Since its first description, boron neutron capture therapy (BNCT) was a special type of radiotherapy for treatment of cancer and with focus mainly on glioma therapeutic. This procedure requires the selective accumulation of boron into the tumoral cells, and due to this requirement, different boron-enriched compounds have been designed and developed. Efforts to circumvent the selectivity-uptake challenge and other problems, such as solubility, stability, and toxicity, have been to driving force behind the medicinal chemistry field in boron-based compounds. In this regard, a wide diversity of medicinal chemistry hypothesis has been used to obtain new and efficient potential BNCT-glioma drugs. In this chapter, these ideas are analyzed focusing on their medicinal chemistry characteristics.
- drug discovery
- boron neutron capture therapy
- boron-bearing compounds
1.1. Boron neutron capture therapy: historical account
In 1932 in the UK, Chadwick discovered neutrons, and for his contribution, he was awarded in 1935 with the Nobel Prize in Physics . One year later in the USA, Gordon Locher introduced the concept of boron neutron capture therapy (BNCT) . He hypothesized that if boron could be selectively concentrated in a tumoral tissue and then exposed to a neutrons beam, a higher radiation dose to the tumor relative to surrounding normal cells would result. A mere 2 years later, Goldhaber, Hall, and Kruger performed the first radiobiological studies using boric acid and slow neutrons in a murine tumoral model . However, the first clinical trials against human brain tumors (glioblastoma multiforme (GBM)) that used BNCT could not be initiated until 1951 at the Brookhaven National Laboratory in collaboration with the Massachusetts General Hospital and the Massachusetts Institute of Technology (MIT) [4, 5]. In this case, ten patients were treated with borax (disodium tetraborate decahydrate, Na2B4O7·10H2O; Figure 1) and thermal neutrons without much success. While there were no serious side effects of BNCT in the patients, the large doses of borax (10B-enriched) infused 200 mg/kg, inducing slight toxicity symptoms. In order to improve this, a second approach was developed comprising nine glioma patients but now with a less toxic compound, sodium pentaborate (NaB5O8; Figure 1) in combination with D-glucose. Unlike the first one, a higher dose of 10B was used, and a higher fluency of incident thermal neutron was applied . Unfortunately, again, serious side effects such as radio-dermatoses of the scalp and deep ulcerations were observed [6, 7]. Simultaneously, in 1963, Sweet and co-workers, from the MIT, treated 18 patients using boron-rich disodium decahydrodecaborate (Na2B10H10; Figure 1) , which was considered to be less toxic and with the ability to deposit more boron atoms per cell. Symptoms of brain necrosis in patients undergoing BNCT were again observed . Due to these disappointing events, the USA halted the progress of research on BNCT in 1961.
On the other hand, in 1968 at Hitachi training reactor, the Japanese neurosurgeon Hiroshi Hatanaka, who in previous years had worked with Sweet in Boston, began treating patients with high-grade malignant gliomas using disodium mercaptoundecahydro-
From the 1990s to the present day with the development of new boron compounds and the improvement in radiation source, the boron neutron capture therapy has been expanded to several centers worldwide, among them in the USA at Brookhaven and Cambridge, in the Netherlands at high flux reactor in collaboration with the Department of Radiotherapy of the University of Essen in Germany, in Finland at FiR-1 Otaniemi reactor, in Sweden at R2–R0 reactor, in the Czech Republic at LVR-15 reactor, in Italy at TRIGA reactor, in Japan at Kyoto University Research Reactor Institute, in Argentina at RA-6 and RA-3 reactors, and in Taiwan at THOR reactor, just to name a few .
1.2. Boron neutron capture therapy: principles and general requirements
BNCT is considered as a rationale and promising binary therapy modality for treatment of several cancers in particular malignant gliomas. The cell-killing effect of BNCT is based on the nuclear reaction 10B(n,α)7Li (Figure 2) that occurs when the nuclide 10B, which is a nonradioactive constituent of natural elemental boron (approximately 20% abundance), is irradiated with neutrons of the appropriate energy, thermal neutrons. The nuclear reaction yields excited boron-11 (11B*) that after instantaneous nuclear fission produces two high-linear energy transfer entities, i.e., α-particle (4He2+) and recoiling lithium-7 nucleus (7Li3+). Because of the very short track length of these heavy particles (<10 μm; roughly one cell diameter), radiation damage is confined to those cells loaded with 10B.
