key: cord-0959062-brza4ahh
authors: Gonzalez, Bárbara Longhini; de Oliveira, Natalia Castelhano; Ritter, Mariane Roberta; Tonin, Fernanda Stumpf; Melo, Eduardo Borges; Sanches, Andréia Cristina Conegero; Fernandez‐Llimos, Fernando; Petruco, Marcus Vinícius; de Mello, João Carlos Palazzo; Chierrito, Danielly; de Medeiros Araújo, Daniela Cristina
title: The naturally‐derived alkaloids as a potential treatment for COVID‐19: A scoping review
date: 2022-03-30
journal: Phytother Res
DOI: 10.1002/ptr.7442
sha: 7a8807e0d9eaa35e5ba6c78d3074eda122a1c8bb
doc_id: 959062
cord_uid: brza4ahh

Coronavirus disease 2019 (COVID‐19) is caused by Severe Acute Respiratory Syndrome Coronavirus 2 (SARS‐CoV‐2), which has a high mortality rate and transmissibility. In this context, medicinal plants have attracted attention due to the wide availability and variety of therapeutic compounds, such as alkaloids, a vast class with several proven pharmacological effects, like the antiviral and anti‐inflammatory activities. Therefore, this scoping review aimed to summarize the current knowledge of the potential applicability of alkaloids for treating COVID‐19. A systematic search was performed on PubMed and Scopus, from database inception to August 2021. Among the 63 eligible studies, 65.07% were in silico model, 20.63% in vitro and 14.28% clinical trials and observational studies. According to the in silico assessments, the alkaloids 10‐hydroxyusambarensine, cryptospirolepine, crambescidin 826, deoxynortryptoquivaline, ergotamine, michellamine B, nigellidine, norboldine and quinadoline B showed higher binding energy with more than two target proteins. The remaining studies showed potential use of berberine, cephaeline, emetine, homoharringtonine, lycorine, narciclasine, quinine, papaverine and colchicine. The possible ability of alkaloids to inhibit protein targets and to reduce inflammatory markers show the potential for development of new treatment strategies against COVID‐19. However, more high quality analyses/reviews in this field are necessary to firmly establish the effectiveness/safety of the alkaloids here described.

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), an enveloped-type virus with positive-sense single-stranded ribonucleic acid (RNA), is the cause of coronavirus disease 2019 , first reported in the city of Wuhan, Hubei province (China). This virus species belongs to Betacoronavirus genus and is capable of causing an intense inflammatory response especially in the respiratory system, which can lead to acute lung injury and respiratory distress, being associated with high rates of mortality and morbidity (Chaomin Wu et al., 2020) .

In this context, the high transmissibility and impact of this disease on worldwide health and economy led the World health Organization (WHO) to declare a Public Health Emergency of International Concern Sohrabi et al., 2020) . Scientific studies for the development of drugs that inhibit viral replication and disease progression have been carried out. Nonetheless, to date, there are no approved therapies with established efficacy and safety against SARS-CoV-2 (Dai et al., 2020) .

In this field, in silico studies can speed up the traditional process of drug discovery by identifying a novel clinical use for compounds that have already proven to be safe and effective in humans, are approved for other indications, or traditionally used by some populations. This strategy can also reduce the costs required for the development of new drugs, with notable savings in preclinical studies, and has been previously used for the development of additional therapeutic options against Ebola, hepatitis C, and zika virus infection (Sultana et al., 2020; Venkatesan, 2021) .

The natural products, including primary and secondary metabolites, work as a model for the synthesis of new antiviral drugs given their availability and variety of compounds with therapeutic potential.

About 50% of all approved drugs for sale between 1981 and 2014 were derived from natural products (Newman & Cragg, 2020) . In view of that, alkaloids, an extensive group of secondary metabolites (more than 12,000 compounds) mostly characterized by the presence of at least one nitrogen atom in a negative oxidation state (at any position in the structure, however it does not include nitrogen in an amide or peptide bond) may be useful in seeking drug treatment for COVID-19 (Bribi, 2018) .

These compounds are found mainly in flowering plants, bacteria, fungi, some animal species, among other sources, being classified according to the biosynthetic route as quinolines, isoquinolines, tropanes, purines, imidazoles, indoles, pyrrolidines, pyrrolizidines, pyridines and other types. The most known pharmacological activities of these compounds include antioxidant, anticarcinogenic, antimalarial, antibacterial, antifungal and antiviral effects (Simões, Schenkel, de Mello, Mentz, & Petrovick, 2017) .

Previous narrative reviews focused on a broader description of the effects of some of the natural products or herbal medicines with antiviral effects. Among the alkaloids, homoharringtonine (HHT), lycorine and emetine were previously mentioned as promising therapies against COVID-19 (Boozari & Hosseinzadeh, 2021; Verma et al., 2020) .

On the basis of the prior discuss, the aim of this study was to summarize the current state of knowledge on the potential applicability of alkaloids for treating COVID-19 by means of a scoping review.

