Michael R. Reithofer

Tuning of lipophilicity and cytotoxic potency by structural variation of anticancer platinum(IV) complexes.

A series of bis(carboxylato)dichlorido(ethane-1,2-diamine)platinum(IV) compounds with IC(50) values ranging between 142 μM and 18 nM was investigated with respect to their lipophilicity (by the shake flask method as well as microemulsion electrokinetic chromatography), reduction potential, as well as their cellular accumulation in cancer cells in vitro. In general, the antiproliferative properties of the complexes correlated with their lipophilicity as well as their accumulation, whereas differences in antiproliferative potency could not be explained by reduction potentials since they do not vary significantly within the investigated series of compounds. Only minor effects for complexes featuring polar end groups were detected.

A SAR study of novel antiproliferative ruthenium and osmium complexes with quinoxalinone ligands in human cancer cell lines.

A series of ruthenium(II) arene complexes with 3-(1H-benzimidazol-2-yl)-1H-quinoxalin-2-one, bearing pharmacophoric groups of known protein kinase inhibitors, and related benzoxazole and benzothiazole derivatives have been synthesized. In addition, the corresponding osmium complexes of the unsubstituted ligands have also been prepared. The compounds have been characterized by NMR, UV-vis, and IR spectroscopy, ESI mass spectrometry, elemental analysis, and by X-ray crystallography. Antiproliferative activity in three human cancer cell lines (A549, CH1, SW480) was determined by MTT assays, yielding IC(50) values of 6-60 μM for three unsubstituted metal-free ligands, whereas values for the metal complexes vary in a broad range from 0.3 to 140 μM. Complexation with osmium of quinoxalinone derivatives with benzimidazole or benzothiazole results in a more consistent increase in cytotoxicity than complexation with ruthenium. For selected compounds, the capacity to induce apoptosis was confirmed by fluorescence microscopy and flow-cytometric analysis, whereas cell cycle effects are only moderate.

The first example of MEEKC-ICP-MS coupling and its application for the analysis of anticancer platinum complexes

MEEKC is a powerful electrodriven separation technique with many applications in different disciplines, including medicinal chemistry; however, up to now the coupling to highly sensitive and selective MS detectors was limited due to the ion suppressive effect of the commonly used surfactant SDS. Herein, the first example of the coupling of MEEKC to ICP‐MS is presented and an MEEKC method for the separation of Pt(II) and Pt(IV) anticancer drugs and drug candidates was developed. Different compositions of microemulsions were evaluated and the data were compared with those collected with standard ultraviolet/visible (UV/vis) spectroscopy detection. The MEEKC‐ICP‐MS system was found to be more sensitive than MEEKC‐UV/vis and the analysis of UV/vis silent compounds is now achievable. The migration behavior of the Pt(II) and Pt(IV) compounds under investigation is correlated to their different chemical structures.

Novel bis(carboxylato)dichlorido(ethane-1,2-diamine)platinum(IV) complexes with exceptionally high cytotoxicity

(OC-6-33)-Dichlorido(ethane-1,2-diamine)dihydroxidoplatinum(IV) (1) was carboxylated using succinic- or 3-methylglutaric anhydride. The resulting bis(carboxylato)platinum(IV) complexes display free, uncoordinated carboxylic acid groups which were further derivatized with primary aliphatic alcohols. The complexes were characterized in detail by elemental analysis, ESI-MS, FT-IR, as well as multinuclear (1H, 13C, 15N, 195Pt) NMR spectroscopy. Cytotoxic properties were evaluated in four human tumor cell lines originating from ovarian carcinoma (CH1, SK-OV-3), cervical carcinoma (HeLa) and colon carcinoma (SW480) by means of the MTT assay (MTT = 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2H-tetrazolium bromide). Structure-activity relationships showed that the cytotoxicity increased with increasing lipophilicity of the alcoholate moiety yielding IC50 values in the low micromolar or even low nanomolar range.

Photoresponsive Liquid Marbles and Dry Water

Stimuli-responsive liquid marbles for controlled release typically rely on organic moieties that require lengthy syntheses. We report herein a facile, one-step synthesis of hydrophobic and oleophobic TiO2 nanoparticles that display photoresponsive wettability. Water liquid marbles stabilized by these photoresponsive TiO2 particles were found to be stable when shielded from ultraviolet (UV) radiation; however, they quickly collapsed after being irradiated with 302 nm UV light. Oil- and organic-solvent-based liquid marbles could also be fabricated using oleophobic TiO2 nanoparticles and show similar UV-induced collapse. Finally, we demonstrated the formation of the micronized form of water liquid marbles, also known as dry water, by homogenization of the TiO2 nanoparticles with water. The TiO2 dry water displayed a similar photoresponse, whereby the micronized liquid marbles collapsed after irradiation and the dry water turned from a free-flowing powder to a paste. Hence, by exploiting the photoresponsive wettability of TiO2, we fabricated liquid marbles and dry water that display photoresponse and studied the conditions required for their collapse.

Novel Endothall‐Containing Platinum(IV) Complexes: Synthesis, Characterization, and Cytotoxic Activity

Two platinum(IV) complexes (OC‐6‐33)‐dichlorido(ethane‐1,2‐diamine)dihydroxidoplatinum(IV) and (OC‐6‐33)‐diammine(dichlorido)dihydroxidoplatinum(IV) were carboxylated using demethylcantharidin as carboxylation agent. The complexes were characterized by elemental analysis, mass spectrometry, multinuclear (1H, 13C, 15N, and 195Pt) NMR spectroscopy, and, in case of (OC‐6‐33)‐diamminebis(3‐carboxy‐7exo‐oxabicyclo[2.2.1]heptane‐2‐carboxylato)dichloridoplatinum(IV) via X‐ray diffraction. Cytotoxicity of the complexes was studied in seven human cancer cell lines representing five tumor entities, i.e., ovarian carcinoma (CH1, SK‐OV‐3), cervical carcinoma (HeLa), colon carcinoma (SW480, HCT‐116), osteosarcoma (U‐2 OS), and hepatocellular carcinoma (Hep G2) by means of the MTT (=3‐(4,5‐dimethylthiazol‐2‐yl)‐2,5‐diphenyl‐2H‐tetrazolium hydrobromide) assay.

