Justin J. Wilson

Synthetic methods for the preparation of platinum anticancer complexes.

The demonstration in the 1960s that cis-diamminedichloroplatinum(II), or cisplatin, inhibits cellular division of Escherichia coli1 led to the subsequent discovery that this simple coordination compound is also an effective antitumor agent in mouse models.2 Subsequent studies validated cisplatin as an effective anticancer agent in humans as well,3–7 and FDA approval of cisplatin for the treatment of metastatic ovarian and testicular cancers was granted in 1978.8 Its introduction as a chemotherapeutic agent significantly improved the survival outlook for many cancer patients; the cure rate for testicular cancer before the approval of cisplatin was less than 10%, significantly lower than the 90% cure rate attained with modern platinum chemotherapy.9,10 Cisplatin kills cancer cells primarily by cross-linking DNA and inhibiting transcription.11 The chemical origin of this process begins when cisplatin enters the cell and undergoes aquation involving loss of one or both chloride ligands. The resulting platinum(II) aqua complexes are potent electrophiles that readily react with a number of biological ligands with loss of the bound water molecules. The purine bases of nucleic acids are strongly nucleophilic at the N7 position. Thus, cisplatin binds readily to DNA, forming primarily bifunctional adducts with loss of both chloride ligands. The major cisplatin-DNA adduct is the intrastrand 1,2-d(GpG) cross-link, which accounts for 60–65% of the bound platinum.12 The resulting Pt-DNA adducts, which distort and bend the DNA structure,13–15 impede transcription.16 The downstream effects of transcription inhibition ultimately lead to cell death. Despite its great curative success in testicular cancer, cisplatin is not universally effective in other cancer types and induces a number of toxic side effects.17–19 Additionally, certain cancers are resistant to cisplatin therapy. This resistance is either intrinsic or developed during prolonged treatment.20,21 To circumvent these problems, new platinum complexes have been pursued and investigated for their antitumor properties. Although well over a thousand complexes have been prepared and tested thus far,22 only two other platinum drugs are approved for clinical use worldwide, and three additional compounds are approved for regional use in Asia.23 These complexes, displayed in Chart 1, operate with a mechanism of action similar to that of cisplatin, which involves DNA binding and transcription inhibition. Open in a separate window Chart 1 Chemical structures of the clinically used platinum-based anticancer drugs. The top three complexes, cisplatin, carboplatin, and oxaliplatin, are approved for use worldwide. The bottom three complexes, nedaplatin, lobaplatin, and heptaplatin, are approved for use in Japan, China, and Korea, respectively.

Phenanthriplatin, a monofunctional DNA-binding platinum anticancer drug candidate with unusual potency and cellular activity profile

Monofunctional platinum(II) complexes of general formula cis-[Pt(NH3)2(N-heterocycle)Cl]Cl bind DNA at a single site, inducing little distortion in the double helix. Despite this behavior, these compounds display significant antitumor properties, with a different spectrum of activity than that of classic bifunctional cross-linking agents like cisplatin. To discover the most potent monofunctional platinum(II) compounds, the N-heterocycle was systematically varied to generate a small library of new compounds, with guidance from the X-ray structure of RNA polymerase II (Pol II) stalled at a monofunctional pyriplatin-DNA adduct. In pyriplatin, the N-heterocycle is pyridine. The most effective complex evaluated was phenanthriplatin, cis-[Pt(NH3)2(phenanthridine)Cl]NO3, which exhibits significantly greater activity than the Food and Drug Administration-approved drugs cisplatin and oxaliplatin. Studies of phenanthriplatin in the National Cancer Institute 60-cell tumor panel screen revealed a spectrum of activity distinct from that of these clinically validated anticancer agents. The cellular uptake of phenanthriplatin is substantially greater than that of cisplatin and pyriplatin because of the hydrophobicity of the phenanthridine ligand. Phenanthriplatin binds more effectively to 5′-deoxyguanosine monophosphate than to N-acetyl methionine, whereas pyriplatin reacts equally well with both reagents. This chemistry supports DNA as a viable cellular target for phenanthriplatin and suggests that it may avoid cytoplasmic platinum scavengers with sulfur-donor ligands that convey drug resistance. With the use of globally platinated Gaussia luciferase vectors, we determined that phenanthriplatin inhibits transcription in live mammalian cells as effectively as cisplatin, despite its inability to form DNA cross-links.