This scoping review was performed according to the recommendations of the Preferred Reporting Items for Systematic Reviews and Meta-Analyses Extension for Scoping Reviews (PRISMA-ScR) Checklist (Tricco et al., 2018) , the Cochrane Handbook for Systematic Reviews of Interventions version 6.2 (Higgins & Thomas, 2021) and the Joanna Briggs Institute methodology for scoping reviews (Joanna Briggs Institute, 2015) , with Open Science Framework (OSF) register doi: 10.17605/OSF.IO/7QXV8.

A systematic search was conducted in the electronic databases Medline (via PubMed) and Scopus with no restriction for publication date or language (the last recovery date of studies was August 18, 2021). The main descriptors used referred to "coronavirus," "COVID-19," "SARS-CoV-2" and "alkaloids" (see full strategy search in Appendix A provided in the supplementary material). Manual search in the reference list of the included studies was also performed.

We included: studies designed as in silico, in vitro, in vivo, clinical trials and observational; Studies that evaluated the use of alkaloids (any subclasses) isolated or associated with other compounds for the treatment of infection caused by the SARS-CoV-2 virus .

Studies that did not evaluate SARS-CoV-2; reviews of any type; letter to editor; comments; editorials or expert opinions were excluded. Synthetic and semi-synthetic alkaloids were not included.

Studies that analyzed crude extract or fractions of plants; articles written in non-Roman characters were excluded from this scoping review (see complete inclusion and exclusion criteria in Appendix B provided in the supplementary material).

The selection process of studies involved two stages, the first being the reading of titles and abstracts (screening phase) for exclusion of irrelevant registers, and the second the reading of full text (eligibility phase) aiming at selecting eligible studies for data extraction. Data was extracted using structured tables containing general characteristics of the trials, evaluated alkaloid, methodological aspects, and main findings in accordance with the study design. Given the nature of the data, evidence was qualitatively synthetized. All steps of this study were conducted by two reviewers independently and disagreements were solved by third referee whenever necessary during consensus meetings.

A total of 1,012 registers were selected from the database after duplicates removal, of which 839 were excluded during screening (title and abstract reading). From the remaining 173 studies, 110 were excluded after full-text appraisal (see the complete list in Appendix C provided in supplementary material). Finally, 63 studies meeting the eligibility criteria had their data extracted and analyzed. No articles were added by manual search (see Figure 1) . Overall, 41 studies (65.07%) were in silico model, 13 (20.63%) in vitro models, 9 clinical trials and observational (14.28%) studies. (Borquaye et al., 2020; El-Demerdash et al., 2021; Firdiana et al., 2021; Ismail et al., 2021; Selvaa Kumar, Kumar, & Wei, 2020; Maiti, Banerjee, & Kanwar, 2021; Maiti, Banerjee, Nazmeen, Kanwar, & Das, 2020; Mao, Bie, Xu, Wang, & Gao, 2021; Ogunyemi et al., 2020; Skariyachan, Gopal, Muddebihalkar, & Uttarkar, 2021) .

The alkaloids nicotine, nigellidine, norboldine, piperine, quinadoline A and trytoquivaline showed binding capacity to the angiotensin-converting enzyme II (ACE2) (Firdiana et al., 2021; Ismail et al., 2021; Maiti et al., 2021; Maurya, Kumar, Prasad, Bhatt, & Saxena, 2020; Selvaa Kumar, Kumar, & Wei, 2020 Caffeine plus ribavirin binding to the complex between SARS-CoV-2 S protein receptor binding domain and ACE2 receptor (RBD-ACE2), quinine, cryptospirolepine binding on RBD-ACE2 and on ACE2, nicotine binding on C-terminal domain of S1 protein bound angiotensin-converting enzyme II receptor (CTD-ACE2) plus favipiravir were described in three studies Lestari, Sitorus, Instiaty, Megantara, & Levita, 2020; Mohammadi et al., 2020) .

The alkaloid 10-hydroxyusambarensine showed binding capacity to human in transmembrane protease serine 2 (TMPRSS2) with high binding energy (À10.40 kcal/mol) .

The jatrorrhizine alkaloid, obtained from Tinospora cordifolia (Willd.)

Miers or Mahonia aquifolium (Pursh) Nutt. or Enantia chlorantha Oliv., Mpro was used as a protein target in 58.53% (n = 24) of the stud-

ies. Among the alkaloids tested for this target protein, the following stood out: emetine deoxynortryptoquivaline, tubocurarine, 10-Hydroxyusambarensine, noscapine plus hydroxychloroquine, cryptomisrine, 18-hidroxy-3-epi-alpha-yohimbine, escholtzine, (S)- 

The 13 available in vitro studies quantitatively evaluated the impact of seven alkaloids (complete table with Cell replication was estimated in two studies using the quantitative real-time polymerase chain reaction (qRT-PCR) assay (Gendrot et al., 2020; Pizzorno et al., 2020) and in one study (Choy et al., 2020) the counting of viral titers was performed using the median tissue culture infectious dose (TCID50). The assays varied in relation to the value of multiplicity of infection (MOI) (0.001 to 3.00) and incubation time (24 to 72 h) (see Table 2 ).

A dose-response assay revealed that the association of berberine with remdesivir results in strong antagonism represented by the Loewe synergy score (À35.12) and by several negative peak values distributed along the synergy map (Pizzorno et al., 2020) . One study performed a time-of-addition assay to decipher a putative mode of action for berberine, which indicated that it acts late in the viral infection cycle affecting the production of infectious virus particles (Varghese et al., 2021) .

