Synthesis, chemical and enzymatic hydrolysis, and aqueous solubility of amino acid ester prodrugs of 3-carboranyl thymidine analogs for boron neutron capture therapy of brain tumors

Prodrugs and their activation mechanisms for brain drug delivery

Prodrugs are masked drugs that first become pharmacologically active after undergoing a structural change in vivo. They are designed to improve physicochemical/biopharmaceutical drug properties and increase site specificity. The prodrug approach is important when developing brain-targeting drugs due to the presence of the brain barriers that seriously limit the brain entry of highly polar, multifunctional drug entities. While several excellent reviews summarize the structural modifications facilitating transport across the brain barriers, a summary of mechanisms used for the activation of the prodrug in the brain is missing. Given the high need for innovative discoveries in brain drug development, we here review the most important tools being developed since 2000 for CNS prodrug activation.

Nucleoside Scaffolds and Carborane Clusters for Boron Neutron Capture Therapy: Developments and Future Perspective

Nucleosides containing carboranes are one of the most important boron delivery agents for boron neutron capture therapy, BNCT, which are good substrates of hTK1. The development of several nucleosides containing carboranes at early stages led to the discovery of the first generation of 3CTAs by incorporating a hydrocarbon spacer between the thymidine scaffold and carborane cluster and attaching dihydroxylpropyl group on the second carbon (C2) atom of the carborane cluster (e.g., N5 and N5-2OH). Phosphorylation rate, tumor cellular uptake, and retention have been evaluated in parallel to change the length of the tether arm of spacers in these compounds. Many attempts were reported and discussed to overcome the disadvantage of the first generation of 3CTAs by a) incorporating modified spacers between thymidine and carborane clusters, such as ethyleneoxide, polyhydroxyl, triazole, and tetrazole units, b) attaching hydrophilic groups at C2 of the carborane cluster, c) transforming lipophilic closo-carboranes to hydrophilic nidocarborane. The previous modifications represented the second generation of 3CTAs to improve the hydrogen bond formation with the hTK1 active site. Moreover, amino acid prodrugs were developed to enhance biological and physicochemical properties. The structure-activity relationship (SAR) of carboranyl thymidine analogues led to the roadmap for the development of the 3rd generation of the 3CTAs for BNCT.

Boron Chemicals in Drug Discovery and Development: Synthesis and Medicinal Perspective

A standard goal of medicinal chemists has been to discover efficient and potent drug candidates with specific enzyme-inhibitor abilities. In this regard, boron-based bioactive compounds have provided amphiphilic properties to facilitate interaction with protein targets. Indeed, the spectrum of boron-based entities as drug candidates against many diseases has grown tremendously since the first clinically tested boron-based drug, Velcade. In this review, we collectively represent the current boron-containing drug candidates, boron-containing retinoids, benzoxaboroles, aminoboronic acid, carboranes, and BODIPY, for the treatment of different human diseases.In addition, we also describe the synthesis, key structure-activity relationship, and associated biological activities, such as antimicrobial, antituberculosis, antitumor, antiparasitic, antiprotozoal, anti-inflammatory, antifolate, antidepressant, antiallergic, anesthetic, and anti-Alzheimer's agents, as well as proteasome and lipogenic inhibitors. This compilation could be very useful in the exploration of novel boron-derived compounds against different diseases, with promising efficacy and lesser side effects.

The Boron Advantage: The Evolution and Diversification of Boron’s Applications in Medicinal Chemistry

In this review, the history of boron’s early use in drugs, and the history of the use of boron functional groups in medicinal chemistry applications are discussed. This includes diazaborines, boronic acids, benzoxaboroles, boron clusters, and carboranes. Furthermore, critical developments from these functional groups are highlighted along with recent developments, which exemplify potential prospects. Lastly, the application of boron in the form of a prodrug, softdrug, and as a nanocarrier are discussed to showcase boron’s emergence into new and exciting fields. Overall, we emphasize the evolution of organoboron therapeutic agents as privileged structures in medicinal chemistry and outline the impact that boron has had on drug discovery and development.

