Localization of skeletal-imaging 99mTc chelates in dead cells in tissue culture: concise communication

Screening new agents for imaging myocardial infarcts and investigating the mechanism of cellular uptake and subcellular localization of these agents prompted us to develop a necrosis model in tissue culture. Live cells obtained from exponentially growing cultures and dead cells obtained from nutritionally depleted plateau-phase cultures were incubated with 99mTc-pyrophosphate, 99mTc-hydroxyethylene diphosphonate (99mTc-HEDP), 67Ga-citrate, and 45Ca-HEDP. For the bone-seeking 99mTc chelates, the ratio of dead to live cell radioactivity was 15--20 after 1 hr of incubation. On the other hand, no preferential binding of 67Ga-citrate was observed, indicating that the mechanism of 67Ga localization in infarcts is different from that of the 99mTc chelates. The death cells accumulated only twice as much 45Ca-HEDP or 45Ca-pyrophosphate as did live cells, possibly due to the carrier effect of calcium in the medium. The receptors for 99mTc chelates could be partially blocked by preincubating the dead cells with microgram amounts of Na-HEDP. Heat-denaturation of live cells also increased the localization of 99mTc-HEDP in the cells, and this uptake was proportional to the duration of heating. Bone-seeking 99mTc chelates could be used for marking dead cells instead of conventional staining methods.