Inhibition of the UCI-107 human ovarian carcinoma cell line by a targeted cytotoxic analog of somatostatin, AN-238.

BACKGROUND: Cytotoxic analogs of somatostatin (SST), such as AN-238, which consists of 2-pyrrolinodoxorubicin (AN-201) linked to the SST carrier RC-121, can be targeted to tumors that express SST receptors. Because SST receptors are present in ovarian carcinoma cells, the authors evaluated the effect of AN-238 on the UCI-107 ovarian carcinoma cell line. METHODS: An analysis of microsatellite alleles in cocultured SST receptor positive and receptor negative cells was used for the demonstration of in vitro targeting. The toxicity and antitumor effects of AN-238 in nude mice bearing UCI-107 human ovarian tumors were investigated with or without pharmacologic inhibition of serum carboxylesterases (CE). The expression of SST receptor subtypes was determined by reverse transcriptase-polymerase chain reaction analysis, and the binding affinity of AN-238 to SST receptors was determined by radioligand assays. RESULTS: The proliferation of SST receptor positive UCI-107 cells in vitro was inhibited preferentially by AN-238. AN-238 showed high-affinity binding to UCI-107 tumor membranes at a 50% inhibition concentration of 3.39 nM +/- 0.74 nM. In vivo, the volume and weights of UCI-107 tumors treated with AN-238 were decreased by more than 60% (P < 0.05) compared with controls. Cytotoxic radical AN-201 or the unconjugated mixture of AN-201 with carrier RC-121 had no significant effects on tumors and were toxic. In mice with inhibited serum CE activity, AN-201 at 400 nmol/kg was lethal, whereas AN-238 at a total dose of 800 nmol/kg caused only 22% mortality and reduced tumor weight by 69% and volume by 70% (P < 0.05 vs. control). CONCLUSIONS: Targeted chemotherapy with the SST conjugate AN-238 inhibits SST receptor positive experimental ovarian tumors. AN-238 may provide a new treatment modality for patients with advanced ovarian carcinoma.

Inhibition of growth of ES-2 human ovarian cancers by bombesin antagonist RC-3095, and luteinizing hormone-releasing hormone antagonist Cetrorelix.

We evaluated the effects of the bombesin/gastrin-releasing peptide (GRP) antagonist RC-3095, and the luteinizing hormone-releasing hormone (LH-RH) antagonist Cetrorelix, administered singly or in combination, on the growth of human ovarian carcinoma cell line ES-2, xenografted into nude mice. RC-3095 at a dose of 20 microg/day and Cetrorelix (100 microg/day), significantly reduced the volume of ES-2 tumors by 63.0% (P<0.01) and 38.0% (P<0.05) respectively, after 44 days of treatment, as compared with controls. The combination of RC-3095 with Cetrorelix inhibited the growth of ES-2 tumors by 66.2% (P<0.01). Serum levels of LH were significantly decreased in the groups treated with Cetrorelix alone and/or in combination with RC-3095. RT-PCR analyses revealed that the expression of mRNA for receptors of GRP (GRPR/BRS-1) and Neuromedin B (NMBR/BRS-2) on tumors was significantly decreased in all the treated groups. The expression of mRNA for epidermal growth factor receptors (EGFR) on tumors was reduced by 36.5 % (P<0.05) in the animals treated with Cetrorelix and by 72.5% (P<0.05) in the group that received the combination of RC-3095 with Cetrorelix. Our results indicate that the bombesin antagonist RC-3095 and the LH-RH antagonist Cetrorelix inhibit effectively the growth of ES-2 ovarian cancers in nude mice. These antagonists and their combination could be considered for the therapy of patients with ovarian cancer.

Targeting of doxorubicin to ES-2 human ovarian cancers in nude mice by linking to an analog of luteinizing hormone-releasing hormone improves its effectiveness.