The probability that a nuclide captures a neutron is measured by the neutron capture cross section, σth, having 10B a value of σth = 3838 barns . However, other abundant endogenous nuclei present in the healthy tissue, such as 1H and 14N, could also capture neutrons yielding after nuclear reactions of a gamma ray in the first case, 1H(n,γ)2H, and a proton in the second one, 14N(n,p)14C. However, the σth of these nuclei is smaller than the value for 10B, i.e., σth,1H = 0.332 and σth,14N = 1.82 barns, and the amount of radiation produced by these nuclear reactions is lesser than the produced by the particle and recoiling nucleus in the case of boron .
On the other hand, for brain tumor such as GBM, usually higher energy epithermal neutron beams which have a greater depth penetration being thermal neutrons unable to act on tumors located below the tissue surface because of scattering effects have been used. Epithermal neutrons do not suffer from the disadvantages of H-recoil processes and, consequently, allow capture reactions to occur at some distance within the tissue; then, epithermal neutrons are progressively slowed into thermal neutrons through heat-releasing interactions with the hydrogen atom and constituents of biological system, that do not cause damage to the tissue .
In order for BNCT to be successful, the 10B-loaded agent must completely fulfill some overriding conditions, namely, (a) selective uptake by tumor tissue relative to normal tissue (preferably accumulating within specific tumoral cell substructure) with ideal tumor:normal tissues and tumor:blood ratios of 3:1 and 5:1, respectively, and (b) appropriate amount of 10B delivered to the tumor tissue, i.e., at least 20 μg 10B/g tumor, corresponding to about 109 atoms of 10B/cell. However, this amount could be lower if the boron delivery system is concentrated in or near the cell nucleus; (c) retention of 10B in tumor during the BNCT process; (d) rapid clearance from blood and healthy tissues; (e) and adequate lipophilicity especially for glioma treatment where the drug should be able to cross blood-brain barrier (BBB) . Furthermore, like any drug in medicinal chemistry, the 10B-loaded agent must meet the following requirements: (f) absence of systemic toxicity, (g) chemical and metabolic stability, and (h) appropriate water solubility.
1.3. Boron neutron capture therapy: current therapeutic agents
After the first efforts, during the 1940s and 1950s (see Section 1.1.), the lack of selectivity and low boron tumor accumulation observed for the simplest boron salts (Figure 1) used until the moment prevented their application in BNCT clinical trials. However, around the 1960s, the first studies of the two compounds currently in clinical began, both 10B-enriched, the polyhedral borane BSH (Figure 1) and L-4-dihydroxyborylphenylalanine, known as L-boronophenylalanine (L-BPA; Figure 3) , which could be accumulated into desired tissues for its structural analogy to some biomolecules.
Due to BSH is a small hydrophilic molecule (Figure 1), it does not cross the intact BBB. It only penetrates into the brain passively when the BBB is disrupted , as it is observed in the GBM. Although BSH has been applied for the treatment of GBM in infusions with no toxic effects, the efficacy has been limited due to low observed tumor:brain (3:1) and tumor:blood (0.9–2.5:1) ratios . The main structural advantage of BSH compared to L-BPA is that BSH contains 12 times more B per molecule yielding a higher number of events after neutron capture than in L-BPA. BSH has been studied in different therapeutic schedules, combined or not with other small molecules, like L-BPA, or vehicles looking for the improvement of the efficacy and the delivery into the glioma . Medicinal chemistry on BSH, structural modifications, has also been done (see below) seeking better biological behavior.
Nowadays, L-BPA (Figure 3) is the standard therapeutic drug used in BNCT [21, 22, 23]. Since the L-amino acid transport system is highly expressed in tumor cells compared with normal cells in most organs including the brain, some natural amino acid boron derivatives have been studied . L-BPA has very limited water solubility (1.6 g/L), and searching to circumvent this problem, the standard strategy used for clinical BNCT treatment, it is as a soluble fructose complex, known as L-BPA-F (Figure 3), which leads to a pharmaceutical product with favorable biodistribution in human GBM, ratios tumor:blood of 3–4:1 , low toxicity, and good capability to cross BBB. Other two strategies include (a) the transformation into the corresponding hydrochloride salt and (b) the esterification of the boronic acid moiety with 1,2- and 1,3-diol producing 1,3,2-dioxaborolanes or 1,3,2-dioxaborinanes, respectively, which is then easily hydrolysable in an aqueous environment (Figure 3) [26, 27]. On the other hand, L-BPA is actively transported across the tumor cell membrane, by the L-amino acid transporter system. It is highly expressed in tumor cells, including the brain, compared with normal tissues and can be stimulated by the previous accumulation of the L-DOPA resulting in a substrate-coupled antiport (exchange) mechanism . At this point, L-BPA is considered to be a better B delivery agent than BSH.
