Effective Treatment of Experimental Androgen Sensitive and Androgen Independent Intraosseous Prostate Cancer With Targeted Cytotoxic Somatostatin Analogue AN-238

New therapies for relapsed castration-resistant prostate cancer based on peptide analogs of hypothalamic hormones

It is a pleasure to contribute our presentation at the International Prostate Forum of the Annual Meeting of the American Urological Association (AUA) to this special issue of the Asian Journal of Andrology.

The ghrelin axis--does it have an appetite for cancer progression?

Ghrelin, the endogenous ligand for the GH secretagogue receptor (GHSR), is a peptide hormone with diverse physiological roles. Ghrelin regulates GH release, appetite and feeding, gut motility, and energy balance and also has roles in the cardiovascular, immune, and reproductive systems. Ghrelin and the GHSR are expressed in a wide range of normal and tumor tissues, and a fluorescein-labeled, truncated form of ghrelin is showing promise as a biomarker for prostate cancer. Plasma ghrelin levels are generally inversely related to body mass index and are unlikely to be useful as a biomarker for cancer, but may be useful as a marker for cancer cachexia. Some single nucleotide polymorphisms in the ghrelin and GHSR genes have shown associations with cancer risk; however, larger studies are required. Ghrelin regulates processes associated with cancer, including cell proliferation, apoptosis, cell migration, cell invasion, inflammation, and angiogenesis; however, the role of ghrelin in cancer is currently unclear. Ghrelin has predominantly antiinflammatory effects and may play a role in protecting against cancer-related inflammation. Ghrelin and its analogs show promise as treatments for cancer-related cachexia. Further studies using in vivo models are required to determine whether ghrelin has a role in cancer progression.

Somatostatin receptor subtypes in hormone-refractory (castration-resistant) prostatic carcinoma

The aim of this study was to examine the tissue expression and localisation of the somatostatin receptors (SSTRs) in hormone-refractory (HR) prostate cancer (PCa). Five SSTRs were evaluated immunohistochemically in 20 radical prostatectomies (RPs) with Gleason score (GS) 3+3=6 PCa, in 20 RPs with GS 4+4=8 and 4+5=9 PCa, and 20 transurethral resection of the prostate specimens with HR PCa. The mean values in the cytoplasm (all five SSTRs were expressed), membrane (only SSTR3 and SSTR4 were expressed) and nuclei (only SSTR4 and SSTR5 were expressed) of the glands in HR PCa were 20-70% lower than in the other two groups, the differences being statistically significant. All five SSTRs were expressed in the smooth muscle and endothelial cells of HR PCa, the mean values being lower than in the other two groups. In conclusion, this study expands our knowledge on the expression and localisation of five SSTRs in the various tissue components in the HR PCa compared with hormone-sensitive PCa.

Targeted Therapy of Breast and Gynecological Cancers with Cytotoxic Analogues of Peptide Hormones

Gynecological cancers such as breast, ovarian, and endometrial carcinoma express receptors for luteinizing hormone-releasing hormone (LHRH), bombesin/gastrin-releasing peptide (BN/GRP), and somatostatin (SST). These tumors are therefore suitable candidates for targeted therapy with cytotoxic hybrid molecules consisting of a cytotoxic radical and a peptide hormone analogue as a carrier. These compounds have been shown to be more active and less toxic in vivo than nontargeted chemotherapy in models of various human cancers which express the respective receptors. The current review summarizes experimental and clinical findings with cytotoxic peptide hormone analogues of LHRH (AN-152 [AEZS 108], AN-207), BN/GRP (AN-215), and SST (AN-238) in breast, ovarian, and endometrial cancers.

Insulin-like growth factors and their binding proteins in prostate cancer: Cause or consequence?☆

Insulin-like growth factors (IGFs) promote growth and survival of many types of tumor cells. Epidemiologic studies have implicated carcinogenesis with high levels of IGFs in circulation or in tissues. The levels of IGF binding proteins (IGFBPs) have been associated with reduced risk for prostate and other cancers. Experimental studies have implicated high levels of IGF-I directly and IGFBP-3 inversely in prostate cancer growth, survival, and progression. However, recent evidence suggests a much weaker association of IGF-I with prostate cancer development and a stronger antagonistic association of IGFBP-3 with prostate cancer progression. Considering the clonal heterogeneity and unpredictable progression pattern of prostate cancer, the role of any single growth factor or its regulator (IGFBP) as a single determining factor is limited. This review is a critical appraisal of the role of IGFs, IGFBP, and IGF-I receptor (the IGF axis) in both experimental and clinical prostate cancer genesis and progression.

