90Y-DOTA-D-Phe1-Try3-octreotide in therapy of neuroendocrine malignancies

Somatostatin receptors type 2 (sst(2)) are expressed in high concentration on numerous neudoendocrine tumors. The successful use of radiolabeled somatostatin analogs in imaging promoted further studies in utilizing them in radiopeptide therapy. The somatostatin analog [(90)Y-DOTA-D-Phe(1)-Try3]octreotide (DOTATOC) (DOTA: 1,4,7,10-tetraazacyclododecane-N,N',N',N'-tetraacetic acid) possesses favorable characteristic for its therapeutic use; shows high affinity for sst(2), moderately high affinity for sst(5), and intermediate affinity for sst(3); high hydrophilicity; stable and facile labeling with (111) In and (90) Y. In this article we report our experience with (90)Y-DOTATOC in neuroendocrine tumors. Eighty-seven patients with neuroendocrine tumors were treated with a cumulated activity ranging from 7.4 to 20.2 GBq. Most patients responded with stabilization of disease (48%); however, objective responses were observed in 28% of patients (5% complete response). No major acute reactions were observed up to the activity of 5.55 GBq per cycle. The dose limiting was bone marrow toxicity and the maximal tolerated dose was defined as 5.18 GBq.     

NODAGATOC,a new chelator-coupled somatostatin analogue labeled with [67/68Ga] and [111In] for SPECT,PET, and targeted therapeutic applications of somatostatin receptor (hsst2) expressing tumors

A monoreactive NOTA (1,4,7-triazacyclononane-1,4,7-triacetic acid) derived prochelator (1-(1-carboxy-3-carbo-tert-butoxypropyl)-4,7-(carbo-tert-butoxymethyl)-1,4,7-triazacyclononane (NODAGA(tBu)(3))) was synthesized in five steps with an overall yield of 21%. It is useful for the coupling to the N-terminus of peptides on solid phase and in solution; it was coupled to [Tyr3]-octreotide (TOC) on solid phase, and the resulting peptide, NODAGA-Tyr3-octreotide (NODAGATOC), was labeled with the radiometals 111In and 67Ga in high yields and good specific activities. [67Ga]- and [111In]-NODAGA-Tyr3-octreotide appear to be useful to visualize primary tumors and metastases which express somatostatin receptors subtype 2 (sstr2), such as neuroendocrine tumors, because of their high affinity to this receptor subtype with IC(50) = 3.5 +/- 1.6 nM and 1.7 +/- 0.2 nM, respectively. NODAGATOC could be used as a SPECT and PET tracer, when labeled with 111In, 67Ga, or 68Ga, and even for therapeutic applications. Surprisingly, [111In]-NODAGATOC shows 2 times higher binding affinity to sstr2, but also a factor of 4 higher affinity to sstr5 compared to [67Ga]-NODAGATOC. [67Ga]-NODAGATOC is very stable in serum and rat liver homogenate. There is no difference in the rate of internalization into AR4-2J rat pancreatic tumor cells; both radioligands are highly internalized, at 4 h a 3 times higher uptake compared to [111In]-DOTA-Tyr3-octreotide ([111In]-DOTATOC) was found. The biodistribution of [67Ga]-NODAGATOC in AR4-2J tumor bearing nude mice is very favorable at short times after injection; there is fast excretion from all nontarget organs except the kidneys and high uptake in sst receptor rich organs and in the AR4-2J tumor. Again it is superior to [111In]-DOTATOC in this respect. The results indicate an improved biological behavior which is likely due to the fact that an additional spacer group separates the chelate from the pharmacophoric part of the somatostatin analogue.     

Tumor targeting of radiometal labeled anti-CEA recombinant T84.66 diabody and t84.66 minibody: comparison to radioiodinated fragments

