Efficient 18 F labeling of cysteine containing peptides and proteins using the tetrazine- trans -cyclooctene ligation

18 F PET has a number of attributes that make it clinically attractive, including nearly 100% positron efficiency, very high specific radioactivity, and short half-life of ~110 min. However, the short half-life of 18 F and the poor nucleophilicity of fluoride introduce challenges for the incorporation of 18 F into complex molecules. Recently, the tetrazine- trans -cyclooctene ligation has been introduced as a novel 18 F labeling method that proceeds with fast reaction rates without catalysis. Herein, we report an efficient method for 18 F-labeling of free cysteines of peptides and proteins based on sequential ligation with a bifunctional tetrazinyl-maleimide and an 18 F-labeled trans -cyclooctene. The newly developed method was tested for site specific labeling of both c(RGDyC) peptide and VEGF-SH protein. Starting with 4 mCi of 18 F- trans -cyclooctene and only 10 μg of tetrazine-RGD (80–100 μM) or 15 μg of tetrazine-VEGF (6.0 μM), 18 F labeled RGD peptide and VEGF protein could be obtained within five minutes in 95% yield and 75% yield, respectively. The obtained tracers were then evaluated in mice. In conclusion, a highly efficient method has been developed for site-specific 18 F labeling of cysteine containing peptides and proteins. The special characteristics of the tetrazine- trans -cyclooctene ligation provide unprecedented opportunities to synthesize 18 F-labeled probes with high specific activity for PET applications.


Introduction
High-molecular-weight biological macromolecules, such as single-stranded oligonucleotides [1], peptides [2], and proteins [3], are being considered with increasing frequency for use as radiopharmaceuticals, and the applications of radiolabeled macromolecules are rapidly gaining importance in nuclear medicine. With a half-life of 109.8 min, low β + -energy (0.64 MeV), and ease of production, 18 F-fluoride represents the ideal radionuclides for routine PET imaging. Its low positron energy results in short positron linear range in tissue, which could lead to higher resolution in PET imaging. Furthermore, the half-life of 18 F is long enough to allow syntheses, transportation, and imaging procedures to be extended over hours, while the patient is subjected to limited amount of radiation exposure. However, the synthesis of 18 F labeled proteins with high specific activity is challenging. 18 F-containing prosthetic groups are often required. Moreover, due to the slow rates of conventional conjugation chemistry, a large excess of protein is often required in order to obtain high radiochemical yield. Fast, efficient labeling methods in 18 F-radiochemistry are therefore necessary for developing imaging agents in nuclear medicine and life science.
Previously, we and others developed a novel bioconjugation that involves reaction between tetrazines and trans-cyclooctenes (TCO) [4][5]. The fast rates of these reactions enable high reactivity at low micromolar concentrations within minutes and without an excess of either reactant [4,[6][7]. With this foundation, we have developed an extremely efficient method for synthesizing 18 F-labeled probes with high specific activity based on tetrazines and an 18 F-labeled trans-cyclooctene ( 18 F-TCO) [8][9]. We also used the tetrazine ligation for the first time in a microPET imaging study using an α v β 3 integrin targeted RGD probe that was constructed through this efficient cycloaddition reaction [10]. Recently, this kind of tetrazine/TCO reactions was successfully applied for a pretargeted imaging strategy [6,11]. These studies demonstrated how the fast rates and efficient reactivity of the tetrazine ligation distinguished it from most existing 18 F labeling methods.
In our previous study, the tetrazine functionality was introduced through lysine acylation. A concern for unselective protein modification of lysine residues is possible interference with biologic activity, as the modification of one or more lysines located at or near the active site can reduce the binding affinity. As cysteines are much less abundant than lysine, aspartic acid and glutamic acid residues, thiol-reactive agents can be used to modify peptides and proteins with higher selectivity than amine and carboxylate-reactive reagents. To further explore the scope of the efficient tetrazine ligation, herein we report a new method for 18 Flableing of free cysteine-containing bioligands. The newly developed method was tested for site specific labeling of c(RGDyC) and VEGF-SH protein.

