Hans-Jürgen Pietzsch

Development of technetium-99m-based CNS receptor ligands: have there been any advances?

Abstract By virtue of its ideal nuclear physical characteristics for routine nuclear medicine diagnostics and its ready availability, technetium-99m is of outstanding interest in the development of novel radiopharmaceuticals. The potential for the development of 99mTc-based radioligands for the study the receptor function in the central nervous system (CNS) is also well recognised despite the difficulties to be overcome. A fundamental challenge is the pharmacologically acceptable integration of the transition metal technetium, with its specific coordination chemistry, into the molecular entity of CNS receptor ligands. Conceptually, the ligand molecule can be assembled by three building blocks: a small neutral chelate unit, an organic linker that may also serve as a pharmacological modifier and a receptor-binding region derived from selective receptor antagonists. The recent introduction of novel technetium chelate units, particularly mixed-ligand complexes and low-valency organometallic compounds of technetium, provides an impetus for the further development of CNS receptor ligands. Moreover, progress in receptor pharmacology and the experience gained with positron emission tomography radiotracers have facilitated the design of numerous 99mTc-based CNS receptor ligands. The formidable challenge of developing 99mTc probes as single-photon emission tomography imaging agents targeting CNS receptors can be viewed with optimism given the successful development of [99mTc]TRODAT-1 as a 99mTc complex for imaging dopamine transporters in the brain, although there are a number of receptor-specific imaging agents that have so far resisted all efforts to develop them. This review presents recent advances and discusses the remaining hurdles in the design of 99mTc-based CNS receptor imaging agents.

SYNTHESIS AND MOLECULAR STRUCTURE OF CHLORO(3-THIAPENTANE-1,5-DITHIOLATO)OXORHENIUM(V)

Abstract Reaction of [BzEt 3 N][ReOCl 4 ] ( 1 ) with 0.9 equiv. of 3-thiapentane-1,5-dithiol (HSSSH) in chloroform delivers [ReO(SSS)Cl] ( 2 ). The chloro ligand can be substituted by mercaptides whth 2 is heated with an excess of a mercaptane, e.g. C 2 H 5 SH, in acetonitrile. Single crystal X-ray analysis of 2 shows square-pyramidal geometry.

Synthesis and reactions of trigonal-bipyramidal rhenium and technetium complexes with a tripodal, tetradentate NS3 ligand

Neutral, trigonal-bipyramidal complexes of technetium and rhenium with the tripodal, tetradentate ligand 2,2′,2″-nitrilotris(ethanethiol), N(CH2CH2SH)3 (H31) have been synthesized and characterized. The technetium complex [99Tc(1) (PPh3)] (2) can be obtained by reduction of K99TcO4 with PPh3 in the presence of H31 or by substitution reaction starting from [99TcCl3(PPh3)2(NCMe)]. The trigonal-bipyramidal complex 2, C24H27NPS3Tc, crystallizes in the monoclinic space group P21/c with a=8.906(2), b=25.804(6), c=11.061(4) A, β=108.42(2)° and Z=4. Rhenium complexes [Re(1) (PR3)] (3) (PR3 = PPh3 (3a, PMe2Ph (3b), PMePH2 (3c), P(n-Bu)3 (3d), P(OEt)3 3e)) have been obtained in analogy to the technetium derivative 2 by reduction of NH4ReO4 with phosphines PR3 in the presence of H31. Complex 3a, C24H27NPReS3, crystallizes in the monoclinic space group P21/n with a=10.855(3), b=16.707(4), c=15.441(5) A, β=92.62(2)° and Z=4. Rhenium complexes containing an isocyanideco-ligand [Re(1) (CNR)] (5) (R=CH2COOMe (5a), t-Bu (5b), Ph (5c), CH2CH2NC4H8O (5d), CH2COOEt (5e)) aan be prepared by substitution of the phosphine ligand in 3 for an isocyanide or by reaction of the isocyanide complexes [ReCl3(PPh3)2(CNR)] (4) (R=CH2COOMe (4a), t-Bu (4b), Ph (4c), CH2CH2NC4H8O (4d)) with H31. The crystal structure of complex 4b has been determined. 4b crystallizes with one molecule of CH2Cl2 per formula unit. Crystals of 4b·CH2Cl2, C42H41Cl5NP2Re, are monoclinic, space group P21/c with a=12.868(3), b=20.454(7), c=16.378(9) A, β=104.71(4)° and Z=4. The substitution reaction starting with complexes of type 3 gives the best yields in the preparation of complexes of type 5. Two complexes of the type [Re(1) (CNR)] were characterized by X-ray crystallography. Crystals of 5a, C10H17N2O2ReS3, are monoclinic, space group P21/c with a=7.827(4), b=13.866(3), c=13.627(6) A, β=93.19(7)° and Z=4. Crystals of 5b, C11H21N2ReS3, are monoclinic, space group P21/c with a=12.084(2), b=11.915(2), c=12.244(3) A, β=114.31(2)° and Z=4. Treatment of 5e with LiOH leads to ester hydrolysis and yields the complex [Re(1) (CNCH2COOH)] (6) while reaction of 5d–5e in the two-phase system toluene/conc. hydrochloric acid gives the carbonyl complex [Re(1) (CO)] (7) which was characterized by X-ray crystallography. Crystals of 7, C7H12NOReS3, are triclinic, space group P1 with a=7.924(2), b=10.467(3), c=13.556(2) A, α=96.61(2), β=90.47(2), γ=101.68(2)° and Z=4 (2 molecules of 7 per asymmetric unit).

