Radiolabeled guanine derivatives for the in vivo mapping of O(6)-alkylguanine-DNA alkyltransferase: 6-(4-[(18)F]Fluoro-benzyloxy)-9H-purin-2-ylamine and 6-(3-[(131)I]Iodo-benzyloxy)-9H-purin-2-ylamine

Covalent 18F-Radiotracers for SNAPTag: A New Toolbox for Reporter Gene Imaging

There is a need for versatile in vivo nuclear imaging reporter systems to foster preclinical and clinical research. We explore the applicability of the SNAPTag and novel radiolabeled small-molecule ligands as a versatile reporter gene system for in vivo nuclear imaging. SNAPTag is a high-affinity protein tag used in a variety of biochemical research areas and based on the suicide DNA repair enzyme O⁶-methylguanine methyl transferase (MGMT). Its ligands are well suited for reporter gene imaging as the benzyl guanine core scaffold can be derivatized with fluorescent or radiolabeled moieties for various applications. Three guanine-based SNAPTag ligands ([¹⁸F]FBBG, [¹⁸F]pFBG and [¹⁸F]mFBG) were synthesized in high yields and were (radio)chemically characterized. HEK293 cells were engineered to express the SNAPTag on the cell surface and served as cell model to assess target affinity by radiotracer uptake assays, Western blotting and SDS-PAGE autoradiography. A subcutaneous HEK293-SNAPTag xenograft model in immunodeficient mice was used for in vivo evaluation of [¹⁸F]FBBG and [¹⁸F]pFBG while the biodistribution of [¹⁸F]mFBG was characterized in naïve animals. The results were validated by ex vivo biodistribution studies and immunofluorescence staining of the xenografts. All three radiotracers were produced in high radiochemical purity, molar activity and good yields. Western blot analysis revealed successful SNAPTag expression by the transfected HEK293 cells. In vitro testing revealed high target affinity of all three tracers with an up to 191-fold higher signal in the HEK293-SNAPTag cells compared to untransfected cells. This was further supported by a prominent radioactive protein band at the expected size in the SDS-PAGE autoradiograph of cells incubated with [¹⁸F]FBBG or [¹⁸F]pFBG. The in vivo studies demonstrated high uptake in HEK293-SNAP xenografts compared to HEK293 xenografts with excellent tumor-to-muscle ratios (7.5 ± 4.2 for [¹⁸F]FBBG and 10.6 ± 6.2 for [¹⁸F]pFBG). In contrast to [¹⁸F]pFBG and its chemical analogue [¹⁸F]mFBG, [¹⁸F]FBBG showed no signs of unspecific bone uptake and defluorination in vivo. Radiolabeled SNAPTag ligands bear great potential for clinical applications such as in vivo tracking of cell populations, antibody fragments and targeted radiotherapy. With excellent target affinity, good stability, and low non-specific binding, [¹⁸F]FBBG is a highly promising candidate for further preclinical evaluation.

A Design of Experiments (DoE) Approach Accelerates the Optimization of Copper-Mediated 18F-Fluorination Reactions of Arylstannanes

Recent advancements in ¹⁸F radiochemistry, such as the advent of copper-mediated radiofluorination (CMRF) chemistry, have provided unprecedented access to novel chemically diverse PET probes; however, these multicomponent reactions have come with a new set of complex optimization problems. Design of experiments (DoE) is a statistical approach to process optimization that is used across a variety of industries. It possesses a number of advantages over the traditionally employed "one variable at a time" (OVAT) approach, such as increased experimental efficiency as well as an ability to resolve factor interactions and provide detailed maps of a process's behavior. Here we demonstrate the utility of DoE to the development and optimization of new radiochemical methodologies and novel PET tracer synthesis. Using DoE to construct experimentally efficient factor screening and optimization studies, we were able to identify critical factors and model their behavior with more than two-fold greater experimental efficiency than the traditional OVAT approach. Additionally, the use of DoE allowed us to glean new insights into the behavior of the CMRF of a number of arylstannane precursors. This information has guided our decision-making efforts while developing efficient reaction conditions that suit the unique process requirements of ¹⁸F PET tracer synthesis.

