Zhenjun Diwu

Photosensitization by anticancer agents 12. Perylene quinonoid pigments, a novel type of singlet oxygen sensitizer

Abstract The marked photosensitizing properties of perylene quinonoid pigments, a promising class of agents for photodynamic therapy of human tumors, have been recognized, but quantitative information on their excited states is lacking. For the first time, some fundamental photophysical parameters of hypocrellins A and B; and cercosporin in benzene have been determined. For hypocrellin A, kF = 1.4 × 108 s−1, ΦF = 0.14, τS = 0.98 ns, ES = 48.5 kcal mol−1, ΦST = 0.86, kST = 8.6 × 108 s−1 and ET = 41.0 kcal mol−. For hypocrellin B, kF = 9.1 × 107 s−1, kC = 2.7 × 109 s−1, kST = 1.1 × 1010 s−1, ΦF = 0.058, ΦIC = 0.18, τS = 0.66 ns, ES = 47.7 kcal mol−1, ΦST = 0.76 and ET = 40.2 kcal mol−1. For cercosporin, kF = 8.3 × 107 s−1, kIC = 1.7 × 109 s−1, kST = 1.1 × 1010 s−1, ΦF = 0.061, ΦIC = 0.13, τS = 0.75 ns, ES = 48.0 kcal mol−1, ΦST = 0.81 and ET = 40.5 kcal mol−1. The quantum yields of singlet oxygen formation by these agents were also determined: ΦO2(hypocrellin A) = 0.83, Φ1O2(hypocrellin B) = 0.76 and Φ1O2(cercosporin) = 0.81. Finally, these perylene quinonoid agents have been shown to act as excellent singlet oxygen sensitizers with several advantages over existing sensitizers, including large molar extinction coefficients, wide UV—visible absorption bands, high quantum yields of singlet oxygen generation, good stability, good solubility and small solvent and concentration effects.

Photosensitization with anticancer agents: 15. Perylenequinonoid pigments as potential photodynamic therapeutic agents: Formation of semiquinone radicals and reactive oxygen species on illumination

Visible light illumination of solutions of perylenequinonoid pigments generates the corresponding semiquinone radicals, singlet oxygen, superoxide anion radical, hydroxyl radical and hydrogen peroxide. In anaerobic solution, the semiquinone radicals are predominantly photoproduced via the self-electron transfer between the excited and ground state species. In aerobic solution, singlet oxygen is the principal product in the photosensitization of perylenequinonoid pigments. The 3,10-dihydroxy-4,9-perylenequinonoid chromophore was shown to be the necessary structural requirement for the generation of singlet oxygen, and the side-chains of the quinones had little effect on the production of singlet oxygen. This conclusion is useful in the development of more efficient photodynamic therapeutic agents than natural perylenequinonoid pigments themselves. Such agents should ideally contain the 3,10-dihydroxy-4,9-perylenequinonoid chromophore to produce singlet oxygen together with appropriate elaborated side-chains to permit the selective localization of the sensitizer in tumor tissue. In addition to singlet oxygen, superoxide anion radical is generated by the perylenequinones on illumination in aerobic solution, but to a lesser extent than singlet oxygen, via the reduction of oxygen by the corresponding semiquinone radicals. This latter process is significantly enhanced by the presence of electron donors.

Design, synthesis and investigation of mechanisms of action of novel protein kinase C inhibitors: perylenequinonoid pigments

