Purpose: No-carrier-added (nca) MIBG is primarily associated with specific uptake (i.e. uptake-1 mechanism). We evaluated the hypothesis that nca MIBG will be less influenced by changes in extra-neuronal uptake (i.e. uptake-2 mechanism) compared with carrier-added (ca) MIBG.
Methods: No-carrier-added MIBG was compared with ca MIBG of two different manufacturers (ca MIBG-1 and ca MIBG-2, with a specific activity of 200 Mq/mumol and 40 MBq/mumol MIBG respectively) in rats (n=6 per group): controls, blocking uptake-1 (desipramine) and blocking uptake-2 (phenoxybenzamine hydrochloride). Dedicated pinhole SPECT was performed 2 h after injection of the radiotracer. After SPECT, biodistribution was assessed [% injected dose per gram tissue (%ID)].
Results: No-carrier-added MIBG had the highest absolute cardiac uptake. Although a clear trend was observed, nca MIBG was not statistically significantly different from ca MIBG-1 (0.31+/-0.05 %ID vs 0.25+/-0.01 %ID,p=0.05). Blocking uptake-1 resulted in a significant decrease in absolute cardiac uptake only for nca MIBG (0.22+/-0.03 %ID,p=0.004). Blocking uptake-2 resulted in a significant reduction in ca MIBG-1 cardiac uptake (0.14+/-0.02 %ID,p=0.0001), but not in the cardiac uptake of nca MIBG or MIBG-2. SPECT showed the highest relative cardiac uptake for nca MIBG. Poor contrast between myocardium and surrounding tissue hampered assessment of relative cardiac uptake on SPECT of both ca MIBG-1 and ca MIBG-2.
Conclusion: No-carrier-added MIBG yields a higher myocardial uptake than ca MIBG and is associated with a higher specific as well as a lower non-neuronal uptake. We therefore conclude that for the scintigraphic assessment of the myocardial sympathetic nervous system, nca MIBG is to be preferred over ca MIBG.