A study on nitroimidazole-99mTc(CO)3 complexes as hypoxia marker: Some observations towards possible improvement in in vivo efficacy

IntroductionHypoxia plays a negative role in the clinical management of cancer. Detection of hypoxic status of a cancer is important for selecting patients for hypoxia directed therapy. Though [18 F]fluoromisonidazole ([18 F]FMISO), a PET radiopharmaceutical, is presently being used in the clinic for the detection of hypoxia, considering the logistical advantages of 99mTc and wider availability of SPECT scanners, a radiopharmaceutical based on this isotope may find wider applicability.MethodsNine nitroimidazole (2-, 4- and 5-nitroimidazole) ligands were synthesized and radiolabeled using [99mTc(CO)3(H2O)3]+ precursor to obtain a group of complexes possessing different single electron reduction potential (SERP), overall charge and lipophilicity, the three attributes which decide the efficacy of the complex to detect hypoxic cells in vivo. The nitroimidazole-99mTc(CO)3 complexes as well as [18 F]FMISO were evaluated in fibrosarcoma tumor bearing mice.ResultsThe 99mTc(CO)3 complexes of nitroimidazole iminodiacetic acid (IDA) showed better tumor uptake and retention than nitroimidazole diethylenetriamine (DETA) and nitroimidazole aminoethylglycine (AEG) complexes. Tumor uptake observed with [18 F]FMISO was higher than any of the nitroimidazole-IDA- 99mTc(CO)3 complexes. However, [18 F]FMISO clearance from tumor was found to be faster compared to 2-nitroimidazole-IDA-99mTc(CO)3 complex. Observed tumor uptake and retention of the radiotracers evaluated could be correlated to its blood clearance pattern and SERP.ConclusionsResults of the present study indicated that uptake of the radiotracer in tumor is closely associated with its rate of clearance from blood. The study also indicated that along with SERP, clearance of radiotracer from blood (net effect of charge and lipophilicity) is a critical factor which decides the in vivo efficacy of the hypoxia detecting radiopharmaceutical.