Pro-drugs

Question 1

Most common method of drugs crossing cells

Answer 1

Passive diffusion through the phospholipid bilayer

Question 2

The rate of passive diffusion is governed by three factors:

Answer 2

The concentration gradient across the membrane
The partition coefficient P for the drug between the lipid phase of the cell membrane and the aqueous phases either side
For very large molecules, increasing molecular size also slows down membrane penetration

Question 3

What is lipophilicity?

Answer 3

This represent s the preference for a drug molecule to dissolve in the lipid phase of the cell membrane rather than aqueous phases

Question 4

What determines lipophilicity?

Answer 4

The structural components of a molecule- polar groups reduce lipophilicity

Question 5

The cell membrane is very difficult to work with experimentally, what is used to approximate it?

Answer 5

In most cases a simple organic hydrophobic solvent is used, and the partition between this solvent and water is measured
Octanol is often used as the lipid

Question 6

Simplest experiment for lipophilicity?

Answer 6

A drug sample is partitioned between layers of octanol and water, and the concentration of sample in each layer is measured

Question 7

Measurement of logP

Answer 7

P= concentration in organic octanol layer/ concentration in aqueous layer
Often represented as Log10P

Question 8

Other solvents (not octanol) that can be used

Answer 8

Particularly for CNS-acting drugs, where access may be especially difficult, combined figures for octanol/water and cyclohexane/water may be more reliable

Question 9

Acidic/basic drugs

Answer 9

If a drug molecule may be protonated at or near physiological pH, its lipophilicity will vary greatly with the pH of measurement
The ionised form of a drug will have a much lower P than the unionised form, this may be approached by using buffer pH 7.4 in place of water

Question 10

What is logD?

Answer 10

LogD describes the ratio of the concentrations of all forms of the compound (ionised plus unionised) in each of the two phases (normally at pH 7.4)

Question 11

How to easily modify a chemical structure to raise the logP?

Answer 11

Block polar functional groups e.g. acid with less polar functional groups e.g. ester, hydrocarbon side chain

Question 12

Effects of logP being too high/low

Answer 12

If P is low, passive diffusion will be slow and drug action limited
If P is too high the majority of drug may reside in the lipid phase, far from the site of action, and correlates with very poor watersolubility

Question 13

Optimum logP

Answer 13

Between 1 and 2 is often regarded as ideal for many drug sites
Lipinski’s rule suggest lower than 5 for good absorption

Question 14

How can drugs with much lower logP access tissues?

Answer 14

The aqueous pore route (entry via gaps between cells), only generally effective for small molecules
Active (specific) membrane transport systems, generally effective for drugs which resemble natural metabolites e.g. nucleosides, peptides

Question 15

In case of a failed drug delivery to target tissue, three approaches may work:

Answer 15

Pharmaceutical technologies
Analogue synthesis
Pro-drug synthesis

Question 16

What is analogue synthesis?

Answer 16

An initial failed lead drug may be very greatly improved in terms of drug access by the synthesis of close structural analogues
Solubility, lipophilicity, stability to metabolism etc. can all be modified by analogue synthesis
However, it is very easy to simultaneously lose potency at the drug target
Very demanding on resources

Question 17

What is a pro-drug?

Answer 17

A pro-drug is a molecular derivative of a drug which requires structural transformation in vivo to exert its pharmacological action

Question 18

Advantages of pro drugs (5):

Answer 18

Improved tissue penetration by altered lipophilicity and or solubility
Reduced first pass metabolism improving bioavailability
Selective transformations of pro-drug in target tissue may reduce or remove unwanted systemic effects of drug
Altered pharmacokinetics e.g. altered duration of action
Improved pharmaceutics e.g. reduced GI irritation

Question 19

Disadvantages of pro-drugs (5):

Answer 19

More complex- more to worry about
Possible chemical instability/ storage problems
Interspecies variability of metabolism
Possible extra toxicity of pro-drug or moiety
Extra production costs

Question 20

Pro-drugs derived from carboxylic acids (not examples!)

