechanisms cannot manage the restoration of cell integrity [7]. The ability of ROS to induce oxidative damage and cell death could be a potential mechanism to destroy tumor cells. Due to the photoactivation ability and subsequent ROS production, hypericin is one of the chemical substances used in the photodynamic therapy [8]. As one of the main components of medicinal plant St. John's wort (Hypericum perforatum L.) [9], hypericin has been widely used as a part of folk medicine in the past. In recent decades, it has been the subject of intensive scientific research due to a wide range of its biomedically important properties [10]. Previous studies identified antiproliferative [11], apoptotic [12,13], and antiangiogenic [14] effects of hypericin. It has also been shown to be effective in the elimination of bacteria [15] and viruses [16].

Our study is focused on monitoring the combined effects of photoactivated hypericin (HY) with manumycin A (MA), representing another secondary metabolite with similar biomedical properties. Manumycin A, a compound originally isolated from Streptomyces parvulus, was identified as an inhibitor of Ras farnesyltransferase [17]. This enzyme plays a key role in the post-translational modification, which ensures the proper activity of the Ras proteins. Therefore, the inhibition of RAS farnesyltransferase prevents the proper functioning of the Ras proteins, commonly abnormally active in cancer tissues. Either alone or in a combination with several substances, MA showed significant antitumor properties due to its antiproliferative [18], apoptotic [19], and antiangiogenic [20] character. Several studies have shown that the treatment of the cells with MA inhibits cell proliferation, activates caspase-3, and thus induces apoptosis in the MCC-2, MSTO-221H and H28 tumor cells [21,22]. Moreover, an increased rate of ROS accumulation has also been identified in colorectal cancer cells (SW480, Caco-2) exposed to MA [23]. However, the usage of a combination of different drugs in clinical practice requires the verification of their safety and reliability and, subsequently, the elucidation of their biochemical properties and mechanisms of action in cells. For that reason, we investigated whether the combination of HY and MA could affect their modes of action in several systems including plasmid DNA, bacteria, and human lymphocytes.