Recombinant antibody fragments offer potential advantages over intact monoclonal antibodies in the radioimmunoscintigraphy (RIS) of solid tumors. Due to their smaller molecular size, antibody fragments have shown rapid tumor targeting and blood clearance, a more uniform tumor distribution and a lower potential to elicit a human immune response. Previously, we have expressed two genetically engineered antibody fragments, the T84.66 diabody (scFv dimer) and the T84.66 minibody (scFv-CH3 dimer), specific to carcinoembryonic antigen (CEA). When radioiodinated, both antibody fragments exhibited rapid tumor targeting and rapid blood clearance in xenografted mice. To extend and optimize their future clinical RIS utility with radiometals, these antibody fragments were conjugated with the macrocycle 1,4,7,10-tetraazacyclododecane N,N',N' ',N' "-tetraacetic acid (DOTA) and labeled with 111In. Tumor targeting and biodistribution studies were carried out in athymic mice xenografted with a human colorectal tumor cell line, LS174T. The [111In]T84.66 diabody (55 kDa) exhibited very rapid tumor targeting with 12.5 +/- 0.4% injected dose per gram (% ID g(-1) +/- standard error) at 2 h and reached a maximum of 13.3 +/- 0.9% ID g(-1) at 6 h. However, kidney uptake was observed to reached a peak of 183.5 +/- 21.0% ID g(-1) at 6 h, a result similar to that reported by others for other low molecular weight fragments labeled with radiometals. Preadministration of an oral dose of D-lysine resulted in a 59% lowering of the renal accumulation at 6 h, but was accompanied by a 31% reduction of tumor uptake to 9.2 +/- 1.2% ID g(-1). The second recombinant antibody fragment, the [111In]T84.66 minibody (80 kDa), displayed rapid tumor targeting of 14.2 +/- 6.1% ID g(-1) at 2 h, and reached a maximum activity of 24.5 +/- 6.1% ID g(-1) by 12 h. Renal uptake achieved a plateau of 12-13% ID g(-1) which cleared to 7.2% ID g(-1) at 72 h. However, hepatic uptake was elevated and reached a maximum of 26.0 +/- 1.0% ID g(-1) at 12 h in these xenograft-bearing mice. Experiments in nontumor bearing mice showed a reduction of hepatic activity at 12 h to 16.6 +/- 1.5% ID g(-1), indicative of an intrinsic hepatic accumulation of the [111In]DOTA-T84.66 minibody or metabolites. While the anti-CEA [111In]DOTA-T84.66 diabody and T84.66 minibody retain the rapid tumor targeting properties of the radioiodinated form, the normal organ accumulation (kidneys and liver, respectively) of the [111In]DOTA forms appeared problematic for RIS and RIT applications. Development of alternative blocking strategies or new metabolizable chelates are under investigation to enhance the utility of the radiometal form of these and other promising recombinant antibody fragments.     

Novel chemically modified analogues of neuropeptide Y for tumor targeting

The successful use of peptides as potential radiopharmaceuticals essentially requires the modification of the bioactive peptide hormones to introduce chelators for radiolabeling. In this study, four Y 1/Y 2 receptor-selective NPY analogues with different receptor subtype specificities have been investigated. For in vitro studies, the cold metal surrogate was used. Gallium and indium complexes were introduced by using 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid as bifunctional chelator. The peptides were synthesized by solid-phase peptide synthesis (SPPS), the chelator was coupled either at the N-terminus or at the N(epsilon) side chain of Lys(4) of the resin-bound peptide, and the labeling was performed in solution after cleavage. Competitive binding assays showed high binding affinity of the receptor-selective analogues at NPY receptor expressing cells. To test internalization of the novel peptide analogues and the metabolic stability in human blood plasma, the corresponding 5(6)-carboxyfluorescein (CF) analogues were prepared and investigated. One of the most promising analogues, the Y 1-receptor selective [Lys(DOTA)(4), Phe(7), Pro(34)]NPY was labeled with (111)In and injected into nude mice that bear MCF-7 breast cancer xenografts, and biodistribution studies were performed. In vitro and in vivo studies suggest that receptor-selective analogues of NPY have promising characteristics for future applications in nuclear medicine for breast tumor diagnosis and therapy.     

Dimeric DOTA-α-Melanocyte-Stimulating Hormone Analogs: Synthesis and In Vivo Characteristics of Radiopeptides with High In Vitro Activity

Dimeric analogs of α-melanocyte-stimulating hormone (α-MSH) labeled with radiometals are potential candidates for diagnosis and therapy of melanoma by receptor-mediated tumor targeting. Both melanotic and amelanotic melanomas (over-)express the melanocortin-1 receptor (MC1-R), the target for α-MSH. In the past, dimerized MSH analogs have been shown to display increased receptor affinity compared to monomeric MSH, offering the possibility of improving the ratio between specific uptake of radiolabeled α-MSH by melanoma and nonspecific uptake by the kidneys. We have designed three linear dimeric analogs containing a slightly modified MSH hexapeptide core sequence (Nle-Asp-His-d-Phe-Arg-Trp) in parallel or antiparallel orientation, a short spacer, and the DOTA chelator for incorporation of the radiometal. In vitro, all three peptides were more potent ligands of the mouse B16-F1 melanoma cell melanocortin-1 receptor (MC1-R) than DOTA-NAPamide, which served as standard. The binding activity of DOTA-diHexa(NC-NC)-amide was 1.75-fold higher, that of diHexa(NC-NC)-Gly-Lys(DOTA)-amide was 3.37-fold higher, and that of DOTA-diHexa(CN-NC)-amide was 2.34-fold higher. Using human HBL melanoma cells, the binding activity of diHexa(NC-NC)-Gly-Lys(DOTA)-amide was sixfold higher than that of DOTA-NAPamide. Uptake by cultured B16-F1 cells was rapid and almost quantitative. In vivo, however, the data were less promising: tumor-to-kidney ratios 4 hr postinjection were 0.11 for [¹¹¹In]DOTA-diHexa(NC-NC)-amide, 0.26 for diHexa(NC-NC)-Gly-Lys([¹¹¹In]DOTA)-amide, and 0.36 for [¹¹¹In]DOTA-diHexa(CN-NC)-amide, compared to 1.67 for [¹¹¹In]DOTA-NAPamide. It appears that despite the higher affinity to the MC1-R of the peptide dimers and their excellent internalization in vitro, the uptake by melanoma tumors in vivo was lower, possibly because of reduced tissue penetration. More striking, however, was the marked increase of kidney uptake of the dimers, explaining the unfavorable ratios. In conclusion, although radiolabeled α-MSH dimer peptides display excellent receptor affinity and internalization, they are no alternative to the monomeric DOTA-NAPamide for in vivo application.     

