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The Use of Carboranes in Cancer Drug Develop-Ment

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The Use of Carboranes in Cancer Drug Develop-Ment ISSN: 2378-3419 Zargham et al. Int J Cancer Clin Res 2019, 6:110 DOI: 10.23937/2378-3419/1410110 Volume 6 | Issue 2 International Journal of Open Access Cancer and Clinical Research REviEW ARtiCLE The Use of Carboranes in Cancer Drug Development Emilia O Zargham, Christian A Mason and Mark W Lee Jr* Check for Department of Chemistry, University of Missouri, Columbia, Missouri, USA updates *Corresponding author: Mark W Lee Jr., Department of Chemistry, University of Missouri, 601 South College Avenue, Columbia, Missouri, 65211, USA, Tel: 573-884-2424 erwise rapidly metabolize [5]. Furthermore, selective Abstract chemical substitution of each carbon or boron atom Over the past decade, there has been a rising interest in these clusters allows for their use as rigid, three di- in the use of carboranes as a potential pharmacophoric moiety in the development of new drugs for the treatment of mensional scaffolds upon which to construct new drug various types of cancer. The unique physical and chemical molecules. properties of carboranes make their use attractive in drug development. In several instances, the inclusion of Nearly all past biomedical research involving carbo- carboranes into a drug structure has increased the agent’s ranes has focused on their use in the design of boron de- binding affinity, potency, or bioavailability. The purpose of livery agents for boron neutron capture therapy (BNCT) this review is to highlight applications of carboranes to the [2]. This binary radiation therapy depends on the selec- medicinal chemistry of cancer. tive delivery of a high concentration of boron-10 atoms to targeted tissues. It is generally accepted that a min- Introduction imum concentration of 30 ppm is required for success- Cancer is the second leading cause of death in the ful BNCT therapy and this concentration is equivalent to 9 United States, exceeded only by heart disease. It is the delivery of approximately 10 boron atoms to each estimated that 1,685,210 new diagnosis and 595,690 targeted cell. As BNCT human clinical trials have been American deaths will be due to cancer this year [1]. The attempted nearly continuously for the past five de- fight against cancer involving early detection and new cades, there is a large body of literature associated with methods of treatment are crucial. Since their initial dis- the use of carboranes for the development of BNCT bo- covery during the 1960’s, researchers have utilized car- ron agents. There are several good reviews and books boranes in the search for new and effective treatments covering that work [6-13]. While BNCT requires the de- for cancer [2,3]. Formally named dicarba-closo-dodeca- livery of large quantities of boron to diseased tissue, boranes, carboranes containing two carbon atoms are (requiring concentrations of boron in the low millimolar icosahedral clusters containing carbon, boron, and hy- range) there is an increasing interest to use carboranes drogen and exist as ortho 1, meta 2, and para 3 isomers, to synthesize new and highly potent drugs which oper- depending on the relative position of the two carbon ate at the opposite extreme of the concentration range, atoms (Figure 1). Carboranes have a general formula namely nanomolar, to picomolar. of C2B10H12 and possess unique chemical and physical Ortho-carborane and its carbon-functionalized properties that suggest their potential use as a moiety derivatives may be prepared by reacting acetylene for the design of new drug molecules with enhanced (or functionalized acetylene) with the decaborane activities and/or target selectivies [3]. Some of these derivative 10B H12L2, where L is a weak Lewis base [3,14]. properties include high oxidative and thermal stability, The acetylenes used in these reactions include a wide hydrophobicity, and low toxicity [4]. Owing to their sta- range of functional groups such as esters, halides, bility, carboranes also may increase the in vivo stability carbamates, ethers, and nitro groups; however, and bioavailability of pharmaceuticals that might oth- such reactions are not successful in the presence of Citation: Zargham EO, Mason CA, Lee Jr MW (2019) The Use of Carboranes in Cancer Drug Develop- ment. Int J Cancer Clin Res 6:110. doi.org/10.23937/2378-3419/1410110 Accepted: April 16, 2019; Published: April 18, 2019 Copyright: © 2019 Zargham EO, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Zargham et al. Int J Cancer Clin Res 2019, 6:110 • Page 1 of 14 • DOI: 10.23937/2378-3419/1410110 ISSN: 2378-3419 Figure 1: Ortho-(1), meta-(2), para-carborane (3), and nido-carboranes (4-6) [3]. nucleophilic species such as alcohols, acids, or amines. the boron carrier approach of carboranes for BNCT to Under an inert atmosphere, ortho-carborane will be used as frameworks in developing newly improved thermally isomerize