Evaluation of viability of microorganisms in ⁶⁸Ga-, 18F-, ^99mTc-, and ¹⁷⁷Lu-labeled radiopharmaceuticals

Parametric release and retrospective sterility test of various diagnostic and therapeutic radiopharmaceuticals (RPs) make it essential to rule out false negative results due to inhibition of microorganisms (MOs) mediated by ionizing radiations emitted from RPs. This study is designed to evaluate the viability of positive control MOs used in sterility test in the presence of RPs and to observe whether viability depends on ionizing radiations or chemical constituents of RPs. Labeling of ¹⁸F, ⁶⁸Ga, ¹⁷⁷Lu, and ^99mTc RPs was performed as per the standard procedures. Cell concentrations of MO (Staphylococcus aureus, Pseudomonas aeruginosa, Bacillus subtilis, and Candida albicans) were maintained at 55-65 CFU. Single patient dose in minimum volume was considered for this study (diagnostic: 15-20 mCi and therapeutic: 195-200 mCi). Two methods were adopted to study the viability of MO: in the first method, MOs were spread on plates which were prior spread with RPs and dried, and in the second method, RPs along with MO were inoculated in liquid media. Viable CFUa of MO were observed (recovery: 90%-95%) in all of our studied RPs, except (^99mTc) DTPA, (¹⁸F) FLT, and (¹⁸F) FMISO, where recovery was <50%. Luxuriant growths were observed in all the media bottles containing inoculums of studied MO and RPs. Research Highlights: Emitted radiations from RPs (single patient dose) do not decrease the viable CFU, do not affect the growth rate, and hence will not lead to false negative results in sterility test. Further, it can be concluded that viability of MO in various RPs largely depends on its chemical constituents. (author)