Opportunities and Challenges in Antiparasitic Drug Discovery

Parasitic diseases continue to impose a significant burden on human health, especially in tropical regions. The primary contributors to this burden are PROTOZOA and HELMINTHS, as shown in Table 1. The existing drugs used to treat these diseases are far from ideal, with many being introduced several decades ago. Issues related to frequently used drugs are highlighted in Table 1. As many authors have emphasized, market incentives alone are inadequate to promote the discovery and development of new drugs for these diseases. Of the more than 1,300 new drugs introduced for all indications between (insert years), only 13 were focused on TROPICAL DISEASES, including those listed in Table 11. In (insert year), merely 0.1% of global investment in health research was dedicated to drug discovery for selected tropical diseases (malaria, leishmaniasis, and trypanosomiasis) and tuberculosis, which collectively account for approximately 5% of the global disease burden2,3.

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Recent developments have invigorated efforts in antiparasitic drug discovery. These include publicly-funded genome sequencing projects for several parasites listed in Tables 1 and 2, along with the establishment of new public-private partnerships (PPPs) focused specifically on tropical diseases4,5. These initiatives counteract, at least in part, the exodus of large pharmaceutical firms from involvement in antiparasitic drug discovery. Notably, increased funding from organizations like the Bill and Melinda Gates Foundation is making a significant difference. Prominent PPPs engaged in antiparasitic drug discovery include the Medicines for Malaria Venture (MMV), the Drugs for Neglected Diseases initiative (DNDi), and the Institute for One World Health (IOWH)4. These collaborations meld a pharmaceutical industry approach to drug discovery with the disease-targeted knowledge from public healthcare organizations.

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In light of these opportunities, we address the challenges related to discovering new antiparasitic drugs, which entails the efforts that lead to a defined drug development candidate (Fig. 1). Drug discovery is an iterative process that generally includes several discrete stages: target identification and validation; assay development; screening (either whole cell or target-based) to identify hits (Box 1); procurement or synthesis and assessment of analogues to develop structure-activity relationships (SAR) and identify leads; iterative medicinal chemistry to optimize leads; and preclinical development before clinical evaluation. We describe various strategies for antiparasitic drug discovery, highlight the potential of high-throughput screening (HTS) on new molecular targets, and underscore the importance of lead optimization. Additionally, we note the contributions that different types of partnerships can make to the discovery process.

Drug discovery involves an iterative process comprising discrete stages. It often begins with basic exploratory biology and biochemistry to identify molecular targets. In other instances, compounds are tested for their activity against whole parasites without prior knowledge of the target. Compounds (actual or potential inhibitors) can be assessed for their activity against the target, if known, and against the entire parasite. Inhibitors of the target are frequently utilized to validate the target. Compounds displaying activity against the whole parasite are classified as hits (see Box 1), which can then be subjected to further testing in animal models of the disease. Other evaluations that monitor the pharmacokinetic properties of the compounds commence at this juncture. Compounds active in animal models and deemed 'druggable' are designated as leads (see Box 1). Lead compounds typically necessitate optimization for efficacy and favorable pharmaceutical properties. The crucial role of medicinal chemistry in both lead identification and the more time-consuming iterative process of lead optimization is noteworthy. Early pharmacokinetic assessments are also emphasized in this diagram. Optimizing pharmaceutical properties (absorption, distribution, metabolism, and excretion (ADME)), ensuring that no significant drug toxicity is present, and maintaining efficacy against the target organism are essential. Once a compound reaches the stage where it might be considered for testing in human patients, it is termed a drug candidate. From that point, it proceeds into preclinical and then clinical trials as part of a standard drug development pathway. Adapted from Ref. 4.

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Challenges in Antiparasitic Drug Discovery

Drug discovery for parasitic diseases is not inherently more expensive or technically complex than that for other indications. Typically, preclinical models for infectious (including parasitic) diseases tend to be more predictive, and clinical trials less complicated and costly compared to non-infectious, chronic disorders. It has been estimated that the cost of bringing a new antimalarial drug to the market is approximately US$300 million, contrasted with a minimum average of US$500 million for new drugs across all indications. The risk of failure in Phase II clinical trials for a new antimalarial is estimated to be 50%, notably lower than for a non-infectious disease4,6.

