Prodrug approach Prodrug: is a pharmacologically inactive compound that is converted into an active drug by metabolic transformations. Soft drug (antedrug): is pharmacologically active drug that will be metabolized to terminate its action and facilitate excretion.

The useful applications behind prodrug design Improving aq. Solubility Improving absorption and distribution Site specificity: depends on the presence of certain enzyme that will release the active form of drug at the site of action Increase biological stability: mainly by interfering with drug metabolism

The useful applications behind prodrug design 5. Minimize possible toxicity: Unpleasant taste or odor Gastric irritation Pain at the site of injection 6. Improving patient compliance 7. Solve some formulation problems

Types of prodrugs Carrier-linked: here the drug is linked to a carrier group that can be removed enzymatically: Bipartate: if the drug is directly attached to the carrier group Tripartate: if there is a linker group between the drug and the carrier moiety Mutual prodrug: here two drugs are linked together, they are synergistic to each other

Types of prodrugs Bioprecursor: Compound that is metabolized by molecular modification into either an active form or into an intermediate that will be farther metabolized. Here oxidation is the main metabolic biotransformation involved in the activation of bioprecursors

Carrier-linked prodrugs Drugs with alcohol and carboxylic acid groups: The common carrier group is the ester (WHY?) Availability of esterase enzyme Possible of either increasing or decreasing the lipophilicity of the parent drug. Can be either easily hydrolyzed or relatively stable ester

Drugs with alcohol and carboxylic acid groups

Drugs with alcohol and carboxylic acid groups Small sized esters will be easily hydrolyzed than sterically hindered esters Electronic factor may play a role in the rate of hydrolysis:

Drugs with alcohol and carboxylic acid groups

Drugs with alcohol and carboxylic acid groups Drugs having carboxylic acid group can be esterified to: Increase lipophilicity. Increase Pka: this can be done by making the choline ester of these drugs:

Drugs having amine moiety The amide derivative is not a suitable choice in most of the case (WHY?)….Stable bond toward hydrolysis compared to the ester bond. Some are more susceptible to cleavage: Amides of amino acids. Phenyl carbamate

Drugs having amine moiety amine prodrugs might have lowered Pka…lower ionization… more lipophilic than the parent drug, the followings are examples of some lipophilic prodrugs from amine:

Drugs having amine moiety

Drugs having amine moiety

Drugs having carbonyl group

Examples Prodrugs to increase water solubility: Esters with anionic solubilizing agent are poorly hydrolyzed while with cationic solubilizing groups will be easily hydrolyzed

Examples Prodrugs to increase water solubility:

Examples Prodrugs to improve absorption through biological membranes:

Examples Prodrugs to improve absorption through biological membranes:

Examples Prodrugs for site specificity: Increasing lipophilicity will increase the access to almost all tissues. Factors affecting the penetration though BBB: Molecular size. Lipophilicity Stability toward enzymes available in BBB

Examples Prodrugs for site specificity: Progabide is an example.. Will b e hydrolyzed after passing the BBB. Tumor cells have elevated amount of phosphatase and amidase.

Examples Prodrugs for site specificity: Approaches of site specific anti-tumor agents: Enzyme-prodrug therapy: Achieve selective activation of prodrug at a desired site Prodrug activating enzyme is incorporated into the target tumor cells, then the prodrug will be administered systemically (what are the drawbacks of this approach?) Antibody-directed enzyme prodrug therapy: An antibody produced against a tumor cell line. Then linked to the exogenous activating enzyme. This antibody-enzyme complex will be accumulated on the tumor cells. Prodrug will be administered then will be activated once reaches tumor cells.

Examples Prodrugs for site specificity: Antibody-directed enzyme prodrug therapy:

Examples Prodrugs for stability: Either physicochemical stability or biological stability:

Examples Prodrugs for stability:

Examples Prodrugs to decrease ionization in GIT…. Gives better oral availability:

Examples Prodrugs to improve chemical stability: Examples are the acid stable penicillins

Examples Prodrugs to improve rectal absorption: Mainly drugs are absorbed from rectum through passive diffusion (the same as from intestine) Passive diffusion depends on the degree of lipophilicity and water solubility. Little amount of fluid is present in the rectum, this means that drug must have greater water solubility in order to be dissolved in rectum.

