Combating Acanthamoeba spp. cysts: what are the options?
© The Author(s). 2018
Received: 19 September 2017
Accepted: 5 December 2017
Published: 9 January 2018
Acanthamoeba spp. are protist pathogens and causative agents of serious infections including keratitis and granulomatous amoebic encephalitis. Its ability to convert into dormant and highly resistant cysts form limits effectiveness of available therapeutic agents and presents a pivotal challenge for drug development. During the cyst stage, Acanthamoeba is protected by the presence of hardy cyst walls, comprised primarily of carbohydrates and cyst-specific proteins, hence synthesis inhibition and/or degradation of cyst walls is of major interest. This review focuses on targeting of Acanthamoeba cysts by identifying viable therapeutic targets.
Targeting the cyst wall
Nanoparticles and antiacanthamoebic drugs
The recent decline in development of newer antimicrobial agents has contributed to increased disease burden as well as mortality rate and drug resistance . As Acanthamoeba infections are considered rare, the development of antiacanthamoebic agents faces an even worse scenario when compared to other communicable diseases. The discovery of various antiacanthamoebic compounds tested in vitro has not been able to enter mainstream drug development due to their limited in vivo potential as well as general inefficacy against the cyst stage. A comprehensive review by our team  identifies and presents targets and mechanism of action of numerous drugs against Acanthamoeba although their in vivo efficacy remains undetermined. A few cases of re-purposing drugs are also reported to work well against Acanthamoeba, including some antibacterial agents: polymyxin B [21, 22], cefazolin , meropenem , moxifloxacin ; antifungal compounds: amphotericin B , azoles [27, 28]; and antineoplastic agents: alkylphosphocholines [29, 30]. Our studies also highlighted the use of re-purposed drugs against Acanthamoeba . These included amiodarone, amlodipine, apomorphine, digoxin, haloperidol, loperamide, prochlorperazine, and procyclidine. Re-purposing of drugs has the advantage of an early start for new applications as they have been approved clinically for other diseases. Finding new sources of antiacanthamoebic drugs has also led several groups including ours to research natural products, and various plant extracts have been tested to exhibit cytotoxicity against Acanthamoeba [32–35].
Targeted antiacanthamoebic therapies
Targeting vehicles such as antibodies for a specific antigen, peptides enabling cells penetration and crossing barriers such as the blood-brain barrier (BBB), parasite specific biosynthetic pathways enzymes inhibitors, and mannose-conjugated antimicrobial agents, are important strategies for the targeted delivery of chemotherapeutic agents against Acanthamoeba infections. These avenues should provide tremendous advantages over non-specific therapies due to minimal side effects and host cells cytotoxicity. The targeted chemotherapy against Acanthamoeba has received limited attention, and only a few reports are published making it an open field for further research. For example, monoclonal IgA antibodies were shown to protect against AK, while the mode of protection was the inhibition of adhesion of Acanthamoeba to the corneal epithelium . Drugs conjugated with parasite-specific adhesins can be of value in specific delivery of compounds. For example, mannose-binding protein on the surface of Acanthamoeba is an efficient target for delivery of mannose-conjugated drugs. A porphyrin molecule conjugated with mannose was used successfully in our study for the development of photodynamic therapy (PDT) against Acanthamoeba . PDT is another promising technology which can be used to target resistant pathogens. The mode of PDT action is based on activation of photosensitizing compound with light of an appropriate wavelength to generate singlet oxygen and/or reactive oxygen species (ROS) which are known to induce cell death in the target pathogen. Targeted PDT is expected show high specificity towards the target cells and minimally toxic effects due to it not binding to host cells. However, such discrimination is sometimes difficult to achieve, hence the selective targeting of pathogenic Acanthamoeba remains a major concern. Additionally, more effective photosensitizers which can generate a burst of toxins at shorter pulsed interval of light will add practicality in their clinical applications. Hence, further work is needed to conjugate Acanthamoeba antibodies with more effective photoactivated compounds and/or drugs in the development of better antiacanthamoebic strategies. Another challenge in the successful treatment of amoebal brain infection is the inefficacy of drugs to cross the BBB to reach the site of infection and target the parasite. Cheng et al.  developed transactivator of transcription (TAT) peptide-modified gold nanoparticles, which are demonstrated to be capable of crossing the BBB and efficiently delivering drugs to brain tumour tissues . These suggested strategies have a strong rationale, and if utilized smartly, could yield a breakthrough in treatment of infections due to Acanthamoeba as well as against other CNS pathogens. However, along with drug development, drugs administration also plays a significant role in the effectiveness of their performance, hence there is a desperate need to revisit the overall drug designing protocol against Acanthamoeba infections for efficient and more practical options.
In summary, Acanthamoeba infections are complex biological disorders associated with very high rate of morbidity and mortality which require therapeutic strategies that recognize and respond to their dynamic nature. The molecular target-inspired approaches such as those presented above, in our opinion represent appealing frontiers for research and need to be evaluated further to develop more effective therapeutic options against Acanthamoeba as a whole and in particular against its cyst stage.
We are grateful to Professor Ed Jarroll, City University New York, USA, and Dr Sutherland Maciver, University of Edinburgh, UK for helpful discussions and constructive comments.
Availability of data and materials
RS proposed the concept. AA searched the literature and prepared the first draft of the manuscript under the supervision of NK. RS and NK corrected the manuscript. All authors read and approved the final manuscript.
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