Ligation of anti-cancer drugs to self-assembling ultrashort peptides by click chemistry for localized therapy

Self-assembling ultrashort peptides from aliphatic amino acids were functionalized with platinum anti-cancer drugs by click chemistry. Oxaliplatin-derived hybrid peptide hydrogels with up to 40% drug loading were tested for localized breast cancer therapy. Stably injected gels showed significant tumor growth inhibition in mice and a better tolerance compared to the free platinum drug.

Synthesis and bioactivity of a conjugate composed of green tea catechins and hyaluronic acid

(−)-Epigallocatechin-3-gallate (EGCG) is a green tea polyphenol that has several biological activities, including anti-cancer activity and anti-inflammation. Hyaluronic acid (HA) is a naturally-occurring polysaccharide that is widely used as a biomaterial for drug delivery and tissue engineering due to its viscoelastic, biocompatible and biodegradable properties. By conjugating HA with EGCG, the resulting HA–EGCG conjugate is expected to exhibit not only the inherent properties of HA but also the bioactivities of EGCG. Toward this end, we report the synthesis of an amine-functionalized EGCG as an intermediate compound for conjugation to HA. EGCG was reacted with 2,2-diethoxyethylamine (DA) under acidic conditions, forming ethylamine-bridged EGCG dimers. The EGCG dimers were composed of four isomers, which were characterized by HPLC, high-resolution mass spectrometry and NMR spectroscopy. The amine-functionalized EGCG dimers were conjugated to hyaluronic acid (HA) through the formation of amide bonds. HA–EGCG conjugates demonstrated several bioactivities which were not present in unmodified HA, including resistance to hyaluronidase-mediated degradation, inhibition of cell growth and scavenging of radicals. The potential applications of HA–EGCG conjugates are discussed.

Influence of metal salts on the hydrogelation properties of ultrashort aliphatic peptides

Over the last decade, a variety of peptides have been designed and studied for their ability to form hydrogels for potential use as functional biomaterials. Peptide hydrogels have been employed as tissue-engineering scaffolds, drug delivery systems, and platforms for biosensors. We have recently added to the existing peptide systems a new class of ultrashort self-assembling peptides containing 3–7 aliphatic amino acids, which are able to easily form solid hydrogels in plain water. These hydrogels can entrap up to 99.9% of water in scaffolds that are made of long helical fibers. Although the aliphatic peptides do not need ionic conditions to form hydrogels, we were interested to investigate if physiological metal salts interfere with hydrogelation. This is particularly important for future in vivo application in biomedicine and biotechnology. Thus, we have studied the effect of multivalent metal salts on the hydrogelation properties of two members of this peptide class, the hexapeptide Ac-LIVAGD and tripeptide Ac-IVD. The majority of the metal salts did not disrupt the hydrogelation process, whereas the trivalent salts could initiate precipitation of the peptides. The characteristic fibrillar network of the hydrogels was still observable by field emission scanning electron microscopy in the presence of moderate and high concentrations of metal salts. Rheological studies of the hexapeptide hydrogel in the presence of monovalent metal salts demonstrated the ability of the salts to reduce the viscoelasticity of the hydrogel. Rheology, together with Fourier transform infrared spectroscopy, confirmed that the ionic strength of the peptide solution affects the bonding interaction between peptide fibers/sheets and the water molecules during hydrogelation. Overall, our results demonstrate that metal salts in general do not significantly affect the self-assembly process of two aliphatic peptides, but exert an influence on their viscoelastic properties, presumably via disruption of the ordered water layer around the peptide fibers. Given that common physiological metal salts have minimal effect on the integrity/morphology of the studied hydrogels, but only slightly influence the mechanical properties, biomedical applications seem to be feasible.

Effect of reactivity on cellular accumulation and cytotoxicity of oxaliplatin analogues

The purpose of this study was to systematically investigate the relationships between reactivity, cellular accumulation, and cytotoxicity of a panel of oxaliplatin analogues with different leaving groups in human carcinoma cells. The reactivity of the complexes towards the nucleotides 2′-deoxyguanosine 5′-monophosphate and 2′-deoxyadenosine 5′-monophosphate was studied using capillary electrophoresis. Cellular accumulation and cytotoxicity were measured in an oxaliplatin-sensitive and oxaliplatin-resistant ileocecal colorectal adenocarcinoma cell line pair (HCT-8/HCT-8ox). Platinum concentrations were determined by flameless atomic absorption spectrometry. The 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyl-2H-tetrazolium bromide (MTT) assay was used to assess cytotoxicity. Early cellular platinum accumulation was predominantly affected by lipophilicity. A relationship between reactivity and cellular accumulation was observed for three of four platinum complexes investigated, whereas the most lipophilic oxaliplatin analogue was an exception. Increased reactivity and reduced lipophilicity were associated with high cytotoxic activity. Resistance was influenced by lipophilicity but not by reactivity. The observed relationships may help in the design of analogues with high antitumoral activity in oxaliplatin-sensitive as well as oxaliplatin-resistant cells.