Synthesis of 2'-fluoro-5-[11C]-methyl-1-beta-D-arabinofuranosyluracil ([11C]-FMAU): a potential nucleoside analog for in vivo study of cellular proliferation with PET

Tracking Docetaxel-Induced Cellular Proliferation Changes in Prostate Tumor-Bearing Mice with 18F-FMAU PET

OBJECTIVES

The aim of this exploratory preclinical study was to evaluate the efficacy of 18F-FMAU PET in quantitatively measuring cellular proliferation changes in response to a chemotherapeutic agent in experimental prostate cancer models.

METHODS AND MATERIALS

Docetaxel (DTX) ‒ a standard therapy agent in castrate-resistant metastatic prostate cancer was used as the chemotherapy drug. Athymic male nu/nu mice were inoculated with PC-3 cells in the right flank. After the tumor diameter reached 5 mm, DTX (24 mg/kg) was injected intravenously twice a week, whereas the control group was intravenously administered with saline. The tumor size and body weight were monitored, and longitudinal PET scans were acquired with 18F-FMAU to evaluate tumor cellular proliferation. 18F-FMAU PET scans were performed at 2 hours post-injection of 18F-FMAU on days 0, 11, 18, and 22. Biodistribution studies were carried out after the PET scan on day 22.

RESULTS

Consecutive administrations of DTX were effective in inhibiting PC-3 tumor growth compared to the control group. For PET imaging, PC-3 tumor uptake of 18F-FMAU in the DTX group was increased significantly from 3.09 ± 0.60 %ID/g (day 0) to 5.32 ± 0.37 %ID/g (day 22), whereas the 18F-FMAU tumor update in the control group remained relatively stable on day 0 (2.37 ± 0.51 %ID/g) vs. day 22 (1.83 ± 0.22 %ID/g). The tumor-to-muscle uptake ratio of 18F-FMAU was increased from 2.63 ± 0.20 (day 0) to 5.91 ± 1.1 (day 22) in the DTX group. On day 22, no statistical significance was observed in the tumor-to-muscle uptake ratio of 18F-FMAU in the DTX group vs. the control group. The tumor-to-liver uptake ratio of 18F-FMAU was also similar on day 22 in the DTX group (4.29 ± 0.09) vs. the control group (3.83 ± 0.59).

CONCLUSION

18F-FMAU uptake in implanted PC-3 tumors increases with DTX despite inhibiting tumor growth. Further investigation is needed to decipher the underlying biological mechanism of this apparent flare effect and its relation to the predictability of tumor response to DTX.

Carbon-11: Radiochemistry and Target-Based PET Molecular Imaging Applications in Oncology, Cardiology, and Neurology

The positron emission tomography (PET) molecular imaging technique has gained its universal value as a remarkable tool for medical diagnosis and biomedical research. Carbon-11 is one of the promising radiotracers that can report target-specific information related to its pharmacology and physiology to understand the disease status. Currently, many of the available carbon-11 (t_1/2 = 20.4 min) PET radiotracers are heterocyclic derivatives that have been synthesized using carbon-11 inserted different functional groups obtained from primary and secondary carbon-11 precursors. A spectrum of carbon-11 PET radiotracers has been developed against many of the upregulated and emerging targets for the diagnosis, prognosis, prediction, and therapy in the fields of oncology, cardiology, and neurology. This review focuses on the carbon-11 radiochemistry and various target-specific PET molecular imaging agents used in tumor, heart, brain, and neuroinflammatory disease imaging along with its associated pathology.

Effect of Androgen on Normal Biodistribution of [18F]-2'-Fluoro-5-methyl-1-beta-D-arabinofuranosyluracil (18F-FMAU) in Athymic Non-tumor-bearing Male Mice

AIM

We assessed the association between the presence and absence of androgen on the normal biodistribution of the positron emission tomography (PET) cellular proliferation imaging biomarker, [¹⁸F]-2'-Fluoro-5-methyl-1-beta-D-arabinofuranosyluracil (¹⁸F-FMAU), in mice.

MATERIALS AND METHODS

Non-castrated (n=4) and castrated (n=4) athymic non-tumor-bearing male mice served as models for presence and absence, respectively, of androgen. MicroPET-CT scans were performed 1 h following tail vein administration of 200 uCi of ¹⁸F-FMAU. Imaging was performed at baseline and then at 7-day intervals longitudinally for 35 days only in castrated mice following subcutaneous introduction of a 12.5 mg, 21-day release, dihydrotestosterone pellet. Mean standardized uptake values (SUV_mean) were obtained for liver, heart, and muscle. Several two-group comparisons of average of SUV_mean were performed.

RESULTS

Pre-pellet baseline average SUV_mean (±s.d.) values in castrated mice were significantly lower than baseline non-castrated values, increased on day 15 and reached peak values on day 28, at which time they were significantly higher than corresponding baseline levels in both non-castrated and pre-pellet castrated mice. The peak values decreased significantly following dihydrotestosterone withdrawal.

CONCLUSION

There is a significant modulatory effect of androgen on normal ¹⁸F-FMAU uptake levels in mice liver, heart and muscle tissues.

Synthesis and Biological Evaluation of Novel (99m)Tc(CO)₃-Labeled Thymidine Analogs as Potential Probes for Tumor Proliferation Imaging

Achieving a ^99mTc labeled thymidine radiotracer for single photon emission tomography (SPECT) is considered to be of interest. In this study, four novel thymidine analogs, 6a, 6b, 6c and 6d, were successfully synthesized via “click reaction” route and then radiolabeled using a [^99mTc(CO)₃]⁺ core to prepare the corresponding ^99mTc(CO)₃ complexes in high yields. These complexes were hydrophilic and had good in vitro stability. Biodistribution of these complexes in mice bearing S180 tumors showed that all of them exhibited accumulation in the tumors, suggesting that they would be potential tumor imaging agents.