Synthesis, Characterization, and Cytotoxicity of Platinum(IV) Carbamate Complexes

The synthesis, characterization, and cytotoxicity of eight new platinum(IV) complexes having the general formula cis,cis,trans-[Pt(NH(3))(2)Cl(2)(O(2)CNHR)(2)] are reported, where R = tert-butyl (4), cyclopentyl (5), cyclohexyl (6), phenyl (7), p-tolyl (8), p-anisole (9), 4-fluorophenyl (10), or 1-naphthyl (11). These compounds were synthesized by reacting organic isocyanates with the platinum(IV) complex cis,cis,trans-[Pt(NH(3))(2)Cl(2)(OH)(2)]. The electrochemistry of the compounds was investigated by cyclic voltammetry. The aryl carbamate complexes 7-11 exhibit reduction peak potentials near -720 mV vs Ag/AgCl, whereas the alkyl carbamate complexes display reduction peak potentials between -820 and -850 mV vs Ag/AgCl. The cyclic voltammograms of cis,cis,trans-[Pt(NH(3))(2)Cl(2)(O(2)CCH(3))(2)] (1), cis,cis,trans-[Pt(NH(3))(2)Cl(2)(O(2)CCF(3))(2)] (2), and cis-[Pt(NH(3))(2)Cl(4)] (3) were measured for comparison. Density functional theory studies were undertaken to investigate the electronic structures of 1-11 and to determine their adiabatic electron affinities. A linear correlation (R(2) = 0.887) between computed adiabatic electron affinities and measured reduction peak potentials was discovered. The biological activity of 4-11 and, for comparison, cisplatin was evaluated in human lung cancer A549 and normal MRC-5 cells by the MTT assay. The compounds exhibit comparable or slightly better activity than cisplatin against the A549 cells. In MRC-5 cells, all are equally or slightly less cytotoxic than cisplatin, except for 4 and 5, which are more toxic.

Detection of Nitric Oxide and Nitroxyl with Benzoresorufin-Based Fluorescent Sensors

A new family of benzoresorufin-based copper complexes for fluorescence detection of NO and HNO is reported. The copper complexes, CuBRNO1-3, elicit 1.5-4.8-fold emission enhancement in response to NO and HNO. The three sensors differ in the nature of the metal-binding site. The photophysical properties of these sensors are investigated with assistance from density functional theory calculations. The fluorescence turn-on observed upon reaction with HNO is an unexpected result that is discussed in detail. The utility of the new sensors for detecting HNO and NO in HeLa cells and RAW 264.7 macrophages is demonstrated.

Physical and structural properties of [Cu(BOT1)Cl]Cl, a fluorescent imaging probe for HNO

Nitroxyl, or HNO, is involved in a number of important physiological processes, such as vascular relaxation and neuroregulation. Effective imaging tools are required in order to gain a deeper understanding of the in vivo mechanisms of these processes and to identify the endogenous sources of HNO. Here, we further investigate the physical properties of our previously reported fluorescent nitroxyl sensor, [Cu(BOT1)Cl]Cl (J. Am. Chem. Soc.2010, 132, 5536; BOT1=BODIPY·triazole, a tetradentate ligand). A new high-yielding synthetic procedure for BOT1 is reported. The X-ray crystal structures of two Cu(II) complexes of BOT1 are described. These structural studies show that the BOT1 ligand can form Cu(II) coordination complexes of both square-pyramidal and trigonal-bipyramidal geometries. Cyclic voltammograms of [Cu(BOT1)Cl]Cl were acquired, revealing the presence of a quasi-reversible feature at 130 mV (vs the ferrocene/ferrocenium couple) in MeCN and at -40 mV (vs Ag/AgCl) in aqueous buffer, which is assigned to the Cu(II)/Cu(I) couple. The reactivity of [Cu(BOT1)Cl]Cl with Angelis salt, a stable source of HNO, was further investigated. A 1000-fold excess of Angelis salt elicits an immediate 10-fold emission turn-on response of the sensor, consistent with our previous report. A new observation, reported here, is that the intensity of this turn-on emission diminishes at longer incubation times. Fluorescent imaging of nitroxyl by [Cu(BOT1)Cl]Cl in HeLa cells was carried out. Upon treatment of the cells with Angelis salt, there was a modest 2-fold intracellular turn-on in emission intensity.

