J. William Lown

Strand scission of DNA by bound adriamycin and daunorubicin in the presence of reducing agents

Abstract Adriamycin and daunorubicin bound to covalently closed circular DNA nick the latter when reduced by sodium borohydride as demonstrated using an ethidium bromide fluorescence assay. The degradation, dependent on oxygen, is strongly inhibited by (i) superoxide dismutase (ii) catalase and (iii) sodium benzoate indicating the intermediacy in the cleavage of superoxide radical anion, hydrogen peroxide and hydroxyl radicals respectively. Less nicking of the DNA is observed by the reduced aglycones, so binding to the DNA by the aminosugar moiety assists the cleavage process. Adriamycin, daunorubicin and both ring C reduced forms bind intercalatively and completely relax supercoiled DNA. The results provide a possible rationale for the degradation of DNA which accompanies anthracycline administration.

The mechanism of the bleomycin-induced cleavage of DNA.

Bleomycin which nicks DNA, contains 0.2 mole % Fe. It enhances the rate of nicking of PM2-CCC-DNA produced by low concentrations of Fe 2+ salts. The reaction under both conditions is dependent on oxygen and is strongly inhibited by (i) superoxide dismutase, (ii) catalase and more efficiently by the two enzymes together, and (iii) free radical scavengers, e.g. isopropyl alcohol, indicating the intermediacy in the cleavage of 0 2 − ·, H 2 O 2 and OH· respectively. 2-Mercaptoethanol, dithiothreitol, ascorbic acid and xanthine/xanthine oxidase, hitherto commonly used with bleomycin studies, separately cleave DNA by a similar mechanism. CuSO 4 (but not Co, Mn, Zn or Mg salts) with NADPH nicks DNA by the superoxide pathway but is inhibited by bleomycin which sequesters the Cu 2+ and prevents its reduction.

HYPOCRELLINS and THEIR USE IN PHOTOSENSITIZATION

Abstract— Hypocrellins A and B are pigments which are isolated from parasitic fungi Hypocrella bambuase (B. et Br) sacc. and Shiraia bambusicola P. Heen found in the Peoples Republic of China (P.R.C.) and Sri Lanka respectively. These agents, which belong to the general class of perylene quinonoid pigments, have a long history as traditional medicinal agents especially in the P.R.C. Recently their marked photosensitizing properties have been established and exploratory studies initiated. This effort has led to the realization of the potential of the hypocrellins for the photodynamic therapy of tumors. The review summarizes the chemical and photophysical properties of the hypocrellins and their derivatives as well as studies on photosensitization to date at the molecular, cellular and in vivo levels, and their prospects as PDT agents.

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.

Anthracycline and anthraquinone anticancer agents: current status and recent developments.

The clinical treatment of neoplastic diseases relies on the complementary procedures of surgery, radiation treatment, immunotherapy and chemotherapy. The latter technique has matured from its earliest applications of mustard alkylating agents in the 1940s to an increasingly rationally based discipline, which is contributing significantly to the management of human malignancies. As the field of chemotherapy matured, several promising natural anticancer agents were identified. However, a more urgent need soon arose from the common experience of clinically limiting toxicities of most anticancer drugs, i.e. the necessity to develop less toxic clinical drug candidates. Thus, the medicinal chemist turned towards analog development involving certain anthraquinones. Hand-in-hand with this considerable synthetic effort, which uncovered several promising clinical leads, biochemical pharmacology, or study of the mechanisms of action of clinical anticancer agents, afforded deeper insight into drug metabolism and mode of action. More recently, therefore, the field of synthetic organic chemistry, which has been complemented by the methods of microbial chemistry, has been faced with new synthetic challenges, occasioned by the identification of hitherto unrecognized cellular targets for anticancer drugs, such as topoisomerases and helicases. The armementarium of the oncologist currently includes about 40-50 clinically useful chemical agents. The paradigm of cytotoxic anticancer agents is doxorubicin, an anthracycline, which is still amongst the most widely prescribed and effective of anticancer agents. The review attempts to summarize the discovery of anthracyclines and the elucidation of their several mechanisms of action and efforts towards improvement of their therapeutic efficacy.

Further studies on the generation of reactive oxygen species from activated anthracyclines and the relationship to cytotoxic action and cardiotoxic effects

The relative ease of generation of reactive oxygen species from a series of reductively activated aglycone and sugar modified anthracyclines was compared by the extents of single strand scission in supercoiled PM2-covalently closed circular (CCC)-DNA. The electrochemical properties of these agents were used as a quantitative measure of the ease of reduction and subsequent reoxidation of the reduced species. The relationship of these processes to various biological properties of the anthracyclines, in particular to the measured cardiotoxicity of the drugs, was examined. An attempt was made to identify those structural changes which ameliorate the cardiotoxic effects measured in other test systems while permitting the expression of antitumor properties.

