Cellular Drug Discovery, Cell Cycle, & Apoptosis Flashcards
Drug discovery
ongoing field of study, for one drug eventually clinically used - about 6000-10,000 new chemical compounds are synthesized (available for potential preclinical screening), not done in patients so lab testing (cells), from time target drug is identified ~12 more years until used clinically, total cost ~$1 billion, half in lab research and half in clinical trails, for one drug successfully clinically trialed about 19 have failed clinical trials
Model systems - example criteria
“druggable” target, pick a model with target that can show a suitable response, validate the target/model combo as an appropriate stand-in
Model systems
Scientifically, ethically, fiscally, medically and pharmacologically appropriate stand-ins (models) for patients eventually treated with the medications
“Druggable” target
biological identity that interacts with a drug, a receptor activated by binding insulin so diabetes, an enzyme inhibited by ibuprofen so inflammation, microtubules affected by taxol so cancer metastasis (assay) before animal and clinical tests, just want to know physical association = what is the appropriate model
Pick a model with target that can show a suitable response
- how do you test binding of insulin derivative to receptor?
- what’s a valid model for testing enzyme inhibition?
- what’s needed to test new anti-metastasis drug?
- will any one preclinical test prove efficacy of new drug?
Validate the target/model combo as an appropriate stand-in
- does model have relevant target regarding delivery, metabolism, binding of drug?
- can you simplify/downsize/reduce use of typical research models (lab mice/rats etc.)
- thousands to be tested
*increase throughput
Drug discovery models
*fish, flies, fungus, & farmacy
- zebra fish
- drosophila (fruit fly)
- yeast (fungus)
Zebra fish
- pre-clinical drug discovery
- easy to monitor embryogenesis, vertebrate cell physiology & gene homologues, small adult size (about 1.5 in) allows high throughput
- b-amyloid (alzheimer’s-associated) protein so defective movement
- in aquarium, small so many fit, similar vertebrae model
Drosophila (fruit fly)
before clinical testing with people, numerous behavior & physiology mutants mapped to specific genes, numerous human gene-equivalents identified, parkinson’s-associated gene so loss of dopaminergic neurons, model nerve degeneration = monitor flight patter to reflect motor neuron function
Yeast (fungus)
extreme example, eukaryote with metabolism mapped to conserved genes, SBDS protein associated with bone marrow failure in humans; protein function was unknown, study in yeast showed protein’s function crucial for ribosome function providing a “druggable” target to improve translation, model stand in for bone marrow
Human cells in petri dishes as models
- about 45 years ago
- sensitivity of human cancer cells to anticancer drugs in petri dish tests was directly related to drug success in treating patient tumor, does not always occur this way, at least for the compounds tested, NOT a universally guaranteed approach; that is part of the validation process, depends on cancer type and drug
Cell cycle 1950s
get cells to replicate in lab
- henrietta lacks & george gey
- cervical carcinoma cells attached to test tube, mitotically active, split to additional tubes (hela cells)
- 1st continuous human cell line
- her cells were immortal and perpetuated
Cell cycle 2020s
cultured cells from diverse sources in addition to sometimes patient tissue, if you can get them to grow in petri-dish environment
- hela and 100’s of other cancer-derived and normal tissue-derived cell types (muscle, skin, cardiac, liver, etc.) used in high through put metabolism studies, cancer gene identification, gene sequencing, pre-clinical drug testing
Imetelstat example of preclinical studies for drug effectiveness
- imetelstat inhibits telomerase (RNA/DNA) and stops cell growing, which decreases cells in petri dish
- imetelstat treated plate slows growth of cancer cells so cancer cell growth inhibited in presence of antisense oligo imetelstat
- sense oligo is the same length/sequence but it cannot interact with RNA template so cancer cells grow in presence of control (sense) oligo tying topics together
- DNA replication & telomerase
- cells in culture (petri dish) for testing of new cancer drugs
Addressing through-put - petri dishes
- 6000-10,000 new potential drugs per year (many chemicals to screen)
- need to increase thru-put for repeats, different concentrations (dose-response), different exposure times
- downsize to gelatin drop with cells, nutrients (glucose), and test compounds “printed” on microscope slide
- reduces amount needed of test compound, target cells, cost
Designing the cell culture model
*interpret what happens to cells
- what cells?
