Drug targeting and nanotechnology 1 Flashcards
What are the two types of controlled drug delivery?
Controlled release and Site-specific drug delivery
Why is site-specific delivery systems desirable?
some drugs never reach the target site
- Because of short half life
> rapid elimination
> inactivation by metabolic action or degradation
- Undesirable distribution of drug, too little at the target, too much at the non-target organs
- Poor transport of drug across biological membrane
> poses a problem if the target is intracellular
> caused by physical-chemical properties of drug such as size, hydrophilicity etc
Presence of drug at non-target sites may cause unacceptable side effects
- Low therapeutic index
What are EIGHT requirements for an ideal drug targeting system?
- Control drug release to achieve the desired therapeutic effects.
- Prevent degradation or inactivation of drug during transit to the target sites.
- Maintain the drug-carrier integrity until the target is reached.
- Target recognition and association.
- Enhance therapeutic index of drug.
- Biocompatible
- Biodegradable
- Simple, reproducible and cost effective
What are the 1st,2nd and 3rd order targetting of site-specific delivery systems?
- 1st Order Targeting (vascular compartment) – delivery to a discrete organ or tissue
- 2nd Order Targeting (cellular) –targeting to a specific cell type(s) within a tissue or organ (eg tumour cells versus normal cells)
- 3rd Order Targeting (intracellular) – this implies delivery to a specific intracellular compartment in the target cells (eg lysosomes )
What is the difference between active and passive targeting for site-specific delivery systems?
Passive: use the natural disposition (set-up) of the carrier in the body
Active: uses some type of “homing” device or principle to change the natural disposition pattern of the carrier to selectively target a particular tissue or cell type
What does the success of drug delivery depend on for site-specific drug delivery systems? Provide THREE different reasons.
- access to the target site
- retention at the target site
- timing of drug release at the target site
The therapeutic efficacy of targeted drug delivery systems depends on the timely availability of the drug in active form at the target site and its intrinsic pharmacological activity
What are THREE factors influencing drug targeting?
- Drug properties - less important if drug is carried by a delivery system
- Properties of delivery system: eg size, charge, surface ligands
- in vivo condition/environment: pH, blood flow; enzyme; vasculature; temperature
What are THREE key biological processes for drug delivery?
- Vascular release (extravasation)
- Cellular uptake
- Lymphatic uptake
What is Vascular release (extravasation)? Describe the process.
Drug or drug delivery system –> blood circulation —> extravasation –> target site (extravascular)
- Most drug/carrier systems need to leave the blood vessels and act on or in cells of target area
- The way drug/carrier leaves vessel depends on:
> permeability of vessel wall
> drug/carrier compound itself
Blood vessel structure varies in different body areas
What are the THREE types of capillaries (blood vessels) which impacts extravasation?
Continuous
- tight interendothelial junctions
- uninterrupted basal lamina
Fenestrated
- 20-80nm gaps in endothelium
- gaps have a thin membrane
Sinusoidal
- ≤150nm interendothelial gaps
- basal membrane discontinuous or absent
fenestrated and sinusoidal present in liver, spleen and bone marrow
For extravasation;
A) What is it governed by?
B) What is extravasation of drug molecules affected by?
C) What is extravasation of drug molecules in drug delivery systems are influenced?
A)
- Extravasation is governed by permeability of blood capillary walls
B)
- Extravasation of drug molecules is also affected by the physicochemical properties of drug such as molecular size, shape, charge & lipophilicity.
C)
- Extravasation of drug molecules in drug delivery systems are influenced by the physicochemical properties of drug delivery systems, not drug molecules.
- Different types of delivery system, their extravasation pathway can be different.
What are the two ways substance pass out of blood vessels (extravasation)
Passively
- Via gaps
- Non-specific phagocytosis
dependent on: molecular size and blood vessel structure
Actively
- receptor-mediated transport
Compare extravasation of soluble macromolecules vs colloidal particles. Include MW in answer.
Macromolecules (soluble)
- (1) by non-specific fluid phase pinocytosis
- (2) passage through inter-endothelia junctions and gaps in the capillary structure
- (3) by receptor-mediated transport systems
If Mw>70kDa, retained in the circulation
Colloidal particles
Escape vasculature through
- (1) the gaps in fenestrated and sinuosoidal regions
- (2) by phagocytes transporting it out of blood
- (3) also via receptor-mediated transport systems.
In certain pathological conditions, more extravasation occurs (eg MW ≤300kDa may escape in tumour tissues.)
For cellular uptake;
A) What is endocytosis?
B) What are the two types of endoyctosis?
A)
- Internalisation of plasma membrane engulfing some extracellular material (fluid or particle)
B)
- Phagocytosis – particle “cell eating”
- Pinocytosis – fluid “cell drinking” –> small particles suspended in the extracellular fluid are moved into the cell
What are the steps involved in phagocytosis (cellular uptake)?
