PET and SPECT imaging of a radiolabeled minigastrin analogue conjugated with DOTA, NOTA, and NODAGA and labeled with (64)Cu, (68)Ga, and (111)In.

Cholecystokinin-2 (CCK-2) receptors, overexpressed in cancer types such as small cell lung cancers (SCLC) and medullary thyroid carcinomas (MTC), may serve as targets for peptide receptor radionuclide imaging. A variety of CCK and gastrin analogues has been developed, but a major drawback is metabolic instability or high kidney uptake. The minigastrin analogue PP-F11 has previously been shown to be a promising peptide for imaging of CCK-2 receptor positive tumors and was therefore further evaluated. The peptide was conjugated with one of the macrocyclic chelators DOTA, NOTA, or NODAGA. The peptide conjugates were then radiolabeled with either (68)Ga, (64)Cu, or (111)In. All (radio)labeled compounds were evaluated in vitro (IC50) and in vivo (biodistribution and PET/CT and SPECT/CT imaging). IC50 values were in the low nanomolar range for all compounds (0.79-1.51 nM). In the biodistribution studies, (68)Ga- and (111)In-labeled peptides showed higher tumor-to-background ratios than the (64)Cu-labeled compounds. All tested radiolabeled compounds clearly visualized the CCK2 receptor positive tumor in PET or SPECT imaging. The chelator did not seem to affect in vivo behavior of the peptide for (111)In- and (68)Ga-labeled peptides. In contrast, the biodistribution of the (64)Cu-labeled peptides showed high uptake in the liver and in other organs, most likely caused by high blood levels, probably due to dissociation of (64)Cu from the chelator and subsequent transchelation to proteins. Based on the present study, (68)Ga-DOTA-PP-F11 might be a promising radiopharmaceutical for PET/CT imaging of CCK2 receptor expressing tumors such as MTC and SCLC. Clinical studies are warranted to investigate the potential of this tracer.

Absence of pituitary adenylate cyclase-activating polypeptide-stimulated transcription of the human glycoprotein alpha-subunit gene in LbetaT2 gonadotrophs reveals disrupted cAMP-mediated gene transcription.

Hormone regulation of anterior pituitary expression of the common glycoprotein hormone alpha-subunit (alphaGSU) is mediated by multiple response elements residing in the first -435 bp of the human promoter. In rat pituitary cells and mouse alphaT3-1 precursor gonadotrophs, the human alphaGSU promoter is strongly responsive to activators of the adenylyl cyclase/cAMP pathway, such as the hypothalamic releasing hormone, pituitary adenylate cyclase-activating polypeptide (PACAP) and forskolin (an adenylyl cyclase activator). However, the role of PACAP and cAMP in regulating alphaGSU transcription in the more differentiated LbetaT2 gonadotroph is unclear. Here, we investigate the regulation of the human alphaGSU promoter by PACAP and forskolin in LbetaT2 and alphaT3-1 gonadotrophs. PACAP failed to stimulate alphaGSU promoter activity or cAMP production in LbetaT2 cells, in marked contrast to alphaT3-1 cells. LbetaT2 gonadotrophs expressed extremely low levels of any PACAP type 1 receptors (PAC(1)-R) isoform by RT-PCR and lacked PAC(1)-R by radioligand binding. Forskolin stimulated the alphaGSU promoter in LbetaT2 cells, but by less than 30% of the response seen in alphaT3-1 gonadotrophs. This blunted cAMP transcriptional effect was not due to different levels of cAMP generation, or altered expression of the cAMP target proteins CREB, Akt, CBP or ICER. However, only LbetaT2 cells showed detectable expression of the protein kinase A type IIalpha regulatory subunit. Binding of activating transcription factor-2 and phosphorylated CREB to the consensus CRE was observed in both LbetaT2 and alphaT3-1 gonadotrophs, yet forskolin failed to stimulate either CRE- or CREB-mediated transcription in LbetaT2 cells. Collectively, these data demonstrate the lack of functional PACAP receptors in LbetaT2 gonadotrophs, and a pronounced attenuation in the responsiveness of this differentiated gonadotroph cell line to cAMP stimulus.

Copper bis(diphosphine) complexes: radiopharmaceuticals for the detection of multi-drug resistance in tumours by PET.

