Diketopiperazine-Based, Flexible Tadalafil Analogues: Synthesis, Crystal Structures and Biological Activity Profile.

Adam Mieczkowski,Krzysztof Woźniak,Krzysztof Drabikowski,Tomasz Przygodzki,Cezary Watala,Patrycja Wińska,Elżbieta Speina,Igor Zhukov,Damian Trzybiński,Maria Winiewska-Szajewska,Ewelina M Borsuk,Lidia Stanczyk,Małgorzata Podsiadła-Białoskórska

doi:10.3390/molecules26040794

Abstract

Phosphodiesterase 5 (PDE5) is one of the most extensively studied phosphodiesterases that is highly specific for cyclic-GMP hydrolysis. PDE5 became a target for drug development based on its efficacy for treatment of erectile dysfunction. In the present study, we synthesized four novel analogues of the phosphodiesterase type 5 (PDE5) inhibitor—tadalafil, which differs in (i) ligand flexibility (rigid structure of tadalafil vs. conformational flexibility of newly synthesized compounds), (ii) stereochemistry associated with applied amino acid building blocks, and (iii) substitution with bromine atom in the piperonyl moiety. For both the intermediate and final compounds as well as for the parent molecule, we have established the crystal structures and performed a detailed analysis of their structural features. The initial screening of the cytotoxic effect on 16 different human cancer and non-cancer derived cell lines revealed that in most cases, the parent compound exhibited a stronger cytotoxic effect than new derivatives, except for two cell lines: HEK 293T (derived from a normal embryonic kidney, that expresses a mutant version of SV40 large T antigen) and MCF7 (breast adenocarcinoma). Two independent studies on the inhibition of PDE5 activity, based on both pure enzyme assay and modulation of the release of nitric oxide from platelets under the influence of tadalafil and its analogues revealed that, unlike a reference compound that showed strong PDE5 inhibitory activity, the newly obtained compounds did not have a noticeable effect on PDE5 activity in the range of concentrations tested. Finally, we performed an investigation of the toxicological effect of synthesized compounds on Caenorhabditis elegans in the highest applied concentration of 6a,b and 7a,b (160 μM) and did not find any effect that would suggest disturbance to the life cycle of Caenorhabditis elegans. The lack of toxicity observed in Caenorhabditis elegans and enhanced, strengthened selectivity and activity toward the MCF7 cell line made 7a,b good leading structures for further structure activity optimization and makes 7a,b a reasonable starting point for the search of new, selective cytotoxic agents.

Highlights

Phosphodiesterase type 5 (PDE5) called the cyclic guanosine monophosphate (cGMP)-specific, cGMP-binding PDE, is the major cGMP-hydrolyzing PDE in platelets, lung, and the vascular smooth muscle of the penile corpus cavernosum
It was revealed that compound 4 kills the parasite via the inhibition of plasmodial PDE activity. These results prove that the cyclic adenosine monophosphate (cAMP)/cGMP pathways could be desirable therapeutic targets against Plasmodium falciparum and antiplasmodial derivatives could act as inhibitors of the hydrolysis of cyclic nucleotides of the parasite
In the most cases, newly synthesized compounds exhibited a weaker effect than tadalafil, except for bromine-substituted compounds 7a and 7b, which showed a stronger cytotoxic effect the parent compound on HEK 293T and MC F7 cell lines

Summary

Introduction

Phosphodiesterase type 5 (PDE5) called the cGMP-specific, cGMP-binding PDE, is the major cGMP-hydrolyzing PDE in platelets, lung, and the vascular smooth muscle of the penile corpus cavernosum. This ubiquitous enzyme decreases 3 ,5 -cyclic guanosine monophosphate (cGMP) or 3 ,5 -cyclic adenosine monophosphate (cAMP) in target cells by catalyzing the hydrolysis of these second messengers. A recent review [19] indicated the importance and the potential of tadalafil as an anticancer agent, which exhibited cytotoxic effects and the reduction of tumor growth in collateral [20,21], HNSCC [22] and thyroid [23] cancers, as well as improved survival in a murine model of brain lymphoma with rituximab treatment [24]. Kassel reported that tadalafil can improve clinical outcome of advanced melanoma patients by enhancing anti-tumor immunity and suggested its potential application in combined melanoma immunotherapy [25]

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Results

Conclusion