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7/21/2019 Review article Pharma Targeting Solid Tumors
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ELSEVIER
Targeting
dv nced
drug deliiry
reviews
Advanced Drug Delivery Reviews 17 (1995) 117-127
solid tumours: challenges, disappointments, and
opportunities
J.C. Murray*, J. Carmichael
Universi ty of Nott ingham Laboratory
of
M olecular Oncol ogy, Cancer Research Campai gn Depart ment of CIi nical 0ncot og.v.
Ci ty Hospi t al , Nott i ngham N G.5 lPB, UK
Received 22 May 1995; accepted 22 May 1995
bstract
The majority of common solid tumours remain essentially refractory to systemic treatment. Many studies have
now shown that the structure and physiology of solid tumours mitigate against the effective delivery of small
molecules as well as macromolecules to the tumour cell. Those molecules which do reach the membrane of the
tumour cell are then confronted with a variety of defensive mechanisms mounted by the cell. Various approaches
have been used to exploit the unusual physiological properties of solid tumours, in particular the development of
agents activated in hypoxic regions of the tumour. Another approach under investigation is to target the supporting
vasculature upon which the growth of the tumour depends, rather than the tumour cells per se, removing a major
hurdle to drug delivery. Novel approaches including ADEPT and gene therapy are being developed with the
intention of enhancing tumour cell specificity. Such treatments should allow the use of more toxic agents, with
simultaneous sparing of the normal tissues.
Keywords: Tumor; Targeting; Chemotherapy; Antibody; ADEPT, Gene therapy; Physiology; Vasculature
Contents
1. Introduction.. _. . . . . . , . .
2. Tumour structure and physiology
. . . .
3. Physiological barriers to delivery
4. Cellular barriers to delivery and efficacy
5. Exploiting tumour physiology
6. Tumour vasculature as a target..
. . .
7. Targeting solid tumours: clinical aspects
8. Achieving selective delivery: the future
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* Corresponding author. Fax: +44 11.5 9627923.
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0169-409X /95/ 29.00 @ 199.5
Elsevier Science B.V. All rights reserved
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1 Introduction
While significant advances in the treatment of
some less common malignancies have taken
place, there has been little progress in the treat-
ment of the common solid tumours, i.e., those of
breast, lung, and colorectum. Despite intensive
efforts to develop new therapeutic modalities,
and to improve upon existing ones, surgery and
radiotherapy remain the front-line treatment for
the majority of common malignancies. We will
discuss some of the reasons systemic therapies
may fail to have a significant impact on the
treatment of the major solid tumours, concen-
trating particularly on the problem of drug deliv-
ery in relation to tumour physiology. We review
current thinking on ways to improve the delivery
and specificity of cytotoxic agents, and suggest
means by which characteristics which render
tumours resistant to drug treatment might be
exploited to therapeutic advantage in novel
therapies.
2. Tumour structure and physiology
At the simplest level, the successful delivery of
cytotoxic agents, whether small molecules, anti-
bodies, or liposomes, to a solid tumour depends
upon the relationship between the tumour cells
and the blood vessels supporting their growth.
Therefore the first requirement for effective
delivery is a fully functional vasculature. In solid
tumours this criterion is rarely met.
Solid tumours comprise sheets or nests of
neoplastic cells interspersed within a supporting
stroma. The stromal component of the tumour is
composed of fibroblasts, inflammatory cells, and
blood vessels, and may represent as much as
90% of the mass of a tumour, depending on the
tumour type [l]. Folkman [2] has emphasized the
critical role of the supporting stroma, in par-
ticular the new blood vessels in the growth of
solid tumours, and has stated that a tumour
cannot grow beyond l-2 mm in diameter without
evoking a new blood supply. This ‘neovascula-
ture’ is responsible for continued growth of the
tumour, through the delivery of nutrients and
removal of catabolites. The process by which the
new vessels are formed, or ‘angiogenesis’, is the
result of a complex programme of proteolytic
and migratory events involving the endothelial
cell (31. There is overwhelming evidence that the
signalling for this programme of neovascularisa-
tion is mediated by growth factors produced by
the tumour cells [4] or by immune effector cells
infiltrating the tumour parenchyma [5] or both.
