RESUMEN
Sodium pertechnetate (99mTcO4) and many99m Tc-products are the radiopharmaceuticals most frequently used in nuclear medicine. Using an in vitro model, we evaluated the effect of cyclophosphamide on per cent radioactivity of 99mTcO4 and methylenedi-phosphonic acid (99mTc-MDP) bound toi isolated blood elements. Blood samples were incubated with the two radiopharmaceuticals, plasma and blood cells were separated and precipitated, and soluble and insoluble fractions were separated. To evaluate the effect of cyclophosphamide, blood was incubated with this drug 1h prior to the addition of the radiopharmaceuticals. The fraction of 99mTcO4 radioactivity was slightly higher in plasma (61.2 to 53.8 per cent) than in blood cells (38.8 to 46.2 per cent) up to 6 h and cyclophosphamide did not interfere with this distribution. The amount of 99mTc-mdp radioactivity was higher in plasma (91.1 to 87.2 per cent) than in blood cells 8.9 to 12.9 per cent) up to 24 h and cyclophosphamide did not modify it. The binding of 99mTcO4 to the insoluble fraction of plasma (4.9 to 6.1 per cent) was low and cyclophosphamide did not interfere with it up to 6h, but a small blockade (9.8 to 4.8 per cent) was observed at 24 h. From 3 h on, cyclophosphamide slightly inhibited 99mTcO4 binding to blood cells (23.1 to 16.6 per cent) and increased it at 24h (31.2 to 14.3 per cent). Cyclophosphamide did not alter 99mTc-MDP binding to the insoluble fraction of blood cells and slightly decreased 99mTc-MDP binding to the insoluble fraction of plasma (29.8 to 23.6 per cent) up to 6 h. The effect of cyclophosphamide was strongest at 24 h, with decreased radioactivity binding to the insoluble fraction of plasma (47.6 to 27.0 per cent) and blood cells (51.2 to 23.2 per cent). The fact that cyclophosphamide can bind to plasma proteins and/or cross the cell membrne explains in part the results obtained. Studies using other chemotherapeutic drugs may lead to the development of an in vitro model for the evaluation of drug interaction with radiopharmaceutical substances