General
All chemicals obtained commercially were of analytic grade and used without further purification. No-carrier-added 18 F-fluoride was produced via the 18 O(p,n) 18 F reaction by the bombardment of an isotopically enriched [ 18 O]water target (95% enrichment, Isonics, Golden, CO) with 11-MeV protons using a Siemens RDS-112 negative ion cyclotron.
Reversed-phase extraction C18 Sep-Pak cartridges were obtained from Waters (Milford MA) and were pretreated with ethanol and water immediately before use. The syringe filter and polyethersulfone membranes (pore size, 0.22 μm; diameter, 13 mm) were obtained from Nalge Nunc International (Rochester, NY). c(RGDyC) was obtained from Peptides International (Louisville, KY). The protein VEGF-SH was provided by Dr. Xiaoyuan Chen at NIH. The semipreparative reversed-phase high performance liquid chromatography (RP-HPLC) using a Vydac protein and peptide column (218TP510; 5 μm, 250 × 4.6 mm) was performed on a Dionex 680 chromatography system with a UVD 170U absorbance detector and model 105S single-channel radiation detector (Carroll & Ramsey Associates). The recorded data were processed using Chromeleon version 7.1 software. With a flow rate of 1.0 mL/min, the mobile phase was changed from 95% solvent A [0.1% trifluoroacetic acid (TFA) in water] and 5% B [0.1% TFA in acetonitrile (MeCN)] (0-2 min) to 5% solvent A and 95% solvent B at 22 min. UV absorbance was monitored at 218 nm and the identification of the peptides were confirmed based on the UV spectrum using a photodiode array detector. MicroPET scans were performed on a microPET R4 rodent model scanner (Siemens Medical Solutions USA, Inc., Knoxville, TN). The scanner has a computercontrolled bed and 10.8-cm transaxial and 8-cm axial fields of view (FOVs). It has no septa and operates exclusively in the 3-dimensional (3D) list mode. Animals were placed near the center of the FOV of the scanner.

Synthesis of 18 F-TTM-VEGF
The tetrazinyl-maleimide (TM, 200 μg, 0.41 μmol, in 100 μL DMSO) and VEGF-SH (100 μg, 5.5 nmol, in 500 μL phosphate buffer) were mixed together at room temperature for 5 h. The conjugate was purified by size exclusion PD-10 column and concentrated by Centricon filter (Millipore, Bedford, MA). The final concentration of tetrazinyl-maleimide-VEGF conjugate (TM-VEGF) was determined based on UV absorbance at 280 nm using unconjugated VEGF of known concentrations as standards. The final concentration was then adjusted to 50 μg/mL. For 18 F labeling, 18 F-TCO (148 MBq, 4 mCi, in 50 μL ethanol) was added to the TM-VEGF (15 μg) in water and incubated for 5 min at 40 °C. The conjugate (shortened as 18 F-TTM-VEGF) was purified by PD-10 column using 1×PBS as the eluent. The 3.0-4.0 mL fraction containing VEGF proteins were collected for in vivo animal experiments.

Cell Culture
Human glioblastoma cell line U87MG was obtained from the American Type Culture Collection (Manassas, VA) and were cultured in DMEM containing high glucose (GIBCO, Carlsbad, CA), which was supplemented with 10% fetal bovine serum (FBS) and 1% penicillin-streptomycin. The cells were expanded in tissue culture dishes and kept in a humidified atmosphere of 5% CO 2 at 37 °C. The medium was changed every other day. A confluent monolayer was detached with 0.05% Trypsin-EDTA, 0.01M PBS (pH 7.4) and dissociated into a single-cell suspension for further cell culture.

MicroPET Imaging
Animal procedures were performed according to a protocol approved by the University of Southern California Institutional Animal Care and Use Committee. For static microPET scans, the mice bearing U87MG xenografts were injected with 3.7 MBq (100 μCi) of 18 F-TTM-RGD via the tail vein (n = 3 for each group). At 0.5, 1, and 2 h post injection (p.i.), the mice were anesthetized with isoflurane (5% for induction and 2% for maintenance in 100% O 2 ) using a knock-down box. With the help of a laser beam attached to the scanner, the mice were placed in the prone position and near the center of the field of view of the scanner. The 3-min static scans were then obtained. Images were reconstructed using a 2-dimensional ordered-subsets expectation maximization (OSEM) algorithm. Regions of interest (ROIs; 5 pixels for coronal and transaxial slices) were drawn over the tumor on decay-corrected whole-body coronal images. The maximum counts per pixel per minute were obtained from the ROI and converted to counts per milliliter per minute using a calibration constant. With the assumption of a tissue density of 1 g/mL, the ROIs were converted to counts per gram per min. Image ROI-derived %ID/g values were determined by dividing counts per gram per minute with injected dose. Similarly, 3.7 MBq (100 μCi) of 18 F-TTM-VEGF was injected to healthy nude mice to study its distribution with microPET. Static scans were performed at 0.5 h and 3 h post injection.

