Bone uptake during 99MTc-rTPA imaging studies - who is the trouble maker?

Full text: Extensive bone uptake was observed in patients administered with 99mTc-labelled rTPA (recombinent tissue plasminogen activator) for detecting deep venous thrombosis. The present study was aimed at identifying the trouble maker. rTPA was prepared in-house by the method described by Butler (JNM, 37 (5), 744-748,1996). The final preparation was stored frozen at -20 deg C for 2-10 weeks. The freshly prepared rTPA samples, when stored for less than 2 weeks seem to give normal biodistribution with unimpressive bone uptake, but when stored for more than 8 weeks showed significant bone uptake. It was hypothesised that free phosphates produced during storage might lead to bone uptake. Since the composition of the preparation contains 114mg of arginine phosphate, we analysed the samples by spectro-photometry. Freshly prepared samples, which were stored for less than 2 weeks or the freeze-dried samples seem to contain intact arginine phosphate and negligible amounts of free phosphates. Samples stored more than 8 weeks and preparations from patients with bone uptake revealed that there was virtually no arginine phosphate present but indicated free phosphates and arginine in the samples. Thus our hypothesis was supported by the available evidence that high levels of phosphates are probably be responsible for elevated bone uptake in these patients. When rTPA is prepared, it is highly recommended to freeze-dry the samples than storing them at -20 deg C, to prevent hydrolysis of the arginine phosphate. Copyright (2003) The Australian and New Zealand Society of Nuclear Medicine Inc