While neovascularisation of certain highly select-
ed experimental tumour models can be shown to
be dependent upon particular growth factors,
many human tumour cell lines express a range of
potentially angiogenic growth factors, and this
phenomenon is unlikely to be under the overall
control of one factor [4].
Angiogenesis in solid tumours represents an
active response of the vascular system to the
nutritional demands of the rapidly expanding
tumour cell population. This expansion is fre-
quently in advance of the growth of the blood
supply; however, the nutritionally deficient en-
vironment elicits angiogenic signals from tumour
cells thus stimulating neovascularisation. Hypo-
xia, in particular, induces the expression in cul-
tured tumour cells of at least one potent endo-
thelial mitogen and angiogenic factor, vascular
endothelial growth factor (VEGF) [6]. Similarly,
in glioblastoma multiforme, a rapidly growing
tumour of the brain, in situ hybridization studies
have shown intense expression of mRNA for
VEGF in regions adjacent to necrotic tumour
cells and presumed to be hypoxic [6,7]. Recent
evidence also suggests that in many human
tumours expression of angiogenic factors is re-
stricted to subsets of tumour cells within a lesion
[S-10], thus giving rise to well and poorly vas-
cularised regions within that tumour.
As a result of intense local angiogenic
pressures, the vasculature of many tumours ap-
pears ‘abnormal’ [ll]. This abnormality occurs at
two levels: the vessel wall itself is often char-
acterised by an interrupted endothelium, and/or
incomplete basement membrane. In melanomas
blood-conducting channels formed entirely of
tumour parenchymal cells and devoid of endo-
thelial cells have been observed [12]. Abnor-
malities of vessel architecture on a macroscopic
scale are also frequently observed; pre-existing
arterioles and venules inevitably incorporated
into the growing tumour mass may become
obstructed and compressed, while other ar-
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J.C. M urray . J. Carmi chael I Adv anced Dr ug Deliv ery Review s 17 (1995) 117-127
119
terioles appear to be maximally dilated, display-
ing a loss of vasomotion [13]. Similarly, the
neovasculature arising from pre-existing venules
displays a range of abnormalities, including in-
creased blood vessel tortuosity and elongation, as
well as abnormal and heterogeneous capillary
density. The overall picture will depend upon the
nature of the tumour, and the environment in
which that tumour is growing [14,15], although in
general the more rapidly growing and poorly
differentiated a tumour, the more bizarre the
associated microvasculature. It should be borne
in mind that much of our accumulated knowl-
edge has been derived from studies on rapidly
growing rodent tumours, and it is clear that such
models may not be representative of anything
other than a minority of tumours seen clinically
[ 161
3. Physiological barriers to delivery
The consequences of abnormal vascular ar-
chitecture for the physiology of the growing
tumour are complex and the subject of intense
investigation, not least because of their implica-
tions for treatment of solid tumours [15]. The
presence of arterio-venous shunting reduces
nutritive blood flow, while increased vessel tor-
tuosity within the neovasculature causes high
flow resistance. Abnormalities in the vessel wall
lead to enhanced permeability with resultant
haemoconcentration and increased viscous resist-
ance. The result is sluggish, poorly nutritive
blood flow in some areas, predisposing to in-
travascular thrombosis and vessel obstruction,
and regions of hypoxic and nutrient-deprived
tumour cells.
As mentioned previously, the first factor likely
to determine available levels of a systemically
administered toxic agent within a tumour is the
relative perfusion with nutritive blood of that
tumour compared to normal tissues. Whereas the
microscopic distribution of blood flow in rodent
tumours has been extensively studied, the data
concerning blood flow within human tumours in
situ is of a more global nature. A variety of
techniques, including positron emission tomog-
raphy with C’“O,, isotope clearance using 85Kr
and ‘““Xe, and thermal washout procedures have
been employed to examine relative perfusion in
human tumours. In an extensive review of the
available data, Vaupel et al. [17] concluded that
(i) blood flow rates can vary considerably among
tumours of similar histological type and primary
site, and (ii) tumours may have flow rates which
are higher or lower than the surrounding normal
tissue. Overall, the pattern of perfusion in human
tumours would appear to conform to that pre-
dicted from the structure of the vasculature;
blood flow is non-uniform, and human tumours
contain well-perfused, rapidly growing regions,
as well as poorly perfused, often necrotic, re-
gions. The first obstacle to effective systemic
treatments is therefore that posed by the hetero-
geneity of distribution within the tumour.