Successful in vivo tumor targeting of prostate-specific membrane antigen with a highly efficient J591/PEI/DNA molecular conjugate

We have utilized a novel polyethylenimine (PEI)/DNA-betagal vector to investigate the specificity and efficiency of immuno-targeting prostate-specific membrane antigen (PSMA). Coupling of the PSMA-specific monoclonal antibody, J591, to the vector was facilitated via the high-affinity interaction between phenyl(di)boronic acid and salicylhydroxamic acid molecules. Highly efficient gene delivery by this prostate cancer (PCA)-targeted J591/polyethylene glycol (PEG)/PEI/DNA-betagal vector was demonstrated in PSMA-positive cells relative to controls, resulting in significant growth inhibition in vitro when the J591/PEG/PEI/DNA-p53 was used. Competition with free antibody resulted in about 90% reduction in both J591 internalization and betagal gene delivery, indicating specificity for PSMA-positive cells. More importantly, testing the efficiency of the J591/PEG/PEI/DNA-betagal targeting vector in an orthotopic PCA model in nude mice resulted in up to a 20-fold increase in gene delivery over the untargeted vector controls. The in vivo organ distribution profile also revealed betagal expression predominantly in the tumor, which was more than 1 log higher than the next highest level of expression in the lung. Furthermore, with the targeted vector containing the gene for yellow fluorescent protein or biotinylated J591, we further demonstrate in vivo that vector-mediated gene delivery is specific for both tumor cells and tumor-associated neovasculature in PSMA-positive tumors. These results suggest the potential for further optimization of this novel vector in the context of therapeutic gene delivery.

Targeted therapy with a cytotoxic somatostatin analog, AN-238, inhibits growth of human experimental endometrial carcinomas expressing multidrug resistance protein MDR-1

BACKGROUND

Chemoresistance mediated by membrane transporters such as multidrug resistance (MDR-1) glycoprotein remains a challenge in the chemotherapy treatment of advanced or recurrent endometrial carcinoma. Targeted chemotherapy might overcome this resistance. The cytotoxic somatostatin (SST) analog, AN-238, consists of a superactive derivative of doxorubicin (DOX), 2-pyrrolino-DOX (AN-201), linked to the SST analog carrier, RC-121. This conjugate binds strongly to SST receptor subtypes (sst) 2a (sst2(a)) and 5 (sst(5)) and can be targeted to tumors that express these receptors.

METHODS

The presence of sst2(a) and sst(5) was determined in 3 human endometrial carcinoma cell lines (HEC-1A, RL-95-2, and AN3CA). Nude mice bearing xenografts of these cancers were treated with AN-238 and its radical, AN-201. The antitumor effects and toxicity were compared. The authors studied the effects of AN-238 and AN-201 on the expression levels of MDR-1, multidrug resistance related protein (MRP-1), and breast carcinoma resistance protein (BCRP) by real-time polymerase chain reaction.

RESULTS

The authors demonstrated the presence of mRNA and receptor protein for sst(2a) and sst(5) on HEC-1A, RL-95-2, and AN3CA tumors. AN-238 significantly (P < 0.05) inhibited the growth of these tumors, whereas AN-201 had no effect. Blockade of SST receptors nullified the effects of AN-238. In all 3 endometrial carcinoma lines, AN-238 caused a weaker induction of MDR-1 than AN-201. No major induction of MRP-1 and BCRP occurred after treatment with AN-238 or AN-201.

CONCLUSIONS

Targeted chemotherapy with the cytotoxic SST analog, AN-238, inhibited powerfully the growth of endometrial carcinoma, which express SST receptors, regardless of their expression level of MDR-1.

Chemotherapy targeted to cancers through tumoral hormone receptors

Work on cytotoxic analogs of luteinizing hormone-releasing hormone (LH-RH), somatostatin and bombesin, designed for targeting chemotherapy to peptide receptors on various cancers, is reviewed here as the project is at advanced stages of development and clinical trials are pending. Cytotoxic analogs of LH-RH, AN-152 and AN-207, containing doxorubicin (DOX) or 2-pyrrolino-DOX (AN-201), respectively, target LH-RH receptors and can be used for the treatment of prostatic, breast, ovarian and endometrial cancers and melanomas. AN-201 was also incorporated into the cytotoxic analog of somatostatin, AN-238, which can be targeted to receptors for somatostatin in prostatic, renal, mammary, ovarian, gastric, colorectal and pancreatic cancers as well as glioblastomas and lung cancers, suppressing the growth of these tumors and their metastases. A cytotoxic analog of bombesin AN-215, containing 2-pyrrolino-DOX, was likewise synthesized and successfully tested in experimental models of prostate cancer, small cell lung carcinoma, gastrointestinal cancers and brain tumors expressing receptors for bombesin/gastrin-releasing peptide. This new class of targeted cytotoxic peptide analogs might provide a more effective therapy for various cancers.