We studied the correlation between the presence of
somatostatin (
SRIH) receptors in GH secreting
tumors and the in vivo GH responsiveness to the SRIH analog
octreotide in 11 acromegalic patients. To test in vivo
octreotide (SMS 201-995) responsiveness, the patients were given a single dose (100 micrograms, sc) and their plasma GH levels were measured for several hours.
Somatostatin receptor content was measured by receptor autoradiography on frozen
tumor tissue sections using either a
somatostatin-28 analog or a SRIH octapeptide analog (Tyr3-octreotide) as iodinated radioligands. In eight acromegalic patients, who had complete and long lasting GH inhibition after
octreotide administration,
SRIH receptors with high affinity for
octreotide were distributed homogeneously in the
tumor tissue. Two patients had moderate, short duration, and incomplete inhibition of GH secretion after
octreotide administration, and their
tumors had very low capacity and/or affinity
SRIH receptors. A further patient who had a moderate response to
octreotide had
SRIH receptors that were nonhomogeneously distributed throughout the
tumor. These data suggest that the therapeutic efficacy of
octreotide may be related to
tumor SRIH receptor levels; the best in vivo responses to
octreotide occurred in patients whose
tumors contained a high density of homogeneously distributed receptors which had a high affinity for
octreotide.