PRODRUG APPROACHES FOR

CNS DRUG DELIVERY

Review Article By : Jarkko Rautio, Krista Laine, Mikko Gynther, And Jouko Savolainen

Published In The AAPS Journal, Vol. 10, No. 1, March 2008

Seminar Presented by Atulkumar Thakur1st Year M.Pharm

Department of PharmaceuticsMSRCP

CONTENTSI. IntroductionII. Prodrug ConceptIII. Role of Increased Lipophilicity In CNS

Drug DeliveryIV. Endogenous Transporters In CNS

Prodrug DeliveryV. Overcoming Efflux TransportVI. Antibody And Gene-Directed Prodrug

TherapiesVII. ConclusionsVIII.References

INTRODUCTION Blood Brain Barrier(BBB):- Structural and functional barrier

for the delivery of therapeutic agents to the CNS.

Passage across BBB:- Main limiting factor for the delivery of potential CNS drugs into the brain parenchyma.

BBB is formed from capillary endothelial cells which are very closely joined together that restrict the paracellular diffusion of hydrophilic drugs.

The brain posseses effective efflux systems that actively remove solutes from brain tissue creating a great challenge for potential neurotherapeutics.

Increased understanding of BBB physiology has led to rational chemistry and biology based drug delivery strategies.

Fig. 1 : Chemistry and biology based approaches to increase brain delivery of CNS targeted

therapeutic agents

For a drug to readily cross the BBB by passive diffusion – less molecular weight , lipid soluble, neutral and capable of forming less than eight H-bonds with water.

Hence prodrug approach is an attractive chemistry based strategy to increase the CNS transport of poorly penetrating therapeutic agents by their chemical modification.

PRODRUG CONCEPT

The term Prodrug or proagent – Introduced in 1958 – Described as bioreversible derivatives of drug molecules that must undergo a chemical or enzymatic biotransformation to the active forms within the body, prior to exerting a pharmacological action.

The major goal in Prodrug design is to overcome the various physicochemical, pharmaceutical, biopharmaceutical, and/or pharmacokinetic limitations of parent drug, which otherwise would hinder its clinical use.

By applying Prodrug technology, the clinical usefulness of a drug molecule may be enhanced without modifying the pharmacological activity of a parent drug.

Prodrug approaches utilize passive drug uptake processes into CNS by chemically modifying a drug to become more lipophilic.

Such chemical derivatives are known as Chemical delivery systems (CDSs) and other prodrug approaches are carrier mediated and receptor mediated prodrug transport as well as gene-directed enzyme prodrug therapy.

ROLE OF INCREASED LIPOPHILICITY IN CNS DRUG DELIVERY

A frequent challenge with new drug candidates, in regard to CNS delivery is high polarity.

Transcellular diffusion through the cell membranes the only feasible passive route for entering the CNS. This makes adequate lipophilicity one of the key elements in passive CNS entry.

A very simple approach to increase the CNS entry of polar molecules would be the masking of polar functionalities within such compounds. This is sometimes referred to as lipidization of molecules.

Lipidization through prodrugs offers a possibility for a more efficient CNS delivery of polar drugs.

Lipidization Approach :

• With increased lipophilicity, for example through simple ester prodrugs, increased CNS access due to the more lipophilic nature of the prodrugs.

• The general criteria of site specific drug delivery through prodrugs :

- The prodrugs must have ready access to the appropriate tissue within the CNS.

- Bioconversion back to the parent drug should be highly siteselective, and

- The parent drug should exhibit prolonged retention within the target tissue.

• Increased lipophilicity through prodrugs does not alone ensure more efficient CNS therapy. It may also lead to concerns of toxicity.

• For this, the peripheral bioconversion of the drug should be slow enough to avoid excess premature bioconversion and the CNS bioconversion fast enough to enable therapeutic drug levels within the CNS before prodrug clearance.

The role of BBB efflux transporters in lipophilic prodrug CNS entry

• The figure illustrates the challenge when the parent

drug is a substrate for an efflux transporter at the BBB. In the case of prodrug A, an active efflux component combined with a relatively slow bioconversion within the CNS gives only minimum parent drug levels in the CNS.

• In the case of prodrug B, the fast bioconversion within the CNS enables substantial levels of parent drug within the CNS, despite the active efflux component.

• Increased lipophilicity still remains one of the key aspects that must be taken into an account when developing prodrugs for CNS delivery.

• With CNS prodrugs, the target tissue bioconversion needs to both rapid and selective enough to compete with elimination from the target tissue, and also to ensure that the premature bioconversion of the prodrug is low enough.

Chemical Drug Delivery Systems :

• A successful prodrug approach that utilizes improved lipophilicity and also requires a sequential bioactivation steps for conversion to an active drug and a brain tissue trapped intermediate is often referred as a chemical drug delivery system.

• The CDS term coined by Bodor and coworkers to distinguish this approach from prodrugs that typically require only a single activation step.

• The principle of CDS, in addition to providing access to the brain by increasing the lipophilicity of a drug, exploits specific properties of the BBB to lock drugs in the brain on arrival and prevent them from re-crossing the BBB.

• CDS exploits the linking of an active drug molecule to a bioremovable lipophilic targetor moiety, 1,4- dihydro-N-methylnicotinic acid, which results in a derivative that readily distributes throughout the body and brain after administration due to its lipophilic character.

