key: cord-0026728-6lth66sj authors: Chen, Yu; Du, Fukuan; Tang, Liyao; Xu, Jinrun; Zhao, Yueshui; Wu, Xu; Li, Mingxing; Shen, Jing; Wen, Qinglian; Cho, Chi Hin; Xiao, Zhangang title: Carboranes as unique pharmacophores in antitumor medicinal chemistry date: 2022-01-10 journal: Mol Ther Oncolytics DOI: 10.1016/j.omto.2022.01.005 sha: 167dbcc449317f05dfbdaa7d962d09f811b38ace doc_id: 26728 cord_uid: 6lth66sj Carborane is a carbon-boron molecular cluster that can be viewed as a 3D analog of benzene. It features special physical and chemical properties, and thus has the potential to serve as a new type of pharmacophore for drug design and discovery. Based on the relative positions of two cage carbons, icosahedral closo-carboranes can be classified into three isomers, ortho-carborane (o-carborane, 1,2-C(2)B(10)H(12)), meta-carborane (m-carborane, 1,7-C(2)B(10)H(12)), and para-carborane (p-carborane, 1,12-C(2)B(10)H(12)), and all of them can be deboronated to generate their nido- forms. Cage compound carborane and its derivatives have been demonstrated as useful chemical entities in antitumor medicinal chemistry. The applications of carboranes and their derivatives in the field of antitumor research mainly include boron neutron capture therapy (BNCT), as BNCT/photodynamic therapy dual sensitizers, and as anticancer ligands. This review summarizes the research progress on carboranes achieved up to October 2021, with particular emphasis on signaling transduction pathways, chemical structures, and mechanistic considerations of using carboranes. Carboranes, boron-carbon molecular cage compounds, are often viewed as the 3D analogs of benzene. 1 They have a wide range of applications as useful functional building blocks in material science, 2-10 organometallic/coordination chemistry, [11] [12] [13] [14] [15] [16] [17] and medicinal chemistry. [18] [19] [20] [21] In this context, considerable progress has been made in carborane functionalization. [22] [23] [24] [25] [26] [27] [28] The special physical and chemical properties of carboranes allow the design of carboranecontaining molecules with new and better antitumor activities, and thus offer medicinal chemists a unique opportunity to explore these new chemical entities for cancer therapy. 1, [18] [19] [20] [21] The most recent comprehensive review regarding carboranes as pharmacophores in medicinal chemistry, by Scholz and Hey-Hawkins, appeared a decade ago, 18 which did not cover the recent research progress in this area. This review highlights the major achievements in the field of carboranes as pharmacophores in antitumor medicinal chemistry, with particular emphasis on signaling transduction pathways, chemical structures, and mechanistic considerations ( Figure 1 ). In boron neutron capture therapy (BNCT), the first step is the selective accumulation of 10 B-containing compounds in cancer cells, which can be irradiated by low-energy and harmless thermal neutrons. 29-36 Subsequently, 10 B atoms break up into a particles and lithium nuclei, yielding high linear energy transfer (LET) particles ( Figure 2 ). [29] [30] [31] [32] [33] [34] [35] [36] As a result, 10 B-containing cancer cells can be destroyed by the high-LET particles. In contrast, the surrounding normal/healthy cells can survive because of the limited path length of these particles of only 5-9 mm, which is smaller than the diameter of a general cell. 29,35 If 10 B-containing compounds only accumulated in cancer cells, the thermal neutron irradiation would selectively eliminate tumors under BNCT conditions ( Figure 2 ). 36 Therefore, it is critical to selectively deliver large amounts of 10 B-containing compounds into cancer cells rather than normal cells. However, only two boron-containing compounds, (L)-4-dihydroxy-borylphenylalanine (BPA, 1) and sodium mercaptoundecahydro-closo-dodecarborate (BSH, 2) ( Figure 3A ), are currently available as BNCT agents in clinical use. 37 This may be ascribed to the low selectivity for cancer cells except for brain tumors as well as head and neck cancers (HNC). 38 Carboranes possess unique physical and chemical properties. They have high content of 10 B atoms and the highest neutron capture cross-section. Thus they can be ideal candidates for BNCT. 39 Currently, a considerable number of carborane-containing compounds have been investigated for BNCT and are discussed in the following. and the polymerized nanoparticles can be formed on a single backbone chain. 47 Carborane derivatives bound to nanoparticles have many advantages, including high stability, high accumulation in cancer cells, and ready preparation, and have become potential drug-delivery systems for nanomedicines and BNCT. 46, 47 Ruthenium and osmium are a class of transition metals widely used in cancer therapy. [48] [49] [50] [51] [52] [53] [54] [55] Complexes containing electron-deficient ruthenium and osmium carborane were reported by the Sadler and Hanna groups. 50,51 The redox-active response of these carborane-containing complexes to biomolecules have resulted in their potential application in cancer therapy. 48 Additionally, Pluronic is a kind of amphiphilic block copolymer with good biocompatibility whose nanostructures have been widely used in biomedical fields. 52-55 Sadler and coworkers employed Pluronic triblock copolymer P123 micelles to encapsulate 16 electron complexes, yielding polymer micelles RuMs and OsMs in water through the self-assembly of nanoparticles ( Figure 3B ). 50 They showed greater selectivity to cancer cells as well as higher intracellular boron concentrations compared with normal cells. 50,52 These findings have provided promising complexes for BNCT. Llop and co-workers recently reported that boron-rich gold nanoparticles (AuNPs, 3) (Figure 3C ) could serve as drug carriers for BNCT. 56 Multifunctional AuNPs (core diameter 4.1 ± 1.5 nm) were synthesized as drug carriers with potential applications in BNCT. 56 On the other hand, the Wang group reported that selfassembled gold nanoclusters were able to combine with carborane amino derivatives with good biocompatibility and stability. 57 They achieved selective delivery of carborane derivatives into tumor tissues through the enhanced permeability and retention effect as well as a nanoscale effect. 