Tumour Targeted Drug Delivery Systems

Introduction:

1. Anti cancer drugs are very toxic and their side effects can be minimized if we can target

them to the specific tumour tissue.

2. Targeting of drugs to tumour tissue will also increase the effectiveness of the treatment.

3. Tumour targeted drug delivery systems are classified basing on three approaches, passive

targeting, active targeting and external stimuli triggering.

Passive targeting:

1. The blood vessels are different in normal tissue and tumor tissue.

2. In normal tissue, the blood vessel wall has an endothelium layer and supportive layers.

They act as a physical barrier between blood and tissue cells. The gap between

endothelial cells in a normal blood vessel is around 2 nm. The tumour targeted drug

delivery systems (liposomes, nano particles etc.) cannot leave blood and go into normal

tissue.

3. In tumor tissue, blood vessels are formed by irregular shaped endothelial cells and

defective supportive layers. These blood vessels are leaky (pore size 200 to 780 nm) and

are highly permeable to macro molecules / liposomes / nano particles etc. Hence, drug

delivery systems with a particle size smaller than 200 nm can cross through leaky blood

vessels into tumor tissue. This is called passive targeting.

4. Normal tissues such as glomerulus of kidney, liver and pulmonary region have blood

vessels with discontinuous endothelium and the tumor targeted drug delivery systems will

reach these tissues also. The anti cancer drug will be released in these normal tissues also

and toxic side effects will be seen in these tissues.

5. The lymphatic system is present in normal tissues and a functional lymphatic system is

absent in tumor tissue. This also helps in targeting of drugs to tumors.

6. The lymphatic network in normal tissues collects and returns interstitial fluid to the blood

to maintain fluid balance in the organs.

7. The inside of tumor tissues does not have a functional lymphatic system and this result in

retention of high molecular weight molecules in tumors.

8. The combined effect of leaky blood vessels and impaired lymphatic systems in tumor

tissue is called as enhanced permeability and retention (EPR) effect. Passive targeting of

anti cancer drugs is due to this EPR effect.

9. The drug delivery system should circulate in the blood for sufficient time to reach the

tumor target. The renal system and reticulo endothelial system try to clear / remove the

drug delivery system from circulation.

10. A tumor-targeted drug delivery system should avoid these clearance mechanisms.

11. Renal clearance depends on molecular weight and size of the polymeric drug carrier.

Hence renal clearance can be avoided by selecting larger polymer molecules that cannot

be filtered in the kidney.

12. The tumor targeted drug delivery systems (liposomes, nano particles) are taken up by

mono cytes and macrophages (RES uptake) in the blood. RES uptake can be significantly

reduced by modifying the tumor targeted drug delivery systems particles surface with

non-immunogenic water-soluble polymers and controlling particle size below 100 nm.

13. A successful example of escaping RES uptake is, small liposomes stabilized by attaching

poly ethylene glycol (PEG) chains on their surface.

Active targeting:

1. In active targeting, tumor targeted drug delivery systems will have a drug, carrier with a

ligand group on its surface that will bind with receptors present on tumor cells.

2. On administration, the ligand will direct the tumor targeted drug delivery system to tumor

tissue. Inside the tumor tissue, drug release is triggered by a change of micro environment

in tumor (such as abnormal low pH or high temperature, or the presence of tumor specific

enzymes).

3. Example: Folate receptor is a membrane protein with high binding affinity to folates.

Folate receptors are negligible in most normal tissues but are present in many human

tumors of ovary, breast, lung and brain.

4. Folic acid is used for folate receptor targeting. It is conjugated via its carboxylic acid

group to the drug.

5. Example: Transferrin receptor is a cell membrane glycol protein that binds to transferrin

to transport iron ion into cells by endocytosis.

6. Human transferrin receptors are present in tumor cells that need iron for rapid growth of

tumor cells. Transferrin receptors have been used in the treatment of brain tumours.

External Stimuli Triggered Systems:

1. External stimuli can also be used for triggering drug release at a specific time and site.

For example, magnetic nano particles or microspheres release drug upon the application

of magnetic field.

