## 1 Introduction  
Platinum-based drugs have been actively exploited for the treatment of oncological diseases since the middle of the twentieth century [1, 2]. Platinum (Pt) antineoplastic agents remain indispensable for the treatment of ovarian cancer and non-small-cell lung carcinoma [3, 4]. The mechanism of action for these drugs is not fully understood; however, the most well-accepted pathway is the binding of active platinum moieties to DNA, forming non-separable adducts and driving cancer cell apoptosis [5]. Currently, novel platinum-binding ligands are actively investigated as they improve the pharmacokinetic profile of the platinum component and exhibit their own biological activity [6].  
In the current study, a novel platinum-containing antineoplastic agent BP-C1 was tested. A platinum atom of BP-C1 originates from cisplatin (Figure 1a) as a raw material. BP-C1 contains two O-donor ligands in the coordination sphere of platinum similar to carboplatin (Figure 1b). BP-C1 is a cis-coordinated complex of platinum (II) with a polymer of benzene-polycarboxylic acids derived from lignin (gross formula C\({}_{83}\)H\({}_{70}\)N\({}_{2}\)O\({}_{27}\)Pt) [7]. A structure of BP-C1 is presented in Figure 1c. The ligand in BP-C1 has a wide spectrum of biological activity [8]; the molecular mechanism is the interaction of the lignin-derived polymer with glucocorticoid and serotonin/5-HT1 receptors, regulating inflammatory responses [9]. Clinical trials of BP-C1 were undertaken in female patients with metastatic breast carcinoma (ClinicalTrials.gov identifiers: NCT04298333, NCT03789019, NCT03603197, NCT02783794 4.0/).  
and NCT01861509). The trials indicated that prolonged intramuscular administration of BP-C1 resulted in the stabilisation of tumour growth and normalised haematological parameters [10,11]. Chemical characterisation of the lignin-derived BP-Cx-1 ligand was previously reported [8].  
The degree of protein binding with the drug can ultimately increase or decrease a drug's performance, depending on how much bound or unbound (free) drug there is [12]. The main target of platinum-based metal drugs at a molecular level is genomic DNA [13]. However, these drugs are able to interact with plasma proteins such as HSA. These drug-protein interactions may be the cause of drug resistance and a multitude of side effects, which may play a role in the drugs' poor therapeutic index [14]. Around 24 h after the administration of cisplatin into the bloodstream, 65-98% of this drug is bound to a variety of different plasma proteins, with the highest fraction bound to HSA [15,16]. Cisplatin can bind to and form stable complexes with two interaction sites on an HSA molecule. These involve sulphur donors that are present in methionine residues and as a free cysteine residue, cysteine-34 [17]. Unlike other xenobiotics, the binding of HSA to cisplatin has been noted as being an irreversible reaction with only 5% of the bound protein being released [18]. Thus, cisplatin binding to HSA has been considered therapeutically inactive [19]. The hydrolysis of cisplatin forms chemically active platinum species that can later convert into a cisplatin complex with HSA, which in turn can have reducing properties. This can also cause the inactivation of the cisplatin drug, also affecting the severe side effects of platinum drugs, such as nephrotoxicity [20, 21, 22].  
Our recent study [23] demonstrated that increased protein binding might be crucial for the reduced side effects of cisplatin under hyperthermic intraperitoneal chemoperfusion compared to the standard infusion administration. The aim of the current study was to study the protein binding of a novel anticancer agent with the polymeric lignin-derived ligand BP-C1 using equilibrium dialysis.