Radioiodinated O(6)-Benzylguanine derivatives containing an azido function

INTRODUCTION

Drug resistance to alkylator chemotherapy has been primarily attributed to the DNA repair protein alkylguanine-DNA alkyltransferase (AGT); thus, personalizing chemotherapy could be facilitated if tumor AGT content could be quantified prior to administering chemotherapy. We have been investigating the use of radiolabeled O(6)-benzylguanine (BG) analogues to label and quantify AGT in vivo. BG derivatives containing an azido function were sought to potentially enhance the targeting of these analogues to AGT, which is primarily present in the cell nucleus, either by conjugating them to nuclear localization sequence (NLS) peptides or by pretargeting via bio-orthogonal approaches.

METHODS

Two O(6)-(3-iodobenzyl)guanine (IBG) derivatives containing an azido moiety-O(6)-(4-azidohexyloxymethyl-3-iodobenzyl)guanine (AHOMIBG) and O(6)-(4-azido-3-iodobenzyl)guanine (AIBG)--and their tin precursors were synthesized in multiple steps and the tin precursors were converted to radioiodinated AHOMIBG and AIBG, respectively. Both unlabeled and radioiodinated AHOMIBG analogues were conjugated to alkyne-derivatized NLS peptide heptynoyl-PK(3)RKV. The ability of these radioiodinated compounds to bind to AGT was determined by a trichloroacetic acid precipitation assay and gel electrophoresis/phosphor imaging. Labeling of an AGT-AIBG conjugate via Staudinger ligation using the (131)I-labeled phosphine ligand, 2-(diphenylphosphino)phenyl 4-[(131)I]iodobenzoate, also was investigated.

RESULTS

[(131)I]AHOMIBG was synthesized in two steps from its tin precursor in 52.2 ± 7.5% (n = 5) radiochemical yield and conjugated to the NLS peptide via click reaction in 50.7 ± 4.9% (n = 6) yield. The protected tin precursor of AIBG was radioiodinated in an average radiochemical yield of 69.6 ± 4.5% (n = 7); deprotection of the intermediate gave [(131)I]AIBG in 17.8 ± 4.2% (n = 9) yield. While both [(131)I]AHOMIBG and its NLS conjugate bound to AGT pure protein, their potency as a substrate for AGT was substantially lower than that of [(125)I]IBG. Uptake of [(131)I]AHOMIBG-NLS conjugate in DAOY medulloblastoma cells was up to eightfold higher than that of [(125)I]IBG; however, the uptake was not changed when the cellular AGT content was first depleted with BG treatment. [(131)I]AIBG was almost equipotent as [(125)I]IBG with respect to binding to pure AGT; however, attempts to radiolabel AGT by treatment with unlabeled AIBG followed by Staudinger ligation using the radiolabeled phosphine ligand, 2-(diphenylphosphino)phenyl 4-[(131)I]iodobenzoate were not successful.

CONCLUSION

Although AHOMIBG, and AIBG were synthesized successfully in both unlabeled and radioiodinated forms, the radioiodinated compounds failed to label AGT either after NLS peptide conjugation or via Staundiger ligation. Currently, other bio-orthogonal approaches are being evaluated for labeling AGT by pretargeting.

Efficient radiosynthesis of 2-[(18)f]fluoroadenosine: a new route to 2-[(18)f]fluoropurine nucleosides

An efficient method to incorporate the fluorine-18 radionuclide in 2-nitropurine-based nucleosides was developed. The nucleophilic radiofluorination of the labeling precursor with [(18)F]KF under aminopolyether-mediated conditions (Kryptofix 2.2.2/K2CO3) followed by deprotection was straightforward and, after formulation, gave 2-[(18)F]fluoroadenosine, ready for injection with a radiochemical yield of 45 ± 5%, a radiochemical purity of >98%, and a specific radioactivity up to 148 GBq/μmol. A micropositron emission tomography imaging and biodistribution study on rodents was reported.