A novel (11)C-labeled thymidine analog, [(11)C]AZT, for tumor imaging by positron emission tomography

Background

Nucleoside analogs labeled with positrons, such as ¹¹C and ¹⁸F, are considered valuable in visualizing the proliferative activity of tumor cells in vivo using positron emission tomography (PET). We recently developed the ¹¹C-labeled thymidine analogs [¹¹C]zidovudine ([¹¹C]AZT) and [¹¹C]stavudine ([¹¹C]d4T) via the Pd(0)-Cu(I) co-mediated rapid C–C coupling reaction. In this study, to examine whether [¹¹C]AZT and [¹¹C]d4T might be useful for visualization of tumors in vivo, we performed PET imaging, tissue distribution studies, and metabolite analysis in tumor-bearing mice.

Methods

Mice bearing tumors (rat glioma C6 and human cervical adenocarcinoma HeLa cells) were injected with 50 MBq of [¹¹C]AZT or [¹¹C]d4T, and PET was performed immediately thereafter. After PET imaging, the radioactivity in several tissues, including tumor tissues, was measured using a γ-counter. In addition, radioactive metabolites in plasma, bile, intestinal contents, and tumor were analyzed using thin layer chromatography (TLC). Cellular uptake of [¹¹C]AZT in C6 was measured in the presence or absence of non-labeled thymidine (0.1 mM).

Results

In PET studies, C6 and HeLa tumors in mice were clearly visualized using [¹¹C]AZT. Time-activity curves using [¹¹C]AZT showed that the accumulation of radioactivity in tumors plateaued at 10 min after injection and persisted for 60 min, while most of the radioactivity in other tissues was rapidly excreted into the urine. In various tissues of the body, tumor tissue showed the highest radioactivity at 80 min after injection (five to six times higher uptake than that of blood). Compared with tumor tissue, uptake was lower in other proliferative tissues such as the spleen, intestine, and bone marrow, resulting in a high tumor-to-bone marrow ratio. Cellular uptake of [¹¹C]AZT in C6 cells was completely blocked by the application of thymidine, strongly indicating the specific involvement of nucleoside transporters. In contrast, the time-activity curve of [¹¹C]d4T in the tumor showed transient and rapid excretion with almost no obvious tumor tissue accumulation.

Conclusions

Tumors can be detected by PET using [¹¹C]AZT; therefore, [¹¹C]AZT could be useful as a novel PET tracer for tumor imaging in vivo.

Electronic supplementary material

The online version of this article (doi:10.1186/s13550-015-0124-0) contains supplementary material, which is available to authorized users.

Nucleoside-based probes for imaging tumor proliferation using positron emission tomography

Cancer is one of the leading causes of human death, and early detection can be beneficial for its timely therapy and management. For the early detection of cancer, positron emission tomography (PET) is more accurate and sensitive than other imaging modalities, such as computed tomography and magnetic resonance imaging. [(18) F]-Labeled fluorodeoxyglucose is the most useful PET probe in early detection of cancer; however, its nonspecific accumulation and consequent false-positive findings warrant the identification of other PET probes. Thymidine (TdR) and its analogs have been radiolabeled for PET imaging of cellular proliferation and DNA synthesis. Because of its in vivo instability, radiolabeled TdR has not been successful in PET imaging. However, some of its radiolabeled analogs have been developed for PET imaging of cellular proliferation and DNA synthesis. In this review, the radiochemistry and production of (11) C-TdR and (11) C/(18) F-labeled TdR analogs published to date are presented.

Molecular imaging of prostate cancer: PET radiotracers

OBJECTIVE

Recent advances in the fundamental understanding of the complex biology of prostate cancer have provided an increasing number of potential targets for imaging and treatment. The imaging evaluation of prostate cancer needs to be tailored to the various phases of this remarkably heterogeneous disease.

CONCLUSION

In this article, I review the current state of affairs on a range of PET radiotracers for potential use in the imaging evaluation of men with prostate cancer.

A new nucleoside analogue with potent activity against mutant sr39 herpes simplex virus-1 (HSV-1) thymidine kinase (TK)

Nucleoside analogues, such as penciclovir, ganciclovir, acyclovir, and their fluoro-substituted derivatives, have wide utility as antivirals. Among these analogues, FHBG ((18)F-Fluorohydroxybutylguanine) is a well-validated PET (positron emission tomography) probe for monitoring reporter gene expression. To evaluate whether or not imposing rigidity into the flexible side chain of FHBG 4 could also impact its interaction, with amino acid residues within the binding site of HSV1-TK (Herpes Simplex Virus-1 Thymidine Kinase), thus influencing its cytotoxic activity. Herein, the synthesis of a new fluorinated nucleoside analogue 6 (conceived via ligand-docking studies) is reported. Agent 6 demonstrates selective activity against HeLa cells stably transfected with mutant HSV1-sr39TK and is also 47-fold more potent than FHBG.

Radio-deoxynucleoside Analogs used for Imaging tk Expression in a Transgenic Mouse Model of Induced Hepatocellular Carcinoma

Purpose: A group of radiolabeled thymidine analogs were developed as radio-tracers for imaging herpes viral thymidine kinase (HSV1-tk) or its variants used as reporter gene. A transgenic mouse model was created to express tk upon liver injury or naturally occurring hepatocellular carcinoma (HCC). The purpose of this study was to use this unique animal model for initial testing with radio-labeled thymidine analogs, mainly a pair of newly emerging nucleoside analogs, D-FMAU and L-FMAU.

Methods: A transgeneic mouse model was created by putting a fused reporter gene system, firefly luciferase (luc) and HSV1-tk, under the control of mouse alpha fetoprotein (Afp) promoter. Initial multimodal imaging, which was consisted of bioluminescent imaging (BLI) and planar gamma scintigraphy with [¹²⁵I]-FIAU, was used for examining the model creation in the new born and liver injury in the adult mice. Carcinogen diethylnitrosamine (DEN) was then administrated to induce HCC in these knock-in mice such that microPET imaging could be used to track the activity of Afp promoter during tumor development and progression by imaging tk expression first with [¹⁸F]-FHBG. Dynamic PET scans with D-[¹⁸F]-FMAU and L-[¹⁸F]-FMAU were then performed to evaluate this pair of relatively new tracers. Cells were derived from these liver tumors for uptake assays using H-3 labeled version of PET tracers.