A synergism trial with emetine (0.195 μM) when associated to remdesivir (6.25 μM) provided an inhibition of up to 64.9% of viral production with a Loewe synergy score of 0.306, which indicates the need of clinical trials and observational studies to confirm these findings (Choy et al., 2020) . Furthermore, an in vitro assay performed with Caco-2 cells testing papaverine and emetine, elucidated that papaverine was eight-fold more potent against SARS-CoV-2 in these cells than against Middle East Respiratory Syndrome Coronavirus (MERS-CoV) (Ellinger et al., 2021) . On the other hand, a study demonstrated that emetine is effective at a low nanomolar range in Vero E6 cells, as well as performed an assessment of emetine's effect on SARS-CoV-2 life cycle showing a highly significant reduction in the total RNA virus (R. Kumar et al., 2021) .

In Authors suggested that, if proven clinically effective, quinine would be a promising candidate for the treatment of hypercytokinaemia due to COVID-19 (Gendrot et al., 2020) .

Other assay treated five cell lines with quinine-sulfate extracted from a tablet (Q-L) and quinine-sulfate as solid material (Q-S). In this study, the median toxic dose (TD 50 ) of Vero B4 cells was $100 μM (Q-S) and A549 cells presented a TD 50 of $150 μM (Q-S)/$300 μM (Q-L). Moreover, a reduction in viral replication by up to 90% was observed when10 μM of quinine was applied in Vero B4 cells (Große et al., 2021) .

Caco-2 cells are capable of expressing TMPRSS2 protein, and for this cell line doses above 50 μM can inhibit SARS-CoV-2 infection (MOI = 3.00), with a dose-dependent effect up to 2 μM, as well as suppressing viral spread and replication of the virus. The authors concluded that quinine exhibits antiviral activity in A549 lung cancer cell lines, which can be modulated but not abrogated by TMPRSS2 expression (Große et al., 2021) . Unlike what the two studies above exposed, Persoons et al. (2020) concludes that quinine only blocked coronavirus replication at higher concentrations. A Chinese single-center pragmatic randomized controlled clinical trial evaluated a total of 39 patients from Wuhan Shelter Hospital, and the same authors also performed a multicenter real- 

The viral cycle begins with the attachment and entry, wherein the coronavirus enters into human cells through glycoprotein spike of SARS-CoV-2 attachment to host-cellular receptors (ACE2 and TMPRSS2) (Islam et al., 2020) . The attachment forms a viral particle that fuses with the cell membrane or endosomal (Figure 2-1 

Due to the large number of alkaloids found in in silico studies, being 45 different compounds, we chose to discuss those that showed higher binding value with more than two targets (host or viral proteins). The possibility of a compound acting simultaneously on more than two targets is interesting for the treatment of COVID-19, owing to the emerging SARS-CoV-2 variants (Tao et al., 2021) . Thus, we found 14 alkaloids (Figure 3 -see complete table with alkaloids chemical structure classification is in Appendix D provided in the supplementary material) that fit in this requisition.

Almost 65.00% of the conducted studies in this scoping review refer to in silico assays. This approach gained strength in the past year given the increasing interest in drug repurposing (i.e., drug repositioning or rediscovery) to accelerate the identification of compounds that can cure or prevent COVID-19 (Y. Kumar, Singh, & Patel, 2020) .

In the context, we found that 10-hydroxyusambarensine, a bisindole alkaloid from Strychnos usambarensis Gilg ex Engl.

(Loganiaceae), showed high binding capacity to the host TMPRSS2 target and Mpro SARS-CoV-2 target (Gyebi, Adegunloye, et al., 2020; . The TMPRSS2 protein performs the proteolytic cleavage of the S1/S2 and S2 sites in the S protein, thus facilitating viral fusion with the host cell (adsorption) (Hoffmann et al., 2020) . The main SARS-CoV-2 target protein, known as Mpro, cleaves polyproteins involved in the production of RNA that encodes structural viral proteins, so its blockage is capable of interrupting the viral cycle at its initial stages (Dai et al., 2020) .

Some species of Loganiaceae family are used in folk African medicine to treat fever and malaria, as well as being known to have an alkaloid rich content (Asuzu & Nwosu, 2020) . The indole alkaloids 10-hydroxyusambarensine, chrysopentamine, strychnopentamine and isostrychnopentamine can be extracted from the species S. usambarensis and demonstrated ability to bind with coronaviruses' main protease .

In addition, 10-hydroxyusambarensine also showed high binding energy and binding affinity with RdRp target, which is responsible for the replication and transcription processes of the viral genome;

besides having an active site in an accessible region (Aftab et al., 2020; Buonaguro, Tagliamonte, Tornesello, & Buonaguro, 2020; Ogunyemi et al., 2020) .