New strategy for the preparation of clickable peptides and labeling with 1-(azidomethyl)-4-[(18)F]-fluorobenzene for PET

The alkyne-azide Cu(I)-catalyzed Huisgen cycloaddition, a click type reaction was used to label a peptide with fluorine-18. A novel solid phase synthesis approach for the preparation of clickable peptides has been developed and has also permitted the straightforward preparation of reference compounds. A complementary azide labeling agent (1-(azidomethyl)-4-[(18)F]-fluorobenzene) has been produced in a four step procedure in 75 min with a 34% radiochemical yield (decay corrected). Conjugation of [(18)F]fluoroazide with a model alkyne-neuropeptide produced the desired (18)F-radiolabeled peptide in less than 15 min with a yield of 90% and excellent radiochemical purity.

Molecular imaging of alkylguanine-DNA alkyltransferase: further evaluation of radioiodinated derivatives of O6-benzylguanine

PURPOSE

An inverse correlation has been established between tumor levels of the DNA repair protein alkylguanine-DNA alkyltransferase (AGT) and a positive outcome after alkylator chemotherapy. Quantitative imaging of AGT could provide important information for patient-specific cancer treatment. Several radiolabeled analogues of O6-benzylguanine (BG), a potent AGT inactivator, have been developed and shown to be capable of labeling pure AGT protein. Herein, two of these analogues--O6-3-[*I]iodobenzylguanine ([*I]IBG) and O6-3-[*I]iodobenzyl-2'-deoxyguanosine ([*I]IBdG)--were further evaluated in two murine xenograft models. (AcO)2-[131I]IBdG, a peracetylated derivative of IBdG, also was investigated as an alternative agent.

METHODS

Several biodistribution studies of radioiodinated IBG and IBdG were performed in TE-671 human rhabdomyosarcoma and DAOY human medulloblastoma murine xenograft models. Mice were treated with BG or its nucleoside analogue dBG to deplete the tumor AGT content. The effect of unlabeled IBG and that of 7,8-benzoflavone (BF), an inhibitor of the cytochrome P-450 isozyme CYP1A2, on the tumor uptake of the tracers was determined. The uptake of (AcO)2-[131I]IBdG along with that of [125I]IBdG in DAOY cells in vitro was determined in the presence and absence of a nucleoside transporter inhibitor, dipyridamole.

RESULTS

Pretreatment of mice either with BG or dBG failed to reduce tumor levels of [*I]IBG or [*I]IBdG even though such treatments completely depleted tumor AGT content. Treatment of mice with BF increased tumor uptake of [125I]IBG by 56%; however, differentiation of tumors with and without AGT still was not possible. (AcO)2-[131I]IBdG, a peracetylated derivative of IBdG, had a higher uptake in vitro in DAOY tumor cells. However, its uptake, like that of [125I]IBdG, was blocked by dipyridamole.

CONCLUSIONS

Taken together, these results suggest that labeled agents that are more specific for cellular AGT and that are more metabolically stable are needed.

Synthesis and preliminary biological evaluation of O6-[4-(2-[18F]fluoroethoxymethyl)benzyl]guanine as a novel potential PET probe for the DNA repair protein O6-alkylguanine-DNA alkyltransferase in cancer chemotherapy

