Youngjoo Byun

Synthesis and Evaluation of Technetium-99m- and Rhenium-Labeled Inhibitors of the Prostate-Specific Membrane Antigen (PSMA)

The prostate-specific membrane antigen (PSMA) is increasingly recognized as a viable target for imaging and therapy of cancer. We prepared seven (99m)Tc/Re-labeled compounds by attaching known Tc/Re chelating agents to an amino-functionalized PSMA inhibitor (lys-NHCONH-glu) with or without a variable length linker moiety. K i values ranged from 0.17 to 199 nM. Ex vivo biodistribution and in vivo imaging demonstrated the degree of specific binding to engineered PSMA+ PC3 PIP tumors. PC3-PIP cells are derived from PC3 that have been transduced with the gene for PSMA. Despite demonstrating nearly the lowest PSMA inhibitory potency of this series, [(99m)Tc(CO)3( L1)] (+) ( L1 = (2-pyridylmethyl)2N(CH2) 4CH(CO2H)NHCO-(CH2) 6CO-NH-lys-NHCONH-glu) showed the highest, most selective PIP tumor uptake, at 7.9 +/- 4.0% injected dose per gram of tissue at 30 min postinjection. Radioactivity cleared from nontarget tissues to produce a PIP to flu (PSMA-PC3) ratio of 44:1 at 120 min postinjection. PSMA can accommodate the steric requirements of (99m)Tc/Re complexes within PSMA inhibitors, the best results achieved with a linker moiety between the epsilon amine of the urea lysine and the chelator.

Radiohalogenated Prostate-Specific Membrane Antigen (PSMA)- Based Ureas as Imaging Agents for Prostate Cancer

To extend our development of new imaging agents targeting the prostate-specific membrane antigen (PSMA), we have used the versatile intermediate 2-[3-(5-amino-1-carboxy-pentyl)-ureido]-pentanedioic acid (Lys-C(O)-Glu), which allows ready incorporation of radiohalogens for single photon emission computed tomography (SPECT) and positron emission tomography (PET). We prepared 2-[3-[1-carboxy-5-(4-[(125)I]iodo-benzoylamino)-pentyl]-ureido]-pentanedioic acid ([(125)I]3), 2-[3-[1-carboxy-5-(4-[(18)F]fluoro-benzoylamino)-pentyl]-ureido]-pentanedioic acid ([(18)F]6), and 2-(3-[1-carboxy-5-[(5-[(125)I]iodo-pyridine-3-carbonyl)-amino]-pentyl]-ureido)-pentanedioic acid ([(125)I]8) in 65-80% (nondecay-corrected), 30-35% (decay corrected), and 59-75% (nondecay-corrected) radiochemical yields. Compound [(125)I]3 demonstrated 8.8 +/- 4.7% injected dose per gram (%ID/g) within PSMA(+) PC-3 PIP tumor at 30 min postinjection, which persisted, with clear delineation of the tumor by SPECT. Similar tumor uptake values at early time points were demonstrated for [(18)F]6 (using PET) and [(125)I]8. Because of the many radiohalogenated moieties that can be attached via the epsilon amino group, the intermediate Lys-C(O)-Glu is an attractive template upon which to develop new imaging agents for prostate cancer.

68Ga-Labeled Inhibitors of Prostate-Specific Membrane Antigen (PSMA) for Imaging Prostate Cancer

Gallium-68 is a generator-produced radionuclide for positron emission tomography (PET) that is being increasingly used for radiolabeling of tumor-targeting peptides. Compounds [(68)Ga]3 and [(68)Ga]6 are high-affinity urea-based inhibitors of the prostate-specific membrane antigen (PSMA) that were synthesized in decay-uncorrected yields ranging from 60% to 70% and radiochemical purities of more than 99%. Compound [(68)Ga]3 demonstrated 3.78 +/- 0.90% injected dose per gram of tissue (%ID/g) within PSMA+ PIP tumor at 30 min postinjection, while [(68)Ga]6 showed a 2 h PSMA+ PIP tumor uptake value of 3.29 +/- 0.77 %ID/g. Target (PSMA+ PIP) to nontarget (PSMA- flu) ratios were 4.6 and 18.3, respectively, at those time points. Both compounds delineated tumor clearly by small animal PET. The urea series of imaging agents for PSMA can be radiolabeled with (68)Ga, a cyclotron-free isotope useful for clinical PET studies, with maintenance of target specificity.