The next barrier to delivery of cytotoxic agents
is the transport of the agents across the blood
vessel wall into the interstitium. In normal tissues
a patent endothelium acts as a selective barrier
to all but the smallest molecules or ions. Larger
molecules may penetrate the endothelial barrier
by para- or trans-cellular pathways, and in some
cases by active transport. As mentioned earlier,
barrier function is often inadequate in tumours
due to compromised endothelial integrity. The
result of this deficiency should be increased ease
of access for drugs, macromolecules such as
antibodies, and liposomes. However, the move-
ment of such agents through the vessel wall is
governed by the laws of hydrodynamics and
solute behaviour, and the net effect of the
diffusive and convective forces may differ con-
siderably from that predicted from observations
of normal tissues (for a rigorous and elegant
analysis of hydrodynamics of tumour blood flow
and transport of macromolecules, the reader is
referred to several excellent reviews by Jain, cf.
Refs. 15, 18, 19).
Diffusion constitutes the movement of solutes
as a result of concentration gradients, whereas
convection moves solutes through bulk fluid flow,
and is proportional to the rate of fluid leakage
from vessels. Convective movement is deter-
mined largely by the difference between vascular
and interstitial hydrostatic pressures. While the
net movement of molecules across the vascular
barrier is in principle the result of both diffusive
and convective forces, diffusion, particularly of
macromolecules, plays a minor role in transport
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120
J C
Murray, .I. Curmichael
I dvanced Drug Delivery
Reviews 17 1995) 117-127
across this barrier [19]. Convection due to ‘leaky’
blood vessels, on the other hand, should enhance
delivery; yet the movement of drugs and macro-
molecules into the interstitium is often surpris-
ingly limited. This is generally attributed to a
diminished hydrostatic pressure gradient be-
tween the vascular compartment and the inter-
stitium, which is explained by decreased vascular
pressure, or increased interstitial pressure, or
both. Several theories have been put forward to
explain the abnormally high interstitial fluid
pressures measured in experimental tumours [20]
and human tumours in situ [21], including the
physical effects of the expansion of the prolifer-
ating tumour cells [22], and the lack of functional
lymphatic vessels in most tumours [23]. Studies
on intra-tumour pressure gradients have also
shown that interstitial pressure is higher at the
centre of a tumour and that it approaches normal
pressures at the periphery [18].
What are the consequences of these anomalies
in pressure gradients for the delivery and dis-
tribution of drugs and macromolecules within the
tumour interstitium? First, high interstitial
pressures mean that the central regions of the
tumour, already poorly perfused, demonstrate
low or non-existent convective flow into the
interstitium. Second, interstitial convective flow
will tend to radiate outward from the centre,
towards the periphery and regions of lower
interstitial pressure. Therefore, significant levels
of drugs or macromolecules will not reach
tumour cells in the centre of the tumour; and at
the tumour periphery, where convective transfer
across the blood vessel wall might take place,
further movement towards the centre of the
tumour will be impeded by bulk flow in the
opposite direction. In conclusion, in solid
tumours the laws of hydrodynamics and trans-
port of solutes mitigate against the successful
delivery of drugs and macromolecules to tumour
cells.
4. Cellular barriers to delivery and efficacy
There are a variety of ways by which the
tumour cell can avoid the toxic effects of cytotox-
ic drugs. The first barrier is the membrane
although the majority of drugs gain access into
cells by passive diffusion. A number of anti-
metabolites are actively transported. In addition
there are certain proteins which act as energy-
dependent efflux pumps for a number of com-
monly used chemotherapy drugs. One protein
has been extensively studied, a 170 kDa glyco-
protein termed P-glycoprotein first described by
Juliano and Ling [24]. This protein is found on
the mucosal surface of a number of tissues in the
body and its expression is increased in a number
of drug-resistant tumours [25]. It has been iden-
tified as a poor prognostic marker in
haematological malignancies, and a number of
inhibitors of this particular protein have been
described. A closely related peptide, MRP, has
recently been described [26], which is thought to
be of greater importance in solid tumours such as
lung cancer.
