| Summary: | One of the greatest benefits of liposomal encapsulation of therapeutic drugs is the tendency for
these carriers to accumulate in sites of disease. In the case of a solid tumor model employed
here, it was the goal of this thesis to maximize the drug delivery to this site. This requires an
understanding of the major factors governing tumor delivery. Of central importance to liposomal
delivery to solid tumors is the circulation lifetime of the carrier. In the first section of
experiments, factors influencing the use of poly(ethylene glycol) (PEG)-lipids to increase the
circulation lifetime of liposomes was examined. Second, the observation that drug loaded
liposomes last significantly longer in the circulation was more fully examined. And finally, the
delivery of drug to a murine solid tumor was assessed and the influence of PEG-lipids in the
drug carrier and the effect of entrapped drug has on its delivery determined.
PEG-lipid anchor conjugates can prolong the circulation lifetimes of liposomes following
intravenous injection, but this can depend upon the nature of the lipid anchor and the chemical
link between the PEG and lipid moieties. Incorporation of various PEG-lipids into large
unilamellar vesicles (LUVs) composed of distearoylphosphatidylcholine (DSPC) and cholesterol
(chol) (DSPC/chol/PEG-lipid, 50:45:5 mol/mol) results in differing liposomal circulation
lifetimes in mice. This is shown to be due to differential removal of the hydrophilic coating in
vivo that arises from exchange of the entire PEG-lipid conjugate from the liposomal membrane,
although chemical breakdown of the PEG-lipid conjugate is also possible. This work establishes
that DSPE is a considerably more effective anchor for PEG₂₀₀₀ than POPE and that the chemical
stability of PEG-PE conjugates is sensitive to the nature of the linkage and exchangeability of the
PEG-PE complex. It is suggested here that retention of the PEG coating is of paramount
importance for prolonged circulation lifetimes.
The influence of entrapped drug on the circulation lifetimes of liposomal carriers was
investigated next. Pre-doses of liposomally entrapped doxorubicin blocked the accumulation of
subsequently injected liposomes in the reticuloendothelial system (RES). This effect is tenned
RES blockade. Liposomal drug doses as low as 2 mg/kg can induce maximum RES blockade
within 24 h after administration, and this effect lasts as long as 8 days. Full recovery is only
achieved by 14 days. Another commonly employed liposomal anti-cancer drug, vincristine, has
effects that are similar in magnitude, but more transient, allowing recovery of the RES within 2
to 4 days. Liposomes incorporating PEG-lipids or ganglioside GM₁ are proposed to avoid the
RES, however it is shown that when loaded with doxorubicin these liposomes also induce RES
blockade and do not avoid uptake by the RES. Rather, these lipids engender a decrease in the rate
of uptake by cells of the RES.
The final set of experiments consisted of a comparison of tumor accumulation and efficacy
properties of doxorubicin entrapped in liposomes incorporating PEG-lipids versus conventional
liposomes by monitoring drug pharmacokinetics and tumor accumulation at the maximum
tolerated dose (MTD)(60 mg/kg liposomal doxorubicin). The tumor model consisted of mice
bearing Lewis Lung carcinoma solid tumors. In contrast to expected behavior, the efficiency of
doxorubicin accumulation at the tumor site, evaluated with an area under the curve analysis, was
higher for conventional liposomes than for the sterically stabilized liposomes. Both formulations,
however, exhibited profound increases of over 500-fold in tumor accumulation of drug as
compared to free drug injected at the MTD (20 mg/kg doxorubicin). These studies suggest that
optimization of factors nominally leading to longer blood circulation times do not provide
therapeutic advantages for liposomal formulation of doxorubicin administered at the MTD. The
dominant factor influencing the circulation lifetime for both liposomal carrier systems appears to
be that of entrapped drug, consistent with RES blockade described in this thesis. Improvement in
other parameters, such as drug leakage rates, hold greater promise for improving therapeutic
properties of liposomal drug carriers.
|
|---|