A series of perylenequinonoid pigments (PQPs) and related compounds were synthesized and screened for the inhibition of protein kinase C (PKC), a key enzyme involved in cellular differentiation and proliferation, and a potential target for anticancer and antiviral chemotherapeutic drugs. This study has established PQPs as efficient PKC inhibitors, and elucidated aspects of the light-enhanced action mode of the PKC inhibitors. Comparative studies between natural and synthetic PQPs led to the recognition of the effect of certain structural features of PQPs on PKC inhibition, including the skeleton of the 3,10-dihydroxy-4,9-perylenequinonoid chromophore and the configuration of the two side chains at positions 1 and 12. Calphostin C was identified as a superior PKC inhibitor of the POP class, and with the latter as a representative structure, we investigated the mechanism of PKC inhibition by PQPs via electron paramagnetic resonance spectroscopy in conjunction with the spin-trapping technique, absorption and fluorescence spectroscopy, photochemical and photobiological studies, and enzyme methodology. Multiple modes of action are suggested for PKC inhibition, comprising the following steps: (1) the binding of PQPs to the PKC regulatory domain via complexation; (2) the photobonding between mercapto groups of PKC cysteine residues and the PQP quinonoid moiety; and (3) the PQP-sensitized photodamage of PKC via Type I and/or Type II photosensitization.

Hypocrellins as photosensitizers for photodynamic therapy : a screening evaluation and pharmacokinetic study

Abstract Hypocrellin compounds were selected as potential photosensitizers for photodynamic therapy (PDT) owing to their high quantum yields of singlet oxygen (1O2), and facility for site-directed chemical modification to enhance phototoxicity, pharmacokinetics, solubility, and light absorption in the red spectral region, among other properties. Parent hypocrellins A and B share an absorption peak at 658 nm. These molecules may therefore be considered useful progenitors of derivatives which absorb more strongly in the red, considering that the ideal sensitizer should absorb in the 650–800 nm range, beyond the absorption range of hemoglobin and melanin, and where light penetration in tissues is maximized through reduced scattering. A series of pure, monomeric hypocrellin derivatives was tested for properties of dark cytotoxicity and photosensitizing potential by clonogenic assay in monolayer cultures of EMT6/Ed murine tumor cells. Their respective toxicities are reported on a molar basis. The in vitro screening assay has, to date, resulted in the selection of four hypocrellin derivatives for further development as photosensitizers for PDT. Cellular uptake for photosensitizing doses of selected compounds was determined by fluorimetry. Dose escalation studies in rodents indicate that potentially photosensitizing doses promote no demonstrable systemic toxicity.

Photosensitization with anticancer agents 14. Perylenequinonoid pigments as new potential photodynamic therapeutic agents: formation of tautomeric semiquinone radicals

Abstract When perylenequinonoid pigments were irradiated with visible light, semiquinone radicals, singlet oxygen, superoxide anion radical, hydroxyl radical and hydrogen peroxide were detected. The formation of the semiquinone radicals and activated oxygen species and the transformations and competitions between them depend on the quinone and oxygen concentrations, time and intensity of irradiation and the nature of the substrate. In anaerobic solution, the semiquinone radicals, which exhibit well-defined electron paramagnetic resonance (EPR) spectra at room temperature, are predominantly photoproduced via the self-electron transfer between the excited and ground species. The EPR spectra of the semiquinone radicals were assigned by reference to simulated spectra. The presence of electron donors significantly promotes the production of the semiquinone radicals. The EPR hyperfine splittings of the semiquinone radicals mainly originate from the coupling interactions of the aromatic, phenolic hydroxyl and methoxyl protons with the unpaired spin. An exception is hypocrellin B in which the two methylene protons of the side-ring are also involved in the hyperfine splitting to a minor extent because the double bond of the side-ring conjugates with the perylenequinonoid chromophore. In addition, compared with cercosporin, the EPR spectra of the hypocrellin semiquinone radicals are further complicated by tautomerizations at ambient temperature. Therefore EPR spectroscopy can also be utilized to study the tautomerization together with nuclear magnetic resonance (NMR), absorption spectroscopy and some specific chemical trapping reactions. In aerobic solution, singlet oxygen is the principal product in the photosensitization of perylenequinonoid pigments. In addition to singlet oxygen, superoxide anion radical is also generated, to a lesser extent, by the quinones on illumination in aerobic solution via the reduction of oxygen by the semiquinone radicals, and this process is significantly enhanced by the presence of electron donors.