Results: The mouse model with dual reporters: HSV1-tk and luc placed under the transcriptional control of an endogenous Afp promoter was used for imaging studies. The expression of the Afp gene was highly specific in proliferative hepatocytes, in regenerative liver, and in developing fetal liver, and thus provided an excellent indicator for liver injury and cancer development in adult mice. Both D-FMAU and L-FMAU showed stable liver tumor uptake where the tk gene was expressed under the Afp promoter. The performance of this pair of tracers was slightly different in terms of signal-to-background ratio as well as tracer clearance.

Conclusion: The newly created knock-in mouse model was used to demonstrate the use of the dual-reporter genes driven by well-characterized cancer-specific transcriptional units in conjunction with in vivo imaging as a paradigm in studying naturally occurring cancer in live animals. While BLI is suitable for small animal imaging with luc expression, PET with L-FMAU seemed be the choice for liver injury or liver cancer imaging with this animal model for future investigations.

Positron emission tomography imaging of cancer biology: current status and future prospects

Positron emission tomography (PET) is one of the most rapidly growing areas of medical imaging, with many applications in the clinical management of patients with cancer. The principal goal of PET imaging is to visualize, characterize, and measure biological processes at the cellular, subcellular, and molecular levels in living subjects using noninvasive procedures. PET imaging takes advantage of the traditional diagnostic imaging techniques and introduces positron-emitting probes to determine the expression of indicative molecular targets at different stages of cancer progression. Although [(18)F]fluorodeoxyglucose ([(18)F]FDG)-PET has been widely utilized for staging and restaging of cancer, evaluation of response to treatment, differentiation of post-therapy alterations from residual or recurrent tumor, and assessment of prognosis, [(18)F]FDG is not a target-specific PET tracer. Over the last decade, numerous target-specific PET tracers have been developed and evaluated in preclinical and clinical studies. This review provides an overview of the current status and trends in the development of non-[(18)F]FDG PET probes in oncology and their application in the investigation of cancer biology.

The improved syntheses of 5-substituted 2'-[18F]fluoro-2'-deoxy-arabinofuranosyluracil derivatives ([18F]FAU, [18F]FEAU, [18F]FFAU, [18F]FCAU, [18F]FBAU and [18F]FIAU) using a multistep one-pot strategy

INTRODUCTION

We and others have previously reported a four-step radiosynthesis of a series of 2'-deoxy-2'-[(18)F]fluoro-5-substituted-1-β-D-arabinofuranosyluracil derivatives including [(18)F]FAU, [(18)F]FEAU, [(18)F]FFAU, [(18)F]FCAU, [(18)F]FBAU and [(18)F]FIAU as thymidine derivatives for tumor proliferation and/or reporter gene expression imaging with positron emission tomography (PET). Although the radiosynthesis has been proven to be reproducible and efficient, this complicated multistep reaction is difficult to incorporate into an automated cGMP-compliant radiosynthesis module for routine production. Recently, we have developed a simple and efficient one-pot method for routine production of [(18)F]FMAU. In this study, we studied the feasibility of radiosynthesizing [(18)F]FAU, [(18)F]FEAU, [(18)F]FFAU, [(18)F]FCAU, [(18)F]FBAU and [(18)F]FIAU using this newly developed method.

METHODS

Similar to the radiosynthesis of [(18)F]FMAU, 5-substituted 2'-[(18)F]fluoro-2'-deoxy-arabinofuranosyluracil derivatives ([(18)F]FAU, [(18)F]FEAU, [(18)F]FFAU, [(18)F]FCAU, [(18)F]FBAU and [(18)F]FIAU) were synthesized in one-pot radiosynthesis module in the presence of Friedel-Crafts catalyst TMSOTf and HMDS.

RESULTS

This one-pot radiosynthesis method could be used to produce [(18)F]FAU, [(18)F]FEAU, [(18)F]FFAU, [(18)F]FCAU, [(18)F]FBAU and [(18)F]FIAU. The overall radiochemical yields of these tracers varied from 4.1%±0.8% to 10.1%±1.9% (decay-corrected, n=4). The overall reaction time was reduced from 210 min to 150 min from the end of bombardment, and the radiochemical purity was >99%.

CONCLUSIONS

The improved radiosyntheses of [(18)F]FAU, [(18)F]FEAU, [(18)F]FFAU, [(18)F]FCAU, [(18)F]FBAU and [(18)F]FIAU have been achieved with reasonable yields and high purity using a multistep one-pot method. The synthetic time has been reduced, and the reaction procedures have been significantly simplified. The success of this approach may make PET tracers [(18)F]FAU, [(18)F]FEAU, [(18)F]FFAU, [(18)F]FCAU, [(18)F]FBAU and [(18)F]FIAU more accessible for preclinical and clinical research.

Prostate cancer

Prostate cancer is biologically and clinically a heterogeneous disease and its imaging evaluation will need to be tailored to the specific phases of the disease in a patient-specific, risk-adapted manner. We first present a brief overview of the natural history of prostate cancer before discussing the role of various imaging tools, including opportunities and challenges, for different clinical phases of this common disease in men. We then review the preclinical and clinical evidence on the potential and emerging role of positron emission tomography with various radiotracers in the imaging evaluation of men with prostate cancer.

Automated synthesis of 2'-deoxy-2'-[18F]fluoro-5-methyl-1-β-D-arabinofuranosyluracil ([18F]-FMAU) using a one reactor radiosynthesis module

2'-Deoxy-2'-[(18)F]fluoro-5-methyl-1-β-D-arabinofuranosyluracil ([(18)F]-FMAU) is an established PET probe used to monitor cellular proliferation. For clinical applications, a fully automated cGMP-compliant radiosynthesis would be preferred. However, the current synthesis of [(18)F]-FMAU requires a multistep procedure, making the development of an automated protocol difficult and complicated. Recently, we have developed a significantly simplified one-pot reaction condition for the synthesis of [(18)F]-FMAU in the presence of Friedel-Crafts catalysts. Here, we report a fully automated synthesis of [(18)F]-FMAU based on a one reactor radiosynthesis module using our newly developed synthetic method. The product was purified on a semi-preparative high-performance liquid chromatography integrated with the synthesis module using 6% EtOH in 10 mM phosphate buffer or 8% MeCN/water. [(18)F]-FMAU was obtained in 12±3% radiochemical yield (decay corrected overall yield based on [(18)F]-F(-), n=4) with 383±33 mCi/μmol specific activity at the time of injection. The α/β anomer ratio was 4:6. The overall reaction time was about 150 min from the end of bombardment and the radiochemical purity was >99%. This automated synthesis should also be suitable for the production of other 5-substituted thymidine analogues.

