Agnieszka B. Olejniczak

Prodrugs: A challenge for the drug development

It is estimated that about 10% of the drugs approved worldwide can be classified as prodrugs. Prodrugs, which have no or poor biological activity, are chemically modified versions of a pharmacologically active agent, which must undergo transformation in vivo to release the active drug. They are designed in order to improve the physicochemical, biopharmaceutical and/or pharmacokinetic properties of pharmacologically potent compounds. This article describes the basic functional groups that are amenable to prodrug design, and highlights the major applications of the prodrug strategy, including the ability to improve oral absorption and aqueous solubility, increase lipophilicity, enhance active transport, as well as achieve site-selective delivery. Special emphasis is given to the role of the prodrug concept in the design of new anticancer therapies, including antibody-directed enzyme prodrug therapy (ADEPT) and gene-directed enzyme prodrug therapy (GDEPT).

Adenosine and 2'-deoxyadenosine modified with boron cluster pharmacophores as new classes of human blood platelet function modulators.

Novel types of adenosine and 2′‐deoxyadenosine derivatives containing boron clusters at positions C2′, N6, or C8 were synthesized. The effect of these modified compounds on platelet function was studied. Modification of adenosine at the C2′ position with a para‐carborane cluster (C2B10H11) results in efficient inhibition of platelet function, including aggregation, protein secretion, and P‐selectin expression induced by thrombin or ADP. These preliminary findings and the new chemistry proposed form the basis for the development of a new class of adenosine analogues that modulate human blood platelet activities.

2′-Deoxyadenosine Bearing Hydrophobic Carborane Pharmacophore

Modification of 2′-deoxyadenosine at position 8 with para-carborane boron cluster is described. Incorporation of boron cluster into nucleic base has been accomplished using Sonogashira palladium-catalyzed cross-coupling reaction or, alternatively, Huisgen “click” type reaction. These are the first examples of adenosine derivatives with hydrophobic carborane pharmacophore attached to purine base.

Towards new boron carriers for boron neutron capture therapy: metallacarboranes bearing cobalt, iron and chromium and their cholesterol conjugates.

A method for the synthesis of cholesterol-metallacarborane conjugates bearing cobalt, iron and chromium was developed. Effective incorporation of the cholesterol conjugate bearing cobalt into liposome membrane was revealed. Using the metallacarborane-encrusted liposomes as boron delivery system in vivo biodistribution experiments in tumor-bearing mice, high accumulation and selective delivery of boron into tumor tissues was observed. The results demonstrate that the cholesterol-metallacarborane conjugates can be considered as a potential candidate for boron delivery vehicle in BNCT.

Synthesis of adenosine containing carborane modification

Abstract The carboranyl cage is a new modifying entity for nucleosides and DNA-oligonucleotides. Most of carborane–nucleoside conjugates described so far belong to pyrimidine series. Herein, the first synthesis of adenosine, nucleoside containing purine nucleic base, modified with carborane cluster, is described.

Electrochemical detection of DNA hybridization using metallacarborane unit.

The evaluation of novel electrochemically active label for electrochemical detection of DNA hybridization is presented. Metallacarborane units modified with iron, cobalt or chromium were investigated. The value of redox potential and relatively strong current signal facilitate usage of Fe-carborane as marker covalently attached to the ssDNA. In electrochemical genosensor the sequence complementary to UL55 gene was labeled and used as a target for biosensor device. Interactions were investigated using electrochemical and piezoelectric methods. Obtained results confirm usefulness of the designed label in electrochemical detection of DNA hybridization.

siRNAs modified with boron cluster and their physicochemical and biological characterization.

RNA interference (RNAi) technology provides a powerful, yet selective, molecular tool to reduce the expression of genes in eukaryotic cells. Despite the success associated with the effective use of siRNA duplexes for gene silencing, there is a need to improve their properties. These properties, related mainly to migration through the cell membranes, stability of siRNA in vivo, and specificity of their silencing activity, can be improved by chemical modifications of siRNA backbone. In this study, we examined the physicochemical and biological properties of siRNA duplexes targeted against BACE1 gene modified at various positions with a lipophilic boron cluster (C2B10H11, CB). The lipophilicity and resistance to enzymatic degradation of the modified oligomers was higher than the unmodified counterparts. As measured in a dual fluorescence assay (BACE1-GFP/RFP), the carboranyl siRNAs (CB-siRNAs) were as active as the parent nonmodified duplexes and their toxicity toward HeLa cells was also similar. The helical structure of CB-siRNAs remained unchanged upon boron cluster introduction, as determined by CD and UV melting experiments.

Phosphorylation of Nucleoside-Metallacarborane and Carborane Conjugates by Nucleoside Kinases

A library of purine and pyrimidine nucleosides modified with carborane or metallacarborane boron clusters at different locations, consisting of new molecules as well as already described compounds, was prepared. The compounds were tested as substrates for human deoxynucleoside kinases. Some conjugates, with modification attached to N3 of thymidine via a linker containing the triazole moiety, were efficiently phosphorylated by cytosolic thymidine kinase 1 and mitochondrial thymidine kinase 2. Higher phosphorylation levels were observed with thymidine kinase 1, the phosphorylation of nucleosides modified with metallacarboranes was observed for the first time.

Nucleoside bearing boron clusters and their phosphoramidites – building blocks for modified oligonucleotide synthesis

This paper describes a general method for the synthesis of four canonical nucleosides T, dC, dA and dG and their phosphoramidites suitable for automated synthesis of DNA modified with a carborane cage. A boron cluster in the form of an electroneutral, lipophilic 1,2-dicarba-closo-dodecaborane (C2B9H11) or negatively charged, redox-active 7,8-dicarba-nido-undecaborate ion (C2B9H12(−1)) was used as a modifying unit. The method is based on the “click chemistry” type Huisgen–Sharpless–Meldal reaction. All boron cluster-nucleoside conjugates have been characterized electrochemically; they have shown different redox potentials allowing for selective electrochemical identification of individual nucleosides in the mixture. There is also the first description of the crystallographic structure of the boron cluster-nucleoside conjugate: N3-{[(o-carboran-1-yl)propyl]-1N-1,2,3-triazol-4-yl}methylenethymidine.

Nucleoside Modification with Boron Clusters and Their Metal Complexes

General methods for the synthesis of nucleosides modified with borane clusters and metallacarborane complexes are presented. These include: (1) the click chemistry approach based on Huisgen 1,3‐dipolar cycloaddition and (2) tethering of the metallacarborane group to the aglycone of a nucleoside via a dioxane ring opening in oxonium metallacarborane derivatives. The proposed methodologies broaden the availability of nucleoside‐borane cluster conjugates and open up new areas for their applications. Curr. Protoc. Nucleic Acid Chem. 38:4.37.1‐4.37.26.