to meta-carborane at temperatures pharmacophores. Compared to aryl rings, the superior ranging from 400-500 °C and to para-carborane at 600- synthetic flexibility of carboranes allow them to be ex- 700 °C [2]. The CH groups in the carboranes are weakly tremely useful in fine tuning the pharmacokinetics of acidic and can be deprotonated using a strong base to newly developed drug candidates [5]. They are optimal produce nucleophiles [5]. The pKa values for ortho-, for chemical modification by functionalizing the weakly meta-, and para-carborane are 22.0, 25.6, and 26.8 acidic CH groups and boron atoms, and have the ability respectively [15,16]. Separately, the boron vertices to employ various isomers (1-3) to optimize the binding may be functionalized by reactive electrophiles. In affinity with other substituents [26]. The closo-carbo- turn, a wide range of carborane derivatives may be rane is air and moisture stable and is known to possess synthesized regioselectively without the need for “superhydrophobicity”. Their high partition coefficient complex protecting groups. Bases, such as pyrrolidine values are known to surpass common bioisosteres such or fluoride, may be used to generate nido-carboranes as aryl, cycloalkyl, and adamantyl groups [5]. Moreover, 4, 5, and 6 (Figure 1) [17-21]. Such reactions convert their high stability and low toxicity towards cells make the extremely hydrophobic closo-carboranes to the them interesting compounds for therapeutic applica- hydrophilic nido-carboranes [22-24]. tions [2]. Carboranes as Pharmacophores Estrogen receptor Pharmacophore approaches involving various li- It has been apparent for close to a century that the gand-based and structure-based methods have evolved steroid estrogen has played a major role in breast cancer to be one of the most successful concepts in medicinal [27]. When molecules, such as estrogen hormones, bind chemistry. A pharmacophore was defined in 1909 by to the ligand binding domain of the estrogen receptor Ehrlich as a molecular framework that carries the essen- the protein forms a dimer. The dimerized form of the tial features responsible for a drug’s biological activity estrogen receptor binds to DNA promotor elements [25]. Considerable progress has been made on phar- which results in the initiation of gene transcription [2,5]. macophore technology in the past two decades and Antagonism of the estrogen receptor is a mechanism has made pharmacophore approaches a major tool in used for the treatment of hormone responsive cancers drug discovery. Recent developments have expanded [28-32]. The molecules depicted in Figure 2 represent Zargham et al. Int J Cancer Clin Res 2019, 6:110 • Page 2 of 14 • DOI: 10.23937/2378-3419/1410110 ISSN: 2378-3419 Figure 2: Hormone and ERα antagonists (7-9), estrogen receptor agonist carborane derivatives (10a-g), and estrogen receptor antagonist carborane derivatives (11-12) [28]. the hormone 17β-estradiol 7 and the estrogen concentrations of 1 × 10-9 M. The binding affinity for the receptor alpha (ERα) antagonists tamoxifen 8 and ERα receptor for 7, 11, and 12 were 0.1 nM, 1.0 nM, 4-hydroxytamoxifen 9. Endo and coworkers synthesized and 60 nM respectively [28]. The most potent inhibition a number of carborane containing estrogen receptor of the transcriptional response to 17β-estradiol was agonists 10a-g and antagonists 11-12 depicted in Figure exhibited by 12, thus the authors noted that the 2 [28,29]. The activities of the estrogen agonists 10a-g antagonistic response does not directly correlate with were analyzed; the carborane derivative 10c exhibited the drugs binding affinity. They also noted that while at least 10-fold greater potency than 17β-estradiol 7 inducing an agonistic effect on the estrogen receptor in both in vitro and in vivo [29]. Remarkably, in comparison the bone tissue of the mice, 11 exhibited antagonistic with the natural molecule, these carborane containing effects on the estrogen receptor in the uterine tissue. agonists are quite simple in both structure and This observation supports the potential of using synthesis, containing no carbon steriocenters. Based on similar carborane-based compounds for the long term the inhibition of ERα, derivative 12 demonstrated the prevention of estrogen dependent malignancy without greatest antiestrogen response, exhibiting an activity inducing negative anti-estrogenic responses such as that was comparable to tamoxifen [28]. bone loss [28]. The estrogenic response of these compounds in Dihydrofolate reductase the uterine and bone tissues of ovariectomized mice was measured. The assays measured the inhibition Folate metabolism is involved in processes critical of 17β-estradiol binding to the ERα receptor at in cell proliferation and DNA synthesis. The enzyme Zargham et al. Int J Cancer Clin Res 2019, 6:110 • Page 3 of 14 • DOI: 10.23937/2378-3419/1410110 ISSN: 2378-3419 Figure 3: MXT (13), TMP (14), and carborane DHFR inhibitor (15) [33,35].
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