Synthesis and radioiodination of N-Boc-p-(tri-n-butylstannyl)-L-phenylalanine tetrafluorophenyl ester: preparation of a radiolabeled phenylalanine derivative for peptide synthesis

Radiobromine and radioiodine for medical applications

The halogens bromine and iodine have similar chemical properties and undergo similar reactions due to their closeness in Group 17 of the periodic chart. There are a number of bromine and iodine radionuclides that have properties useful for diagnosis and therapy of human diseases. The emission properties of radiobromine and radioiodine nuclides with half-lives longer than 1 h are summarized along with properties that make radionuclides useful in PET/SPECT imaging and β/Auger therapy, such that the reader can assess which of the radionuclides might be useful for medical applications. An overview of chemical approaches that have been used to radiolabel molecules with radiobromine and radioiodine nuclides is provided with examples. Further, references to a large variety of different organ/cancer-targeting agents utilizing the radiolabeling approaches described are provided.

A convenient and reproducible method for the synthesis of astatinated 4-[211At]astato-l-phenylalanine via electrophilic desilylation

The 211At-labeled compound, 4-[211At]astato-l-phenylalanine, is one of the most promising amino acid derivatives for use in targeted alpha therapy (TAT) for various cancers. Electrophilic demetallation of a stannyl precursor is the most widely used approach for labeling biomolecules with 211At. However, the low acid-resistance of the stannyl precursor necessitates the use of an N- and C-terminus-protected precursor, which results in a low overall radiochemical yield (RCY) due to the multiple synthetic steps involved. In this study, a deprotected organosilyl compound, 4-triethylsilyl-l-phenylalanine, was employed for the direct synthesis of astatinated phenylalanines. 211At was separately recovered from the irradiated 209Bi target using chloroform (CHCl3) and N-chlorosuccinimide-methanol (NCS-MeOH) solution. The RCYs of 4-[211At]astato-l-phenylalanine obtained from the triethylsilyl precursor with the use of 211At, isolated in CHCl3 and NCS-MeOH solution, were 75% and 64% respectively. In both cases, the retention time of the 4-[211At]astato-l-phenylalanine was found to be about 20 min, which showed reasonable correlation with the retention time of non-radioactive 4-halo-l-phenylalanines (4-chloro-, 4-bromo-, and 4-iodo-l-phenylalanine). The one-step reaction examined in this study involved mild reaction conditions (70 °C) and a short time (10 min) compared to the other currently reported procedures for astatination. Electrophilic desilylation was found to be very effective for the labeling of aromatic amino acids with 211At.

Labeling Single Domain Antibody Fragments with Fluorine-18 Using 2,3,5,6-Tetrafluorophenyl 6-[18F]Fluoronicotinate Resulting in High Tumor-to-Kidney Ratios

ImmunoPET agents are being investigated to assess the status of epidermal growth factor receptor 2 (HER2) in breast cancer patients with the goal of selecting those likely to benefit from HER2-targeted therapies and monitoring their progress after these treatments. We have been exploring the use of single domain antibody fragments (sdAbs) labeled with ¹⁸F using residualizing prosthetic agents for this purpose. In this study, we have labeled two sdAbs that bind to different domains on the HER2 receptor, 2Rs15d and 5F7, using 2,3,5,6-tetrafluorophenyl 6-[¹⁸F]fluoronicotinate ([¹⁸F]TFPFN) and evaluated their HER2 targeting properties in vitro and in vivo. The overall decay-corrected radiochemical yield for the synthesis of [¹⁸F]TFPFN-2Rs15d and [¹⁸F]TFPFN-5F7 was 5.7 ± 3.6 and 4.0 ± 2.0%, respectively. The radiochemical purity of labeled sdAbs was >95%, immunoreactive fractions were about 60%, and affinity was in the low nanomolar range. Intracellularly trapped activity from [¹⁸F]TFPFN-2Rs15d and [¹⁸F]TFPFN-5F7 in HER2-expressing SKOV-3 ovarian and BT474M1 breast carcinoma cells were similar to the sdAbs labeled using the previously validated radioiodination residualizing prosthetic agents N-succinimidyl 4-guanidinomethyl-3-[¹²⁵I]iodobenzoate ([¹²⁵I]SGMIB) and N-succinimidyl 3-guanidinomethyl-5-[¹²⁵I]iodobenzoate ( iso-[¹²⁵I]SGMIB). Intracellular activity was about 2-fold higher for radiolabeled 5F7 compared with 2Rs15d for both ¹⁸F and ¹²⁵I. While tumor uptake of both [¹⁸F]TFPFN-2Rs15d and [¹⁸F]TFPFN-5F7 was comparable to those for the coadministered ¹²⁵I-labeled sdAb, renal uptake of the ¹⁸F-labeled sdAbs was substantially lower. In microPET images, the tumor was clearly delineated in SKOV-3 and BT474 xenograft-bearing athymic mice with low levels of background activity in normal tissues, except the bladder. These results indicate that the [¹⁸F]TFPFN prosthetic group could be a valuable reagent for developing sdAb-based immunoPET imaging agents.

