Alice M. Clark

NATURAL PRODUCTS AS A RESOURCE FOR NEW DRUGS

Natural products have served as a major source of drugs for centuries, and about half of the pharmaceuticals in use today are derived from natural products. The aim of this review is to provide an overview of the continuing central role of natural products in the discovery and development of new pharmaceuticals. In this context, selected examples of important natural product-derived drugs are cited, focusing on some of the most recent introductions to the clinical setting, and a brief overview of some of the important recent developments and remaining challenges in the process of discovering and developing bioactive natural products is provided. Interest in natural products research is strong and can be attributed to several factors, including unmet therapeutic needs, the remarkable diversity of both chemical structures and biological activities of naturally occurring secondary metabolites, the utility of bioactive natural products as biochemical and molecular probes, the development of novel and sensitive techniques to detect biologically active natural products, improved techniques to isolate, purify, and structurally characterize these active constituents, and advances in solving the demand for supply of complex natural products. Opportunities for multidisciplinary research that joins the forces of natural products chemistry, molecular and cellular biology, synthetic and analytical chemistry, biochemistry, and pharmacology to exploit the vast diversity of chemical structures and biological activities of natural products are discussed.

Antimicrobial properties of Honduran medicinal plants.

Ninety-two plants used in the traditional pharmacopoeia of the Pech and neighboring Mestizo peoples of central Honduras are reported. The results of in vitro antimicrobial screens showed that 19 of the extracts from medicinal plants revealed signs of antifungal activity while 22 demonstrated a measurable inhibitory effect on one or more bacterial cultures. Bioassay-guided fractionation of extracts from Mikania micrantha, Neurolaena lobata and Piper aduncum produced weak to moderately active isolates. The broad spectrum of activity of the extracts helps to explain the widespread use of these plants for wound healing and other applications.

3-Methoxysampangine, a novel antifungal copyrine alkaloid from Cleistopholis patens.

Further examination of the active ethanolic extract of the root bark of Cleistopholis patens by using bioassay-directed fractionation resulted in the isolation of a new alkaloid, 3-methoxysampangine (compound I), together with three known alkaloids, eupolauridine (compound II), liriodenine (compound III), and eupolauridine N-oxide (compound IV). The proposed structure of compound I was based on its physicochemical properties and spectral data. 3-Methoxysampangine exhibited significant antifungal activity against Candida albicans, Aspergillus fumigatus, and Cryptococcus neoformans. This is the first report of the isolation of liriodenine (compound III) from the root bark of C. patens.

NMR ASSIGNMENTS OF ELLAGIC ACID DERIVATIVES

HMBC spectroscopy optimized for small couplings was employed to determine the four‐bond and two‐bond proton carbon correlations on the aromatic rings of ellagic acid derivatives. Complete 13C NMR assignments of 3′‐O‐methyl‐3,4‐methylenedioxyellagic acid 4′‐O‐β‐D‐glucopyranoside (1), 3,3′‐di‐O‐methylellagic acid 4′‐O‐β‐D‐xylopyranoside (2), 3,3′,4‐tri‐O‐methylellagic acid 4′‐O‐β‐D‐glucopyranoside (3) and ellagic acid (4) were achieved using this technique. This study indicates that optimization of the delay time in the HMBC spectrum is crucial in assigning the 13C NMR signals of phenolic compounds with highly oxygenated quaternary carbons. Copyright

Substituted Indoloquinolines as New Antifungal Agents

Cryptolepine (2) possesses desirable properties to serve as a lead in developing new antifungal agents. Using SAR techniques, several analogues of cryptolepine were designed to increase potency and to broaden the antifungal spectrum over several opportunistic microorganisms. A number of 2-substituted indoloquinolines have been synthesized and evaluated in antifungal screens and several have been shown to increase potency and expand the antifungal spectrum of cryptolepine. Comparison of MICs of a number of these analogues with standard antifungal agents, shows them to be comparable to Amphotericin B and Ketoconazole.

Reversal of Fluconazole Resistance by Sulfated Sterols from the Marine Sponge Topsentia sp.

Bioassay-guided fractionation of the extract of Topsentia sp. led to the identification of two new sulfated sterols, geodisterol-3-O-sulfite (1) and 29-demethylgeodisterol-3-O-sulfite (2), the active constituents reversing efflux pump-mediated fluconazole resistance. Both compounds enhanced the activity of fluconazole in a Saccharomyces cerevisiae strain overexpressing the Candida albicans efflux pump MDR1, as well as in a fluconazole-resistant Candida albicans clinical isolate known to overexpress MDR1. These results provide insight into the clinical utility of combining efflux pump inhibitors with current antifungals to combat the resistance associated with opportunistic fungal infections caused by C. albicans.