Another compound with the ability to bind with two targets was, camptothecin, a pyrroloquinoline alkaloid, originally isolated from the Chinese tree, Camptotheca acuminata Decne. (Nyssaceae) that showed good value binds with Mpro and RdRp targets protein. Traditionally, it is used in Chinese medicine as a natural cancer drug and is F I G U R E 3 Chemical structures of the main alkaloids found in this scoping review for COVID-19 treatment currently an important alkaloid with proven anticancer properties acting as poison to the enzyme DNA topoisomerase 1 (TOP1). When infected by SARS-CoV-2 the human topoisomerase-1 can activate transcriptional inflammatory genes, in view of that, camptothecin may eventually be studied for TOP1 inhibition activity, aiming to suppress inflammation by limitating the action of this enzyme (Devasia et al., 2021; Martino et al., 2017) .

Caffeine is considered a purine alkaloid found in small amounts in fruit, seeds, and leaves of a variety of plants. This methylxanthine showed a significant binding energy with ACE2 when complexed to the active site of protein S plus ribavirin (Mohammadi et al., 2020) , it either had good binding affinity to Mpro (Elzupir, 2020) and RdRp about its phytochemical composition explained that it is a species rich in alkaloids, tannins and flavones (Osafo, Mensah, & Yeboah, 2017) .

Three studies analyzed cryptomisrine (Borquaye et al., 2020) and

cryptospirolepine Ogunyemi et al., 2020) , two secondary metabolites of C. sanguinlenta. (Bouvet et al., 2014) .

Another alkaloid found was deoxynortryptoquivaline, isolated from the mangrove-derived fungus Cladosporium sp., showed high binding energy with the already mentioned main targets of SARS-CoV-2, Mpro and spike glycoprotein. This quinazoline alkaloid can also inhibit ACE2 target, in addition, this compound exhibits good pharmacokinetic and safety profiles. Nonetheless, more studies must be carried out to investigate it as possible natural multi-target drug against COVID-19 (Ismail et al., 2021) .

The polypeptide ergot alkaloid, ergotamine, commonly used for migraine demonstrated high binding energy with 2 0 -O-MTase , whose action is essential for viral replication and expression of coronaviruses in host cells, giving the virus the ability to escape from the immune system, ensuring less recognition of the host's immune response (Khan et al., 2021) . It also has binding ability with more four targets showing high binding affinity with spike glycoprotein (Mao et al., 2021; Qiao et al., 2020) , significant binding energy with RdRp (Gul et al., 2020) , PLpro (Jade et al., 2021) and with Mpro complexes (Mostafa et al., 2021) . et al., 2020) .

Nigella sativa L., commonly known as black-cumin, offers an indazole alkaloid named nigellidine. This herb is extensively employed traditional medicine like Ayurveda and Unani due to its extract safety, anti-oxidant, anti-bacterial, anti-inflammatory, anti-hypertensive activity and immunomodulatory effects. Therefore, this compound found in N. sativa has been associated with high energy binding with spike glycoprotein, NSP2, nucleocapsid N, ACE1, ACE2, AT1, AT2, in addition to having strong interactions with Mpro (Maiti et al., 2020 (Maiti et al., , 2021 .

Renal and hepatic toxicities of the alkaloid nigellidine were evaluated in female Wistar rats, revealing no important safety concerns for the use of this substance (Maiti et al., 2020) .

Norboldine, a compound classified as aporphine alkaloid that can be extract from Lindera aggregate (Sims) Kosterm., had binding potential with three target proteins, namely ACE2, Mpro and PLpro (Firdiana et al., 2021) . The last target, PLpro acts as multifunctional protein with an essential role in the processing of viral polyproteins, maturation and assembly of RTC, it can also act on host cell proteins by interrupting the immune response and facilitating viral replication (Osipiuk et al., 2021) . It has been an important target against several coronaviruses, such as MERS-CoV (Kandeel et al., 2020; Maiti et al., 2020) .

Quinadoline B, a fumiquinazoline alkaloid isolated from culture broth of Aspergillus sp., showed binding potential with three SARS-CoV-2 proteins, namely RdRp, NSP15 and protein S. A in silico profile of absorption, distribution, metabolism, excretion and toxicity (ADMET) showed a high gastrointestinal absorption, poor blood-brain barrier penetrability and high drug-likeness. Also, it did not confer mutagenic, tumorigenic and reproductive toxicities, despite the highrisk irritant (Quimque et al., 2020) . As a side note, the NSP15 target has endoribonuclease activity, participates in viral RNA synthesis and plays a role in suppressing IFN-α/INF-β associated with the innate immune response of the host. For this reason, it limits the exposure of viral mRNA by the host's double-stranded RNA (dsRNA) sensors, increasing its evasion capacity. Thus, quinadoline B may prevent virus evasion by inhibiting its replication and fusion to the host cells (Batool, Bibi, Amin, & Amjad, 2020; Deng et al., 2017; Quimque et al., 2020) .

Finally, quinine, a quinoline alkaloid extracted from bark of Cinchona species that showed capability of binding with peptidase domain of the ACE2 receptor (Lestari et al., 2020) and to NSP12

(RdRp) target protein (Sumitha et al., 2020) . The antiviral activities of several antimalarial drugs produced from quinine were investigated in an in vitro assay against SARS-CoV-2 (Gendrot et al., 2020) .