57 Among the nanoscale boron carriers, the carborane-loaded nanoscale covalent organic polymers (BCOPs) and magnetic nanoparticles are effective carriers in the BNCT. 58-62 Multifunctional BCOPs were prepared from a Schiff base condensation reaction and further functionalized into BCOP-5T (five octyl chains) through alkyl chain engineering and size adjustment. 58 With the loading of carborane, the 1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-[amino-(polyethyleneglycol)-2000] (M w = 2000) coated with BCOP-5T exhibits excellent physiological stability and biocompatibility, and has been used as a carborane-loaded nanocarrier in BNCT. 58 Hosmane and co-workers showed that through the click reaction, the carborane cage was successfully adsorbed into the modified magnetic nanoparticles (4) ( Figure 3D ). 61 Doxorubicin (DOX) is an anthracycline antitumor antibiotic with a variety of hydroxyl and amino motifs. 63-68 Although DOX has been widely used in the treatment of a variety of cancers, serious cardiotoxicity is a major challenge. 63 Nanoparticle delivery systems can potentially improve the efficacy and reduce the toxicity of DOX. 64-66, 69 The Yan group reported that DOX combined with nanoparticles reduced the toxicity in the treatment of liver cancer. 67 To achieve combined administration of DOX in BNCT and chemotherapy ( Figure 3E ), carborane conjugated amphiphilic copolymer PEG-b-P (LA-co-MPCB) (PLMB, 5) nanoparticles were synthesized by Huang and colleagues. 68 DOX@PLMB (6) was formed by self-assembly of nanoparticles, which prevented boron compounds from leakage into the bloodstream by virtue of the covalent bond between carborane and the backbone chain of polymerization. 68 Also, it was able to protect DOX from bursting release due to its dihydrogen bonding with carborane. Moreover, these authors found that the blood circulation time of DOX@PLMB was prolonged and boron accumulation was increased in tumor tissues. Therefore, DOX@PLMB has reduced systemic toxicity and an improved therapeutic effect. 68 Carboranes bound to PEG/liposomes Hepatocellular carcinoma (HCC) is the most common cause of death from cancer. [70] [71] [72] [73] [74] [75] D'Souza and Devarajan reported that the asialogly-coprotein receptor (ASGPR) could serve as an ideal target specific for delivery to HCC. 76 The Zhou group reported that galactose and lactose residues had a strong affinity for ASGPR. 77 The carboranecontaining clusters self-assembled into micelles with HCC-targeting property were synthesized by Liu and co-workers ( Figure 3F ). 78 Compared with BSH, the carborane-containing micelles enhance the selectivity and absorptive capacity of hepatoma carcinoma cells, and thus the cytotoxicity is reduced. The micelles can weaken the migratory behavior and induce apoptosis of cancer cells by destroying double-stranded DNA during cancer treatment. 78 Among the macromolecular substances, liposomes fused with cell membranes delivered boron-containing elements into tumor tissues. [79] [80] [81] [82] [83] Hawthorne's group reported using liposomes as a transport medium for boron elements. 79, 80 Liposomes were converted into an ammonio derivative, Na 3 [1-(2 0 -B 10 H 9 )-2NH 3 B 10 H 8 ] (TAC), which provided high concentration and long residence time of boron in cancer cells. Hawthorne and co-workers also designed a complementary lipophilic reagent, K[nido-7CH 3 (CH 2 ) 15 -7,8-C 2 B 9 H 11 ] (MAC), which could be stably incorporated into the liposomal bilayer ( Figure 3G ). 79 Subsequently, the liposomal bilayer specifically bound to receptors on the surface of cancer cells and entered into cancer cells through endocytosis. 79 It was demonstrated that the inclusion of amphoteric nido-carborane in the liposomal bilayer accumulated a high concentration of boron in tumor tissues. 79 Also, the application of BNCT was able to encapsulate carboranes in small unilamellar liposomes to selectively deliver 10 B to synovial tissues of patients with rheumatoid arthritis (RA). 84 Moreover, as polyethylene glycol (PEG) is biocompatible, [85] [86] [87] the carborane-PEG conjugate has been used as a new type of boron carrier. [85] [86] [87] The simple encapsulation of nido-carborane anions in PEG liposomes as the boron carriers of BNCT was reported by Lee et al. 85 PEGylated liposomes effectively delivered boron compounds by encapsulating carborane and delivering it into tumor cells. 85 Carboranes bound to peptide ligands G-protein-coupled receptors (GPCRs) refer to a large family of cellsurface receptors. [88] [89] [90] [91] [92] As a seven-pass transmembrane protein, GPCRs that are overexpressed on the membrane of cancer cells bound to peptide ligands and thus could be used as a shuttle for tumor-directed boron absorption systems ( Figure 3H ). 88 Among them, human Y1 receptor (hY1R), 88, 89 gastrin-releasing peptide receptor (GRPR), 90 and ghrelin receptor (GhrR) 91, 92 have become viable targets for BNCT by virtue of their high expression on the surface of cancer cells and their ability to internalize the bound ligands. In this context, the groups of Beck-Sickinger 88 and Hey-Hawkins [89] [90] [91] reported that the combination of carborane and hY1R, GRPR, or GhrR could represent a boron delivery agent in BNCT for the delivery of therapeutic drugs to cancer cells. Neuropeptide Y (NPY), a peptide of the three-membered NPY hormone family, 88, 89 bound to hY1R to form an NPY complex (8) , and the complex was internalized into cells through receptor-mediated endocytosis. Carborane was introduced into the NPY complex by solid-phase peptide synthesis. A boronmodified NPY complex was then used as the boron carrier of BNCT, which selectively delivered therapeutic drugs into breast cancer cells. 88, 89 The significant overexpression of GRPR in various malignant tumor tissues makes it a very attractive target, 90,91 whereby carborane could be attached to the peptide conjugates targeting tumor cells through GPCRs overexpressed on cancer cell membranes. Similarly, the expression of GhrR on a variety of cancer cells makes it a viable target for BNCT. 91, 92 GhrR could serve as a delivery system of BNCT to deliver high doses of boron into cancer cells. 