Somatostatin Receptor-Binding Peptides Suitable for Tumor Radiotherapy with Re-188 or Re-186. Chemistry and Initial Biological Studies

Optimized Production of 188Re-HYNIC-Bombesin: New Therapeutic Agent for GRPR Targeting

Purpose

One of the most interesting methods to deliver therapeutic doses of ionizing radiation to tumor sites is radiolabeled compounds. Bombesin peptide binds to gastrin-releasing peptide receptors (GRPRs) with great affinity. Through its appropriate physical characteristics and accessibility as the 188W/188Re generator, 188Re can be effectively used to develop a therapeutic radio complex. In this study, 188Re-HYNIC-BBN was prepared under optimal conditions.

Methods

Optimization of the effective parameters on 188Re-HYNIC-BBN radio-labeling yield like ligand concentration, pH, reaction time, and temperature were performed. The final product's radiochemical purity was measured by RTLC and HPLC. The stability of the radio-complex was checked in PBS buffer (4 °C) and human blood serum (37 °C). The partition coefficient of the final radio-complex was studied using standard procedure. Finally, the biodistribution of 188Re-HYNIC-BBN and free 188Re in different organs of the rats were compared in various intervals.

Results

The final product was prepared with a specific activity of 7.11 TBq/mmol and radiochemical purity > 95% at the optimized conditions (pH = 4-5, reaction time = 45 min, temp = 95℃). This radio-complex was found to be stable in PBS and blood serum over 24 h. LogPo/w was - 1.78, showing the high hydrophilic nature of the radio-complex. The biodistribution of 188Re-HYNIC-BBN demonstrated the fast clearance of the radio-peptide from the blood circulation. The most portion of the radioactivity was excreted from the body via the urinary tract and the remaining activity was accumulated in GRPR-expressing organs.

Conclusion

The special characteristics of the complex introduce 188Re-HYNIC-BBN as a new therapeutic agent for targeting GRPRs, however, more biological data is still needed.

Rhenium Radioisotopes for Medicine, a Focus on Production and Applications

In recent decades, the use of alpha; pure beta; or beta/gamma emitters in oncology, endocrinology, and interventional cardiology rheumatology, has proved to be an important alternative to the most common therapeutic regimens. Among radionuclides used for therapy in nuclear medicine, two rhenium radioisotopes are of particular relevance: rhenium-186 and rhenium-188. The first is routinely produced in nuclear reactors by direct neutron activation of rhenium-186 via 185Re(n,γ)186Re nuclear reaction. Rhenium-188 is produced by the decay of the parent tungsten-188. Separation of rhenium-188 is mainly performed using a chromatographic 188W/188Re generator in which tungsten-188 is adsorbed on the alumina column, similar to the 99Mo/99mTc generator system, and the radionuclide eluted in saline solution. The application of rhenium-186 and rhenium-188 depends on their specific activity. Rhenium-186 is produced in low specific activity and is mainly used for labeling particles or diphosphonates for bone pain palliation. Whereas, rhenium-188 of high specific activity can be used for labeling peptides or bioactive molecules. One of the advantages of rhenium is its chemical similarity with technetium. So, diagnostic technetium analogs labeled with radiorhenium can be developed for therapeutic applications. Clinical trials promoting the use of 186/188Re-radiopharmaceuticals is, in particular, are discussed.

Bifunctional chelators for radiorhenium: past, present and future outlook

Targeted radionuclide therapy (TRNT) is an ever-expanding field of nuclear medicine that provides a personalised approach to cancer treatment while limiting toxicity to normal tissues. It involves the radiolabelling of a biological targeting vector with an appropriate therapeutic radionuclide, often facilitated by the use of a bifunctional chelator (BFC) to stably link the two entities. The radioisotopes of rhenium, 186Re (t 1/2 = 90 h, 1.07 MeV β-, 137 keV γ (9%)) and 188Re (t 1/2 = 16.9 h, 2.12 MeV β-, 155 keV γ (15%)), are particularly attractive for radiotherapy because of their convenient and high-abundance β--particle emissions as well as their imageable γ-emissions and chemical similarity to technetium. As a transition metal element with multiple oxidation states and coordination numbers accessible for complexation, there is great opportunity available when it comes to developing novel BFCs for rhenium. The purpose of this review is to provide a recap on some of the past successes and failings, as well as show some more current efforts in the design of BFCs for 186/188Re. Future use of these radionuclides for radiotherapy depends on their cost-effective availability and this will also be discussed. Finally, bioconjugation strategies for radiolabelling biomolecules with 186/188Re will be touched upon.