Receptors for luteinizing hormone-releasing hormone (LHRH), expressed by ovarian cancers, can be used for targeting chemotherapeutic compounds more selectively to these tumors. We investigated the effects of cytotoxic LHRH analog AN-152, consisting of doxorubicin (DOX)-14-O-hemiglutarate linked to the epsilon-amino group of [D-Lys6]LHRH, on the growth of LHRH receptor-positive ES-2 human ovarian cancer line xenografted into nude mice. A single injection of AN-152, at a dose of 345 nmol/20 g body weight, caused a 34.5% reduction (P<0.05) in tumor growth after 28 days, while its cytotoxic moiety DOX was inactive at the same dose. Since the overexpression of certain growth factors and/or their receptors, such as vascular endothelial growth factor (VEGF), epidermal growth factor receptor (EGFR) and HER-2/neu, as well as various oncogenes like c-fos and c-jun, is associated with unfavorable prognosis and contributes to progressive growth of ovarian carcinomas, their mRNA levels were analyzed by RT-PCR. Treatment with AN-152 significantly (P<0.05) reduced the expression of EGFR, VEGF, c-fos and c-jun, to 49%, 48%, 55% and 58% respectively, compared to controls. HER-2/neu mRNA expression was also decreased to non-detectable levels. Conversely, DOX decreased non-significantly the expression levels for EGFR by 32%, VEGF 35%, both c-fos and c-jun approximately 20% and HER-2/neu by only 15%. In conclusion, cytotoxic LHRH analog AN-152 could be considered for chemotherapy of ovarian cancers expressing LHRH receptors.

In vitro targeting of a cytotoxic analog of luteinizing hormone-releasing hormone AN-207 to ES-2 human ovarian cancer cells as demonstrated by microsatellite analyses.

Targeting of cytotoxic agents represents a modern approach to the treatment of various cancers, that improves the efficacy and reduces peripheral toxicity. Recently we developed a powerful cytotoxic analog of luteinizing hormone-releasing hormone (LHRH), AN-207, designed to be targeted to tumors that express LHRH receptors. This analog consists of the superactive derivative of doxorubicin (DOX), 2-pyrrolino-DOX (AN-201), linked to [D-Lys6]LHRH carrier. In the present study we investigated the cytocidal effects of AN-207 and AN-201 on the LHRH receptor-positive ES-2 ovarian cancer cells. The targeting of AN-207 to ES-2 cells in the presence of LHRH receptor-negative UCI-107 ovarian cancer cells was also evaluated by semi-quantitative polymerase chain reaction (PCR) amplification of microsatellite markers. Ligand competition assays showed a single class of high-affinity and low-capacity binding sites in ES-2 cells with a mean dissociation constant (KD) of 3.93 +/- 0.1 nM and a mean maximal binding capacity (Bmax) of 271 +/- 26.1 fmol/mg membrane protein. Kinetic assays indicated that AN-207 caused cell death in a concentration- and time-dependent manner in ES-2 cells, but not in UCI-107 cells, while the kinetics of cytotoxic effects of AN-201 were similar in both cell lines. To investigate targeting, ES-2 cells were co-cultured with UCI-107 cells, treated with 10 nM AN-207 or AN-201 for different times and then cultured for 48 h in the absence of cytotoxic agents. Genomic DNA was extracted for microsatellite analyses using different markers. Semi-quantitative analyses of the intensity of the alleles that correspond to each cell line indicated that AN-207 was selectively targeted to ES-2 cells, while AN-201 showed no selectivity for either cell line. These results extend our previous findings that AN-207 can be targeted to ovarian cancers and other tumors that express receptors for LHRH. Cytotoxic analogs of LHRH, such as AN-207, should be considered for treatment of LHRH receptor-positive tumors.

Administration of a targeted cytotoxic analog of luteinizing hormone-releasing hormone inhibits growth of estrogen-independent MDA-MB-231 human breast cancers in nude mice.

