**5. Conclusion**

killing of *C. elegans* in the presence of 1300 bioactive extracts produced by endophytic fungi associated with medicinal plants [54]. The screen identified 36 extracts that promoted the survival of the infected worms, while 4 extracts were found to inhibit *P. aeruginosa* growth using a disc diffusion assay. Given that these extracts contain a mixture of metabolites, the specific compound against *P. aeruginosa* remains to be determined. Nevertheless, this study illustrates the rich reservoir of small molecules in natural symbiotic organisms with anti-

The *C. elegans* infection model was also used to screen for compounds that prolonged host survival following infection with the human pathogenic fungus *Candida albicans*. [55]. Given that most compounds that have antifungal activity are also toxic to the human host, highthroughput methods can greatly increase the likelihood of discovering specific antifungal inhibitors. From a screen of 1266 compounds with known pharmaceutical activities, 15 small molecules were identified that increased survival of *C. albicans*-infected nematodes and in‐ hibited *in vivo* filamentation of *C. albicans*, a mechanism of pathogenesis seen during mam‐ malian infection. Two compounds, caffeic acid phenethyl ester (CAPE), a natural component of honeybee propolis, and the fluoroquinolone agent enoxacin, were further shown to exhibit antifungal activity in a mouse model, validating the use of a *C. elegans* model for potential targets in a mammalian system. Interestingly, CAPE is known to inhibit the mammalian transcription factor NF-κΒ and to induce immunomodulatory effects in mice [56, 57]. Since *C. elegans* does not express a NF-κΒ homolog, it may be the case that

An automated high-throughput screen using the COPAS Biosort was also applied to *C. albi‐ cans* infection of *C. elegans* to assess a library of 3,228 compounds consisting of 1948 bioactive compounds and 1280 small molecules derived from diversity-oriented synthesis [58]. In to‐ tal, 19 compounds were identified that increased *C. elegans* survival in response to *C. albicans* infection, 7 of which are currently used antifungal agents. Several immunosuppressant agents identified in this screen, including ascomycin, cyclosporin A, and FK-506, were previ‐ ously found to exhibit weak antifungal activity against *Cryptococcus* and *Aspergillus*, in addi‐ tion to *C. albicans* [59, 60]. Other hits were predicted to affect an array of biological activities,

**Table 2** Small molecule screens using C. *elegans* as host model for infection

CAPE affects alternative targets to achieve antifungal activity.

*Discovery of novel antifungal agents*

bacterial activity.

168 Drug Discovery

Chemical library screens are a potent and valuable molecular tool for HTS identification of potential inhibitors of infectious disease. The long-standing paradigm to treat pathogen in‐ fection with small molecules that specifically target pathogen growth or metabolism has led to our current dilemma of microbial drug resistance and re-emergence of once-contained in‐ fectious diseases. Thus, new approaches to target pathogen virulence or host response fac‐ tors rather than essential pathogen functions have become increasingly more attractive strategies that are less likely to induce microbial resistance. Some compounds, such as the FDA-approved anti-psychotic, pimozide, exhibited inhibitory properties against infection by several pathogens, suggesting that small molecules can potentially be developed as broadspectrum anti-infectives. Although the molecular mechanism of inhibition by small mole‐ cules remains unknown in most cases, it may be possible to make an educated guess if targeted pathogens share a common virulence strategy, such as the Type III secretion system in Gram-negative bacteria. In other cases, identification of an inhibitor can lead to a molecu‐ lar understanding of the infection mechanism. For example, the small molecule, tachyplegi‐ nA, was found to post-translationally modify TgMLC1, a myosin light chain component, to drive host cell penetration by the parasite *T. gondii* [17].

From the various studies detailed in this review, it is apparent that the library screens repre‐ sent a first step on the road of drug discovery. There has been a growing realization that fundamental discovery of biological mechanisms oftentimes reaches a 'valley of death', in which potential translation avenues into clinical therapies and diagnostics for disease treat‐ ment comes to a standstill and is lost. NIH is addressing this widening gap between basic and clinical research with the establishment of Clinical and Translational Science Centers across the country. The research community will have to remain pro-active to move promis‐ ing leads from the initial screen stage into downstream validation and development modes in a timely manner. As with any drug development strategy, there still remain multiple technical challenges that need to be overcome before small molecule inhibitors can success‐ fully transition into the clinic. Researchers will need to assess such parameters as compound toxicity, pharmacokinetics and pharmacodynamics, and validation in animal models. How‐ ever, FDA-approved small molecule libraries can be applied to HTS as a cost-effective meth‐ od to identify existing licensed drugs for repurposing from diseases unrelated to microbial infection. Furthermore, the development of the *C. elegans* whole organism model for small molecule screening provides a novel methodology to simultaneously assess compound tox‐ icity and host response to pathogen infection. It would be informative to determine whether small molecules identified from conventional host cell culture studies can also inhibit patho‐ gen infection in the *C. elegans* model. Future anti-infective treatments will most likely be comprised of combination therapies that produce additive or synergistic effects to target key processes in both the pathogen and the host. The overall promise of discovering novel antiinfective compounds has generated great hope in the biomedical community for discovery of new countermeasures against infectious disease.

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