Biology Reference
In-Depth Information
[15]
. Hence, there is a very keen interest in liposomes as vehicles for drug delivery. It is
believed that liposomes can meet the four basic requirements of a successful drug delivery
system: 'Retain (the sequestered drug), Evade (the RES), Target (the diseased cell), and
Release (the drug at the appropriate location)'
[16]
. We have already discussed the role of
liposomes in 'Retain', 'Evade' and 'Target'. Finally, we will consider the issue of 'Release',
the process whereby the drug is rapidly deposited at the target site.
In the late 1970s, Yatvin et al.
[17]
pioneered the use of mild hyperthermia to release lipo-
some-sequestered drugs at the site of solid tumors. They based their approach on the large
increase in leakiness that liposomes exhibit when heated through their main lipid phase tran-
sition temperature (T
m
, see Chapter 14)
[18]
. Permeability of DPPC liposomes was discussed
in Chapter 14. Liposomes in the gel state are poorly permeable while those in the liquid crys-
talline state exhibit considerable permeability. However, maximum permeability is achieved
at the T
m
, where equal amounts of gel and liquid crystalline state domains co-exist. Domain
interfaces are locations of extremely high permeability. Temperature-sensitive, or thermo-
liposomes, are in the impermeable gel state at physiological temperature (37
C), but undergo
a sharp phase transition a few degrees above this. At T
m
they rapidly become leaky, dropping
most of their sequestered drug load. The liposome transition temperature cannot be too high
as mammalian cells start to show damage at ~42
C
[19]
. So how does one make liposomes
with T
m
s between ~40
44
C? Many thermo-liposomes have as their major bilayer compo-
nent DPPC since the T
m
of this phospholipid is 41.3
C (see Chapter 5).
In their initial report, Yatvin et al.
[17]
used, as a model system, neomycin-sequestered
liposomes to affect E. coli protein synthesis and cell survival in culture. Their initial experi-
ments employed sonicated SUVs composed of DPPC/DSPC (3:1). This formulation may
seem strange since DSPC has a higher T
m
(56
C) than that of DPPC (41.3
C) and the liposomes
would have a T
m
of around 44
C. However, it is known that tightly curved SUVs exhibit
permeability maxima
e
4
C below their actual T
m
. Therefore the effective (permeability
maxima) temperature was a very acceptable ~42
C. It was demonstrated that E. coli protein
synthesis was inhibited and cell killing enhanced by heating the E. coli with neomycin-
containing liposomes to this reduced temperature. Maximum drug release and cell killing
with the thermo-liposomes was between 42
w
46
C. This experiment established the potential
of thermo-liposomes as useful drug carriers. A large number of drug-bearing thermo-
liposomes were soon developed and tested in animal models
[20]
. The first generation of
thermo-liposomes concentrated on avoiding the RES by lipid composition and size manipu-
lations. In a typical example, Lindner et al.
[21]
made 175 nm thermo-liposomes from DPPC,
DSPC, and a novel lipid DPPGOG (1.2-dipalmitoyl-sn-glycero-3-phosphoglyceroglycerol).
These thermo-liposomes had a long circulation time and trapped drugs were released under
mildly hyperthermic conditions (41
e
42
C). Thermo-liposomes were taken a step further by
making them 'Stealth' with attached PEG. For example, Needham et al.
[15]
developed a series
of 'Stealth' thermo-liposome drug carriers to target solid tumors. The liposomes were made
from various mixtures of DPPC, MPPC (1-palmitoyl-2-hydroxy-sn-glycero-3-phosphocholine
(a lyso-PC)), HSPC (hydrogenated soy sn-glycero-3-phosphocholine), cholesterol, and DSPE-
PEG-2000. The sequestered drug was doxorubicin and the tumor was a human squamous cell
carcinoma xenograft line (FaDu). DPPC and cholesterol provided the gel state, MPPC was
responsible for the narrow phase transition (39
e
40
C), and DSPE-PEG-2000 made the
thermo-liposomes invisible to the RES. The liposomes proved to be very susceptible to
e