Overview of Radiolabeled Somatostatin Analogs for Cancer Imaging and Therapy

Identified in 1973, somatostatin (SST) is a cyclic hormone peptide with a short biological half-life. Somatostatin receptors (SSTRs) are widely expressed in the whole body, with five subtypes described. The interaction between SST and its receptors leads to the internalization of the ligand-receptor complex and triggers different cellular signaling pathways. Interestingly, the expression of SSTRs is significantly enhanced in many solid tumors, especially gastro-entero-pancreatic neuroendocrine tumors (GEP-NET). Thus, somatostatin analogs (SSAs) have been developed to improve the stability of the endogenous ligand and so extend its half-life. Radiolabeled analogs have been developed with several radioelements such as indium-111, technetium-99 m, and recently gallium-68, fluorine-18, and copper-64, to visualize the distribution of receptor overexpression in tumors. Internal metabolic radiotherapy is also used as a therapeutic strategy (e.g., using yttrium-90, lutetium-177, and actinium-225). With some radiopharmaceuticals now used in clinical practice, somatostatin analogs developed for imaging and therapy are an example of the concept of personalized medicine with a theranostic approach. Here, we review the development of these analogs, from the well-established and authorized ones to the most recently developed radiotracers, which have better pharmacokinetic properties and demonstrate increased efficacy and safety, as well as the search for new clinical indications.

Rhenium-188 Labeled Radiopharmaceuticals: Current Clinical Applications in Oncology and Promising Perspectives

Rhenium-188 (¹⁸⁸Re) is a high energy beta-emitting radioisotope with a short 16.9 h physical half-life, which has been shown to be a very attractive candidate for use in therapeutic nuclear medicine. The high beta emission has an average energy of 784 keV and a maximum energy of 2.12 MeV, sufficient to penetrate and destroy targeted abnormal tissues. In addition, the low-abundant gamma emission of 155 keV (15%) is efficient for imaging and for dosimetric calculations. These key characteristics identify ¹⁸⁸Re as an important therapeutic radioisotope for routine clinical use. Moreover, the highly reproducible on-demand availability of ¹⁸⁸Re from the ¹⁸⁸W/¹⁸⁸Re generator system is an important feature and permits installation in hospital-based or central radiopharmacies for cost-effective availability of no-carrier-added (NCA) ¹⁸⁸Re. Rhenium-188 and technetium-99 m exhibit similar chemical properties and represent a “theranostic pair.” Thus, preparation and targeting of ¹⁸⁸Re agents for therapy is similar to imaging agents prepared with ^99mTc, the most commonly used diagnostic radionuclide. Over the last three decades, radiopharmaceuticals based on ¹⁸⁸Re-labeled small molecules, including peptides, antibodies, Lipiodol and particulates have been reported. The successful application of these ¹⁸⁸Re-labeled therapeutic radiopharmaceuticals has been reported in multiple early phase clinical trials for the management of various primary tumors, bone metastasis, rheumatoid arthritis, and endocoronary interventions. This article reviews the use of ¹⁸⁸Re-radiopharmaceuticals which have been investigated in patients for cancer treatment, demonstrating that ¹⁸⁸Re represents a cost effective alternative for routine clinical use in comparison to more expensive and/or less readily available therapeutic radioisotopes.