Chemistry and Radiochemistry
The synthesis of the bifunctional tetrazinyl-maleimide (TM) is shown in Figure 1, and the syntheses of RGD conjugates are shown in Figure 2A. The conjugation of the bifunctional tetrazinyl-maleimide (TM) with c(RGDyC) in phosphate buffer with pH 6.5-7.0 afforded the tetrazinyl-maleimide-RGD conjugate TM-RGD in 85% yield. The cycloaddition reaction of TM-RGD and nonradioactive 19 F-TCO provided the standard for the 18 F labeled product. The pink color of TM-RGD disappeared immediately upon the addition of 19 F-TCO, indicating that the tetrazine had completely reacted upon mixing. The identity of the 19 F-TTM-RGD was confirmed by MALDI-TOF-MS. 18 F labeled trans-cyclooctene ( 18 F-TCO) was produced using the protocol developed in our laboratories [8]. The radiolabeling yield for 18 F-TTM-RGD was counted nearly quantitatively from 18 F-TCO ( Figure 3A). The radiochemical purity of 18 F-TTM-RGD was more than 95% as determined by radio HPLC. The specific activity of 18 F-TTM-RGD was estimated to be approximately 3-6 Ci/μmol. Following a similar protocol, 18 F-TTM-VEGF was obtained in high yield (75% after purification) from VEGF-SH.
To compare the efficiency of the tetrazine ligation with direct conjugate addition of a sulfhydryl containing peptide to a maleimide, we performed the reaction between 18 F-TCO and the tetrazinyl maleimide TM directly, which produced the cycloadduct 18 F-TTM ( Fig  1B). The conjugate 18 F-TTM-RGD could also be efficiently obtained through the reaction of 18 F-TTM and c(RGDyC); however, a relatively long reaction time (20 min) and a higher concentration of c(RGDyC) (1.8 mM) was needed. By contrast, the approach outlined in Fig  1A requires fewer manipulations of 18 F-labeled intermediates, and is complete within 5 min at room temperature at a concentration that is 18 times lower (0.1 mM).

MicroPET Imaging
Representative coronal microPET images of U87MG tumor-bearing mice (n = 3/group) at different times after intravenous injection of approximately 3.7 MBq (100 μCi) of 18 F-TTM-RGD are shown in Figure 4A. The tumors were clearly visible with good contrast at all time points. The mice showed high abdominal activity accumulation. Prominent uptake of 18 F-TTM-RGD was observed in the kidneys and urinary bladder at early time points, indicating that this radiotracer is excreted through the renal system as well. Quantification of tumor and major organ activity accumulation in the microPET scans was achieved by measuring the regions of interest (ROIs) that encompass the entire organ on the coronal images. The tumor and major organ uptake of 18 F-TTM-RGD is depicted in Figure 5A. The tumor uptake of 18 F-TTM-RGD in U87MG tumor was 1.98 ± 0.33, 1.80 ± 0.15, and 1.27 ± 0.33 %ID/g at 0.5, 1, and 2 h p.i., respectively. As can be seen from Figure 5A, the uptake of the 18 Figure 4B, 4C. The activity was mainly accumulated in kidneys, which correlated well with a previous report [13]. MicroPET quantification was performed by measuring the ROI and the result is shown in Figure 5B. The kidney uptake of 18 F-TTM-VEGF was 23.20 ±2.15 and 16.51 ± 1.52 %ID/g at 0.5 h and 3 h post injection, respectively. Low muscle uptakes were observed (2.98 ± 0.66 and 1.59 ± 0.57 %ID/g at 0.5 and 3 h post injection).