Antiparasitic drug discovery is not primarily motivated by the commercial need to introduce novel compounds. Historically, many antiparasite drugs were initially developed for other medical purposes. This opportunistic strategy of leveraging insights gained from research on non-parasitic indications has proven effective and offers clear cost advantages. However, this approach does not necessarily foster the introduction of chemically novel agents and may be nearing a point of diminishing returns, largely due to the widespread emergence of resistance to certain drug classes. It fails to fully utilize new insights obtained from parasite genome sequences, leading to the assertion that "the next significant challenge in tropical diseases is determining the most effective approach to transform genomic insights into new, robust chemical leads for innovative drug discovery"4.

A major challenge in this field is that various organizations with differing cultural perspectives and foundational aims must collaborate. As many large pharmaceutical companies have abandoned in-house discovery research for antiparasitics, forming effective partnerships for virtual drug discovery has emerged as a key factor in drug development for neglected diseases4,5,7. Recently, progress has been made by re-engaging the private biopharmaceutical industry in these efforts. Public funding for early drug discovery integrates into the drug development projects undertaken by industry in collaboration with public health organizations. Enhancing the involvement of researchers, public health officials, and industry leaders from disease-endemic countries remains a critical challenge. Several examples of diverse partnerships are discussed below.

Finally, for 'neglected' diseases, the focus of drug discovery is primarily field-driven, catering to the needs of disease control programs. This typically emphasizes low costs, short treatment regimens, and the ability to safely administer drugs in the absence of close medical supervision. Therefore, it is crucial to define, in partnership with those managing control programs in affected nations, the desired product profiles based on requirements for use in resource-limited settings (Table 3). Successfully optimizing lead compounds to align with these product profiles is a major limiting factor in preclinical drug development.

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Approaches to Drug Discovery

Various foundational strategies for drug discovery targeting tropical diseases can be classified into short-to-medium term approaches (leveraging existing compounds or compound classes) and long-term approaches (requiring the discovery of new chemical classes). The strategies described here are exemplified with cases, with a focus on diseases listed in Table 1, excluding malaria. They aim for the identification of pure chemical entities: the potential advantages and challenges of promoting the use of traditional medicines, particularly for malaria, are discussed elsewhere10,11.

Combinations of Existing Drugs. The use of existing drug combinations (Table 1), such as eflornithine and melarsoprol for African trypanosomiasis12, or praziquantel and oxamniquine for schistosomiasis13, presents opportunities for synergy, reduced toxicity, shorter treatment courses, and delaying the onset of resistance. The extensive use of drug combinations in malaria therapy is noteworthy8, with recent reviews outlining their rationale14. Whenever feasible, fixed-dose combinations are being formulated to enhance patient adherence, particularly those utilizing artemisinin-like compounds. Drug combinations featuring an antimalarial drug and a RESISTANCE REVERSER are also under consideration8.

New Indications for Existing Drugs. A compelling short-term strategy, this option seeks to extend the applications of drugs initially developed for different indications. This "piggy-back" methodology has demonstrated notable success; many antiparasitic drugs were initially developed for other diseases (see case histories in Table 4). For example, DB289, which was first intended for treating Pneumocystis pneumonia, is undergoing clinical trials as a potential oral treatment for malaria and early-stage African trypanosomiasis15,16. A downside to this strategy may arise from the hesitance of pharmaceutical companies to permit their products to be used in non-commercial patient populations, potentially exposing toxins that might undermine their drugs' economic viability. For instance, some companies have expressed reluctance concerning clinical trials of antifungal triazoles for treating Chagas' disease, despite demonstrated effectiveness against Trypanosoma cruzi in animal models17.

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Improvements to Known Drugs and Compound Classes. In the medium term, analogues of existing antiparasitics (Table 5) could prove effective. For malaria, novel analogues of pyrimethamine designed to counteract drug resistance are under development18, while amodiaquine analogues with potentially reduced toxicity are also being studied19. Ferroquine contains a quinoline nucleus akin to chloroquine, but with a unique ferrocenic group in its side chain and exhibits excellent activity against malaria parasites, including those resistant to chloroquine20. In the field of anthelmintics, moxidectin, an ivermectin analogue, is being pursued for the treatment of lymphatic filariasis and onchocerciasis21. This compound, licensed as a veterinary product, has contrasting pharmacokinetic properties compared to ivermectin, expected to enhance efficacy against onchocerciasis and other filarial infections.