An overview of techniques for multifold enhancement in solubility of poorly soluble drugs

Poor water solubility of newly discovered compounds has become the most common challenge in the drug development process. Indeed, poor solubility is considered as the root cause of failure of drug during drug development phases. Moreover, it has also been reported to be the main reason for bioavailability issues such as poor, inconsistent, incomplete and highly variable bioavailability of the marketed products. As per an estimate, approximately 90% of drug molecules suffer with poor water solubility at early stage and approximately 40% of the marketed drugs have bioavailability problems mainly due to poor water solubility. Solubility enhancement of the newly discovered compounds is primary research area for the pharmaceutical industries and research institutions. The conventional techniques to improve aqueous solubility of drugs employ salt formation, prodrug formation, co-crystallization, complexation, amorphous solid dispersion and use of co-solvent, surfactants or hydrotropic agents. Current advancement in the science and technology has enabled the use of relatively new techniques under the umbrella of nanotechnology. These include the development of nanocrystals, nanosuspensions, nanoemulsions, microemulsions, liposomes and nanoparticles to enhance the solubility. This review focuses on the conventional and current approaches of multifold enhancement in the solubility of poorly soluble marketed drugs, including newly discovered compounds.

Microwave-assisted synthesis and in vitro stability of N-benzylamide non-steroidal anti-inflammatory drug conjugates for CNS delivery

More effective delivery of non-steroidal anti-inflammatory drugs (NSAIDs) to the brain could treat the underlying inflammatory pathology of a range of CNS diseases and conditions. Use of a blood-brain barrier shuttle such as the N-benzylamide moiety, which has been largely unexplored for this purpose, could improve the brain bioavailabilities of NSAIDs. A series of novel N-benzylamide NSAID conjugates was synthesized via a three-step process with a microwave-assisted bimolecular nucleophilic substitution as the final step. We explored conditions to promote substitution over a competing elimination reaction, which was successfully suppressed with isopropyl alcohol solvent. All molecules exhibit physicochemical properties consistent with those of brain-penetrant molecules. Furthermore, they exhibit long (>48 h) half-lives in phosphate-buffered saline (PBS; pH 7.4) and short to moderate half-lives in human plasma. N-Benzylamide NSAID conjugates represent promising CNS drug discovery leads.

Amino Acids in the Development of Prodrugs

Although drugs currently used for the various types of diseases (e.g., antiparasitic, antiviral, antibacterial, etc.) are effective, they present several undesirable pharmacological and pharmaceutical properties. Most of the drugs have low bioavailability, lack of sensitivity, and do not target only the damaged cells, thus also affecting normal cells. Moreover, there is the risk of developing resistance against drugs upon chronic treatment. Consequently, their potential clinical applications might be limited and therefore, it is mandatory to find strategies that improve those properties of therapeutic agents. The development of prodrugs using amino acids as moieties has resulted in improvements in several properties, namely increased bioavailability, decreased toxicity of the parent drug, accurate delivery to target tissues or organs, and prevention of fast metabolism. Herein, we provide an overview of models currently in use of prodrug design with amino acids. Furthermore, we review the challenges related to the permeability of poorly absorbed drugs and transport and deliver on target organs.

Towards carborane-functionalised structures for the treatment of brain cancer

Boron neutron capture therapy (BNCT) is a promising targeted chemoradiotherapeutic technique for the management of invasive brain tumors, such as glioblastoma multiforme (GBM). A prerequisite for effective BNCT is the selective targeting of tumour cells with ¹⁰B-rich therapeutic moieties. To this end, polyhedral boranes, especially carboranes, have received considerable attention because they combine a high boron content with relative low toxicity and metabolic inertness. Here, we review progress in the molecular design of recently investigated carborane derivatives in light of the widely accepted performance requirements for effective BNCT.