Application of ion exchange and extraction chromatography to the separation of actinium from proton-irradiated thorium metal for analytical purposes

Actinium-225 (t1/2=9.92d) is an α-emitting radionuclide with nuclear properties well-suited for use in targeted alpha therapy (TAT), a powerful treatment method for malignant tumors. Actinium-225 can also be utilized as a generator for (213)Bi (t1/2 45.6 min), which is another valuable candidate for TAT. Actinium-225 can be produced via proton irradiation of thorium metal; however, long-lived (227)Ac (t1/2=21.8a, 99% β(-), 1% α) is co-produced during this process and will impact the quality of the final product. Thus, accurate assays are needed to determine the (225)Ac/(227)Ac ratio, which is dependent on beam energy, irradiation time and target design. Accurate actinium assays, in turn, require efficient separation of actinium isotopes from both the Th matrix and highly radioactive activation by-products, especially radiolanthanides formed from proton-induced fission. In this study, we introduce a novel, selective chromatographic technique for the recovery and purification of actinium isotopes from irradiated Th matrices. A two-step sequence of cation exchange and extraction chromatography was implemented. Radiolanthanides were quantitatively removed from Ac, and no non-Ac radionuclidic impurities were detected in the final Ac fraction. An (225)Ac spike added prior to separation was recovered at ≥ 98%, and Ac decontamination from Th was found to be ≥ 10(6). The purified actinium fraction allowed for highly accurate (227)Ac determination at analytical scales, i.e., at (227)Ac activities of 1-100 kBq (27 nCi to 2.7 μCi).

Acetate-bridged platinum(III) complexes derived from cisplatin.

Oxidation of the acetate-bridged half-lantern platinum(II) complex cis-[Pt(II)(NH(3))(2)(μ-OAc)(2)Pt(II)(NH(3))(2)](NO(3))(2), [1](NO(3))(2), with iodobenzene dichloride or bromine generates the halide-capped platinum(III) species cis-[XPt(III)(NH(3))(2)(μ-OAc)(2)Pt(III)(NH(3))(2)X](NO(3))(2), where X is Cl in [2](NO(3))(2) or Br in [3](NO(3))(2), respectively. These three complexes, characterized structurally by X-ray crystallography, feature short (≈2.6 Å) Pt-Pt separations, consistent with formation of a formal metal-metal bond upon oxidation. Elongated axial Pt-X distances occur, reflecting the strong trans influence of the metal-metal bond. The three structures are compared to those of other known dinuclear platinum complexes. A combination of (1)H, (13)C, (14)N, and (195)Pt NMR spectroscopy was used to characterize [1](2+)-[3](2+) in solution. All resonances shift downfield upon oxidation of [1](2+) to [2](2+) and [3](2+). For the platinum(III) complexes, the (14)N and (195)Pt resonances exhibit decreased line widths by comparison to those of [1](2+). Density functional theory calculations suggest that the decrease in the (14)N line width arises from a diminished electric field gradient at the (14)N nuclei in the higher valent compounds. The oxidation of [1](NO(3))(2) with the alternative oxidizing agent bis(trifluoroacetoxy)iodobenzene affords the novel tetranuclear complex cis-[(O(2)CCF(3))Pt(III)(NH(3))(2)(μ-OAc)(2)Pt(III)(NH(3))(μ-NH(2))](2)(NO(3))(4), [4](NO(3))(4), also characterized structurally by X-ray crystallography. In solution, this complex exists as a mixture of species, the identities of which are proposed.

Synthesis, characterization, and photophysical properties of three platinum(II) complexes bearing fluorescent analogues of the Di-2-pyridylmethane ligand.