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Focused Sample Collections. An alternative and arguably more productive approach to screening extensive libraries of compounds against whole parasites is screening focused collections. This method emphasizes identifying compounds with defined biological effects against related parasites or exhibiting biochemical activity against isoenzymes or receptors linked to established molecular targets from other organisms. This strategy is particularly crucial in the hunt for new antifilarials and schistosomicides, as the capacity for screening is limited by the availability of relevant test helminths. For instance, research supported by the United Nations Children's Fund/United Nations Development Programme/World Bank/World Health Organization Special Programme for Research and Training in Tropical Diseases (TDR) has only enabled the evaluation of around 1,000 samples annually against Onchocerca and SCHISTOSOMES. Efforts are underway to enhance the quality of test compounds by sourcing samples with established anthelmintic properties from crop protection and animal health firms.

Work involving parasite genome sequences, coupled with biochemical research, has highlighted enzymes like protein farnesyl transferases, cysteine proteases, histone deacetylases, and fatty-acyl synthases (Table 6) as potential drug targets for malaria and TRYPANOSOMATID diseases. Researchers in both academia and the pharmaceutical sector have formed compound collections centered on inhibitors targeting such enzyme classes. It is essential to seize opportunities to access and evaluate these compounds for their antiparasitic activity, as they may provide valuable new leads. A case in point is the lead-optimization project under MMV, based on protein farnesyl transferase inhibitors sourced from Yale University and a cancer chemotherapy initiative at Bristol-Myers Squibb22.

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De Novo Discovery: Whole Parasite Assays

Longer-term strategies seek to identify novel active substances unrelated to existing drugs. Within the biopharmaceutical industry, molecular target identification and HTS are predominant in most initial drug discovery efforts. For parasitic diseases, however, an alternative approach based on screening and assessing compounds against whole parasites is both valuable and viable. Screening varied compound collections against whole parasites in vitro has been gradually declining over the past two decades but is now experiencing a renaissance, driven primarily by assay advancements. This trend is particularly true for test systems involving Plasmodium falciparum, T. brucei, T. cruzi, and Leishmania species. Today, screening often employs parasites transfected with reporter genes, such as those encoding green fluorescent protein, β-lactamase, or β-galactosidase, to facilitate quick and easy detection of antiparasitic activity. Some progress has also been made in adapting protozoal screens to 384-well plates, particularly for P. falciparum.

A recent example illustrating the capability to test large numbers of compounds against whole parasites is found at the Harvard University Institute of Chemistry and Cell Biology’s Initiative for Chemical Genetics (ICCB-ICG), where whole parasites are utilized in an HTS format to assess tens of thousands of samples (see Further information). The Belgian company Tibotec, in collaboration with TDR, has also developed assays based on whole parasites in 384-well plates to analyze relatively large numbers of compounds. For instance, in one project, Tibotec screened 10,000 compounds sourced from a commercial supplier for activity against four parasites (P. falciparum, T. brucei, L. donovani, and T. cruzi) as well as a mammalian cell line (to evaluate cytotoxicity). Many active compounds were discovered and further pursued by the TDR network of drug discovery laboratories. These hits underwent more in-depth analysis, including accurate IC50 determinations against whole parasites, cytotoxicity assessments, and, for the most promising candidates, in vivo evaluations in animal models of the corresponding parasitic infection. Consequently, one compound was identified as a novel lead active against malaria. Further optimization through analogue synthesis is now required to identify a credible development candidate. These findings demonstrate the high attrition rate in lead identification and the necessity of screening extensive compound collections against whole cells for a reasonable likelihood of success.

The quality of the compound libraries being evaluated is a critical factor influencing the success rate of screening (see also below). Libraries of natural products have special advantages for parasitic diseases, alongside other infectious diseases and cancer. Natural products are appealing due to their remarkable structural diversity, particularly in anticancer screenings, as medicinal plants may serve as potential sources for novel pharmacophores23. For instance, artemisinins, known antimalarial agents, were initially extracted from a traditional Chinese medicine24. Other natural products currently being explored in focused drug discovery initiatives include manzamines25, chalcones26, and borrelidins27, all exhibiting antimalarial actions in animal models. In contrast, large-scale screening of natural products against other parasites has been less extensively pursued. The potential (and associated challenges) of this approach is illustrated by the case of PX-, a glycosylated saponin that exhibited antileishmanial activity during a screen of approximately 10,000 plant extracts, but which development was halted due to toxicity concerns28. This instance exemplifies the hurdles in pursuing natural products—these substances are often chemically intricate, limiting opportunities for rapidly exploring SAR through synthesis or procurement of analogues. The transformation of original natural products into metabolically robust, orally bioavailable drugs can be an exceptionally challenging task with protracted timelines. Furthermore, many natural products serve as biological defense mechanisms, making cytotoxicity a recurrent issue. Numerous plant products are produced at specific times during the growing cycle and in diverse parts of the organism, leading to difficulties in sourcing adequate materials for study. Nonetheless, because of the incredible diversity of plant species and the significant reliance on herbal remedies in tropical and subtropical disease-endemic regions, investigating natural products as antiparasitics remains an enticing possibility.