Once in the cell there are also detoxification
mechanisms within the cytoplasm which can
potentially inactivate cytotoxic drugs, including
glutathione and the glutathione-S-transferase en-
zyme. At the nuclear level there is a wide variety
of proteins able to protect the cell against
chemotherapy-induced damage. The topoisomer-
ase enzymes [27] are common targets for the
development of cytotoxic drugs. Inhibitors of
topoisomerase-1 include agents based on the
camptothecin structure, with new drugs under
development, including topotecan and CPT-11;
inhibitors of topoisomerase-2 include etoposide
and adriamycin. There are many topoisomerase-
2 inhibitors currently in use, which block the
action of both topoisomerase-2cY and p although
it would appear with varying specificities be-
tween the two proteins. In addition the malig-
nant cell, as the normal cell, has a complex array
of enzymes involved in recognising and repairing
DNA damage. Increased levels of DNA repair
enzymes have been identified in models of resist-
ance to cytotoxic drugs, in particular to methylat-
ing agents, with elevations in O-methyltransfer-
ase, and in resistance to platinum-based drugs.
5. Exploiting tumour physiology
Over the last 20 years attempts to take advan-
tage of the unusual physiological characteristics
of tumours to enhance the effectiveness of sys-
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J.C. M urray , J. Carmi chael I Adv anced Dr ug Deli vety Reviews 17 (1995) 117-127 (21
temically delivered agents have advanced on two
main fronts: first, by exploiting the bioreductive
environment within hypoxic regions of tumours
to generate toxic molecules; and second, by
manipulating the tumour vasculature to enhance
the delivery and retention of cytotoxic agents
within the tumour.
The realisation that the existence of hypoxic
subpopulations of tumour cells might compro-
mise the efficacy of therapeutic doses of ionising
radiation led to intense efforts to design
‘radiosensitisers’. In theory, these agents mimic
some of the chemical properties of oxygen, thus
rendering hypoxic cells exposed to the agent
sensitive to radiation. While these agents, the
earliest of which were nitroimidazole derivatives,
showed promise in vitro and in murine tumour
models, early hopes were not borne out by the
clinical experience, which was profoundly dis-
appointing. There is some, admittedly limited,
evidence to show that in selected tumour sub-
types nitroimidazole radiosensitisers may be of
benefit [29]; however, normal tissue toxicity has
been a major problem. Nevertheless continuing
enthusiasm for such agents purely as hypoxic cell
cytotoxins was demonstrated with the develop-
ment of compounds such as RSU 1069, a dual
function agent containing a bioreducible nitroim-
idazole moiety as well as an alkylating aziridine
functional
group
[30], and SR 4233
(tirapazamine), a benzotriazine di-/V-oxide [31].
More recently Denny et al. [32] have reported on
a new class of bioreductive agents, based upon
benzylic mustard quaternary salts, which produce
freely diffusible cytotoxic metabolites upon
bioreduction.
Hypoxic cells within tumours may be ‘resis-
tant’ to conventional cytotoxic agents [33,34].
While this could be an effect of hypoxia per se,
such resistance may simply reflect the fact that
hypoxic cells are normally those furthest from
the supporting blood vessel [35]. Studies of the
effects of cisplatin on a mouse mammary car-
cinoma indicated that despite extensive killing of
aerobic tumour cells there was little or no killing
of hypoxic cells [33]. Cisplatin in combination
with the hypoxic cell cytotoxin SR 4233 was
found to give a significant increase in tumour
growth delay, with no enhancement of systemic
toxicity [36]. Earlier studies had pointed out the
phenomenon of ‘chemopotentiation’, whereby
radiosensitisers such as misonidazole potentiate
the effects of melphalan and other conventional
agents [37]. Studies on blood flow in murine
tumours treated with these combinations sug-
gested that the primary mechanism of action of
the radiosensitiser was as a vasoactive agent,
causing a rapid and prolonged drop in tumour
blood flow, thereby altering the distribution and
retention of melphalan [38]. While there is strong
evidence for vasoactive effects of misonidazole,
little is known about other hypoxic cell cytotox-
ins, and it would be unwise to assume similar
activities in the newer agents.