Modulation of pharmacokinetics of radioiodinated sugar-conjugated somatostatin analogues by variation of peptide net charge and carbohydration chemistry

Sugar conjugation of biooactive peptides has been shown to be a powerful tool to modulate peptide pharmacokinetics. In the case of radiolabeled somatostatin analogues developed for in vivo scintigraphy of somatostatin receptor (sst) expressing tumors, it generally led to tracers with predominant renal excretion and low uptake in nontarget organs, and in some cases also with enhanced tumor accumulation. Especially with respect to endoradiotherapeutic applicability of these tracers, however, understanding the structural requirements for minimal kidney accumulation and maximal tumor uptake is important. The aim of this study was therefore the evaluation of the potential of specific glycoside structures in combination with reduced peptide net charge to reduce kidney accumulation without affecting tumor accumulation. Three glyco analogues of radioiodinated Tyr(3)-octreotate (TOCA) with z = 0 were evaluated in a comparative study using [(125)I]Mtr-TOCA (z = +1), the maltotriose-Amadori analogue of [(125)I]TOCA, as a reference, [(125)I]Glucuron-TOCA, the Amadori conjugate with glucuronic acid, and [(125)I]Gluc-S- and [(125)I]Gal-S-TOCA, the coupling products with glucosyl- and mannosyl-mercaptopropionate. In cells transfected with sst(1)-sst(5), all three new analogues show sst-subtype binding profiles similar to I-Mtr-TOCA with high, but somewhat reduced, affinity for sst(2). In contrast, internalization into sst(2)-expressing cells (in % of [(125)I]Tyr(3)-octreotide ([(125)I]TOC)) as well as the EC(50,R) of unlabeled TOC for internalization determined in dual-tracer experiments are substantially enhanced for [(123)I]Gal-S-TOCA and [(123)I]Gluc-S-TOCA (internalization, 190% +/- 12% and 265% +/- 20%, respectively, vs 168% +/- 6% of [(125)I]TOC for [(123)I]Mtr-TOCA; EC(50,R), 2.62 +/- 0.07 and 2.96 +/- 0.14, respectively, vs 1.81 +/- 0.07 for [(123)I]Mtr-TOCA). The tumor accumulation of [(125)I]Gal-S-TOCA and [(125)I]Gluc-S-TOCA in AR42J tumor-bearing nude mice 1 h p.i. is consequently very high (22.6 +/- 2.2 and 26.2 +/- 5.6%ID/g) and comparable to that of [(125)I]Mtr-TOCA (25.1 +/- 4.4%ID/g). [(125)I]Glucuron-TOCA showed lower uptake in sst-expressing tissues than did [(125)I]Mtr-TOCA, but considerably enhanced accumulation in nontarget organs such as liver, intestine, and kidney. Due to increased lipophilicity, hepatic and intestinal uptake 1 and 4 h p.i. of [(125)I]Gal-S-TOCA and [(125)I]Gluc-S-TOCA was also slightly higher than that of [(125)I]Mtr-TOCA. Kidney accumulation, however, was reduced by approximately 50% for both compounds (2.6 +/- 0.3 and 2.2 +/- 0.4, respectively, vs 4.0 +/- 0.7%ID/g at 1 h p.i.). Because no sugar-specific effect was detected in the latter case, it is concluded that general ligand pharmacokinetics and especially kidney accumulation of the tracers investigated are mainly determined by physicochemical characteristics such as lipophilicity, net charge, and also charge distribution ([(125)I]Glucuron-TOCA vs [(125)I]Gal-S- and [(125)I]Gluc-S-TOCA). With respect to receptor targeting, however, the structure of the carbohydrate moiety plays an important role, leading to dramatically enhanced ligand internalization, especially in the case of [(123)I]Gluc-S-TOCA. Taking into account the combined effects of the Gluc-S-moiety both on kidney and on tumor accumulation, this group seems to be a promising synthon for the synthesis of other radiolabeled peptide analogues with improved pharmacokinetics.     