2-(3-{1-Carboxy-5-[(6-[18F]fluoro-pyridine-3-carbonyl)-amino]-pentyl}-ureido)-pentanedioic acid, [18F]DCFPyL, a PSMA-based PET Imaging Agent for Prostate Cancer

Purpose: We have synthesized and evaluated in vivo 2-(3-{1-carboxy-5-[(6-[18F]fluoro-pyridine-3-carbonyl)-amino]-pentyl}-ureido)-pentanedioic acid, [18F]DCFPyL, as a potential imaging agent for the prostate-specific membrane antigen (PSMA). PSMA is upregulated in prostate cancer epithelia and in the neovasculature of most solid tumors. Experimental Design: [18F]DCFPyL was synthesized in two steps from the p-methoxybenzyl (PMB) protected lys-C(O)-glu urea precursor using 6-[18F]fluoronicotinic acid tetrafluorophenyl ester ([18F]F-Py-TFP) for introduction of 18F. Radiochemical synthesis was followed by biodistribution and imaging with PET in immunocompromised mice using isogenic PSMA PC3 PIP and PSMA- PC3 flu xenograft models. Human radiation dosimetry estimates were calculated using OLINDA/EXM 1.0. Results: DCFPyL displays a Ki value of 1.1 ± 0.1 nmol/L for PSMA. [18F]DCFPyL was produced in radiochemical yields of 36%–53% (decay corrected) and specific radioactivities of 340–480 Ci/mmol (12.6–17.8 GBq/μmol, n = 3). In an immunocompromised mouse model [18F]DCFPyL clearly delineated PSMA+ PC3 PIP prostate tumor xenografts on imaging with PET. At 2 hours postinjection, 39.4 ± 5.4 percent injected dose per gram of tissue (%ID/g) was evident within the PSMA+ PC3 PIP tumor, with a ratio of 358:1 of uptake within PSMA+ PC3 PIP to PSMA− PC3 flu tumor placed in the opposite flank. At or after 1 hour postinjection, minimal nontarget tissue uptake of [18F]DCFPyL was observed. The bladder wall is the dose-limiting organ. Conclusions: These data suggest [18F]DCFPyL as a viable, new positron-emitting imaging agent for PSMA-expressing tissues. Clin Cancer Res; 17(24); 7645–53. ©2011 AACR.

A low molecular weight PSMA-based fluorescent imaging agent for cancer

We synthesized YC-27 3 to provide a fluorescent imaging agent for the prostate-specific membrane antigen (PSMA), a marker for hormone-independent prostate cancer and tumor neovasculature, with suitable pharmacokinetics for use in vivo. Immediate precursor trifluoroacetate salt of 2-(3-{5-[7-(5-amino-1-carboxy-pentylcarbamoyl)-heptanoylamino]-1-carboxy-pentyl}-ureido)-pentanedioic acid 2 was conjugated with a commercially available near-infrared light-emitting dye (IRDye 800CW) to provide 3 in 72% yield. YC-27 3 demonstrated a PSMA inhibitory activity of 0.37nM and was capable of generating target-to-nontarget ratios of at least 10 in PSMA-expressing PC3-PIP vs. PSMA-negative PC3-flu tumors in vivo. YC-27 3 may be useful for study of PSMA-expressing tissue in preclinical models or for intraoperative guidance.

6-Gingerol reduces Pseudomonas aeruginosa biofilm formation and virulence via quorum sensing inhibition.

