The discovery of novel synthetic compounds with drug-like properties is an ongoing challenge in medicinal chemistry. Natural products have inspired the synthesis of compounds for pharmaceutical application, most of which are based on N -heterocyclic motifs. Among these, the pyrrole ring is one of the most explored heterocycles in drug discovery programs for several therapeutic areas, confirmed by the high number of pyrrole-based drugs reaching the market. In the present review, we focused on pyrrole and its hetero-fused derivatives with anticancer, antimicrobial, and antiviral activities, reported in the literature between 2015 and 2019, for which a specific target was identified, being responsible for their biological activity. It emerges that the powerful pharmaceutical and pharmacological features provided by the pyrrole nucleus as pharmacophore unit of many drugs are still recognized by medicinal chemists.

The five-membered pyrrolidine ring is one of the nitrogen heterocycles used widely by medicinal chemists to obtain compounds for the treatment of human diseases. The great interest in this saturated scaffold is enhanced by (1) the possibility to efficiently explore the pharmacophore space due to sp3-hybridization, (2) the contribution to the stereochemistry of the molecule, (3) and the increased three-dimensional (3D) coverage due to the non-planarity of the ring—a phenomenon called “pseudorotation”. In this review, we report bioactive molecules with target selectivity characterized by the pyrrolidine ring and its derivatives, including pyrrolizines, pyrrolidine-2-one, pyrrolidine-2,5-diones and prolinol described in the literature from 2015 to date. After a comparison of the physicochemical parameters of pyrrolidine with the parent aromatic pyrrole and cyclopentane, we investigate the influence of steric factors on biological activity, also describing the structure–activity relationship (SAR) of the studied compounds. To aid the reader’s approach to reading the manuscript, we have planned the review on the basis of the synthetic strategies used: (1) ring construction from different cyclic or acyclic precursors, reporting the synthesis and the reaction conditions, or (2) functionalization of preformed pyrrolidine rings, e.g., proline derivatives. Since one of the most significant features of the pyrrolidine ring is the stereogenicity of carbons, we highlight how the different stereoisomers and the spatial orientation of substituents can lead to a different biological profile of drug candidates, due to the different binding mode to enantioselective proteins. We believe that this work can guide medicinal chemists to the best approach in the design of new pyrrolidine compounds with different biological profiles.

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Heterocyclic rings are recognized as key components of many natural, semi-synthetic and synthetic molecules with a broad spectrum of biological activities. Among these molecules, the indole and imidazo [2,1-b][1,3,4]thiadiazole systems have recently been described as useful scaffolds for the design of anticancer agents. Herein the antitumor activity of a series of 3-(6-phenylimidazo[2,1-b][1,3,4]thiadiazol-2-yl)-1H-indoles, designed as hybrid structures, was assessed. Seven out of 10 compounds (1a-g) were submitted to National Cancer Institute (NCI). Remarkably, compound 1g showed antiproliferative activity against the full panel of sixty human cancer lines, with half-maximal inhibitory concentration of between 1.67 and 10.3 μM. Further studies showed antiproliferative activity of 1a-g and of three additional compounds 1h, 1i and 1l, with different substituents on the indole nucleus and phenyl ring, against three pancreatic cancer cell lines. In particular, derivatives 1g and 1h inhibited both proliferation and migration of SUIT-2 cells at concentrations lower than 10 μM. In conclusion, new indole derivatives are characterized by in vitro antitumor activity, supporting future mechanistic studies.