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Mortality from cancer may surpass that from cardiovascular diseases shortly. Approximately seven million people die from cancer each year, and it is estimated that there will be more than 18 million new cancer cases annually by 2025 (Siegel et al., 2021). Cancer, being a complex, heterogeneous disease is characterized by uncontrolled cell division. After primary tumor formation, cancer cells can further invade other tissues in the process termed metastasis (the spread of cancer to other body parts) (Vogelstein & Kinzler, 2004). To date, chemotherapy remains one of the major approaches to treating cancer by delivering cytotoxic substances to the cells. However, conventional chemotherapy has many disadvantages, such as the inability to deliver the correct amount of drug directly to cancer cells without affecting ‘normal’ cells. Drug resistance, altered biodistribution, biotransformation, and drug clearance are also common problems (Kakde et al., 2011). Hence, targeted chemotherapy and drug delivery systems are emerging as powerful methods to address these challenges. Such an approach enables selective and efficient delivery of drugs to specific targets, such as overexpressed receptors on cancer cells, while minimizing exposure to ‘normal’ cells (Aina et al., 2002; Dorsam & Gutkind, 2007; Howl et al., 2007; Meng et al., 2012; Zhang et al., 2012). For instance, tumor cells often express growth receptors like VEGFR (vascular endothelial growth factor receptor) and EGFR (epidermal growth factor receptor) exclusively on their surface (Padró et al., 2002). Various carriers such as carbohydrates, proteins, or peptides can selectively bind to these receptors (Allen, 2002; Duncan et al., 2005; Haag & Kratz, 2006; Hoskin & Ramamoorthy, 2008; Hynes, 1992; Janin, 2003; Nazarenko et al., 2013; Vicent, 2007).
2 Peptide-Mediated Delivery of Therapeutic Agents
Peptide-mediated delivery of therapeutic agents can be categorized into three distinct classes: (a) homing peptides (HP s), which have no internalization properties and therefore only deliver their cargo to the cell surface; (b) peptides that are linked to a cell-penetrating peptide (CPP), which enables cargo internalization via endocytosis or pore formation; and (c) cell-penetrating homing peptides (CPHP s), which can internalize without the aid of external agents. Several approaches for generating HP s with affinity for tissue-specific markers have been reported (Laakkonen & Vuorinen, 2010; Sugahara et al., 2010). As our understanding of peptide functionality and therapeutic effects continues to advance, peptide-based treatments are expected to play a crucial
Numerous examples from the literature highlight peptide conjugates with organic therapeutics. Well-known anticancer drugs such as daunorubicin, doxorubicin, methotrexate, fluorouracil, and paclitaxel are frequently linked to carriers such as peptides or proteins (Fuertes et al., 2003; Haag & Kratz, 2006; Hudecz et al., 2005). For instance, two peptides that have entered clinical trials from the third CPHP group are the tumor-homing peptide RGD (Arg-Gly-Asp; the first CPHP that has undergone Phase I and II, and Phase III trials have been initiated), and NGR peptide (Asn-Gly-Arg). The latter has been tested in Phase I and II trials, targeting human tumor necrosis factor (hTNF) and enhancing doxorubicin delivery for refractory or resistant solid tumors (Eskens et al., 2003; Lorusso et al., 2012; Zhou et al., 2022). RGD and NGR peptide motifs selectively recognize integrins, i.e., proteins responsible for the growth, division, adhesion, and migration of cancer cells. These systems typically exhibit higher efficacy against tumor cells compared to ‘normal’ cells (Buckley et al., 1999; Majumdar & Siahaan, 2012; Sioud & Mobergslien, 2012; Zhu et al., 2020).
3 The Complexity of Inorganic Compounds
The compounds with transition metal ions offer vast opportunities for designing new therapeutics, beyond the scope of organic chemistry. Metal ions exhibit a wide range of geometries, oxidation states, and coordination numbers, allowing for precise tuning of their reactivity. Many transition metal compounds are being explored as alternatives to platinum-based chemotherapeutic agents. An interesting example of such complexes includes phosphine-diimine copper(I) complexes, known for their robustness and diverse biological properties such as anticancer, antibacterial, antiviral, antifungal, and anti-inflammatory activities, making them promising candidates for therapeutic applications (Bhattacharjee et al., 2021; Bykowska et al., 2014, 2018; Guz-Regner et al., 2020; Komarnicka et al., 2016, 2020; Kyzioł et al., 2018). Furthermore, the strong bond between the phosphine ligand and copper(I) prevents its oxidation to copper(II), a finding supported by previous studies (Bhattacharjee et al., 2021; Guz-Regner et al., 2020; Manna et al., 2019). Additionally, phosphine ligands can be easily functionalized. Notably, aminomethylphosphines derived from
Copper(I) complexes with phosphines derived from fluoroquinolones
In Dr. Komarnicka’s group, a series of stable iodide or thiocyanate copper(I) complexes with phosphine ligands derived from fluoroquinolone antibiotics such as ciprofloxacin (HCp), norfloxacin (HNr), lomefloxacin (HLm), and sparfloxacin (HSf) (Figure 10.1) have been synthesized and their physicochemical properties and biological activity have been characterized (Komarnicka et al., 2018, 2020, 2021).
