Design of Tetrazolium Cations for the Release of Antiproliferative Formazan Chelators in Mammalian Cells.

Cancer cells generally present a higher demand for iron, which plays crucial roles in tumor progression and metastasis. This iron addiction provides opportunities to develop broad spectrum anticancer drugs that target iron metabolism. In this context, prochelation approaches are investigated to release metal-binding compounds under specific conditions, thereby limiting off-target toxicity. Here, we demonstrate a prochelation strategy inspired by the bioreduction of tetrazolium cations widely employed to assess the viability of mammalian cells. We designed a series of tetrazolium-based compounds for the intracellular release of metal-binding formazan ligands. The combination of reduction potentials appropriate for intracellular reduction and an N-pyridyl donor on the formazan scaffold led to two effective prochelators. The reduced formazans bind as tridentate ligands and stabilize low-spin Fe(II) centers in complexes of 2:1 ligand-to-metal stoichiometry. The tetrazolium salts are stable in blood serum for over 24 h, and antiproliferative activities at micromolar levels were recorded in a panel of cancer cell lines. Additional assays confirmed the intracellular activation of the prochelators and their ability to affect cell cycle progression, induce apoptotic death, and interfere with iron availability. Demonstrating the role of iron in their intracellular effects, the prochelators impacted the expression levels of key iron regulators (i.e., transferrin receptor 1 and ferritin), and iron supplementation mitigated their cytotoxicity. Overall, this work introduces the tetrazolium core as a platform to build prochelators that can be tuned for activation in the reducing environment of cancer cells and produce antiproliferative formazan chelators that interfere with cellular iron homeostasis.

Multitargeted Platinum(IV) Anticancer Complexes Bearing Pyridinyl Ligands as Axial Leaving Groups.

Although multitargeted PtIV anticancer prodrugs have shown significant activities in reducing drug resistance, the types of bioactive ligands and drugs that can be conjugated to the Pt center remain limited to O-donors. Herein, we report the synthesis of PtIV complexes bearing axial pyridines via ligand exchange reactions. Unexpectedly, the axial pyridines are quickly released after reduction, indicating their potential to be utilized as axial leaving groups. We further expand our synthetic approach to obtaining two multitargeted PtIV prodrugs containing bioactive pyridinyl ligands: a PARP inhibitor and an EGFR tyrosine kinase inhibitor; these conjugates exhibit great potential for overcoming drug resistance, and the latter conjugate inhibits the growth of Pt-resistant tumor in vivo. This research adds to the array of synthetic methods for accessing PtIV prodrugs and significantly increases the types of bioactive axial ligands that can be conjugated to a PtIV center.

An intramolecular photoswitch can significantly promote photoactivation of Pt(iv) prodrugs.

Selective activation of prodrugs at diseased tissue through bioorthogonal catalysis represents an attractive strategy for precision cancer treatment. Achieving efficient prodrug photoactivation in cancer cells, however, remains challenging. Herein, we report two Pt(iv) complexes, designated as rhodaplatins {rhodaplatin 1, [Pt(CBDCA-O,O)(NH3)2(RhB)OH]; rhodaplatin 2, [Pt(DACH)ox(RhB)(OH)], where CBDCA is cyclobutane-1,1-dicarboxylate, RhB is rhodamine B, DACH is (1R,2R)-1,2-diaminocyclohexane, and ox is oxalate}, that bear an internal photoswitch to realize efficient accumulation, significant co-localization, and subsequent effective photoactivation in cancer cells. Compared with the conventional platform of external photocatalyst plus substrate, rhodaplatins presented up to 4.8 104-fold increased photoconversion efficiency in converting inert Pt(iv) prodrugs to active Pt(ii) species under physiological conditions, due to the increased proximity and covalent bond between the photoswitch and Pt(iv) substrate. As a result, rhodaplatins displayed increased photocytotoxicity compared with a mixture of RhB and conventional Pt(iv) compound in cancer cells including Pt-resistant ones. Intriguingly, rhodaplatin 2 efficiently accumulated in the mitochondria and induced apoptosis without causing genomic DNA damage to overcome drug resistance. This work presents a new approach to develop highly effective prodrugs containing intramolecular photoswitches for potential medical applications.

A Photocaged, Water-Oxidizing, and Nucleolus-Targeted Pt(IV) Complex with a Distinct Anticancer Mechanism.