Radiolabeled alpha-melanocyte-stimulating hormone analogs for receptor-mediated targeting of melanoma: from tritium to indium

Following the first synthesis of tritiated alpha-melanocyte-stimulating hormone (alpha-MSH, alpha-melanotropin) in 1974 by Medzihradszky et al., several alpha-MSH analogs were designed containing between 2 and 12 tritium atoms, the latter of which displayed a specific radioactivity of 12.21 GBq/micromol (330 Ci/mmol). Similarly, radioiodinated alpha-MSH analogs of high purity, full biological activity and a specific radioactivity of approximately 140 GBq/micromol were obtained. Although tritiated and radioiodinated alpha-MSH became indispensable tools as tracer molecules for numerous in vitro and in vivo studies, above all for receptor identification and characterization as well as for structure-activity studies, they did not fulfill the criteria required for therapeutic in vivo targeting of metastatic melanoma. Therefore, we recently developed alpha-MSH analogs containing the universal metal chelator 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) in different positions of the molecule. As DOTA can equally well incorporate diagnostic (e.g. (111)In, (67,68)Ga) and therapeutic (e.g. (90)Y, (67)Cu) radionuclides, DOTA-MSH compounds may serve for both melanoma scintigraphy and therapy. The analog DOTA-[betaAla(3), Nle(4), Asp(5), D-Phe(7), Lys(10)]-alpha-MSH(3-10) (DOTA-MSH(OCT)), which contains the metal chelator at its N-terminal end, displayed good in vitro MC1R affinity (IC(50) 9.21 nm). In vivo, [(111)In]DOTA-MSH(OCT) exhibited a favorable biodistribution profile after injection in B16-F1 tumor-bearing mice. The radiopeptide was rapidly cleared from blood through the kidneys and, most importantly, accumulated preferentially in the melanoma lesions. Lung and liver melanoma metastases could be clearly imaged on tissue section autoradiographs 4 h after injection of [(111)In]DOTA-MSH(OCT). A comparative study of [(111)In]DOTA-MSH(OCT) with [(111)In]DOTA-[Nle(4), D-Phe(7)]-alpha-MSH ([(111)In]DOTA-NDP-MSH) demonstrated the superiority of the DOTA-MSH(OCT) peptide, particularly with respect to the amount of radioactivity taken up by non-malignant organs, including bone, the most radiosensitive tissue. These results demonstrate that [(111)In]DOTA-MSH(OCT) specifically targets melanoma metastases and represents a lead compound for the development of therapeutic DOTA-MSH analogs.     

111In-labeled lactam bridge-cyclized alpha-melanocyte stimulating hormone peptide analogues for melanoma imaging

The purpose of this study was to examine the influence of the lactam bridge cyclization on melanoma targeting and biodistribution properties of the radiolabeled conjugates. Two novel lactam bridge-cyclized alpha-MSH peptide analogues, DOTA-CycMSH (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid-c[Lys-Nle-Glu-His-DPhe-Arg-Trp-Gly-Arg-Pro-Val-Asp]) and DOTA-GlyGlu-CycMSH (DOTA-Gly-Glu-c[Lys-Nle-Glu-His-DPhe-Arg-Trp-Gly-Arg-Pro-Val-Asp]), were synthesized and radiolabeled with (111)In. The internalization and efflux of (111)In-labeled CycMSH peptides were examined in B16/F1 melanoma cells. The melanoma targeting properties, pharmacokinetics, and SPECT/CT imaging of (111)In-labeled CycMSH peptides were determined in B16/F1 melanoma-bearing C57 mice. Both (111)In-DOTA-CycMSH and (111)In-DOTA-GlyGlu-CycMSH exhibited fast internalization and extended retention in B16/F1 cells. The tumor uptake values of (111)In-DOTA-CycMSH and (111)In-DOTA-GlyGlu-CycMSH were 9.53+/-1.41% injected dose/gram (% ID/g) and 10.40+/-1.40% ID/g at 2 h postinjection, respectively. Flank melanoma tumors were clearly visualized with (111)In-DOTA-CycMSH and (111)In-DOTA-GlyGlu-CycMSH by SPECT/CT images at 2 h postinjection. Whole-body clearance of the peptides was fast, with greater than 90% of the radioactivities cleared through urinary system by 2 h postinjection. There was low radioactivity (<0.8% ID/g) accumulated in blood and normal organs except kidneys at all time points investigated. Introduction of a negatively charged linker (-Gly-Glu-) into the peptide sequence decreased the renal uptake by 44% without affecting the tumor uptake at 4 h postinjection. High receptor-mediated melanoma uptakes coupled with fast whole-body clearance in B16/F1 melanoma-bearing C57 mice demonstrated the feasibility of using (111)In-labeled lactam bridge-cyclized alpha-MSH peptide analogues as a novel class of imaging probes for receptor-targeting melanoma imaging.     

Synthesis and evaluation of novel gonadotropin-releasing hormone receptor-targeting peptides