To date, it has been shown that copper(I) complexes with phosphine-fluoroquinolone conjugates exhibit much higher cytotoxic activity than unmodified fluoroquinolone antibiotics, their phosphine derivatives, and even the well-known anticancer drug cisplatin (Figure 10.1). These same copper(I) inorganic compounds can induce apoptosis predominantly, regardless of cell type and incubation time. Additionally, electrochemical studies revealed that copper complexes with phosphine ligands derived from fluoroquinolone antibiotics could generate reactive oxygen species (ROS), as confirmed in cell culture experiments. The copper(I) complexes’ anticancer activity is attributed to their interactions with DNA, causing breaks in the sugar-phosphate backbone and redox reactivity associated with ROS generation. Unfortunately, these compounds were toxic to ‘normal’ cells (Bykowska et al., 2018; Komarnicka et al., 2016, 2018, 2020, 2021). However, after encapsulation in liposomes, the water solubility and therapeutic index of the copper(I) complexes significantly increased
Copper(I) complexes with different phosphine ligands (differences highlighted with a circle; description in text)
To reduce high toxicity towards ‘normal’ cell lines, antibiotic molecules were replaced with the water-soluble, simple, small, and inexpensive dipeptide motif Sar-Gly (HSG; Figure 10.2). It is worth mentioning that 11C-glycylsarcosine (11C-Gly-Sar) has been reported as a PET tracer, which can target H+/peptide transporters (PEPT s) functionally expressed in some human cancer cell lines. Additionally, these peptides offer well-known advantages as drugs, including specificity, potency, and low toxicity (Komarnicka et al., 2018, 2020, 2021).
Dr. Komarnicka’s group also synthesized the diphenylphosphinomethyl derivative (P(CH2SG)Ph2; PSG), its oxide (OP(CH2SG)Ph2; OPSG), and the corresponding copper(I) complex (1-PSG) with the phosphine-peptide conjugate (PSG) (Figure 10.3). Additionally, they obtained a new starting salt (p-OCH3-Ph)2P(CH2OH)2Cl(MPOHC), the derived phosphine ligands P(p-OCH3-Ph)2-CH2-OH (MPOH) and P(p-OCH3-Ph)2-CH2-SarGly-OH (MPSG), and the corresponding copper(I) complexes [Cu(I)(dmp)MPOH] (1-MPOH) and [Cu(I)(dmp)PSG] (1-MPSG) (Figure 10.3). The intention was to increase the cytotoxicity of the copper(I) complexes with the phosphine-peptide conjugate by introducing an –OCH3 group into the phosphine phenyl rings of P(p-OCH3-Ph)2-CH2-SarGly-OH (MPSG). It is well known that substitution at the 4-position of the phenyl ring can significantly enhance the cytotoxicity of several therapeutics (Komarnicka et al., 2018, 2020, 2021).
Schematic route of phosphine ligands with and without peptide, as well as copper(I) complex with phosphine ligands
4
The cytotoxic activity of all presented organic and inorganic compounds, as well as cisplatin, was tested against several cancer cell lines: mouse colon carcinoma (CT26), human lung adenocarcinoma (A549), human breast adenocarcinoma (MCF7), human pancreatic cancer (PANC-1), and human prostate cancer (DU-145). Additionally, it was tested against several ‘normal’ cell lines: primary human pulmonary fibroblasts (MRC-5), human embryonic kidney (HEK293T), and human keratinocyte (HaCat). The obtained results indicate that the presence of –OCH3 groups in the phenyl rings of the phosphine significantly increased the cytotoxic activity of the 1-MPOH and 1-MPSG complexes compared to 1-POH and 1-PSG complexes against the selected human cancer cell lines (MCF7, PANC-1, and DU-145) (Komarnicka et al., 2018, 2020, 2021).
Schematic mechanism of cytotoxic action of copper(I) complexes with phosphine-peptide conjugate
We have begun exploring the antitumor potential of a new class of copper(I) complexes containing a diimine (dmp) and phosphine ligands P(p-OCH3-Ph)2CH2OH and P(p-OCH3-Ph)2CH2SarGly, which feature an –OCH3 group in the phenyl rings and the Sar-Gly peptide motif. The steric and electronic properties of the –OCH3 group significantly influence the cytotoxic activity of these copper(I) complexes. Interestingly, the copper complex 1-MPSG exhibited increased cytotoxicity against MCF7, PANC-1, and DU-145 cell lines, while showing reduced cytotoxicity against CT26 and 7649 tumor cell lines. This suggests
5 Conclusions
This chapter presents a simple concept of using copper(I) complexes and the complex nature of their derivation and application in cancer treatment. It is evident that substituting fluoroquinolone molecules in copper(I) complexes with a simple, lipophilic peptide molecule (Sar-Gly) markedly reduces the toxicity of copper(I) complexes towards ‘normal’ cells while enhancing their cytotoxicity against cancer cells. This design strategy, involving cytotoxic copper(I) complexes with peptides attached via a phosphine linker, holds considerable promise for anticancer therapy.
Cancer is a complex and heterogeneous disease, necessitating therapies that simplify patients’ lives by reducing side effects. The development of targeted metal complexes, with increased specificity, represents a step towards achieving this goal, offering a more selective approach that minimizes harm to healthy tissues while effectively combating cancer cells. Notably, metal complexes have shown promise by inducing cancer cell death mechanisms, such as, for instance, ferroptosis (Dixon et al., 2012; Dixon & Pratt, 2023). Further exploration of these diverse mechanisms broadens the potential avenues for innovative treatments. What is more, future research in this direction holds promise for refining therapies, enhancing cancer treatment strategies, and improving patient outcomes.
Acknowledgements
U.K. acknowledges financial support from the National Science Centre Poland, grant no. 2016/23/D/ST5/00269 and the Ministry of Science and Higher Education (grants no. 1233/M/WCH/13 and 1500/M/WCH/15). U.K. and E.B. also
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