N3-substituted thymidine bioconjugates for cancer therapy and imaging

Targeted strategies to deliver boron agents across the blood-brain barrier for neutron capture therapy of brain tumors

Boron neutron capture therapy (BNCT), as an innovative radiotherapy technology, has demonstrated remarkable outcomes when compared to conventional treatments in the management of recurrent and refractory brain tumors. However, in BNCT of brain tumors, the blood-brain barrier is a main stumbling block for restricting the transport of boron drugs to brain tumors, while the tumor targeting and retention of boron drugs also affect the BNCT effect. This review focuses on the recent development of strategies for delivering boron drugs crossing the blood-brain barrier and targeting brain tumors, providing new insights for the development of efficient boron drugs for the treatment of brain tumors.

Boron Neutron Capture Therapy: Current Status and Challenges

Boron neutron capture therapy (BNCT) is a re-emerging therapy with the ability to selectively kill tumor cells. After the boron delivery agents enter the tumor tissue and enrich the tumor cells, the thermal neutrons trigger the fission of the boron atoms, leading to the release of boron atoms and then leading to the release of the α particles (⁴He) and recoil lithium particles (⁷Li), along with the production of large amounts of energy in the narrow region. With the advantages of targeted therapy and low toxicity, BNCT has become a unique method in the field of radiotherapy. Since the beginning of the last century, BNCT has been emerging worldwide and gradually developed into a technology for the treatment of glioblastoma multiforme, head and neck cancer, malignant melanoma, and other cancers. At present, how to develop and innovate more efficient boron delivery agents and establish a more accurate boron-dose measurement system have become the problem faced by the development of BNCT. We discuss the use of boron delivery agents over the past several decades and the corresponding clinical trials and preclinical outcomes. Furthermore, the discussion brings recommendations on the future of boron delivery agents and this therapy.

Application of Nitroimidazole-Carbobane-Modified Phenylalanine Derivatives as Dual-Target Boron Carriers in Boron Neutron Capture Therapy

Boron neutron capture therapy (BNCT) has received extensive attention as noninvasive cell-level oncotherapy for treating solid cancer tumors. However, boron-containing drugs such as l-boronophenylalanine (BPA) and sodium borocaptate have low boron content and/or poor tumor-targeting ability, limiting their application. In this study, we designed and synthesized a series of nontoxic, dual-target boron carriers (B139, B142, and B151) with the ability to accumulate specifically in tumor cells. We found that the B139 uptake into hypoxic tumor regions was high, with a 70-fold boron content compared to BPA. In addition, in vivo observation showed that B139 can be trapped in tumor cells for a prolonged period and maintains an effective therapeutic concentration, with a peak boron concentration of 50.7 μg/g and a high tumor: blood boron ratio of >3, achieving ideal BNCT conditions. Cytotoxicity evaluation in mice further proved that B139 is safe and reliable. Therefore, B139 has great potential for BNCT application as a dual-target, safe, and efficient boron carrier.

Transition metal-free oxidative and deoxygenative C-H/C-Li cross-couplings of 2H-imidazole 1-oxides with carboranyl lithium as an efficient synthetic approach to azaheterocyclic carboranes

The direct C-H functionalization methodology has first been applied to perform transition metal-free C-H/C-Li cross-couplings of 2H-imidazole 1-oxides with carboranyllithium. This atom- and step-economical approach, based on one-pot reactions of nucleophilic substitution of hydrogen (SN H) in non-aromatic azaheterocycles, affords novel imidazolyl-modified carboranes of two types (N-oxides and their deoxygenative analogues), which are particularly of interest in the design of advanced materials.

The Current Status and Perspectives of Delivery Strategy for Boron-based Drugs

Boron-containing compounds are essential micronutrients for animals and plants despite their low-level natural occurrence. They can strengthen the cell walls of the plants and they play important role in supporting bone health. However, surprisingly, boron-containing compounds are seldom found in pharmaceutical drugs. In fact, there are no inherent disadvantages reported so far in terms of the incorporation of boron into medicines. Indeed, drugs based on boron-containing compounds, such as tavaborole (marked name Kerydin) and bortezomib (trade name Velcade) have been investigated and they are used in clinical treatment. In addition, following the advanced development of boron neutron capture therapy and a new emerging proton boron fusion therapy, more boron-containing medicinals are to be expected. This review discusses the current status and perspectives of delivery strategy for boron-containing drugs.

