Synthesis and Anti-Tumor Evaluation of Carboranyl BMS-202 Analogues-A Case of Carborane Not as Phenyl Ring Mimetic

Abstract

Carborane is considered a three-dimensional mimetic of phenyl rings in medicinal chemistry. BMS-202 is a potent PD-L1 inhibitor that can block the PD-L1/PD-1 interaction and restore the immune response to cancer cells. Herein, we replaced the terminal phenyl group of BMS-202 with carborane and prepared its carboranyl BMS-202 analogues. The results showed a loss of PD-L1 binding affinity due to the bulky size of carborane, suggesting that carborane cannot serve as a phenyl ring mimetic in certain cases. Docking study demonstrated that the narrow binding pocket of PD-L1 could not hold the bulky carborane, resulting in loss of its activity. Compounds 1a and 1b exhibited anti-proliferative activities on a broad scope of cancer cell lines. Further studies indicate that compound 1a can induce cell apoptosis and lead to G1 cell cycle phase arrest. The boron biodistribution study of compound 1a revealed that the brain/blood uptake ratio was 0.60 ± 0.08, exhibiting a good blood-brain penetration capability.

Keywords: BMS-202; antitumor activity; carborane; phenyl ring mimetic.

Figure 1

Representative carborane-containing compounds for BNCT or as biological active molecules.

Figure 2

Molecular design of carboranyl BMS-202 analogues.

Scheme 1

Synthesis of compounds 1a–1c. Reagents and conditions: (a) Ethynyltrimethylsilane, PdCl₂(PPh₃)₂, CuI, Et₃N, 85 °C, 12 h; (b) K₂CO₃, MeOH, rt, 1 h; (c) B₁₀H₁₂(MeCN)₂, AgNO₃, toluene, 100 °C, 4 h; (d) LiAlH₄, THF, rt, 4 h; (e) Pd(OAc)₂, t-BuXphos, Cs₂CO₃, 6-bromo-2-methoxynicotinaldehyde, toluene, 80 °C, 8 h; (f) N-(2-aminoethyl)acetamide, NaBH(OAc)₃, THF, rt, 12 h; (g) LiAlH₄, THF, rt, 4 h; (h) NaH, tert-butyl bromoacetate, THF, rt, 12 h; (i) TFA, DCM, rt, 12 h.

Figure 3

X-ray crystal structure of compound 1a.

Figure 4

(A) PD-1/PD-L1 inhibitory activities of compounds 1a–1c. (B) docking of BMS-202 in the binding pocket of PD-L1 dimer (PDB ID: 5N2F) (C) docking of 1a in PD-L1 shows many collisions between the carborane in 1a and its surrounding residues. The red dashed line represents the collisions. (D) docking of BMS-202 and PD-L1 dimer shows that the biphenyl ring can be accommodated in the pocket crevice. (E) docking of 1a and PD-L1 dimer shows that the narrow crevice of PD-L1 dimer can not accommodate the bulky carborane.

Figure 5

(A) cell cycle distribution of compound 1a (B) cell cycle distribution of compound 1b. The flow cytometry analysis graph shows the result after 24 h of dosing. Statistical analysis is shown on the bar graphs. Data are presented as the mean ± SD of at least three independent experiments. ** p < 0.01, *** p < 0.001, **** p < 0.0001.

Figure 6

(A) The apoptosis analysis of HepG2 cells treated with various concentrations of compound 1a. Cells that went through early apoptosis were marked with Annexin V-PE⁺/7-AAD⁻, cells that went through late apoptosis were marked with Annexin V-PE⁺/7-AAD⁺. (B) The apoptosis rate shown as bar graphs. Data are presented as mean ± SD of three independent experiments. * p < 0.05, ** p < 0.01, *** p < 0.001.

Figure 7

(A) In vivo boron biodistribution analysis of compound 1a at 1 h, 3 h and 5 h post-injection. (B) the brain/blood uptake ratio of compound 1a.