De Novo Discovery: Molecular Targets and HTS

Although HTS against molecular targets has become the favored approach for early drug discovery in much of the biopharmaceutical industry, it has only recently begun to gain traction in the quest for new treatments for neglected parasitic diseases. This strategy is expected to grow in significance as the genomic sequences of relevant parasites become accessible and as HTS facilities and compound libraries become more readily available to research teams in academia.

Target Identification and Validation. Access to parasite genome sequences presents exciting opportunities for drug discovery based on identifying and validating new molecular drug targets. The number of compounds (in the hundreds of thousands) that can be screened during a typical HTS campaign based on a molecular target greatly exceeds the throughput achievable using assays based on whole-cell parasite viability. This discrepancy is especially pronounced for helminths, where the throughput of whole-parasite assays is at least tenfold lower than for protozoa. Therefore, the identification and validation of molecular targets for helminths represents a vital and valuable strategy for de novo drug discovery.

However, it is essential not to overestimate the number of suitable drug targets that parasite genomes might encode. The Plasmodium genome contains roughly 5,000 genes, of which Yeh et al.29 estimated that about 200 (4%) could potentially encode drug targets identified through a computational algorithm that spotted enzymes facilitating 'chokepoint' reactions (those uniquely consuming a specific substrate or generating a specific product). Among these, approximately 30 exhibit no significant similarity to any human enzyme. This compares favorably with Hopkins and Groom's estimate30, which suggested that fewer than 1,500 (5%) of the 30,000 genes within the human genome represent viable drug targets based on quite distinct criteria.

Experience gleaned from genome-driven discovery of new antibacterials indicates that enthusiasm for this approach should be tempered with a realistic understanding of its limitations. None of the 18 new antibacterials currently in clinical trials were derived through a genomics initiative31,32. Numerous potential targets have been identified and explored, yet the bottleneck in forming new antibacterials clearly lies beyond simply characterizing compounds with activity against new targets. The real challenge is the conversion of those compounds into drug candidates that are optimized not just for activity, but also for desirable pharmaceutical and physicochemical characteristics33,34,35.

Ideally, targets chosen for a screening campaign should be genetically and/or chemically validated, biochemically and structurally characterized, amenable to selective inhibition without risk of the parasite developing resistance, and technically adaptable for screening a large volume of compounds (Box 2). Specific parasite-centric points must be acknowledged. Firstly, parasite species distinct from human pathogens are frequently employed to validate hits and identify leads (Table 7). For example, P. berghei is widely utilized in animal models of malaria. This necessitates consideration of homologous targets across different parasite species, as outlined in the following section. Secondly, the development of resistance (as illustrated by dihydrofolate reductase) is well documented, so the potential for parasites to develop resistance against new chemical leads should be explored early in the development process. Lastly, kinetic factors can significantly influence target suitability; for instance, the successful inhibition of trypanosomal ornithine decarboxylase may stem from the differing turnover rates of human and parasite enzymes, resulting in the parasite being incapable of regenerating the enzyme swiftly enough to survive its irreversible blockade36. This example underscores the need for comprehensive knowledge of parasite biochemistry when selecting targets.

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Potential parasite drug targets are being validated using chemical (Table 6) or genetic methods (e.g., gene-expression profiling upon drug treatment, RNA interference (RNAi), or genetic knockout techniques) for most of the protozoa listed in Table 1. Approximately 20 potential targets with known inhibitors have been identified for P. falciparum29, and fewer exist for other parasites (Table 6). Conducting in vitro culture and manipulating helminths found in Table 1 proves more technically demanding than for protozoa. The free-living NEMATODE Caenorhabditis elegans serves as a recommended model organism37, especially in conjunction with RNAi methodology, for systematically identifying new targets in Onchocerca and Brugia species38.