It was suggested many years before these
studies that intentionally altering blood flow to
tumours might enhance the effectiveness of sys-
temic treatments such as chemotherapy, targeted
monoclonal antibodies, and radiotherapy [39].
Algire and Legallais [40] had demonstrated pre-
viously that perfusion of experimental tumours
could be modified by vasoactive agents; sub-
sequently many studies examined the influence
of both vasodilators and vasoconstrictors on
tumour blood flow. While most vasoactive agents
appear to decrease relative perfusion of tumours
[41], some, such as the P-adrenoreceptor blocker
propranolol [42] and the vasoconstrictor an-
giotensin II [43], increase perfusion. However,
whichever direction the changes take, it is now
generally agreed that the net effect represents a
largely passive response of the tumour to
changes in the normal vasculature [see Refs. 44,
451. This view is consistent with several studies
demonstrating the lack of both innervation and
smooth muscle in tumours [for review see Ref.
461.
Early attempts to potentiate the effectiveness
of chemotherapeutic agents by vascular manipu-
lation were aimed at the use of bioreductive
agents, and therefore motivated by the desire to
increase hypoxia through the reduction of blood
flow. It was argued that further activation of
bioreductive agents such as the nitroimidazoles
could be achieved through the intentional induc-
tion of tumour ischaemia. The potent vasodilator
hydralazine, which transiently reduces blood flow
in experimental tumours in a dose-dependent
manner [47,48], was indeed shown to potentiate
the effectiveness of the bioreductives RSU 1069
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[48] and SR 4233 [49]. Experiments with the
conventional chemotherapeutic agent melphalan
in combination with hydralazine [50] also demon-
strated potentiation. In this case it was suggested
that pharmacokinetic effects played a large part
in potentiating the effects of melphalan, probably
in a similar manner to the potentiation of mel-
phalan by misonidazole [38]. Quinn et al. [51]
studied the response of two mouse models of
large bowel cancer to hydralazine in combination
with melphalan or tauromustine (TCNU). Ex-
amining acute bone marrow toxicity as well as
tumour response, they concluded that there was
a therapeutic gain for the TCNU/hydralazine
combination, but not for that with melphalan.
and that further work was required to validate
this approach.
Serious doubts currently surround the clinical
applicability of this approach; recent studies of
blood flow in human lung tumours have failed to
demonstrate consistent reductions in blood flow
following hydralazine administration [52]. These
findings also raise more fundamental questions
concerning the relevance of transplantable mu-
rine tumour models to the clinical scenario. To
underscore this point, Field et al. [16] demon-
strated significant differences in the response of
primary and serially transplanted murine
tumours to hydralazine; a high percentage (64%)
of primary tumours were non-responders, com-
pared to transplanted (6% ). Subsequent serial
transplantation of a non-responder produced
histologically similar tumours, but which were
now responders to hydralazine; the basis of this
change is unknown.
6. Tumour vasculature as a target
We have discussed some of the unusual charac-
teristics of tumour physiology, and characteristics
of tumour cells, which make the effective and
selective delivery of cytotoxic agents difficult.
Indeed we have already considered a number of
ways in which these characteristics might be
exploited for therapeutic benefit. Another ap-
proach which warrants closer inspection is based
on the concept of targeting the supporting vas-
culature of the tumour itself [53,54]. The blood
vessels of solid tumours represent suitable targets
in their own right for several reasons, foremost
among which is the fact that all solid tumours
depend upon these blood vessels for nutrition
and sustained growth [2]. In addition, targeting
the vasculature via a systemic route considerably
reduces problems of drug accessibility, such as
those described earlier.
