Comparative kinetic studies of simultaneously injected 167Tm- and 67Ga-Citrate in Normal and Tumour bearing mice

International Journal of Applied Radiation and Isotopes. Vol. _'29.pp. 673,681 Pergamon Press Ltd.. 1978. Printed in Great Britain

Comparative Kinetic Studies of Simultaneously Injected 167Tm" and 67Ga_Citrate in Normal and Tumour Bearing Mice G.-J. BEYER, W.-G. F R A N K E , * K. H E N N I G , * B. A. J O H A N N S E N , * V. A. K H A L K I N , I " M. K R E T Z S C H M A R , N. A. LEBEDEV,I" R. M L I N Z E , A. F. N O V G O R O D O V ' f a n d K. T H I E M E Department Radiochemistry, Central Institute for Nuclear Research Rossendorf, 8051 Dresden, PF 19, G.D.R. (Received 28 November 1977; in revised form 3 April 1978)

Thulium-167 was produced by spallation reactions during irradiation of tantalum targets with 680 MeV protons in the Dubna synchrocyciotron. After the chromatographic separation of simultaneously produced radiolanthanides, thulium was available carrier-free and with high radiochemical purity. The kinetics of intravenously injected thulium-citrate were studied in comparison with simultaneously applied gallium-67-citrate in normal and tumour-bearing mice. Scans were performed in mice and rabbits for the visual recording. The very fast clearance from blood of thulium-167 and its good storage in tumour may be considered as important results of the present study. This behaviour of the radiopharmaceutical was demonstrated by an ascent of the tumour to blood ratio of thulium-167 to the tenfold of the initial value whereas the corresponding ratio for 67-gallium is constant within 76 h, remaining at the level of the initial value. According to the kinetic of thulium-167, the tumour can be scanned already 4-6 h after the application of the tracer.

INTRODUCTION THE FURTHERinvestigation of radioactive lanthanides regarding their tumour affinity was motivated by promising results in positive tumour scanning with '69yb-citrate on the one hand and by unfavourable decay properties of this radionuclide on the other hand (~. 167Tm seemed to be suited especially for this purpose. According to preliminary investigations by ANDO et al. (z" 3i an accumulation of a carrier-free t67Tm-preparation in Yoshida-sarcomas was observed. These authors produced ~67Tm by (),, n) reactions during irradiation of great amounts of natural ytterbium oxide or enriched t69yb20 3 target material. Chemical separations of the daughter 167Tm from the produced parent ~67Yb were time-consuming and extremely difficult. They obtained 500/~Ci t67Tm after a 3-day chemical separation. The product was denoted carrier-free. EXPERIMENTAL Thulium-167 was produced by spallation-reaction with high energy protons on tantalum targets (target * Department of Nuclear Medicine, Medical Academy "Carl Gustav Carus" Dresden, 8019 Dresden, G.D.R. t Laboratory for Nuclear Problems, Joint Institute for Nuclear Research Dubna, Moscow POB 79, U.S.S.R.

weight approx. 5 g). The Ta-targets were irradiated at the internal proton-beam of the Dubna synchrocyclotron (Ep = 680MeV, I v = 2.3gA, irradiation time 5-10 h). Thulium-167 together with all the other generated neutron-deficient isotopes of the rare earth elements were separated from the target material by coprecipitation as fluorides from a HF-HNO3-target solution. After washing the precipitate and dissolving in HBOa-HCl-mixtures, the separation of all the rare earths was carried out at small cation exchange columns filled with 20gm Dowex 50 x 8 resin in the N H ~ ion form using ~-hydroxyisobutyric acid as an elution agent. The Tm-fraction was purified by a further chromatographic process. The Tm-preparation in a concentrated form of a few drops volume was evaporated to dryness and the residue was dissolved in 2 drops of 0.01 M HCI. This preparation was carried to Dresden by plane. By adding the calculated amounts of isotonic Na-citrate solution (2-5 mi), the final Tin-citrate solution with a radiochemical concentration of 2 mCi/ml was obtained. Two weeks after the end of the bombardment (EOB k the contamination by the by-product 165Tm (T1/2 = 30h) in the preparation was negligible. The ~67Tm-citrate preparation was carrier-free. The isotopic purity was greater than that of other preparations produced by irradiation of enriched Er with

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isochroncyclotrons~s~. At present we are able to produce the activity of approximately 1.0mCi//~Ah (EOB). The radionuclide emits 208 keV ),-radiation with a quantum yield of about 50%. The physical half-life of 9.25 days is adequate to study general kinetic behaviour of tumour-aflinitive materials. We produced 67Ga by Zn(d, xn)67Ga reactions at the cyclotron U-120 of the Central Institute for Nuclear Research Rossendoff. t6~ In order to test the aptitude of 167Tm, we compared its kinetic behaviour with that of 67Ga, which at present is regarded as the most suitable tumour scanning agent. Our measuring system consisting of Ge(Li) detector, 4096-channel-ADC, memory, and computation programmes, allowedto determine simultaneously activities of ~TTm and 67Ga in mixtures. Using this technique the number of tests was extremely reduced. The mice employed were of the XVII/BIn. strain, part of them bearing a virus induced mammary tumour (carcinoma solidum medullare partim adenomatosum). Radioactivities in the order of about 50/~Ci 167Tmcitrate and 50/~Ci ~TGa-citrate were injected simultaneously in the tail vein. After taking several organs, tumour tissue, blood, and urine, we determined activity concentrations and total activities in the samples depending on time after intravenous injections. Furthermore, whole body retentions were determined and scintigraphic studies in mice and rabbits were performed. For scanning, we injected about 50/~Ci ~67Tm-citrate or about 200/~Ci 67Ga-citrate in mice and 300/~Ci ~67Tm-citrate in rabbits. RESULTS

