Structure–Activity Relationship Studies of a Novel Class of Transmission Blocking Antimalarials Targeting Male Gametes

Abstract

Malaria is still a leading cause of mortality among children in the developing world, and despite the immense progress made in reducing the global burden, further efforts are needed if eradication is to be achieved. In this context, targeting transmission is widely recognized as a necessary intervention towards that goal. After carrying out a screen to discover new transmission-blocking agents, herein we report our medicinal chemistry efforts to study the potential of the most robust hit, DDD01035881, as a male-gamete targeted compound. We reveal key structural features for the activity of this series and identify analogues with greater potency and improved metabolic 2 stability. We believe this study lays the groundwork for further development of this series as a transmission blocking agent. 3 130±30 6 128±10 34 > 1000 35 785±48 36 > 25000 37 > 10000 38 196±19 39 83±13 40 364±47 41 79±18 42 > 25000 43 > 25000 44 > 25000 45 > 25000 a Biological evaluation was carried out using the DGFA under the specific "add-in" format. Compounds were incubated at variable concentrations with fresh gametocytes, and their ability to impair exflagellation of male gametes determined. Values below 1 µM are expressed as the mean±SEM obtained from between three and eight biological replicates, each carried out in at least duplicate. All synthesised analogues resulted in a significant loss of activity, with the exception of fluorinated derivatives 38-41 (Table 2). Compounds 39 and 41 even show a slight improvement in potency over 6. Once again, these results suggest the binding site of this series within the target(s) to be reasonably confined. Indeed, in contrast to other substituents, fluorine is very similar in size to hydrogen, which could explain why analogues 38-41 retain activity. It should be highlighted that fluorine is widely used within drug discovery to improve permeability by increasing lipophilicity and to address pharmacokinetic issues such as oxidative metabolism, 21 the latter being interesting in the context of this work (vide infra).

Plasmodium falciparumprotein Pfs16 is a target for transmission-blocking antimalarial drug development

“…To facilitate identification of potential cellular target(s) via photoaffinity labelling we first derivatised DDD01035881 to incorporate both a photo-activatable group and clickable alkyne moiety onto the N-4HCS scaffold (see Supplementary Information for chemistry). Despite a number of designs, the tolerance of the N-4HCS scaffold to large changes in structure was found to be limited ( Table S1 ), consistent with our previous medicinal chemistry study 15 . Ultimately, a strategy to incorporate an alkyne handle and aryl azide moiety separately on the molecule was developed, leading to the synthesis of probe 2 .…”

“…The N-4HCS scaffold was recently identified in a high-throughput screen for transmission blocking antimalarials 14 , with subsequent development to improve its activity by medicinal chemistry 15 . We sought to determine a potential mode of action and cellular target for the N-4HCS scaffold, starting out from the original hit DDD01035881 , given its potency for inhibiting Plasmodium falciparum parasite microgametogenesis, the process of male gamete formation from blood-circulating mature male gametocytes 14 .…”

“…Here, we have validated the cellular effects of a potent transmission blocking compound series based on an N-4HCS scaffold and identified the Plasmodium falciparum 16 kDa protein, Pfs16, as a highly promising cellular target. The hit compound series was first identified in a cell-based high throughput screen 14 , with hits further optimised by medicinal chemistry 15 . The N-4HCS activity profile fits with the Medicines for Malaria Venture target candidate profile 5 (TCP5) that covers transmission blocking interventions 43 .…”

“…Within the library, a series of compounds with an N-((4-hydroxychroman-4-yl)methyl)-sulfonamide (N-4HCS) scaffold were found to potently inhibit formation of Plasmodium male gametes from mature stage V male gametocytes. The most potent hit from this scaffold series, DDD01035881 , has since been extensively studied by structure activity relationship, yielding hits with half maximal inhibitory (IC 50 ) concentrations in the nanomolar range 15 . Importantly, DDD01035881 and derivatives were also shown to have minimal impact on viability of HepG2 mammalian cells, suggesting they have low toxicity 15 .…”

Natural Products: A Potential Source of Malaria Transmission Blocking Drugs?

“…The use of drugs to target the parasite and thereby prevent transmission is therefore an enticing new possibility as add-on to current standard practice. Moreover, the low number of sexual stage parasites marks them for targeting and their non-proliferative nature could decrease the probability of development of resistance to transmission-blocking drugs [19], a fact compromising the use of all antimalarial chemotherapeuticals targeting asexual stages.…”