Targeted anticancer prodrugs that can be controllably activated are highly desired for personalized precision medicine in cancer therapy. Such prodrugs with unique action modes are also promising to overcome drug resistance. Herein, we report coumaplatin, an oxaliplatin-based and photocaged Pt(IV) prodrug, to realize nuclear accumulation along with "on-demand" activation. This prodrug is based on a Pt(IV) complex that can be efficiently photoactivated via water oxidation without the requirement of a reducing agent. Coumaplatin accumulates very efficiently in the nucleoli, and upon photoactivation, this prodrug exhibits a level of photocytotoxicity up to 2 orders of magnitude higher than that of oxaliplatin. Unexpectedly, this prodrug presents strikingly enhanced tumor penetration ability and utilizes a distinct action mode to overcome drug resistance; i.e., coumaplatin but not oxaliplatin induces cell senescence, p53-independent cell death, and immunogenic cell death along with T cell activation. Our findings not only provide a novel strategy for the rational design of controllably activated and nucleolus-targeted Pt(IV) anticancer prodrugs but also demonstrate that accumulating conventional platinum drugs to the nucleus is a practical way to change its canonical mechanism of action and to achieve reduced resistance.

Synthesis, Cytotoxicity, and Mechanistic Investigation of Platinum(IV) Anticancer Complexes Conjugated with Poly(ADP-ribose) Polymerase Inhibitors.

Many clinical trials using combinations of platinum drugs and PARP-1 inhibitors (PARPi) have been carried out, with the hope that such combinations will lead to enhanced therapeutic outcomes against tumors. Herein, we obtained seven potential PARPi with structural diversity and then conjugated them with cisplatin-based platinum(IV) complexes. Both the synthesized PARPi ligands and PARPi-Pt conjugates [PARPi-Pt(IV)] show inhibitory effects against PARP-1's catalytic activity. The PARPi-Pt(IV) conjugates are cytotoxic in a panel of human cancer cell lines, and the leading ones display the ability to overcome cisplatin resistance. A mechanistic investigation reveals that the representative PARPi-Pt(IV) conjugates efficiently enter cells, bind to genomic DNA, disturb cell cycle distribution, and induce apoptotic cell death in both cisplatin-sensitive and -resistant cells. Our study provides a strategy to improve the cytotoxicity of platinum(IV)-based anticancer complexes and overcome cisplatin resistance by using a small-molecule anticancer complex that simultaneously damages DNA and inhibits PARP.

Synthesis and Cytotoxic Study of a Platinum(IV) Anticancer Prodrug with Selectivity toward Luteinizing Hormone-Releasing Hormone (LHRH) Receptor-Positive Cancer Cells.

Platinum drugs including cisplatin are widely used in clinics to treat various types of cancer. However, the lack of cancer-cell selectivity is one of the major problems that lead to side effects in normal tissues. Luteinizing hormone-releasing hormone (LHRH) receptors are overexpressed in many types of cancer cells but rarely presented in normal cells, making LHRH receptor a good candidate for cancer targeting. In this study, we report the synthesis and cytotoxic study of a novel platinum(IV) anticancer prodrug functionalized with LHRH peptide. This LHRH-platinum(IV) conjugate is highly soluble in water and quite stable in a PBS buffer. Cytotoxic study reveals that the prodrug selectively targets LHRH receptor-positive cancer cell lines with the cytotoxicities 5-8 times higher than those in LHRH receptor-negative cell lines. In addition, the introduction of LHRH peptide enhances the cellular accumulation in a manner of receptor-mediated endocytosis. Moreover, the LHRH-platinum(IV) prodrug is proved to kill cancer cells by binding to the genomic DNA, inducing apoptosis, and arresting the cell cycle at the G2/M phase. In summary, we report a novel LHRH-platinum(IV) anticancer prodrug having largely improved selectivity toward LHRH receptor-positive cancer cells, relative to cisplatin.

Synthesis, Structure, and Cytotoxicity of Oxaliplatin-Based Platinum(IV) Anticancer Prodrugs Bearing One Axial Fluoride.

