N6-benzyladenosine derivatives as novel N-donor ligands of platinum(II) dichlorido complexes

State of art in the chemistry of nucleoside-based Pt(II) complexes

After the fortuitous discovery of the anticancer properties of cisplatin, many Pt(II) complexes have been synthesized, to obtain less toxic leads which could overcome the resistance phenomena. Given the importance of nucleosides and nucleotides as antimetabolites, studying their coordinating properties towards Pt(II) ions is challenging for bioorganic and medicinal chemistry. This review aims to describe the results achieved so far in the aforementioned field, paying particular attention to the synthetic aspects, the chemical-physical characterization, and the biological activities of the nucleoside-based Pt(II) complexes.

Towards an Improvement of Anticancer Activity of Benzyl Adenosine Analogs

N6-Isopentenyladenosine (i6A) is a naturally occurring modified nucleoside displaying in vitro and in vivo antiproliferative and pro-apoptotic properties. In our previous studies, including an in silico inverse virtual screening, NMR experiments and in vitro enzymatic assays, we demonstrated that i6A targeted farnesyl pyrophosphate synthase (FPPS), a key enzyme involved in the mevalonate (MVA) pathway and prenylation of downstream proteins, which are aberrant in several cancers. Following our interest in the anticancer effects of FPPS inhibition, we developed a panel of i6A derivatives bearing bulky aromatic moieties in the N6 position of adenosine. With the aim of clarifying molecular action of N6-benzyladenosine analogs on the FPPS enzyme inhibition and cellular toxicity and proliferation, herein we report the evaluation of the N6-benzyladenosine derivatives’ (compounds 2a–m) effects on cell viability and proliferation on HCT116, DLD-1 (human) and MC38 (murine) colorectal cancer cells (CRC). We found that compounds 2, 2a and 2c showed a persistent antiproliferative effect on human CRC lines and compound 2f exerted a significant effect in impairing the prenylation of RAS and Rap-1A proteins, confirming that the antitumor activity of 2f was related to the ability to inhibit FPPS activity.

NMR for screening and a biochemical assay: Identification of new FPPS inhibitors exerting anticancer activity

Farnesyl pyrophosphate synthase (FPPS) is a crucial enzyme for the synthesis of isoprenoids and the key target of nitrogen-containing bisphosphonates (N-BPs). N-BPs are potent and selective FPPS inhibitors that are used in the treatment of bone-related diseases, but have poor pharmacokinetic properties. Given the key role played by FPPS in many cancer-related pathways and the pharmacokinetic limits of N-BPs, hundreds of molecules have been screened to identify new FPPS inhibitors characterized by improved drug-like properties that are useful for broader therapeutic applications in solid, non-skeletal tumours. We have previously shown that N6-isopentenyladenosine (i6A) and its related compound N6-benzyladenosine (2) exert anti-glioma activity by interfering with the mevalonate pathway and inhibiting FPPS. Here, we report the design and synthesis of a panel of N6-benzyladenosine derivatives (compounds 2a-m) incorporating different chemical moieties on the benzyl ring. Compounds 2a-m show in vitro antiproliferative activity in U87MG glioma cells and, analogous to the bisphosphonate FPPS inhibitors, exhibit immunogenic properties in ex vivo γδ T cells from stimulated peripheral blood mononuclear cells (PBMCs). Using saturation transfer difference (STD) and quantitative ¹H nuclear magnetic resonance (NMR) experiments, we found that 2f, the N6-benzyladenosine analogue that includes a tertbutyl moiety in the para position of the benzyl ring, is endowed with increased FPPS binding and inhibition compared to the parent compounds i6A and 2. N6-benzyladenosine derivatives, characterized by structural features that are significantly different from those of N-BPs, have been confirmed to be promising chemical scaffolds for the development of non N-BP FPPS inhibitors, exerting combined cytotoxic and immunostimulatory activities.

Adenosine: Synthetic Methods of Its Derivatives and Antitumor Activity

Since 1929, several researchers have conducted studies in relation to the nucleoside of adenosine (1) mainly distribution identifying, characterizing their biological importance and synthetic chemistry to which this type of molecule has been subjected to obtain multiple of its derivatives. The receptors that interact with adenosine and its derivatives, called purinergic receptors, are classified as A1, A2A, A2B and A3. In the presence of agonists and antagonists, these receptors are involved in various physiological processes and diseases. This review describes and compares some of the synthetic methods that have been developed over the last 30 years for obtaining some adenosine derivatives, classified according to substitution processes, complexation, mating and conjugation. Finally, we mention that although the concentrations of these nucleosides are low in normal tissues, they can increase rapidly in pathophysiological conditions such as hypoxia, ischemia, inflammation, trauma and cancer. In particular, the evaluation of adenosine derivatives as adjunctive therapy promises to have a significant impact on the treatment of certain cancers, although the transfer of these results to clinical practice requires a deeper understanding of how adenosine regulates the process of tumorigenesis.

