Abstract:The field of medicinal inorganic chemistry has grown consistently during the past 50 years; however, metal-containing coordination compounds represent only a minor proportion of drugs currently on the market, indicating that research in this area has not yet been thoroughly realized. Although platinum-based drugs as cancer chemotherapeutic agents have been widely studied, exact knowledge of the mechanisms governing their accumulation in cells is still lacking. However, evidence suggests active uptake and efflux mechanisms are involved; this may be involved also in other experimental metal coordination and organometallic compounds with promising antitumor activities in vitro and in vivo, such as ruthenium and gold compounds. Such knowledge would be necessary to elucidate the balance between activity and toxicity profiles of metal compounds. In this review, we present an overview of the information available on the cellular accumulation of Pt compounds from in vitro, in vivo and clinical studies, as well as a summary of reports on the possible accumulation mechanisms for different families of experimental anticancer metal complexes (e.g., Ru Au and Ir). Finally, we discuss the need for rationalization of the investigational approaches available to study metallodrug cellular transport.

Potentially nonadentate (NO) bifunctional chelator p-SCN-Bn-Hneunpa and its immunoconjugate Hneunpa-trastuzumab for In radiolabeling are synthesized. The ability of p-SCN-Bn-Hneunpa and Hneunpa-trastuzumab to quantitatively radiolabel InCl at an ambient temperature within 15 or 30 min, respectively, is presented. Thermodynamic stability determination with In, Bi, and La resulted in high conditional stability constant (pM) values. In vitro human serum stability assays have demonstrated both In complexes to have high stability over 5 days. Mouse biodistribution of [In][In(p-NO-Bn-neunpa)], compared to that of [In][In(p-NH-Bn-CHX-A″-diethylenetriamine pentaacetic acid (DTPA))], at 1, 4, and 24 h shows fast clearance of both complexes from the mice within 24 h. In a second mouse biodistribution study, the immunoconjugates In-neunpa-trastuzumab andIn-CHX-A″-DTPA-trastuzumab demonstrate a similar distribution profile but with slightly lower tumor uptake of In-neunpa-trastuzumab compared to that ofIn-CHX-A″-DTPA-trastuzumab. These results were also confirmed by immuno-single photon emission computed tomography (immuno-SPECT) imaging in vivo. These initial investigations reveal the acyclic bifunctional chelator p-SCN-Bn-Hneunpa to be a promising chelator for In (and other radiometals) with high in vitro stability and also show Hneunpa-trastuzumab to be an excellent In chelator with promising biodistribution in mice.

A series of novel (C^N) cyclometallated Au(III) complexes of general formula [Au(py(b)-H)L(1)L(2)](n+) (py(b)-H = C^N cyclometallated 2-benzylpyridine, L(1) and L(2) being chlorido, phosphane or glucosethiolato ligands, n = 0 or 1) have been synthesized and fully characterized using different techniques, including NMR, IR and far-IR, mass spectrometry, as well as elemental analysis. The crystal structure of one compound has been solved using X-ray diffraction methods. All compounds were tested in vitro in five human cancer cell lines including the lung, breast, colon and ovarian cancer cells. For comparison purposes, all compounds were also tested in a model of healthy human cells from the embryonic kidney. Notably, all new compounds were more toxic than their cyclometallated precursor bearing two chlorido ligands, and the derivative bearing one phosphane ligand presented the most promising toxicity profile in our in vitro screening, displaying a p53 dependent activity in colorectal cancer HCT116 cells. Finally, for the first time C^N cyclometallated gold(III) complexes were shown to be potent inhibitors of the zinc finger protein PARP-1, involved in the mechanism of cisplatin resistance.

Organometallics with N-heterocyclic carbene (NHC) ligands have triggered major interest in inorganic medicinal chemistry. Complexes of the type Rh(I)(NHC)(COD)X (where X is Cl or I, COD is cyclooctadiene, and NHC is a dimethylbenzimidazolylidene) represent a promising type of new metallodrugs that have been explored by advanced biomedical methods only recently. In this work, we have synthesized and characterized several complexes of this type. As observed by mass spectrometry, these complexes remained stable over at least 3 h in aqueous solution, after which hydrolysis of the halido ligands occurred and release of the NHC ligand was evident. Effects against mitochondria and general cell tumor metabolism were noted at higher concentrations, whereas phosphorylation of HSP27, p38, ERK1/2, FAK, and p70S6K was induced substantially already at lower exposure levels. Regarding the antiproliferative activity in tumor cells, a clear preference for iodido over chlorido secondary ligands was noted, as well as effects of the substituents of the NHC ligand.

Background: PentixaFor is a promising radiopharmaceutical for positron emission tomography in the detection of different tumor entities and other diseases. Until now, the synthesis of [ 68 Ga]Ga-PentixaFor was reported for the automated synthesis module from Scintomics® only. Our aim was to evaluate the automated synthesis of this radiopharmaceutical on a different module in order to make it available for a broader community. Results: The synthesis of [ 68 Ga]Ga-PentixaFor with different amounts of PentixaFor (50 μg, 30 μg and 20 μg) on the Modular Lab PharmTracer (MLPT) from Eckert & Ziegler with the already established synthesis template for [ 68 Ga]Ga-DOTATOC yielded best results with 50 μg PentixaFor for clinical multi-dose application. All different quality control parameters tested (e.g. sterility, stability and radiochemical purity) were in accordance with the European Pharmacopoeia. Conclusions: [ 68 Ga]Ga-PentixaFor was successfully synthesized fully-automated on the synthesis module Modular Lab PharmTracer and can be used for multi-dose application in clinical settings.