Fluorine plays more and more important roles in drug design and development. In recent years, fluorine-containing organic drugs have already been applied in a broad range of therapeutic areas. Herein, we report our attempt to introduce an axial fluorine ligand to Pt(IV) complexes by oxidizing oxaliplatin with electrophilic fluorinating reagents in different protic solvents. The crystal structure of one representative complex is presented. The fluorinated Pt(IV) complexes are further expanded by functionalization with different anhydrides, and their analogues bearing one different axial ligand (OAc or OH group) are also synthesized. Further investigations show that the axial fluorine atom has dramatic effects on the chemical properties of these prodrugs. These new fluorinated Pt(IV) complexes are proved to be stable in physiological conditions. For most of the fluorinated Pt(IV) complexes, a higher reduction potential indicates its greater tendency to be reduced by ascorbate. Introducing an axial fluorine ligand in Pt(IV) complexes does not lead to the increase of their lipophilicity. Moreover, these new fluorinated Pt(IV) complexes show better cytotoxicity than nonfluorinated analogues which may derive from their higher cellular accumulation in cancer cells. Therefore, the good stability and high cytotoxicity of these fluorinated Pt(IV) prodrugs indicate their great potential as a building block for further functionalization.

Monochalcoplatin: An Actively Transported, Quickly Reducible, and Highly Potent PtIV Anticancer Prodrug.

Recently, PtIV prodrugs have attracted much attention as the next generation of platinum-based antineoplastic drug candidates. Here we report the discovery and evaluation of monochalcoplatin, a monocarboxylated PtIV prodrug that is among the most cytotoxic PtIV prodrugs to date. Compared with its dicarboxylated counterpart chalcoplatin, monochalcoplatin accumulates astonishingly effectively and rapidly in cancer cells, which is not ascribed to its lipophilicity. The prodrug is quickly reduced, causes DNA damage, and induces apoptosis, resulting in superior cytotoxicity with IC50 values in the nanomolar range in both cisplatin-sensitive and -resistant cells; these IC50 values are up to 422-fold higher than that of cisplatin. A detailed mechanistic study reveals that monochalcoplatin actively enters cells through a transporter-mediated process. Moreover, monochalcoplatin shows significant antitumor activity in an in vivo colorectal tumor model. Our study implies a practical strategy for the design of more effective PtIV prodrugs to conquer drug resistance by tuning both cellular uptake pathways and activation processes.

A Cancer Cell-Selective and Low-Toxic Bifunctional Heterodinuclear Pt(IV)-Ru(II) Anticancer Prodrug.

Although different types of metal-based anticancer complexes have been synthesized, novel complexes to reduce the serious side effect of cisplatin and conquer cancer metastasis are still highly desired. Here, we report the synthesis, characterization, and biological activity of a novel heterodinuclear Pt(IV)-Ru(II) anticancer prodrug. The Pt(IV)-Ru(II) complex exhibits good stability in both water and PBS solution. Biological evaluation revealed that this bifunctional Pt(IV)-Ru(II) complex utilizes the advantages of two metal centers to have both cytotoxicity and antimetastatic property as designed. Although the complex has comparable cytotoxicities to cisplatin in tested cancer cell lines, this prodrug selectively kills cancer but not normal cells, and the IC50 values of the Pt(IV)-Ru(II) complex are 7-10 times higher than those of cisplatin toward normal cells. The cancer cell selectivity is further demonstrated by a cancer-normal cell coculture system. In addition, the antimetastatic properties of the heterodinuclear complex are assessed by using highly metastatic human breast cancer cells, and the results show that the migration and invasion of cancer cells are effectively restrained after the treatment. Moreover, the Pt(IV)-Ru(II) complex displays lower toxicity than cisplatin in developing zebrafish embryos. We, therefore, report an example of heterodinuclear Pt(IV)-Ru(II) complex not only to defeat both drug resistance and cancer metastasis but also having significantly improved cancer cell selectivity and reduced in vivo toxicity than cisplatin.

A Cisplatin-Loaded Immunochemotherapeutic Nanohybrid Bearing Immune Checkpoint Inhibitors for Enhanced Cervical Cancer Therapy.

The efficacy of conventional chemotherapy is hindered by cancer cell escape from the immune system. A multifunctional nanohybrid system is reported for effective immunochemotherapy against cervical cancer. This nanohybrid contains both immune checkpoint inhibitor and cisplatin anticancer prodrug, showing improved cellular accumulation and increased binding of Pt to DNA and resulting in elevated apoptosis than using cisplatin alone when tested in cervical cancer cells. The immune checkpoint inhibitor enables the inhibition of indoleamine-2,3-dioxygenase and reverses immunosuppressive T cells to recognize cancer cells, leading to T cell proliferation and activation, cancer cell cycle arrest, and ultimately increased cancer cell death. The nanohybrid is also active in vivo against the growth of human cervical tumors. Overall, a strategy is provided using a multifunctional nanohybrid system to boost the antitumor activity of cisplatin.