Radiopharmaceutical chemistry for positron emission tomography

Molecular imaging is an emerging technology that allows the visualization of interactions between molecular probes and biological targets. Molecules that either direct or are subject to homeostatic controls in biological systems could be labeled with the appropriate radioisotopes for the quantitative measurement of selected molecular interactions during normal tissue homeostasis and again after perturbations of the normal state. In particular, positron emission tomography (PET) offers picomolar sensitivity and is a fully translational technique that requires specific probes radiolabeled with a usually short-lived positron-emitting radionuclide. PET has provided the capability of measuring biological processes at the molecular and metabolic levels in vivo by the detection of the gamma rays formed as a result of the annihilation of the positrons emitted. Despite the great wealth of information that such probes can provide, the potential of PET strongly depends on the availability of suitable PET radiotracers. However, the development of new imaging probes for PET is far from trivial and radiochemistry is a major limiting factor for the field of PET. In this review, we provided an overview of the most common chemical approaches for the synthesis of PET-labeled molecules and highlighted the most recent developments and trends. The discussed PET radionuclides include ¹¹C (t₁(/)₂=20.4min), ¹³N (t₁(/)₂=9.9min), ¹⁵O (t₁(/)₂=2min), ⁶⁸Ga (t₁(/)₂=68min), ¹⁸F (t₁(/)₂=109.8min), ⁶⁴Cu (t₁(/)₂=12.7h), and ¹²⁴I (t₁(/)₂=4.12d).

Charge dependent substrate activity of C3' and N3 functionalized, organometallic technetium and rhenium-labeled thymidine derivatives toward human thymidine kinase 1

Human cytosolic thymidine kinase (hTK1) has proven to be a suitable target for the noninvasive imaging of cancer cell proliferation using radiolabeled thymidine analogues such as [(18)F]3'-fluoro-3'-deoxythymidine ([(18)F]FLT). A thymidine analogue for single photon emission computed tomography (SPECT), which incorporates the readily available and inexpensive nuclide technetium-99m, would be of considerable practical interest. hTK1 is known to accommodate modification of the structure of the natural substrate thymidine at the positions N3 and C3' and, to a lesser extent, C5. In this work, we used the copper-catalyzed azide-alkyne cycloaddition to synthesize two series of derivatives in which thymidine is functionalized at either the C3' or N3 position with chelating systems suitable for the M(CO)(3) core (M = (99m)Tc, Re). The click chemistry approach enabled complexes with different structures and overall charges to be synthesized from a common precursor. Using this strategy, the first organometallic hTK1 substrates in which thymidine is modified at the C3' position were identified. Phosphorylation of the organometallic derivatives was measured relative to thymidine. We have shown that the influence of the overall charge of the derivatives is dependent on the position of functionalization. In the case of the C3'-functionalized derivatives, neutral and anionic substrates were most readily phosphorylated (20-28% of the value for the parent ligand thymidine), whereas for the N3-functionalized derivatives, cationic and neutral complexes were apparently better substrates for the enzyme (14-18%) than anionic derivatives (9%).

Positron emission tomography measurement of tumor metabolism and growth: its expanding role in oncology

This work highlights the explosion and evolution of positron emission tomography (PET) for use in oncology research and clinical practice. 2-Deoxy-2-[F-18]fluoro-D-glucose (FDG)-PET is important in the staging of cancer, estimation of prognosis, and for its ability to predict therapeutic outcome. A number of new imaging agents are under development and may find a place in oncology when studies prove their utility. This scientific overview includes a review of the development of a number of thymidine analogs, such as 18F-3'-deoxy-3'-fluorothymidine (FLT) and 18F-1-(2'-deoxy-2'-fluoro-beta-D-arabinofuranosyl)-thymine (FMAU), including chemical structure variations; their application in a variety of tumors; and the role of various kinetic models for understanding cellular proliferation. The greatest unmet need for PET is in further developing and validating its use in the measurement of treatment response.

PET imaging of HSV1-tk mutants with acquired specificity toward pyrimidine- and acycloguanosine-based radiotracers

PURPOSE

The aim of this study was to create an alternative mutant of the herpes simplex virus type 1 thymidine kinase (HSV1-tk) reporter gene with reduced phosphorylation capacity for acycloguanosine derivatives, but not pyrimidine-based compounds that will allow for successful PET imaging.

METHODS

A new mutant of HSV1-tk reporter gene, suitable for PET imaging using pyrimidine-based radiotracers, was developed. The HSV1-tk mutant contains an arginine-to-glutamine substitution at position 176 (HSV1-R176Qtk) of the nucleoside binding region of the enzyme.

RESULTS

The mutant-gene product showed favorable enzymatic characteristics toward pyrimidine-based nucleosides, while exhibiting reduced activity with acycloguanosine derivatives. In order to enhance HSV1-R176Qtk reporter activity with pyrimidine-based radiotracers, we introduced the R176Q substitution into the more active HSV1-sr39tk mutant. U87 human glioma cells transduced with the HSV1-R176Qsr39tk double mutant reporter gene showed high (3)H-FEAU pyrimidine nucleoside and low (3)H-penciclovir acycloguanosine analog uptake in vitro. PET imaging also demonstrated high (18)F-FEAU and low (18)F-FHBG accumulation in HSV1-R176Qsr39tk+ xenografts. The feasibility of imaging two independent nucleoside-specific HSV1-tk mutants in the same animal with PET was demonstrated. Two opposite xenografts expressing the HSV1-R176Qsr39tk reporter gene and the previously described acycloguanosine-specific mutant of HSV1-tk, HSV1-A167Ysr39tk reporter gene, were imaged using a short-lived pyrimidine-based (18)F-FEAU and an acycloguanosine-based (18)F-FHBG radiotracer, respectively, administered on 2 consecutive days.

