The Medicine - Semester 2 Flashcards
What is the difference between a drug or a medicine?
Drug - Active pharmacological agent
Medicine - API within its formulation
How does drug development start?
Drug tested in solution for interactions with isolated target cells
Why aren’t drugs administered as solutions?
Not administered in solution due to expense and impracticality
What are the aims of certain dosage forms?
Favourable manufacture, cost, stability
Sufficient amount of API must reach target cells
Define bioavailability and absolute bioavailability
Bioavailability - Absorption of drug as well as availability at site of action
Absolute Bioavailability - Amount of drug from the dosage form in the systemic circulation
Why is blood considered the central compartment?
Links all the tissues in the body
Describe the general routes of administration
Across epithelial layers (skin, GIT, bronchioles, pulmonary)
Bypassing epithelial layers (intra or extravascular injections)
When are parenteral administration routes used?
Patients who can’t take oral drugs
If quick response is required
If drug is unsuitable for oral administration
How do transdermal drug delivery systems work?
High concentration of drug in formulation - drug is low Mr and lipophilic
Excipients may enhance sc permeability
Rate controlling polymeric membrane controls drug release
Why may GI delivery be preferred over transdermal?
Higher SA and higher permeability (absence of stratum corneum)
What affects availability of absorption for a drug?
GIT Variables: intestinal transit time, properties of GIT fluid, properties of tissue at the absorption site
Drug/Dosage Form Variables: Chemical properties of the drug, formulation, pharmalogical effects of excipients
How does absorption differ at different points in the GIT?
Transit time increases as drug moves further down
Properties of the absorbing tissues changes
How do buccal/sublingual delivery systems work?
Mouth is more permeable than skin but smaller SA
Low Mr, lipophilic drugs
Convenient administration
Fast action and avoidance of first pass metabolism
How are drugs absorbed in the stomach?
Fairly low SA
Little absorption - tight junctions and no nutrient transporters
Weak acids and highly permeable drugs may be absorbed
How are drugs absorbed in the intestine?
High SA (microvilli)
Highly perfused
Nutrient transporters
Potential for paracellular transport
What are the three absorption pathways? Describe them
Transcellular: Small, lipophilic, unionised forms
Paracellular: Hydrophilic drugs with low Mr
Carrier Mediated: Protein transporters in membrane
Describe the absorption of gabapentin
Bioavailability decreases as dose increases due to need for active transport
Pro-Drug can be transported by a number of carriers, increasing bioavailability
How do drugs undergo first pass metabolism?
Drugs pass through liver before entering circulation by hepatic portal flow, may be broken down at this stage
How are excipients chosen and what is their purpose?
Generally dependent on indication
Enhance chemical properties of a medicine
What are the objectives of a dosage form?
Produce a predictable therapeutic response
A product that has capability for large scale manufacture
A quality of product that can be repeatably obtained
What aspects of novel drugs may present a problem during manufacturing?
High MW
Peptide, protein or viral components
What are the pharmaceutical considerations of dosage forms?
How different formulations will impact absorption
Physicochemical properties of drug itself
Therapeutic condition and the patient
Financial considerations
Describe pre formulation
Stage of development before drug is made into a medicine
Optimisation of a drug candidate to form a drug product
one is then picked for further development
What makes a good drug candidate to develop further?
Properties lead to an obvious formulation
Generally easiest to formulate
What aspects of a formulation determine bioavailability?
Stability
Dissolution
Drug-Excipient interactions
What are the intrinsic pre formulation considerations?
Solubility: 1mg/ml as liquid, 10mg/ml as oral product
Hygroscopicity: Increased water content can affect manufacturing processes/Water may result in degradation or interactions/Adsorption alone is not enough water to dissolve a substance
Define hygroscopicity and deliquescence
Hygroscopicity - Attraction of water through adsorption or absorption
Deliquescence - Molecule is so hygroscopic that it can dissolve in the volume of water
What are the macroscopic pre formulation considerations?
Melting point Particle size and shape Powder flow Compaction Amorphous materials Polymorphism
How does powder flow affect formulation?
Important for mixing and optimal filling of press
Angle of repose 15-25 degrees and particle size >50µm
How does compaction affect formulation?
Depends of bulk density and tapped density - bigger difference = better compaction
How do amorphous materials affect formulation?
More soluble than crystalline compounds but eventually become crystalline due to instability
How do polymorphs affect formulation?
Most stable has the highest melting point however may not have the best bioavailability or manufacturing properties
How is solubility of an API measured?
Create saturated solution and filter
Analyse solution to determine solute conc. present
What are the rotating/static disc methods for measuring solubility?
Drug formulated in non-disintegrating disc - one face exposed to solvent
Maintain constant speed of rotation, position of disc/holder
Remove solvent and assay at fixed time intervals
How can solubility be modified?
Esterification reduces aqueous solubility
Co solvents can be used to alter solubility
Metastable polymorphs are less soluble
Addition of solvent molecules into crystalline structure reduces solubility (if solvent is aqueous)
How does ionisation affect solubility and what does it depend on?
More ionisation increases solubility
Depends of pH and pKa of molecule and pH of solvent
Why are oral drugs absorbed in the small intestine?
Usually weak bases so they remain unionised
How drug salts increase solubility of drugs?
Acid/base reacts to form salt - stronger = more complete reaction
Dissolution of salt may result in pH change which further aids dissolution of drug
What is wettability and what does it depend on?
Ability of a liquid to stay in contact with a surface - it is defined by the contact angle of droplet of liquid on a surface
Depends on adhesion and cohesion forces (adhesion improves wetting)
What is the contact angle?