A simple high-yielding approach to perylenequinone from the novel one-step double coupling reaction of 1,2-naphtoquinone

Abstract The transformation from 1,2-naphthoquinone to perylenequinone, which usually requires several steps, is stereo- selectivity realized via one step in excellent yield by a newly developed, double coupling method. The possible mechanism for this novel reaction and the properties of the resulting perylenequinone are discussed. Moreover, an improved procedure for the preparation of a 5-bromo-1,2-naphtalenediol, a reported coupling precursor to perylenequinone, is also described. These synthetic procedures open convenient ways to perlenequinone structures of significant biological potential.

Photosensitization with anticancer agents 19. EPR studies of photodynamic action of calphostin C: Formation of semiquinone radical and activated oxygen on illumination with visible light

When calphostin C was illuminated with visible light, the semiquinone radical, singlet oxygen, and superoxide anion radical were detected. The formation of the semiquinone radical and activated oxygen species and the transformations and competitions between them depend upon the quinone and oxygen concentrations, time and intensity of illumination, and the nature of the substrate. In anaerobic solution, the semiquinone radical was predominantly photoproduced via the self-electron transfer between the excited and ground species. In aerobic solution, singlet oxygen is the principal product in the photosensitization of calphostin C. In addition to singlet oxygen, superoxide anion radical is also generated by the quinones upon illumination in aerobic solution, but to a lesser extent than singlet oxygen. The superoxide anion is produced via the reduction of oxygen by the semiquinone radical, and this process is significantly enhanced by the presence of electron donors.

Photosensitization with anticancer agents 20—EPR studies on the photodynamic action of phleichrome: Formation of semiquinone radical and activated oxygen species on illumination with visible light

When phleichrome was illuminated with visible light, the semiquinone radical, singlet oxygen, and superoxide anion radical were detected. The formation of the semiquinone radical and activated oxygen species and the transformations and competitions between them depend on quinone and oxygen concentration, duration and intensity of illumination, and the nature of the substrate. In anaerobic solution, the semiquinone radical was predominantly photoproduced via the self-electron transfer between the excited and ground species. In contrast, in aerobic solution, singlet oxygen is the principal product in the photosensitization of phleichrome. In addition to singlet oxygen, superoxide anion radical is also generated by the quinones upon illumination in aerobic solution, but to a lesser extent. The generation of the superoxide anion is significantly enhanced by the presence of electron donors.

Photosensitization with anticancer agents 16. The photo-oxidation of hypocrellin A. A mechanism study using 18O labelling

Hypocrellin A was selected as a representative structure to investigate the mechanism of photo-oxidation of perylenequinonoid pigments (PQPs). It was found that singlet oxygen plays a key role in the photo-oxidation of PQPs. The following processes were determined to be involved in the photo-oxidation of PQPs using 18O atom labelling. After the PQP is photoexcited, it tautomerizes and dissociates to produce the anion, which reacts with singlet oxygen generated during irradiation, resulting in the formation of an endoperoxide. The endoperoxide then rearranges to form a dioxetane which undergoes cycloreversion to produce the final product, a di-alpha-naphthoquinone. The factors affecting the photo-oxidation of hypocrellin A were also investigated, including the pH of the solution, nature of the solvent, irradiation wavelength and hydration phenomena. Of these factors, the extremely strong effect of pH on the photo-oxidation of hypocrellin A may be used as a basis for the selective localization of hypocrellin A in tumor tissue. On the basis of the proposed mechanism for the photo-oxidation of PQPs, it may be possible to design and synthesize an optimal tumor-selective PQP photosensitizer for the photodynamic therapy of human tumors.

Synthetic studies in novel hypocrellin B derivatives

Abstract Three novel hypocrellin B (HB) derivatives required for their potential in photodynamic therapy have been synthesized. The possible reaction mechanisms and the properties of resulting HB derivatives are discussed.