A novel fluorine-18-labeled O6-benzylguanine (O6-BG) derivative, O6-[4-(2-[18F]fluoroethoxymethyl)benzyl]guanine (O6-[18F]FEMBG, [18F]1), has been synthesized for evaluation as a potential positron emission tomography (PET) probe for the DNA repair protein O6-alkylguanine-DNA alkyltransferase (AGT) in cancer chemotherapy. The appropriate radiolabeling precursor N(2,9)-bis(p-anisyldiphenylmethyl)-O6-[4-(hydroxymethyl)benzyl]guanine (6) and reference standard O6-[4-(2-fluoroethoxymethyl)benzyl]guanine (O6-FEMBG, 1) were synthesized from 1,4-benzenedimethanol and 2-amino-6-chloropurine in four or six steps, respectively, with moderate to excellent chemical yields. The target tracer O6-[18F]FEMBG was prepared in 20-35% radiochemical yields by reaction of MTr-protected precursor 6 with [18F]fluoroethyl bromide followed by quick deprotection reaction and purification with a simplified Silica Sep-Pak method. Total synthesis time was 60-70 min from the end of bombardment. Radiochemical purity of the formulated product was >95%, with a specific radioactivity of >1.0 Ci/micromol at the end of synthesis. The activity of unlabeled O6-FEMBG was evaluated via an in vitro AGT oligonucleotide assay. Preliminary findings from biological assay indicate that the synthesized analogue has similarly strong inhibiting effect on AGT in comparison with O6-BG and O6-4-fluorobenzylguanine (O6-FBG). The results warrant further in vivo evaluation of O6-[18F]FEMBG as a new potential PET probe for AGT.

O6-3-[125I]iodobenzyl-2'-deoxyguanosine ([125I]IBdG): synthesis and evaluation of its usefulness as an agent for quantification of alkylguanine-DNA alkyltransferase (AGT)

The development of O(6)-(3-[(125)I]iodobenzyl)-2'-deoxyguanosine ([(125)I]IBdG), the glycosylated analogue of the O(6)-3-iodobenzylguanine (IBG), as an agent for the in vivo mapping of the DNA repair protein alkylguanine-DNA alkyltransferase (AGT) is described. Synthesis of its tin precursor, O(6)-3-trimethylstannylbenzyl-2'-deoxyguanosine (TBdG) was achieved in four steps from deoxyguanosine. Radioiodination of TBdG in a single step gave [(125)I]IBdG in 70-85% isolated radiochemical yield. [(125)I]IBdG bound specifically to pure AGT with an IC(50) of 7.1 microM. From paired-label assays, [(125)I]IBdG showed a 2- to 3-fold higher cellular uptake than [(131)I]IBG in DAOY medulloblastoma, TE-671 rhabdomyosarcoma, SK-Mel-28 melanoma, and HT-29 colon carcinoma human cell lines. Uptake of both labeled compounds in these cell lines decreased with increasing concentrations of unlabeled O(6)-benzylguanine (BG) when BG was present in the medium during incubation with the labeled compounds. Compared to BG, unlabeled IBdG diminished the uptake of [(125)I]IBdG and [(131)I]IBG in DAOY cells more efficiently (IC(50)<1 microM vs >10 microM for BG). There was no significant change in cell-bound activity of [(125)I]IBdG and [(131)I]IBG when BG was removed from the incubation medium before incubating cells with the tracers, suggesting that only a very small portion of radioactivity taken up by the cells is AGT bound. This was corroborated by gel-electrophoresis performed on extracts from cells treated with varying amounts of BG and then incubated with [(125)I]IBdG in the presence of BG. No radiolabeled AGT band was discernable by phosphor-imaging, signifying low cellular AGT binding of the radiotracer. In contrast, when cell extracts were prepared from BG pre-treated cells and aliquots were incubated with [(125)I]IBdG subsequently, the intensity of radiolabeled AGT band decreased linearly as a function of BG concentration. This suggests that the low level of [(125)I]IBdG that binds to AGT does so in a concentration dependent manner. These data suggest that IBdG is transported across the cell membrane to a higher degree than IBG. However, to be a practical tracer for quantifying cellular AGT, considerable localization of such derivatives need to occur within the cell nucleus where AGT is present predominantly.