Discovery of an SSTR2-Targeting Maytansinoid Conjugate (PEN-221) with Potent Activity in Vitro and in Vivo

Somatostatin receptor 2 (SSTR2) is frequently overexpressed on several types of solid tumors, including neuroendocrine tumors and small-cell lung cancer. Peptide agonists of SSTR2 are rapidly internalized upon binding to the receptor and linking a toxic payload to an SSTR2 agonist is a potential method to kill SSTR2-expressing tumor cells. Herein, we describe our efforts towards an efficacious SSTR2-targeting cytotoxic conjugate; examination of different SSTR2-targeting ligands, conjugation sites, and payloads led to the discovery of 22 (PEN-221), a conjugate consisting of microtubule-targeting agent DM1 linked to the C-terminal side chain of Tyr³-octreotate. PEN-221 demonstrates in vitro activity which is both potent (IC₅₀ = 10 nM) and receptor-dependent (IC₅₀ shifts 90-fold upon receptor blockade). PEN-221 targets high levels of DM1 to SSTR2-expressing xenograft tumors, which has led to tumor regressions in several SSTR2-expressing xenograft mouse models. The safety and efficacy of PEN-221 is currently under evaluation in human clinical trials.

Synthesis, characterization and biological studies of rhenium, technetium-99m and rhenium-188 pentapeptides

A pentapeptide macrocyclic ligand, KYCAR (lysyl-tyrosyl-cystyl-alanyl-arginine), has been designed as a potential chelating ligand for SPECT imaging and therapeutic in vivo agents. This study shows the synthesis and characterization of KYCAR complexes containing nonradioactive rhenium, ^99mTc, or ¹⁸⁸Re. The metal complexes were also biologically evaluated to determine in vivo distribution in healthy mice. The overall goals of this project were (1) to synthesize the Tc/Re pentapeptide complexes, (2) to identify spectroscopic methods for characterization of syn versus anti rhenium peptide complexes, (3) to analyze the ex vivo stability, and (4) to assess the biological properties of the [^99mTc]TcO-KYCAR and [¹⁸⁸Re]ReO-KYCAR complexes in vivo. Details on these efforts are provided below.

METHODS

^NatRe/^99mTc/¹⁸⁸ReO-KYCAR complexes were synthesized, and macroscopic species were characterized via HPLC, IR, NMR, and CD. These characterization data were compared to the crystallographic data of ReO-KYC to assist in the assignment of diastereomers and to aid in the determination of the structure of the complex.

RESULTS

The radiometal complexes were synthesized with high purity (>95%). HPLC, IR, NMR and CD data on the macroscopic ^natReO-KYCAR complexes confirm the successful complexation as well as the presence of two diastereomers in syn and anticonformations. Tracer level complexes show favorable stabilities ex vivo for 2+ h.

CONCLUSION

Macroscopic metal complexes form diastereomers with the KYCAR ligand; however, this phenomenon is not readily observed on the tracer level due to the rapid interconversion. It was determined through pK_a measurements that the macroscopic ^natReO-KYCAR complex is 0 at physiological pH. The [^99mTc]TcO-KYCAR is stable in vitro while the [¹⁸⁸Re]ReO-KYCAR shows 50% decomposition in PBS and serum. Biologically, the tracer level complexes clear through the hepatobiliary pathway. Some decomposition of both tracers is evident by uptake in the thyroid and stomach.

The somatostatin analog 188Re-P2045 inhibits the growth of AR42J pancreatic tumor xenografts

P2045 is a peptide analog of somatostatin with picomolar affinity for the somatostatin receptor subtype 2 (SSTR2) upregulated in some pancreatic tumors. Studies were conducted in rat AR42J pancreatic tumor xenograft mice to determine whether (188)Re-P2045 could inhibit the growth of pancreatic cancer in an animal model.

METHODS

(188)Re-P2045 was intravenously administered every 3 d for 16 d to nude mice with AR42J tumor xenografts that were approximately 20 mm(3) at study initiation. Tumor volumes were recorded throughout the dosing period. At necropsy, all tissues were assessed for levels of radioactivity and evaluated for histologic abnormalities. Clinical chemistry and hematology parameters were determined from terminal blood samples. The affinity of nonradioactive (185/187)Re-P2045 for somatostatin receptors was compared in human NCI-H69 and rat AR42J tumor cell membranes expressing predominantly SSTR2.