Answer 20

Several different structural types e.g. esters, amides etc.
Either the acid or the other component can be the pro-drug
Protection of these structural features is often desirable as it increases lipophilicity
Many enzyme systems are capable of ester/ amide hydrolysis to release the drug

Question 21

Simple aromatic and aliphatic ester pro-drugs

Answer 21

Very popular as easy to prepare, relatively chemically stable, and pro-moiety often non-toxic
e.g. drug- enalaprilat, pro-drug- enalapril
Enalaprilat ahs <12% absorption, while enalapril has 50-75% absorption due to a higher logP

Question 22

Difference between pro-drug and drug

Answer 22

The pro-drug is the one you take, the drug is the active form

Question 23

Amides as pro-drugs

Answer 23

Less popular than esters due to the slow rate of in vivo hydrolysis of amides (regeneration of drug from pro-drug)
Exceptions involve the use of amino acids/peptides or other substituted amides

Question 24

Pro-drugs derived from inorganic acids

Answer 24

Sulphuric and phosphoric acid derivatives have been used a pro-drugs, largely these are used to increase water solubility rather than lipophilicity

Question 25

Clindamycin/ clindamycin phosphate

Answer 25

The phosphate gives much reduced pain at the injection site, in vivo half life of 10 minutes

Question 26

Fludarabine

Answer 26

Fludarabine monophosphate is a useful oral pro-drug of fludarabine; the patent life of the parent drug was thus extended
Dose- by mouth, adult 40mg/m2 for 5 days every 28 days usually for 6 cycles

Question 27

Fospropofol

Answer 27

Fospropofol is a water soluble sedative/ anaesthetic, which is transformed with alkaline phosphatase to make propofol

Question 28

Pro-drugs activated by oxidation/ reduction

Answer 28

Oxidative/ reductive enzyme systems are widely distributed and are finding increasing use in pro-drugs
Sulindac, which makes sulindac sulphide, has 100x the water solubility of the drug and less GI toxicity

Question 29

Pro-2-PAM

Answer 29

Pro-2-PAM has vastly improved BB penetration, leading to a selective concentraion in the brain due to the oxygen rich environment, compared to 2-PAM (pralidoxine)

Question 30

Site specific delivery of pro-drugs

Answer 30

One of the most promising aspects of pro-drugs is the possibility to achieve site-specific delivery of the active drug following systemic administration of the pro-drug
In principle this could greatly improve the pharmacological potency, whilst removing unwanted systemic effects (toxicities)
In practice, the full potential of this approach has yet to be achieved

Question 31

Requirements for site specific delivery of pro-drugs (4)

Answer 31

Rapid transport/uptake at site of action
No cleavage of pro-moiety at liver, kidney etc.
Efficient cleavage at site of action
Retention of drug at active site for a reasonable period

Question 32

Site specific delivery of dopamine

Answer 32

Dopamine has a wide array of pharmacological effects at many tissue sites
Its vasodilating effects in the liver could be useful in the treatment of shock if it could be delivered selectively
Glutamyl transpeptidase has high activity in the liver, if we can design a dopamine pro-moiety to be cleaved by this enzyme system we may achieve liver selectivity
The dopamine is selectively generated from the glutamyl dopamine in the liver
The potent pharmacological effects of the drug are thus concentrated in the liver

Question 33

Site specific delivery of hexamine

Answer 33

Hexamine acts as a pro-drug of formaldehyde, which is too toxic for systemic use, but achieves site-selective delivery
The pro-drug reversion is chemical mediated, not enzymatic
In the acidic environment of the urine, the tricyclic amine hydrolyses, to liberate the anti-bacterial formaldehyde
To prevent hydrolysis in the stomach an enteric coated tablet must be used

Question 34

ADEPT

Answer 34

Antibody-directed enzyme pro-drug therapy
An activating enzyme is attached to a tumour cell antibody
A pro-drug is the administered that requires this enzyme for activation, pro-drug activation is then confined to the region of the tumour cell
Antibody-linked cytosine deaminase leads to a 17-fold increase in tumour cells following FC dosing, vs. FU dosing alone

Question 35

Site specific delivery of cyclophosphamide

Answer 35

Cyclophosphamide first undergoes an enzyme catalysed activation step, hydroxylation by liver microsomes
This material spontaneously rearranges to a ring opened isomer
Which finally undergoes a chemical beta elimination to yield the actual drug

Question 36

AZT

Answer 36

AZT is entirely inactive and requires three consecutive phosphorylations to the triphosphate in order to be active
Thus, AZT is a pro-drug of AZT-triphosphate, which is common to all nucleoside drugs

Question 37

Problems of nucleoside drugs (5)

Answer 37

Often poor metabolism to the active triphosphates
Deactivation, chemical or enzymatic
Structure based/ RDD difficult
Clinical toxicities
Emergence of resistant virus (tumour)