HTS and Compound Libraries. Examples of the limited number of HTS campaigns targeting parasite enzymes include lactate dehydrogenase, peptide deformylase, glyceraldehyde-3-phosphate dehydrogenase, enoyl-ACP reductase (Fab I), and trypanothione reductase. Not all these campaigns have yielded actionable hits, and several targets have since been deemphasized—for instance, lactate dehydrogenase (Table 6). As more molecular targets emerge from genomics programs, the implementation of HTS campaigns is expected to grow. The selection of compound collections for screening is critical. If the target relates to one already pursued by pharmaceutical companies for other medical conditions—such as protein kinases involved in cancer—it would be prudent to carry out the campaign using a small, focused collection of compounds (perhaps 500-1,000) based around the chemical scaffolds recognized for producing inhibitors of such protein classes. However, for targets not yet thoroughly studied, it becomes necessary to resort to screening extensive, diverse compound or natural product collections, often exceeding 100,000 samples. The rise of commercial suppliers providing large compound collections permits academic institutions to organize screening campaigns using either focused or diversified collections. In all scenarios, chemical libraries must undergo stringent triaging to ensure 'drug-likeness' and eliminate compounds likely to be generally toxic, mutagenic, highly reactive, unstable, or resistant to chemical modification. Surprisingly, a significant number of non-drug-like molecules still infiltrate many libraries, owing to the challenge of formulating general criteria capable of detecting and removing them without discarding numerous useful compounds. For instance, MICHAEL ACCEPTORS commonly found may be susceptible to nucleophilic attack, rendering them non-specific in their biological actions—but some notable exceptions exist, such as the vinylsulphones. At least one member of this potent cysteine protease inhibitor series has demonstrated good target specificity and is being developed by IOWH for treating Chagas' disease39 (Table 6). Furthermore, due to their widespread use as synthetic intermediates, many libraries also include aromatic or heterocyclic nitro compounds. Such compounds are undesirable for screening against parasites, as they often exhibit activity, particularly against protozoa, owing to bio-reduction and the formation of reactive free radicals. However, these properties are typically linked to mutagenicity, making it reasonable to exclude such compounds from screening libraries.

The establishment of chemical libraries has been significantly advanced by new technologies related to combinatorial and parallel synthesis40. These libraries may comprise proprietary or non-proprietary compounds. Companies may seek patents or have existing patents for proprietary compounds within their libraries. This factor is significant, as several pharmaceutical companies are currently permitting academia-driven HTS campaigns to be conducted against parasite proteins using their sample collections. Issues can arise when the chemical structures of hits require disclosure for follow-up study if they belong to commercially sensitive chemical series. Such complications are generally absent when compounds in the library were procured from commercial suppliers and deemed non-proprietary. However, running patent searches on compounds considered for lead-optimization programs remains advisable to garner insights into the chemical class and any claims that could impede future commercial development.

Transitioning from Hits to Leads to Drug Candidates

The progression from 'hit' to 'lead' to 'drug candidate' (Fig. 1; Box 1) adheres to the same general pattern as for other drug discoveries in antiparasitic contexts. Compounds are selected based on enhanced efficacy and pharmaceutical properties through studies of analogues and iterative medicinal chemistry. Knowledge of the target molecule's structure, if accessible, proves advantageous in guiding medicinal chemistry endeavors18. Notably, a benefit for discovering new antiparasitic drugs is having well-established, highly predictive in vitro and in vivo assays for activity, which frequently utilize the same parasitic organism infecting the human host. Nonetheless, protocols lack standardization, making result comparisons across different laboratories challenging. Techniques continuously evolve and adapt to achieve higher throughput (e.g., employing reporter genes to enable fluorescence-based assays instead of microscopy). A recent review has addressed the models employed for antimalarial drug discovery, recommending a streamlined evaluation process for new compounds41. Table 7 enumerates commonly utilized in vitro and in vivo models in antiparasitic disease discovery.

While the parasitic strains used in laboratory assays often closely resemble those infecting human patients, notable differences can exist. The standard animal models employed for malaria infections involve P. berghei, P. chabaudi, P. yoelii, or (to a lesser extent) P. vinckei rather than the human-infecting Plasmodium species. Similarly, the T. brucei brucei parasites utilized in initial evaluations for African trypanosomiasis differ from the T. b. rhodesiense and T. b. gambiense subspecies responsible for human disease. Additionally, Onchocerca gutturosa worms used as in vitro models for onchocerciasis are cow parasites rather than human ones. These discrepancies can become particularly significant when adopting a molecular target-based drug discovery strategy; for instance, the cysteine proteases of P. vinckei (vinckepains) differ from those found in P. falciparum (falcipains), necessitating the expression and investigation of both types of proteases in a falcipain-centered effort42. Introducing genetically modified parasites with the relevant pathogen target replacing the model target gene could serve as one potential solution to this issue.