RESULTS

In the 1.85- and 5.55-MBq groups, tumor growth was inhibited in a dose-dependent fashion. In the 11.1-MBq group, tumor growth was completely inhibited throughout the dosing period and for 12 d after the last administered dose. The radioactivity level in tumors 4 h after injection was 10 percentage injected dose per gram, which was 2-fold higher than in the kidneys. (188)Re-P2045 was well tolerated in all dose groups, with no adverse clinical, histologic, or hematologic findings. The nonradioactive (185/187)Re-P2045 bound more avidly (0.2 nM) to SSTR2 in human than rat tumor membranes, suggesting that these studies are relevant to human studies.

CONCLUSION

(188)Re-P2045 is a promising therapeutic candidate for patients with somatostatin receptor-positive cancer.

Models for B12-conjugated radiopharmaceuticals. Cobaloxime binding to new fac-[Re(CO)3(Me2bipyridine)(amidine)]BF4 complexes having an exposed pyridyl nitrogen

New mononuclear amidine complexes, fac-[Re(CO)3(Me2bipy)(HNC(CH3)(pyppz))]BF4 [(4,4'-Me2bipy (1), 5,5'-Me2bipy (2), and 6,6'-Me2bipy (3)] (bipy = 2,2'-bipyridine), were synthesized by treating the parent fac-[Re(I)(CO)3(Me2bipy)(CH3CN)]BF4 complex with the C2-symmetrical amine 1-(4-pyridyl)piperazine (pyppzH). The axial amidine ligand has an exposed, highly basic pyridyl nitrogen. The reaction of complexes 1-3 with a B12 model, (py)Co(DH)2Cl (DH = monoanion of dimethylglyoxime), in CH2Cl2 yielded the respective dinuclear complexes, namely, fac-[Re(CO)3(Me2bipy)(μ-(HNC(CH3)(pyppz)))Co(DH)2Cl]BF4 [(4,4'-Me2bipy (4), 5,5'-Me2bipy (5), and 6,6'-Me2bipy (6)]. (1)H NMR spectroscopic analysis of all compounds and single-crystal X-ray crystallographic data for 2, 3, 5, and 6 established that the amidine had only the E configuration in both the solid and solution states and that the pyridyl group is bound to Co in 4-6. Comparison of the NMR spectra of 1-3 with spectra of 4-6 reveals an unusually large "wrong-way" upfield shift for the pyridyl H2/6 signal for 4-6. The wrong-way H2/6 shift of (4-Xpy)Co(DH)2Cl (4-Xpy = 4-substituted pyridine) complexes increased with increasing basicity of the 4-Xpy derivative, a finding attributed to the influence of the magnetic anisotropy of the cobalt center on the shifts of the (1)H NMR signals of the pyridyl protons closest to Co. Our method of employing a coordinate bond for conjugating the fac-[Re(I)(CO)3] core to a vitamin B12 model could be extended to natural B12 derivatives. Because B12 compounds are known to accumulate in cancer cells, such an approach is a very attractive method for the development of (99m)Tc and (186/188)Re radiopharmaceuticals for targeted tumor imaging and therapy.

Selection of an optimal cysteine-containing peptide-based chelator for labeling of affibody molecules with (188)Re

Affibody molecules constitute a class of small (7 kDa) scaffold proteins that can be engineered to have excellent tumor targeting properties. High reabsorption in kidneys complicates development of affibody molecules for radionuclide therapy. In this study, we evaluated the influence of the composition of cysteine-containing C-terminal peptide-based chelators on the biodistribution and renal retention of (188)Re-labeled anti-HER2 affibody molecules. Biodistribution of affibody molecules containing GGXC or GXGC peptide chelators (where X is G, S, E or K) was compared with biodistribution of a parental affibody molecule ZHER2:2395 having a KVDC peptide chelator. All constructs retained low picomolar affinity to HER2-expressing cells after labeling. The biodistribution of all (188)Re-labeled affibody molecules was in general comparable, with the main observed difference found in the uptake and retention of radioactivity in excretory organs. The (188)Re-ZHER2:V2 affibody molecule with a GGGC chelator provided the lowest uptake in all organs and tissues. The renal retention of (188)Re-ZHER2:V2 (3.1 ± 0.5 %ID/g at 4 h after injection) was 55-fold lower than retention of the parental (188)Re-ZHER2:2395 (172 ± 32 %ID/g). We show that engineering of cysteine-containing peptide-based chelators can be used for significant improvement of biodistribution of (188)Re-labeled scaffold proteins, particularly reduction of their uptake in excretory organs.

