Evaluation of Re/99mTc-labeled somatostatin receptor-targeting peptide complexes synthesized via direct metal cyclization

With interest in the development of somatostatin receptor (SSTR) targeting agents for potential application in diagnostic SPECT imaging (99mTc) or Peptide Radionuclide Receptor Therapy (PRRT, 186Re or 188Re) of neuroendocrine tumors, we present herein 99mTc/Re (radio)complexes synthesized by the integrated (radio)labeling approach of peptide cyclization via metal complexation. In particular, we utilized the potent SSTR2 peptide antagonist sequence DOTA-4-NO2-Phe-c(DCys-Tyr-DTrp-Lys-Thr-Cys)-DTyr-NH2 (DOTA-sst2-ANT) and report the syntheses and in vitro evaluations of its respective [99mTc]Tc/Re-cyclized peptides ([99mTc]Tc/Re-cyc-DOTA-sst2-ANT). The Re-cyc-DOTA-sst2-ANT complex was synthesized via an on-resin Re(V)-cyclization reaction using the ReOCl3(PPh3)2 precursor and consisted of three isomers characterized by LC–ESI-MS. The [99mTc]Tc-cyclized analogue was prepared via a ligand exchange reaction of the [99mTc][TcO]3+ core through a [99mTc]Tc-glucoheptonate intermediate with linear DOTA-sst2-ANT and was characterized by comparative HPLC studies against Re-cyc-DOTA-sst2-ANT. Good in vitro binding affinity was demonstrated in SSTR-expressing cells (AR42J) by the Re-cyc-DOTA-sst2-ANT major isomer, similar to the potent binder Lu-DOTA-sst2-ANT, in which the Lu metal was complexed by the bifunctional chelator DOTA versus via peptide cyclization. [99mTc]Tc-cyc-DOTA-sst2-ANT was obtained in high radiochemical yield, also with an elution pattern of three isomers observed by HPLC analysis, which were comparable yet not identical to those of Re-cyc-DOTA-sst2-ANT. The [99mTc]Tc-tracer complex was shown to be hydrophilic, and stability studies at 4 h demonstrated that it remained intact in both PBS and in rat serum, with low non-specific rat serum protein binding, while exhibiting more moderate stability in 1 mM cysteine. These findings demonstrate that direct Re/[99mTc]Tc-cyclization of DOTA-sst2-ANT is feasible and may be used as an alternative approach to the bifunctional chelate labeling strategy. However, given that the non-radioactive (Re) and radiotracer (99mTc) analogues are not identical and both form isomeric products in equilibrium, additional design modifications will be necessary prior to in vivo application of [99mTc]Tc/Re-cyc-DOTA-sst2-ANT.

Development and Preclinical Evaluation of 99mTc- and 186Re-Labeled NOTA and NODAGA Bioconjugates Demonstrating Matched Pair Targeting of GRPR-Expressing Tumors

PURPOSE

The goal of this work was to develop hydrophilic gastrin-releasing peptide receptor (GRPR)-targeting complexes of the general formula fac-[M(CO)3(L)]+ [M = natRe, 99mTc, 186Re; L: NOTA for 1, NODAGA for 2] conjugated to a powerful GRPR peptide antagonist (DPhe-Gln-Trp-Ala-Val-Gly-His-Sta-Leu-NH2) via a 6-aminohexanoic acid linker.

PROCEDURES

Metallated-peptides were prepared employing the [M(OH2)3(CO)3]+ [M = Re, 99mTc, 186Re] precursors. Re-1/2 complexes were characterized with HR-MS. IC50 studies were performed for peptides 1/2 and their respective Re-1/2 complexes in a binding assay utilizing GRPR-expressing human PC-3 prostate cancer cells and [125I]I-Tyr4-BBN as the competing ligand. The 99mTc/186Re-complexes were identified by HPLC co-injection with their Re-analogues. All tracers were challenged in vitro at 37 °C against cysteine/histidine (phosphate-buffered saline 10 mM, pH 7.4) and rat serum. Biodistribution and micro-SPECT/CT imaging of [99mTc]Tc-1/2 and [186Re]Re-2 were performed in PC-3 tumor-bearing ICR SCID mice.