A meta-analysis of randomized trials showed that quinine analogue hydroxychloroquine (HCQ) is associated with increased mortality in COVID-19 patients, and there is no benefit of using chloroquine (CQN). Currently, quinine alkaloids are the only effective treatment used against malaria and can be an option for further studies against COVID-19, as their analogues HCQ and CQN have failed (Axfors et al., 2021) .

In short, the therapeutic applications of these bioactive compounds must be explored in future pharmacological research. Overall, the information presented is a summary of the findings from the databases and we cannot support active recommendation of use, but evidence as a whole suggests that these 14 alkaloids can be analyzed more deeply about their simultaneous action on target proteins, of the other alkaloids represented in Table 1 .

Few in vitro studies were found in this scoping review. In vitro models provide a starting point for researchers to gather insights into how a cell responds to new substances in a controlled, isolated environment, but they often fail to recapitulate the complexity of human body systems (e.g., physiologically limited). Once a drug candidate demonstrates effectiveness through a series of in vitro essays, in vivo models can be used to advance drug development studies. Preclinical trials typically involve the use of animals or humans to further evaluate the safety, efficacy and delivery of a drug. However, this type of study is costly, and subject to strict regulations and compliance standards (Rosa et al., 2021) .

HHT, a cytotoxic alkaloid isolated from the evergreen tree (Fielding, da Filho, Ismail, & de Sousa, 2020) . It is an anticancer approved by FDA, which demonstrates significant antiviral activity against diverse species of human and animal coronaviruses with the lowest IC 50 (12 nM). Jointly, a review of natural products for COVID-19 exposed HHT, lycorin and emetine as alkaloids with strong anticoronavirus effect (Boozari & Hosseinzadeh, 2021) . TMPRSS2 inhibition a potential therapeutic target Rahman et al., 2020) .

Futhermore, HHT and another alkaloid, narciclasine, demonstrated the ability to induce a similar host response to IFN-β through a significant proportion of IFN-b-responsive genes and are intensively regulated by these alkaloids, which increase the expression of and IFNB1 (interferon beta 1). Narciclasine is a lycorine type alkaloid isolated for the first time from different varieties of Narcissus bulbs, which has a mechanism of action that involves inhibition of eukaryotic protein synthesis (Ceriotti, 1967; Huang et al., 2020) .

On the other hand, we did not observe only analysis of isolated alkaloids, but also the result of the association between synthetic drugs and these compounds, like emetine and remdesivir, that revealed an important synergism with over 50% inhibition of SARS-CoV-2 production, which should be further investigated (Choy et al., 2020) . Researchers suggested that emetine could affect viral genome synthesis in the target cells. Yet, the combination of drugs is a strategy to increase the effectiveness of the treatment, reducing the EC 50 of the compounds and, thus, restricting the side effects (R. Kumar et al., 2021) .

Conversely, an antagonism between the alkaloid berberine and remdesivir was demonstrated, suggesting that this combination should not be further explored (Pizzorno et al., 2020) . A review concluded that berberine exerts anti-inflammatory activity could be used to treat inflammation and other related diseases, such as downregulation of pro-inflammatory cytokines and upregulation of anti-inflammatory cytokines (Shang et al., 2020) .

Berberine also has been associated with myocardial protective effects from ischemia/reperfusion injury. Moreover, it has shown very low toxicity and side effects in animal studies and in clinical trials only mild gastrointestinal reactions, including diarrhea and constipation were reported (Imenshahidi & Hosseinzadeh, 2019; Shang et al., 2020) .

Lycorine, an indolizidine alkaloid isolated from the extract of the Chinese herb Lycoris radiata (L'Hér.) Herb., had a strong interaction with RdRp (Ren et al., 2021) showing a binding affinity (À6.20 kcal/ mol) higher than the FDA approved drug remdesivir (À4.70 kcal/mol) (Jin et al., 2021) . A review of compounds against coronavirus found that lycorine has antiviral activity in nanomolar concentration against SARS-CoV with an EC 50 value of 15.7 nM (Islam et al., 2020) .

Isoquinoline is a subclass of alkaloids that can be extracted from plants of family Rubiaceae. Cephaeline is a terpenoid tetrahydroisoquinoline compound found in dried rhizome and roots of Cepahaelis ipecacuanha (Brot.) A. Rich (Rubiaceae), which may be more tolerable to patients when compared to its analogue, emetine, due to the similarly mechanism of action (inhibition of Zika and Ebola virus replication in vitro) (Ren et al., 2021; .

Emetine is another isoquinoline alkaloid isolated from C. ipecacuanha, as well as plant families Alangiaceae and Icacinaceae, which displayed potent in vitro anti-CoV and anti-MERS-CoV activity with EC 50 value of 0.34 μM, moreover it demonstrated a strong interaction with SARS-CoV-2 replication protein, RdRp (Ren et al., 2021; Shen et al., 2019) .

Among the articles that reported IC 50 , papaverine had the third lowest value. This alkaloid is extracted from opium poppy Papaver somniferum L. (Papaveraceae) and is classified as a benzylisoquinoline.

