Indole Alkaloids from Marine Sources as Potential Leads against Infectious Diseases

Indole alkaloids comprise a large and complex class of natural products found in a variety of marine sources. Infectious diseases remain a major threat to public health, and in the absence of long-term protective vaccines, the control of these infectious diseases is based on a small number of chemotherapeutic agents. Furthermore, the emerging resistance against these drugs makes it urgently necessary to discover and develop new, safe and, effective anti-infective agents. In this regard, the aim of this review is to highlight indole alkaloids from marine sources which have been shown to demonstrate activity against infectious diseases.


Introduction
Seas and oceans occupy more than 75% of the Earth's surface and contain nearly all groups of organisms, including representatives of 34 out of 36 phyla described. Thus, marine ecosystems can be considered as having the greatest phyletic biodiversity with virtually unlimited biotechnological potential [1]. The marine environment is massively complex, consisting of extreme variations in pressure, salinity, temperature, and biological habitats that have led to the production of several novel structures with unique biological properties, which may not be found in terrestrial natural products [2]. In the past 30-40 years, marine plants and animals have been the focus of a worldwide effort to define the natural products of the marine environment. A small number of marine plants, animals, and microorganisms have already yielded more than 12,000 novel chemicals, with hundreds of new compounds still being discovered every year. These discovery efforts have yielded several bioactive metabolites that have been successfully developed by the pharmaceutical industry [3]. A variety of marine sources including sponges, tunicates, red algae, acorn worms, and symbiotic bacteria have been shown to generate indole alkaloids, which represent the largest number and most complicated of the marine alkaloids [4][5][6][7]. The alkaloids obtained from marine organisms frequently possess novel frameworks which cannot be found in terrestrially related organisms. Marine metabolites often possess complexities such as halogen substituents [8]. In addition, bearing in mind that several species of organisms are associated with cyanobacteria and bacteria, it is considered that several natural products originating in microorganisms can be isolated from marine animals [9]. Indole alkaloids have been shown to exhibit a wide array of biological activities such as opioid receptor agonistic [10], antibacterial [11,12], antifungal [13,14], anti-inflammatory [15], antileishmanial [16,17], antiplasmodial [18,19], anti-HIV [20], cytotoxic [21], glucose uptake stimulatory [22], larvicidal [23], trypanocidal [24], and vasodilator [25,26] and inhibition of cholinesterase [27], indoleamine-2, 3-dioxygenase [28], calmodulin [29], and CB1 cannabinoid receptor [30]. Infectious diseases caused by bacteria, fungi, viruses, and parasites are still a major threat to public health, despite the tremendous progress in human medicine. The high prevalence of these diseases and the emergence of widespread drug resistance developed by these parasites to current treatments, leading to reduction of their efficacy and consequent increase in the cost of conventional treatments, highlight the need for novel and effective therapeutic alternatives with fewer or no side-effects. Their impact is particularly large in developing countries due to the relative unavailability of medicines [31,32].
In this review, literature was covered in order to highlight alkaloidal compounds with an indole moiety which have been shown to demonstrate activity against infectious diseases.

