Preincubation with Sn-complexes causes intensive intracellular retention of ^99mTc in thyroid cells in vitro

Summary

Technetium radiopharmaceuticals are well established in nuclear medicine. Besides its well-known gamma radiation, ^99mTc emits an average of five Auger and internal conversion electrons per decay. The biological toxicity of these low-energy, high-LET (linear energy transfer) emissions is a controversial subject. One aim of this study was to estimate in a cell model how much ^99mTc can be present in exposed cells and which radiobiological effects could be estimated in ^99mTc-overloaded cells. Methods: Sodium iodine symporter (NIS)- positive thyroid cells were used. ^99mTc-uptake studies were performed after preincubation with a non-radioactive (cold) stannous pyro - phosphate kit solution or as a standard ^99mTc pyrophosphate kit preparation or with pure pertechnetate solution. Survival curves were analyzed from colony-forming assays. Results: Preincubation with stannous complexes causes irreversible intracellular radioactivity retention of ^99mTc and is followed by further pertechnetate influx to an unexpectedly high ^99mTc level. The uptake of ^99mTc pertechnetate in NIS-positive cells can be modified using stannous pyrophosphate from 3–5% to >80%. The maximum possible cellular uptake of ^99mTc was 90 Bq/cell. Compared with nearly pure extracellular irradiation from routine ^99mTc complexes, cell survival was reduced by 3–4 orders of magnitude after preincubation with stannous pyrophosphate. Conclusions: Intra cellular ^99mTc retention is related to reduced survival, which is most likely mediated by the emission of low-energy electrons. Our findings show that the described experiments constitute a simple and useful in vitro model for radiobiological investigations in a cell model.

Zusammenfassung

Technetiumradiopharmaka sind in der Nuklearmedizin fest etabliert. Neben der bekannten Gammastrahlung emittiert ^99mTc fünf Augerund Konversionselektronen pro Zerfall. Die biologische Toxizität dieser niederenergetischen aber hochwirksamen Strahlung mit hohem linearem Energietransfer wird kontrovers diskutiert. Ein Ziel unserer Untersuchungen war es, in einem Zellmodell zu untersuchen, wie viel ^99mTc sich in diesen Zellen anreichern lässt und welche strahlenbiologischen Effekte damit zu erreichen sind. Methoden: Natrium- Symporter(NIS)-positive adhärent wachsende Zellen wurden verwendet. Die Untersuchungen erfolgten nach Präinkubation der Zellen mit „kaltem“ Zinnpyrophosphat und anschließender Gabe von ^99mTc-Pertechnetat, mit ^99mTc markiertem „heißem“ Pyrophosphat oder mit ^99mTc Pertechnetat allein. Nach Analyse mit dem Koloniebildungsassay ergaben sich für die drei Präparate Überlebenskurven. Ergebnisse: Die Präinkubation mit Zinn- (II)-komplexen wurde durch 113Sn kontrolliert und bewirkte die irreversible intrazelluläre ^99mTc-Retention, gefolgt von weiterem Einstrom von ^99mTc Pertechnetat bis zu einem unerwartet hohen intrazellulären ^99mTc-Gehalt. Die Aufnahme von ^99mTc in NIS-positive Zellen konnte so von 3–5% auf >80% im Untersuchungszeitraum von vier Stunden gesteigert werden. Die maximal erreichte Aktivität in einer Zelle betrug 90 Bq. Verglichen mit einer rein extrazellulären Bestrahlung durch in der nuklearmedizinischen Diagnostik übliche ^99mTc-Komplexe (z. B. MDP, DTPA), die nicht in die Zellen aufgenommen werden, reduzierte sich das Zellüberleben nach Präinkubation mit Zinnpyrophosphat um 3–4 Größenordnungen. Schlussfolgerungen: Zunehmende intra zelluläre ^99mTc-Retention korrelierte mit reduziertem Zellüberleben. Dies wird auf niederenergetische Elektronen kurzer Reichweite im Nanometerbereich zurückgeführt. Mit diesem einfachen In-vitro-Modell für radiobiologische Untersuchungen können ^99mTc- Aufnahme und -Retention in Zellkulturen beeinflusst und zukünftig weitere radiobiologische Daten erhoben werden.

Keywords

^99mTc - pyrophosphate - stannous chloride - PCCl3 - colony-forming assay - Auger electrons

Schlüsselwörter

^99mTc - Pyrophosphat - Zinnchlorid - PCCl3 - Kolonie bildung - Auger-Elektronen

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