Effect Of Drug Release Rate On The PK/PD Behaviors Of Liposomal Drugs:using CPT-11 As An Example

Pharmocokinetics（PK）and therapeutic effect（PD）of liposomal drugs are determined by their bio-fate in vivo.It is well recognized that physicochemical parameters including size,shape,surface properties and stiffness of the vesicles have profound effect on the liposomes’bio-fate.Besides theses parameters,drug release rate plays critical role by changing the ratio of the“free”drugs and the encapsulated ones present in systemic circulation and the target tissues/cells.Therefore,in this study,we used CPT-11 liposomes with varied in vitro drug release rate as an example and studied the effect of drug release rate on the systemic PK of CPT-11.The potential effect of drug release rate on the antitumor effect of the CPT-11liposomes is also studied.CPT-11 liposomes with varied release rate were prepared by remote loading method using five different intraliposomal gradients.The gradients included methanesufonate（MS）,4-hydroxybenzenesulfonate（HBS）,1,2-ethanedisulfonate（EDS）,1,3-propanedisulfonate（PDS）and sucrose octasulfate（SOS）as anionic ions and TEAH⁺as cationic ion.The drug to lipid molar ratio was set at 0.3.All the blank liposomes were composed of completely the same lipids to eliminate the effect of surface property on the bio-fate of the liposomes.The five CPT-11 liposomes at drug to lipid ratio of 0.3 were all prepared in the size of～110nm.And their encapsulation efficiencies were higher than 80%.The thermodynamic properties of the CPT-11 liposomes characterized by high-sensitivity DSC（Micro Cal capillary,Malvern,UK）showed that CPT-11 formed salts with the anionic ions used in the loading gradients and melted at high temperature above 95C,suggesting the ordered structure of the intraliposmal CPT-11 salts.The melting temperature（T_m）of the five intraliposmal CPT-11 salts were:SOS salt（107°C）≈PDS salt（109°C）≈EDS salt（109°C）>HBS salt（103°C）>MS salt（95°C）.The five CPT-11 liposomes showed different in vitro release behaviors in saline and BSA-saline solution both at p H 7.4.The CPT-11 liposomes prepared with TEAMS showed the fastest release rate in both mediums.It showed a bi-phasic release in saline and reached almost 90%release at 24hr.Different from it,CPT-11 liposomes prepared with TEAHBS,TEA₂EDS,TEA₂PDS and TEA₈SOS showed linear release in saline.In BSA saline medium,the drug release rate of all the five formulations were much slower than those in saline with ammonium sulfate.Still,liposomes prepared with TEAMS released fastest and there was about 25%payload released at 24hr.The drug release of the liposomes prepared with TEAHBS,TEA₂EDS,TEA₂PDS and TEA₈SOS respectively was less than 10%at 24hr.We selected CPT-11 liposomes prepared with TEAMS,TEA₂PDS and TEA₈SOS to study the effect of drug release rate on CPT-11pharmacokinetics and pharmacodynamics.The three liposomes showed different PK in the healthy SD rats.The liposome prepared with TEAMS had the shortest T_1/2 of 4.9hr,and the T_1/2 for TEA₂PDS and TEA₈SOS were8.6hr and 11.7hr,respectively.Accordingly,the AUC and MRT of the three formulations ranged in the order of TEAMS<TEA₂PDS<TEA₈SOS.The three Lipo-CPT-11 showed anti-tumor effect in A549 lung cancer nude mice model using phosphate buffer as the control,significant difference between the antitumor effects of TEAMS and TEA₈SOS was observed.In conclusion,CPT-11 liposomes with varied in vitro release rate were prepared with different intraliposmal gradients.The liposomes with slower in vitro release rate showed both the higher melting temperature of the intraliposomal salt assembly and the longer half time in systemic circulation.There was significant difference between the antitumor effects of TEAMS and TEA₈SOS in the anti-tumor effect in transplanted A549 nude mice.