Pseudomonas aeruginosa is a well-known pathogenic bacterium that forms biofilms and produces virulence factors via quorum sensing (QS). Interfering with normal QS interactions between signal molecules and their cognate receptors is a developing strategy for attenuating its virulence. Here we tested the hypothesis that 6-gingerol, a pungent oil of fresh ginger, reduces biofilm formation and virulence by antagonistically binding to P. aeruginosa QS receptors. In silico studies demonstrated molecular binding occurs between 6-gingerol and the QS receptor LasR through hydrogen bonding and hydrophobic interactions. Experimentally 6-gingerol reduced biofilm formation, several virulence factors (e.g., exoprotease, rhamnolipid, and pyocyanin), and mice mortality. Further transcriptome analyses demonstrated that 6-gingerol successfully repressed QS-induced genes, specifically those related to the production of virulence factors. These results strongly support our hypothesis and offer insight into the molecular mechanism that caused QS gene repression.

Sequential SPECT and Optical Imaging of Experimental Models of Prostate Cancer with a Dual Modality Inhibitor of the Prostate-Specific Membrane Antigen

We describe a platform for dual modality (radionuclide/optical) imaging of prostate cancer (PCa) based on targeting the prostate-specific membrane antigen (PSMA). An example provided demonstrates that after a single intravenous (IV) injection of tracer amounts (0.1 nmol) of imaging agent to a tumor-bearing mouse, both single photon emission computed tomography (SPECT) and near-infrared fluorescence (NIRF) imaging were capable of delineating tumor specifically. That such small injected amounts could identify tumor in vivo suggests that optical agents, as has long been known for radiopharmaceuticals, may obey the tracer principle, enabling more rapid clinical translation.

Boron-Containing Nucleosides as Potential Delivery Agents for Neutron Capture Therapy of Brain Tumors

The purpose of the present study was to evaluate both in vitro and in vivo a series of boron-containing nucleosides that potentially could be used as delivery agents for neutron capture therapy. The rationale for their synthesis was based on the fact that proliferating neoplastic cells have increased requirements for nucleic acid precursors, and, therefore, they should preferentially localize in the tumor. A series of 3-carboranlyalkyl thymidine analogs has been synthesized and a subset, designated N4, N5, and N7, and the corresponding 3-dihydroxypropyl derivatives, designated N4–2OH, N5–2OH, and N7–2OH, have been selected for evaluation. Using these compounds as substrates for recombinant human thymidine kinase-1 and the mitochondrial isoenzyme thymidine kinase-2, the highest phosphorylation levels relative to thymidine were seen with N5 and the corresponding dihydroxypropyl analog N5–2OH. In contrast, N4, N4-OH, N7, and N7-OH had substantially lower phosphorylation levels. To compare compounds with high and low thymidine kinase-1 substrate activity, N5 and N7 and the corresponding dihydroxypropyl derivatives were selected for evaluation of their cellular toxicity, uptake and retention by the F98 rat glioma, human MRA melanoma, and murine L929 cell lines, all of which are thymidine kinase-1(+), and a mutant L929 cell line that is thymidine kinase-1(−). N5–2OH was the least toxic (IC50, 43–70 μm), and N7 and N7–2OH were the most toxic (IC50, 18–49 μm). The highest boron uptake was seen with N7–2OH by the MRA 27 melanoma and L929 wild-type (wt) cell lines. The highest retention was seen with L929 (wt) cells, and this ranged from 29% for N5–2OH to 46% for N7. Based on the in vitro toxicity and uptake data, N5–2OH was selected for in vivo biodistribution studies either in rats bearing intracerebral implants of the F98 glioma or in mice bearing either s.c. or intracerebral implants of L929 (wt) tumors. At 2.5 hours after convection-enhanced delivery, the boron values for the F98 glioma and normal brain were 16.2 ± 2.3 and 2.2 μg/g, respectively, and the tumor to brain ratio was 8.5. Boron values at 4 hours after convection-enhanced delivery of N5–2OH to mice bearing intracerebral implants of L929 (wt) or L929 thymidine kinase-1(−) tumors were 39.8 ± 10.8 and 12.4 ± 1.6 μg/g, respectively, and the corresponding normal brain values were 4.4 and 1.6 μg/g, thereby indicating that there was selective retention by the thymidine kinase-1(+) tumors. Based on these favorable in vitro and in vivo data, neutron capture therapy studies will be initiated using N5–2OH in combination with two non-cell cycle dependent boron delivery agents, boronophenylalanine and sodium borocaptate.

