Section 1 Flashcards
In vitro studies
Identification of a new drug target
Screening for a lead compound
Target validation -> target identification -> lead compound ID -> Candidate Optimization
Pre-clinical Testing
Animal Testing
2-4 years
Tests: Efficacy, Selectivity, Mechanism
Clinical Testing: Phase I
Phase I: is it safe? What are the pharmacokinetics?
20-100 people
Clinical Testing: Phase II
Does it work in patients with the disease?
100-200 patients
Usually conducted in special clinical centers
Measures: safety and efficacy “proof of concept”
Phase II: have the highest rate of drug failures
Lipinski Rule of 5
No more than 5 H-bond donors No more than 10 H-bond acceptors Molecular mass of less than 500 Da Octane-water partition coefficient NOT greater than 5 (Solute in octanol / solute in water)
Preclinical Testing
Tests acute toxicity: determines maximum tolerated dose
Determines dose that is lethal in 50% of animals
Subacute Toxicity:
- determines the biochemical and physiological effect
Effect on reproductive performance
Carcinogenic Potential
Mutagenic Potential
Limitations of Preclinical Testing
Expensive and time consuming (2-6 years)
Large number of animals needed
Extrapolations of therapeutic index Anand toxicity data from animals
Rare and adverse effects are unlikely to be detected in preclinical testing
Confounds to clinical testing:
Variable hx of disease = overcome by large population evaluated over time
Presence of other diseases/risk factors = over come by cross over technique
Subject/observer bias =
- placebo responses = overcome by single blind, crossover design
- observer bias = double-blind design
IND
Investigational new drug application filed with the FDA
Includes:
- info on composition/source of drug
- chemical information
- all data from preclinical
- proposed clinical trial plans
Phase I Clinical Trial
20-100 healthy volunteers
Effects of drug as function of dosage for expected toxicity
To find the maximum tolerated dose so that a dose can be recommended for phase II
Pharmacokinetics measured
- absorption, distribution, 1/2 life, metabolism, excretion
Dose Escalation Methods of a Phase I Clinical Trial
Amount of drug in dose is increased with each cohort added
- each cohort is called a “dose cohort” (~10)
- new dose cohort cannot be initiated before safety in previous cohort has been fully assessed
Modified Fibronacci Series: add previous dose to the current one to get the new dose
With dose increases, the action between 2 consecutive doses get smaller
Phase 0 Clinical Trial
To test to see how much of a drug is present in tumor, blood, tissue after one dose —> to see if the drug actually got INTO the tumor
To check whether there is a problem with how the drug is:
Absorbed, distributed, metabolized
Pharmacokinetics and dynamics
Phase II Clinical Trial
In patients with the targeted disease (n = 100)
To determine efficacy = proof of concept
Evaluates safety, tolerability, efficacy
Phase III Clinical Trial
Evaluated in larger patients (1000-6000)
Performed in settings similar to those anticipated for ultimate use of the drug
- formulated as intended for the market
- usually expensive due to large number of patients
Safety and Efficacy
NDA
New Drug Application
- if phase III trial meets expectations, NDA is filled out to market agent
(For biological, a BLA is filed)
Takes months/years for FDA to review
Number of subjects avg 5000
Priority review for breakthrough drugs -> accelerated approval might be granted
Orphan Drug Program
Gives incentives for drugs that treat rare disease
(If less than 200,000 puts or R+D costs do not expect to be recovered)
Eg. Gleevec - oral treatment for CML
Accelerated Approval of NDA: use surrogate endpoints for effectiveness
- bio markers that are likely to predict clinical benefits
Phase IV Clinical Trial
Post marketing surveillance begins
NDA approved
Delienate treatment risks, benefits and use under “actual use” conditions
Efficacy Study
Patent Rights
20 year term for a drug patent filed after 1995
- sometimes the patents expire after the drug is approved
Post expiration of the patent = any company can produce the equivalent
- file a ANDA (abbreviated new drug application) to market a generic version
MW of drugs
100-1000 Da
Agonist
Drugs that mimic actions of endogenous compounds
Agonist interacts with receptor to produce a pharmacological response
Partial Agonist
Agonist that produces a partial response when the receptor is fully saturated
Inverse Agonism
Binds to the same receptor as an agonist but produces the opposite effect
Requires that the receptor has a basal level of activity in absence of ligand
Antagonist
Interferes with action of an agonist/partial agonist/inverse agonist by binding to the receptors
- they bind to the receptor and do not produce a pharmacologic response
Total number of receptors on a cell determines…
Maximal drug effect that the drug might produce
E = [D] x Emax/[D] + EC50
Binding of different agonists to the same receptor:
Plotted on semi log
Sigmoidal curve
EC50 corresponds to the inflection point on the graph
Two drugs will show the same Emax, yet different EC50
Low EC50
High Potency
Less drug needed to reach half of maximal response
High EC50
Low potency
Need more concentration to reach 1/2 max
High drug is not desirable, since adverse effects are likely to increase
Potency Ratio
Differences in potency between two drugs
EC50 of Drug A/EC50 of Drug B = potency ratio
What determines the efficacy of the drug?
