Semiautomated radiosynthesis and biological evaluation of [18F]FEAU: a novel PET imaging agent for HSV1-tk/sr39tk reporter gene expression.

2'-deoxy-2'-[(18)F]fluoro-5-ethyl-1-beta-D-arabinofuranosyluracil ([(18)F]FEAU) is a promising radiolabeled nucleoside designed to monitor Herpes Simplex Virus Type 1 thymidine kinase (HSV1-tk) reporter gene expression with positron emission tomography (PET). However, the challenging radiosynthesis creates problems for being able to provide [(18)F]FEAU routinely. We have developed a routine method using a commercial GE TRACERlab FX-FN radiosynthesis module with customized equipment to provide [(18)F]FEAU. All radiochemical yields are decay corrected to end-of-bombardment and reported as means +/- SD. Radiofluorination (33 +/- 8%; n = 4), bromination (85 +/- 8%; n = 4), coupling reaction (83 +/- 6%; n = 4), base hydrolysis steps, and subsequent high-performance liquid chromatography purification afforded purified [(18)F]FEAU beta-anomer in 5 +/- 1% overall yield (n = 3 runs) after approximately 5.5 h and a beta/alpha-anomer ratio of 7.4. Radiochemical/chemical purities and specific activity exceeded 99% and 1.3 Ci/micromol (48 GBq/micromol), respectively. In cell culture, [(18)F]FEAU showed significantly (P < 0.05) higher accumulation in C6 cells expressing HSV1-tk/sr39tk as compared to wild-type C6 cells. Furthermore, [(18)F]FEAU showed slightly higher accumulation than 9-[4-[(18)F]fluoro-3-(hydroxymethyl)butylguanine ([(18)F]FHBG) in cells expressing HSV1-tk (P < 0.05), whereas [(18)F]FHBG showed significantly higher (P < 0.05) accumulation than [(18)F]FEAU in HSV1-sr39tk-expressing cells. micro-PET imaging of mice carrying tumor xenografts of C6 cells stably expressing HSV1-tk or HSV1-sr39tk are consistent with the cell uptake results. The [(18)F]FEAU mouse images also showed very low gastrointestinal signal with predominant renal clearance as compared to [(18)F]FHBG. The routine radiosynthesis of [(18)F]FEAU was successfully semiautomated using a commercial module along with customized equipment to provide the beta-anomer in modest yields. Although further studies are needed, early results also suggest [(18)F]FEAU is a promising PET radiotracer for monitoring HSV1-tk reporter gene expression.

Molecular imaging of EGFR kinase activity in tumors with 124I-labeled small molecular tracer and positron emission tomography.

Positron emission tomography (PET) with epidermal growth factor receptor (EGFR) kinase-specific radiolabeled tracers could provide the means for noninvasive and repetitive imaging of heterogeneity of EGFR expression and signaling activity in tumors in individual patients before and during therapy with EGFR signaling inhibitors. We developed the synthesis and (124)I-radiolabeling of the (E)-But-2-enedioic acid [4-(3-[(124)I]iodoanilino)-quinazolin-6-yl]-amide-(3-morpholin-4-yl-propyl)-amide (morpholino-[(124)I]-IPQA), which selectively, irreversibly, and covalently binds the adenosine-triphosphate-binding site to the activated (phosphorylated) EGFR kinase, but not to the inactive EGFR kinase. The latter was demonstrated using in silico modeling with crystal structures of the wild type and different gain-of-function mutants of EGFR kinases. Also, this was demonstrated by selective radiolabeling of the EGFR kinase domain with morpholino-[(131)I]-IPQA in A431 human epidermoid carcinoma cells and Western blot autoradiography. In vitro radiotracer accumulation and washout studies demonstrated a rapid accumulation and progressive retention postwashout of morpholino-[(131)I]-IPQA in A431 epidermoid carcinoma and in U87 human glioma cells genetically modified to express the EGFRvIII mutant receptor, but not in the wild-type U87MG glioma cells under serum-starved conditions. Using morpholino-[(124)I]-IPQA, we obtained noninvasive PET images of EGFR activity in A431 subcutaneous tumor xenografts, but not in subcutaneous tumor xenografts grown from K562 human chronic myeloid leukemia cells in immunocompromised rats and mice. Based on these observations, we suggest that PET imaging with morpholino-[(124)I]-IPQA should allow for identification of tumors with high EGFR kinase signaling activity, including brain tumors expressing EGFRvIII mutants and nonsmall-cell lung cancer expressing gain-of-function EGFR kinase mutants. Because of significant hepatobiliary clearance and intestinal reuptake of the morpholino-[(124)I]-IPQA, additional [(124)I]-IPQA derivatives with improved water solubility may be required to optimize the pharmacokinetics of this class of molecular imaging agents.