Examples Prodrugs to improve ocular delivery: Problems in ocular delivery: Is to attain the optimal drug concentration Tear turn-over Poor conjunctival absorption due to the specialized corneal membrane Only 1-10% of the topically applied treatment to eye will be absorbed LogP of drug administered to eye must be 2-3

Prodrugs to minimize toxicity: Dipifevrin is an example…less toxic than epinephrine because it will act locally in the ocular cavity. Aspirin prodrugs:

Prodrugs to encourage patient acceptance:

Prodrugs to eliminate formulation problems: Example: formaldehyde is normally used as antiseptic and disinfectant, but it is highly unstable and toxic…converted to more stable adamantane:

Macromolecular drug delivery Drug here is covalently attached to a macromolecule such as synthetic polymers, glycoproteins, lipoproteins, albumin, liposome, DNA and antibodies. Advantages: Targeting specific target. Improving therapeutic index and lowering toxicity Reducing drug metabolism and excretion

Macromolecular drug delivery Disadvantages: Steric hindrance may affect chemical and enzymatic hydrolysis of drug-macromolecule complex. Immunogenic Large –sized complex…might not get into the cell, except by pinocytosis…then drug will be released after biodegradation by lysosomes.

Macromolecular drug delivery By synthetic polymers: Drug here is attached to totally synthetic polymers Sustained release of ibuprofen Prolonged anti-inflammatory effect

Macromolecular drug delivery By synthetic polymers: Sometimes a spacer is added between the drug and the macromolecule…to enhance the biodegradation and the release of drug:

Macromolecular drug delivery By poly amino acids: Here the drug is attached to peptide chain. The advantage is the biodegradability of the peptide chain…mainly when the amino acids are in the L-stereochemistry.

Macromolecular drug delivery To improve site specificity: Solubilizer group such as ionizable hydrophilic group Homing device….for targeting specific site…such as antibodies

Macromolecular drug delivery To improve site specificity: 40X less toxic than nitrogen mustard

Tripartate prodrugs This is used to overcome the problem of too labile or too stable bipartate prodrugs Sometimes called double prodrugs. Here the drug is attached to a linker that is in turn will attach to the carrier group. First the carrier will be removed by hydrolysis, then an unstable intermediate will be formed…rearranged to release the active drug

Tripartate prodrugs The methyl ester did not give the same improvement in absorption and activity (why?).

Dihydropyridine as a carrier for CNS targeting

Dihydropyridine as a carrier for CNS targeting In this delivery system: The dyhydropyridine-drug complex will be lipophilic…better penetrate into the CNS through BBB. Then the dihydropyridine will be enzymatically oxidized to give the pyridinium ion…ionized…water soluble… trapped within the CNS. Then the drug-pyridinium complex will be hydrolyzed to release the active drug and the non toxic N-methyl nicotinic acid.

Redox tribartate drug delivery system

Peptides as drug delivery system Peptides are normally coupled to drugs to: Improve oral availability Improve skin penetration Enhance antibacterial action. This depends on the fact that cell membranes have peptide transport integral proteins called permease that will actively ingest peptides and peptidomimetic agents into the mammalian cells as well as bacterial cells.

Examples of peptide carrier systems

Mutual prodrugs Applied when it is necessary for two synergistic drugs to be at the same time at the same site. Mutual prodrug is a bipartate or tripartate prodrug in which the carrier is the synergistic drug.

Mutual prodrugs If the two synergistic drugs are given at the same time but separately, they will not necessarily reach the same site of action at the same time. Example: the common combination of amoxicillin and clavulanic acid, or Gentamicin with Carbenicillin. Example of mutual drug is sultamicillin

Bioprecursor prodrugs Bioprecursor prodrugs mostly use either oxidative or reductive activation reactions. The main activation pathways are: Proton activation. Hydrolytic activation Elimination activation Oxidative activation Reductive activation Nucleotide activation Phosphorylation activation Sulfation activation Decarboxylation activation

Bioprecursor by elimination activation Isoxazole ring is known to undergo elimination to nitrile

Bioprecursor by N and O alkylation

Bioprecursor by Oxidative deamination

Bioprecursor by aromatic hydroxylation

Bioprecursor by Alkene oxidation

Bioprecursor by Reduction

Bioprecursor by Nucleotide activation

Bioprecursor by Phopshorylation

Bioprecursor by decarboxylation

Bioprecursor by decarboxylation In order to increase the concentration of L-dopa reaching brain, Peripheral L-amino acid decarboxylase inhibitor is normally administered with L-dopa; carbidopa or benserazid are normally given.

Selected topics in drug design Structure-based drug design: The macromolecular target can be isolated and crystallized…then the structure will be determined using X-ray crystallography. This structure will not give information about the binding site. The co-crystal structure (structure of protein with the inhibitor inside) is better (WHY?): Where is the active site. The distance between inhibitor and binding site boundaries The possible bonds between inhibitor and binding site

How to use the co-crystal structure in drug design First the inhibitor will be removed from the active site (in-silico). Then lead compounds will be inserted (docked) into the active site to see how they fit. Best fit compounds will be synthesized and tested for activity.

De-novo Drug Design Involves the design of a novel drug structure based on a knowledge of the binding site alone. In this approach, you must have good supply of enzyme. Sometimes the bacterial version of enzyme will be used (obtained by recombinant DNA technology). The major drawback of this, is the bacterial version is not identical to the human enzyme…. But it can be considered very similar.