Sweet Battle of the Epimers─Continued Exploration of Monosaccharide-Derived Delivery Agents for Boron Neutron Capture Therapy

Abstract

Boron neutron capture therapy (BNCT) is a cancer therapy in which boron delivery agents play a crucial role. In theory, delivery agents with high tumor targeting capabilities can lead to selective eradication of tumor cells without causing harmful side effects. We have been working on a GLUT1-targeting strategy to BNCT for a number of years and found multiple promising hit compounds which outperform the clinically employed boron delivery agents in vitro. Herein, we continue our work in the field by further diversification of the carbohydrate scaffold in order to map the optimal stereochemistry of the carbohydrate core. In the sweet battle of the epimers, carborane-bearing d-galactose, d-mannose, and d-allose are synthesized and subjected to in vitro profiling studies─with earlier work on d-glucose serving as the reference. We find that all of the monosaccharide delivery agents display a significantly improved boron delivery capacity over the delivery agents approved for clinical use in vitro, thus providing a sound foundation for advancing toward in vivo preclinical assessment studies.

Keywords: boron neutron capture therapy; carbohydrates; carboranes; drug delivery; medicinal chemistry.

Figure 1

Left: graphical representation of the BNCT working principle. Step 1: ¹⁰B atoms are delivered selectively to cancer cells. Step 2: cancer cells are irradiated with a beam of low energy thermal neutrons. Step 3: upon irradiation, ¹⁰B undergoes a nuclear capture followed by a fission reaction, giving rise to ⁷Li and ⁴He nuclei (α particles). Step 4: ⁴He causes a destructive effect on the cancer cell. Right: an example of a promising GLUT1-targeting delivery agent from our recent work. Figure created with BioRender (https://biorender.com/).

Figure 2

Molecular structures of the three synthesized and studied glycoconjugates. 1 is based on d-Man, 2 on d-All, and 3 on d-Gal.

Scheme 1. Reaction Routes and Conditions Employed in the Synthesis of Glycoconjugates 1 (Top), 2 (Middle), and 3 (Bottom)

Top: (i) (1) tert-butyldimethylsilyl chloride (TBDMSCl), pyridine, r.t., 2 h, 68%; (2) BnBr, NaH, DMF, 0 °C → r.t., 2 h, 80%; (3) HF·pyridine, THF, 0 °C → r.t., 18 h, 60%; (ii) propargyl bromide, NaH, DMF, r.t., 2 h, 70%; (iii) (1) B₁₀H₁₄, MeCN, 60 °C, 1 h; (2) 5, toluene, 80 °C, 16 h, 53%; (iv) H₂, 10% Pd/C, EtOAc:MeOH 7:1, 4 bar, r.t., 6 h, 83%. Middle: (v) (1) TBDMSCl, pyridine, r.t., 4 h, 78%; (2) BnBr, NaH, DMF, 0 °C → r.t., 3 h, 88%; (3) HF·pyridine, THF, 0 °C → r.t., 18 h, 72%; (vi) propargyl bromide, NaH, DMF, r.t., 2 h, 77%; (vii) (1) B₁₀H₁₄, MeCN, 60 °C, 1 h; (2) 8, toluene, 80 °C, 16 h, 66%; (viii) H₂, 10% Pd/C, EtOAc:MeOH 7:1, 4 bar, r.t., 4 h, 80%. Bottom: (ix) NaH, propargyl bromide, DMF, r.t., 16 h, 70%; (x) (1) B₁₀H₁₄, MeCN, 60 °C, 1 h; (2) 10, toluene, 80 °C, 16 h, 62%; (xi) trifluoroacetic acid (TFA)/H₂O/Et₂O (2:1:2), r.t., 24 h, 97%.

Figure 3

NMR spectral simulation with the ChemAdder software is showcased for selected parts of the 5.20–3.40 ppm range using glycoconjugate 3 as an example. Top: Simulated spectrum. Bottom: Experimentally obtained spectrum.

Figure 4

Inhibition of [¹⁴C]-d-glucose uptake by glycoconjugates 1–3 in the CAL 27 cell line is showcased and the IC₅₀ values provided.

Figure 5

Cytotoxicity studies in the CAL 27 cell line after incubation with cell culture medium (negative control), 1% v/v Triton X-100 (positive control), glycoconjugates 1–3, and BSH at concentrations of 5, 25, 50, 125, and 250 μM (n = 3–4) at the 24 h time point. The comparative cytotoxicity data for BSH is from one of our earlier studies in the same cell line. The statistical significance was analyzed using an unpaired Student’s t-test, where the significance was set at *p < 0.05, **p < 0.01, and ***p < 0.001.

Figure 6

Boron delivery capacity assessed by ICP-MS determination of elemental boron in digested CAL 27 cells after incubation with glycoconjugates 1–3, BPA, or BSH in the concentration range of 10–400 μM over 5, 30, and 120 min.