It is a non-narcotic alkaloid phosphodiesterase (PDE) inhibitor indicated for heart disease, impotence, pyschosis, and it also inhibits multiple strains of influenza virus (Ellinger et al., 2021; Kawase, Shirato, van der Hoek, Taguchi, & Matsuyama, 2012) . SARS-CoV-2 also stimulates the inflammasome by pathogenassociated molecular patterns (PAMPs) and damage-associated molecular Patterns (DAMPs) triggering numerous proinflammatory states that favor and contribute to cardiovascular process of a thrombotic nature (Páramo, 2020) . A systematic review of the impact of colchicine on cardiovascular outcomes showed that results among clinical evidence have diverged, but they suggested a possible benefit from early initiation of colchicine when inflammation is more intense. In spite of this, they concluded that in post-acute myocardial infarction (MI) patients, colchicine does not reduce cardiovascular or all-cause mortality, recurrent MI, or other cardiovascular outcomes (Diaz-Arocutipa et al., 2021) .

Studies reported evidence that SARS-CoV-2 increase NLRP3 inflammasome activation (essential against viral infections, but linked to the inflammatory disorder when in excess, such as cytokine storm observed on COVID-19) (Páramo, 2020) . Colchicine suppresses NLRP3 inflammasome, by disrupting the microtubule-dependent transport of mitochondria to the endoplasmatic reticulum, whose assembly with its adaptor and NLRP3 is required to be activated with inflammasomes (Lopes et al., 2021) .

Another inflammatory marker observed by the researchers was CRP, whose raising levels have been observed in patients with COVID-19. This non-specific acute phase protein increases in acute infection or inflammation (Stringer et al., 2021) . CRP is a considered a valid biomarker associated among severe and expired patients with COVID-19 (p = .001) (Elshazli et al., 2020) .

Like CRP, D-dimer is a plasma biomarker used for the diagnostic investigation of pulmonary embolism. A meta-analysis reported that D-dimer and prothrombin time between severe and non-severe cases (p-value < .001) was statistically significant, in which severe cases presented a higher D-dimer level, as well as CRP (Danwang et al., 2020) .

The chronic use of colchicine, however, was associated with nausea, emesis, abdominal pain and diarrhea (Ribeiro et al., 2020) which can lead to drug discontinuation. In this context, gastrointestinal sideeffects were similar in colchicine studies, however, an incidence of diarrheal condition was observed in five studies (Deftereos et al., 2020; Lopes et al., 2021; Mareev et al., 2021; Tardif et al., 2021; Scarsi et al., 2020) . On this basis, further studies on dose adjustments are needed to strength this evidence.

The greater limitations of emetine randomized controlled clinical trial was the exclusion of severe cases because of the limited conditions. However, in accordance with it emetine could be enriched in lung tissue for more than 12 h and still retain an effective concentration (Fan et al., 2021) .

In view of that, it is suggested that the distinction observed in the clinical outcomes and values of the parameters analyzed was due to the difference among the severity of patients admitted to each study, ranging from patients with mild, moderate or severe course of COVID-19. It is worth pointing out that the difference is also as a result of medicines associated with the alkaloids.

Overall, they were found as common clinical outcomes to the use of alkaloids colchicine and emetine: mortality rate reduction;

decreased need for non-invasive and invasive mechanical respiratory support (intubation); increased blood and percutaneous oxygenation;

and reduction of inflammatory markers such as C-reactive protein and D-dimer. Despite of good outcomes, further studies are needed to confirm the isolated effects of alkaloids in patients with COVID-19.

Our study has some limitations. No statistical comparisons through meta-analyses were possible given the high heterogeneity among studies (i.e., type of alkaloid, study designs, outcome assessment). This study focused on the antiviral potential of alkaloids, however, other natural compounds (alone or combined) may have some effect in this context and should be better evaluated in the future. Furthermore, well-designed and high-quality studies are needed to investigate the safety/efficacy of the alkaloids found and described in this review. In short, given the paramount importance of the COVID-19 pandemic, as soon as more studies are published, this scoping review should be updated.

This scoping review summarized the available evidence from in silico, in vitro, clinical trials and observational studies on the potential effects of some alkaloids to be used alone or combined with other drugs. The compounds found with most promising antiviral effects against SARS-CoV-2 that can be investigated in additional in vitro assays, clinical trials and observational studies were: 10-hydroxyusambarensine, berberine, caffeine, camptothecin, colchicine, crambescidin 786 and 826, cryptomisrine, cryptospirolepine, deoxynortryptoquivaline, emetine, ergotamine, HHT, lycorine, michellamine B, nigellidine, norboldine, papaverine, quinadoline B, and quinine. It is noteworthy, that further high quality studies are needed to firmly establish the possible clinical efficacy of the alkaloids compounds here described. In short, it is a hope that these findings can guide the design of other studies aiming at developing drugs with bioactive compounds and their derivatives to treat COVID-19.

We truly thank all involved in researching treatment against COVID-19.

The authors declare that there is no conflict of interest. 

It is a review paper and this item is not applicable. 