Radiolabeled Cetuximab Conjugates for EGFR Targeted Cancer Diagnostics and Therapy.

The epidermal growth factor receptor (EGFR) has evolved over years into a main molecular target for the treatment of different cancer entities. In this regard, the anti-EGFR antibody cetuximab has been approved alone or in combination with: (a) chemotherapy for treatment of colorectal and head and neck squamous cell carcinoma and (b) with external radiotherapy for treatment of head and neck squamous cell carcinoma. The conjugation of radionuclides to cetuximab in combination with the specific targeting properties of this antibody might increase its therapeutic efficiency. This review article gives an overview of the preclinical studies that have been performed with radiolabeled cetuximab for imaging and/or treatment of different tumor models. A particularly promising approach seems to be the treatment with therapeutic radionuclide-labeled cetuximab in combination with external radiotherapy. Present data support an important impact of the tumor micromilieu on treatment response that needs to be further validated in patients. Another important challenge is the reduction of nonspecific uptake of the radioactive substance in metabolic organs like liver and radiosensitive organs like bone marrow and kidneys. Overall, the integration of diagnosis, treatment and monitoring as a theranostic approach appears to be a promising strategy for improvement of individualized cancer treatment.

Synthesis, in vitro and in vivo characterization of novel 99mTc-‘4+1’-labeled 5-nitroimidazole derivatives as potential agents for imaging hypoxia

UNLABELLED The evaluation of oxygenation status of solid tumors is an important field of radiopharmaceutical research. With the aim to develop new potential 99mTc-radiopharmaceuticals for imaging hypoxia, we have synthesized two novel isocyanide derivatives of metronidazole, which has demonstrated high affinity for hypoxic tumors in vitro and in vivo. METHODS Metronidazole derivatives 4-isocyano-N-[2-(2-methyl-5-nitro-1H-imidazol-1-yl)ethyl]butanamide (M1) and 1-(4-isocyanobutanoyl)-4-[2-(2-methyl-5-nitro-1H-imidazol-1-yl)ethyl]piperazine (M2) were synthesized, and labeling was performed through preparation of their corresponding 99mTc-(4+1) complexes, 99mTc-NS3M1 and 99mTc-NS3M2. The structure of the technetium complexes was corroborated by preparation and characterization of the corresponding rhenium complexes. We have studied the main physicochemical properties (stability, lipophilicity and plasma protein binding). Biological behavior in HCT-15 cells both in oxia and in hypoxia was assessed. Biodistribution in normal mice and in animals bearing induced 3LL Lewis murine lung carcinoma was also studied. RESULTS Metronidazole derivatives were successfully synthesized. Labeling with high radiochemical purity was achieved for both ligands. 99mTc complexes were stable in labeling milieu and human plasma. However, presence of the piperazine linker in M2 resulted in higher lipophilicity and protein binding. Although cell uptake in hypoxic conditions was observed for both radiotracers, 99mTc-NS3M2 biodistribution was considered unsuitable for a potential radiopharmaceutical due to high liver uptake and poor blood clearance. However, 99mTc-NS3M1 demonstrated a very favorable in vivo profile both in normal mice and in mice bearing induced tumors. CONCLUSION Selective uptake and retention in tumor together with favorable tumor/muscle ratio make 99mTc-NS3M1 a promising candidate for further evaluation as potential hypoxia imaging agent in tumors.