Indole Alkaloids from Marine Sources
Manadomanzamines A and B (Figure 1), -carbolines with a novel rearrangement of the manzamine framework, were isolated from an Indonesian sponge Acanthostrongylophora sp. These compounds showed significant activities against Mycobacterium tuberculosis, with MIC values of 1.9 and 1.5 g/mL, respectively. Manadomanzamines A and B were also active against human immunodeficiency virus (HIV-1) with EC 50 values of 7.0 and 16.5 g/mL. Furthermore, manadomanzamine A was active against the fungus Candida albicans with an IC 50 of 20 g/mL, whilst manadomanzamine B exhibited activity against the fungus Cryptococcus neoformans with IC 50 value of 3.5 g/mL [64].
Manzamine A, a -carboline alkaloid present in several marine sponge species, has been shown to inhibit the growth of the rodent malaria parasite Plasmodium berghei in vivo. More than 90% of the asexual erythrocytic stages of P. berghei were inhibited after a single intraperitoneal injection of manzamine A into infected mice. Moreover, it was demonstrated that immunostimulatory effects caused by the compound play an important role in preventing mouse death due to fulminating recurrent parasitemia in animals treated with 100 mmol per kg of manzamine A [65].
Seven -carboline-based metabolites, designated as eudistomins Y 1 -Y 7 (Figure 2), were isolated from a tunicate of the genus Eudistoma collected in South Korea. These new metabolites differ from previously isolated marine metabolites due to the presence of a benzoyl group attached to the -carboline nucleus at C-1. Eudistomin Y 6 exhibited moderate antibacterial activity against Gram-positive bacteria Staphylococcus epidermidis ATCC12228 and B. subtilis ATCC 6633 with MIC values of 12.5 and 25 g/mL, respectively, but showed no inhibitory activity toward the other two strains of Gram-positive bacteria, S. aureus ATCC 6538 and M. lutes ATCC 9341, and the Gram-negative bacteria including E. coli ATCC 11775, Salmonella typhimurium ATCC 14028, and Klebsiella pneumoniae ATCC 4352. Eudistomins Y 1 and Y 4 also displayed the same selectivity as eudistomin Y 6 but demonstrated weak antibacterial activity against the two strains of bacteria S. epidermidis ATCC12228 and B. subtilis ATCC 6633 with MICs of 50 and 200 g/mL, respectively [68].
Investigation of the CH 2 Cl 2 extract from the bryozoan Pterocella vesiculosa, collected in New Zealand, has led to the isolation of 5-bromo-8-methoxy-1-methyl--carboline ( Figure 2). This alkaloid was evaluated for antibacterial and antifungal activities and showed inhibitory action toward the Gram-positive bacterium B. subtilis and the fungi C. albicans and Trichophyton mentagrophytes with MID ranges of 2-4, 4-5 and 4-5 g/mL, respectively [69].
An indole spermidine alkaloid, didemnidine B ( Figure 3), was described in the New Zealand ascidian Didemnum sp. Evaluation of the compound against Trypanosoma brucei rhodesiense, Trypanosoma cruzi, L. donovani, and Plasmodium falciparum K1 chloroquine-resistant strain indicated didemnidine B to be mildly active toward the malaria parasite with IC 50 value of 15 M [70].
In addition, chemical investigations of the tropical marine sponge Hyrtios sp. have resulted in the isolation of several alkaloids, which were evaluated as C. albicans isocitrate lyase inhibitors. Out of the compounds tested, the bisindole alkaloid hyrtiosin B (Figure 4) showed the most potent inhibitory activity with an IC 50 value of 50.7 M. Other compounds comprised simple indole alkaloids, namely, 1carboxy-6-hydroxy-3,4-dihydro--carboline, 5-hydroxy-1Hindole-3-carboxylic acid methyl ester, serotonin, hyrtiosin A, and 5-hydroxyindole-3-carbaldehyde ( Figure 3) revealed only moderate to weak activity against C. albicans isocitrate lyase, with MIC values ranging between 39.8 and 152.9 M. By comparing their chemical structures, it was found that the enzyme inhibitory activities of these 5-hydroxyindoletype alkaloids are markedly affected by a substitution of  BioMed Research International Figure 2: Structures of -carbolines with eudistomin-derived scaffold and simple -carbolines.

