Employment of magnet-susceptible microparticles for the targeting of drugs

On inverse miniemulsion polymerization of conventional water-soluble monomers

Inverse monomer miniemulsions can be generated by sonification of the polar monomer, water, stabilizer and costabilizer in organic solvents as the unpolar continuous phase. The inverse miniemulsion obtains its stability by using a combination of effective surfactant and osmotic pressure agent, so called lypophobe, which is practically insoluble in the continuous phase and prevents the minidroplets from Ostwald ripening. Inverse miniemulsions are typically sterically stabilized with a nonionic surfactant blend so as to provide a relatively condensed interface. The monomer droplet nucleation proceeds under an uncomplete coverage of the monomer and polymer particles with surfactant. Inverse monomer miniemulsions can be easily polymerized to latexes by using water and oil-soluble initiators. The rate of inverse miniemulsion polymerization of water-soluble monomers increased with increasing both initiator and emulsifier concentrations. The inverse polymerization is very fast and the high conversion is reached during a few minutes. The dependence of the polymerization rate vs. conversion can be described by a curve with the two rate intervals. The abrupt increase in the polymerization rate can be attributed to the increased number of reaction loci and the gel effect. The partitioning of unsaturated monomers between the aqueous and continuous phases favours the contribution of homogeneous nucleation. The desorption of monomeric radicals from the small polymer particles favours the polymerization in the continuous phase. The miniemulsion polymerization and copolymerization is ideal process for the preparation of composite nanoparticles with different structures. This procedure can be used to develop novel thermally responsive polymer microspheres, for example, based on N-isopropylacrylamide monomer. The composite magnetic nanoparticles are prepared by polymerization of both water-soluble and oil-soluble monomers in the presence of water- and oil-soluble iron oxide nanoparticles. The inverse miniemulsion copolymerization of acrylic acid and sodium acrylate in the presence of inorganic nanoparticles and substances produces poly(acrylic acid-co-sodium acrylate)/inorganic phase composite nanoparticles. The presence of hydrophobic monomer in the miniemulsion system favours the formation of hollow nanoparticles. The composite latex particles owned better thermal stability and higher colloidal stability than pure latex particles.

Improved method of recombinant AAV2 delivery for systemic targeted gene therapy

A major hurdle in most current gene therapy modalities is the ability to transduce target tissues at very high efficiencies that ultimately lead to therapeutic levels of transgene expression. We have developed a novel method of recombinant adeno-associated virus 2 (rAAV) delivery that results in increased vector transduction efficiencies using microspheres reversibly conjugated to rAAV vectors. We hypothesize that conjugation to microspheres should result in a higher effective concentration of vector as well as longer relative exposure time of vector to target cells as it moves through the tissue vasculature. In vitro experiments demonstrate that the same level of transduction seen with free vector can be achieved using 1% of vector when conjugated to microspheres. In addition, using magnetic microspheres, the region of infection can be targeted. In vivo, we demonstrate that microsphere-mediated delivery of rAAV vector results in higher transduction efficiencies than delivery with free vector alone when administered either intramuscularly or intravenously. Furthermore, we demonstrate targeting of transgene expression to specific tissues by retention of microsphere-bound vector in the capillary bed. These studies demonstrate a novel method to deliver rAAV vectors more effectively that could prove to be a successful alternative mode of virus-mediated human gene therapy.

Magnetic targeting of thermosensitive magnetoliposomes to mouse livers in an in situ on-line perfusion system

We recently reported the preparation and in vitro targeting of dextran magnetite (DM)-incorporated thermosensitive liposomes, namely thermosensitive magnetoliposomes (TMs) [Viroonchatapan et al. Pharm. Res. 12 1176-1183 (1995)]. The current study was designed to determine whether these novel liposomes can be targeted to the mouse liver with the aid of an extracorporeal magnet. An on-line liver perfusion system consisting primarily of a sample injector, permanent magnets, and a fluorescence detector was established for a real-time measurement of targeting efficiency of TMs containing calcein as a fluorescent marker. Normal and reticuloendothelial system (RES)-blocked livers from mice were used for the perfusion experiments. In the RES-blocked livers, percentage holdings of TMs were 73-80% and 26-45% in the presence and absence of magnetic field, respectively, indicating an efficient targeting of TMs with a targeting advantage index (TAI) of 1.6-3.1. On the other hand, TAI in the normal livers was found to be 1.1-1.4 and less than that in the RES-blocked livers, suggesting a role of RES uptake of TMs. The effects of DM concentrations in TM suspensions on the percentage holding of TMs were shown to be minor. Liposome concentration dependence was observed for hepatic uptake of TMs, possibly because of the saturation of phagocytosis by Kupffer cells. The present results suggest that TMs would be useful in future cancer treatment by magnetic targeting combined with drug release in response to hyperthermia.

Effective targeting of magnetic radioactive 90Y-microspheres to tumor cells by an externally applied magnetic field. Preliminary in vitro and in vivo results

Magnetic biodegradable poly(lactic acid) microspheres that incorporate both magnetite and the beta-emitter 90Y were prepared. By applying a directional external magnetic field gradient in excess of 0.02 Tesla/cm across a 96-well plate containing neuroblastoma cells incubated with the 90Y magnetite loaded microspheres, the radiation dose to the cells could be enhanced or reduced relative to the dose from a uniform loading of the well with 90Y-DTPA. Using the MTT assay, cell survival was measured for the magnetic field directed from above (cell sparing) and from below (cell targeting) the well plate, resulting in 65 +/- 8% or 18 +/- 5% survival respectively. This method was then applied to an in vivo murine tumor model. The biodistribution of intraperitoneally injected magnetic radioactive microspheres, after 24 h in mice, showed that 73 +/- 32% of the radioactivity was found on the subcutaneous tumor that had a rare earth magnet fixed above it. In contrast, the tumor radioactivity with no attached magnet was 6 +/- 4%. Magnetically targeted radiopolymers such as 90Y-microspheres show great promise for regional or intracavitary radiotherapy.

Magnetically directed poly(lactic acid) 90Y-microspheres: novel agents for targeted intracavitary radiotherapy

High energy beta-emitting radioisotopes like Yttrium-90 have a radiotoxic range of about one centimeter. For cancer treatment they must be brought near the tumor cells and kept there for as long as they are radioactive. We developed as carriers for the ionic form of 90Y a matrix-type polymeric drug delivery system, poly(lactic acid) (PLA) microspheres. This radiopharmaceutical could be selectively delivered to the target site after incorporating 10% Fe3O4 (magnetite) which made the magnetic microspheres (MMS) responsive to an external magnetic field. Furthermore, MMS are biodegradable and slowly hydrolyze into physiologic lactic acid after the radioactivity is completely decayed. Previously prepared 10-40 microns MMS were radiochemically loaded to high specific activity with 90Y at a pH of 5.7. Stability studies showed that approximately 95% of added 90Y is retained within the PLA matrix after 28 days (> 10 half-lives) at 37 degrees C in serum, and electron microscopy showed that the microspheres retained their characteristic morphologic appearance for the same time period. Cytotoxicity studies with SK-N-SH neuroblastoma cells growing in monolayer showed that the radiocytotoxicity of the microspheres could be directed magnetically to either kill or spare specific cell populations, thus making them of great interest for targeted intracavitary tumor therapy. We are currently optimizing this system for use in the treatment of neoplastic meningitis.