Inorganic chemistry in nuclear imaging and radiotherapy: current and future directions

Radiometals play an important role in diagnostic and therapeutic radiopharmaceuticals. This field of radiochemistry is multidisciplinary, involving radiometal production, separation of the radiometal from its target, chelate design for complexing the radiometal in a biologically stable environment, specific targeting of the radiometal to its in vivo site, and nuclear imaging and/or radiotherapy applications of the resultant radiopharmaceutical. The critical importance of inorganic chemistry in the design and application of radiometal-containing imaging and therapy agents is described from a historical perspective to future directions.

New monodentate amidine superbasic ligands with a single configuration in fac-[Re(CO)3(5,5'- or 6,6'-Me2bipyridine)(amidine)]BF4 complexes

Treatment of two precursors, fac-[Re(CO)(3)(L)(CH(3)CN)]BF(4) [L = 5,5'-dimethyl-2,2'-bipyridine (5,5'-Me(2)bipy) (1) and 6,6'-dimethyl-2,2'-bipyridine (6,6'-Me(2)bipy) (2)], with five C(2)-symmetrical saturated heterocyclic amines yielded 10 new amidine complexes, fac-[Re(CO)(3)(L)(HNC(CH(3))N(CH(2)CH(2))(2)Y)]BF(4) [Y = CH(2), (CH(2))(2), (CH(2))(3), NH, or O]. All 10 complexes possess the novel feature of having only one isomer (amidine E configuration), as established by crystallographic and (1)H NMR spectroscopic methods. We are confident that NMR signals of the other possible isomer (amidine Z configuration) would have been detected, if it were present. Isomers are readily detected in closely related amidine complexes because the double-bond character of the amidine C-N3 bond (N3 is bound to Re) leads to slow E to Z isomer interchange. The new fac-[Re(CO)(3)(L)(HNC(CH(3))N(CH(2)CH(2))(2)Y)]BF(4) complexes have C-N3 bonds with essentially identical double-bond character. However, the reason that the Z isomer is so unstable as to be undetectable in the new complexes is undoubtedly because of unfavorable clashes between the equatorial ligands and the bulky N(CH(2)CH(2))(2)Y ring moiety of the axial amidine ligand. The amidine formation reactions in acetonitrile (25 °C) proceeded more easily with 2 than with 1, indicating that the distortion in 6,6'-Me(2)bipy resulting from the proximity of the methyl substituents to the inner coordination sphere enhanced the reactivity of the coordinated CH(3)CN. Reaction times for 1 and 2 exhibited a similar dependence on the basicity and ring size of the heterocyclic amine reactants. Moreover, when the product of the reaction of 1 with piperidine, fac-[Re(CO)(3)(5,5'-Me(2)bipy)(HNC(CH(3))N(CH(2)CH(2))(2)CH(2))]BF(4), was challenged in acetonitrile-d(3) or CDCl(3) with a 5-fold excess of the strong 4-dimethylaminopyridine ligand, there was no evidence for replacement of the amidine ligand after two months, thus establishing that the piperidinylamidine ligand is a robust ligand. This chemistry offers promise as a suitable means for preparing isomerically pure conjugated fac-[(99m)Tc(CO)(3)L](n±) imaging agents, including conjugates with known bioactive heterocyclic amines.

Use of a HEHEHE purification tag instead of a hexahistidine tag improves biodistribution of affibody molecules site-specifically labeled with (99m)Tc, (111)In, and (125)I

Affibody molecules are a class of small (∼7 kDa) robust scaffold proteins suitable for radionuclide molecular imaging in vivo. The attachment of a hexahistidine (His(6))-tag to the Affibody molecule allows facile purification by immobilized metal ion affinity chromatography (IMAC) but leads to high accumulation of radioactivity in the liver. Earlier, we have demonstrated that replacement of the His(6)-tag with the negatively charged histidine-glutamate-histidine-glutamate-histidine-glutamate (HEHEHE)-tag permits purification of Affibody molecules by IMAC, enables labeling with [(99m)Tc(CO)(3)](+), and provides low hepatic accumulation of radioactivity. In this study, we compared the biodistribution of cysteine-containing Affibody molecules site-specifically labeled with (111)In, (99m)Tc, and (125)I at the C-terminus, having a His(6)-tag at the N- or C-terminus or a HEHEHE-tag at the N-terminus. We show that the use of a HEHEHE-tag provides appreciable reduction of hepatic radioactivity, especially for radiometal labels. We hope that this information can also be useful for development of other scaffold protein-based imaging agents.