RESULTS

High in vitro receptor affinity (IC50 2-3 nM) was demonstrated for all compounds. The 99mTc/186Re-tracers were found to be hydrophilic (log D7.4 ≤ - 1.35) and highly stable. Biodistribution in PC-3 xenografted mice revealed good tumor uptake (%ID/g at 1 h: 4.3 ± 0.7 for [99mTc]Tc-1, 8.3 ± 0.9 for [99mTc]Tc-2 and 4.2 ± 0.8 for [186Re]Re-2) with moderate retention over 24 h. Rapid renal clearance was observed for [99mTc]Tc-2 and [186Re]Re-2 (> 84 % at 4 h), indicating favorable pharmacokinetics. Micro-SPECT/CT images for the 99mTc-tracers clearly visualized PC-3 tumors in agreement with the biodistribution data and with superior imaging properties found for [99mTc]Tc-2.

CONCLUSIONS

[99mTc]Tc-2 shows promise for further development as a GRPR-imaging agent. [186Re]Re-2 demonstrated very similar in vivo behavior to [99mTc]Tc-2, and further studies are therefore justified to explore the theranostic potential of our approach for targeting of GRPR-positive cancers.

Direct labeling of a somatostatin receptor antagonist via peptide cyclization with Re, 99mTc and 186Re metal centers: Radiochemistry and in vitro evaluation

INTRODUCTION

With the long-term goal of developing a diagnostic (^99mTc) and therapeutic (¹⁸⁶Re) agent pair for targeting somatostatin receptor (SSTR)-positive neuroendocrine tumors (NETs), we developed novel metal-cyclized peptides through direct labeling of the potent SSTR2 antagonist Ac-4-NO₂-Phe-c(DCys-Tyr-DTrp-Lys-Thr-Cys)-DTyr-NH₂ (1) with Re (in Re-1), ^99mTc (in [^99mTc]Tc-1) and ¹⁸⁶Re (in [¹⁸⁶Re]Re-1).

METHODS

Re-1 was characterized by LC-ESI-MS and HR-ESI-MS and was tested for receptor affinity in SSTR-expressing cells (AR42J). Radiolabeling of the peptide was achieved via ligand exchange from ^99mTc-labeled glucoheptonate or [¹⁸⁶Re]ReOCl₃(PPh₃)₂, yielding [^99mTc]Tc-1 or [¹⁸⁶Re]Re-1, respectively. In vitro stability of [^99mTc]Tc-1/[¹⁸⁶Re]Re-1 in PBS (10 mM) at pH 7.4 and 37 °C was determined by HPLC analysis. Moreover, [^99mTc]Tc-1 stability was tested in cysteine (1 mM) and rat serum under the same conditions.

RESULTS

Re-1 consisted of two isomers, confirmed by LC-ESI-MS, with good SSTR2 affinity (IC₅₀ = 43 ± 6 nM). Optimization of the ^99mTc labeling through varying reaction parameters such as pH, reaction time, and Sn²⁺ and ligand concentrations resulted in high radiochemical yield (RCY ≥92%). Similarly, [¹⁸⁶Re]Re-1 was prepared in reasonable RCY (≥50%). Both ^99mTc/¹⁸⁶Re-tracers consisted of two product isomers as identified by HPLC co-injection with Re-1. While [^99mTc]Tc-1 was sufficiently stable in vitro (≥71% intact through 4 h in PBS, cysteine and rat serum), [¹⁸⁶Re]Re-1 exhibited more moderate in vitro stability (58% intact after 1 h in PBS).

CONCLUSIONS

Novel ^99mTc/¹⁸⁶Re-cyclized SSTR2 antagonist peptides were synthesized and characterized using the Re-cyclized analogue as a reference. Due to the nanomolar SSTR2 affinity of Re-1 and good in vitro stability of [^99mTc]Tc-1, the latter shows early promise for development as a radiodiagnostic agent for SSTR-expressing NETs.

ADVANCES IN KNOWLEDGE AND IMPLICATIONS FOR PATIENT CARE

The ^99mTc-cyclized complex showed promising in vitro properties, and future in vivo studies will determine the potential for translating such a design into the human clinic.