92 Cyclooxygenase-2 (COX-2) is highly expressed in HNC, [93] [94] [95] which can be used as a potential target for HNC. [96] [97] [98] [99] The Chen group developed a novel carborane-containing COX-2 inhibitor (9), which was able to induce apoptosis of cancer cells through BNCT and was effectively used for the treatment of HNC. 93 It was shown that COX-2 inhibitor causes DNA double-strand breaks and forms reactive oxygen species, followed by downregulation of expression of phosphatidylinositol 3-kinase/protein kinase B (PI3K/Akt) and mitogen-activated protein kinase, finally inducing apoptosis of cancer cells in BNCT ( Figure 4 ). 93 Matrix metalloproteinases (MMPs) are a type of zinc-dependent endopeptidase that participate in the remodeling and degradation of all components of the extracellular matrix. [100] [101] [102] [103] [104] It has been reported that carborane can combine with MMP ligands to deliver boron atoms into cancer cells, achieving the use of BNCT for dual therapy of tumors. 104 Carbonic anhydrase IX (CAIX, 10) is an enzyme overexpressed in mesothelioma and breast cancer cells. 105-108 CAIX inhibitors specifically bind to the low-density lipoprotein (LDL) receptor on the surface of cancer cells to form receptor-LDL complexes, and is delivered into cancer cells through endocytosis ( Figure 3I ). 105 Sulfonamidofunctionalized-carborane (CA-SF) was discovered by the Geninatti-Crich group. 105 CA-SF served as a CAIX inhibitor and boron delivery agent and has been used in BNCT ( Figure 3I ) to inhibit the growth of mesothelioma and breast cancer cells. Receptor tyrosine kinases (RTKs) are a subclass of tyrosine kinases that are involved in mediating cell-to-cell communication and controlling a wide range of complex biological functions. [109] [110] [111] [112] Carborane-containing RTK inhibitors (11) (12) (13) (14) (15) (16) (17) (18) (19) were designed for the treatment of glioblastoma and prostate cancer ( Figure 3J ). [113] [114] [115] [116] Recently, the combination of RTK inhibitors and BNCT, i.e., combination therapy, was suggested by Cerecetto and co-workers. 113 They proposed a plausible mechanism whereby Hedgehog signal cascade non-canonically upregulated glioma zinc finger transcription factors via the FLT3/PI3K signaling pathway. RTK inhibitors specifically bound to receptors on the surface of cancer cells and were delivered into cancer cells through endocytosis ( Figure 3K ). 113 Prostate-specific membrane antigen (PSMA), i.e., glutamate carboxypeptidase II, is an enzyme highly expressed on the surface of prostate cancer cells. PSMA is commonly used as a target for prostate cancer imaging and drug delivery. [117] [118] [119] [120] [121] As boron-containing inhibitors generally had high binding affinity to PSMA, Flavell and co-workers combined PSMA inhibitor scaffolds with boric acids/carborane derivatives, delivering boron into prostate cancer cells and prostate tumor xenograft models (20, Figure 3L ). 119 The results showed that it was feasible to treat low-metastatic prostate cancer with PSMA containing boric acids or carboranes, thus demonstrating the potential role of PSMA in BNCT for the treatment of prostate cancer. 119 Epidermal growth factor receptor (EGFR), a member of the transmembrane RTK family, is involved in promoting growth, proliferation, migration, angiogenesis, and chemotherapy resistance in tumors. 113, [122] [123] [124] [125] Antisense oligonucleotides conjugated with boron clusters (B-ASOs) could serve as potential gene expression inhibitors and boron carriers for BNCT, 126-129 providing a dual-action therapeutic platform. Some B-ASOs were designed to inhibit the biosynthesis of the EGFR for BNCT. [126] [127] [128] Another example of combining carborane with EGFR was to integrate L-carboranylalanine ( L Cba), an artificial cluster-type amino acid, into a peptide ( Figure 3M ), by which Suga and co-workers established a macrocyclic peptide library containing L Cba residues. 130 In this case, macrocyclic peptides have the advantages of high affinity for human EGFR (hEGFR) and high selectivity for hEGFR-expressing cells, as well as ready synthesis. Curcumin, which is naturally present in turmeric plants, has been utilized to treat a variety of cancers and prevent Alzheimer's disease. [131] [132] [133] [134] [135] Deagostino and co-workers found a new type of boronated analog of curcumin (21) that could be used in combination with BNCT. 132 In this carborane-derived compound, b-diketone functionality was replaced by a carbonyl group while two phenolic rings were replaced by an o-carboranyl cage ( Figure 3N ). Sinomenine is a natural bioactive alkali extracted from the root of climbing ivy and has been widely used to relieve the symptoms of RA. 136 Recently, Zhu and co-workers designed and synthesized 137 a carborane-containing sinomenine derivative from sinomenine to treat RA ( Figure 3O ). [137] [138] [139] They found that the uptake of boron and compound 22 in rat C6 glioma cells was significantly higher than that of BPA and BSH. Moreover, the concentration of boron in the cancer cells indicated that compound 22 had a higher permeability to the cell membrane, which was consistent with the results of the effectiveness of killing cancer cells in vitro. 137 Monosaccharides have been proven as another type of ideal candidate for BNCT, mainly due to their high water solubility, high biocompatibility, and low systemic toxicity. [140] [141] [142] [143] The Ekholm group reported that monosaccharides could bind to carbohydrate transporters such as glucose transporters, and a glucoconjugate bearing an o-carboranylmethyl substituent (23) was designed and synthesized ( Figure 3P ). 142 In addition, immune protein was proposed as a general boron delivery agent. [144] [145] [146] [147] The bispecific antibody (BsMAb, 24, Figure 3Q ) was used to target tumor tissues by virtue of the tumor antigen specificity of BsMAb. 144 In this context, BsMAb was discovered by Hawthorne and co-workers, providing an alternative method for site-directed boron targeting. 144 This bispecific antibody can potentially be used as a boron delivery agent of BNCT for cancer treatment. Photodynamic therapy (PDT) and BNCT are promising cancer treatment modalities. [148] [149] [150] [151] [152] Both approaches are based on the selective accumulation and retention of non-toxic sensitizer molecules (light or neutron sensitizers) in the target cells, whereby the target cells are treated by external radiation to activate the sensitizer, destroying the target cells. 