O6-3-[131I]iodobenzylguanine: improved synthesis and further evaluation of a potential agent for imaging of alkylguanine-DNA alkyltransferase

O(6)-Benzylguanine derivatives with suitable radionuclides attached to the benzyl ring are potentially useful in the noninvasive imaging of the DNA repair protein, alkylguanine-DNA alkyltransferase (AGT). Previously, O(6)-3-[(131)I]iodobenzylguanine ([(131)I]IBG) was prepared using a two-step approach; we now report its synthesis in a single step by the radioiododestannylation of O(6)-3-(trimethylstannyl)benzylguanine in 85-95% radiochemical yield. The in vitro specific uptake of [(131)I]IBG in DAOY human medulloblastoma cells, in TE-671 human rhabdomyosarcoma cells and a CHO cell line transfected to express AGT was linear (r(2) = 0.9-1.0) as a function of cell density. After intravenous injection of [(131)I]IBG in athymic mice bearing TE-671 xenografts, tumor uptake was 1.38 +/- 0.34% ID/g at 0.5 h and declined at 2 and 4 h. Preadministration of O(6)-(3-iodobenzyl)guanine (IBG) at 0.5 h increased uptake not only in tumor but also in several normal tissues. Notable exceptions were thyroid (p < 0.05), lung (p <0.05) and stomach. After intratumoral injection of [(131)I]IBG in the same xenograft model, the uptake in tumors that were depleted of AGT by BG treatment (165.8 +/- 27.5% ID/g) was about 60% of that in control mice (272.4 +/- 48.2% ID/g; p < 0.05).

Inhibition of O6-methylguanine-DNA methyltransferase by glucose-conjugated inhibitors: comparison with nonconjugated inhibitors and effect on fotemustine and temozolomide-induced cell death

The DNA repair protein O(6)-methylguanine-DNA methyltransferase (MGMT) is an important suicide enzyme involved in the defense against O(6)-alkylating mutagens. It also plays a role in the resistance of tumors to anticancer drugs targeting the O(6)-position of guanine, such as temozolomide and fotemustine. Several potent MGMT inhibitors have been developed sensitizing cells to O(6)-alkylating agents. Aimed at targeting MGMT inhibitors to tumor cells, we synthesized MGMT inhibitory compounds conjugated with glucose to improve uptake in tumor cells. Here, we compared O(6)-benzylguanine, O(6)-2-fluoropyridinylmethylguanine (O(6)FPG), O(6)-3-iodobenzylguanine, O(6)-4-bromothenylguanine, and O(6)-5-iodothenylguanine with the corresponding C8-linker beta-d-glucose derivatives. All glucose conjugated inhibitors were 3- to 5-fold less effective than the corresponding nonconjugated drugs as to MGMT inhibition that was measured in cell extracts (in vitro) and cultivated HeLaS3 cells (in vivo). Except for O(6)FPG, IC(50) values of the guanine derivatives applied in vitro and in vivo were correlated. A similar correlation was not obvious for the corresponding glucosides, indicating differences in cellular uptake. C8-alpha-d-glucosides were less effective than beta-glucosides. From the newly developed glucose-conjugated inhibitors tested, O(6)-4-bromothenylguanine-C8-beta-d-glucoside (O(6)BTG-C8-betaGlu) was most potent in inhibiting MGMT both in vitro and in vivo. At a concentration of 0.1 microM, it inhibited cellular MGMT to completion. It was not toxic, even when applied chronically to cells at high dose (up to 20 microM). O(6)BTG-C8-betaGlu strongly potentiated the killing effect of fotemustine and temozolomide, causing reversal from MGMT+ to MGMT- phenotype. Therefore, O(6)BTG-C8-betaGlu seems to be especially suitable for approaching MGMT inhibitor targeting in tumor therapy.

