The effectiveness of nanoparticle-bound ampicillin was tested in the treatment of experimental Listeria monocytogenes infection in congenitally athymic nude mice. Nanoparticles of polyisohexylcyanoacrylate (PIHCA) 187 13 nm in diameter were bound to ampicillin at an ampicillin/PIHCA ratio of 0.2:1. The proportion of ampicillin bound was 90% + 3%. After adsorption onto nanoparticles, the therapeutic activity of ampicillin increased dramatically over that in the free state. Thus, 2.4 mg of nanoparticle-bound ampicillin (three doses of 0.8 mg each) had a greater therapeutic effect than 48 mg of free ampicillin (three doses of 16 mg each). These results might provide an incentive for further development of intracellular targeting of antibiotics on biodegradable polymeric carriers.Infections caused by obligate or facultative intracellular microorganisms such as Listeria monocytogenes are difficult to treat, especially in immunocompromised patients (15). The need for intracellular chemotherapy of these infections has been recognized for many years (19). It was recently observed that the use of particulate carriers able to undergo endocytosis increased intracellular delivery of antibiotics (18,20). For instance, when ampicillin was entrapped in liposomes it had an enhanced therapeutic effect on listeriosis in normal mice (1). This was probably because the liposomal encapsulation of ampicillin made it more readily available to intracellular bacteria and allowed lysosomal uptake of the drug (2).Although these results are interesting, large-scale manufacture of liposomes is still difficult. The disruptive effects of human serum on liposomes may lead to some extrusion of trapped drugs (11), and the limited stability of liposomes in circulating blood has been stressed (17). For these reasons, we investigated the possibility of using biodegradable polymeric nanoparticles as an alternative carrier to enhance intracellular drug delivery (4). Owing to their polymeric nature, polyalkylcyanoacrylate nanoparticles may be more stable than liposomes in biological fluids and during storage, and they can generally entrap molecules in a stable and reproducible way (5). This is why we recently described the conditions required for the binding of ampicillin to nanoparticles of polyisohexylcyanoacrylate (PIHCA) and showed that the antimicrobial activity of the drug remained unaltered after linkage to this carrier (8). Desorption of ampicillin from PIHCA nanoparticles is only 10% of the loaded dose after 2 h of incubation in phosphate buffer at 37°C (8).In the present study of experimental murine listeriosis, we found that such nanoparticle-bound ampicillin was significantly more effective than free ampicillin.
MATERIALS AND METHODSIsohexylcyanoacrylate monomer (Sopar, Saint-DarmesAvelines, Belgium) (100 ,u) was mechanically stirred at room temperature into 10 ml of an aqueous polymerization medium (1% dextran 70 [Sigma], 5% glucose, 0.001 N HCI) containing 2 mg of ampicillin trihydrate (Negma, Buc, France) per ml. After a 6-h polymerizat...