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A meta-analysis of potential biomarkers associated with severity of coronavirus disease 2019 (COVID-19)

An investigation into the identification of potential inhibitors of SARS-CoV-2 main protease using molecular docking study

Anti-HIV reverse transcriptase plant polyphenolic natural products with in silico inhibitory properties on seven non-structural proteins vital in SARS-CoV-2 pathogenesis

Effect of colchicine versus standard care on cardiac and inflammatory biomarkers and clinical outcomes in patients hospitalized with coronavirus disease 2019: The GRECCO-19 randomized clinical trial

Coronavirus nonstructural protein 15 mediates evasion of dsRNA sensors and limits apoptosis in macrophages

Enhanced production of camptothecin by immobilized callus of Ophiorrhiza mungos and a bioinformatic insight into its potential antiviral effect against SARS-CoV-2

Efficacy and safety of colchicine in post-acute myocardial infarction patients: A systematic review and meta-analysis of randomized controlled trials

Comprehensive virtual screening of the antiviral potentialities of marine polycyclic guanidine alkaloids against SARS-CoV-2 (COVID-19)

A SARS-CoV-2 cytopathicity dataset generated by high-content screening of a large drug repurposing collection

Diagnostic and prognostic value of hematological and immunological markers in COVID-19 infection: A meta-analysis of 6320 patients

Caffeine and caffeine-containing pharmaceuticals as promising inhibitors for 3-chymotrypsin-like protease of SARS-CoV-2

Clinical efficacy of low-dose emetine for patients with COVID-19: A real-world study

Alkaloids: Therapeutic potential against human coronaviruses

Silico study of the active compounds of Lindera aggregata (Sims) Kosterm as anti-coronavirus. Current Nutrition & Food Science

Bioinformatic study to discover natural molecules with activity against COVID-19

In silico analysis of selected alkaloids against main protease (Mpro) of SARS-CoV-2

Antimalarial drugs inhibit the replication of SARS-CoV-2: An in vitro evaluation

Identification of alkaloids from Justicia adhatoda as potent SARS CoV-2 main protease inhibitors: An in silico perspective

Quinine inhibits infection of human cell lines with SARS-CoV-2

In silico identification of widely used and well-tolerated drugs as potential SARS-CoV-2 3C-like protease and viral RNA-dependent RNA polymerase inhibitors for direct use in clinical trials

Structure-based virtual screening of phytochemicals and repurposing of FDA approved antiviral drugs unravels lead molecules as potential inhibitors of coronavirus 3C-like protease enzyme

The potential of Paritaprevir and emetine as inhibitors of SARS-CoV

Prevention of SARS-CoV-2 cell entry: Insight from in silico interaction of drug-like alkaloids with spike glycoprotein, human ACE2, and TMPRSS2

Potential inhibitors of coronavirus 3-chymotrypsin-like protease (3CLpro): An in silico screening of alkaloids and terpenoids from African medicinal plants

Cochrane handbook for systematic reviews of interventions

Chloroquine does not inhibit infection of human lung cells with SARS-CoV-2

Enhancement of the IFN-β-induced host signature informs repurposed drugs for COVID-19

Potential antiviral options against SARS-CoV-2 infection

Berberine and barberry (Berberis vulgaris): A clinical review

Natural products and their derivatives against coronavirus: A review of the non-clinical and pre-clinical data

Quinoline and quinazoline alkaloids against Covid-19: An in silico multitarget approach

Virtual high throughput screening: Potential inhibitors for SARS-CoV-2 PLPRO and 3CLPRO proteases

Lycorine, a non-nucleoside RNA dependent RNA polymerase inhibitor, as potential treatment for emerging coronavirus infections

Methodology for JBI scoping reviews Joanna Briggs Institute

Repurposing of FDA-approved antivirals, antibiotics, anthelmintics, antioxidants, and cell protectives against SARS-CoV-2 papain-like protease

Simultaneous Treatment of Human Bronchial Epithelial Cells with Serine and Cysteine Protease Inhibitors Prevents Severe Acute Respiratory Syndrome Coronavirus Entry

Targeting SARS-CoV-2: A systematic drug repurposing approach to identify promising inhibitors against 3C-like proteinase and 2 0 -O-ribose methyltransferase

Understanding the binding affinity of noscapines with protease of SARS-CoV-2 for COVID-19 using MD simulations at different temperatures

Antitussive noscapine and antiviral drug conjugates as arsenal against COVID-19: A comprehensive chemoinformatics analysis

Comparative docking studies to understand the binding affinity of nicotine with soluble ACE2 (sACE2)-SARS-CoV-2 complex over sACE2

Molecular binding mechanism and pharmacology comparative analysis of noscapine for repurposing against SARS-CoV-2 protease

Emetine supresses SARS-CoV-2 replication by inhibiting interaction of viral mRNA with eIF4E

In silico prediction of potential inhibitors for the main protease of SARS-CoV-2 using molecular docking and dynamics simulation based

Molecular docking of quinine, chloroquine and hydroxychloroquine to angiotensin converting enzyme 2 (ACE2) receptor for discovering new potential COVID-19 antidote

Special article-The impact of 2019 novel coronavirus on heart injury: A systematic review and meta-analysis. Progress in Cardiovascular Diseases

Remdesivir in coronavirus disease 2019 (COVID-19) treatment: A review of evidence