In vitro structure-activity relationship of Re-cyclized octreotide analogues

INTRODUCTION

Development of radiolabeled octreotide analogues is of interest for targeting somatostatin receptor (SSTR)-positive tumors for diagnostic and therapeutic purposes. We are investigating a direct labeling approach for incorporation of a Re ion into octreotide analogues, where the peptide sequences are cyclized via coordination to Re rather than through a disulfide bridge.

METHODS

Various octreotide analogue sequences and coordination systems (e.g., S(2)N(2) and S(3)N) were synthesized and cyclized with nonradioactive Re. In vitro competitive binding assays with (111)In-DOTA-Tyr(3)-octreotide in AR42J rat pancreatic tumor cells yielded IC(50) values as a measure of SSTR affinity of the Re-cyclized analogues. Three-dimensional structures of Re-cyclized Tyr(3)-octreotate and its disulfide-bridged analogue were calculated from two-dimensional NMR experiments to visualize the effect of metal cyclization on the analogue's pharmacophore.

RESULTS

Only two of the 11 Re-cyclized analogues investigated showed moderate in vitro binding affinity toward somatostatin subtype 2 receptors. Three-dimensional molecular structures of Re- and disulfide-cyclized Tyr(3)-octreotate were calculated, and both of their pharmacophore turns appear to be very similar with minor differences due to metal coordination to the amide nitrogen of one of the pharmacophore amino acids.

CONCLUSIONS

Various Re-cyclized analogues were developed and analogue 4 had moderate affinity toward somatostatin subtype 2 receptors. In vitro stable studies that are in progress showed stable radiometal cyclization of octreotide analogues via NS(3) and N(2)S(2) coordination forming five- and six-membered chelate rings. In vivo biodistribution studies are underway of (99m)Tc-cyclized analogue 4.

Preparation and evaluation of 99mTc-labeled cyclic arginine-glycine-aspartate (RGD) peptide for integrin targeting

Technetium coordination chemistry has been a subject of interest in the development of radiopharmaceuticals, especially imaging radiotracers. Due to the extensive work done on developing chelates for (99m)Tc, various chelators have been investigated and applied to radiopharmceuticals. Previous studies on the coordination chemistry of the [(99m)Tc=O] core have established peptide-derived sequences as effective chelating ligands. These observations led to the design of tetradentate ligands derived from amino acid sequences. Such amino acid sequences provide a tetradentate coordination site for chelation to the radionuclide and an effective functional group for conjugation to biomolecules using conventional solid-phase synthetic routes. A derivative of a novel tripeptide chelating sequence, Pro-Gly-Cys (PGC) has been developed where it is possible to form stable technetium complexes with the [(99m)Tc=O] via N(3)S(1) tetradentate coordination core that serves this function and can be readily incorporated into biomolecules using solid-phase synthesis techniques. As a model system, the RGD peptide was selected which has been well known to target the integrin receptor for angiogenesis and tumor imaging agents. The results of in vivo studies with these novel radiolabeled compounds in tumor xenografts demonstrated a distribution in tumor targeting and other organs, such as kidney, liver and intestines.

99m-Technetium carbohydrate conjugates as potential agents in molecular imaging

This feature article covers recent reports of work towards the development of (99m)Tc-carbohydrate based agents for use in SPECT imaging, particularly of cancerous tissue. An outline of some of the key biological functions and coordination chemistry of carbohydrates is given, along with an introduction to bioconjugation and molecular imaging. Technetium coordination chemistry and the subset of this involving bioconjugates are discussed before moving into the focus of the article: glycoconjugates of (99m)Tc(v) and the more recently developed [(99m)Tc(I)(CO)(3)](+). Significant work in the last decade has featured the very attractive [(99m)Tc(CO)(3)](+) core, and the ligand sets designed for this core are discussed in detail.