There is considerable evidence that (i) damag-
ing the vasculature of solid tumours, or inhibiting
further blood vessel formation, can have a signifi-
cant effect on the growth of that tumour, and (ii)
a number of conventional treatment modalities,
including chemotherapy, may act in part through
such mechanisms. To date, perhaps the most
elegant demonstration of the power of this ap-
proach has come from studies with ricin A-chain
conjugated to monoclonal antibodies recognising
tumour endothelium-specific antigens in a mu-
rine solid tumour model [55]. In this instance the
solid tumour was a human neuroblastoma en-
gineered to produce the cytokine interferon-y,
growing in nude (immunodeficient) mice.
Interferon-y induces expression of specific his-
tocompatibility antigens (MHC Class II) only on
the tumour-associated endothelial cells, thus of-
fering a target for systemically delivered mono-
clonal antibodies. In this model, treatment of
tumour-bearing mice with the toxic immuno-
conjugates caused significant growth delay. The
implication of these studies is that it may not be
necessary to deliver toxic agents to the tumour
cells; it may suffice to destroy the supporting
vasculature.
Further support for this hypothesis comes from
studies attempting to inactivate receptors for
angiogenic factors, located on endothelial cells
[56], or to block the angiogenic factor [57]; in
both cases significant reductions in tumour
growth were reported. The inflammatory cyto-
kine tumour necrosis factor (TNF-CX), which
causes rapid necrosis in murine tumour models
[58], appears to act in part by inducing vascular
occlusion within the tumour, thus bringing about
necrosis. The potency of such biological effecters
has led to the search for simpler synthetic mole-
cules with similar activities. One such compound,
flavone-g-acetic acid (FAA), showed enormous
promise in murine models, inducing widespread
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J.C. Mur ray, J. Carmi chael I Adv anced Dr ug Deli very Reviews 17 (1995) 117-127
123
necrosis within 24 h in transplanted solid
tumours, with few if any side-effects [59]. On the
other hand, FAA showed little or no cytotoxicity
toward tumour cells in vitro, but produced a
rapid (within 20 min) change in tumour blood
flow accompanied by changes in the coagulation
properties of the blood of the tumour-bearing
mice, in a similar manner to TNF-cu [60]. The
data suggested in addition that the presence of
the tumour primed the coagulation system in
some unknown way. In vitro studies [61] showed
that the coagulant properties of endothelial cells
were altered by this agent, and that this effect
was potentiated by a soluble factor produced by
tumours. It was hypothesised that the primary
mechanism of action of this agent was to arrest
blood flow within the tumour, thus causing ne-
crosis and regression [61]. Further critical studies
revealed, however, that depriving mice of a
subset of T-cells, namely CD4’ cells, abolished
the effects of FAA on tumour growth, without
altering the response in terms of blood flow
changes [62].
In spite of the early enthusiasm for this agent,
FAA has proven a huge disappointment in the
clinic [63]. It is now recognised that the metabo-
lism of the inactive parent compound is different
in man, resulting in the production of an inactive
metabolite. Indeed hope for this type of agent
now appears to rest with the xanthenone acetic
acid analogues, which show greater dose potency
than their predecessor [64] and are about to
enter Phase I clinical trials. To date the mecha-
nism behind the dramatic effects seen in murine
tumours with FAA and its analogues remains a
mystery. It is clear, however, that the harnessing
of the enormous power of natural effector mech-
anisms such as seen here, represents a particu-
larly attractive and challenging approach in the
development of novel anti-cancer agents.
7. Targeting solid tumours: clinical aspects
The vast majority of adult solid tumours are
not curable with currently used combinations of
cytotoxic drugs. This lack of response, or resist-
ance to cytotoxic chemotherapy, is multi-factori-
al. Some factors are host determined; some
patients are unable to tolerate effective doses of
chemotherapy due to unacceptable toxicity, usu-
ally haematological. There is variability between
patients in pharmacokinetic handling of cytotoxic
drugs, e.g. oral bioavailability of drugs such as
etoposide, altered metabolism through variations
in cytochrome P-450 iso-enzymes, and altered
clearance via the hepatic or renal routes, par-
ticularly in the elderly. The vast majority of
cytotoxic drugs are metabolised via cytochrome
P-450-dependent mechanisms, with many ex-
creted through the kidneys. The site of the
cancer is also important. Certain sites are par-
ticularly resistant to cytotoxic chemotherapy,
including the brain and testes, both sanctuary
sites. With the development of metastatic disease,
bone and liver metastases are frequently associ-
ated with a poor prognosis.
