Metal compounds, particularly transition metal complexes, are extensively explored as a novel class of drugs for treating various diseases due to their unique properties and versatile modes of action, with significant applications in cancer therapy [1,2], antimicrobial treatments [3,4], and neurological disorders [5,6]. One of the most studied applications is in cancer therapy, owing to the success of platinum-based drugs [7,8]. More recently, metal-based compounds have been investigated across several compound classes [[9], [10], [11]] for potential use in photodynamic therapy (PDT), a clinical modality based on the interaction of oxygen, light irradiation, and photosensitizers to selectively destroy target cells, particularly in cancer treatment.

Ongoing research has focused on enhancing their photophysical, photochemical, and photobiological properties to overcome limitations of traditional photosensitizers and improve cancer treatment efficacy. Transition metal complexes, in particular, show significant potential due to their tuneable properties, improved light penetration depth, reduced oxygen dependence, and the ability to integrate with other therapeutic modalities such as chemotherapy and immunotherapy [[12], [13], [14], [15]]. These principles are exemplified by the development of TLD1433, the first Ru(II)-based photosensitizer to enter human clinical trials [16].

The primary biological mechanism of cancer cell death induced by these compounds appears to be directly related to the generation of reactive oxygen species (ROS) such as hydroxyl radicals, hydrogen peroxide, and superoxide upon interaction with oxygen. However, oxygen levels in cancerous tissues are generally very low, a condition known as hypoxia, typically ranging from 0.3% to 4.2% O2 [[17], [18], [19], [20]]. Recently, we proposed the use of a meso-substituted porphyrin ruthenium(II) complex as a photosensitizer for PDT (Fig. 1).

This compound displays a synergistic mechanism of action, involving the generation of reactive nitrogen species, particularly nitric oxide (NO), together with reactive oxygen species (ROS) during the death of lung cancer cells. In previous studies from our group, its activity was evaluated in both 2D monolayer cultures and 3D spheroid models of A549 tumor cells. The experiments included phototoxicity assays under visible-light irradiation, which revealed a significant decrease in cell viability, and ROS detection assays, confirming reactive oxygen species as the primary mediators of cell death. Flow cytometry and immunostaining analyses further demonstrated apoptosis induction in treated cells. Moreover, cellular uptake studies indicated efficient internalization of the complex, while the 3D spheroid models enabled evaluation of both tissue penetration and photodynamic efficacy in a more physiologically relevant context. Collectively, these results highlight the compound's photodynamic activity at the cellular level and provide mechanistic insights that support the molecular interactions explored in the present manuscript [21]. These findings strongly support the potential of ruthenium complexes as effective phototherapeutic agents against lung cancer, motivating our investigation into the interactions of the meso-substituted porphyrin ruthenium(II) complex with human serum albumin (HSA), the most abundant transport protein, as well as its role in DNA interactions [22]. Understanding these interactions is critical for drug development, optimizing delivery, and predicting in vivo behavior.