ENDOGENOUS TRANSPORTERS IN CNS PRODRUG DELIVERY

The BBB expresses several different transport mechanisms that enable the CNS delivery of compounds that are fundamental to the normal functions of the brain.

These transporters have become a target to drug or prodrug design in an attempt to ferry drug molecules across the BBB via carrier-mediated transport.

The endogenous BBB transporters can therefore be classified into two categories:-

- Carrier-mediated transport.

- Active efflux transport.

Carrier-mediated Transport:

• Several specific endogenous influx transporters have been identified at the BBB for nutrients, such as amino acids, glucose and vitamins.

• Many drug molecules have similar structural properties

to endogenous substrates.

• Chemical drug modification in a way that the drug can be recognized by specific transporters.

• One attractive approach is to conjugate an endogenous transporter substrate to the active drug molecule in a bioreversible manner, i.e., to utilize the prodrug approach.

• The prodrug should be recognized by the specific transporter mechanism at the BBB, and more importantly transported across the BBB to brain tissues, where the release of an active drug from the prodrug should take place.

Large neutral amino acid transporter:- This transport system is expressed on the luminal and abluminal membranes of capillary endothelial cells, and efficiently transports neutral L-amino acids into the brain.

Several clinically useful amino acid mimicking drugs, such as gabapentin and melphalan, have been shown to be delivered into the brain predominantly via cerebrovascular LAT1-mediated transport, thus demonstrating the ability of LAT1 to be utilized in drug delivery.

The only prodrug that is used clinically for entering the brain predominantly through LAT1-dedicated transport is L-dopa.

The conversion of dopamine into its alpha-amino acid, L-dopa, enables the brain to uptake dopamine via LAT1.

Example of LAT1 utilizing prodrugs is 4-chlorokynurenine, a prodrug of 7-chlorokynurenic acid.

7-Chlorokynurenic acid is an N-methyl-D-aspartate antagonist that crosses the BBB poorly because of its high hydrophilicity. But conversion into 4-cholorokynurenine enable it to pass through BBB via LAT-1 mediated transport.

OVERCOMING EFFLUX TRANSPORT

Transport out of the brain also plays a critical role in efficient drug delivery.

Different efflux transporters present in BBB act as clearance systems for compounds and also efflux the incoming drugs thus resulting in low brain interstitial concentration.

Important functional groups of drug molecules that afford recognition of efflux transporters could be masked with promoities.

Prodrug approach by inhibiting the function of efflux transporters by co-administration of efflux substrate drugs with efflux inhibitors.

An efflux inhibitor could also be conjugated with therapeutic drugs to form a co drug.

• Co drug approach explored to improve L-dopa brain delivery with a potent inhibitor entacapone, as a form of L-dopa-entacapone co-drug. • This approach applicable to lipophilic drugs able to cross the BBB, due to their lipophilicity, but restricted from efficient brain entry by efflux transporters.

• Some metabolites causing neuro diseases are cleared from the CNS by efflux systems.

• But the chronic inhibition of these efflux systems might lead to an accumulation of neurotoxins in the brain.

• Chronic dosing of efflux inhibitors may possibly upregulate other efflux systems at the BBB as a compensatory mechanism raising further issues associated with clinical use of these inhibitors.

Transporter Substrate InhibitorP-glycoprotein Saquinavir, Vincristine VerapamilMRP1-6 Melphalan, AZT ProbenecidBCRP Topotecan CF 120918Oatp1-3 Rosuvastatin, Enalapril Rifampicin, ProbenecidOATP-A Bile acids RifampicinOAT3 Cephalosporin antibiotics Acyclovir, Probenecid

ANTIBODY AND GENE-DIRECTED PRODRUG THERAPIES

• More selective prodrug activation in targeted tissues can be achieved by exogenous prodrug bioconverting enzymes that are delivered via monoclonal antibodies or generated from genes encoding an exogenous enzyme.

• These approaches are referred to as ADEPT and GDEPT, and also known as suicide gene therapy, both useful means of targeting prodrugs especially towards various tumors.

• Bacterial gene cytosine deaminase, sensitizes tumor cells to the antifungal drug 5-fluorocytosine.

• 5-FC is then bioconverted to the anticancer drug, 5-fluorouracil.

• However, no significant benefit when compared to standard therapy such as surgical resection and radiotherapy.

• Further optimization of gene transfer vectors is essential for improving the clinical effectiveness of GDEPT.

CONCLUSIONS Recent advances in biotechnology and pharmaceutical sciences

have greatly expanded the number of drugs that are being developed for the treatment of CNS disorders.

However, the BBB presents a major structural and functional barrier for drugs that have pharmacological targets within the brain.

A potential option to improve CNS drug delivery is in the utilization of prodrug technology.

Prodrugs have already become an integral part of the drug design and delivery processes, it has been estimated that 5-6% of all marketed drugs worldwide are prodrugs; in recent years, approximately 15% of all new approved drugs were prodrugs.

There is a strong belief to justify the idea that prodrugs hold great potential for CNS drug delivery.

A better understanding of molecular biology will certainly provide more insight in the mechanisms of enzymes and endogenous transporters that exist at the BBB.

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