A stable meta-carborane enables the generation of boron-rich peptide agonists targeting the ghrelin receptor

Boron neutron capture therapy (BNCT) is a binary cancer therapy, which combines the biochemical targeting of a boron-containing drug with the regional localization of radiation treatment. Although the concept of BNCT has been known for decades, the selective delivery of boron into tumor cells remains challenging. G protein-coupled receptors that are overexpressed on cancer cells in combination with peptidic ligands can be potentially used as shuttle system for a tumor-directed boron uptake. In this study, we present the generation of short, boron-rich peptide conjugates that target the ghrelin receptor. Expression of the ghrelin receptor on various cancer cells makes it a viable target for BNCT. We designed a novel hexapeptide super-agonist that was modified with different specifically synthesized carborane monoclusters and tested for ghrelin receptor activation. A meta-carborane building block with a mercaptoacetic acid linker was found to be optimal for peptide modification, owing to its chemical stability and a suitable activation efficacy of the conjugate. The versatility of this carborane for the development of peptidic boron delivery agents was further demonstrated by the generation of highly potent, boron-loaded conjugates using the backbone of the known ghrelin receptor ligands growth hormone releasing peptide 6 and Ipamorelin.

Novel 99mTc-labelled complexes with thymidine isocyanide: radiosynthesis and evaluation as potential tumor imaging tracers

A novel thymidine isocyanide (CN-TdR) functionalized at the N3 position of thymidine was synthesized and then radiolabelled with 99mTc(i) and [99mTc(i)(CO)3]+ cores to produce [99mTc(CN-TdR)6]+ and [99mTc(CO)3(CN-TdR)3]+, respectively. Both of them were prepared with high radiochemical purity and were stable over 6 h in saline at ambient temperature and in serum at 37 °C. The partition coefficient results demonstrated that they were hydrophilic. The cell internalization studies showed that their uptake might be mediated by nucleoside transporters. Biodistribution of these complexes in mice bearing the S180 tumor showed that they accumulated in the tumor with high uptake and cleared rapidly from blood and muscles, producing high tumor/blood and tumor/muscle ratios. Between them, [99mTc(CN-TdR)6]+ exhibited advantages concerning a higher tumor uptake, tumor/blood ratio and tumor/muscle ratio at 60 min post-injection. Single photon emission computed tomography imaging studies showed that there was a clear accumulation in tumor sites, suggesting that [99mTc(CN-TdR)6]+ could be a promising candidate for tumor imaging.

Nucleic Acid Bioconjugates in Cancer Detection and Therapy

Nucleoside- and nucleotide-based chemotherapeutics have been used to treat cancer for more than 50 years. However, their inherent cytotoxicities and the emergent resistance of tumors against treatment has inspired a new wave of compounds in which the overall pharmacological profile of the bioactive nucleic acid component is improved by conjugation with delivery vectors, small-molecule drugs, and/or imaging modalities. In this manner, nucleic acid bioconjugates have the potential for targeting and effecting multiple biological processes in tumors, leading to synergistic antitumor effects. Consequently, tumor resistance and recurrence is mitigated, leading to more effective forms of cancer therapy. Bioorthogonal chemistry has led to the development of new nucleoside bioconjugates, which have served to improve treatment efficacy en route towards FDA approval. Similarly, oligonucleotide bioconjugates have shown encouraging preclinical and clinical results. The modified oligonucleotides and their pharmaceutically active formulations have addressed many weaknesses of oligonucleotide-based drugs. They have also paved the way for important advancements in cancer diagnosis and treatment. Cancer-targeting ligands such as small-molecules, peptides, and monoclonal antibody fragments have all been successfully applied in oligonucleotide bioconjugation and have shown promising anticancer effects in vitro and in vivo. Thus, the application of bioorthogonal chemistry will, in all likelihood, continue to supply a promising pipeline of nucleic acid bioconjugates for applications in cancer detection and therapy.

Evaluation of TK1 targeting carboranyl thymidine analogs as potential delivery agents for neutron capture therapy of brain tumors

In this report we describe studies with N5-2OH, a carboranyl thymidine analog (CTA), which is a substrate for thymidine kinase 1 (TK1), using the F98 rat glioma model. In vivo BNCT studies have demonstrated that intracerebral (i.c.) osmotic pump infusion of N5-2OH yielded survival data equivalent to those obtained with i.v. administration of boronophenylalanine (BPA). The combination of N5-2OH and BPA resulted in a modest increase in MST of F98 glioma bearing rats compared to a statistically significant increase with the RG2 glioma model, as has been previously reported by us (Barth et al., 2008). This had lead us to synthesize a second generation of CTAs that have improved in vitro enzyme kinetics and in vivo tumor uptake (Agarwal et al., 2015).