Emax
Agonist versus partial agonist behavior = PA shows lower efficacy
How are drugs selected?
On efficacy or Emax
The amount of drug given is based on the determination of the potency factor
Antagonists
Drugs that block actions of endogenous agents
Carry no activity
Observing Effects:
- plot dose-response curve of an agonist
- determine how that curve is affected by differing antagonist concentrations
Competitive Antagonists
One whose effects can be overcome by adding more agonist
Agonist and antagonist compete with one another
Commonly observed if they have reversible binding property to receptor
By adding more:
- curve shifts to the right (EC50 increases)
- Emax is unchanged
Better antagonist = tighter binding to the receptor
Non Competitive Antagonists
Prevents agonist from reaching maximum effect
Irreversible binding = like a covalent modification
Antagonist bound receptors are effectively removed from the pool of agonist targets
By adding more non-competitive antagonist:
- Emax is reduced
- EC50 remains unchanged
Makes the agonist look like a PARTIAL agonist (Emax lowered, EC50 unchanged)
Space receptors
Increase sensitivity of tissue to the drug (use little bc the target is larger)
Agonist can produce max response without binding to all of the available receptors
Irreversible Antagonists in Spare Receptor Situation
IA decrease the number of spare receptors
- increase EC50
- Emax left unchanged
Once all space receptors are quenched, further addition of antagonist no longer effects the EC50
- EC50 remains unchanged
- Emax decreases
Therapeutic Dose
ED50
Lethal Dose
LD50
Toxic dose
Dose Response Curves
Difficult to construct when pharm response is an “either-or” event
Inter-individuality limits the applicability of 1 dose-response relationship to another
Quantal Dose Response Curve
Y axis = number of individuals responding
X axis = concentration of drug (log[D])
Median effective dose = 50% of individuals exhibit the quantal response
Therapeutic Index
Margin of safety to be expected for a drug
Therapeutic Index = TD 50/ED 50
On Dose-Response Quantal Curve, it is the range of dose between TD50 and ED50
What is the major route of elimination in drugs?
Excreted through the kidney unchanged
Lipophillicity of most drugs:
Facilitates cross through membrane
Renal excretion of lipophillic drugs are POOR due to reabsoportion through the tubular membranes
What is the 2nd Major Route of Drug Elimination?
Bio transformation of drugs in hydrophilic metabolites
- making them more polar is critical for the termination of biological activity and excretion through the body
What are Phase I Reactions in Xenobiotic Metabolizing System?
Introduce/exposes a polar functional group
- OH, COOH, NH2, SH on compound
Enzymes that Catayzle Phase I Functionalization Rxns exist on ER membrane
What are Phase II Enzymes in Metabolism?
Phase II Rxns (Conjugation Reactions)
Conjugation of compounds to yield even more polar conjugated
Located in the cyto soul
Where is the main site of drug metabolism/biotransformation?
Liver = principal organ
GI tract, lungs, skin, kidneys = secondary
Cytochrome 450
Involved in Phase I reactions
Oxidative Rxns require:
- molecular oxygen
- NADPH - reducing agent
- Cytochrome 450
CYPs
Large capacity to metabolize large number of diverse chemicals
Not high specificity
Major Form:
CYP3A4
Phase II Conjugation Reactions in Drug Metabolism
Functional group by Phase I makes compound reactive
- neutralize functional group, make more soluble
Phase II
- involve specific transferases located in the cytosol
- couples a endogenous substance + exogenous
HIGHLY POLAR NATURE OF CONJUGATES PROMOTE elimination
Glucuronidation (Phase II reaction of Metabolism)
Endogenous reactant = UDP-Glucuronic Acid
Uses UDP-glucuronosyltransferase (UGT)
Glutathione Conjugation in Phase II Reactions
Major detoxification pathway
- conjugates reactive electrophillic compounds with tripeptide glutathione
Glutathione S Transferase
Endogenous Reactant = Glutathione
Sulphation in Phase II Conjugation Reaction
Sulphotransferase
Endogenous reactant = PAPS
The metabolic products are…
Often less active than the parent drugs
- they might have enhanced ability (inactive prodrugs might convert to metabolically active drugs
Drug metabolism…
Increases clearance
Decreases 1/2 life
Before a New Drug Application…
Route of metabolism and enzymes involved must be known
Isoniazid (INH)
Is the exception where Phase II reactions precede Phase I reactions
Acetylation (Phase II) occurs 1st
Hydrolysis (Phase I) occurs 2nd
Induction of metabolizing Enzymes
Increases the number of enzymes
Induces Cytochrome P450 gene (of which makes more enzyme)
- increases metabolism rate of some drugs
Inhibiting Metabolizing Enzyme
Decreases the amount of enzymes
Grapefruit juice
Alcohol
Might impair elimination of the drug, prolong its effects, increased incidence of drug toxicity
Liver dysfunction
Affects the liver function and diminishes they metabolism of some drugs
- increases the half life of drugs
As one ages, there is..