148 Dual therapies could thus improve therapeutic effectiveness by targeting different cellular components. 148 Therefore, the synthesis of drugs with PDT and BNCT dual sensitizers have attracted much research interest. [153] [154] [155] Conjugates of chlorin e 6 with iron bis(dicarbollide) nanoclusters Chlorins can accumulate in tumor tissues and have been widely used as photosensitizers for PDT. [156] [157] [158] [159] Moreover, chlorins have been utilized in conjugated boron nanoclusters, such as cobalt bis(dicarbonides), which were particularly attractive as boron-containing partial conjugates of chlorins. 160 Semioshkin et al. and Viñas and co-workers demonstrated that cobalt bis(dicarbollides) were non-toxic both in vivo and in vitro. 161, 162 Conjugate of chlorin e 6 with iron bis(dicarbollide) nanocluster (27), a dual sensitizer of BNCT and PDT, was synthesized by the Feofanov group ( Figure 5A ). 163 They found that conjugate 27 accumulated effectively in rat glioblastoma, delivering >10 9 boron atoms per C6 cell (rat glioblastoma C6 cell), which resulted in 50% and 90% of photoinduced cell death with the concentrations of 35 ± 3 and 80 ± 3 nM, respectively. Therefore, this conjugate provided an alternative direction for further research regarding the combination of PDT and BNCT. 163 Phthalocyanine-ortho-carborane conjugates Boronated tetrapyrrole derivatives are promising dual sensitizers for BNCT and PDT by virtue of their low cytotoxicity under dark conditions, high boron content, and good tumor affinity. 164 Among these compounds, phthalocyanine (PC) has attracted much interest in cancer treatment due to its high singlet oxygen production capacity, high molar extinction coefficient, high optical stability, and strong near-infrared absorption capacity. 165 In this context, a water-soluble, o-carborane-derived PC complex (29) was designed and obtained by Hamuryudan and co-workers. 166 Carboranes served as the boron source of BNCT while PC played a role in PDT A B C Figure 5 . Synthesis of carborane-derived PDT/BNCT dual sensitizers activation ( Figure 5B ). This combination greatly enhances tumor killing efficiency. Boronated porphyrins and their derivatives can preferentially accumulate in cancer cells with low dark cytotoxicity, [167] [168] [169] and thus have potential applications as BNCT/PDT dual sensitizers. [167] [168] [169] [170] The Vicente group reported that boronated chlorin killed T98G cells of human glioma, 171 while Pandey and co-workers found that fluorinated substituents promoted photosensitivity. 172 It was also found that fluorinated porphyrins had higher photokinetic activity than their non-fluorinated counterparts. 171, 172 Tetrafluorophenyl porphyrin (TPPF, 30) was synthesized by Drain and co-workers through microwave reaction. 153 Vicente and co-workers synthesized a novel carborane-derived sensitizer, tetrakis(p-carboranylthio-tetrafluorophenyl) chlorin (TPFC, 31) from TPPF ( Figure 5C ). 154 The same group also reported that F98 rat glioma cells and F98 rat glioma brain tumor model could be used to evaluate the applicability of TPFC as a sensitizer. In in vitro studies, TPFC was located close to the nuclei and was highly photosensitive. 173 According to the results from Vicente and colleagues, the efficacy of TPFC in the treatment of F98 rat glioma was comparable with that of BPA. 154, 173 Therefore, TPFC is potentially a promising dual sensitizer for PDT and BNCT. Estrogen has important functions in cardiovascular, reproductive, skeletal, and central nervous systems. 174-181 E2 (17b-estradiol) and E1 (precursor of E2) were synthesized from estrone sulfate by steroid sulfatase (STS). 174 Therefore, STS could be considered as a promising target for the treatment of breast cancer. In this context, Ohta and coworkers employed carborane as a hydrophobic pharmacophore, and several carborane-containing compounds were synthesized for the treatment of breast cancer. 174, 182 Among these, 1-(4-hydroxyphenyl)-12-hydroxymethyl-p-carborane (BE120) showed a potent binding ability to the estrogen receptor a ( Figure 6A ). 182 Similarly, the Ohta/Endo group developed carborane-derived compounds to target androgen receptor (AR). 183, 184 The AR homodimer translocated to nucleus and bound to the androgen response elements (AREs) in DNA, after which the AR-ARE complexes interacted with the promoters to regulate the target genes ( Figure 7A ). 185 Subsequently, several carborane-containing AR ligands were developed as candidates for prostate cancer therapeutics. [186] [187] [188] The carborane cage served as a hydrophobic pharmacophore for the AR ligand binding domain (AR LBD) of antagonists (33, 34) ( Figure 6B ). 183, 184 Vitamin D receptor ligands Vitamin D nuclear receptor (VDR) is expressed in a variety of tumors and can be used as a potential target for cancer treatment. [189] [190] [191] [192] A potent VDR agonist (35) with the combination of 1a,25-dihydroxyvitamin D3 (1,25D) and a carborane motif was designed and synthe-sized by Mouriño and colleagues, 193 which represented the first example of vitamin D analog binding to the LBD of VDR ( Figure 6C ). 1,25D hormone regulates various physiological and pathological processes, such as cell proliferation and differentiation. 193 In the nucleus, vitamin D and its analogs bind to VDR and then form the VDR-retinoid X receptor complexes, and bind to the vitamin D response element ( Figure 7B ). 189 These results showed that carborane-derived vitamin D analogs could be employed for specific molecular recognition as well as anticancer drug design and discovery. Epidermal growth factor receptor ligands EGFR/ErbB1 is a member of the ErbB protein family of RTKs. [194] [195] [196] [197] The signaling pathways regulated by EGF-EGFR play key roles in regulating basic cell functions, such as cell proliferation, survival, differentiation, and migration ( Figure 7C ). 