2.1. Study Selection and Analysis

The flow diagram (Figure 1) demonstrated the search strategy (identification, inclusion and exclusion) used in the present study. A total of 77 articles were retrieved from the databases (PubMed, Web of Science and Scopus). Then, the duplicated records were excluded (n = 14). Thus, 66 full-text articles were assessed for eligibility, and 30 studies were excluded for different reasons, such as the following: some studies reported only the extraction of compounds and did not report the antiplasmodial activity; others described only the mechanism of the compounds; some studies were only computational. Finally, 36 studies were included and analyzed in this systematic review (Figure 1).

Figure 1. Flow diagram of literature search and selection criteria used in the present review adapted from PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analysis).

A summary of the studies is presented in Table 1. The articles analyzed were published from 1992 to 2019 in different countries. The antimalarial activity was assessed in vitro using Plasmodium falciparum culture [20] for quantification of cell viability over 24-96 h. For the in vitro assays, different lab strains were used (such as 3D7, W2, DD2, NF54), and a wide variety of methods were used for assessing P. falciparum viability ([3H] hypoxanthine, LDH, Microscopy, SYBR Green) presenting as IC₅₀ values instead of the option of XC₅₀. The Demospongiae sponge class was the most explored, where 30 studies evaluated their antiplasmodial activity. Among the genera in Table 1, most belong to the Demospongiae class except for Plakortis (Plakortis simplexs, Plakortis lita, Plakortis halichondrioides), which is from the Homoscleromorpha class. In addition, a great geographical variety was observed, which shows that sponges from different regions of the globe have this potential antiplasmodial activity. The inhibitory concentration for 50% of the parasites (IC₅₀) varied from low micromolar to low nanomolar range, and the species Xestospongia sp showed the best bioactive potential, from which the compound Saringosterol was extracted, which had an IC₅₀ of 0.25 nM. The individual IC₅₀ for each extracted compound is reported in Table 1. The IC₅₀ value units in µg/mL and ng/mL were converted to µM and nM for data comparison, and then some of compounds in Table 1, which IC₅₀ was below 10 µg/mL became higher than 10 µM (see Section 3.2.2, exclusion criteria), as were compounds 10 and 11 [21], 52 [22], 2 and 3 [23], 99 [24].

Table 1. Summary of descriptions of characteristics of included articles.

Author	Sponge Genus	Material Collection Location	Extracted Material (P. falciparum Strain and IC50 Value)
Campos et al., (2019) [21]	Fascaplysinopsis reticulata	Mayotte (Indian Ocean)
Jeong., et al. (2019) [25]	Coscinoderma sp.	Chuuk Island, Federated States of Micronesia
Ju et al., (2019) [25]	Hyrtios erectus	Chuuk Island, Federated States of Micronesia
Murtihapsari. et al., (2019) [26]	Xestospongia sp	Kaimana, West Papua, Indonesia
Parra et al., (2018) [27]	Tedania Brasiliensis	Brazil
Campos et al., (2017) [28]	Monanchora unguiculata	Mitsio islands, Madagascar
Yang et al., (2016) [29]	Diacarnus megaspinorhabdosa	SouthChina Sea Sponge
Gros et al., (2015) [30]	Biemna laboutei	Madagascar
Chianese et al., (2014) [31]	Plakortis simplexs	South China Sea
Gros et al., (2014) [32]	Biemna laboutei	Madagascar at Salary Ba
Yang et al., (2014) [17]	Diacarnus megaspinorhabdosa	South China Sea
Alvarado et al., (2013) [33]	Spongosorites sp	Not reported
Davis et al., (2013) [34]	Plakortis lita	Not reported
Farokhi et al., (2013) [35]	Axinyssa djiferi	Senegalese coasts
Sirirak et al., (2013) [36]	Pachastrissa nuxs	Thailand
Chanthathamrongsiri et al., (2012) [37]	Stylissacf. massa	Not reported
Davis et al., (2012) [38]	Zyzzya sp	Not reported
Ilias et al., (2012) [39]	Petrosia	Eastern Fields north of Australia
Mudianta et al., (2012) [40]	Aplysinella strongylata	Tulamben, Bali, Indonesia
El Sayed et al., (2011) [22]	Diacarnus erythraeanus	Red Sea
Galeano et al., (2011) [41]	Verongula rigida	Urabá Gulf is located in the Southwestern Caribbean
Sirirak et al., (2011) [42]	Pachastrissa nux	Koh-Tao, Surat-Thani ProvinceChumphon IslandsNational Park, Chumphon Province,
Xu et al., (2011) [43]	Pseudoceratina sp	Australian biota
Jiménez-Romero et al., (2010) [44]	Plakortis halichondrioides	Puerto Rico
Samoylenko et al., (2009) [45]	Acanthostrongylophora ingens	Pacific
Ueoka et al., (2009) [46]	Agelas gracilis	southern Japan
Wright et al., (2009) [47]	Cymbastela hooperi	Not reported
Appenzeller et al., (2008) [48]	Agelas cf. mauritiana	Solomon Islands
Desoubzdanne et al., (2008) [49]	New Caledonian	Norfolk Rise (New Caledonia)
Tasdemir et al., (2007) [23]	Agelas oroides	Northern Aegean Sea, Turkey	- fractions: fatty acid mixtures FAME (3.4 μg/mL) and FAMF (8.7 μg/mL)
Laurent et al., (2006) [50]	Xestospongia	Vanuatu
Mancini et al., (2004) [51]	Oceanapia fistulosa	New Caledonia Main Island	-crude mixture (0.98 μM)    -N-methyl derivatives from the crude mixture (8 μM)
Fattorusso et al., (2002) [52]	Plakortis simplex	Berry Island (Bahamas)
Gochfeld et al., (2001) [24]	Plakortis sp.	Jamaica
Kirsch et al., (2000) [53]	Hyrtios cf. erecta	Fiji
Angerhofer et al., (1992) [54]	Acanthella klethra	Australia

To assess the study quality, we used the GRADE method [55]. The 36 studies analyzed were categorized as moderate quality (17) because (i) there were no controls in the experiments; (ii) the toxicity of the compounds was not assessed in parallel, which made it impossible to determine the selectivity of compounds; (iii) all compounds analyzed presented a high cytotoxicity, which demonstrates the unspecified use against P. falciparum; (iv) the methods used to measure the antiplasmodial activity were not described. A total of 19 studies were classified as being of high quality (Table S1).

After this detailed review of the articles reporting the activity of compounds from marine sponges, we made a brief survey of data in the literature to compare the number of articles published reporting the activity of marine organisms with the number of articles published reporting the activity of extracts from plants. To do so, the following combinations of keywords were used: "new antimalarials and plants" or "new antimalarials and marine" and selected the works published in the last 10 years. Figure 2 represents the number of studies reporting antiplasmodial activity of new compounds found. The search for new compounds from marine sources is still uncommon compared to the search for natural products from plants. Other recent reviews have also reported this comparison, which reinforces the importance of seeking new products from marine sources, especially considering that the diverse nature of metabolites produced by these alternative sources presents a compelling case for intensive exploration [56].

Figure 2. Number of published papers reporting the antimalarial activity of new compounds from marine sources or plants in the past 10 years.