Roadmap for development of therapeutics for helminths

What are we trying to achieve and why? What is the problem we are trying to solve?

High throughput screening of existing compound libraries to identify candidate compounds with anthelmintic activity

What are the scientific and technological challenges (knowledge gaps needing to be addressed)?

• Having robust and relevant parasite screening assays with a high probability of identifying potential lead compounds
• Depending of the mechanism of action of a given compound, different types of assays will be more or less likely to discover potential lead compounds
• Motility assays are frequently used for screening compounds for anthelmintic activity because this is a phenotype that is easily measured in a high throughput format. However, many compounds may not demonstrate activity with short-term motility assays.
• Some drugs may require a competent immune system for activity, and these will likely not yield a positive hit in an in vitro assay; for example ivermectin does not have significant in vitro activity against filarial nematodes but is highly effective in vivo. Similarly, motility-based assays fail to reliably detect resistance to ivermectin (and other macrocyclic lactone anthelmintics) across multiple species
• All current in vitro assays used for screening compounds utilize non-parasitic (free-living) stages, which are not the stages in the animal that are targeted by the drug. Thus, some compounds may not kill the stage of parasite used in the assays even though it might have activity against the parasitic relevant stage. Such compounds will thus be overlooked. Consequently, developing in vitro or ex vivo culture systems that can maintain adult nematodes for extended periods of time could facilitate their use as a target in screening assays.
• Non drug phytochemical compounds with alternative mode of action may not demonstrate activity using traditional screening assays. Many of these types of compounds require long-term in vivo exposure to demonstrate activity, and many in vitro assays may not proceed long enough to demonstrate
  the activity.

What approaches could/should be taken to address the research question?

• Multiple phenotypes in addition to motility assays should be utilized, such as larval development assays
• For filarial worms, in vivo screening may be necessary. To minimize the number of in vivo tests, compound hits using other assays could be tested against filarial worms in an invitro assay and if the activity is significantly less than in the original screen then the compound could be tested against filarial worms in an in vivo model.
• For phytochemical compounds, new novel in vitro assays may need to be developed that can better replicate the ruminant digestive system.

What else needs to be done before we can solve this need?

• Presently high throughput assays are expensive to set up; consequently a large investment is required in both
  equipment and personnel. This restricts such screening to large pharmaceutical companies and large centralized
  laboratories with significant funding. Consequently, developing less expensive technologies could facilitate a
  more decentralized system, and therefore the screening of more compounds in a variety of assay systems.
• Making compound libraries more easily available for laboratories to access
• Having a centralized publically accessible database system will facilitate compound screening and anthelmintic discovery by smaller more specialized laboratories.

Existing knowledge including successes and failures

New image analysis software has made the development of inexpensive systems achievable and several are now available and practical for smaller scale laboratories.