target identification and validation and lead optimisation

Question 1

What makes a good pharmacological target

Answer 1

disease association - causal (possible genetic association), such as CTFR mutation in cystic fibrosis, or if the target has an ability to reduce or overcome symptoms - e.g., B2-adrenoceptor agonist bronchodilators.

target has to be expressed in the target organ

the target must be druggable. any protein with an active site/binding pocket is considered drugable by low MW compounds. the more points of interaction between the compound and the binding pocket the higher the affinity/selectiviyt of the drug.

Question 2

Purpose of target identification and validation

Answer 2

Links the target with the disease, determining whether modulation will help treat the disease. also determines whether modulation of target would cause side effects (on-target).

it requires an understanding of the disease and the target in order to identify whether it would treat the cause of the disease, or treat its symptoms. this info can be determined from bioinformatics (datamining for changes in the disease), data from publications, and genetic changes in the disease.

this can be done functionally, such as in primary human cells using target knockout or pharmacological modulators, or using animal models/human tissue experiments. transgenic animals which decrease or increase the protein of interest can identify phenotype changes.

compensatory mechanisms and species differences bring limitations to these models however.

Question 3

Lead identification and methods

Answer 3

development of an assay that would measure the target’s activity. e.g., in CHO or HEK cells over expressed with the target (or addition of recombinant protein - e..g, enzymes). Will be done as HTS such as in a colorimetric or fluorescent plate reader.

must consider the pharmacological relevance, reproducibility, costs, quality and the impact of the vehicle in these assays.

HTS of hundreds/thousands of compounds would identify hits. can also be done virtually using chemical structures to predict interactions. Physiological screens may be done, such as muscle contraction assays

Question 4

Lead optimisation process

Answer 4

takes hits from the HTS and then optimises those compounds to give better parameters. will do assays identifying the potency, functional responses (+ relevance in disease models), selectivity, PK (e.g., Caco cell permeability and BBB permeability), undesirable effects/toxicology (+ CYP enzyme inhibition), and begins to develop structure-activity relationships based on multiple compounds physiochemical interactions with the target. Will also look at the metabolic and chemical stability of the drug in the plasma

Structural changes to the compound will be done an if the change increases potency, then it will be retained. this will generate a structure activity relationship for the target. repeatedly done

Question 5

Factors that make a good drug

Answer 5

Activity (binding and efficacy), selectivity, PK, safety, chemical and metabolic stability, permeability (e.g., cell membrane, GIT, BBB), solubility, pH, etc…

Chemical stability determines shelf-life. physiochemical parameters determine the metabolism (thus t1/2) and potential enzyme inhibition, as well as bioavailability

Question 6

Factors causing drugs to fail pre-clinically

Answer 6

Assays cannot be developed to screen compound

No HTS hits

HTS hits don’t produce functional tissue responses

Hits are toxic - in vitro or in vivo - or undesirable side effects

Poor PK - e.g., permeability, distribution, bioavailability or halflife