2. Drug delivery systems that are ultrasound sensitive are also being investigated.

TUMOR-TARGETED DRUG DELIVERY SYSTEMS:

1. There are two classes of tumor - targeted drug delivery systems — macromolecular

conjugates and particulate systems. (Fig 2)

2. Macromolecular conjugates contain an anti cancer drug bound to a large polymeric carrier

molecule. (Fig. 3). The conjugated drugs have to be freed to exert antitumor activity, and

drug release is usually controlled by selecting suitable linkers.

3. Particulate drug delivery systems contain drugs embedded in the particles (liposomes,

nano particles). Once these particles enter tumour tissue, they are destabilized, and the

drug is released.

Fig. 3 A tumor-targeted polymer-drug conjugate. The major elements include: (i) a polymeric

drug-carrier that is water soluble, bio compatible or bio degradable, non-immunogenic; (ii)

targeting moieties; (iii) a linker between a drug and the carrier. The linker can be: a) a

chemical bond such as ester or amide. An ester bond is more stable at lysosomal pH than at

plasma pH (7.4) while an amide bond is stable at both lysosomal and plasma pH; b) an oligo

peptide linker that is degradable by specific enzymatic hydrolysis; and c) an acid labile linker

that is degradable at lysosomal pH but stable at plasma pH.

Macro molecular conjugates:

Macromolecular conjugates contain an anti cancer drug bound to a large polymeric carrier

molecule like PEG, poly glutamic acid, hydroxyl propyl methacrylamide. (Fig. 3). The

conjugated drugs have to be freed to exert antitumor activity, and drug release is usually

controlled by selecting suitable linkers.

PEG:

1. PEG is a water-soluble and biocompatible polymer.

2. PEG conjugates of small anticancer drug molecules were developed to enhance water

solubility, improve in vivo stability, and eliminate formulation-related side effects.

3. The molecular weight of PEG was 20KDa or higher to reduce renal clearance and to

achieve desired passive targeting.

4. Preclinical studies of PEG - paclitaxel and PEG – ala –camptothecin (PROTHECAN)

demonstrated significantly increased aqueous solubility, enhanced antitumor activity, and

reduced toxicity.

Poly glutamic acid:

1. As a polymeric drug carrier, poly (glutamic acid) has several advantages. It is made of a

naturally occurring amino acid.

2. It is water-soluble, biodegradable, and non-immunogenic.

3. It has multiple pendent carboxyl groups that can be conjugated to therapeutic molecules.

4. A number of small molecule anticancer drugs have been conjugated to poly (glutamic

acid) directly through an ester or an amide bond, or via an oligo peptide linker.

5. Example: DOX, paclitaxel, camptothecin, daunorubicin, mitomycin C, etc.

6. In general, the conjugates have shown greater in vivo activity than the parent drugs due to

EPR effect.

Immunoconjugates:

An immunoconjugate is an efficient antitumor agent produced by covalently coupling a

cytotoxic drug to a MAb (monoclonal anti body) that is specific to tumors containing

antigens. On administration, the immune conjugates are taken up by tumor cells and drug is

released inside the cell.

Other conjugating agents are N-(2-hydroxypropyl) methacrylamide and Styrene-co-maleic acid-half-

butylate.

Particulate drug delivery systems:

Liposomes:

1. Liposomes are vesicles (small pouch of 0.1 to 0.5 micro meter) made up of lipid bi layers

enclosing an aqueous phase.

2. Hydrophilic drug molecules are present in the interior aqueous compartment and

lipophillic drug molecules are trapped between the lipid bi layer region.

3. Liposomes are rapidly eliminated from the blood circulation by the RES; hence their

surface is modified with PEG polymer.

4. PEG polymer chains sterically prevent hydrophobic and electrostatic interactions of blood

components with the liposome surface. It has been reported that up to 72 hr of half-life in

the blood has been obtained with pegylated liposomes.

5. These pegylated liposomes extravasate into tumors with leaky vasculature, liposomes get

destabilized and drug is released.

6. The anticancer drugs delivered by liposomes include doxorubicin, daunorubicin,

camptothecin, etc.

7. Mechanisms of drug release from liposomes are not thoroughly understood. pH /

temperature / enzymes of tumour cells destabilize the liposome and drug is released.

