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

SSDI 0169-409X(95)00044-5



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-



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



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-



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



[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



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.



124

.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



J.C. Mur ray, J. Carmi chael I Adv anced Dr ug Deli very Reviews 17 (1995) 117-127

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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review article pharma targeting solid tumors

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