Multi-Functional Boron-Delivery Agents for Boron Neutron Capture Therapy of Cancers

Boron neutron capture therapy (BNCT) is a binary cancer treatment that involves the irradiation of ¹⁰B-containing tumors with low-energy neutrons (thermal or epithermal). The alpha particles and recoiling Li nuclei that are produced in the ¹⁰B-capture nuclear reaction are high-linear-energy transfer particles that destroy boron-loaded tumor cells; therefore, BNCT has the potential to be a localized therapeutic modality. Two boron-delivery agents have been used in clinical trials of BNCT in patients with malignant brain tumors, cutaneous melanoma, or recurrent tumors of the head and neck region, demonstrating the potential of BNCT in the treatment of difficult cancers. A variety of potentially highly effective boron-delivery agents have been synthesized in the past four decades and tested in cells and animal models. These include boron-containing nucleosides, peptides, proteins, polyamines, porphyrins, liposomes, monoclonal antibodies, and nanoparticles of various types. The most promising agents are multi-functional boronated molecules and nanoparticles functionalized with tumor cell-targeting moieties that increase their tumor selectivity and contain a radiolabel or fluorophore to allow quantification of ¹⁰B-biodistribution and treatment planning. This review discusses multi-functional boron agents reported in the last decade, but their full potential can only be ascertained after their evaluation in BNCT clinical trials.

8(meso)-Pyridyl-BODIPYs: Effects of 2,6-Substitution with Electron-Withdrawing Nitro, Chloro, and Methoxycarbonyl Groups

The introduction of electron-withdrawing groups on 8(meso)-pyridyl-BODIPYs tends to increase the fluorescence quantum yields of this type of compound due to the decrease in electronic charge density on the BODIPY core. A new series of 8(meso)-pyridyl-BODIPYs bearing a 2-, 3-, or 4-pyridyl group was synthesized and functionalized with nitro and chlorine groups at the 2,6-positions. The 2,6-methoxycarbonyl-8-pyridyl-BODIPYs analogs were also synthesized by condensation of 2,4-dimethyl-3-methoxycarbonyl-pyrrole with 2-, 3-, or 4-formylpyridine followed by oxidation and boron complexation. The structures and spectroscopic properties of the new series of 8(meso)-pyridyl-BODIPYs were investigated both experimentally and computationally. The BODIPYs bearing 2,6-methoxycarbonyl groups showed enhanced relative fluorescence quantum yields in polar organic solvents due to their electron-withdrawing effect. However, the introduction of a single nitro group significantly quenched the fluorescence of the BODIPYs and caused hypsochromic shifts in the absorption and emission bands. The introduction of a chloro substituent partially restored the fluorescence of the mono-nitro-BODIPYs and induced significant bathochromic shifts.

Amino Acid Derivatives of Chlorin-e6-A Review

Details of the structural elucidation of the clinically useful photodynamic therapy sensitizer NPe6 (15) are presented. NPe6, also designated as Laserphyrin, Talaporfin, and LS-11, is a second-generation photosensitizer derived from chlorophyll-a, currently used in Japan for the treatment of human lung, esophageal, and brain cancers. After the initial misidentification of the structure of this chlorin-e₆ aspartic acid conjugate as (13), NMR and other synthetic procedures described herein arrived at the correct structure (15), confirmed using single crystal X-ray crystallography. Interesting new features of chlorin-e₆ chemistry (including the intramolecular formation of an anhydride (24)) are reported, allowing chemists to regioselectively conjugate amino acids to each available carboxylic acid on positions 13¹ (formic), 15² (acetic), and 17³ (propionic) of chlorin e₆ (14). Cellular investigations of several amino acid conjugates of chlorin-e₆ revealed that the 13¹-aspartylchlorin-e₆ derivative is more phototoxic than its 15²- and 17³-regioisomers, in part due to its nearly linear molecular conformation.

New Boron Delivery Agents

This proceeding article compiles current research on the development of boron delivery drugs for boron neutron capture therapy that was presented and discussed at the National Cancer Institute (NCI) Workshop on Neutron Capture Therapy that took place on April 20-22, 2022. The most used boron sources are icosahedral boron clusters attached to peptides, proteins (such as albumin), porphyrin derivatives, dendrimers, polymers, and nanoparticles, or encapsulated into liposomes. These boron clusters and/or carriers can be labeled with contrast agents allowing for the use of imaging techniques, such as PET, SPECT, and fluorescence, that enable quantification of tumor-localized boron and their use as theranostic agents.

