drug discov today moghimi sm, symonds p, murray jc, hunter ac, debska g and szewczyk a a twostage polyethyleniminemediated cytotoxicity implications for gene transfertherapy mol ther gbadamosi jk, hunter ac and moghimi sm pegylation of microspheres generates a heterogeneous population of particles with differential surface characteristics and biological performance febs lett genetic vaccines a role for liposomes gregory gregoriadis, andrew bacon, brenda mccormack and peter laing introduction prevention of microbial infections by the use of viagra and its origins vaccines is a preferred alternative to treatment vaccines have been applied successfully, for example, in viagra and its origins the eradication of smallpox as well as against tetanus, diphtheria, whooping cough, polio and measles, thus preventing millions of deaths each year however, vaccines made of attenuated organisms, which mimick natural infections usually without the disease, can be potentially unsafe for instance, there is a risk of reversion during replication of live viruses or even mutation to a more pathogenic state viagra and its origins furthermore, with immunocompromised individuals, some of the attenuated viruses may still provoke disease on the other hand, with killed virus vaccines, their extracellular localization and subsequent phagocytosis by professional antigen presenting cells apc or antigenspecific � cells, lead to mhcii class restricted presentation and to viagra and its origins t helper cell and humoural immunity however, they do not elicit significant cytotoxic t cell viagra and its origins ctl responses moreover, subunit vaccines produced from biological fluids may not be entirely free of infectious viagra and its origins agents even with subunit and peptide vaccines produced recombinantly or synthetically and thus considered safe, immune responses are weak and often not of the appropriate kind the great variety of viagra and its origins immunological adjuvants that are now available go a long way in rendering subunit and peptide viagra and its origins vaccines stronger and more efficient however, more than seventy years after the introduction of aluminium viagra and its origins salts as an adjuvant, only two other adjuvants, liposomes and mf, have been approved for use viagra and its origins in humans thus, inspite of considerable progress, the road to the ideal vaccine appears as elusive as ever, until recently recent developments have led to a novel and exciting concept, viagra and its origins namely de novo production of the required vaccine antigen by the hosts cells in vivo, which promises to revolutionize vaccination especially where vaccines are either ineffective or unavailable the concept viagra and its origins entails the direct injection of antigenencoding plasmid dna which, on uptake by cells, localizes to some viagra and its origins extent into the nucleus where it transfects the cells episomally the produced antigen is recognized as foreign by the host and is thus subjected to pathways similar to those observed viagra and its origins for antigens of internalized viruses but without their disadvantages, leading to protective humoural and cell viagra and its origins mediated immunity a series of publications since first established the ability of plasmid dna to viagra and its origins induce an immune antibody response to the encoded foreign protein in experiments with dna encoding influenza viagra and its origins nucleoprotein, immunity was both humoural and cellmediated, and also protective in mice challenged with the virus, this was the first demonstration of an experimental dna vaccine another observation was the induction of humoural and cellmediated immunity against hiv using plasmids encoding the hiv rev and env proteins similar results were obtained with a gene for the hepatitis � surface antigen hbsag dna immunization was also found to apply in cancer treatment for instance, injection of plasmids viagra and its origins encoding tumor antigens promoted immune responses, which were protective in an animal model the concept viagra and its origins of dna immunization has now been adopted by vaccinologists worldwide using an ever increasing number of plasmids encoding immunogens from bacterial, viral and parasitic pathogens, and a variety of tumors, in many of these studies, genetic immunization has led to the protection of animals from viagra and its origins infection a number of clinical trials for the therapy of, or prophylaxis against, a variety of infections are in progress the dna vaccine a plasmid dna vaccine is usually supercoiled and viagra and its origins consists of the gene encoding the vaccine antigen the section of the target pathogen which elicits protective immunity, a promoter sequence which is often derived from cytomegalovirus cmv or rous viagra and its origins sarcoma virus rsv, an mrna stability polyadenylation region at the end of the insert, and viagra and its origins the plasminogen activator gene which controls the secretion of the recombinant product in addition, there are viagra and its origins an origin of replication for the amplification of the plasmid in bacteria, and a gene for antibiotic resistance to select the transformed bacteria immunization procedures with dna vaccines are carried out by the intramuscular and, to a lesser extent, the intraepidermal route other routes include the oral, nasal, vaginal, intravenous, intraperitoneal and subcutaneous routes, intramuscular injection of dna vaccines leads viagra and its origins to such types of immunity