Synthesis and evaluation of thymidine kinase 1-targeting carboranyl pyrimidine nucleoside analogs for boron neutron capture therapy of cancer

A library of sixteen 2nd generation amino- and amido-substituted carboranyl pyrimidine nucleoside analogs, designed as substrates and inhibitors of thymidine kinase 1 (TK1) for potential use in boron neutron capture therapy (BNCT) of cancer, was synthesized and evaluated in enzyme kinetic-, enzyme inhibition-, metabolomic-, and biodistribution studies. One of these 2nd generation carboranyl pyrimidine nucleoside analogs (YB18A [3]), having an amino group directly attached to a meta-carborane cage tethered via ethylene spacer to the 3-position of thymidine, was approximately 3-4 times superior as a substrate and inhibitor of hTK1 than N5-2OH (2), a 1st generation carboranyl pyrimidine nucleoside analog. Both 2 and 3 appeared to be 5'-monophosphorylated in TK1(+) RG2 cells, both in vitro and in vivo. Biodistribution studies in rats bearing intracerebral RG2 glioma resulted in selective tumor uptake of 3 with an intratumoral concentration that was approximately 4 times higher than that of 2. The obtained results significantly advance the understanding of the binding interactions between TK1 and carboranyl pyrimidine nucleoside analogs and will profoundly impact future design strategies for these agents.

Nanoscaled boron-containing delivery systems and therapeutic agents for cancer treatment

Significant efforts have recently been made to develop nanoscaled boron-containing delivery systems for improving drug delivery in cancer therapy. On one hand, borate ester chemistry has shown importance in ligand-mediated tumor targeting owing to the recognition ability of boronic acid to polyol residues in cell membranes. In particular, the phenylboronic acid-functionalized nanocarriers for specific targeting to sialic acid groups which are overexpressed on tumor cells have made great achievements. On the other hand, nanoscaled boron neutron capture therapy agents show growing potential in efficiently transporting boron to tumor. The current review outlines the recent developments in the application of borate ester chemistry in tumor targeting by nanoparticles, then summarizes recent work on the development of boron-based nanomaterials as boron neutron capture therapy agents.

Iodine monochloride facilitated deglycosylation, anomerization, and isomerization of 3-substituted thymidine analogues

The reaction of thymidine, 3-mono-, and 3,3',5'-trialkylsubstitued thymidine analogues with iodine monochloride (ICl) was investigated. Treatment with ICl resulted in rapid deglycosylation, anomerization, and isomerization of thymidine and 3-substituted thymidine analogues under various reaction conditions leading to the formation of the nucleobases as the major products accompanied by minor formation of α-furanosidic-, α-pyranosidic-, and β-pyranosidic nucleosides. On the other hand, 3,3',5'-trisubstitued thymidine analogues were only deglycosylated and anomerized. These results are similar to those observed for the acidic hydrolysis of the glycoside bond in nucleosides, but were presumably caused by the Lewis acid character of an iodine electrophile.

Cellular influx, efflux, and anabolism of 3-carboranyl thymidine analogs: potential boron delivery agents for neutron capture therapy

3-[5-{2-(2,3-Dihydroxyprop-1-yl)-o-carboran-1-yl}pentan-1-yl]thymidine (N5-2OH) is a first generation 3-carboranyl thymidine analog (3CTA) that has been intensively studied as a boron-10 ((10)B) delivery agent for neutron capture therapy (NCT). N5-2OH is an excellent substrate of thymidine kinase 1 and its favorable biodistribution profile in rodents led to successful preclinical NCT of rats bearing intracerebral RG2 glioma. The present study explored cellular influx and efflux mechanisms of N5-2OH, as well as its intracellular anabolism beyond the monophosphate level. N5-2OH entered cultured human CCRF-CEM cells via passive diffusion, whereas the multidrug resistance-associated protein 4 appeared to be a major mediator of N5-2OH monophosphate efflux. N5-2OH was effectively monophosphorylated in cultured murine L929 [thymidine kinase 1 (TK1(+))] cells whereas formation of N5-2OH monophosphate was markedly lower in L929 (TK1(-)) cell variants. Further metabolism to the di- and triphosphate forms was not observed in any of the cell lines. Regardless of monophosphorylation, parental N5-2OH was the major intracellular component in both TK1(+) and TK1(-) cells. Phosphate transfer experiments with enzyme preparations showed that N5-2OH monophosphate, as well as the monophosphate of a second 3-carboranyl thymidine analog [3-[5-(o-carboran-1-yl)pentan-1-yl]thymidine (N5)], were not substrates of thymidine monophosphate kinase. Surprisingly, N5-diphosphate was phosphorylated by nucleoside diphosphate kinase although N5-triphosphate apparently was not a substrate of DNA polymerase. Our results provide valuable information on the cellular metabolism and pharmacokinetic profile of 3-carboranyl thymidine analogs.