A Comparison of the Photophysical, Electrochemical and Cytotoxic Properties of meso-(2-, 3- and 4-Pyridyl)-BODIPYs and Their Derivatives

Boron dipyrromethene (BODIPY) dyes bearing a pyridyl moiety have been used as metal ion sensors, pH sensors, fluorescence probes, and as sensitizers for phototherapy. A comparative study of the properties of the three structural isomers of meso-pyridyl-BODIPYs, their 2,6-dichloro derivatives, and their corresponding methylated cationic pyridinium-BODIPYs was conducted using spectroscopic and electrochemical methods, X-ray analyses, and TD-DFT calculations. Among the neutral derivatives, the 3Py and 4Py isomers showed the highest relative fluorescence quantum yields in organic solvents, which were further enhanced 2-4-fold via the introduction of two chlorines at the 2,6-positions. Among the cationic derivatives, the 2catPy showed the highest relative fluorescence quantum yield in organic solvents, which was further enhanced by the use of a bulky counter anion (PF₆^-). In water, the quantum yields were greatly reduced for all three isomers but were shown to be enhanced upon introduction of 2,6-dichloro groups. Our results indicate that 2,6-dichloro-meso-(2- and 3-pyridinium)-BODIPYs are the most promising for sensing applications. Furthermore, all pyridinium BODIPYs are highly water-soluble and display low cytotoxicity towards human HEp2 cells.

Click Conjugation of Boron Dipyrromethene (BODIPY) Fluorophores to EGFR-Targeting Linear and Cyclic Peptides

Through a simple 1,3-cycloaddition reaction, three BODIPY-peptide conjugates that target the extracellular domain of the epidermal growth factor receptor (EGFR) were prepared and their ability for binding to EGFR was investigated. The peptide ligands K(N₃)LARLLT and its cyclic analog cyclo(K(N₃)larllt, previously shown to have high affinity for binding to the extracellular domain of EGFR, were conjugated to alkynyl-functionalized BODIPY dyes 1 and 2 via a copper-catalyzed click reaction. This reaction produced conjugates 3, 4, and 5 in high yields (70–82%). In vitro studies using human carcinoma HEp2 cells that overexpress EGFR demonstrated high cellular uptake, particularly for the cyclic peptide conjugate 5, and low cytotoxicity in light (~1 J·cm⁻²) and darkness. Surface plasmon resonance (SPR) results show binding affinity of the three BODIPY-peptide conjugates for EGFR, particularly for 5 bearing the cyclic peptide. Competitive binding studies using three cell lines with different expressions of EGFR show that 5 binds specifically to EGFR-overexpressing colon cancer cells. Among the three conjugates, 5 bearing the cyclic peptide exhibited the highest affinity for binding to the EGFR protein.

An Examination of Factors Deterring the Pursuit of Advanced Degrees Among Alumni of a Minority Research and Training Program

Minority research and training (MRT) programs utilize interventions aimed at increasing student persistence toward the completion of undergraduate degrees and continuation into doctoral degrees. Numerous studies identify these support interventions as essential components for positive outcomes among underrepresented students, including research experiences, social integration, mentorship, and financial support. Many of these previous studies utilize quantitative design involving current program participants or program alumni who continued into graduate programs. This pilot study employed a multiple case study with a narrative approach involving participants of an MRT program who completed a BSc degree but did not subsequently matriculate into a doctoral program. The cross-case analysis identified perceived barriers deterring continuation into doctoral degree programs among former MRT participants. Results from this study add student voice to the current literature and provide recommendations to better serve underrepresented students in the STEM disciplines.

Results from a 14-Year Intervention Program Designed to Impact Pursuit of a PhD in Research among Underrepresented Students in STEM Disciplines

Minority research and training (MRT) programs have been used across U.S. colleges and universities as a method to close the educational achievement gap and generate a highly skilled and diverse workforce. Previous studies have improved our understanding of the need to diversify the science, technology, engineering and mathematics (STEM) disciplines and the various interventions that have been developed to support these efforts. However, there is still little evidence about what strategies are most effective in promoting interest, continuation, and matriculation into STEM graduate programs among underrepresented groups. The study herein utilized a case study design with a mixed methods approach to evaluate the program impacts and outcomes of an MRT program at a research-intensive institution in the southern part of the U.S., and for program replication. This evaluation study examines the types of activities and services provided, the measurable outcomes of those activities and services, the resources used to deliver the services, the practical problems encountered, and the ways in which problems were resolved.

