Nuclear trafficking of a gadolinium conjugate in nude mice xenografted with human LN-229 glioma

Strategies for the Nuclear Delivery of Metal Complexes to Cancer Cells

The nucleus is an essential organelle for the function of cells. It holds most of the genetic material and plays a crucial role in the regulation of cell growth and proliferation. Since many antitumoral therapies target nucleic acids to induce cell death, tumor-specific nuclear drug delivery could potentiate therapeutic effects and prevent potential off-target side effects on healthy tissue. Due to their great structural variety, good biocompatibility, and unique physico-chemical properties, organometallic complexes and other metal-based compounds have sparked great interest as promising anticancer agents. In this review, strategies for specific nuclear delivery of metal complexes are summarized and discussed to highlight crucial parameters to consider for the design of new metal complexes as anticancer drug candidates. Moreover, the existing opportunities and challenges of tumor-specific, nucleus-targeting metal complexes are emphasized to outline some new perspectives and help in the design of new cancer treatments.

Lanthanide-based tools for the investigation of cellular environments

Biological probes constructed from lanthanides can provide a variety of readout signals, such as the luminescence of Eu(iii), Tb(iii), Yb(iii), Sm(iii) and Dy(iii), and the proton relaxation enhancement of Gd(iii) and Eu(ii). For numerous applications the intracellular delivery of the lanthanide probe is essential. Here, we review the methods for the intracellular delivery of non-targeted complexes (i.e. where the overall complex structure enhances cellular uptake), as well as complexes attached to a targeting unit (i.e. to a peptide or a small molecule) that facilitates delivery. The cellular applications of lanthanide-based supramolecules (dendrimers, metal organic frameworks) are covered briefly. Throughout, we emphasize the techniques that can confirm the intracellular localization of the lanthanides and those that enable the determination of the fate of the probes once inside the cell. Finally, we highlight methods that have not yet been applied in the context of lanthanide-based probes, but have been successful in the intracellular delivery of other metal-based probes.

Exploring the cellular accumulation of metal complexes

Transition metal complexes offer great potential as diagnostic and therapeutic agents, and a growing number of biological applications have been explored. To be effective, these complexes must reach their intended target inside the cell. Here we review the cellular accumulation of metal complexes, including their uptake, localization, and efflux. Metal complexes are taken up inside cells through various mechanisms, including passive diffusion and entry through organic and metal transporters. Emphasis is placed on the methods used to examine cellular accumulation, to identify the mechanism(s) of uptake, and to monitor possible efflux. Conjugation strategies that have been employed to improve the cellular uptake characteristics of metal complexes are also described.