Identification of Bis-Cyclic Guanidines as Antiplasmodial Compounds from Positional Scanning Mixture-Based Libraries

Abstract

The screening of more than 30 million compounds derived from 81 small molecule libraries built on 81 distinct scaffolds identified pyrrolidine bis-cyclic guanidine library (TPI-1955) to be one of the most active and selective antiplasmodial libraries. The screening of the positional scanning library TPI-1955 arranged on four sets of sublibraries (26 + 26 + 26 + 40), totaling 120 samples for testing provided information about the most important groups of each variable position in the TPI-1955 library containing 738,192 unique compounds. The parallel synthesis of the individual compounds derived from the deconvolution of the positional scanning library led to the identification of active selective antiplasmodial pyrrolidine bis-cyclic guanidines.

Keywords: antiplasmodial; combinatorial chemistry; guanidines; heterocyclic peptidomimetics; malaria; parallel synthesis; solid-phase synthesis.

Figure 1

Screening of scaffold ranking library for antiplasmodial activity.

Figure 2

Deconvolution of positional scanning pyrrolidine bis-cyclic guanidine library TPI-1955 and synthesis of individual compounds TPI-2359.

Scheme 1

Synthesis of pyrrolidine bis-cyclic guanidines from resin-bound acylated tetrapeptides.

Figure 3

Identification of active groups at positions R¹–R⁴ in the compound mixture.

Figure 4

Structures and activity of identified top antiplasmodial hits.

Figure 5

Stage-specific inhibition of Plasmodium growth by TPI 2359-47. Tightly synchronized parasites were treated at 6-h post-invasion of merozoites with 5 × EC₅₀ concentration of the chemotype. (A) Microscopic evaluation of Giemsa-stained thin smears in the left. Histogram plot of YOYO-1 labeled cells following treatment of the compound were shown in the right. (B) Distribution of developmental stages at different time intervals following treatment of the compound at 6 h postinvasion. Three replicates of 1000 parasitized erythrocytes were counted to determine stage-specific distribution. DHA, dihydroartemisinin.

Figure 6

Stage-specific inhibition of Plasmodium growth by TPI 2359-47. (A) Microscopic evaluation of Giemsa-stained thin smears and flow cytometric analysis of the YOYO-1-labeled cells to measure DNA content when the culture was exposed to the inhibitor (5 × EC₅₀) at 30 h postinvasion. (B) Distribution of different stages when the culture was treated at 30 h postinvasion. (C,D) Effect following exposure of the parasite to the compound at 42 h postinvasion. Three replicates of 1000 parasitized erythrocytes were counted to determine stage-specific distribution. DHA, dihydroartemisinin.