Synthesis and reactivity of β-stannylated phenylalanines

The free radical hydrostannation of a series of N-benzoyl and N-acetyl dehydrophenylalanine esters 2a – h yields β-stannylated phenylalanine derivatives 3 and 4. This addition of tin hydride to such unsaturated compounds simultaneously creates two new chiral centers leading to mixtures of two diastereomeric pairs of enantiomers. The reaction of 3-stannylated phenylalanine 3 with methanolic HCl yields chlorostannyl-substituted compounds 5 and 6 and, with one equivalent of bromine, the bromostannylated compounds 7 and 8 are formed. The bromostannylated phenylalanine derivative 7 reacts with one further equivalent of bromine to produce the dibromostannylated compound 9. Even the chlorostannylated phenylalanine derivative 5 reacts with one further equivalent of HCl to give the dichlorostannylated compound 10. The products were characterized by elemental analysis, infrared (IR), and multinuclear (1H, 13C, 119Sn) nuclear magnetic resonance (NMR) spectroscopy. Attempts were made to assign the preferred conformation of the stannylated phenylalanine derivatives using Karplus-type relationship of coupling constants 3 J(H,H), 3 J(Sn,H), and 3 J(Sn,C=O). The results of these analyses have been confirmed by three crystal structure determinations.

Synthesis of fluorine-18 radio-labeled serum albumins for PET blood pool imaging

We sought to develop a practical, reproducible and clinically translatable method of radiolabeling serum albumins with fluorine-18 for use as a PET blood pool imaging agent in animals and man. Fluorine-18 radiolabeled fluoronicotinic acid-2,3,5,6-tetrafluorophenyl ester, [(18)F]F-Py-TFP was prepared first by the reaction of its quaternary ammonium triflate precursor with [(18)F]tetrabutylammonium fluoride ([(18)F]TBAF) according to a previously published method for peptides, with minor modifications. The incubation of [(18)F]F-Py-TFP with rat serum albumin (RSA) in phosphate buffer (pH9) for 15 min at 37-40 °C produced fluorine-18-radiolabeled RSA and the product was purified using a mini-PD MiniTrap G-25 column. The overall radiochemical yield of the reaction was 18-35% (n=30, uncorrected) in a 90-min synthesis. This procedure, repeated with human serum albumin (HSA), yielded similar results. Fluorine-18-radiolabeled RSA demonstrated prolonged blood retention (biological half-life of 4.8 hours) in healthy awake rats. The distribution of major organ radioactivity remained relatively unchanged during the 4 hour observation periods either by direct tissue counting or by dynamic PET whole-body imaging except for a gradual accumulation of labeled metabolic products in the bladder. This manual method for synthesizing radiolabeled serum albumins uses fluorine-18, a widely available PET radionuclide, and natural protein available in both pure and recombinant forms which could be scaled up for widespread clinical applications. These preclinical biodistribution and PET imaging results indicate that [(18)F]RSA is an effective blood pool imaging agent in rats and might, as [(18)F]HSA, prove similarly useful as a clinical imaging agent.

Development and Preliminary Evaluation of TFIB, a New Bimodal Prosthetic Group for Bioactive Molecule Labeling

The new readily available prosthetic group, tetrafluorophenyl 4-fluoro-3-iodobenzoate (TFIB), designed for both molecular imaging and targeted radionuclide therapy purposes was radiolabeled either with fluorine or iodine radionuclides with excellent radiochemical yields and purities. These radiolabeled tags were conjugated to N,N-diethylethylenediamine to give melanin-targeting radiotracers [ (125) I]9 and [ (18) F]9, which were successfully evaluated by PET and gamma scintigraphic imaging in B16F0 pigmented melanoma-bearing C57BL/6J mice. Then, radiolabeled [ (125) I]/[ (18) F]TFIB was used to tag tumor-targeting peptides (i.e., PEG3[c(RGDyK)]2 and NDP-MSH targeting αvβ3 integrin and MC1R receptors, respectively) in mild conditions and with good radiochemical yields (47-83% d.c.) and purities (>99%). The resulting radiolabeled peptides were assessed both in vitro and by PET imaging in animal models.