Phagocytes belong to mononuclear phagocyte system (MPS)
- Recognition (opsonisation/complement activation)
> Foreign particle is “opsonised” (IgG, complement C3b and fibronectin adsorb onto particle and phagocyte recognises it)
- Adhesion
- Internalisation and Digestion –> Phagocyte cell engulfs particle and phagosome fuses with lysosome inside cell
What are the types of pinocytosis (cellular uptake)
Universal phenomenon, no external stimulus
Fluid phase pinocytosis
- non-specific, continuous process, avoid binding with the membrane constitutes
Adsorptive pinocytosis: adsorption of macromolecule to cell surface before endocytosis
- nonspecific pinocytosis
- receptor-mediated pinocytosis
Pinocytic vesicles can interact among themselves or with endosomes/lysosomes
What does the lymphatic system start as?
Starts as capillaries in tissues and leads back to venous circulation via vessels of increasing size
- Lymph capillaries have single layer of cells with gaps varying from 12nm to 70nm.
- Pressure build-up in tissues from extravasation promotes formation of lymph
- High MW drugs (SC or IM injected drugs) goes into the lymph.
What type of particles can enter the lymph and what particles are reabsorbed into the blood vessels? What increases absorption into lymphatic system?
- Soluble macromolecules ≤30nm can enter lymph
- Particles >50nm are retained in the interstitium
- Low MW (<10 kDa) substances can be reabsorbed into blood vessels
Presence of negative charge or lipid in formulation increases absorption into lymphatic system
What does enhanced permeability and retention (EPR) mean in the lymphatic system? What does it bring an opportunity for?
- Blood vessels in solid tumours “leaky” –> enhanced extravasation in these areas
- Larger molecules can pass out of blood vessels
- Underactive lymph system
Opportunity for passive targeting of cytotoxic drugs. Tumour enzymes can help release drug intra- or extra-cellularly
What are THREE general components for carrier-based target drug delivery?
- an active moiety (drug): for therapeutic effect
- a carrier: for (metabolic) protection, changing the disposition of the drug
- a homing device: for specificity, directing the system to assigned target site
What are the types of site-specific delivery systems?
- Soluble macromolecules
- Micelles
- Dendrimers
- Colloidal carriers:
Liposome (Stealth Liposomes), Nanoparticles, Microparticles
What are the FOUR features of soluble macromolecular drug delivery systems?
- They are soluble and can move relatively freely around the body, therefore more accessible to many organs and tissues.
- The uptake of soluble macromolecules by cells occurs by pinocytosis
- Because of their size, they alter pharmacokinetic profile of the drug.
- The distribution and elimination properties of this system depend on the Mw, hydrophilicity/lipophilicity, ionic nature of macromolecular carrier and its interactions with physiological surroundings
What are the things that make up soluble macromolecular drug carriers
- Macromolecule backbone
- Optional Targeting Ligand
- Intracelluarly cleavable linker
- Drug attachment (direct or via spacer)
What are examples of PEG-conjugated drugs (soluble macromolecular drug carriers)?
- Pegfilgrastim (Neulasta®) – used with chemotherapy to stimulate production of neutrophils. Half-life increased from <4 hours to up to 42 hours compared to filgrastim. One dose per cycle instead of daily.
- Peginterferon alfa-2a (Pegasys®) – treatment of Hepatitis C. Half-life ≈80 hours cf 5 hours for interferon alfa-2a.
- Peginterferon alfa-2b (PEG-Intron®) – 5x increase in half-life
What are the two types of targeting for soluble macromolecular drug carriers? Describe them.
Passive targeting
- Extravasation
- Tumour sites (EPR effects)
Active targeting
- Receptor mediated pinocytsois
- Then combines with lysosomes
What are the major concerns with macromolecule-drug conjugates carrying antibodies (immunoconjugates)?
- Immunogenicity –> causes immune response
- Loss of activity due to chemical coupling procedure
- Low drug carrying capacity
- Loss of specificity due to premature binding with free antigens in general circulation
- Poor diffusion and extravasation due to the increase of the dimension (size)
What are the barriers to tumor targeting by antibody?
- Tumour heterogeneity
- Antigen shedding
- Antigen modulation
(tumour cell-surface specific molecules (antigens) are structures that not necessarily unique - can occur in lower density level in non-target cells)
What are the strategies for overcoming barriers in antibody targeting?
- Use cocktails of different MAb (monoclonal antibodies)
- Bystander effect - release drug at the target but before the antigen-conjugate complex is endocytosed.
- Select antigens which do not show shedding or modulation.
- Inject free MAb prior to the injection of immuno-conjugate (to mop up any free circulating antigen).
> Active targeting of drug requires attachment of homing device
> Antibodies can be used as targeting moieties (immunoconjugates)
> Other ligands that recognise specific sites may be used
What are the types of other targeting ligands that can be used for active targeting? Give examples.
Endogenous ligands
- Transferrin
- Folic acid
> Small molecule & over-expressed on cancer cell
> Efficiently internalised after binding with ligands
- Lipoprotein
Immunological ligands
- Interleukins
- Interferons
Glycoconjugates
- Lectin
- glycolipids