One of the main challenges in targeted alpha therapy is assuring delivery of the α‐particle dose to the targeted cells. Thus, it is critical to identify ligands for α‐emitting radiometals that will form complexes that are very stable, both in vitro and in vivo. In this investigation, thorium‐227 (t1/2 = 18.70 days) chelation of ligands containing hydroxypyridinonate (HOPO) or picolinic acid (pa) moieties and the stability of the resultant complexes were studied. Chelation reactions were followed by reversed‐phased HPLC and gamma spectroscopy. Studies revealed that high 227Th chelation yields could be obtained within 2.5 h or less with ligands containing four Me‐3,2‐HOPO moieties, 1 (83%) and 2 (65%), and also with ligands containing pa moieties, H4octapa 3 (65%) and H4py4pa 6 (87%). No reaction occurred with H4neunpa‐p‐Bn‐NO2 4, and the chelation reaction with another pa ligand H4pypa 5 gave inconsistent yields with a very broad radio‐HPLC peak. The ligands spermine‐(Me‐3,2‐HOPO)4 1, H4octapa 3, and H4py4pa 6 had high stability (i.e., 87% of 227Th still bound to the ligand) in phosphate‐buffered saline at room temperature over a 6‐day period. Preliminary studies with ligand 6 demonstrated efficient chelation of thorium‐226 (t1/2 = 30.57 min) when heated to 80°C for 5 min.

Background [68Ga]Ga-FAPI-46 is a promising radiopharmaceutical for in vivo detection of the fibroblast activation protein by positron emission tomography. Until now, the synthesis of [68Ga]Ga-FAPI-46 has been only performed manually. Our aim was to evaluate the automated synthesis of this radiopharmaceutical on two different commercially available synthesis modules in order to make the tracer readily available for clinical application. Results The synthesis of [68Ga]Ga-FAPI-46 with different amounts of precursor (10–50 μg) on the Modular Lab PharmTracer (MLPT) and Modular Lab eazy (ML eazy) from Eckert & Ziegler with a customized synthesis template and a customized single-use cassette yielded best results with 50 μg FAPI-46 for clinical multi-dose application. All relevant quality control parameters tested (e.g. sterility, stability and radiochemical purity) were in accordance with the European Pharmacopoeia. Conclusions [68Ga]Ga-FAPI-46 was successfully synthesized fully-automated on the synthesis modules Modular Lab PharmTracer and ML eazy and is, thus, available for multi-dose application in clinical settings.

The molecular mechanisms of toxicity and cellular transport of anticancer metallodrugs, including platinum-based agents, have not yet been fully elucidated. The aim of our study was to investigate the relevance of copper transporters (CTR1 and ATP7A/B), organic cation transporters (OCT2) and the multidrug and toxin extrusion proteins (MATE) in the intracellular accumulation of a novel organometallic cytotoxic Au(III) compound in cancer cells in comparison to cisplatin. Specifically, the synthesis and characterization of the gold complex [Au(pyb-H)(PPh2Ar)Cl]PF6 (PPh2Ar = 3-[4-(diphenylphosphino)phenyl]-7-methoxy-2H-chromen-2-one] (1), featuring a coumarin ligand endowed with “smart” fluorescence properties, have been achieved. Initially, the cytotoxic effects of both cisplatin and 1 were studied in a small panel of human cancer cells, and against a non-tumorigenic cell line in vitro. Thus, the human ovarian cancer cell line A2780 and its cisplatin resistant variant A2780cisR, were selected, being most sensitive to the treatment of the gold complex. Co-incubation of the metallodrugs with CuCl2 (a CTR1 substrate) increased the cytotoxic effects of both the Au(III) complex and cisplatin; while co-incubation with cimetidine (inhibitor of OCT2 and MATE) showed some effect only after 72 h incubation. ICP-MS (Inductively Coupled Plasma Mass Spectrometry) analysis of the cell extracts showed that co-incubation with CuCl2 increases Au and Cu accumulation in both cancer cell lines, in accordance with the enhanced antiproliferative effects. Conversely, for cisplatin, no increase in Pt content could be observed in both cell lines after co-incubation with either CuCl2 or cimetidine, excluding the involvement of CTR1, OCT2, and MATE in drug accumulation and overall anticancer effects. This result, together with the evidence for increased Cu content in A2780 cells after cisplatin co-treatment with CuCl2, suggests that copper accumulation is the reason for the observed enhanced anticancer effects in this cell line. Moreover, metal uptake studies in the same cell lines indicate that both 1 and cisplatin are not transported intracellularly by CTR1 and OCT2. Finally, preliminary fluorescence microscopy studies enabled the visualization of the sub-cellular distribution of the gold compound in A2780 cells, suggesting accumulation in specific cytosolic components/organelles.