Anticancer, Anti-inflammatory and Analgesic Activity Evaluation of Heterocyclic Compounds Synthesized by the Reaction of 4-Isothiocyanato-4-methylpentan-2-one with Substituted o-Phenylenediamines, o-Diaminopyridine and (Un)Substituted o

4,5-Dimethyl-1,2-phenylenediamine and 4-chloro-1,2-phenylenediamine react with 4-isothiocyanato-4-methylpentan-2-one (15) to give compounds (3a) and (3b), respectively. 3,4-Diaminobenzoic acid reacts similarly with (15) to give a mixture of compounds, possibly (2a) and (2b), which could be cyclized at pH ~5 to compound (3c). 3,4-Diaminopyridine reacted with (15) in DMF to give compounds (5) and (6), whereas condensation of 5,6-diaminopyrimidine and 4,5,6-triaminopyrimidine sulfate under similar conditions gave compounds (8a) and (8b), respectively. Compounds (8a) and (8b) at pH ~4 gave a mixture of compounds (9a), (10a) and (9b), (10b), respectively. Condensation of 4,5-diamino-6-hydroxy-2-mercaptopyrimidine, 4,5-diamino-2,6-dimercapto-pyrimidine and 5,6-diamino-1,3-dimethyluracil hydrate with (15) gave corresponding mercaptopyrimidines (12a), (12b) and (14), respectively. The evaluation of (3a–c), (8a,b), (12a,b) and (14) aganist a small panel of six cancer cell lines, consisting of prostate (DU145), colon (HT29), melanoma (LOX), breast (MCF, MCF7/ADR), ovarian (OVCAR3) and CNS (U251) is reported. The most active was compound (8b), against colon (HT29) (44.2 M). Anti-inflammatory and analgesic activity is also reported.

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.

Diminished superoxide anion generation by reduced 5-iminodaunorubicin relative to daunorubicin and the relationship to cardiotoxicity of the anthracycline antitumor agents

Abstract 5-Iminodaunorubicin, when reductively activated, produces single strand scission in PM2-CCC-DNA by a mechanism which proceeds via production of Superoxide anion and hydroxyl radicals. Under comparable conditions, 5-iminodaunorubicin produces much less nicking than daunorubicin. With the aid of N -cyclohexyl-5-iminoquinizarin, as a model, assignment of polarographic waves to the quinone moiety of 5-iminodaunorubicin was possible. The electrochemical results indicate that 5-iminodaunorubicin is more difficult to reduce than daunorubicin and that reoxidation of the reduced form, 5,11-dihydro-5-iminodaunorubicin, is much more difficult than reoxidation of reduced daunorubicin. The latter conclusion is supported by independent chemical studies. By comparison of 5-iminodaunorubicin with daunorubicin and N -cyclohexyl-5-iminoquinizarine, the unusual stability of the reduced 5-iminodaunorubicin, is tentatively attributed to strong hydrogen bonding. The results suggest a correlation between the diminished generation of Superoxide anion by 5-iminodaunorubicin and its observed suppressed cardiotoxicity relative to other anthracyclines.

The mechanism of action of quinone antibiotics

SummaryThe review describes recent studies designed to elucidate the molecular mechanism of action of certain quinone antibiotics which exhibit or have potential for clinical treatment of malignant diseases. Although a large number of quinone antibiotics has been described the review will concentrate on four types, the anthracyclines, the mitomycins, streptonigrin, and the saframycin antibiotics because of their biological significance and because the understanding of their underlying modes of action is perhaps more advanced than in the case of other antibiotics. It will be evident that although the antibiotics bear a common quinone moiety this does not confer a commonality of mechanism. Indeed the variety and precision of the different chemical lesions induced by quinone antibiotics on nucleic acids, their principal cell targets, is remarkable. The particular lesions identified include (i) equilibrium binding, (ii) ‘permanent’ single covalent attachment, (iii) reversible covalent binding, (iv) metal ion sequestration and subsequent DNA binding, (v) DNA groove and base specific binding, (v) interstrand cross-linking, (vi) intercalation with concomitant supercoil relaxation and duplex extension, (vii) redox cycling with production of reactive oxygen species and DNA single strand breaks, and (viii) single strand breaks as a result of phosphotriester formation.In many cases the chemical mechanisms involved in these individual processes may be elucidated in in vitro experiments on purified DNAs by the application of ethidium binding assays in conjuction with certain cellular repair enzymes and utilizing techniques including high field nuclear magnetic resonance and electron paramagnetic resonance spectroscopy. The data obtained in this way complement and extend information from cell culture and in vivo experiments. A coherent description of the multiple cellular effects of these reactive agents is emerging. Such reactions involve bioreductive activation of the quinone the subsequent course of which is precisely controlled by structural and stereochemical factors within the individual antibiotic. The concomitant chemical reactions on cellular macromolecules are beginning to be related to pharmacological properties including in the case of the anthracyclines, a plausible rationale for the molecular origin of the dose limiting cardiotoxicity.