194, 198 EGFR-mediated cellular events are interfered with by inhibiting EGF from binding to EGFR on the surface of cancer cells. [199] [200] [201] By employing the carborane cage as a pharmacophore, Viñas' group demonstrated that a carborane-containing erlotinib derivative, 1,7-closo-carboranylanilinoquinazoline (36, Figure 6D ), had a higher affinity than its parent compound erlotinib. 201 Nicotinamide phosphoribosyltransferase (NAMPT) is a first ratelimiting enzyme in the cycle of mammalian nicotinamide adenine dinucleotide (NAD + ). [202] [203] [204] [205] Recent studies showed that NAMPT plays an essential role in metabolism, cell proliferation/survival, and inflammation. [202] [203] [204] [205] In this context, a series of carborane-containing NAMPT inhibitors (37-42) were designed and synthesized by Nakamura and co-workers ( Figure 6E ). 202 Among these inhibitors, compounds 41 and 42 showed significant NAMPT inhibitory activity with IC 50 values of 0.098 ± 0.008 and 0.057 ± 0.001 mM, respectively ( Figure 6E ). 202 CAIX is an enzyme expressed on the surface of hypoxic tumor cells. [206] [207] [208] [209] This enzyme promoted the survival of tumor cells and could be a target for anticancer therapy. 206, 207 In this context, biscarborane-containing CAIX inhibitors (43) were designed and synthesized by the Grüner group ( Figure 6F) . 206, 210 These cobalt bis(dicarbollide) ions acted as highly potent and specific CAIX inhibitors both in vitro and in vivo. Mechanistically, the crystal structure of the cobaltacarborane inhibitor bound to the CAIX active site; therefore, the enzyme cavity was able to easily accommodate the cobalt bis(dicarbollide) cluster ( Figure 6F ). 206, 210 HIV protease receptor ligands Human immunodeficiency virus 1 (HIV-1) protease is a potent target in the treatment of HIV-1 infection, [211] [212] [213] and is also a target for anti-HIV drug design. [214] [215] [216] The Konvalinka group discovered a series of novel non-peptide protease inhibitors that were able to inhibit a variety of protease inhibitor-resistant protease species (44-47, Figure 6G) . [217] [218] [219] These substituted metallacarboranes were effective and selective inhibitors of wild-type and mutated HIV proteases. 5-Lipoxygenase (5-LO) acts as a catalyst for the conversion of arachidonic acid to leukotrienes. [220] [221] [222] [223] The activity of 5-LO is affected by 5-lipoxygenase activating protein (FLAP), and Rev-5901 is an early inhibitor of FLAP-mediated 5-LO activation. 224 As the introduction of carborane can improve the pharmacokinetic behavior of metabolically unstable drugs, 224, 225 the Hey-Hawkins group introduced carborane as a highly metabolically stable pharmacophore to the traditional 5-LO inhibitors ( Figure 6H ). 224, 225 Carborane-containing Rev-5901 derivatives showed the isosteric replacement of the phenyl ring by a carborane cage, leading to improved cytotoxicity in melanoma and colon cancer cells. 224 Cyclooxygenase ligands COX-2 is involved in carcinogenesis, and increasing studies have been conducted on the potential cytotoxic properties of COX-2 selective inhibitors. [226] [227] [228] [229] The incorporation of carborane units into the established anti-inflammatory drugs improved their metabolic stability. 226 Hey-Hawkins and co-workers designed and synthesized several carborane-containing rofecoxib derivatives (49, The discovery of delocalized lipophilic cations (DLCs) is a milestone of organelle-specific drug delivery. [230] [231] [232] [233] Owing to the high selectivity of growth arrest of DLC-functionalized carboranes for cancer cells, such as primary glioblastoma cancer stem cells, Vizirianakis and colleagues reported that DLC-functionalized carboranes (50-52) had potential applications in selective anticancer therapeutics (Figure 6J ). [230] [231] [232] [233] They also demonstrated that the target-specific DLCfunctionalized carboranes could act as BNCT agents. [230] [231] [232] [233] Hyaluronic acid ligands Hyaluronic acid is a highly biocompatible polysaccharide that plays an important role in cancer metastasis. [234] [235] [236] [237] [238] [239] [240] [241] [242] It also interacts with various types of receptors that are overexpressed in cancer tissues. [234] [235] [236] Introduction of carborane motifs can enhance the hydrophobic interaction between the bioactive compounds and their receptors and thus improve their stability and bioavailability in vivo. [237] [238] [239] [240] [241] [242] Crescenzi and co-workers synthesized hyaluronan-amidoazido-carborane (HAAACB) (55) through a click-type coupling of hyaluronan-amidoazide and carboranyl alkyne ( Figure 6K ). 237, 238 They found that HAAACB could specifically interact with the CD44 receptor, leading to accumulation of boron atoms in cancer cells, which will have potential application in BNCT. 237 Histone deacetylases (HDACs) are known to be responsible for the global silencing of tumor-suppressor genes. [243] [244] [245] [246] [247] [248] [249] Overexpression of HDACs was shown to be linked to several cancer types, and their selective inhibition results in potentiated anticancer effects. 249 Treatment with histone deacetylase inhibitors (HDACis) can reverse this process and restore normal cell function. Therefore, HDACis have emerged as valuable epigenetic modulators for the treatment of cancers. 243, 249 Hansen and co-workers identified the meta-carboranyl hydroxamate 56 as the hit compound with an IC 50 value of 0.006 mM and a more than 280-fold selectivity for HDAC6. 249 To investigate the influence of the carborane moiety, they synthesized aryl analogs for the best pan-inhibitory compound 57 and the most selective HDAC6 inhibitor 56 ( Figure 6L ). 249 Both 56 and 57 demonstrated synergistic anticancer activity when combined with the proteasome inhibitor bortezomib. 249 This review summarizes the recent advances in carborane-containing compounds with potential applications in antitumor treatments. Carboranes have been proven as useful pharmacophores in boron delivery agents for BNCT as well as hydrophobic drug carriers for certain biological targets. The introduction of carborane moieties into the skeletons of traditional organic compounds (hits, leads, drugs) can change the activity of the drugs or drug candidates and thus regulate their effects on cancer cells, as shown in the various examples discussed in this review. Although this research area is still in its infancy, selective functionalization of carboranes to obtain various carboranecontaining derivatives has received increasing research attention. Application of carboranes as antitumor pharmacophores will open a new and specialized avenue for novel drug design and discovery. p aromaticity and threedimensional aromaticity: two sides of the same coin? 