Preclinical Evaluation of Novel 64Cu-Labeled Gastrin-Releasing Peptide Receptor Bioconjugates for PET Imaging of Prostate Cancer

We report herein the preclinical evaluation of new [⁶⁴Cu]Cu-gastrin-releasing peptide receptor (GRPR)-targeting tracers, employing the potent peptide antagonist _DPhe-Gln-Trp-Ala-VaI-Gly-His-Sta-Leu-NH₂ conjugated to NOTA (in 1) or NODAGA (in 2) chelators via a 6-aminohexanoic acid linker. The Cu-1/2 metalated peptides were synthesized by reacting 1/2 with CuCl₂ and were characterized by LC-ESI-MS and HR-ESI-MS. Cu-1/2 exhibited high GRPR-binding affinities with IC₅₀ values <3 nM, as measured in a competition assay using the GRPR-expressing human PC-3 prostate cancer cell line and [¹²⁵I]I-Tyr⁴-BBN as the competing ligand. Tracers [⁶⁴Cu]Cu-1/2 were prepared in quantitative radiochemical yield (by radio-HPLC), and their identities were confirmed by coelution with their Cu-1/2 standards via comparative HPLC studies. Lipophilicity was measured in 1-octanol/PBS (pH 7.4), and the negative log D_7.4 values (≤-1) confirmed the anticipated hydrophilic character for [⁶⁴Cu]Cu-1/2. Both tracers demonstrated excellent in vitro stability, with ≥98% remaining intact through 24 h at physiological conditions (PBS, pH 7.4, 37 °C). Biodistribution in PC-3 tumor-bearing mice demonstrated good tumor uptake (%ID/g at 4 h: 4.34 ± 0.71 for [⁶⁴Cu]Cu-1, 3.92 ± 1.03 for [⁶⁴Cu]Cu-2) and rapid renal clearance (≥87% ID at 4 h). Tumor uptake was receptor-mediated, as verified by parallel GRPR-blocking studies. Small-animal PET/CT imaging studies validated the biodistribution data. These preclinical data support that the [⁶⁴Cu]Cu-1/2 tracers show promise for further development as diagnostic PET imaging agents of GRPR-expressing tumors.

NOTA and NODAGA [99mTc]Tc- and [186Re]Re-Tricarbonyl Complexes: Radiochemistry and First Example of a [99mTc]Tc-NODAGA Somatostatin Receptor-Targeting Bioconjugate

With the long-term goal of developing theranostic agents for applications in nuclear medicine, in this work we evaluated the well-known NOTA and NODAGA chelators as bifunctional chelators (BFCs) for the [^99mTc/¹⁸⁶Re]Tc/Re-tricarbonyl core. In particular, we report model complexes of the general formula fac-[M(L)(CO)₃]⁺ (M = Re, ^99mTc, ¹⁸⁶Re) where L denotes NOTA-Pyr (1) or NODAGA-Pyr (2), which are derived from conjugation of NOTA/NODAGA with pyrrolidine (Pyr). Further, as proof-of-principle, we synthesized the peptide bioconjugate NODAGA-sst₂-ANT (3) and explored its complexation with the fac-[Re(CO)₃]⁺ and fac-[^99mTc][Tc(CO)₃]⁺ cores; sst₂-ANT denotes the somatostatin receptor (SSTR) antagonist 4-NO₂-Phe-c(DCys-Tyr-DTrp-Lys-Thr-Cys)-DTyr-NH₂. Rhenium complexes Re-1 through Re-3 were synthesized and characterized spectroscopically, and receptor binding affinity was demonstrated for Re-3 in SSTR-expressing cells (AR42J, IC₅₀ = 91 nM). Radiolabeled complexes [^99mTc]Tc/[¹⁸⁶Re]Re-1/2 and [^99mTc]Tc-3 were prepared in high radiochemical yield (>90%, determined by radio-HPLC) by reacting [^99mTc]/[¹⁸⁶Re][Tc/Re(OH₂)₃(CO)₃]⁺ with 1-3 and correlated well with the respective Re-1 through Re-3 standards in comparative HPLC studies. All radiotracers remained intact through 24 h (^99mTc-labeled complexes) or 48 h (¹⁸⁶Re-labeled complexes) against 1 mM l-histidine and 1 mM l-cysteine (pH 7.4, 37 °C). Similarly, rat serum stability studies displayed no decomposition and low nonspecific binding of 9-24% through 4 h. Biodistribution of [^99mTc]Tc-3 in healthy CF-1 mice demonstrated a favorable pharmacokinetic profile. Rapid clearance was observed within 1 h post-injection, predominantly via the renal system (82% of the injected dose was excreted in urine by 1 h), with low kidney retention (% ID/g: 11 at 1 h, 5 at 4 h, and 1 at 24 h) and low nonspecific uptake in other organs/tissues. Our findings establish NOTA and NODAGA as outstanding BFCs for the fac-[M(CO)₃]⁺ core in the design and development of organometallic radiopharmaceuticals. Future in vivo studies of [^99mTc]Tc- and [¹⁸⁶Re]Re-tricarbonyl complexes of NODAGA/NOTA-biomolecule conjugates will further probe the potential of these chelates for nuclear medicine applications in diagnostic imaging and targeted radiotherapy, respectively.