The purpose of this study was to develop novel radiolabeled gonadotropin-releasing hormone (GnRH) receptor-targeting peptides for breast cancer imaging. Three novel 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA)-conjugated GnRH peptides were designed and synthesized. The radiometal chelator DOTA was conjugated to the epsilon or alpha amino group of D-lysine, or the epsilon amino group of L-lysine via an Ahx {aminohexanoic acid} linker to generate DOTA-Ahx-(D-Lys(6)-GnRH1), DOTA-Ahx-(D-Lys(6)-GnRH2) and DOTA-Ahx-(L-Lys(6)-GnRH3), respectively. The conjugation of the DOTA to the epsilon amino group of D-lysine (rather than alpha amino group of D-lysine nor epsilon amino group of L-lysine) maintained the nanomolar GnRH receptor binding affinity. The IC(50) values of DOTA-Ahx-(D-Lys(6)-GnRH1), DOTA-Ahx-(D-Lys(6)-GnRH2) and DOTA-Ahx-(L-Lys(6)-GnRH3) were 36.1 nM, 10.6 mM and 4.3 mM, respectively. Since only DOTA-Ahx-(D-Lys(6)-GnRH1) displayed nanomolar receptor binding affinity, the specific GnRH receptor binding of (111)In-DOTA-Ahx-(D-Lys(6)-GnRH1) was determined in human GnRH receptor membrane preparations. Furthermore, the biodistribution and tumor imaging properties of (111)In-DOTA-Ahx-(D-Lys(6)-GnRH1) were examined in MDA-MB-231 human breast cancer-xenografted nude mice. (111)In-DOTA-Ahx-(D-Lys(6)-GnRH1) exhibited specific GnRH receptor binding and rapid tumor uptake (1.76 ± 0.58% ID/g at 0.5 h postinjection) coupled with fast whole-body clearance through the urinary system. The MDA-MB-231 human breast cancer-xenografted tumor lesions were clearly visualized by single photon emission computed tomography (SPECT)/CT at 1 h postinjection of (111)In-DOTA-Ahx-(D-Lys(6)-GnRH1). The profound impact of DOTA position on the binding affinity of the GnRH peptide provided a new insight into the design of novel radiolabeled GnRH peptides. The successful imaging of MDA-MB-231 human breast cancer-xenografted tumor lesions using (111)In-DOTA-Ahx-(D-Lys(6)-GnRH1) suggested its potential as a novel imaging probe for human breast cancer imaging.     

Conjugation of DOTA using isolated phenolic active esters: the labeling and biodistribution of albumin as blood pool marker

A convenient method for the functionalization of proteins with DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) has been developed. For this purpose DOTA was converted into a series of different monoreactive activated phenolic esters. The esters were prepared in a single step from commercially available DOTA, using 1-ethyl-3-[3-(dimethylamino)propyl]carbodiimide or 1,3-dicyclohexylcarbodiimide as coupling agent. The resulting activated esters were isolated by HPLC, lyophilized, and stored for future applications. In solid form the compounds exhibit high hydrolytic stability. The reactions with proteins proceeded in good yields. The conjugation and subsequent radiolabeling of the 4-nitrophenol ester of DOTA with 67Ga was investigated with rat serum albumin. A time-dependent biodistribution study in tumor bearing rats was conducted to demonstrate the integrity of the albumin conjugate. These results suggest that phenolic esters of DOTA represent versatile reagents to conjugate DOTA with proteins and other biomolecules in high yields.     

Discovery of substituted 3H-pyrido[2,3-d]pyrimidin-4-ones as potent,biased, and orally bioavailable sst2 agonist

The discovery of a novel 3H-pyrido[2,3-d]pyrimidin-4-one series as potent and biased sst2 agonists is described. This class of molecules exhibits excellent sst2 potency and selectivity against sst1, sst3, and sst5 receptors, and they are significantly more potent at inhibiting cAMP production than inducing internalization. The orally bioavailable 6-(3-chloro-5-methylphenyl)-3-(3-fluoro-5-hydroxyphenyl)-5-({methyl[(2S)-pyrrolidin-2-ylmethyl]amino}methyl)-3H,4H-pyrido[2,3-d]pyrimidin-4-one (36) also suppresses GH secretion in GHRH-challenged rats in a dose-dependent manner.     

Design, synthesis, and biological evaluation of an antagonist-bombesin analogue as targeting vector

The gastrin releasing peptide receptor (GRP-R) is overexpressed on a number of tumors and cancer cell lines including pancreas, prostate, breast, gastrointestinal, and small cell lung cancer (SCLC). Radiolabeled bombesin (BBN) analogues have exhibited high binding affinity and specificity to the GRP-R. A bombesin analogue with an antagonist targeting vector at the C-terminus, DOTA-aminohexanoyl-[D-Phe(6), Leu-NHCH 2CH 2CH3(13), des Met(14)] BBN[6-14] (1, "Bomproamide"), has been synthesized and displays high binding affinity (IC50 = 1.36 +/- 0.09 nM) against (125)I-Tyr (4)-BBN in in vitro competitive assays using PC-3 cells. Maximum internalization of (111)In-1 reached 14% in PC-3 cells after 45 min of incubation. Rapid (0.25 h PI) and high (12.21 +/- 3.2%ID/g) pancreatic uptake of (111)In-1 was observed in healthy CF-1 mice, and 90% of the activity was blocked by coinjection of 100 mug of BBN. Rapid (0.25 h PI) and high uptake (6.90 +/- 1.06%ID/g) was observed in PC-3 prostate cancer xenografts in SCID mice, as well as visualized clearly in a SPECT/CT study. These results support the use of a bombesin construct with an antagonist C-terminal vector as a candidate of choice for specific in vivo imaging of tumors overexpressing GRP-receptors.     