Beneficial effects of colchicine for moderate to severe COVID-19: A randomised, double-blinded, placebo-controlled clinical trial

In silico Nigellidine (N. sativa) bind to viral spike/active-sites of ACE1/2, AT1/2 to prevent COVID-19 induced vaso-tumult/vascular-damage/comorbidity

Activesite molecular docking of nigellidine with nucleocapsid-NSP2-MPro of COVID-19 and to human IL1R-IL6R and strong antioxidant role of Nigella-sativa in experimental rats

Reduced mortality in COVID-19 patients treated with colchicine: Results from a retrospective, observational study

Antiviral drug design based on the opening mechanism of spike glycoprotein in SARS-CoV-2

Proactive antiinflammatory therapy with colchicine in the treatment of advanced stages of new coronavirus infection. The first results of the COLORIT study

The long story of camptothecin: From traditional medicine to drugs

Structure-based drug designing for potential antiviral activity of selected natural products from Ayurveda against SARS-CoV-2 spike glycoprotein and its cellular receptor

In silico investigation on the inhibiting role of nicotine/caffeine by blocking the s protein of sars-cov-2 versus ace2 receptor. Microorganisms

Repurposing of FDA approved alkaloids as COVID 19 inhibitors; in silico studies

Natural products as sources of new drugs over the nearly four decades from 01/1981 to 09/2019

Alkaloids and flavonoids from African phytochemicals as potential inhibitors of SARS-Cov-2 RNA-dependent RNA polymerase: An in silico perspective

Phytochemical and pharmacological review of Cryptolepis sanguinolenta (Lindl.) Schlechter

Structure of papain-like protease from SARS-CoV-2 and its complexes with non-covalent inhibitors

Natural RNA dependent RNA polymerase inhibitors: Molecular docking studies of some biologically active alkaloids of Argemone mexicana

Respuesta inflamatoria en relaci on con COVID-19 y otros fenotipos protomb oticos

Broad spectrum anti-coronavirus activity of a series of anti-malaria quinoline analogues

In vitro evaluation of antiviral activity of single and combined repurposable drugs against SARS-CoV-2

Unravelling high-affinity binding compounds towards transmembrane protease serine 2 enzyme in treating SARS-CoV-2 infection using molecular modelling and docking studies

Computational view toward the inhibition of SARS-CoV-2 spike glycoprotein and the 3CL protease

Virtual screening-driven drug discovery of SARS-CoV2 enzyme inhibitors targeting viral attachment, replication, post-translational modification and host immunity evasion infection mechanisms

Virtual screening of natural products against type II transmembrane serine protease (TMPRSS2), the priming agent of coronavirus 2 (SARS-CoV-2)

A multi-targeting drug design strategy for identifying potent anti-SARS-CoV-2 inhibitors

The therapeutic potential of colchicine in the complications of COVID-19. Could the immunometabolic properties of an old and cheap drug help? Metabolism Open

Possible beneficial actions of caffeine in sars-cov-2

In vitro and in vivo models for studying SARS-CoV-2, the etiological agent responsible for COVID-19 pandemic

A case control study to evaluate the impact of colchicine on patients admitted to the hospital with moderate to severe COVID-19 infection

Association between treatment with colchicine and improved survival in a single-Centre cohort of adult hospitalised patients with COVID-19 pneumonia and acute respiratory distress syndrome

Biologically active isoquinoline alkaloids covering

Computational guided drug repurposing for targeting 2 0 -O-ribose methyltransferase of SARS-CoV-2

High-throughput screening and identification of potent broadspectrum inhibitors of coronaviruses

Farmacognosia: do produto natural ao medicamento

Plant-derived chemicals as potential inhibitors of SARS-CoV-2 main protease (6LU7), a virtual screening study

Structural insights on the interaction potential of natural leads against major protein targets of SARS-CoV-2: Molecular modelling, docking and dynamic simulation studies

World Health Organization declares global emergency: A review of the 2019 novel coronavirus (COVID-19)

The role of C-reactive protein as a prognostic marker in COVID-19

Challenges for drug repurposing in the COVID-19 pandemic era. Frontiers in Pharmacology

COVID-19-In silico structure prediction and molecular docking studies with doxycycline and quinine

The biological and clinical significance of emerging SARS-CoV-2 variants

Colchicine for community-treated patients with COVID-19 (COLCORONA): A phase 3, randomised, double-blinded, adaptive, placebo-controlled, multicentre trial

PRISMA extension for scoping reviews (PRISMA-ScR): Checklist and explanation

Coronavirus biology and replication: Implications for SARS-CoV-2

Berberine and obatoclax inhibit SARS-CoV-2 replication in primary human nasal epithelial cells in vitro

Repurposing drugs for treatment of COVID-19

Anti-SARS-CoV natural products with the potential to inhibit SARS-CoV-2 (COVID-19)

Risk factors associated with acute respiratory distress syndrome and death in patients with coronavirus disease

Molecular interaction and inhibition of SARS-CoV-2 binding to the ACE2 receptor

Emetine inhibits Zika and Ebola virus infections through two molecular mechanisms: Inhibiting viral replication and decreasing viral entry

The naturally-derived alkaloids as a potential treatment for COVID-19: A scoping review