Abstract
Boron neutron capture therapy (BNCT) is a unique binary cancer therapy modality that a drug of boronated compound is injected in a patient and accumulated trap in a tumor followed by bombardment with thermal neutrons. Being RGD peptide able to specifically bind to αvβ3 integrin receptor that can be highly expressed in brain tumors such as glioblastoma, we have attempted to develop boronated RGD compounds as to be BNCT drugs for treatment of brain tumors. To incorporate RGD peptide with boron, 4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2- yl)benzoic acid (STDBA) was synthesized and then conjugated to monomeric RGD peptide, c(RGDyK) and dimeric RGD peptide, E[c(RGDyK)]2 to produce the two boronated RGD peptides, TDB-c(RGDyK) and TDB-E[c(RGDyK)]2. The yields of TDB-c(RGDyK) and TDB-E[c(RGDyK)]2 were 65% and 48%, respectively. By 131I labeling, the labeled yields of the two boronated RGD compounds were both over 95%. The stabilities of 131I-[TDB-c(RGDyK)] and 131I-{TDB- E[c(RGDyK)]2} were high, being maintained at about 90% when stored either in both PBS and serum at ambient temperature for 48 h. 131I-[TDB- c(RGDyK)] and 131I-{TDB-E[c(RGDyK)]2} were found to be hydrophilic with log P = -1.44 ± 0.01 and -1.67 ± 0.02, respectively. In toxicity study, LD50 values of TDB-c(RGDyK) and TDB-E[c(RGDyK)]2 to U87-MG cells were found to be 2.57 nM and 2.75 nM for incubation of 48 h. The binding percentages for U87-MG cells of 131I-[TDB-c(RGDyK)] and 131I-{TDB-E[c(RGDyK)]2} were 12.96 ± 2.63% and 14.49 ± 0.24%, respectively for incubation of 3.5 h. By blocking with the large amounts of TDB-c(RGDyK) and TDB-E[c(RGDyK)]2, the binding percentages decreased to 2.47 ± 0.06% and 6.16 ± 0.87%, respectively. In competitive binding with 131I-E[c(RGDyK)]2, the obtained IC50 values for TDB- c(RGDyK) and TDB-E[c(RGDyK)]2 were 101.88 nM and 61.62 nM, respectively. Upon thermal neutron irradiation with a fluence of 2.69×1012 n•cm-2, the survival fractions of U87-MG cells incubated with 500 nM TDB-c(RGDyK) and 500 nM TDB-E[c(RGDyK)]2 were 27.97 ± 5.75% and 24.10 ± 2.84%, respectively, in contrast with 58.93 ± 5.50% in the case of irradiation of the cells alone.