8. The most successful development is Doxil in the U.S. market. Doxil has shown

significant clinical advantages over free DOX (doxorubicin).

Actively Targeted Liposomes:

1. By coupling targeting moieties to the surface of liposomes, we get actively targeted

liposomes.

2. Liposomes containing tumor-targeting ligands deliver the drug to tumor tissues with a

high selectivity.

3. Example: Folate - targeted liposomes, obtained by coupling folate using PEG were

prepared to deliver doxorubicin to tumor cells having large amounts of folate receptors.

The cellular uptake of doxorubicin was increased 45-fold over similar non - targeted

liposomes, but an in vivo study revealed that the tumor deposition of folate - targeted

liposomes was not significantly different from that of non-targeted liposomes.

4. Immuno liposomes containing monoclonal antibodies is an emerging strategy for tumor

targeting. The immune liposomes generally showed more significant therapeutic

responses than non-targeted liposomes or the free drug.

Polymeric Micelles:

1. Polymeric micelles are formed by the self-assembly of amphiphilic block copolymers.

2. Polymeric micelle possesses a hydrophobic core and a hydrophilic surface.

3. These polymeric micelles have good stability, hydrophilic surface, and are of small size.

They stay stable during circulation in the blood, unrecognizable by RES, and do not bind

with biological components.

4. The most commonly seen hydrophilic block is PEG with molecular weight in the range of

1,500 – 20,000 g/mol, and examples of hydrophobic blocks are poly (amino acids), such

as poly (aspartic acids), poly (glutamic acids), or poly lysine, and their derivatives, and

biodegradable polyesters, such as poly lactide, poly capro lactone, and poly(ethylene

imine) or poly (propylene oxide).

5. A variety of drugs such as hydrophobic drugs, ionic drugs, or nucleotides can be loaded

into polymeric micelles. Block copolymers containing poly (amino acids) also provide

pendent functional groups for drug conjugation.

6. Temperature or pH-sensitive polymeric micelles were also investigated to facilitate drug

release processes (Fig. 6).

7. Dox (doxorubicin) was entrapped inside polymeric micelles made from PEG – Poly

aspartic acid block co polymer. Compared to free DOX, the micelle system showed a

higher level of DOX in tumor tissues.

8. Preclinical studies on the DOX-loaded pluronic micelles revealed superior tumor

inhibition and extended lifespan to free drug.

9. A folate - targeted pH-sensitive polymeric micelle system was developed for active

targeting. DOX was loaded into micelles of PEG - block - poly histidine. The system was

designed to deliver cyto toxic molecules into tumour cells having folate receptors. Drug is

released intra cellular at acidic conditions by the destabilization of the histidine block.

Polymeric Nano particles:

1. Polymeric nano particles are nano scale aggregates of biocompatible or biodegradable

polymers. The size of nanoparticles varies from 10 to 1000 nm, and particles with a size

smaller than 200 nm are preferable for tumor-targeted drug delivery.

2. Transferrin - targeted PEG coated poly cyano acrylate nano particles were developed for

the delivery of paclitaxel. The encapsulation efficiency was 93.4% and the average size

was 101.4 nm. Sustained release of the drug was observed over 30 days in mice.

Transferrin-targeted nano particles showed significantly higher antitumor activity than

non-targeted nano particles and free paclitaxel.

Biological Ghost Delivery Systems:

1. Biological ghosts are derived natural particles from endogenous cells, bacteria, or viruses.

They are biocompatible, biodegradable, and non-immunogenic drug carriers.

2. Membrane vesicles from biological ghosts are obtained by removing the enclosed

contents of cells, bacteria, and viruses.

3. Erythrocyte and bacterial ghosts have been studied for the delivery of small molecule

anticancer drugs and gene delivery, while virus envelops have been extensively used as

vehicles for gene delivery.

4. Folate-targeted erythrocyte ghosts of doxorubicin were prepared for targeting to tumour

cells.

Conclusion: Targeting to tumour cells can be done by passive and active targeting using

liposomes, polymer micelles and nano particles.

Dr. M. Eswar Gupta

Professor, Sir C.R.Reddy College of Pharmaceutical Sciences,

Eluru, A.P., India- 534001, Cell: 9885523760, email: meguptas@yahoo.com