Antiviral 6-amino-quinolones: Molecular basis for potency and selectivity Sara N. Richter, a Barbara Gatto, a Oriana Tabarrini, b Arnaldo Fravolini b and Manlio Palumbo a, * a Department of Pharmaceutical Sciences, University of Padova, via Marzolo 5, 35131 Padova, Italy b Department of Chemistry and Technology of Drugs, University of Perugia, 06123 Perugia, Italy Received 14 April 2005; revised 17 June 2005; accepted 23 June 2005 Available online 27 July 2005 Abstract—Structural modifications introduced in 6-amino-quinolones to increase antiviral activity can strongly affect cytotoxicity to host cells. By competition to Tat–TAR complex and binding experiments to viral and cellular DNA and RNA structures, we show that the nature of the substituent at position 7 modifies drug affinity and specificity for the nucleic acid. Interestingly, the basicity of the above substituent modulates chelation of the quinolone template to magnesium ions, which, in turn, critically affects the potency and target selectivity in the antiviral quinolone family. Ó 2005 Published by Elsevier Ltd. 1. Introduction Quinolones represent an important class of broad-spec- trum antibacterials, whose main structural feature is a 1,4 dihydro-4-oxo-piridinyl moiety bearing an essential carbonyl group at position 3. Activity of quinolones is due to the inhibition of prokaryotic type II topoisome- rases, namely DNA gyrase and, in few cases, topoiso- merase IV. 1 Quinolone derivatives have been shown to inhibit HIV-1 replication in acutely and chronically infected cells. 2–9 Recently, our group developed a new class of 6-substituted quinolones and tested their anti- bacterial and anti-HIV-1 activities. 10,11 A 6-amino- quinolone, bearing a methyl substituent at the N1 posi- tion and a 4-(2-pyridyl)-1-piperazine moiety at the C-7 position (WM5) (Fig. 1A) was found to be the most ac- tive compound in inhibiting HIV-1 replication in acutely and chronically infected cells; its mechanism of action involves inhibition of HIV-1 Tat–TAR interactions, resulting in virus life cycle arrest at the Tat-trans-activa- tion level. 12,13 Subsequent structural investigation on a number of analogues of this lead compound permitted us to draw basic structure–activity relationships (SAR); 14 stringent requirements are a small moiety such as methyl or cyclopropyl at position N1, an amino group at C6 and a hydrogen atom at C8. In contrast, po- sition 7 tolerates a variety of substituents, which resulted in the best way to modulate the anti-HIV activity of these quinolone derivatives. However, among the compounds that exhibited promi- nent activity compared to the parent drug WM5, quino- lone WC12 (28c in 14 ) displayed counterbalancing cytotoxicity in CEM cells with dramatic loss of selectiv- ity. This 6-amino-quinolone exhibits a cyclopropyl func- tion at position 1 and a 3-trifluoromethyl-phenyl- piperazine at position 7 (Fig. 1A). To gain insight into the molecular basis for selectivity displayed by 6-amino-quinolones, we tested the ability of the above compound to interfere with Tat–TAR interaction, recognize specific sequences of nucleic acids, among which TAR RNA structure, and form complexes with magnesium ions. The results were compared to those obtained using the parent potent drug WM5. Interestingly, magnesium binding appears to play a crit- ical role in determining the potency and target selectivity of the test drugs. 2. WC12 inhibits Tat–TAR complex formation It has been shown recently that the lead of the 6- amino-quinolone series, WM5, is able to inhibit HIV-1 0960-894X/$ - see front matter Ó 2005 Published by Elsevier Ltd. doi:10.1016/j.bmcl.2005.06.074 Keywords: Quinolones; Cytotoxicity; Selectivity index; Magnesium binding. * Corresponding author. Tel./fax: +39 049 8275711; e-mail: manlio.palumbo@unipd.it Bioorganic & Medicinal Chemistry Letters 15 (2005) 4247–4251