Discussion
The development of 18 F labeled PET probes is rapidly gaining importance in nuclear medicine. 18 F-fluoride is generally incorporated into the bioligands through three types of functional groups: amino groups [14][15][16], carboxylic acid groups [17], and sulfhydryl groups. As cysteines are much less abundant than lysines, aspartic acid, and glutamic acid residues, thiol-reactive agents have been used to site-selectively modify peptides and proteins [18][19]. Previously, several thiol-reactive 18 F-synthons have been reported, all of which bear a maleimide group for conjugate addition of thiols under mild conditions [12,[20][21][22]. These labeling methods use mild reaction conditions and give high radiolabeling yields. However, the preparation of 18 F-fluorinated probes by conventional methods requires a relatively large amount of bioligand (100-1000 μg) in the bioconjugation step. For peptides or proteins with high cost and low availability, it is desirable to minimize the amount of wasted ligand. Furthermore, for large peptides and for protein labeling, the unlabeled ligands cannot necessarily be efficiently separated from the 18 F-labeled ligands. As unlabeled ligands are competitive inhibitors of the 18 F-labeled probes, the unlabeled ligands can dramatically decrease the sensitivity of radiolabeling. Thus, it is a premium consideration to decrease the equivalency of labeling precursor while maintaining fast and efficient reactivity.
In this study, we developed a bifunctional tetrazinyl-maleimide (TM) that is capable of selectively introducing tetrazine functionality to bioligands through the sulfhydryl group, and subsequent bioconjugation with 18 F-TCO. In order to evaluate the new labeling method, two ligands, c(RGDyC) and VEGF-SH, were selected for labeling reaction.
The conjugation between TM and c(RGDyC) or VEGF-SH is efficient at room temperature. After purification, the radiolabeling of TM-RGD and TM-VEGF was performed with low loadings of bio-ligands. Starting from 18 F-TCO, the synthesis of 18 F-TTM-RGD was achieved in nearly quantitative yield. The total radiosynthesis time was about 90 min from the end of bombardment. For 18 F-TTM-VEGF, the 75% labeling yield was calculated by collecting and measuring the radioactivity from intermediate fractions (3-4 mL) that eluted from a PD-10 size-exclusion column (Later fractions were excluded from the yield calculation).
Previously, several thiol reactive 18 F-tagging molecules have been described [12,[20][21][22], all of which bear a maleimide group allowing for thiol-specific Michael addition reaction. The total synthesis time of those 18 F-tags range from 100-150 min with 10%-20% non-decay corrected radiochemical yields. As 18 F-TCO can be prepared with a total synthesis time of 50 min and in 71% radiochemical yield, the present method represents an efficient alternative for the introduction of 18 F-labels to the sulfhydryl functions of biological molecules.
In addition to RGD peptides, we also tested our method for protein labeling. The interaction between VEGF and its receptor (VEGFR) has been extensively studied for angiogenesisrelated pathways, which are very important for the tumor growth [26][27][28][29]. Previously, VEGF protein based tracers have been developed for PET, single photon emission computed tomography (SPECT) and optical imaging of tumor angiogenesis, all of which showed high renal uptake due to the VEGF-R1 receptor expression in kidneys [30][31][32][33][34]. In the present study, we evaluated the 18 F-TTM-VEGF in normal mice and the biodistribution pattern correlates well with previous reports. The kidney uptake was 23.20 ± 2.15 and 16.51 ± 1.52 %ID/g at 0.5 and 3 h p.i., respectively ( Figure 5B). The high kidney uptake is consistent with the literature [13,27,[35][36]. These results indicate that the radiolabeling was successful and the labeling moiety had a minimum effect on VEGF binding to its receptor. In conclusion, a highly efficient method has been developed for site-specific 18 F labeling of cysteine containing peptides and proteins. The special characteristics of the tetrazine-transcyclooctene ligation provide unprecedented opportunities to synthesize 18 F-labeled probes with high specific activity for PET applications.

Conclusions
Despite the great potential of using 18 F-labeled proteins for both diagnoses and therapeutic monitoring, their application has been limited because it is very challenging to synthesize 18 F-labeled proteins in adequate yield with high specific activity. In this study, we developed an efficient method for the construction of 18 F labeled proteins with high specific activity based on the tetrazine-trans-cyclooctene ligation. We demonstrated that tetrazinyl-maleimide can conjugate with peptides and proteins bearing a free cysteine residue and that the resulting bioconjugates can be efficiently 18 F-labeled through 18 F-TCO ligation. The major advantage of this strategy lies in the fact that this cycloaddition reaction is highly efficient and enables very low bioligand loading. This tetrazinyl maleimide reagent should be useful for the labeling of a variety of peptides, proteins, or oligonucleotides containing free sulfhydryl groups. Future studies will be directed toward the creation of even more reactive bioconjugation partners, and improving the selectivity for tumor uptake relative to kidney uptake. This advance in 18 F labeling methodology should enable the discovery of critical imaging probes for cancer diagnosis and treatment monitoring in both basic and clinical research. Synthesis of N1-(2-(2,5-dioxo-2,5-dihydro-1H-pyrrol-1-yl)ethyl)-N5-(6-(6-(pyridin-2yl)-1,2,4,5-tetrazin-3-yl)pyridin-3-yl)glutaramide    MicroPET quantification of tumor and major organs after injection of (A) 18 F-TTM-RGD and (B) 18 F-TTM-VEGF, respectively.