A Critical Evaluation of the Quality of Published 13C NMR Data in Natural Product Chemistry

Nuclear Magnetic Resonance spectroscopy contributes very efficiently to the structure elucidation process in organic chemistry. Carbon-13 NMR spectroscopy allows direct insight into the skeleton of organic compounds and therefore plays a central role in the structural assignment of natural products. Despite this important contribution, there is no established and well-accepted workflow protocol utilized during the first steps of interpreting spectroscopic data and converting them into structural fragments and then combining them, by considering the given spectroscopic constraints, into a final proposal of structure. The so-called "combinatorial explosion" in the process of structure generation allows in many cases the generation of reasonable alternatives, which are usually ignored during manual interpretation of the measured data leading ultimately to a large number of structural revisions. Furthermore, even when the determined structure is correct, problems may exist such as assignment errors, ignoring chemical shift values, or assigning lines of impurities to the compound under consideration. An extremely large heterogeneity in the presentation of carbon NMR data can be observed, but, as a result of the efficiency and precision of spectrum prediction, the published data can be analyzed in substantial detail.This contribution presents a comprehensive analysis of frequently occurring errors with respect to ¹³C NMR spectroscopic data and proposes a straightforward protocol to eliminate a high percentage of the most obvious errors. The procedure discussed can be integrated readily into the processes of submission and peer-reviewing of manuscripts.

Interaction of selected platinum(II) complexes containing roscovitine-based CDK inhibitors as ligands with human liver microsomal cytochrome P450

BACKGROUND

We studied the interaction of oxaliplatin derivatives involving cytotoxic adenine-based cyclin-dependent kinase inhibitors, with human liver microsomal cytochrome P450.

METHODS AND RESULTS

The activities of 9 human liver microsomal CYP forms (CYPs 1A2, 7-ethoxyresorufin O-deethylation; 2A6, coumarin 7-hydroxylation; 2B6, 7-ethoxy-4-(trifluoromethyl) coumarin O-deethylation; 2C8, luciferin-6´ methyl ether demethylation; 2C9, diclofenac 4´-hydroxylation, 6´-deoxyluciferin hydroxylation; 2C19, (S)-mephenytoin 4´-hydroxylation; 2D6, bufuralol 1´-hydroxylation, 2E1, chlorzoxazone 6-hydroxylation; 3A4, testosterone 6β-hydroxylation, luciferin-6´ benzyl ether debenzylation) were tested using HPLC, fluorescence and luminescence product detection. At 100 µM platinum(II) oxalato complex concentration, CYP inhibition was in general 25%-50%, except for the CYP3A4 form which showed roughly twice the inhibition (72%-95%). At low complex concentration (10 µM), the difference in inhibition of CYP3A4 and other forms was even more pronounced. Dixon and Lineweaver-Burk plots indicated a partially noncompetitive mechanism of CYP3A4 inhibition.

CONCLUSIONS

The tested complexes significantly inhibit human liver microsomal CYP3A4 activity even at clinically relevant concentrations. This could be a serious drawback for the use of these compounds in clinical practice.

Synthesis and pharmacological evaluation of modified adenosines joined to mono-functional platinum moieties

The synthesis of four novel platinum complexes, bearing N6-(6-amino-hexyl)adenosine or a 1,6-di(adenosin-N6-yl)-hexane respectively, as ligands of mono-functional cisplatin or monochloro(ethylendiamine)platinum(II), is reported. The chemistry exploits the high affinity of the charged platinum centres towards the N7 position of the adenosine base system and a primary amine of an alkyl chain installed on the C6 position of the purine. The cytotoxic behaviour of the synthesized complexes has been studied in A549 adenocarcinomic human alveolar basal epithelial and MCF7 human breast adenocarcinomic cancer cell lines, in order to investigate their effects on cell viability and proliferation.

Platinum(II) oxalato complexes involving adenosine-based N-donor ligands: synthesis, characterization and cytotoxicity evaluation

A one-step synthetic procedure using the reaction of potassium bis(oxalato)platinate(II) with the corresponding N6-benzyladenosine derivative (nL) provided the [Pt(ox)(nL)₂]∙1.5H₂O oxalato (ox) complexes 1-5, involving the nL molecules as monodentate coordinated N-donor ligands. The complexes were thoroughly characterized by elemental analysis, multinuclear (¹H, ¹³C, ¹⁵N, 1¹⁹⁵Pt) and two dimensional NMR, infrared and Raman spectroscopy, and mass spectrometry, proving their composition and purity as well as coordination of nL through the N7 atom of the purine moiety. Geometry of [Pt(ox)(4FL)₂] (5) was optimized at the B3LYP/LANLTZ/6-311G** level of theory. The complexes were screened for their in vitro cytotoxicity against two human cancer cell lines (HOS osteosarcoma and MCF7 breast adenocarcinoma), but they did not show any effect up to the concentration of 50.0 µM (compounds 1, 2) or 20.0 µM (compounds 3-5).