The Metabolism and Fate of Tritiated Thymidine in Man *†

Abstract

The incorporation of thymidine (thymine deoxyriboside) into deoxyribonucleic acid (DNA) of proliferating cells has been studied in the past with various isotopic labels and biochemical techniques. Reichard and Estborn showed that thymidine-N15 was a specific precursor of DNA thymine in rats (1). Friedkin, Tilson, Roberts and Wood (2, 3) used thymidine-2-CI4 to show that this compound was incorporated into DNA thymidine of proliferating embryo and animal tissues with negligible partition of radioactivity into other components of DNA or into ribonucleic acid.The recent synthesis of tritium-labeled thymidine by Taylor, Woods and Hughes (4) and in vitro by serial autoradiographic evaluation of labeled cell populations (6-9). In the course of these in vivo investigations it became important to assess the availability time of tritiated thymidine for incorporation into DNA following rapid intravenous injection. It is essential for serial study of proliferating cell populations that the H3-thymidine label be available for incorporation into new DNA for only a small part of the DNA synthesis time, and that afterward there be no further label availability. This short availability time of H3-thymidine then permits study of the various components of the generation cycles (preand postmitotic rests, mitosis and DNA synthesis) by serial study of the appearance of labeled mitotic figures, the appearance of labeled nondividing cells and the decrease of labeling upon subsequent successive mitoses. From these data a kinetic analysis should determine the flow rate between compartments and the time spent in each state (8,9).The metabolic studies reported here indicate that the effective availability time for intravenously administered H3-thymidine is a matter of minutes. The metabolic fate of this tritium-labeled compound was examined by analyses of serial specimens of plasma, stool and urine as well as by evaluation of autoradiograms of tissues. These studies revealed that catabolism of some H3-thymidine to tritiated water (THO) as well as to nonvolatile tritiated compounds occurs. From the data to be presented here it appears that at least 50 per cent of injected H3-thymidine was retained in vivo and was presumably incorporated into new DNA of proliferating cells.

AN AUTORADIOGRAPHIC AND HISTOLOGIC STUDY OF SPLEEN WHITE PULP GERMINAL CENTERS DURING EARLY INTERVALS OF THE PRIMARY IMMUNE RESPONSE (thesis)

“…Although the nuclear-cytoplasmic ratio of these cells appeared not to have changed, due to the morphologic alteration these cells were considered intermediary between a C or dispersed germinal center cells and immature cells of the plasma cell family. Rubini et al (1958) have studied the plasma and body clearance 3 of H -thymidine in man. The plasma is almost completely cleared of 3 H -thymidine in thirty minutes.…”

Is there a role for thymidine in cancer chemotherapy?

“…After rapid injection of tracer dose_s of 3H-labeled TdR to human subjects, a ve~ rapid plasma clearance has been reported with a T~a of 1-2 rain and ~ ofapproximately 25 rain (91). Nlost ofthe injected TdR appeared to be metabolized to thymine, which was further broken down in the liver to fl-aminoisobutyfic acid and COz (26).…”

“…Nlost ofthe injected TdR appeared to be metabolized to thymine, which was further broken down in the liver to fl-aminoisobutyfic acid and COz (26). Between 30 and 80% of the radioactivity was recovered from the urine as ~H-water with only a small amount of aH-fl-aminoisobutyric acid (91). Within one min of injection, 3H-TdR appeared to be incorporated into newly formed DNA ofprollferadng cells (91).…”

“…However, when exposure is maintained for long perils, TdR is also cytotoxic (30). TdR has been used for a number of yea~ in the tHtiated form for cytoldnetie and pharmacokinedc studies in m2ro as well ~ in human subjec~ without noticeable toxicity (91). In the early fifti~ TdR w~ introdue~ as a therapeutic agent for the treatment of megaloblasfic anemias, based on the assumption that in vitamin Bl2 defidency the conversion of dUMP to dTMP was impaired (59, 99).…”

Excretion Pattern of Labelled Spermatozoa and the Timing of Spermatozoa Formation and Epididymal Transit in Rabbits Injected With Thymidine-3h

“…The problem of pre¬ cursor specificity can be overcome by use of adenine-8-14C (Foote & KoefoedJohnsen, 1959) or especially thymidine-3H (Sirlin, 1958; Hupp, Austin & Murphree, 1961). Rubini, Cronkite, Bond & Fliedner (1960) found that all nuclear labelling occurred in less than 60 min following thymidine-3H in¬ jection and that by 90 (Sirlin & Edwards, 1958;Foote, 1959).…”