Structural Investigations on Mono- and Di-Acrolein Substituted Ni(II) Porphyrins and a Ni(II) Benzochlorin. Model Compounds for Photosensitizers in Photodynamic Therapy

The structures of four Ni complexes and a free base related to tetrapentanoporphyrin (TC7P) and octaethylporphyrin (OEP) have been determined by X-ray crystallograpic methods. The Ni(II) complexes exhibit considerable S4-ruffling induced by coordination hole contraction due to the low spin Ni(II). The degree of conformational distortion is enhanced by one or two additional acrolein substituents at the meso positions. The steric strain imposed by interaction of the meso substituent with the neighboring β-pyrrole substituents leads to a significant displacement from the mean plane of the molecule of the meso carbon(s) involved. Increasing non-planarity of the macrocycles is correlated with shifts to longer wavelenghts in the absorption spectra. Ring closures of the acrolein group to afford benzochlorin type pigments results in further bathochromic shifts and a very non-planar molecular conformation. Long wavelength absorbing pigments such as benzochlorins are useful photosensitizers for photodynamic therapy and thus one criterion for a good photosensitizer might be its conformational distortion. Crystal data – 3. C40H46N4 · CH2Cl2, Mr = 667.7, triclinic, P1̄, a = 7.776(5)Å, b = 8.556(4)Â, c = 14.509(8)Å, α = 73.96(1)°, β = 82.04(2)°, γ = 82.58(2)°, V = 914.5(9)Å3, Ζ = 1, Dx = 1.212 Mg/m3, (ΜοΚα) λ = 0.71069Å, μ = 0.209 mm-1, F(000) = 356, 130 Κ, R = 0.082 for 2217 reflections with I > 1.5σ(Ι). Ni4. C44H40N4NiO · CH2Cl2, Mr = 770.4, triclinic, P1̄, a = 11.283(3)Å, b = 11.800(4)Å, c = 15.871(7)Å, α = 79.28(3)°, β = 88.95(2)°, γ = 62.41(1)°, V = 1838.8(11)Å3, Ζ = 2, Dx = 1.391 Mg/m3, (ΜοΚα), μ = 0.715 mm-1, F(000) = 804, 130 Κ, R = 0.072 for 5309 reflections with I > 2.0σ(Ι). Ni5. C46H28N4NiO2 • CH2CH2, Mr = 812.4, triclinic, P1̄, a = 8.959(4)Å, b = 14.420(7)A, c = 15.471(5)Å, α = 102.73(4)°, β = 101.83(4)°, γ = 92.01(5)°, V = 1901.3(14)Å3, Z = 2, Dx = 1.419 Mg/m3, (ΜοΚα), μ = 0.679 mm-1, F(000) = 836, 130 Κ, R = 0.059 for 4197 reflections with I > 1.75σ(Ι). Ni7. C43H45N4NiO2·CH2Cl2, Mr = 793.5, triclinic, P1̄, a = 8.948(3)Å, b = 13.855(3)Å, c = 15.451(3)Å, α = 82.43(2)°, β = 78.79(2)°, γ = 81.24(2)°, V = 1846.7(8)Å3, Z = 2, Dx = 1.427 Mg/m3, (CuKa) λ = 1.54178 Å, μ = 2.448 mm-1, F(000) = 834, 126 K, R = 0.09 for 4039 reflections with I > 2.0σ(Ι). Ni8. C42H48N4NiO, Mr = 683.6, monoclinic, Ρ21/n, a = 15.168(6)Å, b = 14.796(3)Å, c = 15.983(4)Å, β= 97.17(3)°, V = 3552(2)Å3, Z = 4, Dxx = 1.278 Mg/m3, (ΜοΚα), μ = 0.584 mm-1, F(000) = 1456, 130 Κ, R = 0.042 for 5904 reflections with I > 2.5 σ(I).