Three new ligands of the general formula [RNHCH(py)(2)] (py = pyridine; R = tosyl, Ts-dpm; R = dansyl, Ds-dpm; R = 7-nitro-1,2,3-benzoxadiazole, NBD-dpm) have been synthesized and characterized. Reactions of these ligands with cis-[Pt(DMSO)(2)Cl(2)] (DMSO = dimethyl sulfoxide) in methanol affords [Pt(Ts-dpm)Cl(2)] (1), [Pt(Ds-dpm)Cl(2)] (2), and [Pt(NBD-dpm)Cl(2)] (3). The crystal structures of these complexes reveal bidentate coordination of the ligands to the Pt center with nonplanar chelate rings. Because of inequivalent substituents on the methine carbon atom of the ligands, distinct exo and endo isomers exist in the three complexes. X-ray analyses indicate that 1 crystallizes in the endo conformation, 2 in the exo conformation, and 3 as a mixture of the two conformers. The (1)H NMR and (195)Pt NMR spectra of the complexes display two sets of independent signals corresponding to the chemically inequivalent exo and endo conformers. The exo conformer was determined by 2D NMR spectroscopy to be thermodynamically favored for all three complexes. Density functional theory (DFT), time-dependent DFT, and atoms in molecules calculations were carried out for both conformers of 3 to investigate differences in their electronic structures and to explore intramolecular interactions. In the presence of dioxygen, 1 thermally decomposes at 60 degrees C to form several unidentified products. Compound 2 is thermally stable even in the presence of dioxygen and water but upon light exposure decomposes to form a new platinum(II) species with a (195)Pt NMR shift of -2177 ppm. Compound 3 reacts both thermally and photochemically in the presence of dioxygen and trace amounts of water to form both 4-amino-7-nitro-2,1,3-benzoxadiazole and [Pt(dpk)Cl(2)] (dpk = di-2-pyridyl ketone). Oxidation of 1 and 3 with H(2)O(2) in acetic acid affords a mixture of compounds, two of which contain dpm ligands bound in a tridentate manner to platinum.

Spectroscopic and computational investigation of actinium coordination chemistry

Actinium-225 is a promising isotope for targeted-α therapy. Unfortunately, progress in developing chelators for medicinal applications has been hindered by a limited understanding of actinium chemistry. This knowledge gap is primarily associated with handling actinium, as it is highly radioactive and in short supply. Hence, AcIII reactivity is often inferred from the lanthanides and minor actinides (that is, Am, Cm), with limited success. Here we overcome these challenges and characterize actinium in HCl solutions using X-ray absorption spectroscopy and molecular dynamics density functional theory. The Ac–Cl and Ac–OH2O distances are measured to be 2.95(3) and 2.59(3) Å, respectively. The X-ray absorption spectroscopy comparisons between AcIII and AmIII in HCl solutions indicate AcIII coordinates more inner-sphere Cl1– ligands (3.2±1.1) than AmIII (0.8±0.3). These results imply diverse reactivity for the +3 actinides and highlight the unexpected and unique AcIII chemical behaviour.

Bis(thiosemicarbazone) Complexes of Cobalt(III). Synthesis, Characterization, and Anticancer Potential

Nine bis(thiosemicarbazone) (BTSC) cobalt(III) complexes of the general formula [Co(BTSC)(L)2]NO3 were synthesized, where BTSC = diacetyl bis(thiosemicarbazone) (ATS), pyruvaldehyde bis(thiosemicarbazone) (PTS), or glyoxal bis(thiosemicarbazone) (GTS) and L = ammonia, imidazole (Im), or benzylamine (BnA). These compounds were characterized by multinuclear NMR spectroscopy, mass spectrometry, cyclic voltammetry, and X-ray crystallography. Their stability in phosphate-buffered saline was investigated and found to be highly dependent on the nature of the axial ligand, L. These studies revealed that complex stability is primarily dictated by the axial ligand following the sequence NH3 > Im > BnA. The cellular uptake and cytotoxicity in cancer cells were also determined. Both the cellular uptake and cytotoxicity were significantly affected by the nature of the equatorial BTSC. Complexes of ATS were taken up much more effectively than those of PTS and GTS. The cytotoxicity of the complexes was correlated to that of the free ligand. Cell uptake and cytotoxicity were also determined under hypoxic conditions. Only minor differences in the hypoxia activity and uptake were observed. Treatment of the cancer cells with the copper-depleting agent tetrathiomolybdate decreased the cytotoxic potency of the complexes, indicating that they may operate via a copper-dependent mechanism. These results provide a structure-activity relationship for this class of compounds, which may be applied for the rational design of new cobalt(III) anticancer agents.