99mTc-citrate versus 67Ga-citrate for the scintigraphic visualization of inflammatory lesions

Abstract

Citric acid was labeled with 99mTc with an efficiency of > 99%. The biodistribution of 99mTc-citrate was studied in mice with turpentine-induced abscesses in comparison to 67Ga-citrate. The max. abscess muscle concentration ratios were 4.61 ± 1.92 (3 h) for 99mTc-citrate and 4.76 ± 2.04 (4h) for 67Ga-citrate. Arthritis was induced in 10 rabbits by intra-articular injection of ovalbumin Scintigrams obtained 4 days later and at 3 h post-injection of 99mTc-citrate showed increased activity involving the synovium. The max. arthritic contralateral knee ratio was 3.19 ± 1.29 (3 h) and 6.47 ± 3.71 (24 h) for 99mTc- and 67Ga-citrate, respectively. The blood clearance curve of 99mTc-citrate in rabbits was biexponential with a fast ( T 1 2 = 36 min ) and a slow ( T 1 2 = 18 h ) component, compared to mono-exponential clearance of 67Ga-citrate ( T 1 2 = 23 h ). In 10 patients with rheumatoid arthritis whole-body scintigrams and spot images of involved joints indicated localization of the tracer in inflamed tissues. The mean target-to-soft tissue ratios were 3.04 ± 0.81 and 4.95 ± 2.56 for 99mTc-citrate and 99mTc-MDP, respectively. Renal clearance of radioactivity was evident from the scintigrams. Our results demonstrated that 99mTc-citrate is effective as a radiopharmaceutical for the visualization of inflammatory lesions and may be preferred to 67Ga-citrate due to the ideal physical characteristics of the radionuclide, easy preparation, low cost, early accumulation and the preference for the renal route of excretion.