Result of interfacial energies of surface tension of the liquid and the different interfacial energies
Wetting is favourable if contact angle is less than 90°
What is viscosity?
Resistance to movement
Describe properties exhibited by Newtonian Fluids
Rate of flow directly proportional to stress applied
What types of flow do pharmaceutical fluids exhibit?
Plastic flow - Needs a minimum amount of pressure to begin to flow, elastic at pressures before this
Pseudoplastic flow - Material flows with application of pressure, pressure disentangles long molecules to reduce flow resistance
What is dilatant flow?
Addition of pressure increases viscosity of the fluid
Particle disruption causes clumping and larger voids for fluid to fill
Increased in viscosity reversed with removal of shear
Why is dissolution important?
Drug has to be in solution in order to get into bloodstream, other fluids and tissues
What is the Notes-Whitney equation and what does it indicate?
Rate of dissolution
dm/dt = DA(Cs-C)/h
D = Diffusion coefficient, A = SA of particle, h = thickness of diffusion layer, Cs = saturation concentration in diffusion layer, C = concentration in GI fluid
What factors affect the rate of dissolution?
A - Depends on particle size, porosity, dispersibility and wettability
Cs - Depends on solubility in diffusion layer
C - Low conc. maximises Cs - C
D - Depends on the viscosity of the solution
What are the different crystal forms of APIs?
Polymorphs - Metastable or stable, most stable not necessarily most soluble
Amorphous/Crystal - Amorphous more soluble but eventually converts to crystalline structure
Solvates - Higher degree of solvation, lower solubility
How does drug concentration vary in GI fluid?
- Adsorption of the drug to other materials may prevent absorption
- If the drug is chemically unstable it may be broken down in the stomach so less is available for absorption in intestine (enteric coating and pro drug prevents this)
What is the intrinsic dissolution rate of a molecule?
Rate of mass transfer per area of dissoluting surface
What is the importance of in vitro testing?
Predict rate and extent of drug release in vivo to predict absorption
What is the effect of limited solubility?
Dissolution is the rate limiting step of the reaction
What are the experimental conditions used for testing intrinsic solubility?
0.1M HCl, 0.1M NaOH and water used
UV spectroscopy for analysis
Test in temperatures of 4°, 25° and 37°
pH and common ion effect
What are the steps of absorption?
Disintegration
Dissolution
Permeation
Pre-systemic metabolism
How do transit time and stability affect absorption of oral formulations?
If transit time is too fast, medicine is excreted before it is absorbed
Drug may be broken down in stomach/gut or by extensive 1st pass metabolism so low bioavailability
What are the solubility and permeability parameters for formulation?
Solubility - Max dose in 250mL or less over pH 1-8
Permeability - Predicted over 90% absorption
What are the limitations of testing drugs before formulation?
Stability varies in different conditions
Binding interactions of drugs and excipients are not considered
What are the BCS drug classes?
1: HP/HS
2: HP/LS
3: LP/HS
4: LP/LS
What is the main formulation consideration for classes 2 and 3?
2: Maximise dissolution rate
3: Maximise transit time in intestine
How may class 4 drugs be formulated?
Pro-drugs
New formulations
What are the physicochemical factors affecting rate and extent of absorption?
Size and wettability of particles
Solubility and pKa of drug molecule
Lipophilicity of drugs
Stability of drug
What are the biological factors affecting rate and extent of absorption?
Gut motility GIT transit time Local pH Enzymes/surfactants Epithelial SA and physiology Gut contents
How does pH influence drug absorption?
Solubility (esp. for WA/WB)
Stability (drug susceptible to acid hydrolysis)
Gastric pH increases after food
Intestinal pH is affected by food
How does food influence transit time?
Slower gastric emptying after food therefore longer transit time
How can food be used to improve stability of the drug?
Advise to take with or after a meal to increase gastric pH
How can the drug be altered to improve stability in the stomach?
Increase particle size to slow dissolution
Formulate as a salt to alter dissolution
Use enteric coatings to delay disintegration until intestine
What is a pro-drug?
API not released until medicine is absorbed, protects API
Increases solubility
What is the aim of drug absorption?
Peak concentration within therapeutic range
What decides bioavailability and dosage regimen?
Time taken for absorption and elimination
What are the practical issues with assessing bioavailability?
Drug amount has to measured as a conc. and scaled to approximate known volume of blood
Have to account for DME by using a control measurement of injecting the drug
What is the equation for absolute bioavailability?
AUCoral/AUCiv
What is the most common route of administration?
Oral
What are the advantages of tablets?
Increased patient compliance (convenience and aesthetics)
Easy to store and dispense
Better stability
Low cost, easy bulk manufacture
Drug release can be altered
Taste can be masked
Different forms of tablets available to suit patient need
What are the disadvantages of tablets?
Multi stage manufacture results in product loss
Absorption depends on gastric emptying
Powder properties affects compression
May be inappropriate for some patient groups
What is defined as a high dose or low dose API?
High: >80% of total tablet weight
Low: <5% of total tablet weight
How does the size of tablet vary?
Size of tablet is proportional to amount of API so low dose tablets will be smaller
What is the maximum total tablet weight and what is the normal range for an API?
<800mg total weight
5-500mg of API per tablet
How should tablet doses be considered in relation to compliance?
Dose should require no more than two tablets at any time to improve patient compliance
What is the minimum tablet weight and what may be used to achieve this? What properties does this excipient have?
Min. weight 50mg
Low dose tablets will require a filler/diluent to reach this
Fillers/diluents have good compression/solubility properties
What excipients aid disintegration and how?
What percentage of tablet makes up this excipient?
Disintegrants
Facilitate water uptake or rupture from within to break up tablet
1-10% per every 100g of tablet