molecules Review Influenza Neuraminidase Inhibitors: Synthetic Approaches, Derivatives and Biological Activity Pedro Laborda, Su-Yan Wang and Josef Voglmeir * Glycomics and Glycan Bioengineering Research Center, College of Food Science and Technology, Nanjing Agricultural University, 1 Weigang, Nanjing 210095, China; [email protected] (P.L.); [email protected] (S.-Y.W.) * Correspondence: [email protected]; Tel.: +86-25-8439-9512; Fax: +86-25-8439-9553 Academic Editor: Vito Ferro Received: 17 October 2016; Accepted: 3 November 2016; Published: 11 November 2016 Abstract: Despite being a common viral disease, influenza has very negative consequences, causing the death of around half a million people each year. A neuraminidase located on the surface of the virus plays an important role in viral reproduction by contributing to the release of viruses from infected host cells. The treatment of influenza is mainly based on the administration of neuraminidase inhibitors. The neuraminidase inhibitors zanamivir, laninamivir, oseltamivir and peramivir have been commercialized and have been demonstrated to be potent influenza viral neuraminidase inhibitors against most influenza strains. In order to create more potent neuraminidase inhibitors and fight against the surge in resistance resulting from naturally-occurring mutations, these anti-influenza drugs have been used as templates for the development of new neuraminidase inhibitors through structure-activity relationship studies. Here, we review the synthetic routes to these commercial drugs, the modifications which have been performed on these structures and the effects of these modifications on their inhibitory activity. Keywords: influenza treatment; neuraminidase inhibitors; organic synthesis; total synthesis; sialic acid analogues 1. Introduction Influenza is a serious viral illness which can lead to hospitalization and death, especially in the elderly [1–3]. Influenza spreads around the world in yearly outbreaks, resulting in about three to five million cases of severe illness and approximately 250,000 to 500,000 deaths [3,4]. Newly mutated forms of the flu virus appear every year and some of them show high levels of resistance to the standard antiviral drugs [5–7]. Furthermore, during the last century four influenza pandemics have occurred: “Spanish influenza” in 1918, “Asian influenza” in 1958, “Hong Kong influenza” in 1968, and “avian influenza” in 2004 [8]. Influenza is an RNA virus and is subdivided into three genera: influenza A, influenza B and influenza C. Influenza A is the most virulent virus and has provoked the devastating pandemics of the past. Influenza A can be divided into different serotypes based on the antibody response to these viruses. The most famous serotypes are H1N1, which caused the “Spanish influenza” and the 2009 pandemic, H2N2, which caused the “Asian influenza”, H3N2, which caused the “Hong Kong influenza”, and H5N1, which caused the “avian influenza”. Influenza type B and C are much less common than influenza A. The two glycoproteins found on the influenza virus surface envelope are hemagglutinin and neuraminidase (EC 3.2.1.18) (Figure1)[ 9–12]. Hemagglutinin is responsible for viral attachment to the cell surface receptor, which is a terminal sialic acid (N-acetylneuraminic acid, Neu5Ac) residue usually linked to a galactose in a α-(2,3) or α-(2,6) glyosidic linkage [9]. The functional neuraminidase, on the other hand, is anchored to the viral membrane by a hydrophobic sequence near the N-terminus [13–17]. Molecules 2016, 21, 1513; doi:10.3390/molecules21111513 www.mdpi.com/journal/molecules Molecules 2016, 21, 1513 2 of 40 Molecules 2016, 21, 1513 2 of 40 [13–17].It has been It has shown been in shown tissue culturein tissue that culture neuraminidase that neuraminidase activity is required activity tois required destroy viral to destroy receptors viral by receptorsremoving by the removing sialic acid the of the sialic hemagglutinin-sialic acid of the hemagg acidlutinin-sialic linkage, thereby acid linkag contributinge, thereby to thecontributing release of progenyto the release viruses of progeny from infected viruse cellss from [18 infected–20]. cells [18–20]. Figure 1. Influenza virus replication pathway and the effect of neuraminidase inhibitors. (A) The Figure 1. Influenza virus replication pathway and the effect of neuraminidase inhibitors. (A) The virus virus is linked to the host cell via hemagglutinin employing the terminal Neu5Ac residue of the surface is linked to the host cell via hemagglutinin employing the terminal Neu5Ac residue of the surface receptor. Then, Then, the the endocyto endocytosissis of ofthe the virus virus occurs; occurs; (B) Viral (B) Viral RNA RNA is released is released into the into cytoplasm the cytoplasm where