AND DISCUSSION

Previous kinetic studies performed in Wistar-rats after simultaneous i.v, injection of 4 radiolanthanides (lSSTb, 167Tm, 169yb, 171Lu) allowed us to assume that thulium exhibits some advantages in its behaviour in comparison with heavier and lighter rare earth elements <~. As listed in Table 1, ~67Tm showed a faster urinary excretion and a lower liver uptake. Time course curves of activity concentrations and total activities in organs ag well as the ratios of the activities of ~6~Tm to 67Ga after the correction of decay demonstrate (Fig. 1): 167Tm was accumulated in the tumour but in the beginning with a 2.5 times lower intensity than 67Ga was. However, t67Tm showed a delayed and small excretion so that concentrations of both radionuclides were similar after 3 days (Fig. la). Thus we obtained good scintigraphic images of the tumour with both nuclides not only 4 h but also 24 h p.i. (Fig. 2). The slower blood clearance of 76Ga (Fig. lb) and the greater 67Ga accumulation in liver and gastrointestinal tract (Fig. lc, d) were the reasons for obtaining satisfactory scintigrams only 2-4 days after administration/ 8~ At this time, the 6~Ga contents in

tumour tissue amounted only 1/4-1/5 of the initial contents. The kinetics of 167Tm in organs pointed out the possibility of performing scintigraphic studies relatively soon after injection (4-6 h) and that later no further essential improvement of the contrast is to be expected. Five hours after administration, the contents of 167Tm in the tumour was about 1.5-2 times greater than the contents of 67Ga 50h after injeciion. These results were confirmed by scanning the mice (Fig. 2). The variation of activity with time in the tumour and the very fast blood clearance caused a rapid increase of the tumour-to-blood ratios of t67Tm activity by more than one order of magnitude in respect to the initial ratio, whereas in the case of 67Ga no significant change of these ratios was observed during 76 h (Fig. 3). It is supposed that the rapid blood clearance of t67Tm corresponded with a fast accumulation of a greater portion of the administered material in skeletons and a partial urinary excretion (Fig. le). This assertion was verified by the course of the whole body retention (Fig. 4). Approximately 409"o of administered 167Tm activity were excreted during 24h. After this time, the rate of excretion was only very small and not detectable any more by whole body counting because the effective half-life was nearly equal to the physical half-life. Gallium-67 showed an essential higher rate of excretion and was eliminated with a biological half-life of about 15 h. The parts of 67Ga and 167Tm accumulated in skeleton were not removed (Fig. le). Gallium-67 also showed an approximately parallel uptake into bones, but values of uptake for ~67Tm were much greater and reached nearly 30~o of the injected activity for the whole skeleton. The maxima of the intrahepatic accumulation were almost the same for both the radionuclides. Tumour-bearing mice accumulated more 67Ga in the liver than normal mice so that the ratios of the uptake of 167Tm to 67Ga differed between the two groups (Fig. lc). Accordingto the somewhat different excretion of both the nuclides from the liver also, ttVTm activity was removed faster than 67Ga activity from the gastrointestinal tract (Fig. ld). Contrary to 67Ga, we observed a prolonged intrarenal retention of t67Tm (Fig. If). Scans reflected this kinetic behaviour (Figs. 2, 5): a long lasting representation of the whole skeleton after administration of 167Tm and an accumulation of 67Ga in the entire abdomen. Considering our present results, it can be stated that the investigated mammary carcinoma as well as the already reported 5123C Miiusehepatom c9~ and Yoshida-sarcoma~2~ must be regarded as ttVTm accumulating tumour tissue. The kinetic of intratumoural uptake we found in this study was similar to the results obtained in rats with Yoshida-sarcoma. Also the time-dependent uptake in bone, liver, and kidney were in good agreement during the observed period.

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Kinetic studies in normal and tumour bearing mice 4. KHALKIN V. A., HERMANN E. and MOLNAR F. Atomizdat 4, 1044 (1973). 5. SCHOLZ K. L., SODO V. J. and BLUE J.-W. Int. J. appl. Radiat. Isotopes 27, 263 (1976). 6. PETZOLD O. and BEYER G.-J. Jahresbericht 1976, ZfK-312, (Edited by Mi)NZE R.), p. 69 (1976).

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7. BEYERG.-J., KRETZSCHMAR M., MigNZE R. and THIEME K. Jahresbericht 1976. ZfK-312 (Edited by MiJNZE R.), p. 165 (1976). 8. JOHNSTON G. S. and JONES A. E. (Eds). Atlas o.I" Gallium-67, Scintigraphy. Plenum Press, New York (1973). 9. HAYES R. l., BROWN D. H., BYRD B. L. and CARLTON J. E. J. nucl. Med. 15, 526 (1975).