CONCLUSION

We conclude that in combination with acycloguanosine-specific HSV1-A167Ysr39tk reporter gene, a HSV1-tk mutant containing the R176Q substitution could be used for PET imaging of two different cell populations or concurrent molecular biological processes in the same living subject.

Preferential tumor targeting and selective tumor cell cytotoxicity of 5-[131/125I]iodo-4'-thio-2'-deoxyuridine

PURPOSE

Auger electron emitting radiopharmaceuticals are attractive for targeted nanoirradiation therapy, provided that DNA of malignant cells is selectively addressed. Here, we examine 5-[123/125/131I]iodo-4'-thio-2'-deoxyuridine (ITdU) for targeting DNA in tumor cells in a HL60 xenograft severe combined immunodeficient mouse model.

EXPERIMENTAL DESIGN

Thymidine kinase and phosphorylase assays were done to determine phosphorylation and glycosidic bond cleavage of ITdU, respectively. The biodistribution and DNA incorporation of ITdU were determined in severe combined immunodeficient mice bearing HL60 xenografts receiving pretreatment with 5-fluoro-2'-deoxyuridine (FdUrd). Organ tissues were dissected 0.5, 4, and 24 h after radioinjection and uptake of [131I]ITdU (%ID/g tissue) was determined. Cellular distribution of [125I]ITdU was imaged by microautoradiography. Apoptosis and expression of the proliferation marker Ki-67 were determined by immunohistologic staining using corresponding paraffin tissue sections.

RESULTS

ITdU is phosphorylated by thymidine kinase 1 and stable toward thymidylate phosphatase-mediated glycosidic bond cleavage. Thymidylate synthase-mediated deiodination of [123/125/131I]ITdU was inhibited with FdUrd. Pretreatment with FdUrd increased preferentially tumor uptake of ITdU resulting in favorable tumor-to-normal tissue ratios and tumor selectivity. ITdU was exclusively localized within the nucleus and incorporated into DNA. In FdUrd-pretreated animals, we found in more than 90% of tumor cells apoptosis induction 24 h postinjection of ITdU, indicating a highly radiotoxic effect in tumor cells but not in cells of major proliferating tissues.

CONCLUSION

ITdU preferentially targets DNA in proliferating tumor cells and leads to apoptosis provided that the thymidylate synthase is inhibited.

N(3)-Substituted thymidine analogues V: synthesis and preliminary PET imaging of N(3)-[(18)F]fluoroethyl thymidine and N(3)-[(18)F]fluoropropyl thymidine

INTRODUCTION

[(18)F]-Labeled analogues of thymidine have demonstrated efficacy for PET imaging of cellular proliferation. We have synthesized two [(18)F]-labeled N(3)-substituted thymidine analogues, N(3)-[(18)F]fluoroethyl thymidine (N(3)-[(18)F]-FET) and N(3)-[(18)F]fluoropropyl thymidine (N(3)-[(18)F]-FPrT), and performed preliminary PET imaging studies in tumor-bearing mice.

METHODS

Thymidine was converted to its 3',5'-O-bis-tetrahydropyranyl ether, which was then converted to the N(3)-ethyl and propyl-substituted mesylate precursors. Reactions of these mesylate precursors with n-Bu(4)N[(18)F] or K[(18)F]/kryptofix followed by acid hydrolysis and HPLC purification yielded N(3)-[(18)F]-FET and N(3)-[(18)F]-FPrT, respectively. Subcutaneous (sc) xenografts of H441 human non-small cell lung cancer were established in two groups of mice (each n=6). Micro-PET images of the tumor-bearing animals were acquired after intravenous injection of N(3)-[(18)F]-FET or N(3)-[(18)F]-FPrT (3700 KBq/animal).

RESULTS

The radiochemical yields were 2-12% (d.c.) for N(3)-[(18)F]-FET and 30-38% (d.c.) for N(3)-[(18)F]-FPrT. Radiochemical purity was >99% and calculated specific activity was >74 GBq/mumol at the end of synthesis. The accumulation of N(3)-[(18)F]-FET and N(3)-[(18)F]-FPrT in the tumor tissue at 2 h postinjection was 1.81+/-0.78 and 2.95+/-1.14 percent injected dose per gram (%ID/g), respectively; tumor/muscle ratios were 5.57+/-0.82 and 7.69+/-2.18, respectively; the unidirectional influx rates (K(i)) were 0.013 and 0.018 ml/g per minute, respectively.

CONCLUSION

Two novel [(18)F]- N(3)-substituted thymidine analogues have been synthesized in good yields, high purity and high specific activity. Preliminary in vivo studies demonstrated the efficacy of these [(18)F]- N(3)-substituted thymidine analogues for PET imaging of tumors.

Analysis and reproducibility of 3'-Deoxy-3'-[18F]fluorothymidine positron emission tomography imaging in patients with non-small cell lung cancer

PURPOSE

Imaging tumor proliferation with 3'-deoxy-3'-[(18)F]fluorothymidine (FLT) and positron emission tomography is being developed with the goal of monitoring antineoplastic therapy. This study assessed the methods to measure FLT retention in patients with non-small cell lung cancer (NSCLC) to measure the reproducibility of this approach.

EXPERIMENTAL DESIGN

Nine patients with NSCLC who were untreated or had progressed after previous therapy were imaged twice using FLT and positron emission tomography within 2 to 7 days. Reproducibility (that is, error) was measured as the percent difference between the two patient scans. Dynamic imaging was obtained during the first 60 min after injection. Activity in the blood was assessed from aortic images and the fraction of unmetabolized FLT was measured. Regions of interest were drawn on the plane with the highest activity and the adjacent planes to measure standardized uptake value (SUV(mean)) and kinetic variables of FLT flux.

RESULTS

We found that the SUV(mean) obtained from 30 to 60 min had a mean error of 3.6% (range, 0.6-6.9%; SD, 2.3%) and the first and second scans were highly correlated (r(2) = 0.99; P < 0.0001). Using shorter imaging times from 25 to 30 min or from 55 to 60 min postinjection also resulted in small error rates; SUV(mean) mean errors were 8.4% and 5.7%, respectively. Compartmental and graphical kinetic analyses were also fairly reproducible (r(2) = 0.59; P = 0.0152 and r(2) = 0.58; P = 0.0175 respectively).