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and applications Transition metal-carboryne complexes: synthesis, bonding, and reactivity Fusing dicarbollide ions with N-heterocyclic carbenes Blue phosphorescent zwitterionic iridium(III) complexes featuring weakly coordinating nido-carborane-based ligands Doxorubicin-loaded carborane-conjugated polymeric nanoparticles as delivery system for combination cancer therapy Biomineralization: an opportunity and challenge of nanoparticle drug delivery systems for cancer therapy Epidemiology of hepatocellular carcinoma Evolution of systemic therapy for hepatocellular carcinoma Global epidemiology of NAFLD-related HCC: trends, predictions, risk factors and prevention Hepatocellular carcinoma Immunotherapy for advanced hepatocellular carcinoma: a focus on special subgroups Pursuing efficacious systemic therapy for hepatocellular carcinoma Asialoglycoprotein receptor mediated hepatocyte targeting-strategies and applications Galactosylated fluorescent labeled micelles as a liver targeting drug carrier Asialoglycoprotein receptor targeted micelles containing carborane clusters for effective boron neutron capture therapy of hepatocellular carcinoma Boron neutron capture therapy demonstrated in mice bearing EMT6 tumors following selective delivery of boron by rationally designed liposomes Validation and comparison of the therapeutic efficacy of boron neutron capture therapy mediated by boron-rich liposomes in multiple murine tumor models Aptamer-functionalized liposomes for targeted cancer therapy Liposomal nanocarriers for statins: a pharmacokinetic and pharmacodynamics appraisal Liposome-based drug delivery systems in cancer immunotherapy Model studies directed toward the application of boron neutron capture therapy to rheumatoid arthritis: boron delivery by liposomes in rat collagen-induced arthritis PEGylated liposome encapsulating nido-carborane showed significant tumor suppression in boron neutron capture therapy (BNCT) PEG-derivatized dual-functional nanomicelles for improved cancer therapy Combined strategies with poly (ADP-Ribose) polymerase (PARP) inhibitors for the treatment of ovarian cancer: a literature review Selective neuropeptide Y conjugates with maximized carborane loading as promising boron delivery agents for boron neutron capture therapy Modular triazine-based carborane-containing carboxylic acids-synthesis and characterisation of potential boron neutron capture therapy agents made of readily accessible building blocks A selective carborane-functionalized gastrin-releasing peptide receptor agonist as boron delivery agent for boron neutron capture therapy Tuning a modular system-synthesis and characterisation of a boron-rich s-triazine-based carboxylic acid and amine bearing a galactopyranosyl moiety A stable meta-carborane enables the generation of boron-rich peptide agonists targeting the ghrelin receptor Novel carborane compounds based on cyclooxygenase-2 inhibitors for effective boron neutron capture therapy of tongue squamous cell carcinoma Up-regulation of cyclooxygenase 2 gene expression in human colorectal adenomas and adenocarcinomas Cyclooxygenase-2 expression is up-regulated in squamous cell carcinoma of the head and neck Cyclooxygenase-2 in gastrointestinal malignancies MicroRNA expression and its implications for diagnosis and therapy of tongue squamous cell carcinoma MicroRNAs in human tongue squamous cell carcinoma: from pathogenesis to therapeutic implications Identification of miR-139-5p as a saliva biomarker for tongue squamous cell carcinoma: a pilot study Role of matrix metalloproteinases in angiogenesis and cancer Matrix metalloproteinases participation in the metastatic process and their diagnostic and therapeutic applications in cancer The roles of matrix metalloproteinases and their inhibitors in human diseases Extracellular matrix contribution to skin wound re-epithelialization Carborane-containing matrix metalloprotease (MMP) ligands as candidates for boron neutron-capture therapy (BNCT) In vitro and in vivo BNCT investigations using a carborane containing sulfonamide targeting CAIX epitopes on malignant pleural mesothelioma and breast cancer cells The role of carbonic anhydrase IX in cancer development: links to hypoxia, acidosis, and beyond Inhibition of carbonic anhydrase IX targets primary tumors, metastases, and cancer stem cells: three for the price of one How and why are cancers acidic? Carbonic anhydrase IX and the homeostatic control of tumour extracellular pH Mechanisms of receptor tyrosine kinase activation in cancer Function of the c-Met receptor tyrosine kinase in carcinogenesis and associated therapeutic opportunities AXL receptor tyrosine kinase as a promising anti-cancer approach: functions, molecular mechanisms and clinical applications Receptor tyrosine kinase activation: from the ligand perspective Sunitinib-containing carborane pharmacophore with the ability to inhibit tyrosine kinases receptors FLT3, KIT and PDGFR-b, exhibits powerful in vivo anti-glioblastoma activity Bimodal therapeutic agents against glioblastoma, one of the most lethal forms of cancer In vitro and in vivo evaluation of fluorescently labeled borocaptate-containing liposomes Small-molecule kinase-inhibitors-loaded boron cluster as hybrid agents for glioma-cell-targeting therapy New prostate cancer targets for diagnosis, imaging, and therapy: focus on prostate-specific membrane antigen Targeting prostate cancer: prostate-specific membrane antigen based diagnosis and therapy Synthesis and initial biological evaluation of boron-containing prostate-specific membrane antigen ligands for treatment of prostate cancer using boron neutron capture therapy Lutetium-177 prostatespecific membrane antigen (PSMA) theranostics: practical nuances and intricacies More than meets the eye: scientific rationale behind molecular imaging and therapeutic targeting of prostate-specific membrane antigen (PSMA) in metastatic