A key consideration when selecting suitable animal models is the desired product profile (Table 3). Most diseases necessitate testing across various animal models. Primary models typically represent the acute form of the disease, whereas secondary, more complex assays reflect the chronic or drug-resistant stages targeted for treatment. For example, compounds would not qualify as useful leads for late-stage African trypanosomiasis or chronic Chagas' disease unless they exhibited efficacy in the corresponding secondary infection model (Table 7). Primary models also serve to filter compounds before progressing to time-consuming and costly chronic tests, which typically involve tracking infected animals for six weeks or longer.

Helminth models collectively present unique challenges. Conventional in vitro assays evaluating antischistosome activity lack standardization, and their predictive capability concerning animal model activity remains unclear. For many in vitro helminth screens, readout relies on tracking parasite motility, though dye reduction may be used as a secondary measure to assess worm viability. While reasonable overall correlation exists between motility and dye reduction readouts, this consistency is not complete. Generating genetically modified C. elegans could elevate throughput during early discovery phases. Moreover, for Onchocerca infections, the primary animal model concentrates on microfilariae rather than adult worms, yet drug efficacy against adult forms (macrofilariae) is required. Time-consuming secondary assays are necessary to identify quality lead compounds for onchocerciasis or lymphatic filariasis.

Throughout the animal studies, the influence of formulation on compound activity warrants consideration. To maximize the odds of success (i.e., achieving at least some level of activity), compounds are usually tested initially in water-based formulations comprising 10% dimethylsulfoxide (DMSO). This can significantly affect oral bioavailability, particularly for water-insoluble compounds. DMSO-containing formulations are generally not suitable for assessing toxicity, making it imperative to reevaluate active compounds in vehicles devoid of DMSO, such as 'standard suspending vehicle' (SSV)43.

Significance of Lead Optimization. As delineated in Fig. 1, lead optimization is an iterative endeavor in which medicinal chemistry directs the design and synthesis of new compounds, which are then scrutinized for enhanced properties. As the cycle described in Fig. 1 progresses, costs often rise, and timelines extend, with the lead-optimization phase being crucial in constraining the overall drug discovery process. This stage fundamentally constitutes an exercise in problem-solving where considerations of bioavailability, metabolism, and toxicity converge to shape a robust drug candidate. The involvement of the pharmaceutical industry becomes critical during this phase. Historically, lead optimization has been the most overlooked and underfunded component of the antiparasitic drug discovery pipeline over the past two decades. However, recent advents in PPPs have markedly improved this scenario by re-engaging the pharmaceutical industry and providing essential funding.

An illustrative example of the critical role of lead optimization in creating a drug candidate is the antimalarial synthetic peroxide OZ277 (RBx-), which has now proceeded into clinical trials44. Under the TDR 'malperox' initiative45, meetings were convened for TDR-funded biologists and chemists to discuss pivotal issues surrounding the optimization of artemisinin-like compounds. More than 1,000 semi-synthetic and synthetic artemisinins from at least seven different laboratories were evaluated. Biologists provided vital biological data back to chemists to support the optimization process. A particularly promising project focused on a group of synthetic peroxides. Subsequently, under the auspices of a newly established PPP, MMV, a dynamic 'virtual' discovery team linking US-based chemists, Swiss parasitologists, and Australian pharmacokinetic experts was formed. An optimized drug candidate was selected in (insert year) and transferred to an Indian pharmaceutical company (Ranbaxy) for scaled-up production aimed at producing material now undergoing clinical trials44.

This scenario emphasizes how public funding allocated for a diverse array of early-stage discovery initiatives culminated in the selection of impactful lead compounds for further development through a PPP. This collaborative effort successfully moved the lead compound into drug candidate status by leveraging funding capabilities and uniting essential expertise, particularly from the pharmaceutical sector.

Partnerships for Drug Discovery

The contribution of PPPs to drug development for neglected diseases has been thoroughly examined4,5,46,47. They have achieved encouraging outcomes in advancing drug candidates to clinical trials, while stimulating discussions on the potential contributions of the industry to this process48. Nevertheless, drug discovery presents higher levels of risk compared to development, leading the PPPs outlined in Table 6 to adopt portfolio approaches that typically prioritize development. Basic research and early discovery efforts are mainly funded through public resources. Early discovery initiatives often entail collaborations among various public institutions and public-private agreements. Below, we present several examples of collaborative early discovery research backed by public funding, and in some cases, bolstered by in-kind industry contributions.