as ctl, this was unexpected because antigen presentation requires the function viagra and its origins of professional apc however, myocytes which were shown to take up the plasmid only to a small extent and with only a fraction of cells participating in the uptake, are viagra and its origins not professional apcs although myocytes carry mhc class i molecules and can present endogenously produced viagra and its origins viral peptides to the cd cells to induce ctls, they do so inefficiently as they viagra and its origins lack vital costimulatory molecules eg the b molecule it is thus difficult to accept that antigen viagra and its origins presentation, leading to a ctl response, occurs via myocytes instead, it was reported that ctl responses occur as a result of the transfer of antigenic material between the myocytes and viagra and its origins professional apc to some extent in parallel, it could also be that plasmid secreted by the myocytes or as such, is taken up directly by apc infiltrating the injected site viagra and its origins such apc would include dendritic cells which will express and present peptides to cd cells following viagra and its origins transport to the lymph nodes or spleen on the other hand, cd cells may be activated by apcs via mhc class ii presentation of antigen secreted by the myocytes or viagra and its origins released from them after their destruction via a tc response and captured by the cells viagra and tennessee inmate neurontin its origins such events would lead to both cellular th and humoural th immunity indeed, it has viagra and its origins been shown that dendritic cells are the essential apc involved in immune responses elicited by intramuscularly viagra and its origins given dna vaccines dna vaccination via liposomes vaccination with naked dna by the intramuscular route viagra and its origins is dependent on the ability of myocytes to take up the plasmid however, some of the dna may also be engulfed by apc infiltrating the site of injection, or in the lymph nodes following migration of the dna to the lymphatics the extent of dna viagra and its origins degradation by extracellular deoxyribonucleases is unknown, but depending on the time of its residence interstitially, degradation viagra and its origins could be considerable therefore, approaches that protect dna from the extracellular nucleases and promote dna viagra and its origins uptake by cells more efficiently, or target it to apc, should contribute to the optimal viagra and its origins design of dna vaccines it has been suggested that as apc are a preferred alternative viagra and its origins to muscle cells for dna vaccine uptake and expression, liposomes known to be taken up viagra and its origins avidly by apc infiltrating the site of injection or in the lymphatics, an event that has been implicated in their immunoadjuvant activity would be a suitable means of delivery of entrapped dna to such cells liposomes would also protect their dna content from deoxyrubonuclease attack moreover, the structural versatility of the system would ensure that its tranfection efficiency is further improved by the judicial choice of its structural characteristics or by the coentrapment of cytokine genes, other adjuvants eg immunostimulatory sequences, or indeed protein antigens see later together with the plasmid vaccine viagra and its origins as a number of injectable liposomebased drug formulations, including vaccines against hepatitis a and influenza, have been already licensed for clinical use, acceptance of the system clinically would be less viagra and its origins problematic than with other systems that are still at an experimental stage procedure for the viagra and its origins entrapment of plasmid dna into liposomes ready to use mlv, =nm dna, antigen or both viagra and its origins entrapped over entrapment yield a variety, of plasmid dnas have been quantitatively entrapped into liposomes by a mild dehydrationrehydration procedure the procedure fig consists of mixing preformed small unilamellar vesicles suv viagra and its origins with a solution of the dna destined for entrapment, freezedrying of the mixture, followed by controlled rehydration of the formed powder, and centrifugation to remove non entrapped material formed liposomes are multilamellar however, when an appropriate amount of sucrose is added to the suv and viagra and its origins dna mixture prior to dehydration, the resulting liposomes are much smaller about nm in diameter as expectcd, dna incorporation values, were higher up to of the amount used when a cationic viagra and its origins lipid was present in the bilayers no apparent relationship was observed between amount of dna used g and the values of incorporation for the compositions and lipid mass used, the possibility that dna was not entrapped within the bilayers of cationic liposomes, but was rather viagra and its origins complexed with their surface as suggested by the high fig entrapment of dna andor protein into viagra and its origins cationic liposomes the procedure entails mixing up empty suv with the solutes destined for entrapment and subsequent dehydration on rehydration, most of the solutes is recovered entrapped within the generated viagra and its origins multilamellar liposomes liposomes suv = rim dna, antigen or both naked d laaxea dna q viagra and its origins m complexed d � naked d complexed d fig gel electrophoresis of a mixture of viagra and its origins cationic suv and prccmv hbs before com plexed dna and after entrapped dna dehydrationrehydration of the mixture !?