prostate cancer and beyond Epidermal growth factor receptor: structure-function informing the design of anticancer therapeutics Breast cancer Receptor tyrosine kinases: principles and functions in glioma invasion Epidermal growth factor receptor (EGFR) involvement in epithelial-derived cancers and its current antibody-based immunotherapies Versatile method for the site-specific modification of DNA with boron clusters: anti-epidermal growth factor receptor (EGFR) Antisense oligonucleotide case High boron-loaded DNA-oligomers as potential boron neutron capture therapy and antisense oligonucleotide dual-action anticancer agents Boron clusters as a platform for new materials: composites of nucleic acids and oligofunctionalized carboranes (C(2)B(10)H(12)) and their assembly into functional nanoparticles Metallacarborane complex boosts the rate of DNA oligonucleotide hydrolysis in the reaction catalyzed by snake venom phosphodiesterase De novo carborane-containing macrocyclic peptides targeting human epidermal growth factor receptor Versatile role of curcumin and its derivatives in lung cancer therapy Design, synthesis and preliminary in-vitro studies of novel boronated monocarbonyl analogues of Curcumin (BMAC) for antitumor and b-amyloid disaggregation activity Curcumin analogues and their hybrid molecules as multifunctional drugs Anti-viral potential and modulation of Nrf2 by curcumin: pharmacological implications Role of curcumin and (-)-epigallocatechin-3-O-gallate in bladder cancer treatment: a review HPLC and LC-MS analysis of sinomenine and its application in pharmacokinetic studies in rats Synthesis, molecular docking, and in vitro boron neutron capture therapy assay of carboranyl sinomenine Substituted carborane-appended water-soluble single-wall carbon nanotubes: new approach to boron neutron capture therapy drug delivery Addressing the biochemical foundations of a glucose-based "Trojan horse"-strategy to boron neutron capture therapy: from chemical synthesis to in vitro assessment Exploring the biochemical foundations of a successful GLUT1-targeting strategy to BNCT: chemical synthesis and in vitro evaluation of the entire positional isomer library of ortho-carboranylmethyl-bearing glucoconjugates Bispecific antibody mediated targeting of nido-carboranes to human colon carcinoma cells Critical evaluation of bispecific antibodies as targeting agents for boron neutron capture therapy of brain tumors Bispecific antibodies: a mechanistic review of the pipeline Overcoming challenges for CD3-bispecific antibody therapy in solid tumors Photodynamic therapy for cancer Synthesis and biological evaluation of new boron-containing chlorin derivatives as agents for both photodynamic therapy and boron neutron capture therapy of cancer Boron neutron capture therapy and photodynamic therapy for high-grade meningiomas Synthesis and biological evaluation of new BSH-conjugated chlorin derivatives as agents for both photodynamic therapy and boron neutron capture therapy of cancer Ru(II) and Ir(III) phenanthroline-based photosensitisers bearing o-carborane: PDT agents with boron carriers for potential BNCT Efficient microwaveassisted synthesis of amine-substituted tetrakis(pentafluorophenyl)porphyrin Synthesis and biological investigations of tetrakis(p-carboranylthio-tetrafluorophenyl)chlorin (TPFC) Cobaltacarborane-phthalocyanine conjugates: syntheses and photophysical properties Chlorin e6-based photosensitizers for photodynamic therapy and photodiagnosis Cellular uptake and photosensitizing properties of quantum dot-chlorin e6 complex: in vitro study Photodynamic therapy of melanoma skin cancer using carbon dot-chlorin e6-hyaluronate conjugate Chlorin e6 conjugated poly(dopamine) nanospheres as PDT/PTT dual-modal therapeutic agents for enhanced cancer therapy Recent progress in the syntheses and biological evaluation of boronated porphyrins for boron neutron-capture therapy Cyclic oxonium derivatives of polyhedral boron hydrides and their synthetic applications Metallacarboranes on the road to anticancer therapies: cellular uptake, DNA interaction, and biological evaluation of cobaltabisdicarbollide Conjugate of chlorin f(6) with iron bis(dicarbollide) nanocluster: synthesis and biological properties Application of a novel boronated porphyrin (H 2 OCP) as a dual sensitizer for both PDT and BNCT Unveiling Ga(III) phthalocyanine-a different photosensitizer in neuroblastoma cellular model A phthalocyanineortho-carborane conjugate for boron neutron capture therapy: synthesis, physicochemical properties, and in vitro tests Porphyrinic ionic liquid dyes: synthesis and characterization How does the encapsulation of porphyrinic photosensitizers into polymer matrices affect their self-association and dynamic properties? Probing the interactions of porphyrins with macromolecules using NMR spectroscopy techniques Photodynamic therapy in the treatment of microbial infections: basic principles and perspective applications Synthesis and cellular studies of a carboranylchlorin for the PDT and BNCT of tumors Structure-activity relationship among purpurinimides and bacteriopurpurinimides: trifluoromethyl substituent enhanced the photosensitizing efficacy Tetrakis(p-carboranylthio-tetrafluorophenyl)chlorin (TPFC): application for photodynamic therapy and boron neutron capture therapy Design and synthesis of p-carborane-containing sulfamates as multitarget anti-breast cancer agents Recent progress in the development of steroid sulphatase inhibitors-examples of the novel and most promising compounds from the last decade Menopause-related estrogen decrease and the pathogenesis of HFpEF: JACC review topic of the week Estrogen receptor involvement in vascular cognitive impairment and vascular dementia pathogenesis and treatment Estrogen signaling and its potential as a target for therapy in ovarian cancer Estrogen-mediated gaseous signaling molecules in cardiovascular disease Estrogen, estrogen-like molecules and autoimmune diseases Structure-activity relationship of para-carborane selective estrogen receptor b agonists Design, synthesis, and anti-proliferative activity of 1-(4-methoxyphenyl)-12-hydroxymethyl-p-carborane derivatives Novel androgen receptor full antagonists: design, synthesis, and a docking study of glycerol and