Improved analogs and novel delivery systems for somatostatin octapeptides

Synthesis, in vitro biological activity and docking of new analogs of BIM-23052 containing unnatural amino acids

Somatostatin (SST) is an endogenous cyclic tetradecapeptide hormone that exerts multiple biological activities via a family of five receptors. BIM-23052 (DC-23-99) D-Phe-Phe-Phe-D-Trp-Lys-Thr-Phe-Thr-NH2 is a linear SST analog with established in vitro GH-inhibitory activity and high affinity to sstr5, sstr3 and sstr2. The different SSTR subtypes are expressed in different tissues and in some tumor cells. Based on this finding, a series of new analogs of BIM-23052 with expected antitumor activity have been synthesized. The Thr at position 6 in BIM-23052 was replaced by the conformationally hindered Tle, Aib, Ac5c and Ac6c of the new analogs. The peptides were synthesized by standard solid-phase peptide chemistry methods, Fmoc strategy. The cytotoxic effects of the compounds were tested in vitro against a panel of tumor cell lines: HT-29, MDA-MB-23, Hep-G2, HeLa and the normal human diploid cell line Lep-3. All five somatostatin receptor subtypes were modeled and docking was performed to determine the binding affinity of the analogs. The new peptides exhibited different concentration-dependent antiproliferative effect on the tumor cell lines after 24 h of treatment. The compound 3B (Aib⁶) demonstrated the most pronounced antiproliferative effects on HepG-2 cells with the IC₅₀ = 0.01349 nM. Docking confirmed that all compounds bind well to SST receptors with preference to sstr3 and sstr5, which is most probably the reason for the observed biological effects.

Treatment of Metastatic Disease in Patients with Neuroendocrine Tumors

Gastroenteropancreatic neuroendocrine tumors (GENTs) compromise a heterogeneous group of relatively uncommon neoplasms with a yearly incidence rate of 1.2 to 3.0 per 100,000 population. These tumors share numerous histologic and biologic features, allowing their consideration as a common entity. They are postulated to arise from neuroendocrine cells, but most are not from neural crest origin. Their predominant site of origin is the gastrointestinal tract, where most involve the small intestine and appendix, but are also found in the adrenal medulla, bronchopulmonary system, pancreas, thyroid, parathyroid, and paraganglia cells. A common feature is their often indolent course, but some tumors are poorly differentiated and behave aggressively. This article addresses the surgical management of endocrine malignancies and the treatment of metastatic disease in patients with neuroendocrine tumors.

Somatostatin receptors in pituitary and development of somatostatin receptor subtype-selective analogs

Somatostatin receptor (SSTR) subtypes 1, 2, and 5 are expressed in the normal human pituitary. SSTR2 and SSTR5 are expressed in almost all growth hormone (GH) cell adenomas, and prolactin (PRL)-secreting tumors express SSTR5 more than SSTR2. SSTR4 is not detected in all pituitary adenoma subtypes, and SSTR1 and SSTR3 are expressed in about 50% of tumors. Human GH is regulated through ligand binding to both SSTR2 and SSTR5, but octreotide and lanreotide, the two clinically available somatostatin analogs, bind to human SSTR2 much better than to SSTR5. Novel SSTR2- and SSTR5- selective analogs with improved binding affinity for these receptor subtypes are highly potent in suppressing GH release from cultures of human fetal pituitaries or GH-cell adenomas. Only SSTR5-selective analogs suppress in vitro PRL secretion from cultured prolactinomas. A new SSTR2+5 bispecific analog with high affinity and selectivity for both SSTR2 and SSTR5, and a somatostatin analog with a unique broad receptor (SSTR1, 2, 3, and 5) binding profile, are both able to inhibit in vitro GH release in GH cell adenomas partially sensitive to octreotide. Recently, a somatostatindopamine hybrid molecule was introduced with potentially functional synergy on GH and PRL release. Using the expanding knowledge on SSTRs and their ligand activation, the development of novel pharmacologic concepts may open new opportunities for effective manipulation of this complex intracellular signaling system. These concepts may achieve better control of pituitary hormone hypersecretion, pituitary size, as well as antitumor effects in patients with SSTR-expressing tumors.