Receptor targeted chemotherapy is less toxic and more effective than conventional chemotherapy. Receptors for luteinizing hormone-releasing hormone (LH-RH) are found in about 50% of human breast cancers. Highly potent cytotoxic radical 2-pyrrolinodoxorubicin (AN-201) was linked to the agonistic analog [D-Lys6]LH-RH to form cytotoxic LH-RH analog AN-207. We evaluated whether AN-207 could be targeted to the hormone-independent MDA-MB-231 human breast cancers. Nude mice bearing MDA-MB-231 tumors were injected i.v. with 250 nmol/kg doses of cytotoxic radical AN-201, cytotoxic LH-RH analog AN-207, the unconjugated mixture of AN-201 and carrier [D-Lys6]LH-RH, [D-Lys6]LH-RH alone and vehicle (control). The growth of MDA-MB-231 tumors in animals given a single dose of AN-207 was inhibited significantly (p = 0.01) for 3 weeks after injection, whereas tumors in all the other groups grew steadily. All cytotoxic compounds produced leukopenia, but the strongest lymphocyte suppression was caused by cytotoxic radical AN-201. Three weeks after treatment, the presence of mRNA for LH-RH receptors was demonstrated by RT-PCR in all the groups and radioreceptor assays demonstrated high-affinity binding sites for LH-RH on tumor cell membranes of control animals and those treated with AN-201, the carrier peptide alone or in combination with AN-201. At this time point binding assays did not reveal the expression of membrane proteins in tumors treated with AN-207, but 60 days after administration of AN-207, high affinity LH-RH binding sites were found again in MDA-MB-231 tumors. These results indicate that cytotoxic LH-RH analog AN-207 could be utilized for receptor targeted chemotherapy of breast cancers expressing receptors for LH-RH.

Luteinizing hormone-releasing hormone as an autocrine growth factor in ES-2 ovarian cancer cell line.

Agonistic and antagonistic analogs of luteinizing hormone-releasing hormone (LHRH) inhibit the growth of various cancers in vivo. This effect is mainly exerted through the suppression of the pituitary-gonadal axis and the creation of a state of sex steroid deprivation. In addition, much evidence has been accumulated in the past few years that LHRH analogs can also have direct effects on tumor growth mediated by specific LHRH receptors (-R) on tumor cells. Although an involvement of LHRH in the proliferation of some cancer cells has been postulated, it is still not clear at present whether LHRH produced locally has a stimulatory or inhibitory effect. In the present study we investigated whether LHRH can function as an autocrine growth factor in ovarian cancer. ES-2 human ovarian cancer cell line expresses mRNA for LHRH, which is apparently translated into peptide LHRH and then secreted by the cells, as demonstrated for the first time by the detection of LHRH-like immunoreactivity in conditioned media from the cells cultured in vitro. ES-2 cells also express mRNA for LHRH receptors. [D-Trp6]LHRH agonist at 10 ng/ml stimulates the proliferation of ES-2 in vitro after 48 h, but is inhibitory after 72 h and at concentrations of 1000 ng/ml. LHRH antagonist Cetrorelix inhibits growth of ES-2 cell line only at 1000 ng/ml. The incubation of ES-2 ovarian cancer cells in vitro with an LHRH antibody inhibited cell proliferation in a time and concentration-dependent manner. Collectively, our results suggest that LHRH may function as an autocrine growth factor in ovarian cancer.

Inhibition of growth of MDA-MB-468 estrogen-independent human breast carcinoma by bombesin/gastrin-releasing peptide antagonists RC-3095 and RC-3940-II.

