Brief Articles Osteoadsorptive Bisphosphonate Derivatives of Fluoroquinolone Antibacterials Pa´ l Herczegh, ² Thomas B. Buxton, ‡ James C. McPherson III, ‡ AÄ rpa´d Kova´ cs-Kulyassa, ²,⊥ Phyllis D. Brewer, ‡ Ferenc Sztaricskai, ² Gary G. Stroebel, § Kent M. Plowman, ‡ Dan Farcasiu, | and John F. Hartmann* ,| Research Group for Antibiotics, Hungarian Academy of Sciences, University of Debrecen, Hungary, Department of Clinical Investigation, Eisenhower Army Medical Center, Fort Gordon, Georgia, Augusta State University, Augusta, Georgia, and ElizaNor Biopharmaceuticals, Inc., Princeton Junction, New Jersey Received November 19, 2001 Bisphosphonates conjugated to fluoroquinolone antibacterials through an intermediate carbon had better activity than conjugates lacking the carbon. Virtually all molar-based activity of these esterified bisphosphonate derivatives was identical to that of its parent. De-esterified free-acid forms retained good activity against most Gram-negative bacteria, but not against Gram-positives. A free-acid derivative remained bound to washed bone and completely inhibited Staphylococcus aureus growth. The more potent parent, ciprofloxacin, failed to bind significantly, and bacterial growth occurred. Introduction Bisphosphonates, carbon analogues of pyrophosphate, strongly chelate metal ions and adsorb to bone. 1 They have been used to treat osteoporosis 2 and as radio- imaging agents 3 and have reduced metastases in breast cancer. 4 The introduction of a bisphosphonate moiety into pharmacologically active molecules, like fluoroquinolone antibacterial drugs, may enhance their ability to bind to, concentrate in, and/or be retained by infected bone, a site often difficult to treat clinically. 5 Their excellent pharmacological distribution make the fluoroquinolones the drug of choice in treatment of many chronic infec- tions. For example, ciprofloxacin’s broad-spectrum bac- tericidal activity affects many pathogens, including methicillin-susceptible Staphylococcus aureus and Pseu- domonas aeruginosa, 6 both associated with osteomyeli- tis. 5,7 In this study, the bisphosphonate derivatives of three fluoroquinolone antibacterials, norfloxacin 8 (com- pound 1), enoxacin 9 (compound 2), and ciprofloxacin 10 (compound 3), were prepared. 11 Results The biological properties of modified fluoroquinolone antibacterials were investigated by comparing deriva- tives in which bisphosphonate and fluoroquinolone moieties were linked with or without an intermediate carbon, i.e., positioned between the nitrogen of the fluoroquinolone’s piperazine group and the carbon of the bisphosphonate. For compounds linked by one carbon, compounds 1, 2, or 3 were allowed to react with tetraethyl ethenylidenebisphosphonate 12 (compound 4) in dichloromethane in the presence of triethylamine resulting in bis-(diethoxy-phosphoryl)-ethyl derivatives 5, 6, or 7, respectively (Scheme 1). The ester groupings were then removed by bromotrimethylsilane and sub- sequent hydrolysis, 13 giving rise to compound 8, 9, or 10, respectively, as hydrobromide salts. When the linkage between the bisphosphonate and the fluroqui- nolone lacked a carbon, a bis-phosphonomethyl moiety was introduced into compound 1, 2, and 3. By allowing them to react with a mixture of triethyl orthoformate and diethyl phosphite at 145 °C, 14 compounds 11, 12, and 13, respectively, were obtained (Scheme 2, Sup- porting Information). Bromotrimethylsilane-mediated hydrolysis of ester groups resulted in the free bisphos- * Corresponding author. Address: ElizaNor Biopharmaceuticals, Inc., One Woodmeadow Lane, Princeton Junction, NJ 08550. Phone: 609-799-2812. Fax: 609-897-9660. E-mail: elizanor@aol.com. ² University of Debrecen. ‡ Eisenhower Army Medical Center. § Augusta State University. | ElizaNor Biopharmaceuticals, Inc. ⊥ Present address: Laboratoire de Chimie The´rapeutique, Faculte´ de Pharmacie, Universite´ de Reims, France. Scheme 1. Synthesis of Bis-phosphono-ethyl Derivatives (5-10) of Fluoroquinolone Antibacterials: Norfloxacin (1), Enoxacin (2), and Ciprofloxacin (3) a a Reaction conditions: (a) N(C2H5)3; (b) (CH3)3SiBr; (c) H2O. 2338 J. Med. Chem. 2002, 45, 2338-2341 10.1021/jm0105326 CCC: $22.00 © 2002 American Chemical Society Published on Web 04/09/2002