Mangiferin functionalized radioactive gold nanoparticles (MGF-198AuNPs) in prostate tumor therapy: green nanotechnology for production, in vivo tumor retention and evaluation of therapeutic efficacy

We report here production of MGF-¹⁹⁸AuNPs and its application for prostate tumor therapy.

Lox-dependent gene expression in transgenic plants obtained via Agrobacterium-mediated transformation

Lox sites of the Cre/lox recombination system from bacteriophage P1 were analyzed for their ability to affect on transgene expression when inserted upstream from a gene coding sequence adjacent to the right border (RB) of T-DNA. Wild and mutated types of lox sites were tested for their effect upon bar gene expression in plants obtained via Agrobacterium-mediated and biolistic transformation methods. Lox-mediated expression of bar gene, recognized by resistance of transgenic plants to PPT, occurred only in plants obtained via Agrobacterium-mediated transformation. RT-PCR analysis confirms that PPT-resistant phenotype of transgenic plants obtained via Agrobacterium-mediated transformation was caused by activation of bar gene. The plasmid with promoterless gus gene together with the lox site adjacent to the RB was constructed and transferred to Nicotiana tabacum as well. Transgenic plants exhibited GUS activity and expression of gus gene was detected in plant leaves. Expression of bar gene from the vectors containing lox site near RB allowed recovery of numerous PPT-resistant transformants of such important crops as Beta vulgaris, Brassica napus, Lactuca sativa and Solanum tuberosum. Our results demonstrate that the lox site sequence adjacent to the RB can be used to control bar gene expression in transgenic plants.

High-level expression of human interferon alpha-2b in transgenic carrot (Daucus carota L.) plants

In this study, we report the obtaining of carrot plants expressing human interferon alpha-2b via Agrobacterium-mediated transformation using two vector constructs containing the sequence coding for interferon gene fused with Nicotiana plumbagenifolia calreticulin apoplast targeting signal driven by 35S CaMV promoter and root-specific Mll promoter. The human interferon alpha-2b gene was correctly translated in carrot plants according to Western blot analysis. The recombinant protein exhibited antiviral activity in vitro by inhibition of vesicular stomatitis virus replication in established piglet testicular cells. The results demonstrated the higher activity of interferon accumulated in carrot plants for young leaves (up to 50.7 × 10(3) IU/g FW) compared to the mature ones probably due to the degradation-susceptible nature of this protein. The taproot-expressing system could have also provided the sufficient protein amounts (up to 16.5 × 10(3) IU/g FW) and could possibly be used for generating interferon alpha-2b protein in planta for preventing and curing infectious diseases.

Impaired migration of trophoblast cells caused by simvastatin is associated with decreased membrane IGF-I receptor, MMP2 activity and HSP27 expression

BACKGROUND

Statins inhibit 3-hydroxy-3-methylglutaryl coenzyme-A reductase, the rate-limiting enzyme of the mevalonate pathway, and are used successfully in the treatment of hypercholesterolaemia. Statins are contraindicated during pregnancy. Lately, we have shown that simvastatin has adverse affects on human first trimester placental explants' proliferation and migration. The objective of the present study was to investigate the molecules involved in mediating simvastatin's effect on trophoblast cell migration. We hypothesized that simvastatin attenuates insuline-like growth factor-I (IGF-I) receptor expression (involved in trophoblast motility), matrix metalloproteinase (MMP) activities, and heat shock protein 27 (HSP27) levels (whose mRNA is actively transcribed during trophoblast differentiation) in trophoblast cells thus consequently effecting their migration.

METHODS

Human placental explants were cultured above a matrigel with/without simvastatin (10 microM) for 5 days. In this model, trophoblast migrates from the villi into the matrigel. Western-blot and immunohistochemistry served for analysing HSP27 expression. Immunohistochemistry was used for assessing IGF-I receptor localization. MMPs activity was assayed by gel zymography.

RESULTS

Simvastatin reduced IGF-I receptor membranal expression, MMP2 activity and HSP27 expression in trophoblast cells (P < 0.05).

CONCLUSIONS

The inhibitory effect of simvastatin on trophoblast cell migration is associated with a significant decrease in the tested molecules, which probably contributes to the impaired migration.