BACKGROUND: The growth of breast carcinoma is promoted by autocrine growth factors such as the bombesin (BN)-like peptides and epidermal growth factor (EGF). The stimulatory action of BN-like peptides can be blocked by the use of BN/gastrin-releasing peptide (GRP) antagonists. METHODS: The authors investigated the effects of synthetic BN/GRP antagonists RC-3095 and RC-3940-II on tumor growth and the expression of mRNA for EGF receptors and three BN receptor subtypes in MDA-MB-468 human breast carcinoma. Athymic nude mice with xenografts of MDA-MB-468 human breast carcinoma were injected subcutaneously for 6 weeks with RC-3940-II at doses of 20 or 40 microg/day. In another study, the effects of RC-3940-II and RC-3095 were compared. RESULTS: RC-3940-II caused a significant and dose-dependent growth inhibition of MDA-MB-468 tumors in nude mice; therapy with either dose of RC-3940-II significantly (P<0.01) reduced the mean final tumor volume and weight compared with controls. RC-3940-II induced a persistent regression of > 50% of all tumors. One of 3 tumors treated with 20 microg of RC-3940-II and 3 of 5 tumors treated with 40 microg were found to have regressed completely by the end of the study. When RC-3940-II and RC-3095 were compared at the dose of 20 microg/day, both powerfully suppressed growth of MDA-MB-468 tumors, with RC-3940-II causing a complete regression of 2 tumors and RC-3095 a complete regression of 1 tumor. Receptor analyses of untreated MDA-MB-468 tumors revealed an overexpression of EGF receptors and two classes of binding sites for BN/GRP. mRNAs for receptors of GRP, neuromedin B, and BN receptor subtype-3 were detected by reverse transcriptase-polymerase chain reaction. CONCLUSIONS: A virtual arrest of growth or regression of MDA-MB-468 human breast carcinoma after therapy with RC-3940-II and RC-3095 indicates that these BN/GRP antagonists could provide a new treatment modality for breast tumors expressing BN and EGF receptors.

High incidence of receptors for luteinizing hormone-releasing hormone (LHRH) and LHRH receptor gene expression in human prostate cancers.

PURPOSE: Agonistic analogs of luteinizing hormone-releasing hormone (LHRH) are widely used for therapy of advanced prostate cancer based upon their ability to suppress testosterone secretion in patients. Various studies also indicate that LHRH analogs might have direct inhibitory effects on prostate tumors mediated by specific LHRH receptors. In this study we investigated the presence and characteristics of receptors for LHRH and their messenger (m) ribonucleic acid (RNA) expression in specimens of human prostate adenocarcinomas and benign prostatic tissue. MATERIALS AND METHODS: In vitro ligand competition assays as well as reverse transcriptase polymerase chain reaction (RT-PCR) were performed to investigate the expression of receptors for LHRH in surgical specimens of human prostate cancers and benign prostatic tissue. RESULTS: Sixty-nine of 80 (86%) cancers exhibited specific, medium to high-affinity binding for [D-Trp6]LHRH with a dissociation constant (Kd) of 6.55+/-0.4 nM and a maximal binding capacity (Bmax) of 483.6+/-25.4 fmol./mg. membrane protein. Two prostate cancer patients who were treated with the LHRH agonist goserelin prior to prostatectomy did not show tumor LHRH receptors. The expression of mRNA for LHRH receptors was observed in 19 of 22 (86%) prostate cancers. Benign prostatic tissue also displayed LHRH receptor gene expression, but exhibited lower Bmax value. There was a negative correlation (p <0.001) between LHRH receptor binding capacity and cancer grade (Gleason score); higher Gleason scores were associated with significantly lower binding capacity but an increased binding affinity. CONCLUSIONS: The expression of specific receptor proteins for LHRH in human prostate cancer provides a rationale for the improvement in methods for therapy of this malignancy based on LHRH analogs.

Growth inhibition of experimental pancreatic cancers and sustained reduction in epidermal growth factor receptors during therapy with hormonal peptide analogs.