Synthesis and preliminary biological evaluation of radiolabeled O6-benzylguanine derivatives, new potential PET imaging agents for the DNA repair protein O6-alkylguanine-DNA alkyltransferase in breast cancer

Novel radiolabeled O(6)-benzylguanine (O(6)-BG) derivatives, 2-amino-6-O-[(11)C]-[(methoxymethyl)benzyloxy]-9-methyl purines ([(11)C]p-O(6)-AMMP, 1a; [(11)C]m-O(6)-AMMP, 1b; [(11)C]o-O(6)-AMMP, 1c), 2-amino-6-O-benzyloxy-9-[(11)C]-[(methoxycarbonyl)methyl]purine ([(11)C]ABMMP, 2), and 2-amino-6-O-benzyloxy-9-[(11)C]-[(4'-methoxycarbonyl)benzyl]purine ([(11)C]ABMBP, 3), have been synthesized for evaluation as new potential positron emission tomography (PET) imaging agents for the DNA repair protein O(6)-alkylguanine-DNA alkyltransferase (AGT) in breast cancer. The appropriate precursors for radiolabeling were obtained in two to three steps from starting material 2-amino-6-chloropurine with moderate to excellent chemical yields. Tracers were prepared by O-[(11)C]methylation of hydroxymethyl or acid precursors using [(11)C]methyl triflate. Pure target compounds were isolated by solid-phase extraction (SPE) purification procedure in 45-65% radiochemical yields (decay corrected to end of bombardment), and a synthesis time of 20-25 min. The activity of unlabeled standard samples of 1-3 was evaluated via an in vitro AGT oligonucleotide assay. Preliminary findings from biological assay indicate the synthesized analogs have similar strong inhibitory effectiveness on AGT in comparison with the parent compound O(6)-BG. The results warrant further evaluation of these radiotracers as new potential PET imaging agents for the DNA repair protein AGT in breast cancer in vivo.

Synthesis and preliminary biological evaluation of 6-O-[11C]-[(methoxymethyl)benzyl]guanines, new potential PET breast cancer imaging agents for the DNA repair protein AGT

Novel radiolabeled O(6)-benzylguanine derivatives, 6-O-[(11)C]-[(methoxymethyl)benzyl]guanines ([(11)C]p-O(6)-MMBG, 1a; [(11)C]m-O(6)-MMBG, 1b; ([(11)C]o-O(6)-MMBG, 1c), have been synthesized for evaluation as new potential positron emission tomography (PET) breast cancer imaging agents for DNA repair protein, O(6)-alkylguanine-DNA alkyltransferase (AGT).

An approach to the evaluation of the activity of the DNA repair enzyme O6-methylguanine-DNA-methyl-transferase in tumor tissue in vivo: syntheses of 6-benzyloxy-9-(2-[18F]fluoroethyl)-9H-purin-2-yl-amine and 6-benzyloxy-7-(2-[18F]fluoroethyl)-7H-purin-2-yl-amine

The resistance of tumor cells to the cytostatic activity of methylating and chloroethylating anticancer drugs is determined by the level of expression of the DNA repair protein O6-methylguanine-DNA-methyl-transferase (MGMT). The synthesis of labelled 6-benzyloxy-9H-purin-2-ylamine derivatives should hence allow a quantification of the MGMT status of tumor and non-target tissue in vivo. 6-benzyloxy-9-(2-fluoroethyl)-9H-purin-2-yl-amine and 6-benzyloxy-7-(2-fluoroethyl)-7H-purin-2-yl-amine were synthesized and evaluated in vitro, both showing an affinity of 1.8 microM. 6-benzyloxy-9-(2-[18F]fluoroethyl)-9H-purin-2-yl-amine and 6-benzyloxy-7-(2-[18F]fluoroethyl)-7H-purin-2-yl-amine were synthesized by alkylation of 6-benzyloxy-9H-purin-2-ylamine with 1-[18F]fluoro-2-tosylethane in optimized yields of 41% and 20%, respectively. Biodistribution studies were performed in nude mice, carrying mex+ (MGMT expressing) and mex- tumors.