Radiopharmaceuticals
Tc 99m IDA (Iminodiacetic acid) analogs:
Tc-iminodiacetic acid derivatives are organic anions. These agents predominantly bind to albumin in the circulation and are carried to the liver. Protein binding helps to decrease renal excretion. Dissociation of the tracer from albumin takes place at the space of Disse and the tracer enters the anion exchange pathway of bilirubin. Since the compounds share the same hepatic uptake and excretion pathways, IDA compounds are subject to competitive inhibition by bilirubin. IDA compounds are taken up by hepatocytes and are NOT conjugated prior to their excretion (they are excreted unchanged). In general, increasing the length of the alkyl chain substitution on the benzene ring of IDA, will increase the biliary excretion of the agent and decrease the renal clearence. This is because substitutions of non-polar groups on the phenyl ring make the molecule more lipophilic and the more lipophilic a compound, the greater its protein binding. (Protein binding prevents renal excretion and promotes hepatic uptake)
HIDA: dimethyl
85% excreted by the liver, 15% by the kidneys
Good visualization at bili levels of 5-7 mg/dl
discontinued
DISIDA: diisopropyl (Disofenin)
Approximately 88% hepatic excretion, 10% renal
Good visualization at bilirubin levels approaching 20
BRIDA: (Mebrofenin)
An analog of lidocaine m-3-bromo-trimethyl-IDA, has greatest resistance to displacement by bilirubin. There is about 98% hepatic excretion of the agent.
T1/2 of uptake is about 6 minutes, and the T1/2 of excretion is about 14 minutes.
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