Reduction in receptors for epidermal growth factor (EGF) in cancers appears to be one of the principal mechanisms through which peptide hormone analogs can inhibit tumor growth. In this study, hamsters with nitrosamine-induced pancreatic cancers were treated for 8 weeks with bombesin/gastrin-releasing peptide (GRP) antagonist RC-3095, somatostatin analog RC-160 or the luteinizing hormone-releasing hormone antagonist Cetrorelix, using sustained delivery systems releasing 20, 35 and 20 microg analog/ day respectively. To establish the pattern of changes in the number and affinity of EGF receptors on tumors, groups of animals were sacrificed at regular intervals during therapy. Chronic treatment with RC-3095 or Cetrorelix resulted in an early (day 10) and sustained reduction (71% or 69% respectively) in EGF receptors on pancreatic tumors. In contrast, RC-160 decreased receptor concentration by 60% only after 20 days. Among the histological characteristics of proliferation, the decrease in argyrophilic nucleolar organizer regions, but not apoptotic and mitotic indices, showed a correlation with the fall in EGF receptors. The concentration of the receptors returned to the control level 4 days after cessation of chronic treatment with RC-3095. The effect of single injections of RC-3095, RC-160 and Cetrorelix on EGF receptors was also investigated. RC-160 decreased the number of EGF receptors on pancreatic cancers by 31% 3 h after administration, but the receptors had returned to normal level at 6 h. RC-3095 and Cetrorelix caused a 67% and 59% decline, respectively, in EGF receptors only 6 h after injection and the concentration of receptors remained low for 24 h. Thus, the pattern of downregulation of EGF receptors in pancreatic cancers appears to depend on the peptide used for therapy. Since the antitumor effect may be the result of the fall in EGF receptors in cancers, information on the time course of changes in these receptors during treatment with these analogs may lead to an improvement in therapeutic regimens.

Complete regression of MX-1 human breast carcinoma xenografts after targeted chemotherapy with a cytotoxic analog of luteinizing hormone-releasing hormone, AN-207.

BACKGROUND: Receptors for luteinizing hormone-releasing hormone (LH-RH) are found in about 50% of human breast carcinomas. A highly potent cytotoxic agent, 2-pyrrolinodoxorubicin (AN-201), was linked to the agonist [D-Lys6]LH-RH to form a cytotoxic LH-RH analog, AN-207, that can be targeted to LH-RH receptors on breast carcinomas. METHODS: Nude mice bearing MX-1 hormone-independent, doxorubicin-resistant human breast carcinomas were injected intravenously with vehicle (control), 250 nmol/kg doses of AN-201, AN-207, or an unconjugated mixture of AN-201 and [D-Lys6]LH-RH. Tumor growth and changes in hematologic parameters were evaluated. Receptors for LH-RH were investigated by radioreceptor assays, and the expression of their mRNA was determined by reverse transcriptase-polymerase chain reaction. RESULTS: AN-207 caused complete regression of MX-1 tumors in all 10 animals, and they were still tumor free 60 days after treatment. In contrast, after therapy with AN-201 or the mixture of AN-201 and [D-Lys6]LH-RH, the regression of most MX-1 tumors was only transitory. AN-201 caused the death of 1 of the 10 animals and significantly greater leukopenia than AN-207, which produced no toxic deaths. Radioreceptor assays revealed high affinity binding sites for LH-RH on tumor cell membranes. The expression of mRNA for LH-RH receptors also was found in tumors. CONCLUSIONS: The results of this study indicate that powerful, targeted cytotoxic LH-RH analogs such as AN-207 could be considered for the treatment of human breast carcinomas that possesses receptors for LH-RH.

Expression of growth hormone-releasing hormone (GHRH) messenger ribonucleic acid and the presence of biologically active GHRH in human breast, endometrial, and ovarian cancers.

