Linker design for the modular assembly of multifunctional and targeted platinum(ii)-containing anticancer agents

Development of Prodrug-Payloads for Targeted Therapeutic Applications of Platinum-Acridine Anticancer Agents

A synthetic platform has been developed that provides access to platinum(IV) prodrugs of highly cytotoxic platinum-acridine anticancer agents and allows them to be incorporated into conjugation-ready prodrug-payloads (PPLs). The PPLs can be conveniently assembled in highly efficient microscale reactions utilizing strain-promoted azide-alkyne cycloaddition chemistry. Model reactions were performed to study the stability of the PPLs in buffers and media and to assess their compatibility with cysteine-maleimide Michael addition chemistry. Amide coupling was a successful strategy to generate a conjugate containing integrin-targeted cyclo[RGDfK] peptide. Reactions with ascorbate were performed to mimic the reductive activation of the PPLs and the latter conjugate, and a cyanine (Cy5) fluorophore-labeled PPL was used to probe the reduction of platinum(IV) in cancer cells by confocal microscopy. The PPL concept introduced here should be evaluated for treating solid tumors with PAs using cancer-targeting vehicles, such as antibody-drug conjugates.

Evaluation of a Platinum-Acridine Anticancer Agent and Its Liposomal Formulation in an in vivo Model of Lung Adenocarcinoma

Liposomal formulations have been developed for a highly cytotoxic platinum-acridine agent, [PtCl(pn)(C18 H21 N4 )](NO3 )2 (PA, pn=propane-1,3-diamine), and fully characterized. Nanoliposomes consisting of hydrogenated soybean phosphatidylcholine (HSPC), 1,2-dihexadecanoyl-sn-glycero-3-phospho-(1'-rac-glycerol) (DPPG), and polyethylene glycol-2000-distearoylphosphatidylethanolamine (DSPE-mPEG2k ) were able to stably encapsulate PA at payload-to-lipid ratios of 2-20 %. The fusogenic properties of the liposomes promote efficient cellular uptake of PA across the plasma membrane, which results in vesicular transport of payload to the nucleus in cultured lung cancer cells. Unencapsulated PA and one of the newly designed liposomal formulations show promising tumor growth inhibition in tumor xenografts derived from A549 lung adenocarcinoma cells of 76 % and 72 %, respectively. Cisplatin showed no significant efficacy at a 10-fold higher dose. These findings underscore the utility of platinum-acridine agents for treating aggressive, chemoresistant forms of cancer and validate nanoliposomes as a biocompatible, expandable platform for their intravenous delivery and other potential routes of administration.

Combinatorial Synthesis to Identify a Potent, Necrosis-Inducing Rhenium Anticancer Agent

Combinatorial synthesis can be applied for developing a library of compounds that can be rapidly screened for biological activity. Here, we report the application of microwave-assisted combinatorial chemistry for the synthesis of 80 rhenium(I) tricarbonyl complexes bearing diimine ligands. This library was evaluated for anticancer activity in three different cancer cell lines, enabling the identification of three lead compounds with cancer cell growth-inhibitory activities of less than 10 μM. These three lead structures, Re-9B, Re-9C, and Re-9D, were synthesized independently and fully characterized by NMR spectroscopy, mass spectrometry, elemental analysis, and X-ray crystallography. The most potent of these three complexes, Re-9D, was further explored to understand its mechanism of action. Complex Re-9D is equally effective in both wild-type and cisplatin-resistant A2780 ovarian cancer cells, indicating that it circumvents cisplatin resistance. This compound was also shown to possess promising activity against ovarian cancer tumor spheroids. Additionally, flow cytometry showed that Re-9D does not induce cell cycle arrest or flipping of phosphatidylserine to the outer cell membrane. Analysis of the morphological changes of cancer cells treated with Re-9D revealed that this compound gives rise to rapid plasma membrane rupture. Collectively, these data suggest that Re-9D induces necrosis in cancer cells. To assess the in vivo biodistribution and stability of this compound, a radioactive ^99mTc analogue of Re-9D, ^99mTc-9D(H₂O), was synthesized and administered to naı̈ve BALB/c mice. Results of these studies indicate that ^99mTc-9D(H₂O) exhibits high metabolic stability and a distinct biodistribution profile. This research demonstrates that combinatorial synthesis is an effective approach for the development of new rhenium anticancer agents with advantageous biological properties.

Cysteine-Directed Bioconjugation of a Platinum(II)-Acridine Anticancer Agent

Classical maleimide Michael addition chemistry in conjunction with copper-free click chemistry was investigated as a synthetic strategy to attach cytotoxic platinum-acridine hybrid agents to carrier proteins. The structural integrity and selectivity of the model payloads, which were validated in human serum albumin (HSA) using mass spectrometric analysis and heteronuclear 2D ¹H-¹⁵N HSQC NMR experiments, may have broad utility for the targeted delivery of highly cytotoxic platinum acridines and other nonclassical platinum containing anticancer agents.