2. Medicinal chemistry of boron-bearing compounds for BNCT
From a medicinal chemistry point of view, different strategies have been studied in order to identify new and more selective molecules to glioma cells, with adequate ability to cross the BBB, with higher tumor concentration in the path of the neutron beam and drug-like properties. The third-generation products, which potentially may accumulate into glioma for its structural analogy to some biomolecules, could be classified  in (a) macromolecular species and (b) low molecular weight molecules. In reference to the first group, we could mention monoclonal and bispecific antibodies, epidermal growth factor, and encapsulating agents such as boron-containing nanovehicles (liposomes). Here, we will discuss compounds belonging to the second group, like polyhedral boron cluster derivatives, boronic acid derivatives, and other boron-containing small molecules (e.g., oxaborolanes, dioxaborolanes, and azaboro-heterocycles, among others).
2.2. Polyhedral boron clusters
2.2.1. Implications for drug discovery: structural features of
closo-carboranes and metallacarboranes. How do they influence on the drug-like properties?
The most known and commonly used class of polyhedral boron compounds in the medicinal chemistry field are the icosahedral dicarba-
Among the outstanding and widely explored properties of carboranes and metallacarboranes for medicinal chemistry research are (a) the geometry and the electron-deficient nature of the boron atoms, which generate a strong hydride character in the BH shell, which are some of the main features that determine the intermolecular interactions with the biological targets because they make the B-clusters extremely hydrophobic; (b) both the pharmacokinetics and the bioavailability can be modulated according to the chemotype of boron cluster selected, so the hydrophilicity and lipophilicity, or both, could be tuned; (c) the globular architecture and rigid geometry allow for molecular construction in three dimensions improving the docking, or not, with bio-targets; (d) high boron content per molecule and stability to catabolism are important criteria for the development of agent for BNCT; and (e) the well-established chemistry that makes boron clusters attractive synthons to construct novel pharmaceuticals .
Nevertheless, some problems persist today that delay the application of boron clusters in the development of new drugs: (a) the relatively high cost of carboranes and their derivatives even more if they will be used in BNCT because 10B-enriched compound will be needed; (b) the difficulty of in silico drug design and screening of boron cluster drugs, due to the lack of appropriate descriptors for the interaction potentials of boron and the attached hydrogen atoms; and (c) the lack of libraries of boron cluster compounds for high-throughput screening.
Closo-carboranes and metallacarboranes designed and studied for BNCT-glioma treatment
In the past decade, saccharides have been widely studied due to their low toxicities, generally having high water solubility, high expression of specific receptor on the tumor cell surface (specifically in the brain capillary endothelial cells that form the brain barriers, resulting in optimal BBB penetration), and an increased rate of glycolysis in cancer cells. According to this and in order to improve boron uptake, carbohydrates such as ribose, mannose, maltose, and galactose have been chosen to generate carbohydrate-based boron delivery platforms for successful BNCT approach [32, 33]. Since the first description by Hawthorne’s group , there have been numerous works on the synthesis and biological evaluation of carboranyl-carbohydrate conjugates. Later, Tietze and co-workers developed and evaluated both in vitro and in vivo, against rat glioma cells (C6) and melanoma cell line (B16), a series of carboranyl glycosides including glucoside, lactoside, and maltoside conjugates (
On the other hand, in the last few years, boron-bearing purines, pyrimidines, thymidines, nucleosides, and nucleotides have been widely explored as a novel approach to improve boron uptake in glioma tumor cells. This strategy is based on the fact that tumor cells have higher metabolic activity and an increased requirement for DNA and RNA precursors [39, 40]. Although in recent years several strategies have been addressed, the main focus has been on thymidine analogs substituted with
Barth and co-workers evaluated 10B-enriched derivative
Since the first description by Haushalter and Rudolph in 1978 (
In the last few years, a large number of boron-containing natural porphyrins have been synthesized and evaluated both in cellular and animal models (
The success of derivative
In order to overcome this drawback, several researchers designed and synthesized a highly boron water-soluble porphyrins as a possible BNCT agents. In this sense, Vicente and co-workers described
Carborane-containing nucleosides and analogues as the means to concentrate boron within the tumor cell nucleus were described. However, another class of derivatives, which could interact directly to chromosomal DNA, has been developed and widely explored for boron neutron capture therapy. Among this the following could be mentioned (Figure 9): (a) alkylating agents (i.e.,
More recently, Vicente’s group proposed other interesting strategies that involved the use of porphyrins bearing
2.2.3. BSH structural modifications
As it is indicated above, BSH inadequate drug-like properties, i.e., lack tumor selectivity and poor BBB crossing ability, have conducted to develop BSH hybrid compounds containing other pharmacophoric moieties seeking improvement biological behavior.
