| Summary: | Studies in this thesis were aimed at developing a versatile strategy to study tumor influx
and efflux pharmacokinetics of liposomes, and comparing these kinetic parameters
between conventional and sterically stabilized lipid formulations under specified
conditions.
This research effort was initiated following recently published studies that indicate the
lack of benefits associated with steric stabilization of liposomal drug formulations. In
these studies, an improvement in the plasma lipid pharmacokinetics was not translated
into improved pharmacokinetics and pharmacodynamics in solid tumors. It was thought
that a better understanding for liposomal accumulation in tumors can be gained by
characterizing the individual influx and efflux kinetics of liposomes that contribute to the
net flux, or accumulation, in solid tumors.
Avidin-induced cross-linking of biotinylated liposomes was utilized to rapidly deplete
liposomes from the blood compartment, thereby allowing liposome tumor efflux and
influx rates to be determined. In the conventional [1,2 distearoyl-sn-glycero-3-
phosphocholine (DSPC)/cholesterol (Choi) (55:45 mol ratio)] liposomal system (100 nm
in diameter), incorporation of 0.5 mol% N-((6 (Biotinoyl)amino)hexanoyl)-l,2-
distearoyl-sn-glycero-3-phosphoethanolamine (Biotin-X-DSPE) into the formulation and
administration of 50 μg neutravidin resulted in greater than 90% of plasma liposomal
lipid (10 mg/kg and 100 mg/kg) removed within 1 hour in female CD-I mice. This rapid
removal was achievable at 1, 4 and 8 hours post-injection of liposomes. Using the LSI 80
human colon carcinoma solid tumor xenograft model in SCID/RAG2 mice, the rapid
lipid removal permitted characterization and determination of tumor influx and efflux
rate constants, which were estimated to be 0.022 hour⁻¹ and 0.041 hour⁻¹respectively
when neutravidin was injected 4 hours after liposome injection. Therefore, it appears that
DSPC/Chol liposomal accumulation, in LSI80 solid tumor is dictated primarily by
plasma liposome concentrations and liposome rate constant is higher for efflux than
influx into the tumor.
Rapid elimination (>90% plasma lipid removed within 1 hour) of sterically stabilized or
PEGylated [DSPC/Chol/l,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-
[Poly(ethyleneglycol)2000] (PEG₂₀₀₀-DSPE) 5%] liposome systems (100 nm in diameter,
at 10 mg/kg and 100 mg/kg lipid dose) in female balb/c mice was achieved with 100 jug
of avidin and a double biotinylated system formulated with 0.5 mol% biotin-X-DSPE and
0.5 mol% N-[w-(Biotinoylamino)poly(ethyleneglycol)2000] 1,2-distearoyl-sn-glycero-3-
phosphoethanolamine (Biotin-PEG2ooo-DSPE). Using the LSI80 solid tumor xenograft
model in SCID/RAG2 mice, the rapid lipid removal led to characterization of tumor
influx and efflux rate constants, which were estimated to be 0.062 hour⁻¹ and 0.011 hour⁻¹
respectively when avidin was injected 4 hours after liposome injection. Therefore, it
appears that DSPC/Chol/PEG₂₀₀₀-DSPE 5% liposomal accumulation, in LSI80 solid
tumor is dictated primarily by plasma liposome concentrations and liposome tumor influx
rate constant is greater than efflux rate constant.
Comparisons between conventional and sterically stabilized formulation in our studies
show that PEGylation results in favourable tumor influx and efflux pharmacokinetics.
The question related to the conflicting data of PEGylation arising from the few studies
still remains unanswered. Nevertheless, the present thesis introduces a novel
methodology that allows characterization of the detailed tumor accumulation properties
of liposomes, and the application has the potential to be fully utilized to characterize the
impact of various liposome parameters such as dose, composition, and size on the tumor
accumulation kinetics.
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