Scintigraphic imaging and turnover studies with iodine-131 labelled serum amyloid P component in systemic amyloidosis

Radiolabelled serum amyloid P component (SAP) is a specific tracer for amyloid. Iodine-123 has ideal physical characteristics for scintigraphy but is expensive and not widely available. Here we report serial imaging and turnover studies in which we labelled SAP with iodine-131, a cheap alternative isotope which would be expected to yield poorer images but permit more prolonged turnover measurements. Imaging and plasma clearance and whole body retention (WBR) of tracer were studied for up to 7 days in ten patients with proven systemic AL amyloidosis and two patients in whom the diagnosis was suspected, after i.v, administration of about 37 MBq of ¹³¹I-SAP. Normal blood pool images were obtained in the latter two subjects and amyloidosis was subsequently refuted histologically. WBR at 48 h was <60% and 6-h plasma activity was > 65% of the injected dose (i.d.). Among the other ten patients, amyloid deposits were identified in the spleen in eight cases, liver in five and kidneys in four, other sites that gave positive results included bone, joints and soft tissues, and the myocardium in one case. Up to 95% of the tracer localised into amyloid within 6-h, and the values became progressively more discriminating during the study period, exceeding the normal reference value (<25%) in all cases by day 7. The optimal imaging time was found to be between 24 and 48 h. The duration of the study enabled us to measure the tracer elimination half-life which was increased in all cases by up to tenfold. Follow-up studies performed after 2-24 months in four patients who were treated with iododoxorubicin showed regression of amyloid in one patient and a small increase in one case; in the other two patients the imaging and turnover studies were identical to baseline. Despite its unfavourable imaging characteristics, ¹³¹I-SAP produced diagnostic scans in every patient in this series and, coupled with the detailed turnover information, is adequate for monitoring disease progress.