Prodrug approach: An overview of recent cases

In this review we highlight the most modern trends in the prodrug strategy. In drug research and development, the prodrug concept has found a number of useful applications. Selected examples of this approach are provided in this paper and they are classified according to the aim of their design.

Multi-small molecule conjugations as new targeted delivery carriers for tumor therapy

In response to the challenges of cancer chemotherapeutics, including poor physicochemical properties, low tumor targeting ability, and harmful side effects, we developed a new tumor-targeted multi-small molecule drug delivery platform. Using paclitaxel (PTX) as a model therapeutic, we prepared two prodrugs, ie, folic acid-fluorescein-5(6)-isothiocyanate-arginine-paclitaxel (FA-FITC-Arg-PTX) and folic acid-5-aminofluorescein-glutamic-paclitaxel (FA-5AF-Glu-PTX), composed of folic acid (FA, target), amino acids (Arg or Glu, linker), and fluorescent dye (fluorescein in vitro or near-infrared fluorescent dye in vivo) in order to better understand the mechanism of PTX prodrug targeting. In vitro and acute toxicity studies demonstrated the low toxicity of the prodrug formulations compared with the free drug. In vitro and in vivo studies indicated that folate receptor-mediated uptake of PTX-conjugated multi-small molecule carriers induced high antitumor activity. Notably, compared with free PTX and with PTX-loaded macromolecular carriers from our previous study, this multi-small molecule-conjugated strategy improved the water solubility, loading rate, targeting ability, antitumor activity, and toxicity profile of PTX. These results support the use of multi-small molecules as tumor-targeting drug delivery systems.

COSAN as a molecular imaging platform: synthesis and "in vivo" imaging

A labelling method for the covalent attachment of radioiodine to the boron-rich 8-I-cobaltabisdicarbollide (I-COSAN) and a bi-functional (iodine and PEG) COSAN derivative, [3,3'-Co(8-I-1,2-C2B9H10)(8'-(OCH2CH2)2COOC6H5-1',2'-C2B9H10)], is reported. Biodistribution studies in rodents using dissection/gamma counting and in vivo nuclear imaging have been performed. The general strategy reported here can be applied in the future to COSAN derivatives bearing a wide range of functionalities.

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.

Targeting (cellular) lysosomal acid ceramidase by B13: design, synthesis and evaluation of novel DMG-B13 ester prodrugs

Acid ceramidase (ACDase) is being recognized as a therapeutic target for cancer. B13 represents a moderate inhibitor of ACDase. The present study concentrates on the lysosomal targeting of B13 via its N,N-dimethylglycine (DMG) esters (DMG-B13 prodrugs). Novel analogs, the isomeric mono-DMG-B13, LCL522 (3-O-DMG-B13·HCl) and LCL596 (1-O-DMG-B13·HCl) and di-DMG-B13, LCL521 (1,3-O, O-DMG-B13·2HCl) conjugates, were designed and synthesized through N,N-dimethyl glycine (DMG) esterification of the hydroxyl groups of B13. In MCF7 cells, DMG-B13 prodrugs were efficiently metabolized to B13. The early inhibitory effect of DMG-B13 prodrugs on cellular ceramidases was ACDase specific by their lysosomal targeting. The corresponding dramatic decrease of cellular Sph (80-97% Control/1h) by DMG-B13 prodrugs was mainly from the inhibition of the lysosomal ACDase.

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.

N3-substituted thymidine bioconjugates for cancer therapy and imaging

The compound class of 3-carboranyl thymidine analogues (3CTAs) are boron delivery agents for boron neutron capture therapy (BNCT), a binary treatment modality for cancer. Presumably, these compounds accumulate selectively in tumor cells via intracellular trapping, which is mediated by hTK1. Favorable in vivo biodistribution profiles of 3CTAs led to promising results in preclinical BNCT of rats with intracerebral brain tumors. This review presents an overview on the design, synthesis, and biological evaluation of first- and second-generation 3CTAs. Boronated nucleosides developed prior to 3CTAs for BNCT and non-boronated N3-substituted thymidine conjugates for other areas of cancer therapy and imaging are also described. In addition, basic features of carborane clusters, which are used as boron moieties in the design and synthesis of 3CTAs, and the biological and structural features of TK1-like enzymes, which are the molecular targets of 3CTAs, are discussed.