Comparison of the stability of Y-90-, Lu-177- and Ga-68- labeled human serum albumin microspheres (DOTA-HSAM)

INTRODUCTION Microparticles derived from denatured human serum albumin (DOTA-derivatized human serum albumin microspheres, or DOTA-HSAM) are attractive carriers of radionuclides for both therapeutic and diagnostic purposes. In this article, we describe a labeling procedure for diagnostic (Ga-68) and therapeutic (Y-90, Lu-177) radionuclides and report on the results of stability studies of these products. METHODS DOTA-HSAM was labeled in 0.5 M ammonium acetate buffer, pH 5.0, containing 0.02 mg/ml detergent. After adding the radionuclide, the mixture was shaken for 15 min at 90°C. Labeling yields and in vitro stability were determined by thin-layer chromatography. For determination of the in vivo stability of Ga-68 and Y-90 DOTA-HSAM, the particles were injected intravenously in Wistar rats. RESULTS Labeling yields up to 95% in the case of Ga-68 and Lu-177 were achieved. Ga-68-labeled DOTA-HSAM showed high in vitro and in vivo stability. The amount of particle-bound radioactivity of Lu-177 DOTA-HSAM declines slowly in a linear manner to approximately 72% after 13 days. For Y-90, the labeling yield decreased with increasing radioactivity level. We presume radiolysis as the reason for these findings. CONCLUSION The labeling of DOTA-HSAM with different radionuclides is easy to perform. The radiation-induced cleavage of the labeled chelator together with the rather short half-life of radioactivity fixation in vivo (3.7 days) is, in our opinion, opposed to therapeutic applications of DOTA-HSAM. On the other hand, the high stability of Ga-68 DOTA-HSAM makes them an attractive candidate for the measurement of regional perfusion by PET.

Sugar-Decorated Dendritic Nanocarriers: Encapsulation and Release of the Octahedral Rhenium Cluster Complex [Re6S8(OH)6]4−

The encapsulation of a nanometer-sized octahedral anionic rhenium cluster complex with six terminal hydroxo ligands [Re(6)S(8)(OH)(6)](4-) in maltose-decorated poly(propylene amine) dendrimers (POPAM, generation 4 and 5) has been investigated. Ultrafiltration experiments showed that maximal loading capacity of the dendrimers with the cluster complex is achieved after about ten hours in aqueous solution. To study the inclusion phenomena, three different methods have been applied: UV/Vis, time-resolved laser-induced fluorescence spectroscopy (TRLFS), and laser-induced liquid bead ion desorption mass spectrometry (LILBID-MS). From the results obtained, it could be concluded that: a) the hydrolytic stability of the rhenium cluster complex is significantly enhanced in the presence of dendritic hosts; b) the cluster anions are preferentially bound inside the dendrimers; c) the number of cluster complexes encapsulated in the dendrimers increases with rising dendrimer generation. On average, four to five cluster anions can preferentially be captured in the interior of sugar-coated dendritic carriers. An asymptotic progression of the release of cluster complexes from the loaded dendrimers was observed under physiologically relevant conditions (isotonic sodium chloride solution: approximately 93 % within 4 days for loaded POPAM-G4-maltose; approximately 86 % within 4 days for loaded POPAM-G5-maltose). These encapsulation and release properties of maltose-decorated nanocarriers imply the possibility for the development of the next generation of dendritic nanocarriers with specific targeting of destined tissue for therapeutic treatments.

Synthesis and characterization of rhenium and technetium-99m tricarbonyl complexes bearing the 4-[3-bromophenyl]quinazoline moiety as a biomarker for EGFR-TK imaging