In this sense, some approaches were based on the particular tumoral amino acid requirement and the special ability of some peptides to interact with receptors that, via endocytosis, are internalized into tumoral cells. In the first strategy, hybrid
As it was already mentioned, the biochemical peculiarities of the porphyrin system also allowed the delivery of BSH within the tumor. In this sense, hybrid BSH porphyrin
GBM contains areas with different oxygenation levels, and consequently different metabolic patterns, which make difficult the therapeutic strategies . Highly oxygenated regions are close to blood vessels, present high proliferation rate and oxidative metabolism and are susceptible to metabolism-active drugs and radiotherapy. While hypoxic regions present low proliferation rate, reductive metabolism and are resistant to chemo- and radiotherapy. The hypoxic condition in GBM tumors, which induces metastasis and promotes angiogenesis and resistance, has been attributed to contribute to tumor regrowth and, therefore, in a relapse. The troublesome characteristics of hypoxic regions have been exploited to generate cancer therapeutics known as hypoxia-selective bioreductive prodrugs which are compounds able to undergo reduction producing cytotoxic events. Some of the well-known bioreductive pharmacophores, nitroimidazoles, and
2.3. Boronic acids and their esters
Boronic acid (R-B(OH)2) has been utilized as a pharmacophore in the searching of new agents to be employed in BNCT. This functional group has some relevant drug-like properties turning it into a useful moiety for biological applications, for example [66, 67, 68], (a) the
The success of the L-BPA in preliminary studies in patients with GBM, followed by PET studies with 1-aminocyclobutane-1-[11C]-carboxylic acid, revealing that cyclic amino acids were located preferentially in this tumor , led to the development of series of these cycloalkane-boronic acids (
As it is mentioned above, polyamines are essential for differentiation and growth of mammalian cell, and their depletion has growth inhibitory effects on tumors. Furthermore, a polyamine-facilitated transport system is present and allows the uptake by malignant cells . For that, boronic acid-containing polyamines have been also studied as potential BNCT agents . Compound
Thinking about DNA as target to metabolically incorporate boron atoms to the tumoral cell, boronic acid-containing nucleic acids have been studied. One of the first described compounds was the uracil derivative
The combination of PDT and BNCT has been also proposed using boronic acid-containing porphyrins, i.e.,
As previously stated the particular hypoxic condition of some GBM regions was used as strategy to produce boronic acid derivatives that could be selectively accumulated in this tissue. For example, the well-known 2-nitroimidazolyl hypoxia pharmacophore was employed as structural guide to boron-enriched tumoral cells generating the boronic acid ester
2.4. Miscellaneous compounds
Other boron-containing moieties have been described for BNCT, for example, in compounds
2.5. Other approaches: site-specific delivery
Beyond the strategies of therapy combination mentioned above, such as phototherapy or hypoxia, another hypothesis has been choosing a bio-system overexpressed in tumoral cells, but not in healthy cells, as a way to selectively accumulate boron drugs in the desired tissue to further BNCT. In this sense, Nakamura and colleagues have described the use of protein tyrosine kinases (PTKs), that its uncontrolled activation is often associated with uncontrolled cell growth and tumor progression, to generate boronic acid and esters hybridized with PTK pharmacophores as new drugs (i.e.,
The development of boron-bearing compound for BNCT has been a vast field of research within medicinal chemistry. New and interesting pharmacophores have been described in order to fulfill the BNCT requirements. Despite this, very few reached the stage of clinical studies, being currently L-BPA and BSH, the only two potential therapeutic agents. However, due to the multidisciplinary approach of the BNCT and the emerging novel structures, we expect with optimism the new developments in the years to come.
The authors are SNI-ANII researchers and PEDECIBA members.
Conflict of interest
The authors declare no conflict of interest.