While experimental models have shown dose-
dependent responses to a range of cytotoxic
drugs both in vitro and in vivo, the situation is
less clear cut in human studies. No doubt dose
intensity of chemotherapy is important, particu-
larly for the more sensitive tumours such as
lymphomas. It is recognised that lower doses are
less effective than standard doses of chemother-
apy, but whether increased doses such as those
used in bone marrow or stem cell transplantation
offer additional benefits remains unclear. Such
approaches are being evaluated widely in many
tumour types, in particular breast and ovarian
cancer. In addition there are several ways in
which normal tissues may now be protected from
the effects of cytotoxic chemotherapy, allowing
the administration of larger doses to patients,
thereby increasing the intratumoural concentra-
tion of drugs.
Another way of increasing local levels of
cytotoxic drugs is to use a targeting approach,
exemplified by the use of infusional chemother-
apy, aimed directly to the tumour. This approach
has been used in melanoma at peripheral sites,
where one is able to isolate the blood supply to a
limb for instance, and perfuse high doses of
cytokines and/or chemotherapy drugs with mini-
mal spill-over into the systemic circulation. In
patients with localised recurrences, where this
approach is appropriate great benefit has been
derived,
although it remains experimental.
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.J.C. M urrav , J. Carmi chael I Advanced Dr ug Deli ver,v Reviews 17 (1995) 117-127
Another area where local infusion of chemo-
therapy is under evaluation is in the use of
hepatic arterial administration. Liver metastases
are a major cause of treatment failure in patients
with colorectal cancer. Approximately 50% of
patients who die of colorectal cancer will die of
hepatic metastasis and frequently this is the only
site of metastatic spread. The administration of
this treatment produces high drug levels in the
liver. The blood supply of liver metastasis is
predominantly from the hepatic arterial circula-
tion in contrast to hepatocyte where the portal
circulation is more important. Tumour responses
to 5fluorouracil (or FUDR) are greater with
arterial infusion than with portal vein infusions.
In view of the high level of first pass metabolism
of these drugs during infusion through the liver,
little in the way of systemic toxicity is observed,
even with relatively high doses. Likewise hepatic
artery infusions have been shown to be more
effective than systemic chemotherapy with the
same drugs. In contrast to systemic infusion,
where diarrhoea, mucositis and myelo-suppres-
sion are the most common problems, abnormal
liver function and sclerosing cholangitis are the
problems most commonly encountered with ar-
terial infusions [65,66].
There are therefore a number of hurdles to be
overcome in attempting to enhance tumour cell
kill; the first step is to increase the local con-
centration of the chemotherapy agent around the
tumour cell. In addition to the use of high dose
chemotherapy approaches with concomitant
protection of normal tissues, a number of other
approaches have been developed. Local perfu-
sion, as highlighted above, is used with significant
benefit in particular cancers; however, this ap-
proach is essentially limited to cancers localised
to a single site. Unfortunately, in the majority of
cases cancers are more widely disseminated, and
other approaches have been explored in attempt-
ing to achieve a degree of specificity or targeting.
These include encapsulation of cytotoxic drugs in
small particles such as microspheres or lipo-
somes, as has been described elsewhere in this
issue. The other area where considerable de-
velopment has occurred in the treatment of
malignancy involves the development of anti-
bodies to antigens up-regulated in tumour tissue
compared to normal tissues. It is now well
recognised that a number of these antibodies can
recognise tumour cells and some are in use as
imaging agents in cancer detection and diagnosis.
However, there is the potential to use these
agents therapeutically, with the aim of inhibiting
growth by a variety of mechanisms. Such ap-
proaches include (i) the inhibition of tumour
growth factors or their receptors, (ii) direct
tumour cell kill using antibodies tagged with
cytotoxic radionuclides or toxins, and (iii) the
use of anti-idiotype antibodies as vaccines de-
signed to stimulate host response to malignant
cells. While such approaches are considered
experimental at the current time, they represent
an exciting and challenging new area of cancer
research.
