Targeting hypoxia in tumors using 2-nitroimidazoles with peptidic chelators for technetium-99m: effect of lipophilicity

Molecular imaging of tumor hypoxia: Evolution of nitroimidazole radiopharmaceuticals and insights for future development

Though growing evidence has been collected in support of the concept of dose escalation based on the molecular level images indicating hypoxic tumor sub-volumes that could be radio-resistant, validation of the concept is still a work in progress. Molecular imaging of tumor hypoxia using radiopharmaceuticals is expected to provide the required input to plan dose escalation through Image Guided Radiation Therapy (IGRT) to kill/control the radio-resistant hypoxic tumor cells. The success of the IGRT, therefore, is heavily dependent on the quality of images obtained using the radiopharmaceutical and the extent to which the image represents the true hypoxic status of the tumor in spite of the heterogeneous nature of tumor hypoxia. Available literature on radiopharmaceuticals for imaging hypoxia is highly skewed in favor of nitroimidazole as the pharmacophore given their ability to undergo oxygen dependent reduction in hypoxic cells. In this context, present review on nitroimidazole radiopharmaceuticals would be immensely helpful to the researchers to obtain a birds-eye view on what has been achieved so far and what can be tried differently to obtain a better hypoxia imaging agent. The review also covers various methods of radiolabeling that could be utilized for developing radiotracers for hypoxia targeting applications.

Tumor microenvironment targeted nanotherapeutics for cancer therapy and diagnosis: A review

Recent findings suggest that the cellular and extracellular materials surrounding the cancerous cells from an atypical tumor microenvironment (TM) play a pivotal role in the process of tumor initiation and progression. TM comprises an intricate system involving diverse cell types including endothelial cells, pericytes, smooth muscle cells, fibroblasts, various inflammatory cells, dendritic cells, and cancer stem cells (CSCs). The TM-forming cells dynamically interact with the cancerous cells through various signaling mechanisms and pathways. The existence of this dynamic cellular communication is responsible for creating an environment suitable for sustaining a reasonably high cellular proliferation. Presently, researchers are showing interest to use these TM conditions to mediate effective targeting measures for cancer therapy. The use of nanotherapeutics-based combination therapy; stimuli-responsive nanotherapeutics targeting acidic pH, hypoxic environment; and nanoparticle-induced hyperthermia are some of the approaches that are under intense investigation for cancer therapy. This review discusses TM and its role in cancer progression and crosstalk understanding, opportunities, and epigenetic modifications involved therein to materialize the capability of nanotherapeutics to target cancer by availing TM. STATEMENT OF SIGNIFICANCE: This article presents various recent reports, proof-of-concept studies, patents, and clinical trials on the concept of tumor microenvironment for mediating the cancer-specific delivery of nanotechnology-based systems bearing anticancer drug and diagnostics. We highlight the potential of tumor microenvironment; its role in disease progression, opportunities, challenges, and allied treatment strategies for effective cancer therapy by conceptual understanding of tumor microenvironment and epigenetic modifications involved. Specifically, nanoparticle-based approaches to target various processes related to tumor microenvironment (pH responsive, hypoxic environment responsive, targeting of specific cells involved in tumor microenvironment, etc.) are dealt in detail.

Post-functionalization of platinum–NHC complexes by oxime ligation for ligand targeted therapy

Carbonyl condensation reactions have been used for accessing N-heterocyclic carbene (NHC) platinum bioconjugate complexes.

Toward hypoxia-selective rhenium and technetium tricarbonyl complexes

With the aim of preparing hypoxia-selective imaging and therapeutic agents, technetium(I) and rhenium(I) tricarbonyl complexes with pyridylhydrazone, dipyridylamine, and pyridylaminocarboxylate ligands containing nitrobenzyl or nitroimidazole functional groups have been prepared. The rhenium tricarbonyl complexes were synthesized with short reaction times using microwave irradiation. Rhenium tricarbonyl complexes with deprotonated p-nitrophenyl pyridylhydrazone ligands are luminescent, and this has been used to track their uptake in HeLa cells using confocal fluorescent microscopy. Selected rhenium tricarbonyl complexes displayed higher uptake in hypoxic cells when compared to normoxic cells. A (99m)Tc tricarbonyl complex with a dipyridylamine ligand bearing a nitroimidazole functional group is stable in human serum and was shown to localize in a human renal cell carcinoma (RCC; SK-RC-52) tumor in a mouse.