Matemone Tryptophol
Isoplysin 6-Bromoaplysinopsin  functional group at the C-3 position. A substitution by a hydrophilic group at the C-3 position results in an increase of the isocitrate lyase inhibitory activity [71]. Tryptophol (Figure 3), a simple indole alkaloid from sponge Ircinia spinulosa, was screened for antitrypanosomal activity. The compound was active against T. b. rhodesiense with an IC 50 value of 5.89 g/mL, but it was 8-fold less active against T. cruzi, with IC 50 value of 49.37 g/mL, than against T. b. rhodesiense. Toward L. donovani, tryptophol displayed an IC 50 value of 9.60 g/mL, compared with 0.20 g/mL of the standard drug, miltefosine. For P. falciparum, it was shown that tryptophol presented an IC 50 value of 5.08 g/mL, compared with 0.056 g/mL of chloroquine [72].
Dendridine A (Figure 4), a C2-symmetrical 4,4 -bis(7hydroxy) indole alkaloid, was reported in extracts of an Okinawan sponge Dictyodendrilla sp. Dendridine A exhibited inhibitory activities against Gram-positive bacteria Bacillus subtilis and Micrococcus luteus with MIC values of 8.3 and 4.2 g/mL, respectively, and the fungus C. neoformans with MIC of 8.3 g/mL [74].
In order to discover active compounds with inhibitory activity against methicillin-resistant S. aureus pyruvate kinase (MRSA-PK), screening of an extract library of marine invertebrates resulted in the identification of bis-indole alkaloids from the Topsentia pachastrelloides. The most active compounds, cis-3,4-dihydrohamacanthin B and bromodeoxytopsentin (Figure 4), were identified as highly potent MRSA-PK inhibitors with IC 50 values of 16-60 nM and with at least 166-fold selectivity over human PK isoforms. These novel anti-PK natural compounds exhibited significant antibacterial activities against MRSA with MIC values of 12.5 and 6.25 g/mL, respectively, and selectivity indices (CC 50 /MIC) > 4 [75].
The bis-indole alkaloid nortopsentin A (Figure 4) present in enriched fractions of marine sponges from genus Spongosorites exhibited potent inhibition of Plasmodium falciparum growth. Assays were performed in chloroquinesensitive (3D7) and chloroquine-resistant (Dd2) strains, and IC 50 values obtained were 460 nM against the former and 560 nM against the latter [76].
Bis-indole alkaloids isolated from the Jamaican sponge Smenospongia aurea were tested against the D6 clone of P. falciparum for their in vitro antimalarial activity. The compound 6-bromoaplysinopsin ( Figure 4) exhibited activity at endpoints of 0.087 and 0.34 g/mL with a selectivity index of   [80].
From the CH 2 Cl 2 extract of the sponge Hyrtios cf. erecta, collected in Fiji, -carbolines homofascaplysin A and fascaplysin ( Figure 6) were isolated. Evaluation of the biological activity of the compounds toward P. falciparum revealed that homofascaplysin and fascaplysin are both potently active in vitro against the parasite. Homofascaplysin A also inhibited the growth of Escherichia coli (50 g/9 mm) and Bacillus megaterium (50 g/11 mm). Fascaplysin inhibited the growth of E. coli (50 g/6 mm) and B. megaterium (50 g/10 mm). Further biological activity for fascaplysin was found against Trypanosoma b. rhodesiense, displaying moderate activity with IC 50 value of 0.17 g/mL compared with melarsoprol, which showed IC 50 value of 2 ng/mL.   showed cytopathogenicity at 0.038 g/mL of fascaplysin, while untreated FRhK-4-cells remained unchanged. Homofascaplysin A and fascaplysin were shown to be potent in vitro inhibitors of chloroquine-susceptible (NF54) and chloroquine-resistant P. falciparum strains. Positive control substances were chloroquine and artemisinin. The potency against the K1 strain of homofascaplysin A was stronger than that of chloroquine. Compared with artemisinin (K1 strain) and with both positive control substances (NF54 strain), homofascaplysin was approximately 10-fold less active [81]. Alkaloids obtained from the fermentation broth of Marinactinospora thermotolerans SCSIO 0652 were tested for their antiplasmodial activities against P. falciparum line 3D7, a drug-sensitive strain, and Dd2, a multi-drug-resistant strain. The results of the antiplasmodial assays revealed that marinacarboline A (Figure 7) and methylpendolmycin-14-O-glucoside ( Figure 6 ( Figure 8). The compound was screened for anti-infective activities and showed significant antiparasitic activity against Leishmania major with IC 50 5.30 M and T. b. brucei with IC 50 0.022 M [83]. Staurosporine was first isolated in 1977 from Streptomyces staurisporeus and later also from other actinomycetes as well as cyanobacteria. In the meantime several staurosporine analogues were isolated from actinomycetes and also from marine invertebrate, including among others sponges, mollusks, and tunicates. Interestingly, in several cases staurosporine and related derivatives were isolated from Streptomyces sp. The occurrence of such compound may suggest the presence of associated microorganisms responsible for the biosynthesis of staurosporine [84].
Meridianins A-G ( Figure 13) comprise indole alkaloids substituted at the C-3 position by a 2-aminopyridine ring and had been previously isolated from the tunicate Aplidium meridianum. Investigations on the antimalarial and antileishmanial activity of meridianin C and G were carried out and showed that these two compounds inhibited P. falciparum  [90]. Hyrtioerectines D-F ( Figure 14) were isolated from Hyrtios species and had their antimicrobial activities screened. These -carboline-derived compounds showed inhibition zones of 17, 9, and 14 mm against C. albicans compared to 35 mm illustrated by clotrimazole at the same concentration. In addition, they caused inhibition zones of 20, 10, and 16 mm against S. aureus, respectively, compared to 30 mm illustrated by ampicillin, as well as inhibition zones of 7-9 mm against Pseudomonas aeruginosa compared to 30 mm illustrated by imipenem [91].
As noted in the study above, the main source of marine indole alkaloids are sponges, whilst pharmacological assays focus on activity toward parasites from the genus Plasmodium and Trypanosoma. Although few indole alkaloids from marine sources have been tested for anti-infective activity, there are some promising compounds which demonstrate high level of activity against several infectious diseases agents including bacteria, fungi, and protozoa. These active alkaloids show simple as well as complex frameworks and unlike terrestrial organisms, they frequently contain halogenated moieties. Therefore this review highlighted some marine indole alkaloids which may be considered as potential starting points for the development of novel agents for the treatment of infectious diseases.

Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.