Sequencing parasite genomes necessitated the establishment of international genome networks (Table 2). To translate the resultant knowledge into new therapeutic options, TDR, via its Pathogenesis and Applied Genomics group, is supporting the development of regional networks in South America, Asia, and Africa aimed at enhancing training in bioinformatics and its applications to parasite genome research. This includes potential drug target identification. Through its Genomics and Discovery Research group, TDR can advance potential targets followed by validation studies and assay development. With validated targets and appropriate assay formats, principal investigators can seek collaborations with partners having expertise in HTS and access to compound collections. These partners can be from academia, as seen at ICCB-ICG Harvard mentioned earlier, or from institutions such as the Walter & Eliza Hall Institute of Medical Research in Melbourne, Australia. Alternatively, industrial partners may be involved, as demonstrated through the ongoing collaborations between TDR and Serono, facilitating scientists from two disease-endemic countries to screen enzyme targets from Plasmodium and other parasites. Various other collaborative networks aim to connect suppliers of natural products with academic laboratories capable of assessing their antiparasitic efficacy (e.g., the Multilateral Initiative on Malaria (MIM) and the University of Mississippi websites referred to in Further information). The Kitasato Institute in Japan has screened thousands of natural products from its inventory27, in addition to around 30,000 synthetic compounds supplied by Japanese pharmaceutical firms for antimalarial activity, under a broad collaboration involving WHO/TDR, the Japan Ministry of Health & Welfare, and 14 Japanese drug manufacturers.

Such collaborations unite specialized groups with disparate expertise in different facets of drug discovery for a specific objective. Formal agreements should outline project objectives and delineate each stakeholder's rights and responsibilities; such arrangements often include clauses for preferential pricing of any resultant products for developing nations, along with provisions detailing the protection and availability of intellectual property rights4. Successful projects resulting from these collaborative early drug discovery efforts can transition into candidates for support from PPPs, given their experience and focus on managing projects through targeted development teams, tailored to advance discovery projects to fruition.

Outlook

Although the introduction of new drugs targeting tropical diseases significantly lagged between (insert years), a flurry of activity has unfolded since (insert year), leading to the development or emergence of over 20 new agents for parasitic diseases49 (Table 6). This surge of activity is primarily attributed to a renewed spirit of partnership among the public and private sectors, with industry and public health interests becoming increasingly aligned. The establishment of public-private agreements, especially the PPP model of project management, along with a rise in philanthropic funding, has contributed to a greater number of antiparasitic drug candidates entering clinical trials over the past five years. While many of these candidates may be combinations or drugs already utilized for alternative indications, the current discovery project pipeline is significantly more robust compared to a decade prior. The historically diminished participation of the pharmaceutical sector is being offset, at least in part, by its involvement in PPPs, individual agreements concerning specific projects, and the advent of industry-backed research institutes with designated drug discovery mandates targeting ailments such as malaria, tuberculosis, and dengue4.

For sustained success, PPPs must prioritize the identification of new compounds for further development. Concerns exist that most of the easily accessible "low-hanging fruit" have been harvested, necessitating stronger early-stage discovery research in both public and private domains, supported by financial backing from public sources. The efficacy of the PPP model should not obscure the necessity of maintaining a thriving backdrop of discovery-oriented research, which predominantly relies on public funding. Relevant publicly funded early-stage research encompasses parasite genetics, biochemistry, molecular target identification and validation, HTS targeting these candidates, screening compounds against whole parasites, and chemistries advancing hits to lead compounds. Once promising lead compounds are identified, attracting new funding sources becomes more feasible—for instance from PPPs, which could provide the essential financial resources and drug development expertise necessary to transform leads into drug candidates (as exemplified by the OZ compound discussed earlier44).

Disease-endemic nations are increasingly becoming pivotal in the discovery of new drugs. Certain countries, such as Brazil, China, India, and South Korea, possess drug manufacturing industries and research institutions engaged in drug discovery. Other nations may support research institutes with expertise in fields like natural product chemistry but presently lack well-established pharmaceutical sectors capable of advancing compounds from discovery through the entire drug development pathway depicted in Fig. 1. Growing awareness (see, for instance, Ref. 50) indicates that countries most affected by these diseases should take an active role in addressing challenges, including research aimed at developing improved therapeutic strategies. The ability of a country to respond to disease threats closely correlates to its research capacity51. Support from both private and public sectors is on the rise: for example, the Gates Foundation recently granted $20 million to science academies in Nigeria, South Africa, and Uganda, and the Department for International Development in Britain plans to enhance its spending on research and development in Africa. TDR has emphasized research capability enhancement in developing countries for 30 years, making it a core aspect of its mission. Many leaders in tropical disease research now originate from disease-endemic regions and have benefitted from TDR's support during their early career stages. This core of expertise located within disease-endemic countries requires encouragement and more active engagement not only in initial drug discovery projects but also in more advanced drug discovery and development initiatives bolstered by PPPs.