Tricarbonyltechnetium(I) and tricarbonylrhenium(I) complexes of amino acids: crystal and molecular structure of a novel cyclic dimeric Re(CO)3-amino acid complex comprised of the OON donor atom set of the tridentate ligand

Radiolabeled complexes of monoamino polycarboxylic, polyamino monocarboxylic and thiol containing amino acid ligands were prepared from a fac-[(99m)Tc(CO)3(H2O)3](+) precursor.The overall radiochemical yield was 94-98%. The complexes exhibited substantial in vitro and in vivo stability. The corresponding Re(I) complexes of the ligands DAPA, Asp and CysH were prepared and characterized by means of IR, NMR, and MS spectroscopic studies, as well as X-ray crystallography (for those containing D,L-DAPA and D,L-Asp). The rhenium complexes have been structurally correlated with the technetium complexes by means of HPLC studies. The reaction of Re(CO)5Cl with D,L-Asp in presence of triethylamine led to the formation of a new class of cyclic dimeric complexes formed by the OON donor atom set of the tridentate ligands. The amino carboxylate ligand system formed well defined complexes with a fac-[M(CO)3(H2O)3](+) core and shows good promise in (99m)Tc(CO)3 tracer development.

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.

Synthesis and in vitro and in vivo evaluation of three radioiodinated nitroimidazole analogues as tumor hypoxia markers

Three novel nitroimidazole-based thioflavin-T derivatives, N-[4-(benzothiazol-2-yl)phenyl]-3-(4-nitroimidazole-1-yl)propanamide, N-[4-(benzothiazol-2-yl) phenyl]-3-(4-nitroimidazole-1-yl)-N-methylpropanamide and N-[4-(benzothiazol-2-yl)phenyl]-3-(2-nitroimidazole-1-yl) propanamide were synthesized and radiolabeled with iodine-131. Three (131)I-labeled compounds continuously accumulated in hypoxic murine sarcoma S180 cells in vitro but not in aerobic cells. Biodistribution results in mice bearing S180 tumor indicated that the tracers could localize in the tumor and eliminate from it slowly. In contrast, the uptake in other organs (stomach excluded) was little and the clearance was quick. The tumor-to-tissue ratios of three compounds all increased with time.

Imaging hypoxia in tumors

For many years, it has been known that hypoxia affects the response to radiotherapy in human cancers. Hypoxic regions can develop as a tumor grows beyond the ability of its blood supply to deliver oxygen to the full extent of the tumor, exacerbated by vascular spasm or compression caused by increased interstitial fluid pressure. However, hypoxia is heterogeneous, and tumors that appear identical by clinical and radiographic criteria can vary greatly in their extent of hypoxia. Several invasive procedures to measure hypoxia in tumors have been developed and are predictive of response to therapy, but none of these is in routine clinical use because of technical complexity, inconvenience, and inability to obtain repeated measures. Noninvasive imaging with a hypoxia-directed radiopharmaceutical could be of great clinical utility. Most such radiopharmaceuticals under development use 2-nitroimidazole as the targeting moiety. 2-Nitroimidazole, which is selectively reduced and bound in hypoxic tissues, has been labeled with F-18, Cu-64/67, I-123, and Tc-99m. Of these, F-18-fluoromisonidazole and I-123-iodoazomycin arabinoside (IAZA) have been most widely studied clinically. Non-nitro-containing bioreductive complexes, such as the Cu-60/62/64 thiosemicarbazone ATSM and Tc-99m butylene amineoxime (BnAO or HL91), have also been evaluated. In particular, 1-123-IAZA and Cu-60-ATSM have shown correlation with response to radiotherapy in preliminary clinical studies. However, more preclinical studies comparing imaging with validated invasive methods and clinical studies with outcome measures are required. Nuclear medicine is poised to play an important role in optimizing the therapy of patients with hypoxic tumors.