Dimeric DOTA-alpha-melanocyte-stimulating hormone analogs: synthesis and in vivo characteristics of radiopeptides with high in vitro activity

Dimeric analogs of alpha-melanocyte-stimulating hormone (alpha-MSH) labeled with radiometals are potential candidates for diagnosis and therapy of melanoma by receptor-mediated tumor targeting. Both melanotic and amelanotic melanomas (over-)express the melanocortin-1 receptor (MC1-R), the target for alpha-MSH. In the past, dimerized MSH analogs have been shown to display increased receptor affinity compared to monomeric MSH, offering the possibility of improving the ratio between specific uptake of radiolabeled alpha-MSH by melanoma and nonspecific uptake by the kidneys. We have designed three linear dimeric analogs containing a slightly modified MSH hexapeptide core sequence (Nle-Asp-His-d-Phe-Arg-Trp) in parallel or antiparallel orientation, a short spacer, and the DOTA chelator for incorporation of the radiometal. In vitro, all three peptides were more potent ligands of the mouse B16-F1 melanoma cell melanocortin-1 receptor (MC1-R) than DOTA-NAPamide, which served as standard. The binding activity of DOTA-diHexa(NC-NC)-amide was 1.75-fold higher, that of diHexa(NC-NC)-Gly-Lys(DOTA)-amide was 3.37-fold higher, and that of DOTA-diHexa(CN-NC)-amide was 2.34-fold higher. Using human HBL melanoma cells, the binding activity of diHexa(NC-NC)-Gly-Lys(DOTA)-amide was sixfold higher than that of DOTA-NAPamide. Uptake by cultured B16-F1 cells was rapid and almost quantitative. In vivo, however, the data were less promising: tumor-to-kidney ratios 4 hr postinjection were 0.11 for [(111)In]DOTA-diHexa(NC-NC)-amide, 0.26 for diHexa(NC-NC)-Gly-Lys([(111)In]DOTA)-amide, and 0.36 for [(111)In]DOTA-diHexa(CN-NC)-amide, compared to 1.67 for [(111)In]DOTA-NAPamide. It appears that despite the higher affinity to the MC1-R of the peptide dimers and their excellent internalization in vitro, the uptake by melanoma tumors in vivo was lower, possibly because of reduced tissue penetration. More striking, however, was the marked increase of kidney uptake of the dimers, explaining the unfavorable ratios. In conclusion, although radiolabeled alpha-MSH dimer peptides display excellent receptor affinity and internalization, they are no alternative to the monomeric DOTA-NAPamide for in vivo application.     

Synthesis of aminooxy-functionalized lanthanide(III) chelates for carbonyl-group conjugation

The syntheses of three new aminooxy-tethered lanthanide(III) chelates, compounds 1-3, incorporating DOTA (= 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid), DTPA (= diethylenetriaminepentaacetic acid), or a substituted terpyridine (2,2',2',2'-[2,2': 6',2'-terpyridine-6,6'-diylbis(methylenenitrilo)]tetraacetic acid), respectively, are described. Reagents 1-3 can be used for carbonyl 'labeling', as shown by the formation of the corresponding oxime-ether bioconjugates of naltrexone (16) and 2-deoxy-beta-D-glucose (17) (Scheme 4).     

Distribution and characterisation of somatostatin receptor mRNA and binding sites in the brain and periphery.

The distribution and nature of (somatostatin) SRIF receptors and receptor mRNAs was studied in the brain and periphery of various laboratory animals using in situ hybridisation, autoradiography and radioligand binding. The messenger RNA (mRNA) expression of SRIF receptors msst1, msst2, msst3, msst4 and msst5 was studied in the adult mouse brain by in situ hybridisation histochemistry using specific oligonucleotide probes and compared to that of adult rats. As observed in rat brain, sst3 receptor mRNA is prominently expressed across the mouse brain, although equivalent binding has not yet been identified in situ. Sst1 and sst2 receptor mRNA expression, was prominent and again comparable to that observed in rat brain, whereas sst4 and especially sst5 receptor mRNA show comparatively low levels, although the former appears to be widely distributed while the latter could only be identified in a few nuclei. Altogether, the data are compatible with current knowledge, i.e. sst1 and sst2 receptor mRNA is prominent (both receptors have been functionally identified in the brain and for sst2 in the periphery), sst3 mRNA is highly expressed but in the absence of any functional correlate remains elusive. The expression of sst4 mRNA is comparatively low (especially when compared to what is seen in the lung, where high densities of sst4 receptors are present) and it remains to be seen whether sst5 receptor mRNA, which is confined to a few nuclei, will play a role in the brain, keeping in mind that high levels are found in the pituitary. Radioligand binding studies were performed in CCL39 cells expressing the five human recombinant receptors and compared to binding in membranes of rat cerebral cortex with [125I]Tyr11-SRIF14 which in the presence of 120 mM labels primarily sst1 receptor as suggested by the better correlation hsst1 and similar rank order of potency. The profile of [125I]Tyr3-octreotide labelled sites in rat cortex correlates better with recombinant sst2 than sst3 or sst5 binding profiles. Finally, [125I]LTT-SRIF28-labelled sites in rat lung express a sst4 receptor profile in agreement with previous findings. SRIF receptor autoradiography was performed in the brain and peripheral tissue of rat and/or guinea-pig using a number of ligands known to label recombinant SRIF receptors: [125I]LTT-SRIF28, [125I]CGP 23996, [125I]Tyr10-CST, or [125I]Tyr3-octreotide. Although, [125I]Tyr10-CST has been shown to label all five recombinant SRIF receptors, it is apparent that this radioligand is not useful for autoradiographic studies. By contrast, the other three ligands show good signal to noise ratios in rat or guinea-pig brain, rat lung, rat pancreas, or guinea-pig ileum. In most tissues, [125I]Tyr3-octreotide represents a prominent part of the binding (when compared to [125I]LTT-SRIF28 and [125I]CGP 23996), suggesting that sst2 receptors are strongly expressed in most tissues; it is only in rat lung that [125I]LTT-SRIF28 and [125I]CGP 23996 show marked binding, whereas [125I]Tyr3-octreotide does apparently label no sites, in agreement with the sole presence of sst4 receptors in this tissue.     