Pharmacokinetics and dosimetry of 188 Re-pharmaceuticals

The main objective of this review is to apportion current and new insight into the biodistribution, radiopharmacokinetics, dosimetry and cell targeting of rhenium-188 labeled radiopharmaceuticals used as therapeutic drugs. The emphasis lies on the generator obtained rhenium-188, its physical, therapeutic, dosimetric and coordinated compounds. Its use in radioimmunotherapy for lymphoma and other hematological diseases with monoclonal antibodies is discussed. Radiolabeled peptides to target cell receptors are an important field in nuclear medicine and in some research facilities are already being used, especially, somatostatin, bombesin and other peptides. Small molecules labeled with 188 Re are promising as therapeutic drugs. A review about some of the non-specific targeting molecules with therapeutic or pain palliation effect such as phosphonates, lipiodol, microparticles and other interesting molecules is included. Research on the labeling of biomolecules with the versatile rhenium-188 has contributed to the development of therapeutics with favorable pharmacokinetic and dosimetric properties for cancer treatment.

MO tripeptide diastereomers (M=99/99mTc, Re): models to identify the structure of 99mTc peptide targeted radiopharmaceuticals

Biologically active molecules, such as many peptides, serve as targeting vectors for radiopharmaceuticals based on 99mTc. Tripeptides can be suitable chelates and are easily and conveniently synthesized and linked to peptide targeting vectors through solid-phase peptide synthesis and form stable TcVO complexes. Upon complexation with [TcO]3+, two products form; these are syn and anti diastereomers, and they often have different biological behavior. This is the case with the approved radiopharmaceutical [99mTcO]depreotide ([99mTcO]P829, NeoTect) that is used to image lung cancer. [99mTcO]depreotide indeed exhibits two product peaks in its HPLC profile, but assignment of the product peaks to the diastereomers has proven to be difficult because the metal peptide complex is difficult to crystallize for structural analysis. In this study, we isolated diastereomers of [99TcO] and [ReO] complexes of several tripeptide ligands that model the metal chelator region of [99mTcO]depreotide. Using X-ray crystallography, we observed that the early eluting peak (A) corresponds to the anti diastereomer, where the Tc=O group is on the opposite side of the plane formed by the ligand backbone relative to the pendant groups of the tripeptide ligand, and the later eluting peak (B) corresponds to the syn diastereomer, where the Tc=O group is on the same side of the plane as the residues of the tripeptide. 1H NMR and circular dichroism (CD) spectroscopy report on the metal environment and prove to be diagnostic for syn or anti diastereomers, and we identified characteristic features from these techniques that can be used to assign the diastereomer profile in 99mTc peptide radiopharmaceuticals like [99mTcO]depreotide and in 188Re peptide radiotherapeutic agents. Crystallography, potentiometric titration, and NMR results presented insights into the chemistry occurring under physiological conditions. The tripeptide complexes where lysine is the second amino acid crystallized in a deprotonated metallo-amide form, possessing a short N1-M bond. The pKa measurements of the N1 amine (pKa approximately 5.6) suggested that this amine is rendered more acidic by both metal complexation and the presence of the lysine residue. Furthermore, peptide chelators incorporating a lysine (like the chelator of [TcO]depreotide) likely exist in the deprotonated form in vivo, comprising a neutral metal center. Deprotonation possibly mediates the interconversion process between the syn and anti diastereomers. The N1 amine group on non-lysine-containing metallopeptides is not as acidic (pKa approximately 6.8) and does not deprotonate and crystallize as do the metallo-amide species. Three of the tripeptide ligands (FGC, FSC, and FKC) were radiolabeled with 99mTc, and the individual syn and anti isomers were isolated for biodistribution studies in normal female nude mice. The main organs of uptake were the liver, intestines, and kidneys, with the FGC compounds exhibiting the highest liver uptake. In comparing the diastereomers, the syn compounds had substantially higher organ uptake and slower blood clearance than the anti compounds.