Experience with imaging of the multi-drug resistance (MDR) phenotype in tumours using technetium-99m sestamibi, a substrate of the P-glycoprotein (Pgp) transporter, suggests that better quantification of images and separation of MDR from other variables affecting tracer uptake in tumours are required. One approach to these problems is the development of short half-life positron-emitting tracers which are substrates of Pgp. Several lipophilic cationic copper(I) bis(diphosphine) complexes labelled with copper-64 have been synthesised and evaluated in vitro as substrates for Pgp. The synthesis is rapid and efficient with no need for purification steps. The chemistry is suitable for use with very short half-life radionuclides such as copper-62 (9.7 min) and copper-60 (23.7 min). Incubation of the complexes with human serum in vitro showed that they are sufficiently stable in serum to support clinical imaging, and the more lipophilic members of the series are taken up rapidly by cells (Chinese hamster ovary and human ovarian carcinoma) in vitro with great avidity. Uptake in human ovarian carcinoma cells is significantly reduced after several months of conditioning in the presence of doxorubicin, which induces increased Pgp expression. Uptake in hooded rat sarcoma (HSN) cells, which express Pgp, is significantly increased in the presence of the MDR modulator cyclosporin A. Biodistribution studies in hooded rats show rapid blood clearance, excretion through both kidneys and liver, and low uptake in other tissues. The one complex investigated in HSN tumour-bearing rats showed uptake in tumour increasing up to 30 min p.i. while it was decreasing in other tissues. We conclude that diphosphine ligands offer a good basis for development of radiopharmaceuticals containing copper radionuclides, and that this series of complexes should undergo further evaluation in vivo as positron emission tomography imaging agents for MDR.

99mTc-HYNIC-[Tyr3]-octreotide for imaging somatostatin-receptor-positive tumors: preclinical evaluation and comparison with 111In-octreotide.

UNLABELLED: In this paper we describe the preclinical evaluation of 99mTc-hydrazinonicotinyl-Tyr3-octreotide (HYNIC-TOC) using different coligands for radiolabeling and a comparison of their in vitro and in vivo properties with 111In-diethylenetriaminepentaacetic acid (DTPA)-octreotide. METHODS: HYNIC-TOC was radiolabeled at high specific activities using tricine, ethylenediaminediacetic acid (EDDA), and tricine-nicotinic acid as coligand systems. Receptor binding was tested using AR42J rat pancreatic tumor cell membranes. Internalization and protein binding studies were performed, and biodistribution and tumor uptake were determined in AR42J tumor-bearing nude mice. RESULTS: All 99mTc-labeled HYNIC peptides showed retained somatostatin-receptor binding affinities (Kd < 2.65 nM). Protein binding and internalization rates were dependent on the coligand used. Specific tumor uptake between 5.8 and 9.6 percentage injected dose per gram (%ID/g) was found for the 99mTc-labeled peptides, compared with 4.3 %ID/g for 111In-DTPA-octreotide. Tricine as coligand showed higher activity levels in muscle, blood, and liver, whereas tricine-nicotinic acid produced significant levels of activity in the gastrointestinal tract. EDDA showed the most promising overall biodistribution profile, with tumor-to-liver and tumor-to-gastrointestinal tract ratios similar to those obtained with 111In-DTPA-octreotide, lower ratios in blood and muscle, but considerably higher tumor-to-kidney ratios. CONCLUSION: TOC can be radiolabeled to high specific activities using HYNIC as a bifunctional chelator. The high specific tumor uptake, rapid blood clearance, and predominantly renal excretion make 99mTc-EDDA-HYNIC-TOC a promising candidate for an alternative to 111In-DTPA-octreotide for tumor imaging.

Metal complexes of bleomycin: evaluation of [Rh-105]-bleomycin for use in targeted radiotherapy.

Bleomycin has been used as a carrier for several radioisotopes; however, its potential for clinical use has been limited either by the in vivo stability of the complexes or the half-life of the isotope used. The chemical, biological, and radiological properties of 105Rhodium appear to make it an ideal choice for targeted radiotherapy. The synthesis and purification of a hereto unreported 105Rhodium-bleomycin (105Rh-BLM) complex is described. The stability of this complex in plasma is sufficient to allow targeted delivery of the radioisotope. 57Cobalt-bleomycin was studied under identical conditions for comparative purposes. The suitability of 105Rh-BLM for targeted therapy, which appears to be limited by the renal clearance of this agent, is discussed.