**1. Introduction**

Boron (B) is an element that is found in ultratrace amounts in mammalian cells and consists of two stable isotopes, boron-10 (10B) and boron-11 (11B), with a natural abundance ratio (10B/11B = 19.9/80.1). The most important properties of boron compounds with respect to biological and medical sciences would be: (1) 11B atoms have a higher NMR sensitivity (16.5% for 11B and 2.0% for 10B relative to 1 H NMR), thus permitting the detection of B-containing drugs themselves and analytes that react with B-containing probes in living systems [1]; and (2) the 10B nucleus possesses a high reactivity with thermal neutrons resulting in the generation of two radioactive species ( 4 He and 7 Li particles), which induce the excitation and ionization of molecules within short path lengths [2]. For the above reasons, boron compounds can be useful in biological applications for the treatment and diagnosis of cancer and other diseases [3].

In 1936, Locher proposed the concept of boron neutron capture therapy (BNCT) based on the aforementioned nuclear reaction between 10B and thermal neutrons [4]. Because the destructive effect of the two heavy particles (4 He and 7 Li particles) that are generated by the decomposition of 10B lies within 5–9 μm, which is close to the size of living cells, single-cell treatment would be possible by the achievement of cancer-specific delivery of 10B and irradiation with a sufficient intensity of thermal neutrons [5–7].

BNCT systems have been installed in clinical facilities as a method for the noninvasive treatment of certain types of cancers such as recurrent head and neck cancer and malignant gliomas [8]. The selective and efficient accumulation of boron into tumor tissues is one of the important clues for successful BNCT and, as described below, two boron compounds have been approved for use as BNCT drugs. In addition, monitoring the distribution of boron in patients is required for planning treatment protocols to determine the irradiation doses and positions of the patient [9].

In this review, we introduce the applications of boron compounds to 11B NMR (nuclear magnetic resonance)/MRI (magnetic resonance imaging) probes for the sensing of intracellular metal ions and BNCT agents for use in the treatment of cancer. The d-block metal ion probes take advantage of changes in the chemical shift in 11B NMR spectra due to the cleavage of the carbon-boron bond in phenylboronic acid-pendant cyclen (1,4,7,10-tetraazacyclododecane) and the decomposition of the *ortho*-carborane moieties of carborane-metal chelator hybrids upon complexation with metal ions in aqueous solution at neutral pH. In the second half of this review, the development of novel BNCT agents bearing sugar and macrocyclic polyamine scaffolds is described.