Potent in vitro antifungal activities of naturally occurring acetylenic acids.

ABSTRACT Our continuing effort in antifungal natural product discovery has led to the identification of five 6-acetylenic acids with chain lengths from C16 to C20: 6-hexadecynoic acid (compound 1), 6-heptadecynoic acid (compound 2), 6-octadecynoic acid (compound 3), 6-nonadecynoic acid (compound 4), and 6-icosynoic acid (compound 5) from the plant Sommera sabiceoides. Compounds 2 and 5 represent newly isolated fatty acids. The five acetylenic acids were evaluated for their in vitro antifungal activities against Candida albicans, Candida glabrata, Candida krusei, Candida tropicalis, Candida parapsilosis, Cryptococcus neoformans, Aspergillus fumigatus, Aspergillus flavus, Aspergillus niger, Trichophyton mentagrophytes, and Trichophyton rubrum by comparison with the positive control drugs amphotericin B, fluconazole, ketoconazole, caspofungin, terbinafine, and undecylenic acid. The compounds showed various degrees of antifungal activity against the 21 tested strains. Compound 4 was the most active, in particular against the dermatophytes T. mentagrophytes and T. rubrum and the opportunistic pathogens C. albicans and A. fumigatus, with MICs comparable to several control drugs. Inclusion of two commercially available acetylenic acids, 9-octadecynoic acid (compound 6) and 5,8,11,14-eicosatetraynoic acid (compound 7), in the in vitro antifungal testing further demonstrated that the antifungal activities of the acetylenic acids were associated with their chain lengths and positional triple bonds. In vitro toxicity testing against mammalian cell lines indicated that compounds 1 to 5 were not toxic at concentrations up to 32 μM. Furthermore, compounds 3 and 4 did not produce obvious toxic effects in mice at a dose of 34 μmol/kg of body weight when administered intraperitoneally. Taking into account the low in vitro and in vivo toxicities and significant antifungal potencies, these 6-acetylenic acids may be excellent leads for further preclinical studies.

Probing the N-5 Region of the Indoloquinoline Alkaloid, Cryptolepine for Anticryptococcal Activity

N-5 Alkylated analogues of cryptolepine were synthesized and tested for anticryptococcal activity. Evidence provided in this study suggests that the active form of cryptolepine consists of the flat tetracyclic aromatic ring with the methyl group on the N-5 atom. It was also found that changes in the electronic density around the N-5 atom do not appear to affect activity. Steric hindrance of the N-5 substituents seems to decrease activity. Through systematic modification of the N-5 alkyl groups, o-phenylpentyl group was shown to possess the highest potency thus far.

Anti-Cryptococcal and Nitric Oxide Synthase Inhibitory Imidazole Alkaloids from the Calcareous Sponge Leucetta cf chagosensis

Abstract Antifungal imidazole alkaloids were isolated from the Egyptian Red Sea sponge Leucetta cf chagosensis using HPLC. These compounds were the previously reported naamidine A, B, D and G and the unreported symmetric imidazole alkaloid naamine D. Naamine D possesses moderate antifungal and nitric oxide synthase inhibitory activity. The structure of naamine D was determined using 1D and 2D NMR experiments including 1H–15N HMBC and high resolution mass spectrometry.

High-performance liquid chromatographic analysis of the metabolism of primaquine and the identification of a new mammalian metabolite.

Using rats that had been dosed with 20 mg/kg of primaquine diphosphate (11.4 mg/kg free base), it was found that the drug underwent a metabolic oxidative deamination to give 8-(3-carboxy-1-methylpropylamino)-6-methoxyquinoline. The presence of this new mammalian metabolite was verified using high-performance liquid chromatographic, gas chromatographic, and mass spectral methods. A quantitative high-performance liquid chromatographic method for the determination of primaquine and the carboxylic acid metabolite in plasma using only 50 microliters of whole blood from the rat was developed and the method could be used to detect levels as low as 0.05 microgram/ml of the metabolite. Following intravenous administration of the drug, it was found that the plasma levels of primaquine fell very rapidly and after 30 min, the levels of the metabolite were much higher than those of primaquine.