CONCLUSION

FLT imaging of patients with NSCLC was quite reproducible with a worst case SUV(mean) error of 21% when using a short imaging time.

Probing DNA polymerase activity with stereoisomeric 2′-fluoro-β-D-arabinose (2′F-araNTPs) and 2′-fluoro-β-D-ribose (2′F-rNTPs) nucleoside 5′-triphosphates

2′-Deoxy-2′-fluoro-β-D-ribonucleosides (2′F-rN) and 2′-deoxy-2′-fluoro-β-D-arabinonucleosides (2′F-araN) differ solely in the stereochemistry at the 2′-carbon of the furanose sugar ring. 2′F-rN 5′-triphosphates (2′F-rNTPs) are among the most commonly used sugar-modified nucleoside 5′-triphosphates (NTPs) for in vitro selection; however, the epimeric 2′F-araN 5′-triphosphates (2′F-araNTPs) have only recently been applied to polymerase-directed biosynthesis [C.G. Peng and M.J. Damha. J. Am. Chem. Soc. 129, 5310 (2007)]. The present study describes primer extension assays that compare, for the first time, the incorporation efficiency of the two isomeric NTPs, namely, 2′F-araNTPs or 2′F-rNTPs, by four DNA polymerases [Deep Vent (exo-), 9°Nm, HIV-1 RT, and MMLV-RT]. Under the conditions used, incorporation of 2′F-araTTP proceeded more efficiently relative to 2′F-rUTP, while the incorporation of 2′F-araCTP is comparable or slightly less efficient than that observed with 2′F-rCTP. Interestingly, these preferences were observed for all four of the DNA polymerases tested. Unexpected differences in NTP incorporation were observed for 2′F-rCTP vs. rCTP. Despite their seemingly similar conformation, they behaved striking differently in the in vitro polymerization assays. 2′F-rCTP is a much better substrate than the native counterpart (rCTP), an observation first made with human DNA polymerases [F.C. Richardson, R.D. Kuchta, A. Mazurkiewicz, K.A. Richardson. Biochem. Pharmacol. 59, 1045 (2000)]. In contrast, 2′F-rUTP behaved like rUTP, providing poor yield of full-length products. Taken together, this indicates that 2′F-rCTP is very unusual with regard to enzyme/substrate recognition; an observation that can be exploited for the production of DNA oligomers enriched with both ribose and arabinose modifications. These findings are timely given the significant interest and growing need to develop chemically modified oligonucleotides for therapeutic and diagnostic research. By examining the structure-activity relationship (SAR) of the ribose and arabinose sugar, this study furthers our understanding of how the nature of the 2′ substituent (e.g., α vs. β; F vs. OH) and the heterocyclic base affect NTP selection (specificity) by DNA polymerases.Key words: 2′F-rNTPs, 2′F-araNTPs, DNA polymerases, biosynthesis, modified nucleoside triphosphates.

Imaging of cell proliferation: status and prospects

Increased cellular proliferation is an integral part of the cancer phenotype. Several in vitro assays have been developed to measure the rate of tumor growth, but these require biopsies, which are particularly difficult to obtain over time and in different areas of the body in patients with multiple metastatic lesions. Most of the effort to develop imaging methods to noninvasively measure the rate of tumor cell proliferation has focused on the use of PET in conjunction with tracers for the thymidine salvage pathway of DNA synthesis, because thymidine contains the only pyrimidine or purine base that is unique to DNA. Imaging with 11C-thymidine has been tested for detecting tumors and tracking their response to therapy in animals and patients. Its major limitations are the short half-life of 11C and the rapid catabolism of thymidine after injection. These limitations led to the development of analogs that are resistant to degradation and can be labeled with radionuclides more conducive to routine clinical use, such as 18F. At this point, the thymidine analogs that have been studied the most are 3'-deoxy-3'-fluorothymidine (FLT) and 1-(2'-deoxy-2'-fluoro-1-beta-d-arabinofuranosyl)-thymine (FMAU). Both are resistant to degradation and track the DNA synthesis pathway. FLT is phosphorylated by thymidine kinase 1, thus being retained in proliferating cells. It is incorporated by the normal proliferating marrow and is glucuronidated in the liver. FMAU can be incorporated into DNA after phosphorylation but shows less marrow uptake. It shows high uptake in the normal heart, kidneys, and liver, in part because of the role of mitochondrial thymidine kinase 2. Early clinical data for 18F-FLT demonstrated that its uptake correlates well with in vitro measures of proliferation. Although 18F-FLT can be used to detect tumors, its tumor-to-normal tissue contrast is generally lower than that of 18F-FDG in most cancers outside the brain. The most promising use for thymidine and its analogs is in monitoring tumor treatment response, as demonstrated in animal studies and pilot human trials. Further work is needed to determine the optimal tracer(s) and timing of imaging after treatment.

Evaluation of 2'-deoxy-2'-[18F]fluoro-5-methyl-1-beta-L: -arabinofuranosyluracil ([18F]-L: -FMAU) as a PET imaging agent for cellular proliferation: comparison with [18F]-D: -FMAU and [18F]FLT

PURPOSE

Clevudine (L: -FMAU) an un-natural analogue of thymidine, is in clinical trials for the treatment of hepatitis B virus (HBV). L: -FMAU is phosphorylated by cellular kinases such as thymidine kinase 1 and deoxycytidine kinase, and its triphosphate form inhibits HBV deoxyribonucleic acid synthesis. Thus, L: -FMAU, radiolabeled with an appropriate isotope, may be useful for positron emission tomography (PET) imaging of tumor proliferation. We evaluated [18F]-L-FMAU as a PET imaging agent in tumor-bearing mice and compared the results with those of two other radiotracers, [18F]-d-FMAU and [18F]-FLT.

METHODS

Subcutaneous xenografts of the human lung cancer cell lines H441 and H3255 were established in mice. A micro-PET scanner was used to obtain images of the tumor-bearing animals with [18F]-L-FMAU, [18F]-D-FMAU, and [18F]-FLT.

