Twenty Five Years of Drug Discovery at the University of California Targeting Kinetoplastid Parasites

This review summarizes 25 years of screening compounds against three major kinetoplastid parasites, Trypanosoma brucei, Trypanosoma cruzi, and Leishmania spp. The work was carried out at two University of California campuses by a consortium of scientists. The history of this effort is summarized beginning with DARPA and NIAID TDRU projects. The compound collections that were screened came from both academic and industry sources. To facilitate screening, high throughput or high content microtiter plate-based assays were developed. Three approaches to discovery of new drugs for kinetoplastid diseases are presented. These include structure-based drug design against specific parasite molecular targets, repurposing of already approved drugs, and screening of marine natural products. As similar screening efforts against other molecular targets or with other compound libraries are ongoing, one conclusion is that the current bottleneck in drug development for neglected tropical diseases is downstream of compound screening and hit to lead. More medicinal chemistry efforts for lead optimization and more preclinical package work needs to be done. For more information see (www. cdipd.org). Abstract ISSN: 2348-9782


Results
A summary of the approximate number of compounds screened, and how far they progressed, is given in Figures 1, 2, 3 and 4. A more comprehensive review of all the compound series tested can be found in the over 70 publications that resulted from this work as listed in References .

Notable molecular targets
Both phenotypic screens (against the parasites themselves) as well as screens against specific molecular targets were carried out. Phenotypic screens have the advantage of identifying hits directly against the relevant parasite stage [6][7][8]. Target-based screens have more capacity and are more focused. That is they can use inhibitor libraries that might have been produced and validated against homologous enzyme or receptor targets. The molecular targets screened against at the two UC campuses were by no means comprehensive, but rather reflected the research interests of collaborators and whether they had been shown to be "druggable". "Druggable" targets are herein defined as proteins (usually enzymes) which have homologues (usually human) successfully targeted by approved drugs in current clinical use. Two notable molecular targets in Figures 1, 2, 3 and 4 are proteases [29,38,48,66] and sterol biosynthesis enzymes [60,61] both of which represent drug target families for which multiple drugs are in clinical use. In some cases compounds were de-prioritized for PK/PD reasons but helped identify new targets like cytochrome b [67,68].
From phenotypic screens the most advanced hit was an oxyborole from Anacor Pharmaceuticals that is now in clinical trials for HAT. SCYX-7158 was identified as a promising hit by Zachary Mackey in the original HTS assay [6] and subsequently optimized at Scynexis for blood-brain barrier penetration [49].

Conclusion
References Other promising phenotypic screen hits included compounds from Collaborative Drug Design [65] and the Memorial Sloan Kettering Cancer Center (manuscript in preparation).
One potentially promising and cost effective approach to identifying drugs for NTDs is "repurposing" drugs already approved and in clinical use targeting other diseases (ncats.nih/preclinical/repurpose

Repurposed drugs
One of the most promising sources of potential drugs is marine natural products provided by several laboratories. Natural products are the origin of 60% of drugs approved by the FDA. Most of the antibiotics now used for bacterial infections are natural products. But industry has until recently abandoned natural product development for reasons of complex synthesis challenges and scale-up issues. These issues have been addressed with new technology in HPLC, mass spectrometry, and genome sequencing. As Figures 1,  2, 3 and 4 indicate, several marine natural products are still being evaluated and represent some of the most effective "hits" to date [51,53]. In some cases the parasite molecular target of a specific marine natural product is now known [58,59]. Figures show numbers of compounds tested and how far they progressed for each of the three kinetoplastids assayed.

Marine Natural Products
Double bars indicate that a program was stopped because of lack of efficacy at that stage of screening or issues with ADME, toxicity, or pharmacokinetics.
Arrow heads indicate programs still in progress. When available and relevant, Reference numbers are given in brackets [ ]