Thymidine kinase 1 as a molecular target for boron neutron capture therapy of brain tumors.

The purpose of the present study was to evaluate the effectiveness of a 3-carboranyl thymidine analogue (3CTA), 3-[5-{2-(2,3-dihydroxyprop-1-yl)-o-carboran-1-yl}pentan-1-yl] thymidine, designated N5–2OH, for boron neutron capture therapy (BNCT) of brain tumors using the RG2 rat glioma model. Target validation was established using the thymidine kinase (TK) 1(+) wild-type, murine L929 cell line and its TK1(−) mutant counterpart, which were implanted s.c. (s.c.) into nude mice. Two intratumoral (i.t.) injections of 10B-enriched N5–2OH were administered to tumor-bearing mice at 2-hour intervals, after which BNCT was carried out at the Massachusetts Institute of Technology (MIT) Research Reactor. Thirty days after BNCT, mice bearing TK1(+) L929 tumors had a 15× reduction in tumor volume compared with TK1(−) controls. Based on these favorable results, BNCT studies were then initiated in rats bearing intracerebral (i.c.) RG2 gliomas, after i.c. administration of N5–2OH by Alzet osmotic pumps, either alone or in combination with i.v. (i.v.) boronophenylalanine (BPA), a drug that has been used clinically. The mean survival times (MSTs) of RG2 glioma bearing rats were 45.6 ± 7.2 days, 35.0 ± 3.3days, and 52.9 ± 8.9 days, respectively, for animals that received N5–2OH, BPA, or both. The differences between the survival plots of rats that received N5–2OH and BPA alone were highly significant (P = 0.0003). These data provide proof-of-principle that a 3CTA can function as a boron delivery agent for NCT. Further studies are planned to design and synthesize 3CTAs with enhanced chemical and biological properties, and increased therapeutic efficacy.

Synthesis and biological evaluation of low molecular weight fluorescent imaging agents for the prostate-specific membrane antigen.

Targeted near-infrared (NIR) optical imaging can be used in vivo to detect specific tissues, including malignant cells. A series of NIR fluorescent ligands targeting the prostate-specific membrane antigen (PSMA) was synthesized and each compound was tested for its ability to image PSMA+ tissues in experimental models of prostate cancer. The agents were prepared by conjugating commercially available active esters of NIR dyes, including IRDye800CW, IRDye800RS, Cy5.5, Cy7, or a derivative of indocyanine green (ICG) to the terminal amine group of (S)-2-(3-((S)-5-amino-1-carboxypentyl)ureido)pentanedioic acid 1, (14S,18S)-1-amino-8,16-dioxo-3,6-dioxa-9,15,17-triazaicosane-14,18,20-tricarboxylic acid 2 and (3S,7S)-26-amino-5,13,20-trioxo-4,6,12,21-tetraazahexacosane-1,3,7,22-tetracarboxylic acid 3. The K(i) values for the dye-inhibitor conjugates ranged from 1 to 700 pM. All compounds proved capable of imaging PSMA+ tumors selectively to varying degrees depending on the choice of fluorophore and linker. The highest tumor uptake was observed with IRDye800CW employing a poly(ethylene glycol) or lysine-suberate linker, as in 800CW-2 and 800CW-3, while the highest tumor to nontarget tissue ratios were obtained for Cy7 with these same linkers, as in Cy7-2 and Cy7-3. Compounds 2 and 3 provide useful scaffolds for targeting of PSMA+ tissues in vivo and should be useful for preparing NIR dye conjugates designed specifically for clinical intraoperative optical imaging devices.