GHRH is produced in a variety of extrahypothalamic tissues, including some neoplasms. We have previously reported that GHRH antagonists can inhibit the growth of various human cancers xenografted into nude mice. These observations suggest that locally produced GHRH might directly affect tumor cell proliferation. To investigate this possibility, we have examined the local production of GHRH in human endometrial, ovarian, and breast cancers obtained after surgery or grown in nude mice as xenografts. We have also examined whether the GHRH produced in these tumors is biologically active. RT-PCR and Southern blotting showed expression of messenger ribonucleic acid for GHRH in 17 of 22 endometrial and 17 of 22 ovarian cancer specimens and in all of the human endometrial, ovarian, and breast cancer xenografts studied. Acid extracts of endometrial cancer specimens and breast cancer xenografts that expressed the GHRH gene contained immunoreactive GHRH peptide, as assessed by RIA for GHRH. The level of immunoreactive GHRH detected was equivalent to 2.7-6.4 ng GHRH-(1-29)/g tissue. Purified extract from one of these tumor samples induced a powerful stimulation of GH release from rat pituitary cells. The presence of biologically and immunologically active GHRH and messenger ribonucleic acid for GHRH in human breast, endometrial, and ovarian cancers supports the hypothesis that locally produced GHRH may play a role in the proliferation of these tumors.

Targeted cytotoxic analog of luteinizing hormone-releasing hormone AN-207 inhibits the growth of PC-82 human prostate cancer in nude mice.

BACKGROUND: Receptors for luteinizing hormone-releasing hormone (LH-RH) found in prostate cancers might be used for targeting of chemotherapeutic agents. Doxorubicin derivative 2-pyrrolinodoxorubicin (AN-201) can be linked to carrier analog [D-Lys6]LH-RH to form the targeted cytotoxic analog of LH-RH, AN-207. METHODS: We evaluated the effects of AN-207 and its components on the growth of LH-RH receptor-positive human prostate cancer PC-82 xenografted into nude mice. Analog AN-207, radical AN-201, carrier [D-Lys6]LH-RH, or a mixture of [D-Lys6]LH-RH and AN-201 were injected intravenously once at doses of 200 nmol/kg. Tumor growth, body weight, total WBC counts, and serum prostate-specific antigen (PSA) were determined. Receptors for LH-RH on PC-82 tumors were evaluated, and the expression of mRNA for LH-RH receptors was assessed by RT-PCR. RESULTS: Eight weeks after administration of cytotoxic analog AN-207, there was a 67.8% reduction in tumor volume (P < 0.01), 70.7% decrease in tumor burden (P < 0.01), and 36.5% decrease in serum PSA levels (P < 0.01) as compared with controls. Only one of 8 animals treated with AN-207 died. Cytotoxic radical AN-201 caused a 34.2% (not significant, NS) reduction in tumor volume with no change in serum PSA, and killed 3 of 8 mice due to toxicity. Carrier [D-Lys6]LH-RH and the unconjugated mixture of [D-Lys6]LH-RH and AN-201 had no effect on tumor growth. LH-RH receptors as well as the expression of their mRNA were found in PC-82 tumors.

Bombesin/gastrin-releasing peptide antagonists RC-3095 and RC-3940-II inhibit tumor growth and decrease the levels and mRNA expression of epidermal growth factor receptors in H-69 small cell lung carcinoma.

BACKGROUND: Antagonists of bombesin/gastrin-releasing peptide (BN/GRP) have been developed to block the autocrine stimulatory effect of BN/GRP on tumors such as small cell lung carcinoma (SCLC). Although several studies have addressed the intracellular events that follow the formation of the receptor-ligand complex, the mechanism of action of BN/GRP antagonists remains unclear. METHODS: In this study the authors investigated the effect of synthetic BN/GRP antagonists RC-3095 and RC-3940-II on tumor growth and the expression of epidermal growth factor receptors (EGF-R) in H-69 SCLC. Athymic nude mice xenografted with H-69 SCLC were treated subcutaneously for 5 weeks with RC-3095 and RC-3940-II at the dose of 10 microg/animal/day. RESULTS: RC-3095 decreased tumor volume by approximately 50% (P < 0.05) and RC-3940-II by 70-60% (P < 0.01). Tumor burden also was significantly decreased in the groups treated with RC-3095 and RC-3940-II. Receptor analyses demonstrated high affinity binding sites for BN/GRP and EGF on the untreated H-69 SCLC tumors. After treatment with RC-3095 and RC-3940-II, the concentration of receptors for BN/GRP was decreased by 29.0% and 36.5%, respectively (both, P < 0.01) compared with controls, and EGF-R levels were reduced by 62.3% and 63.0%, respectively (both, P < 0.01). Reverse transcriptase-polymerase chain reaction and Southern blot analyses revealed that the levels of mRNA for EGF-R in tumors were lowered by 31% (P < 0.05) and 43% (P < 0.01), respectively, after treatment with RC-3095 and RC-3940-II. CONCLUSIONS: This study indicates that the inhibition of growth of H-69 SCLC by BN/GRP antagonists RC-3095 and RC-3940-II is accompanied by a marked decrease in the levels and mRNA expression of EGF-R.

