Labelling of thymidine with 99mTc-carbonyl and in vivo biodistribution studies

Full text: Labeled thymidine is used for tumour imaging, since it is incorporated into DNA and therefore provides a measure of cell proliferation. It is reported that high human thymidine kinase levels occur in breast carcinomas, in lung cancer and other proliferating and malignant cells. For the current study thymidine was functionalized at the C5' position of the sugar moiety with the tridentate iminodiacetic acid chelator for complexation and radiolabeling with 99mTc(I)-tricarbonyl core. The aim of this study was the labeling of thymidine analogue with 99mTc-tricarbonyl technique, and the exploration of its potential as radiopharmaceutical in vivo. The preparation of the 99mTc-precursor consisted in flushing the mixture of 4.4 mg of sodium carbonate, 5.5 mg of sodium borohydride and 20 mg of sodium-potassium tartrate tetrahydrate with CO gas during 30 min. Pertechnetate was added and the vial was heated for 35 min at 75 deg. C. The reaction was stopped in ice bath, and pH was adjusted to 7. Then 50 μL of the precursor was added to 10-4 M of the ligand and heated again for 50 min at 75 deg. C. Radichemical purity of the precursor and the product was checked by HPLC, TLC and paper chromatography. Biodistribution studies were performed in normal Swiss mice at 1, 4 and 24 h post injection of the drug and also in breast-cancer-bearing Sprague-Dawley rats (induced by dimethylbenzaanthracene), 1h after administration of the drug. Results: Yield of the 99mTc(CO)3-thymidine complex was 98.3 ± 0.8%. Radiochemical purity of the product was 97.3 ± 0.4%. Biodistribution studies showed the highest uptake by intestine, followed by liver and kidneys. A low amount of radioactivity was observed in the blood 1 hour post-injection (0.1 %ID/mL). Biodistribution studies in tumour-bearing rats showed a higher uptake in the kidney than in the liver, probably because these animals were anesthetized. In these animals it was observed that 0.32 ± 0.05 % ID/g remained in the blood 1 hour post-injection. Tumour/blood and tumour/muscle ratios were 0.56 and 1.38, respectively. Uptake by the tumour was 0.18 ± 0.2 %ID/g. Preparation of the 99mTc-precursor and labeling of thymidine were achieved with very high yield. Uptake by the breast-tumour-bearing rats was low. Other tumour models and labeling techniques should be used to permit better results. (author)