Aiming at the development of technetium-99m ((99m)Tc) complexes for early detection and staging of EGFR positive tumors, the tyrosine kinase inhibitor 6-amino-4-[(3-bromophenyl)amino]quinazoline was derivatized with pyridine-2-carboxaldehyde to generate the imine 6-(pyridine-2-methylimine)-4-[(3-bromophenyl)amino]quinazoline suitable for reacting with the fac-[(99m)Tc(CO)(3)](+) core as an N,N bidentate ligand. The labelling was performed in high yield (>90%) by ligand exchange reaction using fac-[(99m)Tc(OH(2))(3)(CO)(3)](+) as precursor. The (99m)Tc complex was characterized by comparative HPLC analysis using the analogous rhenium (Re) complex as reference. The Re complex was prepared by ligand exchange reaction using the fac-[ReBr(3)(CO)(3)](2-) as precursor and was fully characterized by NMR and IR spectroscopies and elemental analysis. In vitro studies indicate that both the ligand and its Re complex inhibit the EGFR autophosphorylation (IC(50): 17+/-3.7 and 114+/-23 nM respectively) in intact A431 cells, bind the receptor in a reversible mode, and inhibit A431 cell growth (IC(50): 5.2+/-1.1 and 2.0+/-0.98 microM respectively). Biodistribution of the (99m)Tc complex in healthy animals showed a rather fast blood and soft tissue clearance between 1 and 15 min p.i. with excretion occurring mainly via the hepatobiliary system.

New insights into the pretargeting approach to image and treat tumours

Tumour pretargeting is a promising strategy for cancer diagnosis and therapy allowing for the rational use of long circulating, highly specific monoclonal antibodies (mAbs) for both non-invasive cancer radioimmunodetection (RID) and radioimmunotherapy (RIT). In contrast to conventional RID/RIT where the radionuclides and oncotropic vector molecules are delivered as presynthesised radioimmunoconjugates, the pretargeting approach is a multistep procedure that temporarily separates targeting of certain tumour-associated antigens from delivery of diagnostic or therapeutic radionuclides. In principle, unlabelled, highly tumour antigen specific mAb conjugates are, in a first step, administered into a patient. After injection, sufficient time is allowed for blood circulation, accumulation at the tumour site and subsequent elimination of excess mAb conjugates from the body. The small fast-clearing radiolabelled effector molecules with a complementary functionality directed to the prelocalised mAb conjugates are then administered in a second step. Due to its fast pharmacokinetics, the small effector molecules reach the malignant tissue quickly and bind the local mAb conjugates. Thereby, corresponding radioimmunoconjugates are formed in vivo and, consequently, radiation doses are deposited mainly locally. This procedure results in a much higher tumour/non-tumour (T/NT) ratio and is favourable for cancer diagnosis and therapy as it substantially minimises the radiation damage to non-tumour cells of healthy tissues. The pretargeting approach utilises specific non-covalent interactions (e.g. strept(avidin)/biotin) or covalent bond formations (e.g. inverse electron demand Diels-Alder reaction) between the tumour bound antibody and radiolabelled small molecules. This tutorial review descriptively presents this complex strategy, addresses the historical as well as recent preclinical and clinical advances and discusses the advantages and disadvantages of different available variations.

Labeling of fatty acid ligands with the strong electrophilic metal fragment [99mTc(N)(PNP)]2+ (PNP=diphosphane ligand).

The electrophilic metal fragment [(99m)Tc(N)(PNP)](2+) (PNP=diphosphane ligand) has been employed for the labeling of fatty acid chains of different lengths. To provide a site-specific group for the attachment of the metallic moiety, the fatty acid derivatives were functionalized by appending a bis-mercapto or, alternatively, a dithiocarbamato pi-donor chelating systems to one terminus of the carbon chain to yield both dianionic and monoanionic bifunctional ligands (L). The resulting complexes, [(99m)Tc(N)(PNP)(L)] (0/+), exhibited the usual asymmetrical structure in which a Tc(triple bond)N group was surrounded by two different bidentate chelating ligands. Dianionic ligands gave rise to neutral complexes, while monoanionic ligands afforded monocationic species. Biodistribution studies were carried out in rats. An isolated perfused rat heart model was employed to assess how structural changes in the radiolabeled fatty acid compound affect the myocardial first pass extraction. Results showed that only monocationic complexes accumulated in myocardium to a significant extent. Conversely, neutral complexes were not efficiently retained into the heart region and rapidly washed out. In isolated perfused rat heart experiments, monocationic complexes exhibited a behavior similar to that of the monocationic flow tracers (99m)Tc-MIBI and (99m)Tc-DBODC with almost identical extraction values, a result that could be attributed to the presence of the monopositive charge. Instead, a slightly lower myocardial extraction was found for neutral complexes. Comparison of the observed kinetic behavior of neutral complexes in the isolated perfused rat heart model with that of the myocardial metabolic tracer [(123)I]IPPA revealed that the introduction of the metallic moiety partially hampers recognition of the labeled fatty acids by cardiac enzymes, and consequently, their behavior did not completely reflect myocardial metabolism.