A potent and selective agonist for NK-2 tachykinin receptor

Replacement of the glycine in position 8 of the C-terminal heptapeptide NKA(4-10) with beta-alanine give rise to a potent and selective agonist for the NK-2 tachykinin receptor. The affinity of [beta-Ala8]-NKA(4-10) to the NK-2 receptor is enhanced by almost one order of magnitude as compared to NKA(4-10), while affinity decreases at about the same extent at NK-1 and NK-3 receptors, respectively. Synthesis and biological activities of a series of NKA(4-10) analogues systematically replaced in each position with beta-alanine are also reported.     

Somatostatin analogs and radiopeptides in cancer therapy

Since the discovery of somatostatin (sst) in 1973, numerous chemical and biological studies have been carried out to develop sst analogs with enhanced resistance to proteases and prolonged activity. Three highly potent sst analogs-octreotide, lanreotide, and vapreotide-are now available in the clinic, and demonstrate efficacy in the treatment of tumors of the pituitary and the gastroenteropancreatic tract. The most striking effect is the control of hormone hypersecretion associated with these tumors. Available data on growth suppression in patients indicate a limited antiproliferative action, tumor shrinkage is observed in 10-20% patients, and tumor stabilization in about half of the patients for duration of 8-16 months. Eventually, however, all patients escape from sst analog therapy with regard to both hormone hypersecretion and tumor growth, the only exception being observed in acromegalic patients who do not experience tachyphylaxis even after more than 10 years of daily octreotide injection. The mechanism underlying the escape phenomenon is not yet clarified. Regarding the molecular mechanisms involved in sst antineoplastic activity, both indirect and direct effects via specific somatostatin receptors (SSTRs) expressed in the target cells have be described. Direct action may result from blockade of mitogenic growth signal or induction of apoptosis following interaction with SSTRs. Indirect effects may be the result of reduced or inhibited secretion of growth-promoting hormones and growth factors that stimulate the growth of various types of cancer; also, inhibition of angiogenesis or influence on the immune system are important factors. Five SSTR subtypes have been identified so far, which are variably expressed in a variety of tumors such as gastroenteropancreatic (GEP) tumors, pituitary tumors, and carcinoid tumors. Although all five SSTR subtypes are linked to adenylate cyclase, they are now known to affect multiple other cellular signaling systems and hence they differentially participate in the regulation of the various cellular processes. The finding of several laboratories that SSTR-expressing tumors frequently contain two or more SSTR subtypes, and the recent discovery that SSTR subtypes might form homo/heterodimers to create a novel receptor with different functional characteristics, expand the array of selective SSTR activation pathways and subsequent intracellular signaling cascades. This may lead to improved clinical protocols that take into account possible synergistic interactions between the SSTR subtypes present on the same cancer cell. Radiolabeled sst analogs, such as [(111)In]-[diethylenetriamine pentaacetic acid (DTPA)-D-Phe(1)]-octreotide (OcreoScan), have proved to be very useful for tumor scintigraphy and internal radiotherapy of SSTR overexpressing tumors. The recent introduction of the metal chelator DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) considerably improved the stability of the radioconjugates, making possible the incorporation of a variety of radionuclides, such as (90)Y for receptor-mediated radionuclide therapy or (68)Ga for positron emission tomography (PET). Another promising area is the development of sst conjugates incorporating cytotoxic anticancer drugs.     