Boron chemicals in diagnosis and therapeutics

Advances in the field of boron chemistry have expanded the application of boron from material use to medicine. Boron-based drugs represent a new class of molecules that possess several biomedical applications including use as imaging agents for both optical and nuclear imaging as well as therapeutic agents with anticancer, antiviral, antibacterial, antifungal and other disease-specific activities. For example, bortezomib (Velcade(®)), the only drug in clinical use with boron as an active element, was approved in 2003 as a proteasome inhibitor for the treatment of multiple myeloma and non-Hodgkin's lymphoma. Several other boron-based compounds are in various phases of clinical trials, which illustrates the promise of this approach for medicinal chemists working in the area of boron chemistry. It is expected that in the near future, several boron-containing drugs should become available in the market with better efficacy and potency than existing drugs. This article discusses the current status of the development of boron-based compounds as diagnostic and therapeutic agents in humans.

Remarkable cage deboronation and rearrangement of a closo-1,12-dicarbadodecaborane to form a neutral nido-7,9-dicarbaundecaborane

Deboronation and cage rearrangement of the closo-1,12-carborane salt [1,12-(PPh2Me)2-1,12-C2B10H10]I2 occurs in refluxing methanol to give the zwitterionic nido-7,9-carborane 7,9-(PPh2Me)2-7,9-C2B9H9. Notably, deboronation and cage substitution of the isomeric closo-1,7-carborane salt [1,7-(PPh2Me)2-1,7-C2B10H10]I2 takes place in methanol to afford the salt [10-OMe-7,9-(PPh2Me)2-7,9-C2B9H9]I.

Synthesis of N3-substituted carboranyl thymidine bioconjugates and their evaluation as substrates of recombinant human thymidine kinase 1

Four different libraries of overall twenty three N3-substituted thymidine (dThd) analogues, including eleven 3-carboranyl thymidine analogues (3CTAs), were synthesized. The latter are potential agents for Boron Neutron Capture Therapy (BNCT) of cancer. Linker between the dThd scaffold and the m-carborane cluster at the N3-position of the 3CTAs contained amidinyl-(3e and 3f), guanidyl-(7e-7g), tetrazolylmethyl-(9b1/2-9d1/2), or tetrazolyl groups (11b1/2-11d1/2) to improve human thymidine kinase 1 (hTK1) substrate characteristics and water solubilities compared with 1st generation 3CTAs, such as N5 and N5-2OH. The amidinyl- and guanidyl-type N3-substitued dThd analogues (3a-3f and 7a-7g) had hTK1 phosphorylation rates of <30% relative to that of dThd, the endogenous hTK1 substrate, whereas the tetrazolyl-type N3-substitued dThd analogues (9a, 9b1/2-9d1/2 and 11a, 11b1/2-11d1/2) had relative phosphorylation rates (rPRs) of >40%. Compounds 9a, 9b1/2-9d1/2 and 11a, 11b1/2-11d1/2 were subjected to in-depth enzyme kinetics studies and the obtained rk(cat)/K(m) (k(cat)/K(m) relative to that of dThd) ranged from 2.5 to 26%. The tetrazolyl-type N3-substitued dThd analogues 9b1/2 and 11d1/2 were the best substrates of hTK1 with rPRs of 52.4% and 42.5% and rk(cat)/K(m) values of 14.9% and 19.7% respectively. In comparison, the rPR and rk(cat)/K(m) values of N5-2OH in this specific study were 41.5% and 10.8%, respectively. Compounds 3e and 3f were >1900 and >1500 times, respectively, better soluble in PBS (pH 7.4) than N5-2OH whereas solubilities for 9b1/2-9d1/2 and 11b1/2-11d1/2 were only 1.3-13 times better.