Decrease in: liver mass Hepatic enzyme activity Hepatic blood flow P450 activity
Females: decreased oxidation of estrogens and benzodiazepines
Oral Route Absorption of Drugs
Absorption of drug from H2O across the GI epithelium
GI -> Blood
Drug Lipophillicity
Factor involved in absorption
- lipid/water partition coefficient is the best predictor of drug entry measured by octanol/water
- higher the coefficient means that the drug will leave the water phase and cross into the GI epithelium
PkA or Ka of a adrug
Factor involved in absorption
Most drugs are weak bases or weak acids
Uncharged weak acid
Protonated form
Can move across the biological membrane
Uncharged form of weak base
Unprotonated form
Can move across the biological membrane
Lipid/Water Partition Coefficient
Best predictor of drug entry into body
If above one, or higher, the drug will more readily leave the water phase and cross the GI epithelium
10 (pH-pKa) =
[A-]/[HA]
For carboxlyic acids, they are ionic in the deprotonated state.
1O (pH-pKa) = (for amines)
[B]/[BH+]
Partitioning of Drug
Distribution of a drug will proceed until the uncharged form of drug achieves equal concentration in both water compartments
At equilibrium, the total drug concentration is higher in compartment with higher pH-dependent ionization.
-> if it is ionized, it is stuck in place!
Oral Bioavailability of a drug is:
Fraction of the drug that gains access to systemic circulation in chemical all unaltered forms
- if it is absorbed well and survives the liver metabolism
Mouth -> GI -> liver (1st pass metabolism)
Infusion Rate
R = CL (body) x CL (plasma)
Half Life Equation
T1/2 = .693 x Vd / CL(body)
Vd = L/kg
This means that you need to correct for Kg of the patient
Vd = 1.5L/Kg x 70kg
What is the time for a drug to reach steady state concentration?
Four 1/2 lives
Eg:
(8 hr half-life)(4) = 32 hours to reach steady state
To create an IV regimen, you begin with twice the effective dose of the drug. Then what?
Repeat with the effective dose every 1/2 life of the drug.
Only works if:
- half-life is between 8-24 hrs
- 2 fold fluctuation in the drug is acceptable
Example:
Loading dose = 500 mg
Maintenance Dose = 250 mg/8 hours
Switch from IV -> Oral Considerations:
Maintenance Dose in IV is 100mg/2x day
Oral Dose with a bioavailability of 50% = 200mg/2x
Fixed Dose and Fixed Time Regimen = most common pattern of administration
Drugs are eliminated exponentially
- some of the 1st dose is present at the time of the 2nd dose
Drug accumulates until its concentration increases to a SS point where the rate of input = rate of output
When is steady state reached?
After 4 half lives, the amount of drug lost during the dosing interval is exactly what is being put in
Volume of Distribution (Vd)
Vd must be normalized by body weight (L/Kg)
Vd = total amount of drug in the body/plasma concentration of drug Vd = Ab / Cpl
Clearance (CL)
Volume of plasma from which all drug is removed
CL(body) = L/hr, mL/min
- should be normalized for body weight
CL(renal)+CL(hepatic)=CL(body)
Clearance of drug at an organ
CLorgan = Organ Plasma Flow (OPF) x Extraction Ratio (ER)
Extraction Ratio: fractional decline in drug concentration from arterial->venous side of the organ
Half Life
Time it takes for plasma concentration or amount of drug in body to be reduced by half
T1/2 = .693 x Vd/CLbody
As Vd goes up, so does the half-life
IV infusion
Continuous administration at a constant rate
Upon infusion, plasma concentration rises until the rate of loss = rate of input
- Cpl-ss = therapeutic level / efficacious level
At the steady state, the rate of the infusion =
R = CLbody x Cpl-s
Increase in Rate, changes SS-level in what way?
2R will cause the steady state level to double
Therefore, increasing rate does not allow SS to be reached faster
2 in 2R controls where you end up, not how fast you get there.
