There has been accumulated reports (1, 3) in various aspects of diagnosis of the malignant tumor by means of radioactive isotopes, most of which has gone so far as to get an indirect negative delineation, while it is more reasonable to get a positive delineation through specific uptake of radio-isotopes by the malignant tumor per se. To meet the demand of this problem, a number of investigations (4-10, 45) have recently been reported, and yet it still poses problems to seek a radioactive compound a cancer tissue can uptake specifically.
In view of the fact that the halogenated uracil derivatives are closely correlated with metabolism of cancer tissues, Park and Chin (11) put forward a possibility, by studying distribution, absorption, and excretion rate in vitro of 5-bromouracil-82Br in the tissues of tumor-bearing animals, that tumor localization could be visualized by macroautoradiography of the radio-compound.
Uracil, a precursor of the ribonucleic acid biosynthesis, is highly utilized by the malignant tumor tissue (12, 13), and the same is also of true in the rapidly proliferating epithelial cell of normal gastrointestinal tract and bone marrow, which underlies that the uracil utilization may well be said as not necessarily specific in the cancer tissue.
Since it has been brought into light, however, that 3-halogenated pyrimidine derivatives display an antagonistic role to cellular metabolism, attention of many workers has been focussed on their affinity as well as antagonism to the cancer cell (14, 17). Reports have been made on the potential merit of ^(131)I-labelled pyrimk ine derivatives in the cancer diagnosis (18-20). Being more specific on the problem, the rate of 5-fluoro-uracil uptake in the tumor-transplanted animal is more marked in the tumor cell than any of the host cell (21-26), while, on the other hand, the rate of iodinated uracil uptake is reported to exert less specificity (27). But it is recognized that 5-iodo -2-deoxyuridine inhibits the growth of tumor so strongly that the iodinated compound of pyrimidines deserve further investigations with regard to their diagnostic significances.
Strong as ^(18)F is, with regard to its affinity to the cancer tissue, it carries experimental difficulties because of its too short a half life of 112 min, and for the same reason ^(82)Br is not quite convenient to handle for the sake of animal experiments due to its rather short half-life of 35.9 hr. ^(131)I, on the other hand, merits so much attention by virtue of its long half-life of 7.8d. and of its r-ray, emitting 0.36 MeV adequate for its energy monitoring, that the author synthesized 5-iodo-uracil-131I to inject it intravenously into the Rhodamine-B fibrosarcoma-transplanted albino rat of Wistar strain, and to scan the radioactivity distribution of the synthesized compound as a
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