whereis transferred is transferred to the host to the cell host nucleus; cell nucleus; (C) Viral (C )RNA Viral replication RNA replication and mRNA and mRNA synthesis synthesis occurs occursinside theinside nucleus. the nucleus. The RNA The is RNA then isre thenleased released into the into cytoplasm; the cytoplasm; (D) Viral (D )proteins Viral proteins are synthetized are synthetized using mRNAusing mRNA and directed and directed to the to cell the cellmembrane membrane for forvirus virus assembly. assembly. After After incorporation incorporation of of the the genome fragments, virusvirus budding budding begins; begins; (E )(E After) After budding, budding, the virusthe virus surface surfac is attachede is attached at the Neu5Ac at the Neu5Ac receptor. receptor.The neuraminidase The neuraminidase is able to hydrolyze is able to Neu5Ac hydrolyze allowing Neu5Ac the releaseallowing of the virus;release (F )of Neuraminidase the virus; (F) Neuraminidaseactivity is inhibited activity in presence is inhibited of neuraminidasein presence of neuraminidase inhibitors preventing inhibitors the pr releaseeventing of thethe virusrelease and of thereducing virus and pathogenicity. reducing pathogenicity. Medical treatment treatment of ofinfluenza influenza is generally is generally based based on the onadministration the administration of neuraminidase of neuraminidase inhibitors [21–26].inhibitors 2,3-Didehydro-2-deoxy- [21–26]. 2,3-Didehydro-2-deoxy-N-acetylneuraminicN-acetylneuraminic acid, also calle acid,d DANA, also calledwas the DANA, first influenza was the neuraminidasefirst influenza inhibitor neuraminidase reported inhibitor (Figure reported2) [27]. While (Figure DANA2)[ 27 has]. never While been DANA commercialized, has never been its structurecommercialized, has been its used structure as a template has been for usedthe discov as aery template of inhibitors for the which discovery are both of inhibitorsmore potent which and betterare both tolerated more potent by humans. and better Of tolerated these, zanamivir, by humans. oseltamivir, Of these, zanamivir, laninamivir oseltamivir, and peramivir laninamivir have emergedand peramivir as promising have emerged long-acting as promising neuraminidase long-acting inhibitors neuraminidase for the treatment inhibitors and for prophylaxis the treatment of humanand prophylaxis influenza ofvirus human infection influenza (Figure virus 2) infection[28–35]. However, (Figure2)[ severa28–35l ].naturally However, occurring several naturallyinfluenza neuraminidaseoccurring influenza mutations, neuraminidase such as mutations,H274Y mutations such as in H274Y H1N1 mutations and H5N1 in H1N1influenza and H5N1A strains, influenza have Ademonstrated strains, have significant demonstrated resistance significant towards resistance the abov towardse drugs [5,36,37]. the above For drugs this [reason,5,36,37 ].the For development this reason, ofthe antiviral development drugs of which antiviral are drugseffective which against are effectivenew strains against of influenza new strains virus of influenzathrough the virus creation through of novelthe creation influenza of novelneuraminidase influenza inhibitors neuraminidase or by improving inhibitors the or inhibitory by improving activity the of inhibitory existing antiviral activity drugsof existing is a vibrant antiviral research drugs field is a [38]. vibrant Nowadays, research the field development [38]. Nowadays, of new theneuraminidase development inhibitors of new isneuraminidase generally based inhibitors on the is synthesis generally of based derivatives on the synthesis of the previously of derivatives mentioned of the previously commercial mentioned drugs. Here,commercial we review drugs. all Here,the synthetic we review strategies all the which synthetic have strategies been reported which for have the been production reported of for these the productioncompounds, of the these modifications compounds, perf theormed modifications on their structures, performed and on their the effects structures, of these and modification the effects of onthese their modification biological activities. on their biological activities. Molecules 2016, 21, 1513 3 of 40 Molecules 2016, 21, 1513 3 of 40 Molecules 2016, 21, 1513 3 of 40 Figure 2. Retrosynthetic analysis of the evolution of influenza neuraminidase inhibitors. Figure 2. Retrosynthetic analysis of the evolution of influenza neuraminidase inhibitors. 2. Zanamivir,2. Zanamivir,Figure Laninamivir Laninamivir 2. Retrosynthetic and and Other Otheranalysis DerivativesDerivatives of the evolution of influenza neuraminidase inhibitors. Zanamivir, commercialized under the name Relenza, received regulatory approval as a 2.Zanamivir, Zanamivir, Laninamivir commercialized and Other under Derivatives the name