8. Achieving selective delivery: the future
Killing tumour cells is not difficult; the prob-
lem is how to avoid killing normal cells. The
perpetual search for selectivity and specificity in
cancer therapy has led to an explosion of exciting
new concepts. In particular, molecular biology
has suggested a range of new ways of directing
cytotoxins, or the machinery for making cytotox-
ins, to particular sites. The archetype for this
approach was of course the monoclonal anti-
body, or ‘magic bullet’. While the concept of
specifically targeting a highly toxic molecule,
such as the ricin A-chain, conjugated to a mono-
clonal antibody, still looks attractive, there clear-
ly remain significant problems of distribution. It
is not surprising then that many see immuno-
toxins as being more appropriate for the ‘mop-
ping up’ of solid tumour cells within the circula-
tion, or for leukaemias. A significant advance on
the strict immunotoxin approach is ADEPT
(antibody-directed enzyme pro-drug therapy)
[67]. The basis of this approach is that an enzyme
is targeted to the vicinity of tumour cells by
conjugating the enzyme to a tumour cell-specific
monoclonal antibody. A pro-drug, only activated
by that enzyme, is then administered systemical-
ly. In principle, active drug concentrations will be
highest immediately around the tumour cells
with minimal systemic toxicity. This approach is
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125
now entering clinical trial for the treatment of
colorectal cancer.
Perhaps the most exciting and technologically
challenging new approaches are those coming
under the heading of ‘gene therapy’. The basis of
these approaches is the delivery of autologous
genes into the tumour or associated host cells in
such a manner that they may be expressed in
those cells. The genes may code for a toxic
molecule, or may, as in the ‘VDEPT’ approach
[68], code for an enzyme not normally found in
human cells, which will activate a pro-drug. The
question of specificity has been addressed in two
ways: either the therapeutic gene is delivered
using a vehicle which recognises only certain cell
types; or, alternatively, the gene is presented to
all cells, encapsulated in a suitable viral vector, as
a chimaeric construct whose expression is con-
trolled by an upstream promoter sequence pre-
ferably active in particular cell types.
Two interesting gene therapy models using the
latter approach have recently been described.
These treatments are now planned to enter
clinical trial for malignant melanoma and breast
carcinoma. Vile and Hart [69] have designed a
gene construct which codes for a viral enzyme,
thymidine kinase, coupled to the promoter re-
gion for the tyrosinase gene, which is only
expressed in
melanocytes and pigmented
melanoma cells. This ‘tissue-specific’ construct
can be delivered systemically, encapsulated in a
retroviral vector. The enzyme will only be ex-
pressed in cells which have been transfected with
the vector, and which are inherently able to
‘switch on’ the tyrosinase gene. Upon administra-
tion of the pro-drug ganciclovir, a substrate for
thymidine kinase, a highly toxic agent is gener-
ated within these cells. Harris et al. [70] have
used a ‘tumour-specific approach’, targeting their
gene construct specifically to tumour cells. In this
case the gene for a bacterial enzyme, cytosine
deaminase, is linked to the promoter sequence of
erbB-2, a gene frequently up-regulated in breast
cancer cells. This construct produces active en-
zyme within breast cancer cells, which converts
the non-toxic pro-drug Sfluorocytosine to highly
toxic 5-fluorouracil in the tumour cells.
Finally, a very exciting combination of gene
therapy coupled to radiotherapy has recently
been described. Weichselbaum and colleagues
[71] have designed a gene construct which con-
tains within its promoter region a radiation-re-
sponsive element. Upon irradiation with conven-
tional doses of X-rays, this construct initiates
transcription of the gene coding for the toxic
cytokine TNF-a. This combination of gene
therapy and conventional irradiation has demon-
strated a significant increase in tumour cures in
murine models compared with radiotherpay or
gene therapy alone.
The central problem of cancer treatment is
now one of specificity; exquisitely toxic agents
are already available. The question remains how
to get those agents to the cancer cells and to no
others. It is clear that, in spite of the set-backs
and disappointments, progress is being made in
this field. The solution lays in the application of
the clear advances that are being made within
the wider range of physical and chemical sciences
to this problem.
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