Efficacy of chimeric molecules directed towards multiple somatostatin and dopamine receptors on inhibition of GH and prolactin secretion from GH-secreting pituitary adenomas classified as partially responsive to somatostatin analog therapy

Objective: This study compared the potency of a somatostatin receptor (sstr)2-sstr5 analog, BIM-23244, of an sstr2-dopamine D2 receptor (sstr2-DAD2) molecule, BIM-23A387 and of new somatostatin-dopamine chimeric molecules with differing, enhanced affinities for sstr2, sstr5 and DAD2, BIM-23A758, BIM-23A760 and BIM-23A761, to suppress GH and prolactin (PRL) from 18 human GH adenomas that are partially responsive to octreotice or lanreotide. Materials and methods: The sstr2, sstr5 and DAD2 mRNA levels were determined by RT-PCR. The effect of drugs was tested in cell cultures at various concentrations. Results: In all tumors, the sstr2, sstr5 and DAD2 mRNA levels were coexpressed (mean levels +/- S.E.M. 0.4 +/- 0.1, 5.3 +/- 1.9 and 2.0 +/- 0.4 copy/copy beta-glucuronidase). In 13 tumors, the maximal suppression of GH secretion produced by BIM-23A387 (30 +/- 3%) and BIM-23244 (28 +/- 3%) was greater than that produced by octreotide (23 +/- 3%). In six out of 13 tumors, BIM-23A758, BIM-23A760 and BIM-23A761 produced greater maximal suppression of GH secretion than octreotide (33 +/- 5, 38 +/- 2 and 41 +/- 2 vs 24 +/- 2%). Their EC50 values were 10, 2 and 4 pmol/l. BIM-23A761 was more effective than BIM-23A387 in GH suppression (41 2 vs 32 +/- 4%). The new chimeric molecules produced maximal PRL suppression greater than octreotide (62 +/- 8 to 74 +/- 5 vs 46 +/- 11%). Conclusions: Novel dopamine-somatostatin chimeric molecules with differing, enhanced activity at sstr2, sstr5 and DAD2, consistently produced significatly greater suppression of GH and PRL than either octreotide or single-receptor-interacting ligands in tumors from patients classified as only partially responsive to octreotide therapy. The higher efficacy of the chimeric compounds was, at least partially, linked to their high affinity for sstr2 (IC50 1-10 pmol/l). The other mechanisms by which such molecules produce an enhanced inhibition of GH remain to be elucidated.