RESULTS

At 2 h postinjection, the tumor uptake (% ID/g) of 18F]-L: -FMAU, 18F]-D: -FMAU, and [18F]-FLT in the faster-growing H441 cells was 3.13 +/- 1.11, 7.74 +/- 1.39, and 5.10 +/- 1.45, respectively. The corresponding values for the slower-growing H3255 cells were 1.38 +/- 0.81, 4.49 +/- 1.08, and 0.57 +/- 0.33. Tumor/muscle ratios of accumulation for [18F]-L: -FMAU, [18F]-D: -FMAU, and [18F]-FLT in H441 cells were 4.15 +/- 1.82, 3.37 +/- 1.19, and 12.94 +/- 4.38, respectively, and the corresponding values in H3255 cells were 1.62 +/- 0.50, 1.96 +/- 0.74, and 1.50 +/- 0.90.

CONCLUSIONS

[18F]-L: -FMAU may be a useful agent for imaging tumor proliferation in fast-growing human lung cancers by PET.

Feasibility studies of 4'-[methyl-(11)C]thiothymidine as a tumor proliferation imaging agent in mice

This study reports on the radiosynthesis and feasibility studies of 4'-[methyl-(11)C]thiothymidine ([methyl-(11)C]S-dThd) as a tumor proliferation imaging agent. [Methyl-(11)C]S-dThd was synthesized by rapid methylation of corresponding 5-trimethylstannyl- or 5-tributylstannyl-precursor via a palladium-promoted Stille cross-coupling reaction with [(11)C]methyl iodide. The decay-corrected radiochemical yields of [methyl-(11)C]S-dThd synthesized by the corresponding 5-trimethylstannyl-precursor and 5-tributylstannyl-precursor based on [(11)C]CO(2) were 18.9% and 14.5%, respectively. The radiochemical purity of [methyl-(11)C]S-dThd was always greater than 99%. The specific activities of [methyl-(11)C]S-dThd synthesized by the corresponding 5-trimethylstannyl-precursor and 5-tributylstannyl-precursor were 47 GBq/mumol and 121 GBq/mumol, respectively, at the end of the synthesis. The total synthesis time was 30 min after the end of bombardment. The comparison between in vivo distribution of [methyl-(14)C]S-dThd and that of [methyl-(3)H]FLT showed that tracer uptake was comparable in nonproliferating tissues. In contrast, [methyl-(14)C]S-dThd showed significantly higher uptake in proliferating tissues than did [methyl-(3)H]FLT. [Methyl-(11)C]S-dThd uptake levels in five different tumor tissues were well correlated with the DNA synthesis levels determined by [2-(14)C]thymidine DNA incorporation. At 30 min after injection, plasma analysis found 95% of the activity in unmetabolized form. The microPET imaging of the C6 glioma xenograft showed significantly high uptake in the tumor and urinary bladder, followed by the intestine and marrow. Our results demonstrated that the tumor uptake of [methyl-(11)C]S-dThd was higher than that of [methyl-(3)H]FLT and was well correlated with the DNA synthesis level. Consequently, 4'-[methyl-(11)C]thiothymidine has promise for the imaging of tumor cell proliferation by positron emission tomography.

Radiosynthesis of 2'-deoxy-2'-[18F]-fluoro-5-methyl-1-beta-L-arabinofuranosyluracil ([18F]-L-FMAU) for PET

Radiosynthesis of 2'-deoxy-2'-[(18)F]-fluoro-5-methyl-1-beta-L-arabinofuranosyluracil ([(18)F]-L-FMAU) is reported. Compound 1 was synthesized and converted to 2-triflate 2. Compound 3 was prepared from 2 using tetrabutylammonium[(18)F]fluoride, converted to 4, and then coupled with 5. The crude product was hydrolyzed, and purified by HPLC to obtain 7a. The radiochemical yield of [(18)F]-L-FMAU was 26% decay corrected (d.c.) in four runs with radiochemical purity >99% and specific activity 2200 mCi/micromol. The synthesis time was 3.3-3.5h from the end of bombardment (EOB).

Synthesis and evaluation of a 99mTc-MAMA-propyl-thymidine complex as a potential probe for in vivo visualization of tumor cell proliferation with SPECT

INTRODUCTION

Cytosolic thymidine kinase (TK1) catalyzes phosphorylation of thymidine to its monophosphate. TK1 activity is closely related with DNA synthesis, and thymidine analogs derivatized with bulky carboranylalkyl groups at the N-3 position were reported to be good substrates for TK1. Accordingly, we have synthesized (99m)Tc-MAMA-propyl-thymidine and evaluated it as a potential tumor tracer.

METHODS

The bis(S-trityl)-protected MAMA-propyl-thymidine precursor (3-N-[S-trityl-2-mercaptoethyl]-N-[N'-(S-trityl-2-mercaptoethyl)amidoacetyl]-aminopropyl-thymidine) was prepared in three steps, and its structure was confirmed with (1)H NMR and mass spectrometry. Deprotection of the thiols and labeling with (99m)Tc were done in a two-step, one-pot procedure, yielding (99m)Tc-MAMA-propyl-thymidine, which was analyzed with high-performance liquid chromatography, radio-LC-MS analysis (ESI+) and electrophoresis, and its log P was determined. The biodistribution in normal mice was evaluated, and its biodistribution in a radiation-induced fibrosarcoma (RIF) tumor mouse was compared with that of 3'-deoxy-3'-[(18)F] fluorothymidine [(18)F]FLT.

RESULTS

(99m)Tc-MAMA-propyl-thymidine was obtained with a radiochemical yield of 70%. Electrophoresis indicated that the complex is uncharged, and its log P was 1.0. The molecular ion mass of the Tc complex was 589 Da, which is compatible with the hypothesized N(2)S(2)-oxotechnetium structure. Tissue distribution showed fast clearance from plasma primarily by the hepatobiliary pathway. Whole-body planar imaging after injection of (99m)Tc-MAMA-propyl-thymidine in an RIF tumor-bearing mouse showed high uptake in the liver and the intestines. No uptake was observed in the tumor, in contrast to the clear uptake observed for [(18)F] FLT visualized with muPET.