Fluorescent biphenyl derivatives of phenylalanine suitable for protein modification

In a recent study, we demonstrated that structurally compact fluorophores incorporated into the side chains of amino acids could be introduced into dihydrofolate reductase from Escherichia coli (ecDHFR) with minimal disruption of protein structure or function, even when the site of incorporation was within a folded region of the protein. The modified proteins could be employed for FRET measurements, providing sensitive monitors of changes in protein conformation. The very favorable results achieved in that study encouraged us to prepare additional fluorescent amino acids of potential utility for studying protein dynamics. Presently, we describe the synthesis and photophysical characterization of four positional isomers of biphenyl-phenylalanine, all of which were found to exhibit potentially useful fluorescent properties. All four phenylalanine derivatives were used to activate suppressor tRNA transcripts and incorporated into multiple positions of ecDHFR. All phenylalanine derivatives were incorporated with good efficiency into position 16 of ecDHFR and afforded modified proteins that consumed NADPH at rates up to about twice the rate measured for wild type. This phenomenon has been noted on a number of occasions previously and shown to be due to an increase in the off-rate of tetrahydrofolate from the enzyme, altering a step that is normally rate limiting. When introduced into sterically accessible position 49, the four phenylalanine derivatives afforded DHFRs having catalytic function comparable to wild type. The four phenylalanine derivatives were also introduced into position 115 of ecDHFR, which is known to be a folded region of the protein less tolerant of structural alteration. As anticipated, significant differences were noted in the catalytic efficiencies of the derived proteins. The ability of two of the sizable biphenyl-phenylalanine derivatives to be accommodated at position 115 with minimal perturbation of DHFR function is attributed to rotational flexibility about the biphenyl bonds.

One step radiosynthesis of 6-[(18)F]fluoronicotinic acid 2,3,5,6-tetrafluorophenyl ester ([(18)F]F-Py-TFP): a new prosthetic group for efficient labeling of biomolecules with fluorine-18

The labeling of biomolecules for positron emission tomography (PET) with no-carrier-added fluorine-18 is almost exclusively accomplished using prosthetic groups in a two step procedure. The inherent complexity of the process renders full automation a challenge and leads to protracted synthesis times. Here we describe a new (18)F-labeled prosthetic group based on nicotinic acid tetrafluorophenyl ester. Reaction of [(18)F]fluoride at 40 degrees C with the trimethylammonium precursor afforded 6-[(18)F]fluoronicotinic acid tetrafluorophenyl ester ([(18)F]F-Py-TFP) directly in 60-70% yield. [(18)F]F-Py-TFP was conveniently purified by Sep-Pak cartridge prior to incubation with a peptide containing the RGD sequence. The desired conjugate was formed rapidly and in good yields. An in vitro receptor-binding assay for the integrin alpha(v)beta(3) was established to explore competition with peptide and peptidomimetic prepared from F-Py-TFP with (125)I-echistatin. The nonradioactive conjugates were found to possess high binding affinities with calculated K(i) values in the low nanomolar range.

2-Substitution of adenine nucleotide analogues containing a bicyclo[3.1.0]hexane ring system locked in a northern conformation: enhanced potency as P2Y1 receptor antagonists

Preference for the northern (N) ring conformation of the ribose moiety of adenine nucleotide 3',5'-bisphosphate antagonists of P2Y(1) receptors was established by using a ring-constrained methanocarba (a bicyclo[3.1.0]hexane) ring as a ribose substitute (Nandanan et al. J. Med. Chem. 2000, 43, 829-842). We have now combined the ring-constrained (N)-methanocarba modification with other functionalities at the 2-position of the adenine moiety. A new synthetic route to this series of bisphosphate derivatives was introduced, consisting of phosphorylation of the pseudoribose moiety prior to coupling with the adenine base. The activity of the newly synthesized analogues was determined by measuring antagonism of 2-methylthio-ADP-stimulated phospholipase C (PLC) activity in 1321N1 human astrocytoma cells expressing the recombinant human P2Y(1) receptor and by using the radiolabeled antagonist [(3)H]2-chloro-N(6)-methyl-(N)-methanocarba-2'-deoxyadenosine 3',5'-bisphosphate 5 in a newly developed binding assay in Sf9 cell membranes. Within the series of 2-halo analogues, the most potent molecule at the hP2Y(1) receptor was an (N)-methanocarba N(6)-methyl-2-iodo analogue 12, which displayed a K(i) value in competition for binding of [(3)H]5 of 0.79 nM and a K(B) value of 1.74 nM for inhibition of PLC. Thus, 12 is the most potent antagonist selective for the P2Y(1) receptor yet reported. The 2-iodo group was substituted with trimethyltin, thus providing a parallel synthetic route for the introduction of an iodo group in this high-affinity antagonist. The (N)-methanocarba-2-methylthio, 2-methylseleno, 2-hexyl, 2-(1-hexenyl), and 2-(1-hexynyl) analogues bound less well, exhibiting micromolar affinity at P2Y(1) receptors. An enzymatic method of synthesis of the 3',5'-bisphosphate from the corresponding 3'-monophosphate, suitable for the preparation of a radiophosphorylated analogue, was explored.