One-step ¹8F-labeling of carbohydrate-conjugated octreotate-derivatives containing a silicon-fluoride-acceptor (SiFA): in vitro and in vivo evaluation as tumor imaging agents for positron emission tomography (PET).

The synthesis, radiolabeling, and initial evaluation of new silicon-fluoride acceptor (SiFA) derivatized octreotate derivatives is reported. So far, the main drawback of the SiFA technology for the synthesis of PET-radiotracers is the high lipophilicity of the resulting radiopharmaceutical. Consequently, we synthesized new SiFA-octreotate analogues derivatized with Fmoc-NH-PEG-COOH, Fmoc-Asn(Ac3AcNH-ß-Glc)-OH, and SiFA-aldehyde (SIFA-A). The substances could be labeled in high yields (38 ± 4%) and specific activities between 29 and 56 GBq/µmol in short synthesis times of less than 30 min (e.o.b.). The in vitro evaluation of the synthesized conjugates displayed a sst2 receptor affinity (IC50 = 3.3 ± 0.3 nM) comparable to that of somatostatin-28. As a measure of lipophilicity of the conjugates, the log P(ow) was determined and found to be 0.96 for SiFA-Asn(AcNH-ß-Glc)-PEG-Tyr³-octreotate and 1.23 for SiFA-Asn(AcNH-ß-Glc)-Tyr³-octreotate, which is considerably lower than for SiFA-Tyr³-octreotate (log P(ow) = 1.59). The initial in vivo evaluation of [¹8F]SiFA-Asn(AcNH-ß-Glc)-PEG-Tyr³-octreotate revealed a significant uptake of radiotracer in the tumor tissue of AR42J tumor-bearing nude mice of 7.7% ID/g tissue weight. These results show that the high lipophilicity of the SiFA moiety can be compensated by applying hydrophilic moieties. Using this approach, a tumor-affine SiFA-containing peptide could successfully be used for receptor imaging for the first time in this proof of concept study.

Treatment of peritoneal carcinomatosis by targeted delivery of the radio-labeled tumor homing peptide bi-DTPA-[F3]2 into the nucleus of tumor cells.

BACKGROUND: Alpha-particle emitting isotopes are effective novel tools in cancer therapy, but targeted delivery into tumors is a prerequisite of their application to avoid toxic side effects. Peritoneal carcinomatosis is a widespread dissemination of tumors throughout the peritoneal cavity. As peritoneal carcinomatosis is fatal in most cases, novel therapies are needed. F3 is a tumor homing peptide which is internalized into the nucleus of tumor cells upon binding to nucleolin on the cell surface. Therefore, F3 may be an appropriate carrier for alpha-particle emitting isotopes facilitating selective tumor therapies. PRINCIPAL FINDINGS: A dimer of the vascular tumor homing peptide F3 was chemically coupled to the alpha-emitter (213)Bi ((213)Bi-DTPA-[F3](2)). We found (213)Bi-DTPA-[F3](2) to accumulate in the nucleus of tumor cells in vitro and in intraperitoneally growing tumors in vivo. To study the anti-tumor activity of (213)Bi-DTPA-[F3](2) we treated mice bearing intraperitoneally growing xenograft tumors with (213)Bi-DTPA-[F3](2). In a tumor prevention study between the days 4-14 after inoculation of tumor cells 6x1.85 MBq (50 microCi) of (213)Bi-DTPA-[F3](2) were injected. In a tumor reduction study between the days 16-26 after inoculation of tumor cells 6x1.85 MBq of (213)Bi-DTPA-[F3](2) were injected. The survival time of the animals was increased from 51 to 93.5 days in the prevention study and from 57 days to 78 days in the tumor reduction study. No toxicity of the treatment was observed. In bio-distribution studies we found (213)Bi-DTPA-[F3](2) to accumulate in tumors but only low activities were found in control organs except for the kidneys, where (213)Bi-DTPA-[F3](2) is found due to renal excretion. CONCLUSIONS/SIGNIFICANCE: In conclusion we report that (213)Bi-DTPA-[F3](2) is a novel tool for the targeted delivery of alpha-emitters into the nucleus of tumor cells that effectively controls peritoneal carcinomatosis in preclinical models and may also be useful in oncology.

(18)F-Fluoroglucosylation of peptides, exemplified on cyclo(RGDfK).

