Biodistribution, tumor detection and radiation dosimetry of F-18 5-Fluoro-2'-Deoxycytidine ( 18 F-FdCyd) with Tetrahydrouridine in solid tumors.

In pre-clinical studies, 5-fluoro-2'-deoxycytidine (FdCyd), an inhibitor of DNA methyltransferase and DNA hypermethylation, has shown treatment efficacy against multiple malignancies by suppressing epigenetic hypermethylation in tumor cells. Several clinical trials are undergoing using FdCyd and while some patients may respond to this drug, in the majority of patients it is ineffective. Thus, by establishing a non-invasive imaging modality to evaluate the distribution of the drug may provide insight into the variable responses. Objective: A novel experimental radiopharmaceutical, Fluorine-18 labeled FdCyd (18 F-FdCyd), was developed as a companion imaging agent to the non-radioactive form of the drug, FdCyd. We present the first-in human radiation dosimetry results and biodistribution of 18 F-FdCyd, administered along with tetrahydrouridine, an inhibitor of cytidine/deoxycytidine deaminase, in patients with a variety of solid tumors undergoing FdCyd therapy. Methods: This phase 0 imaging trial examined the 18 F-FdCyd biodistribution and radiation dosimetry in 5 human subjects enrolled in companion therapy trials. In each subject, four sequential PET scans were acquired to estimate whole body and individual organ effective dose, using OLINDA/EXM v1.0. Tumor to background (T:B) ratios were also calculated for the tumor sites visualized on PET/CT imaging. Results: Average whole body effective dose for the experimental radiopharmaceutical 18 F-FdCyd administered in conjunction with tetrahydrouridine was 2.12E-02 ± 4.15E-03 mSv/MBq. This is similar to the radiation dose estimates for 18 F-FDG PET. The critical organ, with the highest absorbed radiation dose was the urinary bladder wall at 7.96E-02 mSv/MBq. Other organ doses of note were the liver (6.02E-02mSv/MBq), kidneys (5.26E-02 mSv/MBq), and gallbladder (4.05E-02 mSv/MBq). Tumor target to background ratios ranged from 2.4 to 1.4, which potentially enable tumor visualization in static PET images. Conclusion: This phase 0 imaging clinical trial provides evidence that 18 F-FdCyd administered in conjunction with tetrahydrouridine yields acceptable individual organ and whole body effective doses, as well as modest T:B ratios that potentially enable tumor visualization. Dose estimates for 18 F-FdCyd are comparable to those of other PET radiopharmaceuticals such as 18 F-FDG. Further studies with larger study population are warranted to assess 18 F-FdCyd imaging as a predictor of FdCyd treatment effectiveness.