Aims, Concerns and Current Interest in Series 4

Current Series Aims

1. Lead optimisation, to improve solubility and metabolic stability while maintaining potency.
   
2. Validation of PfATP4 activity as mechanism of action via experimental and computational means.
   
3. To source more chemical and biological inputs from the widest possible community.
   

Notable Points about The Series

1. Compounds in this series have been identified down to 16 nM potency.
2. Seems to have good in vitro HLM and hHEP stability: CL_int < 8.1 is compatible with 10 nM potency.
3. RLM remains stubbornly high, particularly for the more potent analogues translating to short half-lives in rat PK. It's possible that rat metabolism may not be a good model for human metabolism for this series.
4. Series appears to have little polypharmacology or cytotoxicity.
5. There is strong correlation between activity vs. Pfal and activity in Kiaran Kirk’s ion regulation assay, implying the mechanism of action is inhibition of PfATP4.

Concerns about The Series

1. Although dofetilide binding looks weak or nil, the series has shown activity in a patch clamp assay at Essen (1-10 μM) which is quite potent though with a window of >100 fold over Pfal potency.
2. In Kip Guy’s resistant mutants the picture is mixed, but there is still support for the idea that some members of the series are weaker in the resistant strains. The series has no or weak >>1 μM activity against gametocytes, no activity against Winzeler’s Pb liver stage and may have weak activity against ookinetes but the dose-response data has not been completed.

Project Strands of Current Interest

The biggest issue is metabolic stability, as measured in rat in particular. There are few toxicity concerns. Thus possible future directions:

1. Small scale changes around the side chains, particularly phenethyl to attempt to balance solubility, potency and metabolism. Other possibilities: a) N is tolerated in the pendant rings, but hasn’t been explored much recently. b) Is 3,4-diF the best substitution pattern? c) Some evidence (eg MMV669848) that the phenethyl side chain can be rigidified, perhaps the iso-indoline of that compound could be improved on with other ring systems and by more optimal substitution of the aromatic benzene ring of the isoindoline. d) The amide MMV670944 is interesting and shows good RLM stability, but many other amides failed to match its potency.
2. Incorporation of a basic centre to increase volume as a potential fix for half-life. However, this might come at the expense of plasma concentration so would require high potency. Of the 29 compounds with a basic centre only one (MMV670437, below) has a measured potency < 100 nM (actually 44 nM).

3. More significant structural changes. Of the changes made to the basic skeleton, the most successful might be the recent evaluation of the substitution position changes (e.g., MMV670945), possibly in combination with modifying the disposition of the N atoms in the core (though the triazolopyrazine has shown the best data).