PURPOSE: Oxime formation between an aminooxy-functionalized peptide and an (18)F-labelled aldehyde has recently been introduced as a powerful method for the rapid one-step chemoselective synthesis of radiofluorinated peptides. MATERIALS AND METHODS: Here, the potential of using routinely produced and thus readily available [(18)F]fluorodeoxyglucose ([(18)F]FDG) as the aldehydic prosthetic group was investigated using an aminooxyacetyl-conjugated cyclic RGD peptide (cyclo(RGDfK(Aoa-(Boc)) as a model peptide. RESULTS: The use of [(18)F]FDG from routine production ([(18)F]FDGTUM) containing an excess of D: -glucose did not allow the radiosynthesis of [(18)F]FDG-RGD in activities >37 MBq in reasonable yield, rendering the direct use of clinical grade [(18)F]FDG for the routine clinical synthesis of (18)F-labelled peptides impossible. Using no-carrier-added (n.c.a.) [(18)F]FDG obtained via HPLC separation of [(18)F]FDGTUM from excess glucose, however, afforded [(18)F]FDG-RGD in yields of 56-93% (decay corrected) and activities up to 37 MBq. Suitable reaction conditions were 20 min at 120 degrees C and pH 2.5, and a peptide concentration of 5 mM. In a preliminary in vivo biodistribution study in M21 melanoma-bearing nude mice, [(18)F]FDG-RGD showed increased tumour accumulation compared to the "gold standard" [(18)F]galacto-RGD (2.18 vs 1.49 %iD/g, respectively, at 120 min after injection), but also slightly increased uptake in non-target organs, leading to comparable tumour/organ ratios for both compounds. CONCLUSION: These data demonstrate that chemoselective (18)F-labelling of aminooxy-functionalized peptides using n.c.a. [(18)F]FDG represents a radiofluorination/glycosylation strategy that allows preparation of (18)F-labelled peptides in high yield with suitable pharmacokinetics. As soon as the necessary n.c.a. preparation of [(18)F]FDG prior to reaction with the Aoa-peptide can be implemented in a fully automated [(18)F]FDG-synthesis, [(18)F]fluoroglucosylation of peptides may represent a promising alternative to currently used chemoselective one-step (18)F-labelling protocols.

Preclinical evaluation of the alpha-particle generator nuclide 225Ac for somatostatin receptor radiotherapy of neuroendocrine tumors.

PURPOSE: Peptide receptor radionuclide therapy (PRRT) using somatostatin analogues labeled with beta-particle-emitting isotopes such as 90Y or 177Lu has been a promising treatment strategy for metastasized neuroendocrine tumors. Although remission can be accomplished in a high percentage of neuroendocrine tumors, some tumors do not respond to this treatment. alpha-Emitting isotopes-such as the 10-day half-life alpha-emitting generator nuclide Actinum-225 (225Ac)-are characterized by extremely high cytotoxic activity on the cellular level, and may be superior in the treatment of neuroendocrine tumors not responding to PRRT using beta-emitting isotopes. EXPERIMENTAL DESIGN: Radiolabeling of 225Ac 1,4,7,10-tetra-azacylododecane N,N',N'',N'''-J-tetraacetic acid-Tyr3-octreotide (DOTATOC) was done at pH 5 (60 minutes at 70 degrees C) without further purification. Biodistribution in nude mice bearing AR42J rat pancreas neuroendocrine tumor xenografts were measured for up to 24 hours. Toxicity was tested by weight changes, retention variables (blood urea nitrogen and creatine), and histopathology in mice 7 months after treatment with 10 to 130 kBq (n = 4-5). Therapeutic efficacy was assessed by tumor weighing in animals treated 4 days after xenotransplantation and compared with 177Lu-DOTATOC as a reference. RESULTS: Activities up to 20 kBq had no significant toxic effects in mice. In contrast, activities higher than 30 kBq induced tubular necrosis. Biodistribution studies revealed that 225Ac-DOTATOC effectively accumulated in neuroendocrine xenograft tumors. 225Ac-DOTATOC activities were shown to be nontoxic (12-20 kBq), reduced the growth of neuroendocrine tumors, and showed improved efficacy compared with 177Lu-DOTATOC. CONCLUSIONS: 225Ac might be suitable to improve PRRT in neuroendocrine tumors.