64Cu-labeled alpha-melanocyte-stimulating hormone analog for microPET imaging of melanocortin 1 receptor expression

The alpha-melanocyte-stimulating hormone (alpha-MSH) receptor (melanocortin type 1 receptor, or MC1R) plays an important role in the development and growth of melanoma cells. It was found that MC1R was overexpressed on most murine and human melanoma, making it a promising molecular target for melanoma imaging and therapy. Radiolabeled alpha-MSH peptide and its analogs that can specifically bind with MC1R have been extensively explored for developing novel agents for melanoma detection and radionuclide therapy. The goal of this study was to evaluate a 64Cu-labeled alpha-MSH analog, Ac-Nle-Asp-His-D-Phe-Arg-Trp-Gly-Lys(DOTA)-NH2 (DOTA-NAPamide), as a potential molecular probe for microPET imaging of melanoma and MC1R expression in melanoma xenografted mouse models. 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) conjugated NAPamide was synthesized and radiolabeled with 64Cu (t1/2=12 h) in NH4OAc (0.1 M; pH 5.5) buffered solution for 60 min at 50 degrees C. Cell culture studies reveal rapid and high uptake and internalization of 64Cu-DOTA-NAPamide in B16F10 cells. Over 90% of receptor-bound tracer is internalized at 3 h incubation. A cellular retention study demonstrates that the receptor-bound 64Cu-DOTA-NAPamide is slowly released from the B16F10 cells into the medium; 66% of the radioactivity is still associated with the cells even after 3 h incubation. The biodistribution of 64Cu-DOTA-NAPamide was then investigated in C57BL/6 mice bearing subcutaneous murine B16F10 melanoma tumors with high capacity of MC1R and Fox Chase Scid mice bearing human A375M melanoma with a relatively low number of MC1R receptors. Tumor uptake values of 64Cu-DOTA-NAPamide are found to be 4.63 +/- 0.45% and 2.49 +/- 0.31% ID/g in B16F10 and A375M xenografted melanoma at 2 h postinjection (pi), respectively. The B16F10 tumor uptake at 2 h pi is further inhibited to 2.29 +/- 0.24% ID/g, while A375M tumor uptake at 2 h pi remains 2.20 +/- 0.41% ID/g with a coinjection of excess alpha-MSH peptide. MicroPET imaging of 64Cu-DOTA-NAPamide in B16F10 tumor mice clearly shows good tumor localization. However, low A375M tumor uptake and poor tumor to normal tissue contrast were observed. This study demonstrates that 64Cu-DOTA-NAPamide is a promising molecular probe for alpha-MSH receptor positive melanoma PET imaging as well as MC1R expression imaging in living mice.     

Synthesis of Poly[APMA]-DOTA-64Cu conjugates for interventional radionuclide therapy of prostate cancer: assessment of intratumoral retention by micro-positron emission tomography

To develop new radiopharmaceuticals for interventional radionuclide therapy of locally recurrent prostate cancer, poly[N-(3-aminopropyl)methacrylamide] [poly(APMA)] polymers were synthesized by free radical precipitation polymerization in acetone-dimethylsulfoxide using N,N'-azobis(isobutyronitrile) as the initiator. The polymers were characterized with nuclear magnetic resonance, size exclusion chromatography, and dynamic light scattering (M(n) = 2.40 x 10(4), M(w)/M(n) = 1.87). Subsequently, poly[APMA] was coupled with 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) using 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride as an activator, followed by conjugation with (64)Cu radionuclide. Prolonged retention of poly[APMA]-DOTA-(64)Cu conjugates within the tumor tissues was demonstrated by micro-positron emission tomography at 24 hours following intra-tumoral injection of the conjugates to human prostate xenografts in mice. The data suggest that the poly[APMA]-DOTA-(64)Cu conjugates might be useful for interventional radionuclide therapy of locally recurrent prostate cancer in humans.     

Evaluation of maleimide derivative of DOTA for site-specific labeling of recombinant affibody molecules

Affibody molecules are a new class of small (7 kDa) scaffold affinity proteins, which demonstrate promising properties as agents for in vivo radionuclide targeting. The Affibody scaffold is cysteine-free and therefore independent of disulfide bonds. Thus, a single thiol group can be engineered into the protein by introduction of one cysteine. Coupling of thiol-reactive bifunctional chelators can enable site-specific labeling of recombinantly produced Affibody molecules. In this study, the use of 1,4,7,10-tetraazacyclododecane-1,4,7-tris-acetic acid-10-maleimidoethylacetamide (MMA-DOTA) for 111 In-labeling of anti-HER2 Affibody molecules His 6-Z HER2:342-Cys and Z HER2:2395-Cys has been evaluated. The introduction of a cysteine residue did not affect the affinity of the proteins, which was 29 pM for His 6-Z HER2:342-Cys and 27 pM for Z HER2:2395-Cys, comparable with 22 pM for the parental Z HER2:342. MMA-DOTA was conjugated to DTT-reduced Affibody molecules with a coupling efficiency of 93% using a 1:1 molar ratio of chelator to protein. The conjugates were labeled with 111 In to a specific radioactivity of up to 7 GBq/mmol, with preserved binding for the target HER2. In vivo, the non-His-tagged variant 111 In-[MMA-DOTA-Cys61]-Z HER2:2395-Cys demonstrated appreciably lower liver uptake than its His-tag-containing counterpart. In mice bearing HER2-expressing LS174T xenografts, 111 In-[MMA-DOTA-Cys61]-Z HER2:2395-Cys showed specific and rapid tumor localization, and rapid clearance from blood and nonspecific compartments, leading to a tumor-to-blood-ratio of 18 +/- 8 already 1 h p.i. Four hours p.i., the tumor-to-blood ratio was 138 +/- 8. Xenografts were clearly visualized already 1 h p.i.