- what’s to be determined?
- what scale of testing is to be done?
What cells?
*do they appropriately model target?
- normal and cancer cells (test on normal to see if you want to treat cancer so if it is toxic it might be bad)
- validate cells retain relevant processes (ex. cyp450 enzymes)
- demonstrate target is present (ex. receptor)
What’s to be determined?
*what’s the endpoint
- survival (maintain number) and/or replication (increase number) - test with neutral red
- metabolism (biotransformation, ex. in liver) of drug
- cell migration, intracellular movement
- cell death (necrosis vs. apoptosis); toxicity - test with neutral red (lethal effect)
What scale of testing is to be done?
- biological repeats within assay
- technical repeats of entire assay
- varied times of exposure (many days)
- dose - response correlation
- cell biology meets bio-engineering
*each variation expands through put
Cell culture - drug testing
*neutral red (NR) assay - lysosomes
- NR weak cationic dye - readily penetrates cell membrane
- more neutral red so more cells grow with lysosome (healthy)
NR in healthy cells
- retained within lysosomes in healthy cells (selectively accumulates)
- NR binds anionic proteins in lysosome
- amount is therefore related to cell number
- assay endpoint; experimentally measure amount of dye
- replaces counting individual cells
- dye incorporated into cells is detected visually (microscope) or extraction from cells at end of incubation
- red dye measured by spectrometry at 540nm
- dye amount gives quantifiable response to drug
- lysate cell (break open with red dye) to give an objective number
- saves time of counting individual lysosomes
NR in stressed/damaged lysosome
dye leaks from stressed/damaged lysosome, drug toxic effects stress cell or directly damage lysosome, decrease in intracellular (lysosomal) or extractable dye reflects cell damage and/or dying cells, less red = less absorbed
Neutral red (NR) assay - putting it to use
individual cell (accumulated red) to increase concentration of candidate drug in cell causes increased damaged cells, damaged cells do not spread around bottom of well, do many times to accumulate repeatable data, maximum redness when all alive (quantitative readout)
Addressing multiple organ interaction
most drug delivery ultimately has systemic distribution, test new colon cancer drug: drug effects on other organs (possible bone marrow toxic) OR other organs (cells) affecting drug?, cells-on-chip technology = microscale systems biology (organ-on-chip), very small, cells in chamber (organs) and connecting for circulatory system, +/- liver metabolism, marrow toxicity, colon target
Assay potential chemotherapy drug: cell survival
liver chamber empty and full is same output, no liver cells with drug, and liver cells and drug
*impact on tumor cells
Interpretation on toxicity? any negative effect on bone marrow cells? regarding effectiveness (reducing tumor cell number)
many cancer chemotherapy drugs decrease WBC counts, all bone marrow cells survived with and without liver but with parent compound or predicted metabolite (drug), drug has increased impact on colon cancer cells in presence of liver because less live tumor cells (metabolize to more active form)
*would not have seen without complex model system (whole physiology)
Cell cycle steps
M: mitosis, G1: gap 1, G0: quiescent, S: synthesis, G2: gap 2
Mitosis
nuclear & cell division (cytokinesis) - relatively short in time compared to interphase
G1: Gap 1
hours to days or more (some are so long that they may not go back into cycle)
G0: Quiescent
Apparently non-dividing cells - long term temporary or permanent (extended G1)
S: synthesis of DNA
Remember, most cells will wind up with shortened telomeres (need nutrients, growth factors, etc.)
G2: gap 2
Completion of G1 replication and replicated genome ready to undergo mitosis
Interphase
Metabolically active with euchromatin & heterochromatin observed, G1 to G2 (no mitosis)
Gap
named because apparent gap in activity under microscope, seems like this but actually replicating and actively metabolizing (intracellular), G1, G0 and G2 are still metabolically, biochemically active, etc., G1 variation on theme (become longer or shorter), permanent G0 (cells with extended G1 phase) - post-mitotic and will not reenter cell cycle