Box 1 | Criteria for Antiparasite Hits, Leads, and Drug Candidates

Hit

At the start of a screening campaign, the compound must be:

• Active in vitro against whole protozoa with IC50 of <1 µg per ml* (for protozoa), or inhibiting motility of helminths in vitro to at least 75% at 10 µg per ml*

• Selective (at least tenfold more active against parasites than against a mammalian cell line, such as MRC-5*)

Lead

Initially, the compound must be:

• Active in vivo against parasites at a dosage of < 100 mg per kg*

• Not overtly toxic in animals at the efficacious dose

• Active in vitro against relevant parasite types (for example, drug-resistant variants—refer to product profiles)

• Chemically tractable (analogues can be obtained)

As the campaign proceeds, criteria will become stricter. A candidate for lead optimization (see text) should be:

• Active in vitro with activity nearing that of established drugs

• Active in vivo against parasites in the relevant small animal model (for example, chronic or late-stage illness), administered via an appropriate route (preferably oral) in an acceptable formulation at a reasonable dose (< 100 mg per kg)*

• Exhibit good selectivity when assessed against multiple mammalian cell lines

Drug Development Candidate

A compound that has emerged from a lead optimization process (see text) and appears likely to satisfy at least the essential criteria set forth in the desired product profile (Table 3). It should:

• Be active in vivo with efficacy comparable to or exceeding that of established drugs in the most relevant animal models

• Effectively target the desired range of parasites (such as drug-resistant strains and various species)

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• Pass early toxicity/mutagenicity criteria (such as the Ames Test)

• Possess an acceptable metabolic profile in vitro and in vivo (preferably without significant species differences)

• Exhibit an acceptable pharmacokinetic profile

• Be amenable to cost-effective scale-up

• Ideally, possess a well-understood mode of action

Clinical Development Candidate

A drug development candidate that has fulfilled additional criteria in pharmacology studies, pharmacokinetics/absorption, distribution, metabolism and excretion, mutagenicity and toxicity evaluations, formulation development, production scale-up, and cost of goods, and similar considerations.

*Illustrative values only, based on experience from the screening network laboratories mentioned in the acknowledgments. Actual values can vary depending on the specific parasite, assay, and compound type under investigation.

Box 2 | Points to Consider in Selecting Parasite Molecular Targets

Selectivity

• Is the target absent from mammals; or:

• Does the target exhibit any molecular or pharmacological attributes that set it apart from related mammalian proteins?

• Are any related mammalian proteins present in both humans and the animal species used for in vivo efficacy, toxicity, and pharmacokinetic evaluations?

Validation

• Is there supporting evidence (from RNA interference, gene knockouts, inhibitors, etc.) that the target is vital for growth, survival, or reproduction?

• Are proteins with comparable properties also found in any model parasite species utilized?

• Is the target expressed in a parasite lifecycle stage suitable for drug intervention?

Potential for Resistance Development

• Absence of isoforms of the target within a species

• Absence of biochemical 'bypass' reactions or transport mechanisms that could mitigate the inhibition of the target

Biochemical Properties

• If the target protein is an enzyme within a multi-step pathway, is it rate-limiting to the extent that its inhibition can reduce flow through the pathway to non-viable levels?

• Will the turnover rate of the target permit inhibition over reasonable time frames?

Structure and 'Druggability'

• Is the amino acid sequence of the target known?

• Is there a crystal or NMR structure of related proteins available or obtainable, preferably with known binding cofactors, inhibitors, or agonists/antagonists?

• Does the target have a small molecule ligand-binding pocket?

• Are there precedents for the target type, indicating existing drugs or ligands?

'Assayability'

Key characteristics include:

• Is expression precedent available?

• Does existing biochemistry/enzymology support the target?

• Is there a single subunit where feasible?

• Are there specific and inexpensive readouts that can be predicted, particularly optical ones compatible with high-throughput screening?

• Is the active-site chemistry well characterized?

Other desirable features include:

• Is a focused chemical library already available for the target class?

• Are there cell-based assays?

• Are there assays featuring functional endpoints?

• Do the assays involve minimal steps (e.g., washes)?