A novel somatostatin conjugate with a high affinity to all five somatostatin receptor subtypes

BACKGROUND

Somatostatin receptors (SRS, five subtypes) are expressed in a variety of human tumors, including most tumors of neuroendocrine origin, breast tumors, certain brain tumors, renal cell tumors, lymphomas, and prostate cancer. Somatostatin (SMS) triggers cytostatic and cytotoxic effects and has a general inhibitory effect on secretion mediated through its interaction with SRS. That is the basis for its use in the treatment of SRS-positive tumors. Radiolabeled SMS analogs can also be used for systemic radiotherapy and for diagnostic investigations.

METHODS

Sms-14 was conjugated to a periodate-activated dextran70 (mean molecular weight, 70 kD) by reductive amination. The human tumor cell line LCC-18, from a neuroendocrine colonic tumor, was used for stable transfection with each SRS gene separately; transfection was achieved with the expression system TETon (Clontech, Palo Alto, CA). Clones were selected by culturing with G418 and hygromycin B, and positive clones were identified by reverse transcriptase-polymerase chain reaction and binding of iodine-125-labeled SMS-14. The binding affinity for each SRS subtype was then determined for the SMS-dextran conjugate (with SMS-14 used as a positive control).

RESULTS

Sms-dextran70 showed high affinity binding to all five receptor subtypes. The IC50 values were between 3 and 80 nM.

CONCLUSIONS

This conjugate has a long circulation half-life (i.e., approximately 27 hours after subcutaneous administration in mice) and, with high SRS pan-affinity demonstrated in this study, it has potential in the therapy of SRS-positive tumors. Currently, the clinical significance of SMS-dextran70 is being explored in a clinical Phase I-II study of patients with hormone-refractory prostate cancer. The outcome of this study will be reported when it is available.

Somatostatin receptor subtype specificity in human fetal pituitary cultures. Differential role of SSTR2 and SSTR5 for growth hormone, thyroid-stimulating hormone, and prolactin regulation

Somatostatin (SRIF), a hypothalamic inhibitor of pituitary growth hormone (GH) and thyroid-stimulating hormone (TSH) secretion, binds to five distinct receptor (SSTR) subtypes. We therefore tested SSTR subtype-specific SRIF analogs in primary human fetal pituitary cultures (23-25-wk gestation) to elucidate their role in regulating human pituitary function. Using reverse transcription-PCR, mRNA expression of SSTR2 and SSTR5 were detected in fetal pituitary by 25 wk. SRIF analog affinities were determined by membrane radioligand binding in cells stably expressing the human SSTR forms. GH secretion was suppressed equally (40-60%, P < 0.005) by analogs preferential for either SSTR2 (IC50 for receptor binding affinity, 0.19-0.42 nM) or SSTR5 (IC50, 0.37 nM), and compounds with enhanced affinity for SSTR2 were more potent (EC50 for GH suppression, 0.05-0.09 nM) than Lanreotide (EC50, 2.30 nM) and SRIF (EC50, 0.19 nM). Similarly, analogs with high affinity for SSTR2 or SSTR5 decreased TSH secretion (30-40%, P < 0.005). However, prolactin was effectively inhibited only by compounds preferentially bound to SSTR2 (20-30%, P < 0.05). Luteinizing hormone was modestly decreased (15-20%) by SSTR2- or SSTR5-specific analogs. An SSTR5-specific analog also exclusively inhibited GH in acromegalic tumor cells. Thus, SRIF regulation of GH and TSH in primary human fetal pituitary cells is mediated by both SSTR2 and SSTR5, both of which are abundantly expressed by 25 wk. In contrast, suppression of prolactin is mediated mainly by SSTR2. These results indicate that SSTR5 is critical for physiologic regulation of GH and TSH. SRIF analogs with selective affinity for this receptor may therefore be more effective in the treatment of hormone-secreting pituitary adenomas.