aminoglycerol derivatives that contain p-carborane cages Design and synthesis of carborane-containing androgen receptor (AR) antagonist bearing a pyridine ring A critical update on the strategies towards modulators targeting androgen receptors 11-Oxygenated androgens in health and disease Impact of androgens on inflammation-related lipid mediator biosynthesis in innate immune cells Prostate carcinogenesis: inflammatory storms New insights into the role of vitamin D in hepatocellular carcinoma Vitamin D signaling in inflammation and cancer: molecular mechanisms and therapeutic implications New aspects of vitamin D metabolism and action -addressing the skin as source and target Vitamin D and its potential interplay with pain signaling pathways Carborane-based design of a potent vitamin D receptor agonist HER1/EGFR tyrosine kinase inhibitors for the treatment of glioblastoma multiforme Rac inhibition as a novel therapeutic strategy for EGFR/HER2 targeted therapy resistant breast cancer Implementing anti-epidermal growth factor receptor (EGFR) therapy in metastatic colorectal cancer: challenges and future perspectives EGFR-mediated autophagy in tumourigenesis and therapeutic resistance Role of epidermal growth factor receptor in breast cancer Epidermal growth factor receptor (EGFR) signaling in cancer EAI045: the fourth-generation EGFR inhibitor overcoming T790M and C797S resistance Discovery of potent EGFR inhibitors through the incorporation of a 3D-aromatic-boron-rich-cluster into the 4-anilinoquinazoline scaffold: potential drugs for glioma treatment Structure-based drug design of novel carborane-containing nicotinamide phosphoribosyltransferase inhibitors Structural basis of beneficial design for effective nicotinamide phosphoribosyltransferase inhibitors Targeting metabolic reprogramming in metastatic melanoma: the key role of nicotinamide phosphoribosyltransferase (NAMPT) Location, location, location: compartmentalization of NAD(+) synthesis and functions in mammalian cells Cobalt bis(dicarbollide) alkylsulfonamides: potent and highly selective inhibitors of tumor specific carbonic anhydrase IX Inhibitors of CA IX enzyme based on polyhedral boron compounds Intrinsically disordered features of carbonic anhydrase IX proteoglycan-like domain Carbonic anhydrase IX and acid transport in cancer Metallacarborane sulfamides: unconventional, specific, and highly selective inhibitors of carbonic anhydrase IX Darunavir-resistant HIV-1 protease constructs uphold a conformational selection hypothesis for drug resistance The recent application of 3D-QSAR and docking studies to novel HIV-protease inhibitor drug discovery Natural bioactive compounds from fungi as potential candidates for protease inhibitors and immunomodulators to apply for coronaviruses Recent progress in the development of HIV-1 protease inhibitors for the treatment of HIV/AIDS Multifaceted HIV integrase functionalities and therapeutic strategies for their inhibition Cabergoline: a review of its use in the inhibition of lactation for women living with HIV From nonpeptide toward noncarbon protease inhibitors: metallacarboranes as specific and potent inhibitors of HIV protease Inorganic polyhedral metallacarborane inhibitors of HIV protease: a new approach to overcoming antiviral resistance Design of HIV protease inhibitors based on inorganic polyhedral metallacarboranes Emerging roles of 5-lipoxygenase phosphorylation in inflammation and cell death 5-Lipoxygenase pathway and its downstream cysteinyl leukotrienes as potential therapeutic targets for Alzheimer's disease 5-Lipoxygenase as an emerging target against age-related brain disorders Targeting the enzymes involved in arachidonic acid metabolism to improve radiotherapy CarbORev-5901: the first carborane-based inhibitor of the 5-lipoxygenase pathway Carborane-based analogues of 5-lipoxygenase inhibitors Co-inhibit heat shock protein 90 in HCT116 cells Carboranyl derivatives of rofecoxib with cytostatic activity against human melanoma and colon cancer cells Cardiovascular effects and safety of (non-aspirin) NSAIDs Cyclooxygenase-2 inhibitor: a potential combination strategy with immunotherapy in cancer The role of brain cyclooxygenase-2 (Cox-2) beyond neuroinflammation: neuronal homeostasis in memory and anxiety Carborane derivatives of 1,2,3-triazole depolarize mitochondria by transferring protons through the lipid part of membranes Conjugation of natural triterpenic acids with delocalized lipophilic cations: selective targeting cancer cell mitochondria Mitochondria as a novel target for cancer chemoprevention: emergence of mitochondrial-targeting agents Pharmacological development of target-specific delocalized lipophilic cation-functionalized carboranes for cancer therapy Association of hyaluronic acid family members (HAS1, HAS2, and HYAL-1) with bladder cancer diagnosis and prognosis Revisiting the hallmarks of cancer: the role of hyaluronan Chitinase-3 like-protein-1 function and its role in diseases Hyaluronan as carrier of carboranes for tumor targeting in boron neutron capture therapy Novel types of carborane-carrier hyaluronan derivatives via "click chemistry Application of hyaluronic acid as carriers in drug delivery Hyaluronic acid-based biopharmaceutical delivery and tumor-targeted drug delivery system Hyaluronic acid: the influence of molecular weight on structural, physical, physico-chemical, and degradable properties of biopolymer Hyaluronic acid-decorated carborane-TAT conjugation nanomicelles: a potential boron agent with enhanced selectivity of tumor cellular uptake Histone deacetylase 2 (HDAC2) inhibitors containing boron Histone deacetylase 2: a potential therapeutic target for cancer and neurodegenerative disorders Epigenetics and therapeutic targets in gastrointestinal malignancies Dissecting histone deacetylase 3 in multiple disease conditions: selective inhibition as a promising therapeutic strategy Vorinostat in autophagic cell death: a critical insight into autophagy-mediated, -associated and -dependent cell death for cancer prevention Role of acetylation in doxorubicin-induced cardiotoxicity Borinostats: solid-phase synthesis of carborane-capped histone deacetylase inhibitors with a tailor-made selectivity profile The authors declare no competing interests.