Lead Optimization of a Pyrazole Sulfonamide Series ofTrypanosoma bruceiN-Myristoyltransferase Inhibitors: Identification and Evaluation of CNS Penetrant Compounds as Potential Treatments for Stage 2 Human African Trypanosomiasis

Abstract

Trypanosoma bruceiN-myristoyltransferase (TbNMT) is an attractive therapeutic target for the treatment of human African trypanosomiasis (HAT). From previous studies, we identified pyrazole sulfonamide, DDD85646 (1), a potent inhibitor of TbNMT. Although this compound represents an excellent lead, poor central nervous system (CNS) exposure restricts its use to the hemolymphatic form (stage 1) of the disease. With a clear clinical need for new drug treatments for HAT that address both the hemolymphatic and CNS stages of the disease, a chemistry campaign was initiated to address the shortfalls of this series. This paper describes modifications to the pyrazole sulfonamides which markedly improved blood–brain barrier permeability, achieved by reducing polar surface area and capping the sulfonamide. Moreover, replacing the core aromatic with a flexible linker significantly improved selectivity. This led to the discovery of DDD100097 (40) which demonstrated partial efficacy in a stage 2 (CNS) mouse model of HAT.

Development of Small‐Molecule Trypanosoma brucei N‐Myristoyltransferase Inhibitors: Discovery and Optimisation of a Novel Binding Mode

“…The selectivity ratio of T. brucei cell versus hNMT enzymep otencyw as used as ap rediction for the in vivo therapeutic window in the pyrazole sulfonamide series, and formed the basis of ranking and selecting ab ack-up series for lead optimisation (Table 3). [10] Hit-to-lead discussion Investigation of simple halogen and methyl group substitution around the core benzomorpholinone scaffold indicated substitution with ah alogen (Cl or Br), as in 36 and 37,i st olerated at the 7-position ( Table 3). The 8-bromoa nalogue 38 showeds imilar potencyt o7 -bromo compound 37.T his implies that the 7-or 8-positions are suitable vectors for extension of 23 toward the Cterminus, in agreement with the X-ray crystal structure ( Figure 6).…”

“…In addition to the pyrazole sulfonamide series previously reported, [8][9][10] severalo ther hits were identified. These were also investigated to determine if they offer opportunities to develop selective and blood-brain barrier penetrant TbNMT inhibitors.…”

“…Work was initially carried out withoutX -ray co-crystal structures;w hen this structurali nformation was established using the Leishmania major enzyme, key early-stage molecules were also co-crystallised with the enzyme. As we discussed in ap revious publication, [10] the L. major NMT shows high sequence homologyt ob oth the T. brucei and humanN MTs. This has been an excellent system to improvet he activity of inhibitors, but given the similarities and lack of high-resolution structures of the T. brucei enzyme, it has much less use in predictings electivities.…”

“…However,t his observed selectivity was lost upon removal of the (R 2 -phenyl)-3-chloro substituent, seen with 9,a lthough this is partly due to the overall diminished potencyo bserved for this compound. Substitution at the ortho (11)o rmeta (12)p ositions of the R 2 phenyl group appeared to be preferred over para substitution (10)w hich may be due to ac lash of this substituent with the side chain of Tyr217.…”

“…[10] Relative selectivities calculated as (hNMT IC 50 )/(T. brucei cell assay EC 50 )w ere used to comparec ompounds. [10] Whilst it is unknown if inhibition of mammalian NMT is dose limiting or whether the compounds have off-target effects, this "selectivity" value turned out to be better at predicting the "therapeutic ratio" in mousem odelso fi nfection than as imple ratio of cellular activities. For example, 44 was found to be > 10-fold more potent in the parasite growth assay than the other five extended compounds.…”