Targeted cytotoxic analogue of somatostatin AN-238 inhibits growth of androgen-independent Dunning R-3327-AT-1 prostate cancer in rats at nontoxic doses.

Receptors for somatostatin (SST) that are found on prostate cancers might be used for targeting of chemotherapeutic agents. Thus, doxorubicin derivative 2-pyrrolinodoxorubicin (AN-201) can be linked to SST analogue RC-121 (D-Phe-Cys-Tyr-D-Trp-Lys-Val-Cys-Thr-NH2) to form targeted cytotoxic SST analogue AN-238. In this study, we evaluated the effects of AN-238 on the growth of SST receptor (SSTR)-positive androgen-independent Dunning R-3327-AT-1 prostate cancers in Copenhagen rats. The dose range and tumor growth-inhibitory effects of AN-238 and AN-201 were investigated in preliminary experiments. Administration of cytotoxic radical AN-201 at single i.v. doses of 110, 125, and 150 nmol/kg resulted in 0, 77.7, and 100% mortality, respectively, within 6-10 days. Four weeks after the injection of 110 nmol/kg AN-201, mean tumor volume was reduced by 35.1 % (P < 0.05), as compared with controls. In contrast, a single i.v. injection of analogue AN-238 at a dose of 300 nmol/kg was nontoxic and remarkably potent in inhibiting the growth of Dunning AT-1 tumors, resulting in a 85.9% (P < 0.01) reduction in tumor volume after 4 weeks. Treatment with AN-238 extended the survival time of tumor-bearing rats from 52.0+/-3.75 to 91.8+/-3.70 days, corresponding to a 76.5% (P < 0.01) increase. In a comprehensive experiment, we compared the effects of radical AN-201 at 115 nmol/kg, analogue AN-238 at 115 and 300 nmol/kg, carrier SST analogue RC-121 at 300 nmol/kg, and a mixture of AN-201 and RC-121 at doses of 300 nmol/kg administered i.v. Administration of AN-201 at 115 nmol/kg led to 90.0% mortality in 12 days, but animals treated with 115 nmol/kg of AN-238 showed no signs of toxicity, their tumor volume was reduced by 40.0% (P < 0.05), and their tumor weight was reduced by 42.8% (P < 0.01) after 4 weeks, as compared with controls. The dose of 300 nmol/kg of AN-238 was also nontoxic and diminished tumor volume by 80.9% (P < 0.01) and tumor weight by 82.0% (P < 0.01). No reduction in tumor growth or toxic effects was observed with carrier RC-121, but after the injection of unconjugated mixture of AN-201 and RC-121 at doses of 300 nmol/kg, all rats died within 4 days. Specific high-affinity receptors for SST were found on Dunning R-3327-AT-1 tumor membranes by radioligand binding assay and were identified by reverse transcription-PCR as SSTR2. Our study indicates that cytotoxic SST analogue AN-238 can be targeted to SSTRs on tumors and produces a powerful inhibition of the growth of Dunning-AT-1 rat prostate cancer at doses that are nontoxic, whereas its cytotoxic component, 2-pyrrolinodoxorubicin, is toxic and ineffective.