A Platinum(IV) Anticancer Prodrug Targeting Nucleotide Excision Repair To Overcome Cisplatin Resistance.

DNA damage response plays a key role not only in maintaining genome integrity but also in mediating the antitumor efficacy of DNA-damaging antineoplastic drugs. Herein, we report the rational design and evaluation of a PtIV anticancer prodrug inhibiting nucleotide excision repair (NER), one of the most pivotal processes after the formation of cisplatin-induced DNA damage that deactivates the drug and leads to drug resistance in the clinic. This dual-action prodrug enters cells efficiently and causes DNA damage while simultaneously inhibiting NER to promote apoptotic response. The prodrug is strongly active against the proliferation of cisplatin-resistant human cancer cells with an up to 88-fold increase in growth inhibition compared with cisplatin, and the prodrug is much more active than a mixture of cisplatin and an NER inhibitor. Our study highlights the importance of targeting downstream pathways after the formation of Pt-induced DNA damage as a novel strategy to conquer cisplatin resistance.

An upconversion nanoplatform for simultaneous photodynamic therapy and Pt chemotherapy to combat cisplatin resistance.

Platinum-based antineoplastic drugs are among the first-line chemotherapeutic agents against a variety of solid tumors, but toxic side-effects and drug resistance issues limit their clinical optimization. Novel strategies and platforms to conquer cisplatin resistance are highly desired. Herein, we assembled a multimodal nanoplatform utilizing 808 nm-excited and biocompatible core-shell-shell upconversion nanoparticles (UCNPs) [NaGdF4:Yb/Nd@NaGdF4:Yb/Er@NaGdF4] that were covalently loaded with not only photosensitizers (PSs), but also Pt(iv) prodrugs, which were rose bengal (RB) and c,c,t-[Pt(NH3)2Cl2(OCOCH2CH2NH2)2], respectively. The UCNPs had the capability to convert near infrared (NIR) light to visible light, which was further utilized by RB to generate singlet oxygen. At the same time, the nanoplatform delivered the Pt(iv) prodrug into cancer cells. Thus, this upconversion nanoplatform was able to carry out combined and simultaneous photodynamic therapy (PDT) and Pt chemotherapy. The nanoplatform was well characterized and the energy transfer efficiency was confirmed. Compared with free cisplatin or UCNPs loaded with RB only, our nanoplatform showed significantly improved cytotoxicity upon 808 nm irradiation in both cisplatin-sensitive and -resistant human ovarian cancer cells. A mechanistic study showed that the nanoparticles efficiently delivered the Pt(iv) prodrug into cancer cells, resulting in Pt-DNA damage, and that the nanoplatform generated cellular singlet oxygen to kill cancer cells. We, therefore, provide a comprehensive strategy to use UCNPs for combined Pt chemotherapy and PDT against cisplatin resistance, and our nanoplatform can also be used as a theranostic tool due to its NIR bioimaging capacity.

Mono- and di-bromo platinum(IV) prodrugs via oxidative bromination: synthesis, characterization, and cytotoxicity.

Platinum(IV)-based anticancer prodrugs have attracted much attention due to their relative inertness under physiological conditions, being activated inside cells, and their capacity for functionalization with a variety of small-molecule or macromolecule moieties. Novel asymmetric platinum(IV) compounds synthesized through expedient and unique methods are desired. Here we utilize N-bromosuccinimide (NBS) and carry out oxidative bromination on platinum(II) drugs, namely cisplatin, carboplatin, and oxaliplatin, to obtain asymmetric and mono-bromo platinum(IV) prodrugs. Different solvents are used to obtain various compounds, and the compounds are further functionalized. Di-bromo compounds are also obtained through NBS-directed oxidative bromination in ethanol. The crystal structures of representative compounds are discussed, and the reduction potentials of some compounds are examined. A cytotoxicity test shows that the mono- and di-bromo platinum(IV) compounds are active against human ovarian cancer cells. Our study enriches the family of asymmetric platinum(IV) prodrugs and provides with a convenient strategy to obtain brominated platinum(IV) complexes.