Distribution and Elimination of the Somatostatin Analogue (¹¹¹In-DTPA-D-Phe¹)-Octreotide (OctreoScan111)

Abstract

The distribution and elimination characteristics of the ¹¹¹In-labelled somatostatin analogue OctreoScan111 were studied in 23 patients with malignant tumours. The substance exhibited a rapid blood elimination following a bi-phasic pattern. The initial part of the elimination curves showed a t 1/2^a of between 0.27 and 3.6 h. The patients investigated had creatinine clearance rates ranging from 33 to 124 ml/min. However, within this range, no apparent correlation was found between the OctreoScan111 elimination rate and kidney function. Also no correlation was observed between the amount of administered activity and the elimination rate of OctreoScan111. The serum radioactivity of 6 patients was analyzed with respect to molecular size. These experiments showed that OctreoScan111 circulated unbound in serum. About 3% of the radioactivity, most probably representing ¹¹¹In-chloride of DTPA-¹¹¹In-chloride, circulated protein-bound. The elimination of OctreoScan111 radioactivity in urine displayed a bi-phasic pattern. Size separation of the radioactivity appearing in the urine after 24 h showed a higher molecular weight when compared with OctreoScanlll, indicating the existence of a metabolite of the injected substance. The results obtained are discussed in the light of a potential role for the substance in systemic radiotherapy.