CONCLUSIONS

Although it has been reported that TK1 accepts large substituents at the N-3 position of the thymine ring, the results of this study show that (99m)Tc-MAMA-propyl-thymidine cannot be used as a single photon emission computed tomography tumor tracer, probably because the (99m)Tc-MAMA ligand is too bulky to be tolerated by TK1.

Carbon-11 labeling chemistry based upon [11C]methyl iodide

Radiochemistry with the short-lived positron emitter 11C (half-life 20.38 min) represents special challenges in terms of synthesis time and labeling techniques. The recent developments in 11C radiochemistry have steadily expanded the number of 11C labeled compounds. This chapter addresses selected chemical and technical aspects of 11C chemistry based on the readily available labeling precursors [11 C]methyl iodide and, to a lesser extent, [11C]methyl triflate. Special emphasis is placed on heteroatom methylation reactions and 11C-C bond formations.

Alkyl-fluorinated thymidine derivatives for imaging cell proliferation

Derivatives of 2'-deoxyuridine that contain fluoroalkyl groups at the C5 position and derivatives of thymidine that contain fluoroalkyl groups at the N3 position were synthesized and examined in three in vitro assays designed to evaluate their potential as radiopharmaceuticals for imaging cellular proliferation. Three of the former nucleosides and five of the latter were synthesized. The three assays were as follows: (a) phosphoryl transfer assay, which showed that all three of the former nucleosides and four of the latter ones were phosphorylated by recombinant human thymidine kinase 1 (TK1) and that N(3)-(2-fluoroethyl)-thymidine (NFT202) was the most potent substrate of the eight nucleosides studied; (b) transport assay, which indicated that all eight nucleosides had good affinity for an 6-[(4-nitrobenzyl)thio]-9-beta-d-ribofuranosylpurine-sensitive mouse erythrocyte nucleoside transporter, with inhibition constants in the range of 0.02-0.55 mM; and (c) degradation assay, which showed that all but one of the former nucleosides and none of the latter were degraded by recombinant Escherichia coli thymidine phosphorylase (an enzyme that catalyzes the glycosidic bond of thymidine and 2'-deoxyuridine derivatives). From these in vitro screening assays, we selected NFT202 as a candidate for subsequent in vivo evaluation because this compound met the three minimum requirements of the in vitro screening assays and had the most potent phosphorylation activity as a substrate for recombinant human TK1.

Synthesis and Evaluation of [18F] Labeled Pyrimidine Nucleosides for Positron Emission Tomography Imaging of Herpes Simplex Virus 1 Thymidine Kinase Gene Expression

Synthesis of three novel 2'-deoxy-2'-[18F]fluoro-1-beta-D-arabinofuranosyluracil derivatives [18F]FPAU, [18F]FBrVAU, and [18F]FTMAU is reported. The compounds were synthesized by coupling of 1-bromo-2-deoxy-2-fluoro sugars with corresponding silylated uracil derivatives. In vitro cell uptake indicated that all three compounds are taken up selectively in RG2TK+ cells with negligible uptake in RG2 cells. The results indicate that [18F]FBrVAU and [18F]FTMAU have better uptake profiles in comparison to [18F]FPAU and have potential as PET probes for imaging HSV1-tk gene expression.

The progress and promise of molecular imaging probes in oncologic drug development

As addressed by the recent Food and Drug Administration Critical Path Initiative, tools are urgently needed to increase the speed, efficiency, and cost-effectiveness of drug development for cancer and other diseases. Molecular imaging probes developed based on recent scientific advances have great potential as oncologic drug development tools. Basic science studies using molecular imaging probes can help to identify and characterize disease-specific targets for oncologic drug therapy. Imaging end points, based on these disease-specific biomarkers, hold great promise to better define, stratify, and enrich study groups and to provide direct biological measures of response. Imaging-based biomarkers also have promise for speeding drug evaluation by supplementing or replacing preclinical and clinical pharmacokinetic and pharmacodynamic evaluations, including target interaction and modulation. Such analyses may be particularly valuable in early comparative studies among candidates designed to interact with the same molecular target. Finally, as response biomarkers, imaging end points that characterize tumor vitality, growth, or apoptosis can also serve as early surrogates of therapy success. This article outlines the scientific basis of oncology imaging probes and presents examples of probes that could facilitate progress. The current regulatory opportunities for new and existing probe development and testing are also reviewed, with a focus on recent Food and Drug Administration guidance to facilitate early clinical development of promising probes.

Imaging DNA synthesis with [18F]FMAU and positron emission tomography in patients with cancer

PURPOSE

FMAU (1-(2'-deoxy-2'-fluoro-beta-D-arabinofuranosyl)thymine) is a thymidine analog that can be phosphorylated by thymidine kinase and incorporated into DNA. This first-in-human study of [18F]FMAU was conducted as a pilot in patients to determine its biodistribution and suitability for imaging DNA synthesis in tumors using positron emission tomography (PET).

METHODS

Fourteen patients with diverse cancers (brain, prostate, colorectal, lung, and breast) were imaged with [18F]FMAU. We obtained dynamic PET images for 60 min and a whole-body image. Blood and urine samples were analyzed by high-performance liquid chromatography to measure metabolites and clearance.

RESULTS

Active tumors in the breast, brain, lung and prostate were clearly visualized with standardized uptake values (SUVs) of 2.19, 1.28, 2.21, and 2.27-4.42, respectively. Unlike with other tracers of proliferation, low uptake of [18F]FMAU was seen in the normal bone marrow (SUV(mean) 0.7), allowing visualization of metastatic prostate cancer (SUV 3.07). Low background was also observed in the brain, pelvis, and thorax, aside from heart uptake (SUV 3.36-8.78). In the abdomen, increased physiological uptake was seen in the liver (SUV 10.07-20.88) and kidneys (SUV 7.18-15.66) due to metabolism and/or excretion, but the urinary bladder was barely visible (SUV(mean) 2.03). On average, 95% of the activity in the blood was cleared within 10 min post injection and an average of 70% of the activity in the urine was intact FMAU at 60 min post injection.

CONCLUSION

Tumors in the brain, prostate, thorax, and bone can be clearly visualized with FMAU. In the upper abdomen, visualization is limited by the physiological uptake by the liver and kidneys.